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Extreme floods in the Pacific coastal region 1986

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EXTREME FLOODS IN THE PACIFIC COASTAL REGION by ANTHONY MICHAEL MELONE A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY i n THE FACULTY OF GRADUATE STUDIES Department of C i v i l E n g i n e e r i n g We a c c e p t t h i s t h e s i s as con f o r m i n g t o the r e q u i r e d s t a n d a r d THE UNIVERSITY OF BRITISH COLUMBIA August 1986 © Anthony M i c h a e l Melone, 1986 In p r e s e n t i n g t h i s t h e s i s i n p a r t i a l f u l f i l m e n t of t h e r e q u i r e m e n t s f o r an a d v a n c e d d e g r e e a t t h e The U n i v e r s i t y of B r i t i s h C o l u m b i a , I a g r e e t h a t t h e L i b r a r y s h a l l make i t f r e e l y a v a i l a b l e f o r r e f e r e n c e and s t u d y . I f u r t h e r a g r e e t h a t p e r m i s s i o n f o r e x t e n s i v e c o p y i n g - of t h i s t h e s i s f o r s c h o l a r l y p u r p o s e s may be g r a n t e d by t h e Head o f my Department o r by h i s or h e r r e p r e s e n t a t i v e s . I t i s u n d e r s t o o d t h a t c o p y i n g or p u b l i c a t i o n of t h i s t h e s i s f o r f i n a n c i a l g a i n s h a l l not be a l l o w e d w i t h o u t my w r i t t e n p e r m i s s i o n . D e p a r t m e n t of C i v i l E n g i n e e r i n g The U n i v e r s i t y o f B r i t i s h C o l u m b i a 2075 Wesbrook P l a c e V a n c o u v e r , Canada V6T 1W5 D a t e : A u g u s t 1986 - i i - ABSTRACT The research program developed hydrograph procedures for estimation of extreme rain-on-snow floods on ungauged watersheds i n the P a c i f i c coastal region. A m u l t i - d i s c i p l i n a r y i n v e s t i g a t i o n was undertaken encompassing the areas of hydrometeorology, snow hydrology and hydrologic modelling. Study components include assessment of flood producing mechanisms in the coastal region; analysis of regional r a i n f a l l c h a r a c t e r i s t i c s for input to a hydrograph model; examination of the role of a snowpack during extreme events; and a p p l i c a t i o n of a hydrograph model. Based on an assessment of atmospheric processes which a f f e c t climate, examination of h i s t o r i c a l flood data, and analysis of flood frequency, i t i s shown that the area bounded by the crests of the coastal mountains forms a hydrologic region with s i m i l a r flood c h a r a c t e r i s t i c s . Extreme floods i n the coastal region are r a i n f a l l - i n d u c e d , either as runoff from r a i n f a l l - o n l y or as a combination of ra i n and snowmelt. Recorded storm r a i n f a l l along the coast was examined to determine whether regional c h a r a c t e r i s t i c s could be i d e n t i f i e d from available data even though the magnitude of r a i n f a l l varies between s t a t i o n s . Multi-storm i n t e n s i t y data available from Atmospheric Environment Service and r a i n - f a l l i n t e n s i t i e s occurring within single storms that were i d e n t i f i e d as part of this study were analyzed. Results show that r a t i o s of shorter duration i n t e n s i t i e s to the 24-hour r a i n f a l l are i n a r e l a t i v e l y narrow range i n the coastal region for both multi and single storm i n t e n s t i t y data, and this range set l i m i t s on the hourly i n t e n s i t i e s that need to be considered as input r a i n f a l l data to a hydrograph model. - i i i - With regard to basin response to extreme rain-on-snow, available l i t e r a - ture suggests that for a ripe snowpack, development of an i n t e r n a l drain- age network within the snowpack i s the dominant routing mechanism for l i q u i d water. Consequences of t h i s conclusion on hydrograph procedures are that a watershed undergoes a t r a n s i t i o n from snow-controlled to more t e r r a i n - c o n t r o l l e d water movement and basin storage c h a r a c t e r i s t i c s approach conditions which would occur on the same basin without a snowcover. Lag and route hydrograph techniques were investigated to assess whether t h i s method can be applied to rain-on-snow floods. Results from analysis of two rain-on-snow floods suggest t h i s procedure can be applied when the following methodology i s adopted: 1) estimate t r a v e l time through the basin from channelized and overland flow considerations; 2) s e l e c t a storage c o e f f i c i e n t which simulates basin response; 3) take water inputs as the sum of snowmelt and r a i n f a l l ; and 4) consider there are no losses to groundwater. The combination of r e s u l t s from each study component provides a metho- dology for estimating input r a i n f a l l data and for undertaking hydrograph analysis for extreme rain-on-snow floods i n the mountainous P a c i f i c c oastal region. - i v - TABLE OF CONTENTS PAGE ABSTRACT i i LIST OF TABLES v i i LIST OF FIGURES X ACKNOWLEDGEMENTS xiv 1. INTRODUCTION 1 2. FLOOD CHARACTERISTICS IN THE COASTAL REGION 13 2.1 CLIMATE 13 2.2 HISTORICAL STREAMFLOW RECORDS 21 2.3 CASE STUDIES 32 2.4 SUMMARY 43 3. CHARACTERISTICS OF STORM RAINFALL IN THE COASTAL REGION ... 45 3.1 INTRODUCTION 45 3.2 OVERVIEW OF PRECIPITATION SYSTEMS 47 3.3 SOURCE OF B.C. RAINFALL INTENSITY DATA 51 3.4 INTENSITY-DURATION-FREQUENCY CURVES 56 3.4.1 Development and Use of IDF Curves 56 3.4.2 Depth-Duration Relationships 60 3.4.2.1 Analysis of B.C. Data 60 3.4.2.2 Formulas for B.C. Data 64 3.4.3 Depth-Frequency Relationships 73 3.4.3.1 Analysis of B.C. Data 73 3.4.3.2 Formulas for B.C. Data 76 3.4.4 Comparison With Other P a c i f i c Northwest Data 78 3.5 TIME DISTRIBUTION OF SINGLE STORM RAINFALL 82 3.5.1 Analysis of B.C. Data 82 3.5.2 Comparison With Other P a c i f i c Northwest Data 93 3.6 ELEVATION EFFECTS ON STORM RAINFALL 97 3.6.1 Background 97 3.6.2 Analysis of Selected Storm Data 100 3.6.3 Relationship to Annual P r e c i p i t a t i o n 109 3.7 SUMMARY 115 - V - TABLE OF CONTENTS (continued) PAGE 4. PHYSICAL ASPECTS OF WATER FLOW THROUGH SNOW 118 4.1 INTRODUCTION 118 4.2 FLOW PATHS AND SNOW METAMORPHISM 120 4.3 WATER INPUTS DURING RAIN-ON-SNOW 126 4.4 SUMMARY 131 5. DEVELOPMENT OF RAIN-ON-SNOW HYDROGRAPH MODEL 133 5.1 PERSPECTIVE ON HYDROLOGIC MODELS 133 5.2 CONTINUOUS FLOW VS EVENT MODELS 138 5.3 SELECTION OF MODELLING PROCEDURE 140 5.4 SOURCES OF RAIN-ON-SNOW DATA 144 5.5 APPROACH TO MODEL DEVELOPMENT 148 5.6 LAG AND ROUTE HYDROLOGIC MODEL 150 5.6.1 Procedures for Computation 150 5.6.2 Travel Time 152 5.6.3 Storage C o e f f i c i e n t 160 5.7 ANALYSIS OF FLOOD HYDROGRAPHS ON MANN CREEK 165 5.7.1 Basin Location 165 5.7.2 R a i n f a l l Flood of October 28, 1950 to November 2, 1950 165 5.7.2.1 Hydrometeorological Data 165 5.7.2.2 Travel Time and Storage C o e f f i c i e n t 168 5.7.2.3 Application of Lag and Route Hydrograph Model 172 - v i - TABLE OF CONTENTS (continued) PAGE 5.7.3 Rain-On-Snow Flood of February 3 - 8 , 1951 175 5.7.3.1 Hydrometeorological Data 175 5.7.3.2 Travel Time and Storage C o e f f i c i e n t 179 5.7.3.3 Application of Lag and Route Hydrograph Model 181 5.8 ANALYSIS OF FLOOD HYDROGRAPH ON LOOKOUT CREEK 184 5.8.1 Basin Location 184 5.8.2 Rain-On-Snow Flood of December 21-24, 1964 184 5.8.2.1 Hydrometeorological Data 184 5.8.2.2 Travel Time and Storage C o e f f i c i e n t 191 5.8.2.3 Appl i c a t i o n of Lag and Route Hydrograph Model 194 5.9 DISCUSSION OF RESULTS 201 REFERENCES 208 APPENDIX I MAXIMUM FLOODS ON RECORD IN COASTAL BRITISH COLUMBIA AND SOUTHEAST ALASKA . 217 APPENDIX II DEPTH-DURATION—FREQUENCY DATA FOR THE. BRITISH COLUMBIA COASTAL REGION ... 224 APPENDIX III MAXIMUM 24-HOUR RAINFALL ON RECORD AT BRITISH COLUMBIA COASTAL STATIONS .. 283 APPENDIX IV WATER PERCOLATION THROUGH SNOW 341 - v i i - LIST OF TABLES PAGE TABLE 2.1 MEAN MONTHLY PRECIPITATION FOR REPRESENTATIVE COASTAL STATIONS 16 TABLE 2.2 MONTHLY DISTRIBUTION OF MAXIMUM FLOODS ON RECORD IN COASTAL BRITISH COLUMBIA 26 TABLE 2.3 MONTHLY DISTRIBUTION OF MAXIMUM FLOODS ON RECORD IN SOUTHEAST ALASKA 27 TABLE 2.4 SUMMARY OF FLOOD REGIMES AT COASTAL BRITISH COLUMBIA STATIONS 28 TABLE 2.5 MONTHLY ADJUSTMENT FACTORS FOR PROBABLE MAXIMUM PRECIPITATION 39 TABLE 3.1 COASTAL B.C. STATIONS WITH RAINFALL INTENSITY DATA 52 TABLE 3.2 DENSITY OF RAIN GAUGE NETWORKS 54 TABLE 3.3 DEPTH-DURATION DATA FOR PITT POLDER 61 TABLE 3.4 DEPTH-DURATION RATIOS FOR IDF CURVES 62 TABLE 3.5 COMPARISON OF DEPTH-DURATION RATIOS 64 TABLE 3.6 FORMULAS RELATING RAINFALL DEPTH TO DURATION .... 65 TABLE 3.7 DEPTH-DURATION FORMULAS FOR COASTAL B.C 68 TABLE 3.8 COMPARISON OF RAINFALL DEPTH RATIOS 72 TABLE 3.9 DEPTH-FREQUENCY DATA FOR PITT POLDER 74 TABLE 3.10 DEPTH-FREQUENCY RATIOS FOR IDF CURVES 75 TABLE 3.11 DEPTH-DURATION RATIOS IN THE PACIFIC NORTHWEST .. 79 TABLE 3.12 DEPTH-FREQUENCY RATIOS IN THE PACIFIC NORTHWEST 80 TABLE 3.13 TIME OF OCCURRENCE OF MAXIMUM HOURLY INTENSITIES 91 - v i i i - LIST OF TABLES (continued) PAGE TABLE 3.14 SINGLE STORM PRECIPITATION DATA IN THE PACIFIC NORTHWEST 94 TABLE 3.15 STORM DATA NEAR MOUNT SEYMOUR (FITZHARRIS, 1975) 105 TABLE 3.16 RELATIONSHIP BETWEEN 24-HOUR AND ANNUAL PRECIPITATION 113 TABLE 3.17 DISTRIBUTION OF SHORT AND LONG DURATION PRECIPITATION 114 TABLE 4.1 HOURLY RAINFALL INTENSITIES 126 TABLE 4.2 REPRESENTATIVE SNOWMELT RATES 129 TABLE 4.3 REPRESENTATIVE RAINFALL AND SNOWMELT INPUTS 130 TABLE 5.1 SOURCES OF RAIN-ON-SNOW FLOOD DATA 146 TABLE 5.2 STORAGE COEFFICIENTS FOR RAIN-ON-SNOW EVENTS .... 163 TABLE 5.3 MANN CREEK RAINFALL DATA: OCTOBER 28 - NOVEMBER 2, 1950 167 TABLE 5.4 MANN CREEK CLIMATOLOGICAL STATIONS 175 TABLE 5.5 MANN CREEK RAINFALL DATA: FEBRUARY 3-8, 1951 .... 176 TABLE 5.6 MANN CREEK AIR TEMPERATURE DATA: FEBRUARY 3-8, 1951 176 TABLE 5.7 MANN CREEK SNOWCOURSE DATA: FEBRUARY, 1951 177 TABLE 5.8 SNOWMELT AND RAINFALL ESTIMATES 177 TABLE 5.9 RAINFALL AT MCKENZIE BRIDGE: DECEMBER 21-24, 1964 186 TABLE 5.10 RAINFALL NEAR LOOKOUT CREEK BASIN 187 TABLE 5.11 AIR TEMPERATURE (°C) NEAR LOOKOUT CREEK BASIN ... 189 - i x - LIST OF TABLES (continued) PAGE TABLE 5.12 SNOWMELT ESTIMATED FOR LOOKOUT CREEK 190 TABLE 5.13 SNOW DEPTHS IN CASCADE RANGE: DECEMBER 1964 190 TABLE 5.14 SNOW DEPTHS AT SANTIAM PASS (Elev. 1448 m) 199 - X - LIST OF FIGURES Figure 2.1 Physiographic Regions of B r i t i s h Columbia 14 Figure 2.2 Mean Monthly Temperatures for Coastal Stations .. 17 Figure 2.3 Mean Monthly Sea Level Pressure (kPa) for December, After Thomas (1977) 19 Figure 2.4 Mean Monthly Sea Level Pressure (kPa) for July, After Thomas (1977) 20 Figure 2.5 Maximum Floods On Record 24 Figure 2.6 Flood Frequency Curves for Cheakamus River Near Mons (1924-47) 29 Figure 2.7 Ratios of Maximum Instantaneous to Maximum Daily Floods 31 Figure 2.8 P r e c i p i t a t i o n at Terrace Airport for Oct. 31 - Nov. 1 , 1975 33 Figure 2.9 Intensity-Duration-Frequency Curves for Terrace Airport 34 Figure 2.10 P r e c i p i t a t i o n i n Vancouver Area on December 25, 1972 36 Figure 2.11 Intensity-Duration-Frequency Curves for Vancouver International A i r p o r t 37 Figure 2.12 Rain-On-Snow Flood Hydrographs 42 Figure 3.1 Twenty-Four Hour P r e c i p i t a t i o n i n B r i t i s h Columbia (After Hogg and Carr, 1985) 46 Figure 3.2 Lengths of Record at Coastal B.C. Stations 55 Figure 3.3 IDF Curves for Vancouver K i t s i l a n o 57 Figure 3.4 Depth-Duration Ratios for IDF Curves 62 Figure 3.5 Depth-Frequency Ratios for IDF Curves 75 - x i - LIST OF FIGURES (continued) PAGE Figure 3.6 Maximum 24-Hour R a i n f a l l on Record 84 Figure 3.7 Time D i s t r i b u t i o n of 12-Hour R a i n f a l l ( a f t e r Hogg, 1980) 87 Figure 3.8 Depth-Duration Ratios for 24-Hour R a i n f a l l 88 Figure 3.9 Monthly D i s t r i b u t i o n of Maximum R a i n f a l l s on Record 90 Figure 3.10 S o i l Conservation Service Type 1A Storm D i s t r i b u t i o n 96 Figure 3.11 Annual P r e c i p i t a t i o n i n the North Cascade Mountains 99 Figure 3.12 Station Locations i n North Vancouver 103 Figure 3.13 Transect A: Elevation vs 24-Hour P r e c i p i t a t i o n 104 Figure 3.14 Transect B: R a i n f a l l D i s t r i b u t i o n for December 6-7, 1970 106 Figure 3.15 Transect B: R a i n f a l l D i s t r i b u t i o n for December 9-11 , 1970 107 Figure 3.16 Transect B: R a i n f a l l D i s t r i b u t i o n for February 13-15, 1971 108 Figure 3.17 Relationship Between 24-Hour and Annual p r e c i p i t a t i o n 111 Figure 4.1 Snowpack Response to Rain-On-Snow (a f t e r Colbeck, 1976) 122 Figure 5.1 Perspective on Hydrologic Models 133 Figure 5.2 Location Map for Oregon Watersheds 147 Figure 5.3 Comparison of Wide Channelized and Overland Flow V e l o c i t i e s 154 - x i i - LIST OF FIGURES (continued) PAGE Figure 5.4 Overland Flow V e l o c i t i e s (after S o i l Conservation Service, 1974) 157 Figure 5.5 Rain-On-Snow Flood Hydrograph on Lookout Creek ... 161 Figure 5.6 Mann Creek Topography 166 Figure 5.7 Recorded Hydrograph on Mann Creek: Oct. 27- Nov. 6, 1950 167 Figure 5.8 Mann Creek Time-Area Graph 170 Figure 5.9 Semi-Log Plot of Mann Creek Hydrograph 171 Figure 5.10 Simulated R a i n f a l l Hydrograph on Mann Creek 174 Figure 5.11 Recorded Hydrograph on Mann Creek: February 3-8, 1951 178 Figure 5.12 Semi-Log Plot of Rain-on-Snow Hydrograph on Mann Creek 180 Figure 5.13 Simulated Rain-on-Snow Hydrograph on Mann Creek 183 Figure 5.14 Lookout Creek Topography 185 Figure 5.15 December 1964 P r e c i p i t a t i o n 187 Figure 5.16 Time D i s t r i b u t i o n of R a i n f a l l 188 Figure 5.17 Rain-on-Snow Flood Hydrograph on Lookout Creek: December 19-27, 1964 192 Figure 5.18 Lookout Creek Time-Area Graph 193 Figure 5.19 Semi-Log Plot of Rain-on-Snow Flood Hydrograph on Lookout Creek 195 - x i i i - LIST OF FIGURES (continued) PAGE Figure 5.20 Simulated Rain-on-Snow Flood Hydrograph on Lookout Creek 197 Figure IV.1 Wave Speed vs Influx Rate (after Colbeck and Davidson, 1973) IV-2 Figure IV.2 Water Percolation Through Snow (after Tucker and Colbeck, 1977) IV-4 Figure IV.3 Percolation Rates for V e r t i c a l Unsaturated Flow IV-7 Figure IV.4 Comparison of Predicted and Observed Outflow Hydrographs (after Dunne et a l . , 1976) IV-9 Figure IV.5 Travel Times for Basal Saturated Flow IV-12 - x i v - AC KNOWLEDGEMENTS I am appreciative of the advice, cooperation and friendship of my thesis supervisor Dr. Michael Quick, P.Eng.; and committee members Dr. Denis Russ e l l , P.Eng.; Dr. B i l l Caselton, P.Eng.; Dr. Al Freeze, p.Eng.; and Dr. olev Slaymaker. m Special thanks to my wife, Jolanta, for her patience and support throughout the Ph.D. program. I am indebted to the late Dr. "Chick" Evans, one of the great humanitar- ians and amateur golfers of our time, for the honor of p a r t i c i p a t i n g i n the Evans Scholars program which he established. Much of the academic and professional success which I have experienced can be attributed to the opportunities that were provided to me by the Evans Scholars Foundation. The generosity of Klohn Leonoff Ltd. i n providing word processing services and the corresponding d i l i g e n c e of Mrs. Linda Davidson were c r i t i c a l to the preparation of th i s manuscript. F i n a n c i a l assitance for th i s research was provided by the E a r l R. Peterson Memorial Scholarship i n C i v i l Engineering and a Natural Sciences and Engineering Research Council of Canada Postgraduate Scholarship. 1 . I N T R O D U C T I O N Most development projects such as dams, r a i l r o a d and highway extensions, mine s i t e s , p i p e l i n e i n s t a l l a t i o n s and new townsite planning require that flood analysis be undertaken for project design purposes. However, due to the r e l a t i v e l y sparse hydrologic data c o l l e c t i o n network i n much of the P a c i f i c Northwest coastal region, flood analysis must be under- taken i n many instances without adequate s i t e s p e c i f i c data. The most common methods i n current engineering practice for estimating flood flows can be categorized as either s t a t i s t i c a l analysis of stream- flow data or the ap p l i c a t i o n of a model which simulates the runoff process for a basin. The ap p l i c a t i o n of these flood estimation techni- ques i n coastal B r i t i s h Columbia i s e s p e c i a l l y d i f f i c u l t . For example, t y p i c a l problems commonly encountered by the p r a c t i c i n g engineer i n B r i t i s h Columbia include: i ) the streamflow gauge network operated by Water Survey of Canada and the p r e c i p i t a t i o n gauge network which reports to the Atmospheric Environment Service are r e l a t i v e l y sparse i n remote regions; i i ) a v a i l a b l e streamflow and p r e c i p i t a t i o n data cannot be re a d i l y transposed with confidence over long distances due to mountainous t e r r a i n with i t s corresponding l o c a l variations i n climate; i i i ) many streamflow and p r e c i p i t a t i o n stations currently i n opera- - 2 - t i o n do not have long term records and, therefore, meaningful s t a t i s t i c a l analysis often cannot be undertaken; iv) a v a i l a b l e streamflow data are often limited to mean d a i l y flow estimates, though an estimate of maximum instantaneous flood d i s - charge i s usually required for design. The o v e r a l l goal of t h i s research i s to overcome the shortcomings i n data described above by esta b l i s h i n g a r a t i o n a l basis for estimating extreme floods i n instances when s u f f i c i e n t s i t e s p e c i f i c design data are not a v a i l a b l e . Extreme flood i s a subjective c l a s s i f i c a t i o n and i s commonly used i n context with a s p e c i f i c design objective. For t h i s study extreme flood generally refers to any flood with a return period greater than about 20 years. While the focus of th i s i n v e s t i g a t i o n i s on coastal B r i t i s h Columbia, the r e s u l t s are also generally applicable to the entire northern p a c i f i c coastal region which includes southeast Alaska and the coastal region of Washington and Oregon. Accordingly, some data and res u l t s from studies i n these other segments of the coastal region are included i n this i n v e s t i g a t i o n of coastal B r i t i s h Columbia f l o o d s . A m u l t i - d i s c i p l i n a r y i n v e s t i g a t i o n i s undertaken encompassing the areas of hydrometeorology, snow hydrology and hydrologic modelling. Research i s presented i n four components whose primary obj ectives are as follows: - 3 - CHAPTER 2: to develop an understanding of the flood producing mechanisms for the region by i d e n t i f y i n g those flood c h a r a c t e r i s t i c s which are common to the coastal hydrologic region. The emphasis of this study component i s on i d e n t i f y i n g those c l i m a t i c and runoff conditions which lead to extreme floods, as these are the flows of i n t e r e s t i n many instances of engineering planning and design. CHAPTER 3: to provide a basis for estimating the time d i s t r i b u t i o n of r a i n f a l l for input to a hydrograph model. This assessment i s undertaken on a region wide scale, although i t i s recognized that when supplemental s i t e data are ava i l a b l e for a basin of i n t e r e s t the more general trends i d e n t i f i e d for the coastal region may be improved. CHAPTER 4: to assess basin conditions which a f f e c t runoff leading to extreme f l o o d s . In p a r t i c u l a r , the role of a snowpack i s investigated with regard to i t s contribution of snowmelt to t o t a l runoff and i t s e f f e c t on the amount and rate of r a i n f a l l runoff through the snow. CHAPTER 5: to develop a hydrograph model that i s capable of producing flood estimates for conditions which lead to extreme floods i n the coastal mountains of the P a c i f i c Northwest. Hydrograph procedures commonly applied for r a i n f a l l events are examined for t h e i r p o t e n t i a l a p p l i c a t i o n to extreme rain-on-snow flood events. - 4 - Even though each study component i s presented separately i n a d i f f e r e n t Chapter, i t i s the combination of re s u l t s which ultimately leads to an understanding of flood mechanisms i n the coastal region and the develop- ment of a n a l y t i c a l procedures for extreme flood hydrograph a n a l y s i s . The i n i t i a l task for any flood analysis i s to e s t a b l i s h the flood produc- ing mechanism which must be simulated by hydrograph methods. In the coastal region, floods are generally either snowmeIt-induced i n spring and summer or r a i n f a l l - i n d u c e d i n f a l l and winter. Floods which are ra i n f a l l - i n d u c e d r e s u l t from r a i n f a l l - o n l y or a combination of rai n and snowmelt runoff. The s i t u a t i o n i s complicated further because some basins experience both types of floods during the year. Based on an assessment of atmospheric processes which a f f e c t climate i n the region, examination of h i s t o r i c a l flood data and analysis of flood frequency undertaken i n this study, i t i s shown that extreme floods on most basins i n the coastal region are generated from rai-n-on-snow events. Therefore, hydrograph procedures capable of simulating rain-on-snow floods are required for the mountainous coastal region. A requirement common to a l l hydrograph models i s that the time d i s t r i b u - t i o n of storm r a i n f a l l onto the basin must be estimated. Therefore, the natural s t a r t i n g point i n the development of rain-on-snow hydrograph pro- cedures i s analysis of storm r a i n f a l l for input to a model. This assess- ment i s an e s s e n t i a l task of hydrograph analysis, and can be undertaken separately from assessment of basin response and runoff c h a r a c t e r i s t i c s . - 5 - At an ungauged watershed two steps are usually required to produce a hyetograph for input to a hydrograph model. F i r s t , storm r a i n f a l l c h a r a c t e r i s t i c s are estimated at a regional s t a t i o n where r a i n f a l l i n t e n s i t y data are a v a i l a b l e , and then these data are transposed to the project s i t e . The e x i s t i n g gauge network which records r a i n f a l l inten- s i t y i n coastal B.C. consists of only 58 stations and i s r e l a t i v e l y sparse compared to recommendations (World Meteorological Organization, 1970) for network density i n mountainous t e r r a i n . Therefore, i t i s common that a p r e c i p i t a t i o n gauge i s not located near a project s i t e . Even when a regional gauge i s a v a i l a b l e , transposing data to a project s i t e i s e s p e c i a l l y d i f f i c u l t i n mountainous regions because r a i n f a l l can vary over short distances both in plan and elevation. Because of the d i f f i c u l t y i n estimating storm r a i n f a l l for hydrograph analysis i n the mountainous coastal region, one goal established for this study i s to examine whether regional c h a r a c t e r i s t i c s can be i d e n t i - f i e d from available data even when the magnitude of r a i n f a l l varies between s t a t i o n s . Assessment of regional r a i n f a l l c h a r a c t e r i s t i c s i n a region as extensive and diverse as the coastal region i s uncommon, and i s undertaken as an exploratory exercise without previous knowledge as to whether the analysis w i l l produce usable r e s u l t s . Two types of r a i n f a l l i n t e n s i t y data can be used to produce synthetic hyetographs for input to a hydrograph model. One type results from analysis of r a i n f a l l i n t e n s t i e s from many d i f f e r e n t storms and the other from analysis of i n t e n s i t i e s occurring within a single storm. Atmos- - 6 - pheric Environment Service summarizes multi-storm i n t e n s i t y data at each of t h e i r stations by producing Intensity-Duration-Frequency (IDF) Curves. IDF curves provide average i n t e n s i t i e s for a given duration and return period, but do not provide information regarding variations in r a i n f a l l i n t e n s i t i e s within a single storm. Development of synthetic hyetographs based on i n t e n s i t y data from IDF curves i s an approach commonly applied only because single storm data are seldom a v a i l a b l e . To improve upon methods employed using IDF curves, analysis of r a i n f a l l i n t e n s i t i e s occurring within single storms i s also undertaken as part of this study. This exercise requires a l l hourly data recorded at each of the 58 st a - tions i n the coastal region be obtained on magnetic tape, and computer programs written to scan the tape, i d e n t i f y extreme r a i n f a l l events and extract hourly i n t e n s i t i e s within the storm for further a n a l y s i s . The procedure adopted for analysis of regional r a i n f a l l c h a r a c t e r i s t i c s i s to examine multi-storm i n t e n s i t y data available from AES and single storm data i d e n t i f i e d i n this study i n a r a t i o format. For example, r a t i o s of 1, 2, 6 and 12-hour to 24-hour p r e c i p i t a t i o n are calculated at each s t a t i o n and then compared to corresponding ratios at a l l other sta- tions . This method i s one approach to i d e n t i f y i n g regional characteris- t i c s even when the amount of r a i n f a l l i s d i f f e r e n t between st a t i o n s . For both sets of i n t e n s i t y data, computer programs are written to extract the necessary data from magnetic tape and undertake the required c a l c u l a t i o n s . - 7 - Results of the analysis show that regional c h a r a c t e r i s t i c s for both IDF and single storm data can be i d e n t i f i e d i n coastal B.C. In pract i c e , these r e s u l t s can be used to set l i m i t s on the range of hourly i n t e n s i - t i e s that need to be considered by a design engineer i n the absence of s i t e data. One concern regarding r e s u l t s of analysis of B.C. data i s that there are no high elevation stations which record r a i n f a l l i n t e n s i t y i n the coastal region. To supplement B.C. data, r a i n f a l l i n t e n s i t y data from Oregon and Washington are also obtained to i l l u s t r a t e further the regional a p p l i c a b i l i t y of r a i n f a l l c h a r a c t e r i s t i c s i d e n t i f i e d i n B.C. and to provide results from stations at higher elevations than are cu r r e n t l y a v a i l a b l e i n B.C. Results of analysis of U.S. data show regional charac- t e r i s t i c s s i m i l a r to those calculated for lower elevations i n B.C. The next step i n developing hydrograph procedures capable of simulating rain-on-snow floods i s to assess the role of a snowpack with regard to i t s contribution of snowmelt to t o t a l runoff and i t s e f f e c t on runoff response from the basin. A fundamental question which arises for extreme rain-on-snow i s whether water percolation through the snow medium or development of i n t e r n a l drainage channels i s the dominant routing mechan- ism. Quantitative formulations have been proposed describing water per- c o l a t i o n through snow in a v e r t i c a l unsaturated zone (Colbeck, 1971, 1972) and a basal saturated layer (Colbeck, 1974a). However, evidence i s also a v a i l a b l e to suggest that an i n t e r n a l drainage network, not water percolation, controls runoff during extreme rain-on-snow floods. - 8 - The approach taken i n this study to assess the role of a snowpack i s : (i) to review a v a i l a b l e l i t e r a t u r e i n the general areas of snow physics and snow hydrology; ( i i ) to assess r e s u l t s of research studies which pertain to the flow of l i q u i d water through snow; and ( i i i ) to i n t e r p r e t r e s u l t s with regard to the i r impact on hydrograph procedures required for rain-on-snow f l o o d s . Once the role of a snowpack on basin response to rain-on-snow i s assessed, then requirements of a hydrograph model can be established. Perhaps the most important concept to recognize i n snow hydrology i s that snowpack response i s not constant, but rather varies with physical properties of the snow. Therefore, discussion of snowpack response must be q u a l i f i e d by a de s c r i p t i o n of snow properties being considered. i n the coastal region, much of the snowpack can be categorized as "warm" (Smith, 1973). Warm snowpacks are those whose i n t e r i o r temperatures remain near 0°C during most of the snow season. Also, snow can be categorized as "wet" when l i q u i d water i s present (Colbeck, 1982a). Some l i q u i d water i s held i n a snowpack as absorbed or c a p i l l a r y water, but once saturation i s achieved water inputs are transmitted by pro- cesses dominated by gravity (Colbeck and Davidson, 1973). A warm, wet snowpack i s commonly referred to as a ripe snowpack. Research r e s u l t s and observations of snow hydrologists for response of r i p e snowpacks to inputs of l i q u i d water show: ( i ) snowpack response i s - 9 - usually less than predicted by theories for water percolation, and the apparent explanation i s formation of d i s t i n c t flow channels; ( i i ) once p r e f e r e n t i a l drainage routes are i n i t i a t e d , they are self-perpetuating and drainage from the snowpack becomes more rapid as melt channels develop; and ( i i i ) development of flow channels causes a snowcovered watershed to undergo a t r a n s i t i o n from snow-controlled to terrain-con- t r o l l e d water movement. The above observations suggest that development of an i n t e r n a l drainage network i s the dominant routing mechanism during extreme rain-on-snow. Consequences of the above conclusions on hydrograph procedures for extreme rain-on-snow are: ( i ) water percolation processes do not need to be simulated i n a hydrograph model, and ( i i ) as water movement in a snowcovered watershed becomes t e r r a i n controlled, i t i s possible that basin response c h a r a c t e r i s t i c s might approach conditions which would occur without a snowcover. This assessment of snowpack response forms the basis for hydrograph procedures developed i n this study for a p p l i c a - tion to extreme rain-on-snow floods i n the coastal region. Unit-hydrograph and lag and route techniques are investigated i n this study to assess whether empirical relationships and c o e f f i c i e n t s em- ployed by each method for r a i n f a l l - o n l y could be modified for applica- t i o n to rain-on-snow floods i n mountainuous regions of the P a c i f i c Northwest. This i n v e s t i g a t i o n i s undertaken as an exploratory exercise without knowing whether snowpack response, even with the formation of an i n t e r n a l drainage network, can be simulated using conventional hydro- graph procedures. - 10 - i n i t i a l screening of the two methods leads to the conclusion that the lag and route hydrograph procedure warrants more detai l e d i n v e s t i g a t i o n i n t h i s study for a p p l i c a t i o n to rain-on-snow. One a t t r a c t i o n of the lag and route method i s that r a i n f a l l and snowmelt inputs to the model can be d i s t r i b u t e d across the basin. This option more accurately repre- sents conditions in mountainous t e r r a i n . Also, t r a v e l time of a water p a r t i c l e through each watershed can be estimated based on hydraulic p r i n c i p l e s rather than having to r e l y on equations developed for other basins and regions. This procedure i s p a r t i c u l a r l y a t t r a c t i v e for ungauged watersheds. The lag and route hydrograph method requires estimates for t r a v e l time through the basin and a storage c o e f f i c i e n t which simulates ch a r a c t e r i s t s of the watershed. Procedures are demonstrated i n t h i s study for estimat- ing t r a v e l time based on channelized and overland flow v e l o c i t y e s t i - mates, without any a d d i t i o n a l time increment added for water movement through the snowpack. Storage c o e f f i c i e n t s calculated from recorded extreme rain-on-snow flood hydrographs are tabulated as preliminary estimates for use with lag and route procedures. No suitable watersheds i n coastal B.C. are i d e n t i f i e d which s a t i s f y data requirements for rain-on-snow hydrograph analysis to a standard needed for research. A l t e r n a t i v e l y , drainage basins are examined i n other segments of the coastal hydrologic region of the P a c i f i c Northwest and su i t a b l e watersheds are i d e n t i f i e d i n the Cascade Mountains i n Oregon. - 1 1 - Two drainage basins, Mann and Lookout Creeks, are selected to examine the p o t e n t i a l for applying lag and route hydrograph procedures to simu- lat e rain-on-snow f l o o d s . Development of hydrograph procedures includes: (i ) analysis of a r a i n f a l l - o n l y event on Mann Creek to confirm that the f a s t runoff contribution to flood peaks i n mountainous regions can be simulated using one storage c o e f f i c i e n t ; ( i i ) analysis of a rain-on-snow event on Mann Creek to examine whether the model can be adapted for rain-on-snow, and to compare the storage c o e f f i c i e n t with that on the same basin for r a i n f a l l - o n l y ; and ( i i i ) analysis of a rain-on-snow event on Lookout Creek to undertake a second a p p l i c a t i o n of the,model, and to assess storage c o e f f i c i e n t s during more extreme flood events. Results from Mann and Lookout Creeks show lag and route hydrograph pro- cedures can be applied to simulate rain-on-snow flood hydrographs when the following methodology i s adopted: ( i ) estimate trave l time through the basin from channelized and overland flow considerations; ( i i ) s e l e c t the appropriate storage c o e f f i c i e n t ; ( i i i ) specify water inputs to the basin as the sum of snowmelt and r a i n f a l l ; and (iv) consider there are no losses to groundwater. Selection of a storage c o e f f i c i e n t for the fa s t component of runoff i s an important consideration i n a p p l i c a t i o n of lag and route procedures. One question which arises i s how does the storage c o e f f i c i e n t for rain-on-snow floods compare on the same basin with that for r a i n f a l l f l o o d s . Storage c o e f f i c i e n t s on Mann Creek for a r a i n f a l l and a r a i n - on-snow hydrograph d i f f e r by a factor of two. However, the rain-on-snow - 12 - flood on Mann Creek i s not a very extreme event and perhaps runoff i s s t i l l p a r t l y snow-con t r o l l e d . If this i s the case, then data from the two Mann Creek floods cannot be used to test the concept that an i n t e r - nal drainage network within a snowpack causes basin response to be si m i l a r to that for r a i n f a l l floods. I t i s worth noting that the storage c o e f f i c i e n t for an extreme rain-on-snow event on Lookout Creek i s s i m i l a r to the c o e f f i c i e n t for r a i n f a l l - o n l y on Mann Creek. This r e s u l t may be coincidence or i t may demonstrate a more t e r r a i n - c o n t r o l l e d basin res- ponse for extreme rain-on-snow floods. U n t i l further research i s under- taken, i t i s recommended that preliminary storage c o e f f i c i e n t s calculated i n this study from recorded extreme rain-on-snow floods be adopted for use with lag and route hydrograph procedures. Available evidence suggests that extreme r a i n f a l l combined with r e l a - t i v e l y high temperatures on Lookout Creek produces much greater snowmelt than predicted by the Corps of Engineers temperature-index equation developed for this region. I t i s l i k e l y that temperature index equa- tions , such as developed by the Corps of Engineers, w i l l continue to be applied to ungauged mountainous watersheds because other c l i m a t i c data needed for a l t e r n a t i v e melt equations are seldom a v a i l a b l e . Therefore, snowmelt occurring during the sp e c i a l case of extreme rain-on-snow i s highlighted as an important topic requiring further analysis i n the development of procedures for estimating extreme rain-on-snow floods. - 13 - 2. FLOOD CHARACTERISTICS IH THE COASTAL REGION 2.1 CLIMATE The climate of the northern P a c i f i c coastal region along B r i t i s h Columbia has been described by various authors including Chapman (1952), Hare and Thomas (1974), Schaefer (1978), and Chilt o n (1981). The coastal c l i m a t i c region extends the entire length of the province and i s generally bounded by the crest of the Coastal Mountains as shown i n Figure 2.1. This c l i m a t i c region extends southward into Washington and Oregon bounded by the Cascade Mountain Range, and includes southeast tAlaska immediately adjacent to northern B r i t i s h Columbia. The primary c l i m a t i c features of the coastal region include r e l a t i v e l y high annual p r e c i p i t a t i o n with the wettest months occurring i n f a l l and winter, and a r e l a t i v e l y small annual range of temperature. Within the coastal region, however, l o c a l v ariations e x i s t i n p r e c i p i t a t i o n and temperature due to the complex i n t e r a c t i o n between atmospheric c i r c u l a - t i o n patterns and major topographic features d i s t r i b u t e d along the coast which serve as b a r r i e r s to the movement of a i r masses. For example, d i s t i n c t zones within the coastal region can be i d e n t i f i e d along west- facing mountain slopes which tend to have more clouds and receive more p r e c i p i t a t i o n than eastern faces of the mountains. Also, the south- eastern lowlands of Vancouver Island, the islands of the S t r a i t of Georgia and the Fraser River estuary comprise a zone which l i e s i n the rainshadow of Vancouver Island and the Olympic Mountains i n Washington - 14 - FORT NELSON G R E A T P L A I N S \ M O U N T A I N S A N D \ S O U T H E R N R O C K Y • M O U N T A I N S ) CRANBROOK s F i g u r e 2.1 P h y s i o g r a p h i c Regions o f B r i t i s h Columbia - 15 - State. This zone i s the d r i e s t segment of the coastal region and i s also the warmest with more hours of bright sunshine during the summer months. Monthly p r e c i p i t a t i o n data are included i n Table 2.1 for representative coastal stations extending from Vancouver, B r i t i s h Columbia in the south to Sit k a , Alaska i n the north. These data i l l u s t r a t e the v a r i a b i l i t y i n p r e c i p i t a t i o n along the coast yet also show that on a monthly percentage basis the d i s t r i b u t i o n of p r e c i p i t a t i o n i s s i m i l a r for the region. For example, annual p r e c i p i t a t i o n ranges from 1259 to 4388 mm for representa- t i v e stations included i n Table 2.1. However, on a percentage basis at each s t a t i o n , a summer month receives i n the order of only 3 to 6 percent of the annual p r e c i p i t a t i o n while each of the wettest winter months receive about 10 to 15 percent. Comparison of these data also shows that the period of high p r e c i p i t a t i o n s t a r t s e a r l i e r i n the northern than in the southern segments of the coastal region. Williams (1948) noted a southward progression in the occurrence of maximum d a i l y p r e c i p i t a t i o n for the year of about one degree of l a t i t u d e for each 4.5 days. TABLE 2.1 MEAN MONTHLY PRECIPITATION FOR REPRESENTATIVE COASTAL STATIONS* Vancouver U.B.C. To f1 no A1r por t Port 1 Hardy A i r p o r t Ocean Fal Is Cape S t . James Pr ince 1 Rupert A i r p o r t S i t k a % o f % of % of % o f % of % o f % o1 annual annual annual annual annual annual annu< mm prec i p. mm p r e c l p . mm prec l p. mm pr ec 1 p. mm prec l p. mm prec l p. mm prec Jan 173 14 404 12 211 12 459 10 162 11 228 9 197 8 Feb 133 11 366 11 159 9 392 9 137 9 222 9 162 7 Mar 116 9 372 11 142 8 346 8 130 8 201 8 177 7 Apr 69 5 234 7 108 6 302 7 107 7 190 8 136 6 May 60 5 143 4 69 4 217 5 85 ' 6 140 6 1 18 5 June 43 3 102 3 71 4 192 4 74 5 130 5 88 4 Ju l y 37 3 86 3 52 3 151 3 58 4 103 4 132 5 Aug 53 4 114 3 69 4 227 5 79 5 158 6 200 8 Sept 72 6 163 5 136 7 376 9 125 8 233 9 292 12 Oct 133 11 392 12 245 14 625 14 198 13 367 15 388 16 Nov 162 13 432 13 245 14 514 12 187 12 268 11 305 12 Dec 208 16 479 15 277 15 587 13 191 12 284 11 258 11 Annual 1259 3287 1784 4388 1533 2524 24 53 * S t a t i o n l o c a t i o n s shown on F igure 2.1 - 17 - Temperature data plotted on Figure 2.2 for three representative stations along the coastal region i l l u s t r a t e the r e l a t i v e l y small annual range at a given s t a t i o n and the s i m i l a r annual trend i n temperature between sta- t i o n s . Chapman (1952) noted an average reduction i n mean annual tempera- ture (corrected to sea lev e l ) along the coast from 60° to 24°20' north l a t i t u d e of about 0.6°C per degree of l a t i t u d e . 18 _2 I I I ' ' ' • 1 ' ' ' J F M A M J J A S O N D M O N T H Figure 2.2 Mean Monthly Temperatures for Coastal Stations The climate of the coastal region i s controlled on a seasonal basis by macro-scale atmospheric processes. During winter months vigorous c i r c u - l a t i o n i s produced by a strong temperature gradient between t r o p i c a l and polar l a t i t u d e s . During t h i s season low pressures over the Gulf of Alaska and high pressures inland combine to produce strong pressure - 18 - gradients over western Oregon, Washington and B r i t i s h Columbia where southerly surface winds p r e v a i l . Mean sea l e v e l atmospheric pressure patterns for December (Thomas, 1977) are shown on Figure 2.3. The winter atmospheric c i r c u l a t i o n pattern causes numerous storms to develop r a p i d l y i n the northern P a c i f i c Ocean and move i n a northeasterly d i r e c t i o n to the Gulf of Alaska where they d i s s i p a t e . On a smaller scale, f r o n t a l systems break away from the storm centers and impinge upon the coast, often bringing strong southwesterly flows of warm moist a i r a l o f t which are responsible for the coastal region's heaviest r a i n - f a l l s . During summer months a weaker atmospheric c i r c u l a t i o n develops (Thomas, 1977). The summer coastal climate i s controlled by the dominance of a large high pressure centre which expands northward as shown for the month of July on Figure 2.4. For this summer condition pressure gradients are weaker than i n the winter, northwesterly winds p r e v a i l along much of the coast, and the frequency and i n t e n s i t y of P a c i f i c storms i s diminished. The summary of atmospheric c i r c u l a t i o n patterns suggests how s i m i l a r annual trends develop for temperature and p r e c i p i t a t i o n for the e n t i r e coastal region. Variations within the region, however, r e s u l t from the e f f e c t of more l o c a l topographic features such as elevation, slope and aspect on c i r c u l a t i o n patterns as f r o n t a l systems impinge of the very diverse c o a s t l i n e . Figure 2.3 Mean Monthly Sea Level Pressures (kPa) for December, af t e r Thomas (1977) - 20 - Figure 2.4 Mean Monthly Sea Level Pressure (kPa) fo r July, a f t e r Thomas (1977) - 21 - 2.2 H I S T O R I C A L S T R E A M F L O W R E C O R D S H i s t o r i c a l flood data for the coastal region of B r i t i s h Columbia and southeast Alaska were reviewed to e s t a b l i s h the t y p i c a l range of extreme floods that have been recorded and to i d e n t i f y general trends and simi- l a r i t i e s among the data. These flood c h a r a c t e r i s t i c s were documented by examining unit discharge (discharge per unit area), ratios of maximum instantaneous to maximum d a i l y discharge, flood producing mechanisms and period of year when extreme floods have occurred. I d e n t i f i c a t i o n of streamflow gauging stations within the coastal region of B r i t i s h Columbia and southeast Alaska and the sel e c t i o n of stations for review i n this study proceeded as follows: i ) stations located within coastal B r i t i s h Columbia were i d e n t i - f i e d i n a reference index (Environment Canada, 1983b) and those i n southeast Alaska were obtained from a report prepared by the U.S. Geological Survey (Lamke, 1979). Flood data through 1982 were re a d i l y a v a i l a b l e for B r i t i s h Columbia (Environment Canada, 1983a), while Alaska flood data from the 1979 report were updated to 1982 by the USGS o f f i c e in Anchorage. i i ) stations which were designated as having regulated flows were omitted. i i i ) major r i v e r s , such as the Fraser, Skeena, Stikine and Taku Rivers which flow through the coastal region but whose drainage basins extend inland beyond the coast were omitted. - 2 2 - iv) only data from those stations with ten or more years of record were reviewed. The screening process resulted i n the s e l e c t i o n of 66 stations i n coast- a l B r i t i s h Columbia and 47 stations i n southeast Alaska. In B r i t i s h Columbia some r i v e r s had more than one station so that only 58 d i f f e r e n t r i v e r s were represented by the 66 st a t i o n s . The l i s t of stations i s included in Appendix I . Before analyzing flood data> available for the coastal region, i t i s important to recognize sources of scatter in any r e s u l t s derived from analysis of these data. As one would i n t u i t i v e l y expect, v a r i a b i l i t y i n l o c a l climate and basin runoff c h a r a c t e r i s t i c s across the entire coastal region produce a range i n the magnitude of floods on record for a given drainage area. In addition there are l i m i t a t i o n s i n the data records themselves which inherently lead to scatter in any r e s u l t s derived from analysis of the a v a i l a b l e flood data. These data l i m i t a t i o n s include: i ) periods of record are not concurrent for a l l st a t i o n s , although i n general most flood data are for more recent years. i i ) length of record varies among s t a t i o n s . A station with a long record i s l i k e l y to have experienced a more rare event flo o d than a station with a shorter record. - 23 - i i i ) flows are published based on a fixed 24-hour time period, when a short duration storm hydrograph spans two days, the corresponding mean d a i l y flow may be deceptively low compared to the recorded peak for each day. iv) the magnitude of an extreme flood i s normally estimated from the portion of the stage-discharge rating curve at each s t a t i o n that i s r e l a t i v e l y i l l - d e f i n e d due to absence of gauged flows i n this range. Maximum floods on record at coastal B r i t i s h Columbia and Alaska stations are plotted on Figure 2.5 as unit discharge versus drainage area. Even though the data points are scattered, a single band of data i s neverthe- less defined when viewed against other regions. For example, the largest floods on record from the adjacent i n t e r i o r plateau of B r i t i s h Columbia are also included on Figure 2.5 for comparison. The single band of flood data reinforces the concept of a single hydrologic region, while the range of data i l l u s t r a t e s the e f f e c t s of l o c a l c l i m a t i c v a r i a t i o n s across the region. 1 0 . 0 5 . 0 C V l E (A 1.0 ro e — 0 . 5 UJ CD or < o CO Q z ZD 0 . 1 Q 0 5 0 . 0 M 1 0 1 0 0 1 0 0 0 1 0 0 0 0 V x x X x 1 0 0 0 0 0 x x CP ••I • Coastal British Columbia x Southeast Alaska o Interior Plateau of Brit ish Columbia o 1 0 too 1 0 0 0 DRAINAGE AREA (sq km ) 1 0 0 0 0 1 0 0 0 0 0 Figure 2.5 Maximum Floods on Record - 25 - Additional i n s i g h t to the c h a r a c t e r i s t i c s of coastal floods can be obtained by examining the period of year when these floods occurred. The monthly d i s t r i b u t i o n of the maximum floods on record for coastal B r i t i s h Columbia and southeast Alaska are shown i n Tables 2.2 and 2.3, res p e c t i v e l y . The tables i l l u s t r a t e that the most extreme floods i n the coastal region have occurred during the f a l l and winter period when over 90 percent have been recorded. - 26 - TABLE 2.2 MONTHLY DISTRIBUTION OF MAXIMUM FLOODS ON RECORD IN COASTAL BRITISH COLUMBIA* Number of Floods Percent Spring/Summer March 0 0 A p r i l 1 2 May 0 0 June 3 5 July 0 0 August 0 0 4 7% Fall/Winter September 3 5 October 13 22 November 10 17 December 16 28 January 10 17 February 2 4 54 93% 1. L i s t of coastal B r i t i s h Columbia stations i n Appendix I. 2. Only one st a t i o n considered for main stem of each r i v e r . TABLE 2.3 MONTHLY DISTRIBUTION OF MAXIMUM FLOODS ON RECORD IN SOUTHEAST ALASKA* Number of Floods Percent Spring/Summer March 0 0 A p r i l 0 0 May 0 0 June 0 0 July 0 0 August 4 9 4 9% Fa11/Winter September 14 32 October ~ 13 30 November 8 18 December 3 7 January 1 2 February 1 2 40 91% L i s t of southeast Alaska stations i n Appendix I. - 28 - As part of a study undertaken by Water Survey of Canada (Environment Canada, 1982), flood data were examined on drainage basins i n B.C. and the Yukon. The primary objective of the study was to conduct a flood frequency analysis at each station with 9 or more years of record. An examination of the flood data revealed, however, that annual maximum discharge alone was not an adequate c r i t e r i o n for sel e c t i n g flows for flood frequency a n a l y s i s . I t was observed that some stations on the B.C. coast experienced floods that were either r a i n f a l l - i n d u c e d i n the f a l l and winter or snowmeIt-induced i n spring and summer, while others experienced both types such that two d i s t i n c t flood regimes were i d e n t i - f i a b l e on the same basin. These flood regimes had to be i d e n t i f i e d at each station to ensure that flood data selected for frequency analysis resulted from a s i m i l a r flood producing mechanism. A summary of flood regimes determined by Water Survey of Canada for the 58 d i f f e r e n t basins considered i n this study i s included i n Table 2.4. TABLE 2.4 SUMMARY OF FLOOD REGIMES AT COASTAL BRITISH COLUMBIA STATIONS Flood Regime Number of Stations Predominantly r a i n f a l l - i n d u c e d floods i n f a l l and winter 43 Predominantly snowmelt-induced floods i n spring and summer Both r a i n f a l l - i n d u c e d floods i n f a l l and winter and snowmelt-induced floods i n spring and summer 14 58 - 29 - For the 14 stations c l a s s i f i e d as having both r a i n f a l l and snowmelt flood regimes on the same basin, Water Survey of Canada conducted a sepa- rate flood frequency analysis with each set of data. Examination of the results of these separate flo o d frequency analyses showed that for each of the 14 stations the 50, 100 and 200-year return period flood e s t i - mates were greater for r a i n f a l l - i n d u c e d floods than those derived for the same basin for snowmelt floods. Flood frequency curves are shown separately for r a i n f a l l and snowmelt-induced floods on Figure 2.6 for a t y p i c a l coastal B.C. s t a t i o n . 20 1.05 1.25 2 5 10 20 50 200 RETURN PERIOD ( years ) Figure 2.6 Flood Frequency Curves for Cheakamus River Near Mons (1924-47) - 30 - Ratios of maximum instantaneous to maximum d a i l y discharge for the max- ium floods on record for a l l coastal B r i t i s h Columbia and southeast Alaska stations reviewed for th i s study are plo t t e d on Figure 2.7. These data indicate that the range of flood r a t i o s for small basins i s s i g n i f i c a n t l y greater than for larger basins. The larger flood r a t i o s i l l u s t r a t e the flashy nature of floods which are r a i n f a l l - i n d u c e d on many basins i n the coastal region. 10 100 1000 10000 100 000 x • l» X • I • Coastal British Columbia x Southeast Alaska X X X X X X i » - X « X X • • X X X I • X • ) • • • X * • • • < • X • X 1 i < • X * ) X X < X X 1 ( 1 x X • I * t ••* X • • • 1 •• • • • I 10 100 1000 10000 100000 DRAINAGE AREA (sq km ) Figure 2.7 Ratios of Maximum Instantaneous to Maximum D a i l y Floods - 32 - 2.3 C A S E S T U D I E S Four case studies are selected to i l l u s t r a t e c h a r a c t e r i s t i c s described i n preceding sections which lead to extreme floods i n the coastal region. These c h a r a c t e r i s t i c s generally include storms which r e s u l t from low pressure systems in f a l l and winter; floods which are r a i n f a l l - i n d u c e d either as r a i n f a l l - o n l y or as rain-on-snow; and flood hydrographs which are very flashy i n the mountainous coastal region. November, 1978 Flood near Terrace, B r i t i s h Columbia (Schaefer, 1979) A multi-day r a i n f a l l event, heaviest on October 31 and November 1, 1978, resulted i n serious flooding i n the area surrounding Terrace, B r i t i s h Columbia. Both the Zymoetz River near Terrace and the Kitimat River to the south experienced t h e i r largest flood on record during this storm, estimated to be about the 100-year return period f l o o d . The storm resulted from a f r o n t a l wave which approached from the south- west with winds a l o f t of about 85 knots and reached the coast north of the Queen Charlotte Islands. The airmass which was the source of the storm's heavy p r e c i p i t a i o n was close to saturation from the surface to 5000 m with considerable moisture at higher l e v e l s . Freezing l e v e l s which averaged 1500 m in the Terrace area p r i o r to the storm increased to 3000 m. By the afternoon of November 1 a cold front impinged on the mainland coast, a f t e r which the airmass cooled and dried out markedly. The time d i s t r i b u t i o n of p r e c i p i t a t i o n recorded at Terrace Airport for the storm period i s shown on Figure 2.8. The greatest rates of accu- - 33 - mulation for periods greater than one hour occurred during the f i r s t morning of the storm on October 31 , although r e l a t i v e l y heavy ra i n also f e l l near the end of the storm during the afternoon of November 1. For durations of less than one hour, peak i n t e n s i t i e s occurred during showers which e f f e c t i v e l y ended the storm. The return periods for a range of storm durations at Terrace^ Airport are shown on Figure 2.9. The storm was unremarkable for durations less than one hour with e s t i - mated return periods less than two years; f o r longer durations from 2 to 4 days estimated return periods ranged from 85 to 95 years. 250 1200 0000 I200 0000 1200 0000 OCT. 30 OCT. 31 NOV. I D A T E AND T I M E ( H O U R S ) Figure 2.8. P r e c i p i t a t i o n at Terrace Airport for Oct. 31 - Nov. 1, 1975 SMORt DURRTION R P I N F P L L 1 N T E N S I T Y - D U R R T I O N FREOUENCY DPTP F O R - OONMEES SUR L ' 1 N T E N S I T E . L P PUREE E l L n FREQUENCE 0 E 9 CHUTES DE P L U I E DE COURTE DUREE P TERRACE R BC DPSED OH RECORDING RP1N CPUCE DPTP FOR THE P E R I O D - BOSEE3 SUR L E 9 D0NNEE9 DU PLUVIO0RPPHE9 POUR L P PERIODS 1869 - 1993 16 YEPRS/PNS is so 3a eo a e is MINUTES HOURS HEURE9 DURRTI ON DUREE Figure 2.9 Intensity-Duration-Frequency Curves for Terrace Airport - 35 - December, 1972 Rainstorm at Vancouver, B r i t i s h Columbia (Eddy, 1979) A storm began i n the early morning hours on December 25 and lasted from 23 to 26 hours at c l i m a t o l o g i c a l stations i n the Vancouver area. This storm produced the largest 24-hour p r e c i p i t a t i o n on record at Vancouver International A i r p o r t (92.9 mm) and i n Vancouver's c i t y centre (141.5 mm) while other stations in the area experienced near record amounts. The storm caused extensive flooding i n the greater Vancouver area. During the morning of December 25th, a deep low pressure area was moving northward over the Gulf of Alaska. Ah associated tongue of warm a i r a l o f t with winds from the southwest was at t h i s time l y i n g across the Queen Charlotte Islands. While the deep low continued to move towards the Alaska coast, the f r o n t a l wave associated with the tongue of warm a i r continued eastward across Vancouver Island and the mainland coast. P r e c i p i t a t i o n ended quite abruptly when this system passed and a weak ridge of high pressure began to bui l d over the area. The time d i s t r i b u t i o n s of r a i n f a l l recorded at Vancouver A i r p o r t and i n Vancouver's c i t y centre are shown on Figure 2.10. The return periods for a range of durations are shown on Figure 2.11 for Vancouver A i r p o r t . The storm c h a r a c t e r i s t i c s plotted on Figure 2.11 show that while p r e c i p i t a - tion i n t e n s i t i e s for durations less than about two hours were r e l a t i v e l y low, longer duration i n t e n s i t i e s were more extreme with the 24-hour amount exceeding an estimated 50-year return period event. I t i s i n t e r - esting to note that on Hollyburn Ridge, a higher elevation s t a t i o n at 951 m overlooking Vancouver, snow changed to r a i n early on December 25, 1972. Figure 2.10. P r e c i p i t a t i o n i n Vancouver Area on December 25, 1972. - LZ - - 38 - Probable Maximum P r e c i p i t a t i o n - Coquitlam Lake watershed (Schaefer,1981) An analysis of meteorological conditions associated with the generation of a probable maximum flood was undertaken for the Coquitlam Lake water- shed located approximately 30 km northeast of downtown Vancouver. Eleva- tions i n the 181 sq km basin range from 153 to 2000 m with several peaks reaching 1400 m around the drainage basin boundary. As part of the study, the most extreme p r e c i p i t a t i o n events on record for durations of one to four days were analyzed at Coquitlam Lake and Vancou- ver International A i r p o r t . At Coquitlam Lake eight multi-day events were analyzed for the period of record from 1924 to 1981, and at Vancou- ver International Airport seven multi-day events were analyzed from 1937 to 1981. The eight most extreme events analyzed at Coquitlam Lake occurred during the period from November through February, while the seven events at Vancouver Ai r p o r t occurred from October through January. Although d e t a i l s d i f f e r e d from case to case., common features i n a l l events included f r o n t a l waves, surface low pressure areas and strong southwesterly a i r flows a l o f t . V e r t i c a l i n s t a b i l i t y was ruled out as a s i g n i f i c a n t contributing factor to the t o t a l p r e c i p i t a t i o n i n a l l storms considered i n the a n a l y s i s . This finding i s consistent with a U.S. Weather Bureau study (1966) which concluded that severe thunderstorms were not a factor i n the region west of the Cascade Mountains. The f i v e l a r g e s t one day storms for each calender month were also ana- lyzed for the two stations i n conjunction with recorded temperature and - 39 - moisture data to estimate the maximum pr e c i p i t a b l e water available i n each storm. Based on t h i s analysis i t was concluded that the probable maximum p r e c i p i t a t i o n (PMP) would occur i n December, and rat i o s were developed for the r e l a t i v e amount of p r e c i p i t a t i o n that could occur i n other months of the year, Table 2.5. TABLE 2 . 5 MONTHLY ADJUSTMENT FACTORS FOR PROBABLE MAXIMUM PRECIPITATION JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC Coqu I t l am Lake 0 . 9 1 0 . 7 6 0 . 6 8 0 . 6 2 0 . 5 9 0 . 5 8 0 . 5 9 0 . 6 5 0 . 7 8 0 . 9 1 0 . 9 9 1 .00 V a n c o u v e r I n t e r n a t i o n a l A i r p o r t 0 . 8 5 0 . 6 9 0 . 5 9 0 . 5 4 0 . 4 8 0 . 4 6 0 . 4 9 0 . 5 6 0 . 6 9 0 . 8 3 0 . 9 6 1 .00 - 40 - Flood of December 1964 i n Coastal Oregon (Waananen et a l , 1971) During the period from December 19-23, 1964 extensive flooding occurred along coastal Oregon and in the Willamette River v a l l e y which l i e s between the Coastal and Cascade Mountain ranges. Many r i v e r s exper- ienced t h e i r largest floods on record during t h i s period and i t i s estimated that without flood regulation, the peak flow on the Willamette River would have been the second largest flood on record behind that which occurred i n 1861. In the Willamette River v a l l e y 85,000 ha of a g r i c u l t u r a l land was inundated, three l i v e s were l o s t and flood losses were more than $65 m i l l i o n . Along coastal Oregon six l i v e s were l o s t and flood losses were more than $60 m i l l i o n . P r i o r to the December 19-23 storm a high pressure airmass over the P a c i f i c Ocean occupied most of the ocean area between Hawaii and Alaska. An a r c t i c airmass spread over Oregon from December 14-18 and p a r t l y froze much of the ground. I n i t i a l storm p r e c i p i t a t i o n from December 18-20 was accompanied by low temperatures and consisted l a r g e l y of snow over much of the region. The P a c i f i c high located northeast of Hawaii eroded on December 20 and allowed storms with warm moist t r o p i c a l a i r to move across the ocean at successively lower latitudes as they approached the west coast. Mixing of warm moist a i r with cold A r c t i c a i r west of the coast caused the storm systems to i n t e n s i f y . - 41 - From December 21-23 temperatures rose sharply and freezing l e v e l s rose to 3000 m causing almost a l l p r e c i p i t a t i o n to occur as r a i n . In the Willamette River basin the storm brought as much as 380 mm of r a i n to the v a l l e y during December 19-23 and 460 mm of rain to higher a l t i t u d e s i n the Cascade Range. Average p r e c i p i t a t i o n along the coast ranged from 150 to 280 mm for the same period with point measurements as high as 550 mm i n the Coast Range. P r e c i p i t a t i o n rates i n excess of 200 mm i n 24-hours were recorded at a few stations i n Oregon. The response of r i v e r flows to this storm was extremely rapid as heavy r a i n f a l l runoff was supplemented by snowmelt. in some instances, flows increased twenty-fold from the s t a r t of the storm on December 19 to a peak on December 22. Examination of flood hydrographs for many stations i n the region showed two-fold increases i n discharge over periods as short as four hours. The flashy nature of streamflow response to extreme r a i n f a l l combined with snowmelt i s i l l u s t r a t e d on Figure 2.12 f o r repre- sentative s t a t i o n s . Each flood hydrograph plotted on Figure 2.12 rep- resents a flood discharge with an estimated return period of at least 50 years. - 42 - Figure 2.12. Rain-On-Snow Flood Hydrographs - 43 - 2.4 SUMMARY 1) A single hydrologic region exists along the coast which extends the entir e length of the province and includes southeast Alaska, and i s bounded to the east by the crest of the Coastal Mountain range. 2) Climatic features i n the coastal region include consistent annual trends i n p r e c i p i t a t i o n with the wettest months occurring i n f a l l and winter. A r e l a t i v e l y small annual temperature range occurs. Within the coastal region, however, l o c a l variations e x i s t i n pre- c i p i t a t i o n and temperature due to the complex i n t e r a c t i o n between atmospheric c i r c u l a t i o n patterns and major topographic features d i s t r i b u t e d along the coast. 3) The climate of the coastal region i s controlled on a seasonal basis by macro-scale atmospheric processes. During winter months low pressure areas over the Gulf of Alaska cause numerous storms to form over the P a c i f i c Ocean and move from the southwest towards the Gulf. During summer months a high pressure centre forms off-shore and the i n t e n s i t y and frequency of P a c i f i c storms i s diminished compared to winter. 4) Detailed' meteorologic analyses conclude that the most extreme storms i n the coastal region w i l l occur during the winter months and w i l l r e s u l t from storm systems which develop from low pressure areas offshore and generally approach the coast from the southwest. - 44 - 5) Extreme floods occur i n the f a l l and winter on most drainage basins i n the coastal region. These floods are r a i n f a l l - i n d u c e d . Rain- f a l l - i n d u c e d floods r e s u l t from r a i n f a l l runoff only or from a combination of ra i n and snowmelt runoff. 6) For those stations i n coastal B r i t i s h Columbia determined to have both a f a l l / w i n t e r r a i n f a l l and a spring/summer snowmelt-induced flood regime, extreme r a i n f a l l - i n d u c e d floods are greater than those estimated on the same basin for snowmelt floods. 7) Any hydrologic analysis undertaken to model coastal basins i n order to p r e d i c t extreme floods must be capable of simulating both r a i n - f a l l runoff and runoff r e s u l t i n g from the i n t e r a c t i o n between ra i n and snow. A model of these runoff processes must undertake c a l c u l a - tions with a time step much less than one day i n order to simulate the flashy nature of most coastal floods. - 45 - 3. CHARACTER!STICS OF STORM RAINFALL IN THE COASTAL REGION 3.1 INTRODUCTION An assessment of storm r a i n f a l l i s required for flood analysis in coastal B.C. since extreme floods on most drainage basins are r a i n f a l l - i n d u c e d during f a l l and winter months. Major obstacles facing the p r a c t i c i n g engineer i n design situations in the coastal region include: the pre- c i p i t a t i o n gauge network i s r e l a t i v e l y sparse in remote regions; most stations are located at r e l a t i v e l y low elevations; many stations in cur- rent operation do not have long term records; a v a i l a b l e data cannot be rea d i l y transposed with confidence over long distance due to mountainous t e r r a i n ; and many stations report only 24-hour data so that shorter duration storm i n t e n s i t i e s are not a v a i l a b l e . The shortcomings i n data described above are not e a s i l y overcome. One recent study by Hogg and Carr (1985) produced a r a i n f a l l frequency atlas as shown on Figure 3.1 which i l l u s t r a t e s general trends in the d i s t r i b u - tion of 24-hour p r e c i p i t a t i o n i n B.C. Engineering design i n the coastal region, however, requires more detai l e d r a i n f a l l d i s t r i b u t i o n data at a much larger scale than i s currently a v a i l a b l e . Analysis of r a i n f a l l data presented i n this chapter was undertaken with the primary goal of i d e n t i f y i n g regional c h a r a c t e r i s t i c s which can be used to estimate r a i n f a l l i n instances where l o c a l data are not a v a i l - able. The premise that such c h a r a c t e r i s t i c s might e x i s t for the diverse coastal region extending the length of the province was based on the following: Figure 3.1 Twenty-Four Hour P r e c i p i t a t i o n in B r i t i s h Columbia (after Hogg and Carr, 1985) - 47 - i ) atmospheric pressure maps presented on Figures 2.3 and 2.4 i n Chapter 2 suggest s i m i l a r macro-scale c i r c u l a t i o n patterns a f f e c t climate along the en t i r e coastal region. i i ) monthly p r e c i p i t a t i o n data for coastal B.C. stations presented i n Table 2.1 i n Chapter 2 i l l u s t r a t e that even though the magnitude of p r e c i p i t a t i o n varies considerably along the coast, monthly d i s t r i b u - t i o n on a percentage basis i s s i m i l a r among the s t a t i o n s . The two observations noted above prompted the region-wide analysis of storm r a i n f a l l presented in the following sections. That i s , i f trends i n the monthly d i s t r i b u t i o n of p r e c i p i t a t i o n e x i s t in the coastal region, then perhaps trends can also be i d e n t i f i e d for storm p r e c i p i t a t i o n . Ratios of r a i n f a l l depth for a given duration to that of a reference duration are used i n the analysis rather than r a i n f a l l magnitude alone. This method i s one approach to i d e n t i f y i n g regional c h a r a c t e r i s t i c s when the amount of r a i n f a l l i s quite d i f f e r e n t between s t a t i o n s . 3.2 OVERVIEW OF PRECIPITATION SYSTEMS The following discussion of p r e c i p i t a t i o n systems i s included in t h i s s e c t i o n , p r i o r to presenting analysis undertaken with r a i n f a l l data i n coastal B.C., to provide an overview of the nature and structure of storms. Oke (1978) used a consensus of the l i t e r a t u r e to develop a c l a s s i f i c a t i o n system for atmospheric phenomena based on h o r i z o n t a l - 48 - scales. Braun and Slaymaker (1981) incorporated s i m i l a r concepts of scale i n t h e i r discussion of atmospheric and hydrologic systems. In general, atmospheric phenomena can be c l a s s i f i e d as follows: i ) Macroscale (or synoptic scale) processes include those with h o r i - zontal scales of 100-10 000 km and l i f e t i m e s from one day to a week. Examples of this scale include low and high-pressure systems which often a f f e c t weather over large regions for a period of a few days. The basis for weather prediction at the synoptic scale i s weather- map analysis which reduces vast amounts of data into meaningful patterns that can be interpreted by a meteorologist. i i ) Microscale processes include those with horizontal scales i n the order of 1 cm to 1 km and with time scales varying from a second to several minutes. These phenomena include convection c e l l s which form from d i f f e r e n t i a l heating of adjacent airmasses and mechanical turbulence caused by a i r flowing over rough t e r r a i n . Tornados are an example of microscale motion and possess t y p i c a l c h a r a c t e r i s t i c s such as rapid growth, vigorous updrafts and downdrafts, and random movement. i i i ) L ocal and mesoscale processes include those which l i e between micro and macroscale phenomena and have hor i z o n t a l scales ranging from 100 m to 500 km with a time scale up to a day. Examples of meso- scale processes include land-sea and mountain-valley breezes, and sq u a l l lines of thunderstorm a c t i v i t y . - 49 - In addition, p r e c i p i t a t i o n i s usually divided into three p r i n c i p a l types according to the primary mode causing u p l i f t of a i r (Barry and Chorley, 1982). These categories of p r e c i p i t a t i o n are described below: i ) Convective p r e c i p i t a t i o n results when a l o c a l i n s t a b i l i t y i s gener- ated as a portion of a i r i s heated and the airmass column r i s e s . i i ) Cyclonic p r e c i p i t a t i o n i s caused by the ascent of a i r through m h o r i z o n t a l convergence of airstreams i n an area of low pressure. In some instances this i s reinforced by u p l i f t of warm moist a i r along an airmass boundary. i i i ) Orographic p r e c i p i t a t i o n i s caused by u p l i f t i n g of an airmass as i t passes over a topographic feature. Meteorological phenomena described above provide a convenient framework for categorizing physical processes for a n a l y t i c a l study. In r e a l i t y , however, meteorological observations are affected by many scales of - motion occurring simultaneously and which continuously change with time. For example, r i s i n g topography may induce orographic p r e c i p i t a t i o n but also tr i g g e r convective i n s t a b i l i t y from d i f f e r e n t i a l heating of moun- ta i n slopes, increase cyclonic p r e c i p i t a t i o n by retarding the rate of movement, and cause u p l i f t through funnelling e f f e c t s of v a l l e y s on airstreams. - 50 - The structure and evolution of f i v e storms as they approached the Van- couver region in coastal B.C. were examined by Bonser (1982) based on radar-derived p r e c i p i t a t i o n measurements. Even though the study area was li m i t e d to the Vancouver region, c h a r a c t e r i s t i c s of p r e c i p i t a t i o n patterns observed by Bonser can be considered in a q u a l i t a t i v e manner as t y p i c a l for many storms as they impinge on coastal B.C. mountains. The range of scales of meteorological phenomena observed for a single storm i n December 1980 i s described below: i ) Macroscale. P r e c i p i t a t i o n resulted from a low pressure system pass- ing over the Vancouver area. i i ) Mesoscale. P r e c i p i t a t i o n was i d e n t i f i e d as a band moving in the d i r e c t i o n of the front and as i r r e g u l a r patches ahead of the fr o n t . As a r a i n f a l l band approached the mountains north of Vancouver i t was retarded r e l a t i v e to other portions of the system behind i t . i i i ) Microscale. Individual convective c e l l s formed within broad r a i n - f a l l areas and appeared to remain in the same p o s i t i o n r e l a t i v e to the r a i n f a l l band. Radar data together with data processing and v i s u a l display software provide a means of examining movement of storm systems and growth and decay of c e l l s within the system. Unfortunately, the radar s t a t i o n i n - 51 - Abbotsford, B.C. used by Bonser i n his study ceased operation i n 1982 and there i s cur r e n t l y no st a t i o n operating anywhere i n B r i t i s h Columbia. The i n t e r a c t i o n of d i f f e r e n t scales of motion i s one of the most d i f f i - c u l t problems of quantitative meteorology, as i t i s not yet possible to tr e a t numerically a l l relevant scales which range from a centimetre to thousands of kilometres i n size and from seconds to months i n time (Anthes et a l . , 1978). Lacking adequate radar techniques and physical models, data recorded at ra i n gauge networks remain the most important source of information a v a i l a b l e for storm analysis by engineering hydro- l o g i s t s . 3.3 SOURCE OF B.C. RAINFALL INTENSITY DATA R a i n f a l l data are available from Atmospheric Environment Service (AES) for stations throughout Canada. In coastal B.C. there are cu r r e n t l y 58 stations (Table 3.1) at which r a i n f a l l i n t e n s i t y data are recorded. Data analyzed i n th i s study were provided by AES on magnetic tape and computer programs were written to extract pertinent data from the tape and to undertake c a l c u l a t i o n s with these data as described i n the follow- ing sections. Of the 58 coastal B.C. st a t i o n s , 27 are located i n the Vancouver/Lower Mainland area, 6 i n the Victoria/Saanich Peninsula area and 25 are d i s - t r i b u t e d across the remainder of the region. The network density for the coastal region outside of the Vancouver and V i c t o r i a areas i s com- pared i n Table 3.2 to recommendations by the World Meteorological Organi- zation (WMO, 1970). - 52 - TABLE 3.1 COASTAL B.C. STATIONS WITH RAINFALL INTENSITY DATA Location No. of North West Elev. Years Station Latitude Longitude (m) of Record Abbotsford A 49 02 122 22 58 7 Agassiz CDA 49 15 121 46 15 26 Alouette Lake 49 17 122 29 117 13 Alta Lake 50 09 122 57 668 13 Bear Creek 48 30 124 00 351 7 Bel l a Coola Hydro 52 22 126 49 14 14 Buntzen Lake 49 23 122 52 17 15 Bur nab y Mtn BCHPA 49 17 122 55 465 9 Campbell River BCFS 50 04 . 125 19 128 10 Campbell River BCHPA 50 03 125 19 30 11 Carnation Creek 48 54 125 00 61 7 Chilliwack Microwave 49 07 121 54 229 17 Clowhom F a l l s 49 43 1 23 32 23 15 Com ox A 49 43 124 54 24 14 Coquitlam Lake 49 22 1 22 48 1 61 13 Courtney Puntledge 49 41 125 02 24 20 Daisy Lake Dam 49 59 123 08 381 15 Estevan Point 49 23 126 33 7 10 Haney Microwave 49 12 1 22 31 320 20 Haney UBC 49 16 122 34 143 20 Jordan River Diversion 48 30 124 00 393 10 Jordan River Generating 48 25 124 03 5 11 Kitimat 54 00 128 42 17 10 Ladner BCHPA 49 05 123 03 2 13 Langley Lo c h i e l 49 03 1 22 35 1 01 12 Mission West Abbey 49 09 122 16 221 21 Nanaimo Departure Bay 49 13 1 23 57 8 1 3 North Vane. Lynn Creek 49 22 123 02 191 19 P i t t Meadows STP 49 13 1 22 42 5 9 P i t t Polder 49 18 122 38 2 19 - 53 - TABLE 3.1 (continued) COASTAL B.C. STATIONS WITH RAINFALL INTENSITY DATA Location No. of North West Elev. Years Station Latitude Longitude (m) of Record Port Alberni A 49 15 124 50 2 15 Port Coquitlam City Yard 49 16 122 46 7 13 Port Hardy 50 41 127 22 22 10 Port Mellon 49 31 123 29 8 11 Port Moody Gulf O i l Ref. 49 17 1 22 53 1 30 13 Port Renfrew BCFS 48 35 124 24 6 11 Prince Rupert A 54 18 1 30 26 34 14 Saanich Densmore 48 30 123 25 38 10 Sandspit A 53 15 131 49 5 12 Spring Island 50 00 127 25 11 8 Stave F a l l s 49 14 1 22 21 55 8 Strathcona Dam 50 00 125 35 201 15 Surrey Kwantlen Park. 49 12 122 52 93 22 Surrey Municipal H a l l 49 06 122 50 76 20 Terrace A 54 28 128 35 217 15 Terrace PCC 54 30 128 37 58 15 Tofino A 49 05 125 46 20 V3 Vancouver A 49 11 .123 10 3 31 Vancouver Harbour 49 18 1 23 07 0 8 Vancouver K i t s i l a n o 49 16 123 11 23 30 Vancouver PMO 49 17 1 23 07 59 10 Vancouver UBC 49 15 123 15 87 6 V i c t o r i a Gonzales Heights 48 25 123 19 69 51 V i c t o r i a Int. A 48 39 123 26 19 19 V i c t o r i a Marine Radio 48 22 1 23 45 32 17 V i c t o r i a Shelbourne 48 28 123 20 38 9 V i c t o r i a U. of V i c t . 48 28 1 23 20 46 19 Whi te Rock STP 49 01 122 46 15 18 Note: Station descriptions from Environment Canada (1981b) 54 TABLE 3.2 DENSITY OF RAIN GAUGE NETWORKS Recommendations by WMO: F l a t regions Mountainous regions Small mountainous islands with i r r e g u l a r p r e c i p i t a t i o n B r i t i s h Columbia: Coastal region excluding Vancouver and V i c t o r i a areas: 600-900 km 2/station 100-250 km 2/station 25 km 2/station 225 000 km2/25 stations = 9 000 km 2/station Information presented in Table 3.2 i l l u s t r a t e s the shortcomings of r a i n - f a l l i n t e n s i t y data in coastal B.C. with regard to network density. I t also emphasizes that for much of the region, engineering design requir- ing r a i n f a l l analysis often has to be undertaken without data from the project s i t e or from the immediately surrounding area. Even i n instances when r a i n f a l l i n t e n s i t y data are available from a l o c a l s t a t i o n the period of record i s often too short to confidently undertake s t a t i s t i c a l analysis of the data. Lengths of record for the 58 stations which record r a i n f a l l i n t e n s i t y i n the coastal region are i l l u s t r a t e d on Figure 3.2. This summary shows that over half of the coastal stations have less than 15 years of data and only two stations have more than 30 years of record. - 55 - 0 ' 1 1 1 1 L-1 0 10 2 0 3 0 4 0 5 0 6 0 NUMBER OF STATIONS WITH L E N G T H OF RECORD E Q U A L TO OR G R E A T E R T H A N V A L U E S H O W N Figure 3.2 Lengths of Record at Coastal B.C. Stations In addition to the 58 stations which record r a i n f a l l i n t e n s i t y , there are approximately 250 stations in coastal B.C. which record only 24-hour data (Environment Canada, 1981a). A p a r t i c u l a r benefit of i d e n t i f y i n g regional c h a r a c t e r i s t i c s of r a i n f a l l i n t e n s i t y would be that these 24-hour stations could then be used to greatly expand the data base curr e n t l y available for design purposes. - 56 - 3.4 INTENSITY-DLTRATION-FREOLTENCY CORVES 3.4.1 Development and Dse of IDF Curves Intensity-duration-frequency (IDF) curves are prepared by Atmospheric Environment Service for each of the 58 coastal B.C. stations which record r a i n f a l l i n t e n s i t y . The procedure used by AES to develop IDF curves at each s t a t i o n consists of producing from recorded data annual maximum series of r a i n f a l l i n t e n s i t i e s for durations ranging from 5 minutes to 24 hours, conducting an extreme value frequency analysis with each annual maximum ser i e s , and f i n a l l y generating a set of best f i t curves for selected return periods and for the range of durations. A t y p i c a l IDF curve produced by AES i s shown on Figure 3.3. Plotted points for each duration are res u l t s of the extreme value frequency anal- ysis and i l l u s t r a t e the "depth-frequency" r e l a t i o n s h i p for that duration. Best f i t lines connecting points with the same return period i l l u s t r a t e the "depth-duration" r e l a t i o n s h i p for a p a r t i c u l a r return period. I t i s important to recognize i n the development of IDF curves that since annual maximum series are generated for each duration, r a i n f a l l i n t e n s i t y for one duration i s not necessa r i l y related to the i n t e n s i t y for another duration with the same return period. That i s , the set of r a i n f a l l i n t e n s i t i e s for durations from 5 minutes to 24 hours do not generally occur within the same storm to produce the 24-hour r a i n f a l l depth. IDF curves provide average i n t e n s i t i e s for a given duration and return period, but do not provide information regarding var i a t i o n s i n r a i n f a l l i n t e n s i t i e s within a single storm. - Z.S - - 58 - Use of IDF re l a t i o n s h i p s to develop synthetic hyetographs for flood analysis has been widely incorporated i n Canadian pra c t i c e (McKelvie, 1982). One procedure i s commonly referred to as the Chicago method and was o r i g i n a l l y proposed by Ke i f e r and Chu (1957). In this method a design storm i s generated such that the r e s u l t i n g hyetograph i s com- prised throughout the storm duration of incremental r a i n f a l l i n t e n s i t i e s with the same return period. As noted above, however, IDF curves are de- veloped with data from a v a r i e t y of storms and generally do not repre- sent a sequence of i n t e n s i t i e s i n a single storm. Nevertheless, t h i s procedure i s used p a r t l y because of l i m i t e d a l t e r n a t i v e s for design. In the Chicago method the time sequence of r a i n f a l l i n t e n s i t i e s within the design storm i s determined from analysis of h i s t o r i c a l storm records. The r e l a t i v e timing of peak i n t e n s i t y within storms on record i s used as a guide for choosing the time sequence for a synthetic hyetograph devel- oped from IDF curves. An a l t e r n a t i v e method i s to d i s t r i b u t e r a i n f a l l i n t e n s i t i e s symmetrically with time. Even though this second approach appears quite a r b i t r a r y , i t has been applied extensively and i s s t i l l recommended by the U.S. Bureau of Reclamation (USBR, 1977) for flood s t u dies. Regional c h a r a c t e r i s t i c s of IDF curves are investigated i n this study by analyzing depth-duration and depth-frequency relationships separately. The i n i t i t a l data base consisted of r a i n f a l l depths produced by AES from extreme value frequency analysis, p r i o r to estimates by AES of best f i t - 59 - curves included on IDF graphs. For each s t a t i o n depth-duration char- a c t e r i s t i c s are assessed by c a l c u l a t i n g r a t i o s of short duration r a i n - f a l l to the 24-hour depth, and depth-frequency c h a r a c t e r i s t i c s for a given duration are assessed by c a l c u l a t i n g r a t i o s to a reference depth taken as the 10 year period. R a i n f a l l depth-duration-frequency data analyzed i n this i n v e s t i g a t i o n are included Ln Appendix II for each of the 58 coastal B.C. s t a t i o n s . - 60 - 3.4.2 Depth-Duration Relationships 3.4.2.1 Analysis of B.C. Data R a i n f a l l i n t e n s i t y data available from AES are analyzed to determine the r e l a t i o n s h i p between r a i n f a l l depth and duration for a given return period and to assess the v a r i a b i l i t y of t h i s r e l a t i o n s h i p throughout the coastal region. Regional "depth-duration" c h a r a c t e r i s t i c s provide a basis for estimating r a i n f a l l for a range of durations i n instances when r a i n f a l l i s known only for one duration. Depth-duration r e l a t i o n s h i p s for the coastal region are assessed by c a l c u l a t i n g r a t i o s of short duration r a i n f a l l to the 24-hour depth. R a i n f a l l depths were obtained at each of the 58 ava i l a b l e coastal B.C. stations included i n Appendix II for durations of 1, 2, 6, 12 and 24 hours. These r a i n f a l l depths were used to calculate the r a t i o of depth for each duration to the 24-hour depth with the same return period. Depth r a t i o s calculated by t h i s procedure are tabulated i n Appendix II for each i n d i v i d u a l coastal s t a t i o n . R a i n f a l l data and depth-duration r a t i o s calculated for P i t t Polder in southwestern B.C. are included i n Table 3.3 to i l l u s t r a t e t y p i c a l r e s u l t s of analysis undertaken i n th i s study. These re s u l t s for P i t t Polder show that the r a t i o of r a i n f a l l on IDF curves for a given duration to the 24-hour depth i s r e l a t i v e l y constant with return period. - 61 - TABLE 3.3 DEPTH-DURATION DATA FOR PITT POLDER R a i n f a l l Data (mm) From AES Return Period (Years) Duration 2 5 10 25 50 100 1 hr 1 2.4 1 4.7 16.2 18.1 19.1 20.9 2 hr 18.9 22.9 25.4 28.7 31 .2 33.6 6 hr 42.7 51 .7 57.7 65.3 70.9 76.4 12 hr 67.3 80.3 88.9 99.8 107.9 115.9 24 hr 98.9 119.0 1 32.2 149.0 1 61 .5 1 73.8 Depth-Duration Relationships Duration 2 5 10 25 50 100 1 hr 0.13 0.12 0.12 0.12 0.12 0.12 2 hr 0.19 0.19 0.19 0.19 0.19 0.19 6 hr 0.43 0.43 0.44 0.44 0.44 0.44 12 hr 0.68 0.67 0.67 0.67 0.67 0.67 24 hr 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 - 62 - Examination of results of analysis of r a i n f a l l i n t e n s i t y data from across the region shows depth-duration r a t i o s have minimal v a r i a t i o n with return period at each of the available coastal B.C. st a t i o n s . Furthermore, the magnitude of these depth r a t i o s from a l l available stations are i n a r e l a t i v e l y narrow range for the coastal region. Mean values of depth- duration r a t i o s for 58 coastal stations are l i s t e d in Table 3.4 and shown g r a p h i c a l l y on Figure 3.4. TABLE 3.4 DEPTH-DURATION RATIOS FOR IDF CURVES Depth Ratios for Coastal Region Duration (Hours) 2 Return Period (Years) 5 10 25 50 100 Mean ( a l l Std. Dev. values) 1 0.16 0.16 0. 16 0. 16 0. 17 0.17 0.16 0.06 2 0.24 0.23 0. 23 0. 23 0. 23 0.23 0.23 0.06 6 0.48 0.46 0. 45 0. 44 0. 43 0.42 0.45 0.06 12 0.72 0.69 0. 68 0. 67 0. 67 0.66 0.68 0.05 24 1 .00 1.00 1 . 00 1 . 00 1 . 00 1 .00 1 .00 DURATION (HOURS) Figure 3.4 Depth-Duration Ratios for IDF Curves - 63 - These r e s u l t s are p a r t i c u l a r l y i n t e r e s t i n g considering that the magni- tude of r a i n f a l l varies considerably between stations across the region. For example, the 24-hour storm r a i n f a l l with a 100-year return period ranges from 75 to 380 mm for the a v a i l a b l e 58 coastal stations; mean annual p r e c i p i t a t i o n at these stations ranges from about 650 to 3500 mm. The regional c h a r a c t e r i s t i c of IDF curves i d e n t i f i e d for depth-duration r a t i o s i s e s p e c i a l l y useful for ap p l i c a t i o n to the approximately 250 coastal B.C. stations which record only 24-hour data. When a project s i t e i s near one of these st a t i o n s , frequency analysis can be undertaken to provide an estimate of a 24-hour r a i n f a l l with a desired return period. Then, depth-duration r a t i o s can be applied to estimate r a i n f a l l depths for any shorter durations which may be required for design purposes. Depth-duration r a t i o s from other regions of B r i t i s h Columbia are i n - cluded i n Table 3.5 for comparison. These data show that even though a range of r a t i o s has been calculated for the coastal region, values within t h i s range are d i s t i n c t from those r a t i o s calculated i n other regions of B r i t i s h Columbia. - 64 - TABLE 3.5 COMPARISON OF DEPTH-DURATION RATIOS Physiographic Duration Location Region 1 hour 6 hours 12 hours Mean of 58 Stations B.C. Coast 0.16 0.45 0.68 Castelgar, B.C. Southeast Mountains 0.44 0.79 0.87 Kamloops, B.C. I n t e r i o r Plateau 0.35 0.62 0.77 Fort St. John, B.C. Great Plains 0.42 0.74 0.87 3.4.2.2 Formulas for B.C. Data Results of analysis of coastal B r i t i s h Columbia data presented on Figure 3.4 provide a graphical d e s c r i p t i o n of how r a i n f a l l depth varies with duration. This section provides formulas r e l a t i n g depth and dura- t i o n which are i n a more convenient format for programming purposes by users of the r e s u l t s . Various formulas which r e l a t e r a i n f a l l depth to duration have been proposed. Chen (1976) summarized the main types of formulas which are commonly applied to r a i n f a l l data as part of a study of r a i n f a l l inten- s i t i e s at 34 c i t i e s across the United States. These formulas and that c u r r e n t l y applied by AES to Canadian data are included i n Table 3.6. 65 TABLE 3.6 FORMULAS RELATING RAINFALL DEPTH TO DURATION No. Equation Reference Comment 3.1 I = t + b Meyer (1921, 1928) Bernard (1932) concluded after analyzing further much of the data i n i t i a l l y reviewed by Meyer that Eq. 3.1 i s only su i t a b l e for short durations of about 5 to 120 minutes. 3.2 I = a a) Bernard (1932) a) Because of the apparent l i m i t a t i o n of Eq. 3.1, Bernard proposed Eq. 3.2 for longer durations of 2 to 24 hours. b) Environment Canada (1983c) b) AES applies Eq. 3.2 to stations across Canada for durations of 5 minutes to 24 hours, except where inspection of IDF curves shows the equation to be inappropriate. 3.3 I = :t + c Sherman (1931) Sherman found Eq. 3.3 to be applicable for the complete range of durations from 5 minutes to 24 hours. Chen (1976) used Eq. 3.3 i n his study of r a i n f a l l i n t e n s i - t i e s i n 34 c i t i e s i n the U.S. tP + c Keif e r and Chu (1957) Used by Keifer and Chu i n thei r development of a proce- dure for generating synthetic hyetographs, commonly r e f e r - red to as the Chicago Method. I = r a i n f a l l i n t e n s i t y ; t = duration; a, b and c = s t a t i o n c o e f f i c i e n t s - 66 - Formulas presented i n Table 3.6 are generally considered as "standard- form" types of r e l a t i o n s . These formulas are a l l empirical and i t i s reasonable to suppose that t h e i r acceptance as standard equations i s based mostly on precedent. Considering that the formulas were proposed i n an era which generally predates computer analysis, commonly applied curve f i t t i n g techniques were l i k e l y very simple compared to those c u r r e n t l y i n use. Review of Eq. 3.2 shows that t h i s r e l a t i o n s h i p repre- sents a st r a i g h t l i n e on a log-log p l o t . Perhaps Eqs. 3.3 and 3.4 were developed as an extension of Eq. 3.2 because r a i n f a l l i n t e n s i t y data often do not p l o t as a s t r a i g h t l i n e over an entire duration range from 5 minutes to 24 hours. R a i n f a l l i n t e n s i t i e s plotted on log-log scales are commonly concave up or down over the 5-60 minute range compared to longer duration i n t e n s i t i e s . Eq. 3.2 i s cu r r e n t l y applied by AES to r a i n f a l l i n t e n s i t y data from sta- tions across Canada. F i t t i n g a curve i n the form of Eq. 3.2 to Canadian data for durations ranging from 5 minutes to 24 hours r e s u l t s i n one of the following: r a i n f a l l i n t e n s i t y data f i t the curve well over the en- t i r e duration range; the curve does not f i t the data well over the entire range but i s nevertheless considered by AES to be an acceptable approxi- mation; or Eq. 3.2 does not f i t the data well enough to be recommended by AES i n which case no a l t e r n a t i v e i n t e n s i t y - d u r a t i o n formula i s provided. The c r i t e r i a described above are generally applied i n a subjective manner by AES based on inspection of plotted r a i n f a l l i n t e n s i t y data (Hogg, 1985). Inspection of IDF curves developed by AES for the 58 available coastal B.C. stations shows many stations are in the second category. - 67 - Based on assessment of procedures cu r r e n t l y applied by AES to develop empirical equations r e l a t i n g i n t e n s i t y to duration for a 5 minute to 24-hour range and on inspection of the r e s u l t s of these methods on IDF curves i n coastal B.C., the equations provided by AES are not included i n t h i s i n v e s t i g a t i o n of regional r a i n f a l l c h a r a c t e r i s t i c s . Alterna- t i v e l y , Eq. 3.2 was applied to i n t e n s i t y data with durations ranging only from 1 to 24 hours and a nonlinear curve f i t t i n g routine, NL2S0L, av a i l a b l e from the UBC Computing Centre (Moore, 1984) was used to calcu- l a t e the c o e f f i c i e n t s a and b in Eq. 3.2 which best f i t the available data. A summary of c o e f f i c i e n t s derived for each coastal s t a t i o n i s included i n Table 3.7. C o e f f i c i e n t a i s quite variable in the coastal region because i t r e f l e c t s magnitude of r a i n f a l l . For a given duration, t h i s c o e f f i c i e n t w i l l vary between stations which receive d i f f e r e n t amounts of r a i n and at the same s t a t i o n because i n t e n s i t y varies with return period. C o e f f i c i e n t b i s more representative of a regional character- i s t i c as i t shows the i n t e r r e l a t i o n s h i p between i n t e n s i t i e s at a station for a range of durations with the same return period. TABLE 3.7 DEPTH-DURATION FORMULAS FOR COASTAL B.C.* Return Period 2-years 5-years 10-years 25-years 50-years 100-years Station a b a b a b a b a b a b Abbotsford A 85 0.46 183 0.58 265 0.60 384 0.64 483 .0.67 588 0.69 Agassiz CDA 53 0.39 69 0.41 80 0.42 94 0.43 104 0.44 115 0.45 Alouette Lake 45 0.31 55 0.32 61 0.33 69 0.33 75 0.33 82 0.33 Alta Lake 40 0.42 43 0.40 45 0.39 47 0.38 49 0.38 51 0. 37 Bear Creek 150 0.45 289 0.51 393 0.54 534 0.56 644 0.57 757 0.59 Bella Coola Hydro 40 0.32 45 0.30 49 0.29 55 0.28 59 0.28 63 0.27 Buntzen Lake 58 0.34 72 0.33 81 0.33 93 0.33 102 0.33 110 0.33 Burnaby Mtn BCHPA 49 0.36 61 0.37 69 0.38 80 0.39 87 0.39 95 0.40 Campbell River BCFS 69 0.45 102 0.49 125 0.51 156 0.52 180 0.53 204 0.54 Campbell River BCHPA 106 0.51 223 0.61 325 0.66 478 0.71 606 0.74 746 0.76 Carnation Creek 45 0.32 53 0.31 59 0.30 66 0.30 71 0.30 77 0.29 Chilliwack Microwave 63 0.45 92 0.49 114 0.51 141 0.52 163 0.54 185 0.55 Clowhom F a l l s 45 0.36 63 0.39 76 0.40 93 0.42 106 0.43 118 0.43 Comox A 56 0.43 80 0.46 96 0.47 119 0.49 135 0.50 152 0.50 Coquitlam Lake 42 0.26 43 0.24 45 0.23 46 0.22 48 0.21 49 0.21 Courtney puntledge 44 0.37 57 0.38 65 0.39 77 0.39 85 0.40 93 0.40 Daisy Lake Dam 46 0.37 85 0.44 115 0.47 155 0.50 187 0.52 220 0.53 Estevan Point 79 0.36 99 0.36 112 0.36 128 0.36 140 0.36 152 0. 36 Haney Microwave 67 0.41 87 0.42 101 0.43 119 0.44 1 32 0.44 145 0.45 Haney UBC 51 0.35 61 0.36 69 0.36 78 0.36 85 0.36 91 0.36 Jordan River Diversion 112 0.39 174 0.42 218 0.43 275 0.44 318 0.45 361 0.45 Jordan River Generating 44 0.34 46 0.32 47 0.30 49 0.29 50 0.28 52 0.27 Kitimat 45 0.32 51 0.30 55 0.29 60 0.29 64 0.28 69 0.28 Ladner BCHPA 52 0.45 65 0.46 74 0.47 85 0.48 94 0.48 102 0.49 Langley Lochiel 71 0.45 101 0.48 1 23 0.49 152 0.51 174 0.52 198 0.53 TABLE 3.7 DEPTH-DURATION FORMULAS FOR COASTAL B.C.* (continued) r Return Period 2-years 5-years 10-years 25-years 50-years 100-years Station a b a b a b a b a b a b Mission West Abbey 95 0.47 151 0.52 194 0.55 251 0.57 296 0.59 342 0.60 Nanaimo Departure Bay 72 0.50 201 0.63 313 0.68 474 0.72 603 0.75 740 0.77 North Vancouver Lynn Creek 52 0.31 58 0.29 62 0.28 67 0.27 71 0.27 75 0.26 P i t t Meadows STP 71 0.43 125 0.48 165 0.50 217 0.53 258 0.54 299 0.55 P i t t Polder 47 0.33 54 0.32 59 0.32 66 0.32 71 0.31 75 0.31 Port Alberni 45 0.34 74 0.39 96 0.42 128 0.45 153 0.47 180 0.48 port Coquitlam C i t y Yard 46 0.35 58 0.36 66 0.37 76 0.38 84 0.39 91 0.39 Port Hardy 33 0.29 32 0.25 32 0.23 32 0.22 33 0.21 33 0.20 Port Mellon 73 0.33 75 0.31 76 0.30 78 0.29 80 0.28 82 0.27 Port Moody Gulf O i l Refin. 39 0.32 51 0.33 58 0.34 68 0.35 75 0.35 83 0.35 Port Renfrew BCFS 73 0.30 1 25 0.36 165 0.39 220 0.42 263 0.43 308 0.45 Prince Rupert A 55 0.37 55 0.33 56 0.31 57 0.30 58 0.28 60 0.28 Saanich Densmore 34 0.36 35 0.33 36 0.32 38 0.31 39 0.30 40 0.29 Sandspit A 66 0.45 79 0.45 88 0.45 99 0.45 107 0.46 114 0.46 Spring Island 61 0.34 68 0.32 73 0.31 79 0.30 83 0.30 88 0.30 Stave F a l l s 50 0.35 49 0.31 48 0.29 49 0.27 50 0.26 50 0. 25 Strathcona Dam 84 0.48 146 0.52 191 0.55 254 0.57 302 0.58 352 0.60 Surrey Kwantlen Park 59 0.41 85 0.43 103 0.45 126 0.46 143 0.46 160 0.47 Surrey Municipal H a l l 50 0.41 78 0.45 97 0.47 123 0.49 143 0.50 163 0.51 Terrace A 68 0.47 91 0.47 106 0.48 125 0.48 139 0.48 1 54 0.48 Terrace PCC 45 0.42 85 0.47 114 0.49 151 0.51 179 0.51 207 0.52 Tofino A 78 0.36 88 0.35 95 0.35 103 0.35 110 0.35 116 0.35 Vancouver A 68 0.47 92 0.49 108 0.50 129 0.50 145 0.51 160 0.51 Vancouver Harbour 95 0.49 182 0.57 252 0.61 353 0.65 436 0.68 522 0.69 Vancouver K i t s i l a n o 47 0.39 58 0.39 65 0.39 74 0.39 80 0.39 86 0.39 TABLE 3.7 DEPTH-DURATION FORMULAS FOR COASTAL B.C.* (continued) Return Period 2-years 5-years 10-years 25-years 50-years 100-years Station a b a b a b a b a b a b Mission West Abbey 95 0.47 151 0.52 194 0.55 251 0.57 296 0.59 342 0.60 Vancouver PMO 43 0.36 44 0.33 45 0.31 47 0.30 49 0.29 50 0.28 Vancouver UBC 60 0.44 87 0.47 106 0.49 131 0.50 150 0.51 169 0.52 V i c t o r i a Gonzales Heights 36 0.39 41 0.36 44 0.35 49 0.34 52 0.33 56 0.33 V i c t o r i a Int. A 42 0.40 47 0.38 50 0.37 54 0.37 57 0.36 59 0.36 V i c t o r i a Marine Radio 43 0.37 55 0.37 63 0.38 73 0.38 80 0.38 87 0.38 V i c t o r i a Shelbourne 46 0.43 51 0.41 55 0.40 60 0.40 63 0.39 67 0.39 V i c t o r i a U. of V i c t o r i a 37 0.37 38 0.34 38 0.32 40 0.31 41 0.30 43 0.29 White Rock STP 105 0.53 291 0.66 453 0.71 685 0.75 874 0.78 1070 0.80 Mean 0.39 0.41 0.41 0.42 0.43 0.43 Std. Dev 0.06 0.09 0.11 0.12 0.13 0.14 * intensity-duration equation j _ a - 71 - The r e l a t i o n s h i p between two i n t e n s i t i e s and I 2 can be shown as follows Converting to r a i n f a l l depth, R, Eq. 3.5 becomes: T2-r2{rJ « 3 . 6 ) Setting R1 = R and t 1 = t to represent r a i n f a l l depth for any time less than 24 hours, and se t t i n g R 2 = R 24 a " d t 2 = t 2 4 hours (1440 minutes) to represent r a i n f a l l depth i n 24 hours, Eq. 3.6 s i m p l i f i e s to: R2i 1440 V t ) ( 3 ' 7 ) F i n a l l y , i n s e r t i n g the mean value of b for the coastal region from Table 3.7 y i e l d s : * - i - ( i ! £ y - " = 0 0 1 3 7 ( 0 . M ,3 .8) R2, 1440 V t I  UU137! Comparison of r a i n f a l l depth r a t i o s presented i n Table 3.4 with those calculated with Eq. 3.8 i s included below. - 72 - TABLE 3.8 COMPARISON OF RAINFALL DEPTH RATIOS Duration Mean Depth Ratios for Coastal Region (Hours) Table 3.4/Figure 3.4 Eq. 3.8 1 0.16 0.15 2 0.23 0.23 6 0.45 0.44 12 0.68 0.66 24 1 .00 1 .00 Depth-duration equations which represent 80 percent confidence l i m i t s on Figure 3.4 are derived by i n s e r t i n g the appropriate value for b as shown below for lower and upper l i m i t s , r e s pectively: R t /1440\°" ... 5%; = l4To(— j A closer examination of c o e f f i c i e n t b l i s t e d for i n d i v i d u a l coastal st a - tions shows some i n t e r e s t i n g l o c a l v a r i a t i o n s . For example, comparison of Terrace A with Terrace PCC shows that c o e f f i c i e n t b can vary between stations i n close proximity. Comparison of Coquitlam Lake and Mission West Abbey, two stations located i n the mountains immediately north of the Fraser River near Vancouver, shows that c o e f f i c i e n t b for Coquitlam Lake i s one of the lower values calculated for the coastal region while .(3.9) (3.10) - 73 - the corresponding value for Mission west Abbey i s one of the higher values i n the region. The above observations suggest that proximity alone i s not adequate to transpose r e l i a b l y p r e c i p i t a t i o n depth r a t i o s c a l c ulated at one s t a t i o n to another s i t e without also considering the range of depth r a t i o s observed for the coastal region. 3.4.3 Depth-Frequency Relationships 3.4.3.1 Analysis of B.C. Data R a i n f a l l i n t e n s i t y data available from AES are analyzed to determine the r e l a t i o n s h i p between r a i n f a l l depth and return period for durations rang- ing from 1 to 24-hours and to assess the v a r i a b i l i t y of t h i s r e l a t i o n s h i p throughout the coastal region. Regional "depth-frequency" c h a r a c t e r i s - t i c s provide a basis for estimating r a i n f a l l for a range of return periods i n instances when r a i n f a l l i s known only for one return period. V a r i a t i o n of r a i n f a l l depth with frequency of occurrence for a given duration i s assessed i n t h i s study by c a l c u l a t i n g r a t i o s to a reference depth. R a i n f a l l depths were obtained at each of the 58 a v a i l a b l e coastal B.C. stations included in Appendix II for return periods of 2, 10, 25, 50 and 100 years. These r a i n f a l l depths were used to calculate r a t i o s of depth for each return period to a reference depth taken as the 10-year return period. Depth ra t i o s calculated by this procedure are tabulated in Appendix II for each i n d i v i d u a l coastal s t a t i o n . R a i n f a l l data and depth-frequency r a t i o s calculated for P i t t Polder in southwestern B.C. are repeated i n Table 3.9 to i l l u s t r a t e t y p i c a l r e s u l t s - 74 - of depth-frequency a n a l y s i s . Results for P i t t Polder show that the ra t i o s on IDF curves for a given return period to the 10-year return period depth i s r e l a t i v e l y constant for a range of durations from 1 to 24-hours. TABLE 3.9 DEPTH-FREQUENCY DATA FOR PITT POLDER R a i n f a l l Data (mm) from AES Return Period (Years) Duration 2 5 10 25 50 100 1 hr 12.4 14.7 16.2 18.1 1 9.1 20.9 . 2 hr 18.9 22.9 25.4 28.7 31 .2 33.6 6 hr 42.7 51 .7 57.7 65.3 70.9 76.4 12 hr 67.3 80.3 88.9 99.8 107.9 115.9 24 hr 98.9 119.0 1 32.2 149.0 1 61 .5 173.8 Depth-Frequency Relationships Return Period (Years) Duration 2 5 10 25 50 100 1 hr 0.77 0.91 1 .00 1.12 1 .21 1 .29 2 hr 0.74 0.90 1 .00 1.13 1 .22 1 .32 6 hr 0.74 0.90 1 .00 1.13 1 .23 1 .32 12 hr 0.76 0.90 1 .00 1.12 1 .21 1 .30 24 hr 0.75 0.90 1 .00 1.13 1 .22 1 .31 Analysis of r a i n f a l l i n t e n s i t y data throughout the B.C. coastal region shows that at each s t a t i o n depth-frequency r a t i o s do not vary g r e a t l y with duration. This r e s u l t i s p a r t i c u l a r l y useful because there are ap- proximately 250 coastal B.C. st a t i o n s , i n addition to 58 stations with IDF data, which record only 24-hour p r e c i p i t a t i o n . These 250 stations g r e a t l y expand the data base across the region with which l o c a l assess- ments of depth-frequency r a t i o s for shorter duration r a i n f a l l i n t e n s i t i e s can be undertaken. That i s , depth-frequency r a t i o s calculated for 24- hour data can be used to develop a frequency curve for shorter duration - 75 - r a i n f a l l which may be required for design purposes. Mean values of depth-frequency ratios for 58 coastal stations are l i s t e d i n Table 3.10 and shown gra p h i c a l l y on Figure 3.5. Results are shown with reference to the 10-year return period, although arithmetic calculations can con- vert these curves to reference any desired return period. Also, the s t a - t i s t i c a l inference of the depth-return period results i s that the c o e f f i - cient of v a r i a t i o n i n the coastal region for the mean, upper and lower 80 percent confidence l i m i t s are 0.28, 0.35 and 0.21, res p e c t i v e l y . TABLE 3.10 DEPTH-FREQUENCY RATIOS f o r IDF CURVES Duration Return Period (Years) (hours) 2 5 10 25 50 100 1 0.70 0.88 1.00 1.15 1.27 1 .38 2 0.74 0.89 1 .00 1 .13 1 .23 1 .33 6 0.76 0.90 1 .00 1.12 1.22 1 . 3 1 12 0.73 0.89 1 .00 1 .14 1 .24 1 .34 24 0.69 0.88 1 .00 1.15 1 .27 1 .38 ( a l l values) Mean 0.72 0.89 1 .00 1 .14 1 .24 1 .35 Std. Dev. 0.05 0.02 - 0.02 0.02 0.03 o r < UJ >- 1.4 1.2 CL O UJ 28 x cr a o . UJ Q 2 i .o cr a CJ UJ o r o . 0.6 0.8 - — r I I 1 1 1 —|—j™ 1 1 1 1 / i / / / - - CONFIC >ENCE .IMITS — / / / • / — r — / - - //,' - - - - • '/ i -1,1 11, 1 .1 ,IJ J i t.L , I. 3 4 5 10 20 50 100 RETURN PERIOD (YEARS) 200 Figure 3.5: Depth-Frequency Ratios for IDF Curves - 76 - Observations noted previously i n discussion of depth-duration r a t i o s also apply to r e s u l t s of analysis of depth-frequency r a t i o s in the coastal region. That i s , the range of r a t i o s i s r e l a t i v e l y small considering the d i v e r s i t y of the region and that the magnitude of r a i n f a l l varies con- siderably between s t a t i o n s . Results of depth-frequency analysis are derived for coastal stations where 24-hour storm r a i n f a l l with a 100-year return period ranges from 75 to 380 mm and mean annual p r e c i p i t a t i o n ranges from about 650 to 3500 mm. 3.4.3.2 Formulas for B.C. Data An expression r e l a t i n g r a i n f a l l depth and frequency for the extremal d i s - t r i b u t i o n was presented by Chow (1951, 1959) as follows: R T = R + KTa (3.11) where R T = r a i n f a l l depth with return period T; R = mean of recorded r a i n f a l l data; o = standard deviation of r a i n f a l l data, and K T = frequen- cy factor which varies with return period and record length. For the extreme value d i s t r i b u t i o n , the r a t i o between any two points establishes the r a t i o between any two other points on the frequency curve. If the r a t i o between two r a i n f a l l depths, R1 and R 2, with respec- t i v e return periods T-| and T 2 i s known, then the r a t i o between any other r a i n f a l l R T with return period T and a known r a i n f a l l depth can be expressed. - 77 - Consider: Rx = R + Kx a (3.12) R2 = R + K2 a (3.13) Solving Eq. 3.12 and 3.13 simultaneously y i e l d s : R Ki R2 ~ K2 R: K\ — K2 1 (3.14) C7 R\ — R2 Ki - K2 (3.15) Substituting Eq. 3.14 and 3.15 into Eq. 3.11 and 3.12 and taking the Eq. 3.16 shows how the r a t i o between r a i n f a l l depths can be determined for the extreme value d i s t r i b u t i o n when the r a t i o between any two depths on the frequency curve i s known. Eq. 3.16 i s i n a convenient form for numerical presentation of depth-frequency r e l a t i o n s h i p s i n the coastal region, while the resu l t s presented on Figure 3.5 provide a more i l l u s - t r a t i v e d e s c r i p t i o n of how depth r a t i o s vary with return period. r a t i o RIJI/R-) y i e l d s : RT = K T - K 2 K X - K T (R£\ Rx Kx — K2 Kx — K2 \Ri) (3.16) - 78 - 3.4.4 Comparison with Other Pacific Northwest Data R a i n f a l l i n t e n s i t y data a v a i l a b l e for the coastal region of Washington and Oregon are compared with r e s u l t s obtained i n this study for B.C. These data from the U.S. were obtained to i l l u s t r a t e further the region- a l a p p l i c a b i l i t y of r a i n f a l l c h a r a c t e r i s t i c s documented for coastal B.C. In addition, U.S. stations provide data for higher elevations than stations c u r r e n t l y available i n coastal B.C. Sources of U.S. p r e c i p i t a t i o n data considered in this study include the precipitation-Frequency Atlas of the Western United States ( M i l l e r et a l , 1973) and r e s u l t s of frequency analyses undertaken at six stations i n the Cascade Mountains of Washington (Brunengo, 1985). Data from i s o - p l u v i a l maps i n the precipitation-frequency atlas were obtained for t h i s study only at points where p r e c i p i t a t i o n gauges are known to be located. Depth r a t i o s were calculated with data from U.S. stations i n the same format that was applied to B.C. data. A summary of depth r a t i o s calcu- lated at coastal stations i n Washington and Oregon and comparison of these values to those calculated for coastal B.C. i s included i n Table 3.11 for depth-duration r a t i o s and in Table 3.12 for depth-fre- quency r a t i o s . - 79 - TABLE 3.11 DEPTH-DURATION RATIOS IN THE PACIFIC NORTHWEST Location Elev. Ratio to 24-Hr Depth Station Latitude Longitude (m) 1-hr 6-hr 12-hr Coastal B.C. - - - 0.16 0.45 0.68 Washington Palmer ( 1) 47 18 121 51 280 0.18 0.45 0.68 Mud Mtn Dam ( 2) 47 09 1.21 56 399 0.18 0.47 0.70 Cedar Lake (1) 47 25 121 44 476 0.19 0.40 0.64 Lester <1) 47 12 121 29 497 0.18 0.47 0.68 Greenwater ( 1) 47 08 1 21 38 527 0.14 0.47 0.70 Rainier Longmire (2) 46 45 121 49 842 0.17 0.47 0.74 Snowqualmie Pass (2) 47 25 121 25 921 0.13 0.41 0.71 Stampede Pass 47 17 121 20 1207 0.11 0.39 0.63 Stevens Pass 47 44 121 05 1241 0.16 0.48 0.73 Mt. Baker Lodge ^ 2) 48 52 121 40 1265 0.13 0.41 0.71 Oregon Haskins Dam ^2) 45 19 123 21 256 0.17 0.46 0.73 H i l l s Creek Dam (2) 43 43 122 26 380 0.16 0.41 0.71 McKenzie Bridge ( 2) 44 10 122 10 419 0.13 0.38 0.69 Sexton Summit WB (2) 42 37 123 22 1170 0.17 0.46 0.73 Data from Brunengo (1985) ( 2> Data from M i l l e r et a l (1973) - 80 - TABLE 3.12 DEPTH-FREQUENCY RATIOS IN THE PACIFIC NORTHWEST Location Elev. Ratio to 10-Yr Return Period Station Lat Long (m) 2 25 50 100 Coastal B.C. - - - 0.72 1 .14 1 .24 1 .35 Washington Palmer ( 1) 47 18 121 51 280 0.74 1 .13 1 .22 1 .32 Mud Mtn Dam ( 2) 47 09 121 56 399 0.71 1 .13 1 .24 1 .35 Cedar Lake ( 1 ) 47 25 121 44 476 0.78 1 .10 1 .20 1 .27 Lester I 1 ) 47 12 121 29 497 0.74 1 .15 1 .23 1 .33 Greenwater ^ ̂  ̂  47 08 121 38 527 0.67 1 .14 1 .29 1 .39 Rainier Longmire ^ 2^ 46 45 121 49 842 0.60 1 .18 1 .31 1 .43 Snowqualmie Pass ^ 2^ 47 25 121 25 921 0.64 1 .19 1 .31 1 .41 Stampede Pass ^ 2^ 47 17 121 20 1207 0.67 1 .15 1 .28 1 .40 Stevens Pass ^ 2) 47 44 121 05 1241 0.64 1 .18 1 .31 1 .40 Mt. Baker Lodge ^ 2) 48 52 121 40 1265 0.63 1 .15 1 .25 1 .42 Oregon Haskins Dam ( 2) 45 19 123 21 256 0.69 1 .17 1 .31 1 .53 H i l l s Creek Dam <2^ 43 43 122 26 380 0.70 1 .14 1 .28 1 .40 McKenzie Creek ( 2) 44 10 122 10 419 0.75 1 .17 1 .29 1 .38 Sexton Summit WB ^ 2) 42 37 123 22 1170 0.70 1 .17 1 .31 1 .43 Data from Brunengo (1985) ( 2> Data from M i l l e r et a l (1973) - 81 - Results of analysis of U.S. data included i n Tables 3.11 and 3.12 show that depth r a t i o s calculated for stations i n the coastal region of Wash- ington and Oregon are i n the same range as those i n coastal B.C. This r e s u l t i s p a r t i c u l a r l y informative as some of the U.S. stations are at r e l a t i v e l y high elevations i n the Cascade Mountains. Also, examination of p r e c i p i t a t i o n data for i n d i v i d u a l stations i n Washington shows the v a r i a t i o n i n depth-duration r a t i o s with return period and depth-fre- quency r a t i o s with duration i s r e l a t i v e l y small j u s t as was observed for coastal B.C. s t a t i o n s . - 82 - 3.5 TIME DISTRIBUTION OF SINGLE STORM RAINFALL 3.5.1 Analysis of B .C . Data Analysis of r a i n f a l l i n t e n s i t i e s within single storms i s undertaken for thi s study to assess whether regional c h a r a c t e r i s t i c s also e x i s t for single storm data i n coastal B.C. As noted i n the preceding sections, development of synthetic hyetographs based on i n t e n s i t y data from IDF curves i s an a l t e r n a t i v e approach commonly applied i n design s i t u a t i o n s only because single storm data are seldom a v a i l a b l e . R a i n f a l l i n t e n s i t i e s occurring within single storms are investigated at the same 58 stations i n coastal B.C. for which AES prepared IDF curves. A computer program was written to scan recorded hourly data provided on magnetic tape by AES. At each s t a t i o n , maximum 24-hour r a i n f a l l on record, hourly increments within the 24-hour r a i n f a l l and time of occur- rence of peak i n t e n s i t i e s within the 24-hour period were i d e n t i f i e d . Analysis was undertaken for continuous 24-hour periods and was not limi t e d to a calendar day time period. Even though maximum 24-hour r a i n f a l l on record occurred within a storm of longer duration i n many instances, analysis of i n t e n s i t i e s within a 24-hour period of maximum r a i n f a l l i s usually s u f f i c i e n t for hydrograph development i n the coastal region. Time d i s t r i b u t i o n of maximum 24-hour r a i n f a l l was analyzed i n a r a t i o format s i m i l a r to that applied to IDF data so that regional assessment could be more r e a d i l y undertaken. Maximum 24-hour r a i n f a l l on record at each s t a t i o n was i d e n t i f i e d and i t s time d i s t r i b u t i o n was analyzed on an - 83 - hourly basis as a percentage of the 24-hour r a i n f a l l . Results are tabu- lated i n Appendix III for each of 58 coastal B.C. stat i o n s . Twenty-one d i f f e r e n t storm periods are represented by the 58 stations because in some instances the same storm produced the maximum r a i n f a l l on record at more than one s t a t i o n . T y p i c a l 24-hour d i s t r i b u t i o n s for maximum r a i n f a l l s on record i n the coastal region are shown on Figure 3.6. Data from Bear Creek, B e l l a Coola and Strathcona Dam are selected to i l l u s t r a t e g r a p h i c a l l y the range in d i s t r i b u t i o n s of storm r a i n f a l l calculated i n this study for coastal B.C. The somewhat l i n e a r d i s t r i b u t i o n for Bear Creek i s common for many of the maximum 24-hour r a i n f a l l s on record at stations across the region. B e l l a Coola and Strathcona Dam are selected as i l l u s t r a t i v e examples of more non-linear d i s t r i b u t i o n s with higher i n t e n s i t y r a i n f a l l occurring at late and early stages of the 24-hour period, respectively. Each of the 58 stations i n the coastal region experienced maximum 24-hour r a i n - f a l l d i s t r i b u t i o n s s i m i l a r to those shown on Figure 3.6. A summary of the 58 r a i n f a l l d i s t r i b u t i o n s i d e n t i f i e d i n this study i s also i l l u s - trated on Figure 3.6 which shows the r e l a t i v e l y narrow band of r a i n f a l l data. The magnitude of 24-hour r a i n f a l l from stations across the region ranged from 65 to 340 mm. - 84 - o . a> _1 o _) <° (T U. ^ o . — rv <ZL CC ° y o 1 i n CN L_ ° o *• I— z s- u ce LJ o Q_ ™ o_ 0 2 4 E 8 10 12 11 16 IB 20 22 21 HOUR (a) Maximum 24-Hour R a i n f a l l at Bear Creek (300.5 mm) HOUR (b) Maximum 24-Hour R a i n f a l l at Be l l a Coola (131.4 mm) Figure 3.6. Maximum 24-Hour R a i n f a l l on Record - 85 - 0 2 1 6 8 10 I2 M IE IS 20 22 21 HOUR Maximum 24-Hour R a i n f a l l at Strathcona Dam (155.2 mm) HOUR Range of 24-Hour R a i n f a l l D i s t r i b u t i o n s Figure 3.5 Maximum 24-Hour R a i n f a l l on Record - 86 - A study of 12-hour r a i n f a l l data across Canada (Hogg, 1980) found that storm d i s t r i b u t i o n s were more variable i n other regions of Canada than i n coastal B.C. Hogg analyzed r a i n f a l l data from Agassiz, Vancouver, V i c t o r i a and Comox i n the coastal region. Twelve-hour r a i n f a l l s were selected at each s t a t i o n to form a p a r t i a l duration series with one event for each year on record. Data from a l l four stations were com- bined to create a "coastal B.C." data base. Similar procedures were also applied to regions designated as the East Coast, Southern Ontario and the P r a i r i e s . Results of analysis undertaken by Hogg are shown on Figure 3.7 which i l l u s t r a t e s the range of 12-hour r a i n f a l l d i s t r i b u t i o n s i n each region for a wide range of return periods represented by the p a r t i a l duration s e r i e s . Storm d i s t r i b u t i o n s for coastal B.C. f i t a much narrower range than corresponding data for the East Coast, Southern Ontario and the P r a i r i e s . This r e s u l t suggests that regional r a i n f a l l c h a r a c t e r i s t i c s of single storms in coastal B.C. may be more r e a d i l y i d e n t i f i a b l e than for other regions of Canada. - 87 - 12 HOUR STORM RRIN D ISTRIBUT ION B. C . CORST NO. OF EVENTS »tl9 SELECTION CRITERIP, 6 HR [2 MR (MM • 10) 381 CURVES SNOW t Of €VENTS WITH X STORM RfltN J VQLUES PLOTTED 12 HOUR STORM R P I N D I S T R I B U T I O N CON EPST COPST NO. OF EVENTS >8S SELECTION CRITERIA' S HR 12 HR ( M M - 10] «39 CURVES SHOU X OF EVENTS WITH i 3T0RN RfltN } YPLUES PLOTTED TIME (HOURS) TIME (HOURS) 12 HOUR STORM RPIN D I S T R I B U T I O N P R R I R I E S NO. OF EVENTS »333 SELECTION CRtTEHIOi 6 HR 12 hP. ( M M • 19) 30S CURVES SHOW X OF EVENTS WITH x STORM ROIN 1 VQLUES PLOTTED 12 HOUR STORM R S I N D I STR I BUT Ci SOUTHERN ONTRRIO NO. Or EVENTS »tflfl SELECTION CRIT£R[p. 6 HR \2 V.?. (MM • 13) 330 221 CURVES SHOW X OF 5v£f*TS -ITH * STORM RAIN S VQLUES PLOTTEt / /y 1 30tf / / / j / S « / / / 1 / / 7 3 7 / \ / / / 9*7 TIME (HOURS) s a i d TIME (HOURS) Figure 3.7 Time D i s t r i b u t i o n of 12-Hour R a i n f a l l ( a f t e r Hogg, 1980) - 88 - Regional r a i n f a l l c h a r a c t e r i s t i c s for coastal B.C. are investigated i n this study by analyzing maximum 1, 2, 3, 4, 6, 8 and 12-hour r a i n f a l l s occurring within the maximum 24-hour r a i n f a l l on record at each s t a t i o n . These data are l i s t e d for each coastal s t a t i o n in Appendix I I I . Analy- s i s of these inter-storm data shows that on a percentage basis, maximum incremental r a i n f a l l s within the largest storms on record do not vary g r e a t l y across the region. Results are shown on Figure 3.8 i n a format s i m i l a r to that used previously f o r IDF data. The lower l i m i t shown on Figure 3.8 represents those storms which tend to be lin e a r and the upper l i m i t i l l u s t r a t e s c h a r a c t e r i s t i c s of 24-hour r a i n f a l l which have periods of higher i n t e n s i t y within the 24-hour period. 0 2 4 6 8 10 12 14 16 18 20 22 24 DURATION (HOURS ) Figure 3.8 Depth-Duration Ratios for 24-Hour R a i n f a l l - 89 - Comparison of hourly r a i n f a l l i n t e n s i t i e s i n Figure 3.4 based on IDF curves and in Figure 3.8 based on single storm data shows that beyond about 6-hour durations the two curves are quite s i m i l a r . For shorter durations, r a i n f a l l i n t e n s i t i e s estimated from IDF curves would produce a synthetic hyetograph with greater maximum hourly i n t e n s i t i e s than have been observed to occur within single storms. In prac t i c e the two curves can be used to set l i m i t s on the range of hourly r a i n f a l l i n t e n s i t i e s to be considered by the design engineer i n the absence of s i t e data. For each coastal s t a t i o n , maximum r a i n f a l l i n t e n s i t i e s on record for durations less than 24-hours and maximum occurring within 24-hour storms are compared i n Appendix I I I . Examination of these data shows maximum short duration i n t e n s i t i e s on record occurred within the maximum 24-hour r a i n f a l l on record i n many instances. This occurrence, sometimes re- ferred to as "nesting", i s consistent with the s i m i l a r results for depth r a t i o s obtained from separate analysis of IDF and single storm r a i n f a l l data. The p o t e n t i a l for nesting i s apparent by examining the period of year of occurrence of maximum 1 , 6, 12 and 24-hour r a i n f a l l s on record i n coastal B.C. as shown on Figure 3.9. - 90 - J F M A M J J A S 0 N 0 M O N T H O F O C C U R R E N C E O F M A X . I - HR R A I N F A L L J F M A M J J A S O N O M O N T H O F O C C U R R E N C E O F M A X . 6 - H R R A I N F A L L J F M A M J J A S O N D M O N T H O F O C C U R R E N C E O F M A X . 1 2 - H R R A I N F A L L J F M A M J J A S O N O M O N T H O F O C C U R R E N C E O F M A X . 2 4 - H R R A I N F A L L Figure 3.9 Monthly D i s t r i b u t i o n of Maximum R a i n f a l l s on Record - 91 - Results presented on Figure 3.8 provide a basis for estimating hourly- r a i n f a l l increments occurring within single storms, but do not provide information on the time sequence of occurrence needed for development of a synthetic hyetograph. Therefore, time of occurrence of maximum hourly i n t e n s i t i e s within the maximum 24-hour r a i n f a l l on record at each of the 58 stations was examined. Maximum 24-hour r a i n f a l l s on record were investigated to determine the time of occurrence of maximum 1 , 3 and 5-hour i n t e n s i t i e s within the 24-hour period. A summary of results for a l l stations i n the coastal B.C. region i s included i n Table 3.13. TABLE 3.13 TIME OF OCCURRENCE OF MAXIMUM INTENSITIES Maximum 1-Hour Maximum 3-Hour Maximum 5-Hour Time of No. of Time of NO. of Time of No. of Occurrence Occurrences Occurrence Occurrences Occurrence Occurrences (Hours) (Hours) (Hours) 0-1 0 0-3 0 0-5 1 1-2 0 1-4 0 1-6 1 2-3 2 2-5 1 2-7 3 3-4 2 3-6 3 3-8 1 4-5 2 4-7 3 4-9 2 - 5-6 2 5-8 1 5-1 0 2 6-7 1 6-9 5 6-11 5 7-8 4 7-10 2 7-1 2 3 8-9 2 8-11 3 8-13 2 9-10 2 9-1 2 2 9-1 4 3 10-11 5 10-13 1 10-15 2 11-12 1 11-14 2 11-16 2 12-13 3 12-15 5 12-17 2 13-14 3 13-16 2 1 3-18 5 14-15 5 14-17 5 14-19 5 15-16 3 15-18 4 15-20 4 16-17 3 16-19 3 16-21 4 17-1 8 3 17-20 5 17-22 4 18-19 3 18-21 2 18-23 1 • 19-20 2 19-22 5 19-23 5 20-21 4 20-23 1 21-22 3 21-24 2 22-23 1 23-24 1 - 92 - Results presented i n Table 3.13 suggest for the coastal B.C. region that there i s no apparent strong bias for periods of high i n t e n s i t y to occur e i t h e r e a r l y or late within large 24-hour r a i n f a l l s . The consequences of t h i s observation to the design engineer i n th i s region are two-fold. F i r s t , hourly r a i n f a l l increments within a synthetic hyetograph for the coastal region can be arranged i n many d i f f e r e n t time sequences and s t i l l produce 24-hour events with s i m i l a r p r o b a b i l i t i e s of occurrence. Second- l y , s e l e c t i o n by a design engineer of a time sequence of maximum hourly i n t e n s i t i e s within a 24-hour synthetic hyetograph can be governed by response c h a r a c t e r i s t i c s of the basin under consideration. - 93 - 3.5.2 Comparison With Other Pacific Northwest Data Single storm r a i n f a l l data a v a i l a b l e for the coastal region of Wash- ington and Oregon are compared to results obtained i n this study for B.C. These data from the U.S. were obtained to i l l u s t r a t e further the regional a p p l i c a b i l i t y of single storm r a i n f a l l c h a r a c t e r i s t i c s docu- mented for coastal B.C. In addition, U.S. stations provide data for higher elevations than stations c u r r e n t l y available i n coastal B.C. Single storm p r e c i p i t a t i o n data were obtained at four stations at r e l - a t i v e l y high elevations i n the Cascade Mountains of Washington and at four stations i n the coast and Cascade Mountains i n Oregon. P r e c i p i t a - tion data presented for each s t a t i o n i n Washington represent the largest 24-hour event i d e n t i f i e d from a v i s u a l inspection of long term records. P r e c i p i t a t i o n data for Oregon are from the same storm period i n December 1964 when extreme floods with return periods ranging from about 50 to 100 years occurred over most of the coastal region. Depth r a t i o s for maximum hourly increments within 24-hour events are calculated with data from U.S. stations i n the same format that was applied to B.C. data. A summary of depth ratios calculated with U.S. data and comparison of these values to those calculated for coastal B.C. i s included i n Table 3.14. Results of analysis of U.S. data show that inter-storm hourly increments calculated with single storm data from coastal Washington and Oregon are i n the same range as those i n coastal B.C. TABLE 3.14 SINGLE STORM PRECIPITATION DATA IN THE PACIFIC NORTHWEST 24-Hour Max. Occurring Within 24-Hours Location Elev Precip (Ratio to 24-Hour P r e c i p i t a t i o n ) Station Latitude Longitude (m) (mm) 1-Hour 2-Hour 6-Hour 12-Hour COASTAL B.C. - - - - 0.08 0.15 0.37 0.63 WASINGTON Snowqualmie Pass^ 1^ 47 25 121 25 921 178 0.10 0.16 0.38 0.72 Stampede Pass^ 1) 47 17 121 20 1207 202 0.08 0.15 0.38 0.64 Stevens Pass( 1) 47 44 121 05 1241 130 0.08 0.14 0.33 0.53 Mt. Baker Lodge( 1) 48 52 121 40 1265 127 0.06 0.12 0.32 0.58 OREGON Haskins Dam ^ 1) 45 19 123 21 256 138 0.09 0.18 0.42 0.72 H i l l s Creek Dam v 1 ) 43 43 122 26 380 92 0.07 0.14 0.36 0.62 McKenzie Bridge RS ( 1 ) 44 10 122 10 419 103 0.08 0.11 0.29 0.55 Sexton Summit WB ( 1 ) 42 37 123 22 1170 113 0.10 0.19 0.37 0.57 SCS TYPE 1A * 2 ) - - - - 0.15 0.25 0.47 0.69 ( 1) data from U.S. National Weather Service (2) af t e r S o i l Conservation Service (1982) - 95 - A regional 24-hour r a i n f a l l d i s t r i b u t i o n developed by the U.S. S o i l Con- servation Service (SCS, 1973, 1982) for use on the coastal side of the S i e r r a Nevada and Cascade Mountains of Oregon, Washington and northern C a l i f o r n i a , and the coastal regions of Alaska i s also compared i n Table 3.14 to B.C. data. Data presented for the coastal storm d i s t r i b u t i o n proposed by SCS are not i n the same range as those calculated at i n d i v i - dual coastal s t a t i o n s . However, this apparent discrepancy can be re- solved by examining procedures used by SCS to develop th e i r regional curve. Procedures outlined by SCS (1973) indicate their single storm d i s t r i b u - tion i s derived from IDF data obtained from r a i n f a l l a t lases. Comparison of hourly increments proposed by SCS for a single storm d i s t r i b u t i o n to r a i n f a l l depth ra t i o s calculated for coastal B.C. based on IDF data (Table 3.4) shows these values are s i m i l a r . Therefore, ap p l i c a t i o n of the SCS storm d i s t r i b u t i o n i n the coastal region produces a hyetograph with greater maximum hourly i n t e n s i t i e s than have been observed from ana- l y s i s of single storm data. F i n a l l y , SCS also proposes a time sequence for hourly increments within a 24-hour r a i n f a l l . The storm d i s t r i b u t i o n i s plotted on Figure 3.10. Examination of the curve shows the maximum 1 -hour r a i n f a l l occurs early i n the storm from hour 7 to 8 and the maximum 4-hour increment occurs from hours 7 through 10. SCS states, however, select i o n of the period of - 96 - maximum i n t e n s i t y i s based on design considerations rather than meteoro- l o g i c a l f a c t o r s . Therefore, i t i s reasonable to conclude that this storm d i s t r i b u t i o n was proposed to provide a standardized synthetic hyetograph which generally produces conservative r e s u l t s , e s p e c i a l l y considering that hourly increments are based on IDF data. Figure 3.10 S o i l Conservation Service Type 1A Storm D i s t r i b u t i o n - 97 - 3.6 ELEVATION EFFECTS ON STORM RAINFALL 3.6.1 Background Regional. c h a r a c t e r i s t i c s of storm r a i n f a l l presented i n the preceding sections can be used to develop synthetic hyetographs at a point within a drainage basin. A point estimate for r a i n f a l l i s sometimes an adequate i n d i c a t o r of r a i n f a l l across an ent i r e basin. Hydrograph analysis i n mountainous regions of coastal B.C., however, usually requires storm r a i n f a l l to be d i s t r i b u t e d with elevation. Examination of elevation e f f e c t s as storm systems i n t e r a c t with moun- tainous t e r r a i n can be undertaken i n two d i f f e r e n t frames of reference. In a Lagrangian reference frame one moves with the storm and observes i t s temporal growth and decay. This approach i s most commonly adopted by meteorologists and i s a basis for weather forecasting. The Lagrangian approach to storm analysis has not yet received widespread a p p l i c a t i o n i n engineering studies, perhaps due i n part to a shortage of necessary f a c i l i t i e s such as p r e c i p i t a t i o n radar s t a t i o n s . There i s l i t t l e p o t e n t i a l i n B.C. for exploring the ap p l i c a t i o n of a Lagrangian approach to storm analysis for engineering studies as there i s no p r e c i p i t a t i o n radar s t a t i o n c u r r e n t l y i n operation. Engineering studies t r a d i t i o n a l l y u t i l i z e a Eulerian reference frame where an observer remains stationary and records r a i n f a l l as a storm passes through a region. This approach leads to development of a hyeto- graph for one point i n the basin, which i n turn i s used to estimate - 98 - r a i n f a l l over the remainder of the drainage basin under study. The d i f f i c u l t y with which a point measurement can be transposed across a basin increases g r e a t l y when there are large variations i n e l e v a t i o n . For example, mountainous t e r r a i n may induce orographic p r e c i p i t a t i o n within storms and also tr i g g e r convective i n s t a b i l i t y from d i f f e r e n t i a l heating of mountain slopes, increase cyclonic p r e c i p i t a t i o n by retarding the rate of movement, and cause u p l i f t through funnelling e f f e c t s of v a l l e y s on airstreams. The complexity of atmospheric processes a f f e c t i n g i n t e r a c t i o n of storm systems and mountainous t e r r a i n can be demonstrated by examining conclus- ions of three d e t a i l e d meteorological studies. Hetherington (1976) analyzed 42 storms in the co a s t a l mountains north of Vancouver, B.C. and concluded the amount of orographic r a i n f a l l i s related p r i m a r i l y to wind speed normal to a mountain b a r r i e r , moisture content of the lower atmos- phere, freezing l e v e l and a i r mass s t a b i l i t y . Another physically-based meteorologic analysis of the d i s t r i b u t i o n of storm r a i n f a l l i n mountain- ous regions ( E l l i o t , 1977) showed the r e l a t i o n of orographic r a i n f a l l to elevation "depends i n a complex way upon the character of the t e r r a i n , on the e f f i c i e n c y with which microphysical mechanisms remove cloud condensate as" p r e c i p i t a t i o n , the wind d i r e c t i o n and speed, the depth of cloud, and the a i r mass s t a b i l i t y " . F i n a l l y , a comprehensive review of the structure and mechanism of orographically enhanced r a i n conducted by Browning (1980) concluded that the p r i n c i p a l influencing factors are the form of a i r f l o w induced by r i s i n g topography, magnitude of r e l a t i v e humidity, wind strength, wet-bulb temperature, existence of p o t e n t i a l i n s t a b i l i t y , and presence and nature of pre-existing p r e c i p i t a t i o n . - 99 - The above discussion i l l u s t r a t e s that l o c a l variations i n storm r a i n f a l l with elevation are affected by the i n t e r a c t i o n of many s i t e s p e c i f i c phe- nomena. Examination of annual p r e c i p i t a t i o n by Rasmussen and Tangborn (1976) at 38 stations i n the Northern Cascade Mountains of Washington i l l u s t r a t e s further that p r e c i p i t a t i o n amounts are affected by factors i n addition to elevation alone, p r e c i p i t a t i o n data are shown on Figure 3.11 for a region extending from the Canadian border southward for about 190 km and for stations only on the western slope of the Cascades. These data i l l u s t r a t e that even within a r e l a t i v e l y l o c a l region there i s wide v a r i a t i o n i n p r e c i p i t a t i o n for a given elevation. • 8001 .8800 .7437 • 5443 .8718 .1233 .3384 .3909 ' .4295 .7781 .8009 4999 > 1 9 9 2 •5840 .2157 .7379 .8089 • 8000 .1479 .3141 8 0. J 4 .77/3 *^48* .527 1484 J7507 . '5523 V 5 5 7 -324 r»2475 7478 .174 i 3500 i z z g ? 2500 < Z z < z < 200 400 800 1000 1200 1400 1400 1800 2000 G A G E ALT ITUDE . I N METRES Figure 3.11 Annual P r e c i p i t a t i o n i n the North Cascade Mountains - 100 - Detailed meteorologic investigations of elevation e f f e c t s on storm r a i n f a l l , s i m i l a r to those noted above by Hetherington .(1976) and E l l i o t t (1977), are not undertaken i n this study of regional r a i n f a l l c h a r a c t e r i s t i c s . A l t e r n a t i v e l y , analysis i s undertaken to e s t a b l i s h those c h a r a c t e r i s t i c s which can be i d e n t i f i e d with limited data cu r r e n t l y a v a i l a b l e i n coastal B.C. The data base and network density currently available for analysis r e s t r i c t s region wide i n v e s t i g a t i o n of elevation e f f e c t s on storm r a i n f a l l . Nevertheless, some trends can s t i l l be i d e n t i f i e d which can be applied immediately to engineering studies, and which serve as a basis for more comprehensive research of elevation e f f e c t s than i s possible i n this study. 3.6.2 Analysis of Selected Storm Data Analysis of elevation e f f e c t s on storm r a i n f a l l i n coastal B.C. i s limited for two reasons. F i r s t , as described i n the preceding section, atmospheric processes a f f e c t i n g i n t e r a c t i o n of storm systems and moun- tainous t e r r a i n are very complex. Secondly, the ex i s t i n g r a i n gauge network i n coastal B.C. i s generally not of s u f f i c i e n t density to examine l o c a l variations i n r a i n f a l l from low v a l l e y bottom elevations to higher elevations near mountain c r e s t s . The approach adopted for this study i s to assess r e s u l t s of meteorologi- c a l i n v e s t i g a t i o n s undertaken by B.C. Hydro at two locations in the coastal region, and then compare trends i d e n t i f i e d i n their reports to recorded r a i n f a l l data at other stations i n the region. Data from gauge networks along mountain slopes immediately north of Vancouver were - 101 - selected for comparison to the meteorologic studies as network densities i n other segments of the B.C. coastal region are not adequate. Two meteorologic investigations of storm r a i n f a l l i n mountainous areas include probable maximum p r e c i p i t a t i o n (PMP) studies for the Coquitlam Lake Watershed (Schaefer, 1981) located about 30 km northeast of Van- couver's c i t y centre and for the Cheakamus Project (B.C. Hydro, 1983) located approximately 100 km north of Vancouver. The methodology adopted for these studies was established by the World Meteorological Organiza- ti o n (WMO, 1973) and i s known as the orographic separation method. The technique involves making separate estimates for an orographic component of p r e c i p i t a t i o n induced by the l i f t i n g of a i r flow over mountains and for a convergence component of p r e c i p i t a t i o n r e s u l t i n g from atmospheric processes. The two components are summed to produce estimates of storm r a i n f a l l with increasing e l e v a t i o n . Each of the meteorologic studies produced PMP estimates for the range of elevations i n each basin. Results were presented i n the Coquitlam basin for an elevation range of 156 - 1750 m, and for the Cheakamus project from 200 - 1800 m. Even though re s u l t s of the PMP studies are s i t e s p e c i f i c for each basin, two trends i n the r e s u l t s are apparent and were considered to merit a d d i t i o n a l i n v e s t i g a t i o n . The two trends are: i ) storm r a i n f a l l increased l i n e a r l y with e l e v a t i o n . i i ) the l i n e a r r e l a t i o n s h i p generally extended to the highest elevation i n the basin. - 102 - Each of the above trends derived from meteorological analysis are inves- tigated further i n this study by examining recorded r a i n f a l l data from other coastal B.C. stations not used i n the PMP studies. Data were ob- tained from stations along two d i f f e r e n t transects i n the mountains imme- d i a t e l y north of Vancouver as shown on Figure 3.12. Stations shown along Transect A from Burrard I n l e t to Grouse Mountain record p r e c i p i t a t i o n data for Atmospheric Environment Service and those included on Transect B to Mount Seymour are temporary locations established as part of a Ph.D. research program ( F i t z h a r r i s , 1975). Elevation e f f e c t s on p r e c i p i t a t i o n along Transect A are shown on Figure 3.13 which presents the average annual maximum 24-hour p r e c i p i t a t i o n at each station versus elevation. This r e l a t i o n s h i p appears l i n e a r j u s t as was calculated i n meteorologic investigations of single storm PMP events. In addition, the highest s t a t i o n i n the transect at elev a t i o n 1128 m i s within about 200 m of crest elevations along the top of the slope. Figure 3.12 Station Locations i n North Vancouver - 104 - I 4 0 0 2 0 0 I 0 0 0 ~ 8 0 0 O r- < LU 6 0 0 _ i LU 4 0 0 2 0 0 / j i / / / / / / / / / / / • • 1 •A -/ • » 40 60 80 100 120 STORM PRECIPITATION (mm) 140 Figure 3.13 Transect A: Elevation vs 24-Hour P r e c i p i t a t i o n - 105 - Storm data along Transect B were c o l l e c t e d by F i t z h a r r i s (1975) for 73 storms ranging from 2 to 81 hours during the 1969-70 winter and for 74 storms ranging from 6 to 91 hours during the 1970-71 winter. Most of the p r e c i p i t a t i o n events examined by F i t z h a r r i s had return periods less than about 2 years. The complexity of atmospheric processes i s apparent from examination of a l l 147 storms which showed no consistent trends i n eleva- t i o n e f f e c t s on storm r a i n f a l l . However, three events l i s t e d on Table 3.15 were i d e n t i f i e d as being of i n t e r e s t to t h i s study of storm r a i n f a l l leading to extreme floods. Each of the three events experienced r e l a t i v e l y large p r e c i p i t a t i o n amounts for the b r i e f period of record and consisted of rain over most of the elevation range .with r a i n mixed with snow at the top of the mountain. Table 3.15 Storm Data Near Mount Seymour ( F i t z h a r r i s , 1975) Date Storm Duration (hrs) P r e c i p i t a t i o n Elevation 120 m Amount (mm) Elevation 1260 m Dec 6-7, 1970 26 53 82 Dec 9-11 , 1970 31 50 107 Feb 13-15 , 1971 43 74 1 37 P r e c i p i t a t i o n data from each of the three storms are plotted on Figures 3.14 through 3.16 for twelve sampling locations over a 120 to 1260 m e l e - vation range. The highest sampling s t a t i o n i s within about 200 m of the peak of Mount Seymour. L i n e a r i t y of these single storm p r o f i l e s of pre- c i p i t a t i o n with elevation along Transect B supports results of two PMP studies for B.C. Hydro and of the p r o f i l e of average annual maximum 24- hour p r e c i p i t a t i o n recorded along Transect A. - 106 - I 4 0 0 2 0 0 I 0 0 0 ~ 8 0 0 o p - < LU 6 0 0 LU 4 0 0 2 0 0 «/ i / / ; / r / / J 7 /. / / / * ./ • / / 40 60 80 100 120 STORM PRECIPITATION (mm) 140 Figure 3.14 Transect B: R a i n f a l l D i s t r i b u t i o n for December 6-7, 1970 - 107 - 4 0 0 2 0 0 0 0 0 ~ 8 0 0 o < > U J 6 0 0 _ i LU 4 0 0 2 0 0 1 1 m • 1 1 I •> 1 /• ! / " • / / • / ,/ / / / • ./ / / . 4 0 6 0 8 0 100 120 140 S T O R M P R E C I P I T A T I O N ( m m ) Figure 3.15 Transect B: R a i n f a l l D i s t r i b u t i o n for December 9-11, 1970 - 108 - I 4 0 0 2 0 0 I 0 0 0 w 8 0 0 h-< LU 6 0 0 _ i 4 0 0 2 0 0 - / • / • /• / / • / / • • •/ / / / • / / . / / . 4.0 6 0 8 0 I00 I20 I 4 0 S T O R M P R E C I P I T A T I O N ( m m ) Figure 3.16 Transect B: R a i n f a l l D i s t r i b u t i o n for February 13-15, 1971 - 109 - P r e c i p i t a t i o n p r o f i l e data currently available for coastal B.C. are not yet s u f f i c i e n t to support d e f i n i t i v e conclusions for the region. Never- theless , available evidence suggests that a l i n e a r increase with eleva- tion of storm r a i n f a l l during extreme events i s a reasonable approxima- t i o n for i n d i v i d u a l slopes with a constant aspect in the coastal region. Inasmuch as p r e c i p i t a t i o n p r o f i l e data analyzed in this study are derived from events with a wide range in frequency of occurrence, the magnitude of the rate of increase with elevation cannot be compared between stud- i e s . A d d i t ional research of elevation e f f e c t s on storm r a i n f a l l i n the coastal region which examines atmospheric processes during extreme events and supports results of a n a l y t i c a l study with recorded data i s greatly needed for engineering use. In the absence of regional meteorologic studies of storm r a i n f a l l , the following section presents an a l t e r n a t i v e method which can serve as a guideline to estimate the d i s t r i b u t i o n of storm r a i n f a l l with e l e v a t i o n . 3.6.3 R e l a t i o n s h i p t o A n n u a l P r e c i p i t a t i o n For engineering design sit u a t i o n s when the d i s t r i b u t i o n of storm r a i n f a l l cannot be assessed d i r e c t l y based on l o c a l - h i s t o r i c a l data, a l t e r n a t i v e methods must be applied. In these instances the approach i s generally to assess other factors which may be indices of storm r a i n f a l l . For this study of regional r a i n f a l l c h a r a c t e r i s t i c s i n coastal B.C. the r e l a t i o n - ship between annual and short duration p r e c i p i t a t i o n i s examined. The premise that annual p r e c i p i t a t i o n d i s t r i b u t i o n may be an index for storm - no - r a i n f a l l i s based on the f a c t that most annual p r e c i p i t a t i o n occurs during the f a l l and winter and results from the same type of low pres- sure system which produce the regions l a r g e s t 24-hour r a i n f a l l events. Annual and 24-hour p r e c i p i t a t i o n data were obtained for a l l available coastal B.C. stations and are p l o t t e d on Figures 3.17. For quick ref- erence, data are plotted separately for Vancouver and Fraser Valley, east coast of Vancouver Island inlcuding V i c t o r i a and the Gulf Islands, west coast of Vancouver Island, and other B.C. coastal s t a t i o n s . - 111 - I000 2 0 0 0 3 0 0 0 4000 MEAN A N N U A L PRECIPITATION (mm) ( a ) VANCOUVER AND FRASER V A L L E Y . 0 I000 2000 3 0 0 0 4000 MEAN A N N U A L PRECIPITATION (mm) ( b ) EAST COAST OF V A N C O U V E R ISLAND Figure 3.17 Relationship Between 24-Hour and Annual P r e c i p i t a t i o n - 112 - ( c ) WEST COAST OF VANCOUVER I S L A N D . cr x i 2 0 0 * E x E < — I 5 0 - J o 3 t ; loo i t < a . < cr tr Q. UJ > < 5 0 0 — ^ — — ^ - ' o 0 o° ° ° ^ — 0 o ° ° ° « ° ° o ° o o o^— i i 1 1 1 1 1 1 0 0 0 2 0 0 0 3 0 0 0 4 0 0 0 M E A N A N N U A L P R E C I P I T A T I O N ( m m ) ( d ) OTHER B.C. COASTAL STATIONS Figure 3.17 Relationship Between 24-Hour and Annual P r e c i p i t a t i o n - 113 - Envelope curves are included on Figure 3.17 to i l l u s t r a t e the consistent r e l a t i o n s h i p i n the coastal region between annual and 24-hour p r e c i p i t a - tion at a s t a t i o n . These relationships are summarized i n Table 3.16 which l i s t s the range i n percentages of 24-hour p r e c i p i t a t i o n versus annual p r e c i p i t a t i o n represented by envelope curves on each Figure. Table 3.16 Relationship Between 24-Hour and Annual P r e c i p i t a t i o n Ratio of 24-Hour to Annual P r e c i p i t a t i o n (percent) Annual West Coast of East Coast of Vane and Other Coastal Precip. (mm) Vane, is l a n d Vane. Island Fraser V a l l e y Stations 1000 - 3.3-7.1 3.6-5.3 2.6-7.0 2000 2.7-4.9 3.3-5.5 3.6-4.7 2.6-4.6 3000 3.2-4.8 3.3-5.5 3.6-4.5 2.6-4.1 4000 3.4-4.7 - 3.6-4.4 2.6-3.7 Results included i n Table 3.16 indicate that a 24-hour p r e c i p i t a t i o n estimate can be made based on annual p r e c i p i t a t i o n at a l o c a t i o n . There- fore, i t i s reasonable to suppose that the d i s t r i b u t i o n of long term p r e c i p i t a t i o n with elevation i s an index for the d i s t r i b u t i o n of storm r a i n f a l l . This conclusion i s supported by examining further p r e c i p i t a - t i o n data from Transects A and B on Figure 3.12. For Transect A, d i s t r i - bution of mean annual p r e c i p i t a t i o n with elevation i s compared to that for average annual maximum 24-hour p r e c i p i t a t i o n on record, and at Transect B the d i s t r i b u t i o n of 1970-71 winter p r e c i p i t a t i o n i s compared to that for three storms. Results are shown i n Table 3.17 where rat i o s of p r e c i p i t a t i o n at 600 and 1200 m to a reference value taken at eleva- ti o n 200 m are included for short and long durations. - 114 - Table 3.17 D i s t r i b u t i o n of short and Long Duration P r e c i p i t a t i o n Ratio of P r e c i p i t a t i o n Amounts 600 m:200 m 1200 m:200 m TRANSECT A Average Annual p r e c i p . Ave. Annual Max. 24-Hr Precip. 1 .13 1.13 1 .28 1 .31 TRANSECT B Winter 1970-71 Dec 6-7, 1970 Dec 9-11, 1970 Feb 13-15, 1971 1 .31 1 .12 1 .35 1 .32 1 .79 1 .31 1 .88 1 .79 Even though results shown i n Table 3.17 suggest annual p r e c i p i t a t i o n can be used as an index for 24-hour p r e c i p i t a t i o n , judgement i s s t i l l re- quired for engineering design situations when l o c a l recording stations are not a v a i l a b l e . However, d i s t r i b u t i o n of longer duration p r e c i p i t a - t i o n i s oftentimes more recognizable than that for storm r a i n f a l l . For example, varia t i o n s i n vegetation and forest cover may be apparent during a s i t e reconnaissance, or interviews with l o c a l residents may be more . r e l i a b l e regarding observations such as areas of deeper snow or wetter f i e l d s . In the absence of s u f f i c i e n t p r e c i p i t a t i o n records on s i t e , i n v e s t i g a t i v e procedures noted above may provide the only basis on which to assess the d i s t r i b u t i o n of storm p r e c i p i t a t i o n for engineering design purposes. - 115 - 3.7 SUMMARY 1 . R a i n f a l l i n t e n s i t y data are available from Atmospheric Environment Service (AES) for 58 stations in the coastal region of B r i t i s h Columbia. Gauge density i n most of the region i s much less than that recommended by the World Meteorological Organization (1970) for mountainous t e r r a i n . 2. Atmospheric Environment Service summarizes r a i n f a l l c h a r a c t e r i s t i c s at each s t a t i o n by producing Intensity-Duration-Frequency (IDF) Curves. IDF curves provide average i n t e n s i t i e s for a given duration and return period, but do not provide information regarding v a r i a - tions i n r a i n f a l l i n t e n s i t i e s within a single storm. 3. Regional c h a r a c t e r i s t i c s of IDF curves are investigated in this study by analyzing the v a r i a t i o n i n r a i n f a l l depth with return period for a given duration (depth-frequency relationships) and v a r i a t i o n of depth with duration for a given return period (depth- duration r e l a t i o n s h i p s ) . 4. Ratios of r a i n f a l l depth are used in the analysis rather than r a i n - f a l l magnitude alone. This method i s one approach to i d e n t i f y i n g regional c h a r a c t e r i s t i c s when the amount of r a i n f a l l i s d i f f e r e n t between s t a t i o n s . For the 58 stations available in coastal B r i t i s h Columbia, 24-hour r a i n f a l l with a 100-year return period ranges from 75 mm to 380 mm, and mean annual p r e c i p i t a t i o n ranges from 650 mm to 3500 mm. - 116 - 5. Results of regional analysis of IDF curves are shown for depth- duration and depth-frequency relationships i n Figure 3.4 and 3.5, re s p e c t i v e l y . The r e l a t i v e l y narrow range of depth r a t i o s on each figu r e i l l u s t r a t e s regional c h a r a c t e r i s t i c s of storm r a i n f a l l i n coas t a l B r i t i s h Columbia. 6. Regional analysis of the time d i s t r i b u t i o n of single storm r a i n f a l l are investigated i n this study at the same 58 stations for which AES prepared IDF curves. Development of synthetic hyetographs based on i n t e n s i t y data from IDF curves i s an approach commonly applied i n design s i t u a t i o n s only because single storm data are seldom a v a i l a b l e . 7. Results of analysis of single storm r a i n f a l l data i n coastal B r i t i s h Columbia are shown in Figure 3.8. The lower curve represents 24-hour r a i n f a l l s which tend to be linear and the upper l i m i t i l l u s t r a t e s those which have periods of higher i n t e n s i t y . T y p i c a l time d i s t r i b u t i o n s of extreme 24-hour r a i n f a l l s recorded i n coastal B r i t i s h Columbia are shown on Figure 3.6. 8. Comparison of hourly r a i n f a l l i n t e n s i t i e s i n Figure 3.4 based on IDF curves and in Figure 3.8 based on single storm data shows that beyond about 6-hour durations the two curves are s i m i l a r . For shorter durations, r a i n f a l l i n t e n s t i t i e s estimated from IDF curves would produce a synthetic hyetograph with greater maximum hourly i n t e n s t i t i e s than have been observed to occur within single storms. - 117 - In practice the two curves can be used to set l i m i t s on the range of hourly r a i n f a l l i n t e n s i t i e s to be considered by the design engineeer i n the absence of s i t e data. 9. Analysis of r a i n f a l l i n t e n s i t y data from Oregon and Washington produces r e s u l t s s i m i l a r to those documented for coastal B r i t i s h Columbia. Analysis of U.S. data, included in Tables 3.11, 3.12 and 3.14, i l l u s t r a t e s further the regional a p p l i c a b i l i t y of r a i n f a l l c h a r a c t e r i s t i c s i d e n t i f i e d i n B r i t i s h Columbia, and provides r e s u l t s from stations at higher elevations than are cu r r e n t l y available i n coastal B r i t i s h Columbia. 10. Regional c h a r a c t e r i s t i c s of IDF curves and single storm r a i n f a l l are e s p e c i a l l y useful for a p p l i c a t i o n to approximately 250 coastal B.C. stations which record only 24-hour data. Regional r a i n f a l l charac- t e r i s t i c s can now be applied to 24-hour data at these stations to estimate shorter duration i n t e n s i t i e s and, therefore, greatly expand the data base c u r r e n t l y a v a i l a b l e for design purposes. 11. Local var i a t i o n s in storm r a i n f a l l with elevation are not constant for a l l storms. R a i n f a l l d i s t r i b u t i o n i s controlled by s i t e s p e c i f - i c meteorologic conditions which e x i s t -during each storm. P r e l i m i - nary assessment of limited a v a i l a b l e data in coastal B.C. suggests that during extreme events, r a i n f a l l increases l i n e a r l y with eleva- t i o n up to mountain c r e s t s . In the absence of h i s t o r i c a l storm data, the d i s t r i b u t i o n of annual p r e c i p i t a t i o n can be used as an i n d i c a t o r for the d i s t r i b u t i o n of extreme storm r a i n f a l l . - 118 - 4. PHYSICAL ASPECTS OF WATER FLOW THROUGH SNOW 4.1 INTRODUCTION Development of hydrograph procedures capable of simulating rain-on-snow floods requires that the role of a snowpack be assessed with regard to i t s contribution of snowmelt to t o t a l runoff and i t s e f f e c t on runoff re- sponse from the basin. A fundamental question which arises for extreme rain-on-snow i s whether water percolation through the snow medium or development of i n t e r n a l drainage channels i s the dominant routing mech- anism. Quantitative formulations have been proposed describing water percolation through snow i n a v e r t i c a l unsaturated zone and a basal saturated l a y e r . However, evidence i s also available to suggest that an in t e r n a l drainage network, not water percolation, controls runoff during extreme rain-on-snow floods. The approach taken i n this study to assess the role of a snowpack i s : (i ) to review available l i t e r a t u r e i n the general areas of snow physics and snow hydrology; ( i i ) to assess re s u l t s of research studies which per t a i n to the flow of l i q u i d water through snow; and ( i i i ) to i n t e r p r e t r e s u l t s with regard to the i r impact on hydrograph procedures required for rain-on-snow floods. Once the role of a snowpack on basin response to rain-on-snow i s assessed, then requirements of a hydrograph model can be established. Only those physical aspects of water flow through snow which a f f e c t r a i n - on-snow flood hydrographs i n the coastal region of the P a c i f i c Northwest - 119 - are assessed i n this study. In p a r t i c u l a r , snowpack response to inputs of l i q u i d water i s examined. Available l i t e r a t u r e describing p h y s i c a l aspects of water flow through snow i s extensive; For example, Gerdel (1945, 1954) provided some of the e a r l i e s t quantitative descriptions of water transmission through snow; Colbeck developed theories for v e r t i c a l percolation through unsaturated homogeneous snow (1971, 1972), saturated flow along the base of a snowpack (1974a), water flow in a dry snowpack (1976) and flow through heterogeneous snow (1979a); more microscopic analyses of water and snow i n t e r a c t i o n and snow metamorphism have been undertaken by deQuervain (1973) and Colbeck (1982a, 1983); water pressure and c a p i l l a r y e f f e c t s within a snowpack were assessed by Colbeck (1974b) and Wankiewicz (1978a); and grain clusters and geometry were analyzed by Colbeck (1979b, 1982b). An overview of much of the research noted above i s a v a i l a b l e from Colbeck (1978) or Wankiewicz (1978b). A summary of water percolation processes through homogeneous snow i s included i n Appendix IV. I t was o r i g i n a l l y envisioned that a contribution of this study would be the incorporation of water percolation processes into a hydrograph model. However, assessment of snow metamorphism and flow path development f o l - lowing inputs of l i q u i d water to a snowpack suggests that an i n t e r n a l drainage network, not water percolation, i s the dominant routing mechan- ism during extreme rain-on-snow. Examination of av a i l a b l e l i t e r a t u r e which leads to this conclusion and the consequences of the conclusion on hydrograph procedures are presented in this Chapter. - 1 2 0 - Development of hydrograph procedures considering than an i n t e r n a l drain- age network i s the primary routing mechanism through the snowpack i s pre- sented i n Chapter 5. Preliminary r e s u l t s of a p p l i c a t i o n of these proce- dures i n Chapter 5 confirms that during extreme rain-on-snow floods, no a d d i t i o n a l runoff delay needs to be included for water percolation through the snow. 4.2 FLOW PATHS AND SNOW METAMORPHISM A mountainous snowpack i s highly variable i n both time and space. A snowcover i s deposited by a sequence of discr e t e storms and i s usually a layered medium. For example, the snow surface may be rearranged by d r i f t i n g which breaks down grains and repacks them into a higher density wind crust, the surface may be glazed by absorption of solar r a d i a t i o n , and freeze-thaw cycles can lead to the formation of ice l a y e r s . The state of snow metamorphism at any given time, therefore, i s the r e s u l t of preceding c l i m a t o l o g i c a l conditions. Layering within snowpacks occurs throughout the season and a f f e c t s flow paths taken by melt water. Wankiewicz (1978b) categorized layers depend- ing on whether the snow horizon would impede, accelerate or have no ef- fe c t on flow through the pack. For example, some layers impede downward percolation, some regions cause a r e d i s t r i b u t i o n of flow, and fin g e r i n g can develop where flow concentrates below impeding horizons. in B r i t i s h Columbia these phenomena are well documented by dye studies of water flow through snow undertaken by Wankiewicz (1976) and Jordan (1978). - 121 - perhaps the most important concept to recognize i n snow hydrology i s that snowpack response i s not-constant, but rather varies with p h y s i c a l properties of the snow. Therefore, discussion of snowpack response must be q u a l i f i e d by a description of snow properties being considered. Much of the snowpack i n the coastal mountains of the p a c i f i c Northwest can be categorized as "warm" (Smith, 1973). Warm snowpacks are those whose i n t e r i o r temperature remain near 0°C during most of the snow season. This snow can also be categorized as "wet" when l i q u i d water i s present (Colbeck, 1982a). A warm, wet snowpack i s commonly referred to as a ripe snowpack. Analysis i s presented i n the following sections for warm snowpacks, isothermal at 0°C, and for wet snow whose l i q u i d water content exceeds the "irreducible-water saturation". Irreducible-water saturation i s a measure of water held i n place as absorbed or c a p i l l a r y water and has been shown through experimentation (Scheidegger, 1957) to equal about 7% of the pore volume. Once irreducible-water saturation i s s a t i s f i e d , a d d i t i o n a l water inputs are transmitted through the snowpack by processes dominated by gravity (Colbeck and Davidson, 1973). The e f f e c t s of water content and grain size on water percolation through homogeneous snowpacks have been investigated i n a t h e o r e t i c a l study by Colbeck (1976). For an input of r a i n , water percolation for three d i f - ferent snow conditions was examined as shown on Figure 4.1. Figure 4.1a shows the r a i n f a l l input, and 4.1b through 4.1d show the corresponding - 122 - I» 10 0.8 0.6 0.4 0.2 la) Roin - on-snow _l ' ' • ' i I i L 0 4 8 (b) Ripe Snow 12 16 - J . l i i i i_ 20 I « 10 h 0.8- o oj in 0.6- £ • 0.4- Z> • 0.2- _l_ 4 8 tile) Refrozen Snow 12 16 I x 10'' 0.8- u a>_ i/> 0.6- £ - 0.4 - 3 0.2- (d) Fresh Snow 24 28x10s 20 24 28x10' 24 28xl0J 12 16 Time (sec) 20 24 28x10' Figure 4.1 Snowpack Response to Rain-On-Snow (after Colbeck, 1976) - 123 - response f o r : ripe snow whose absorbed and c a p i l l a r y water requirements are s a t i s f i e d ; refrozen snow which has the same grain sizes as ripe snow but whose r e s i d u a l water i s refrozen; and fresh snow comprised of smaller grain s i z e s . As i l l u s t r a t e d on Figure 4.1, response of ripe snow i s r e l a t i v e l y f a s t , inflow and outflow shapes are s i m i l a r , and a l l inflow occurs as outflow; refrozen snow requires an i n i t i a l water input to ra i s e snow temperature to 0°C and then responds much as ripe snow; and response time of fresh snow i s longer because water i s needed to s a t i s f y the i r r e - ducible water content, and water movement through f i n e r grain sizes i s slower. These three examples i l l u s t r a t e that i n some instances outflow from a snowpack following a rain-on-snow event may be r e l a t i v e l y small, while for a ripe snowpack outflow can be f a i r l y rapid and equal i n volume to r a i n and snowmelt inputs. Introduction of l i q u i d water into a snowpack causes metamorphic processes to accelerate r a p i d l y . C h a r a c t e r i s t i c s of t h i s aging process are summa- riz e d by Colbeck (1977) and include: i) rapid grain growth occurs u n t i l uniform grain diameters of 2 to 3 mm develop. i i ) permeability of wind crusts and ice-l a y e r s increases r a p i d l y when l i q u i d water moves through the snowpack. i i i ) snow generally d e n s i f i e s during melt metamorphism. - 124 - In conjunction with f i e l d studies of the transmission of water through snow, Gerdel (1954) observed that ice planes i n wet snow r a p i d l y d i s - integrate; h o r i z o n t a l and v e r t i c a l i n t e r n a l drainage channels develop; flow channels are directed to small streams, and when the i n t e r n a l drainage network i s established discharge of snowmelt and rain w i l l be approximately equal to the rate of water input at the snow surface. Colbeck (1974a) admitted i n his paper which presented theories for both v e r t i c a l unsaturated flow and basal saturated flow through snow that "further study i s needed ... to determine the extent of saturated flow versus open channel flow beneath the snowpack". Colbeck (1977) noted that even i n r e l a t i v e l y homogeneous snow, wetting- front advance follows d i s t i n c t routes. Once flow paths are established, t h e i r permeability increases due to grain growth and f r i c t i o n a l melting and they become p r e f e r e n t i a l routes for subsequent flow. Additional f r i c t i o n a l melting causes channels to enlarge and, consequently, decreases the response time of the snow cover. A comprehensive review of snow accumulation, d i s t r i b u t i o n , melt and runoff undertaken by leading researchers i n snow hydrology (Colbeck et a l . , 1979) provides the following summary: i ) delay i n runoff from a snowpack i s usually less than that predicted by theory based on homogeneous snow. The apparent explanation i s the development of d i s t i n c t flow channels. - 125 - i i ) i n i t i a t i o n of flow channels i s probably controlled by the deta i l e d structure of the snow cover rather than the inherently unstable flow known as f i n g e r i n g . i i i ) once p r e f e r e n t i a l drainage routes are i n i t i a t e d , they are s e l f - perpetuating. iv) drainage from the snowpack becomes more rapid as melt channels develop. v) development of flow channels during snow metamorphism e f f e c t i v e l y causes a snowcovered watershed to undergo a t r a n s i t i o n from snow- cont r o l l e d to t e r r a i n - c o n t r o l l e d water movement. The above observations suggest development of an i n t e r n a l drainage net- work i s the dominant routing mechanism during extreme rain-on-snow. Consequences of this conclusion on hydrograph procedures for extreme rain-on-snow are: ( i ) water percolation processes do not need to be simulated i n a hydrograph model, and ( i i ) as water movement i n a snow- covered watershed becomes t e r r a i n c o n t r o l l e d , i t i s possible that basin response c h a r a c t e r i s t i c s might approach conditions which would occur without a snowcover. This assessment of snowpack response forms the basis for hydrograph procedures developed i n Chapter 5 for a p p l i c a t i o n to extreme rain-on-snow floods i n the coastal region. - 126 - 4.3 WATER INPUTS DURING RAIN—ON—SHOW Water transmitted through a snowpack during a rain-on-snow event i s contributed by both r a i n f a l l and snowmelt. Typical input rates for the coastal region are derived i n t h i s section based on hourly r a i n f a l l i n t e n s i t i e s determined i n Chapter 3 and on snowmelt estimates using temperature-index equations. As shown i n Chapter 3, estimates for 24-hour r a i n f a l l with a 100-year return period i n coastal B r i t i s h Columbia range from about 75 mm to 380 mm. During these events minimum and maximum hourly i n t e n s i t i e s are t y p i c a l l y about 3 and 8 percent, respectively, of the 24-hour r a i n f a l l . The corresponding range of hourly r a i n f a l l i n t e n s i t i e s for three 24-hour events chosen for i l l u s t r a t i o n are shown in Table 4.1. TABLE 4.1 HOURLY RAINFALL INTENSITIES 24-Hour R a i n f a l l (mm) Hourly I n t e n s i t i e s (mm/hr) Minimum Average Maximum 75 2 3 6 150 5 6 12 380 11 16 30 - 127 - The generation of snowmelt i s a thermodynamic process where the amount of' melt i s dependent on the net heat exchange between the snowpack and i t s environment. Various sources and processes which influence heat transfer with a snowpack include absorbed shortwave (solar) r a d i a t i o n , net longwave ( t e r r e s t r i a l and atmosphere) r a d i a t i o n , convective heat transfer from the a i r , la t e n t heat released by condensation from the a i r , conduction of heat from the ground, and heat content of r a i n water. Detailed analysis of these processes i s available from the U.S. Army Corps of Engineers (1956), and various s i t e s p e c i f i c studies are a v a i l a b l e in proceedings of annual snow conferences (e.g. Western Snow Conference, 1985). In this study, temperature-index equations are used to estimate t y p i c a l snowmelt rates during rain-on-snow events. Use of temperature-index equations to estimate snowmelt i s an a l t e r n a t i v e approach to deta i l e d thermo-budget snowmelt a n a l y s i s . The U.S. Army Corps of Engineers (1956) showed that temperature can be used as an index for snowmelt by deriving empirical r e l a t i o n s h i p s which simulate more complex physical phenomena. Temperature-index equations have been applied extensively for snowmelt modelling i n the P a c i f i c Northwest both i n the United States (U.S. Army Corps of Engineers, 1972) and Canada (Quick and Pipes, 1976). Seven equations a v a i l a b l e i n the l i t e r a t u r e for estimating snowmelt during rain-on-snow events have re c e n t l y been evaluated by Kattelmann (1985) for isothermal snowpack at 0°C at two s i t e s i n the S i e r r a Nevada Mountains of C a l i f o r n i a . Snowmelt was measured and compared to estimates - 128 - from each of seven snowmelt equations for rain-on-snow events occurring over an 11-year period on one basin and a 24-year period on the other. The snowmelt equation that had the lowest computed root mean square error (RMSE) at each of the two basins was proposed by Dunne and Leopold (1978): M = (0.142 + 0.051J72 + 0.0125.P)T a + 0.25 (4.1) where M = d a i l y snowmelt (cm); U 2 = windspeed at 2 m (m/s); P = d a i l y r a i n f a l l (cm); and Ta = mean a i r temperature (°C). The r e l a t i o n s h i p proposed by Dunne and Leopold i s s i m i l a r to the widely used equation developed by the U.S. Army Corps of Engineers (1956), except the Corps of Engineers equation has a larger c o e f f i c i e n t for the wind term: M = (0.133+ 0.086l7j 5 + 0.0126P)T a +0.23 (4.2) where U ^ = windspeed at 15 m (m/s). The U.S. Army Corps of Engineers (1956) developed another snowmelt equation for rain-on-snow, not included i n the study by Kattleman (1985), for forested areas: M = (0.339 + 0M26P)Ta + 0.13 ( 4 - 3 ) - 129 - Eqn 4.3 i s commonly applied to estimate snowmelt because wind data are seldom av a i l a b l e for a project s i t e . Representative snowmelt rates e s t i - mated using Eqns. 4.1 and 4.3 for rain-on-snow i n the coastal mountains are included for comparison in Table 4.2 for a range of c l i m a t o l o g i c a l conditions shown for i l l u s t r a t i o n . TABLE 4.2 REPRESENTATIVE SNOWMELT RATES 24-Hour Mean Air Daily Snowmelt (mm) R a i n f a l l Temperature Eqn. 4.1 Eqn. 4.1 Eqn. 4.3 (mm) CO u = 0 m/s u = 10 m/s 75 2 7 17 10 75 8 21 62 36 1 50 2 9 19 12 150 8 29 70 44 380 2 15 25 18 380 8 52 93 67 Comparison of r e s u l t s i n Table 4.2 shows the e f f e c t of wind speed on snowmelt estimates and i l l u s t r a t e s the d i f f i c u l t y i n estimating snowmelt on ungauged watersheds where climate data are not a v a i l a b l e . Results from Tables 4.1 for t y p i c a l r a i n f a l l rates and 4.2 for snowmelt estimates based on Eqn. 4.3 are combined for i l l u s t r a t i o n in Table 4.3 to produce representative water inputs to a snowpack in the coastal region for a range of r a i n f a l l events and mean a i r temperatures. - 1 30 - TABLE 4.3 REPRESENTATIVE RAINFALL AND SNOWMELT INPUTS Mean R a i n f a l l Snowmelt Total Temp. Daily Ave. Hrly Daily Ave. Hrly Ave. Hrly ( °C) (mm) (mm/h) (mm) (mm/h) (mm/h) 2 75 3.1 10 0.4 3.5 8 75 3.1 36 1.5 4.6 2 150 6.3 12 0.5 6.8 8 150 6.3 44 1.8 8.1 2 380 15.8 18 0.8 16.6 8 380 15.8 67 2.8 18.6 - 131 - 4.4 SUMMARY 1 . Development of hydrograph procedures capable of simulating rain-on- snow floods requires that the role of a snowpack be assessed with regard to i t s contribution of snowmelt to t o t a l runoff and i t s e f f e c t on runoff response from the basin. A fundamental question which arises for extreme r a i n f a l l on ripe snowpacks i s whether water percolation through the snow medium or development of i n t e r n a l drainage channels i s the dominant routing mechanism. Available evidence suggests that the drainage channel routing mechanism controls runoff during extreme rain-on-snow events. 2. Temperature, water content and grain size a f f e c t response of snow- packs to rain-on-snow as shown, for example, on Figure 4.1. Intro- duction of l i q u i d water into a snowpack causes metamorphic processes to accelerate r a p i d l y such that rapid grain growth occurs, perme- a b i l i t y of wind crusts and i c e - l a y e r s increases and snow d e n s i t i e s . 3. A comprehensive report by Colbeck e t . a l . , 1979 concludes snowpack response time i s usually less than predicted by water percolation theory, and the apparent explanation i s development of d i s t i n c t flow channels. Development of flow channels during snow metamorphism e f f e c t i v e l y causes a snow covered watershed to undergo a t r a n s i t i o n from snow-controlled to t e r r a i n - c o n t r o l l e d water movement. - 132 - If the development of an i n t e r n a l drainage network i s the dominant routing mechanism during extreme rain-on-snow, then the consequences on hydrograph procedures for extreme rain-on-snow are: ( i ) water percolation processes do not need to be simulated in a hydrograph model, and ( i i ) as water movement i n a snowcovered watershed becomes t e r r a i n c o n t r o l l e d , i t i s possible that basin response character- i s t i c s might approach conditions which would occur without a snow- cover. This assessment of snowpack response forms the basis for hydrograph procedures developed i n Chapter 5 for a p p l i c a t i o n to extreme rain-on-snow floods i n the coastal region. - 1 3 3 - 5. DEVELOPMENT OF RAIN-ON— SNOW HYDROGRAPH MODEL 5.1 PERSPECTIVE ON HYDROLOGIC MODELS Development of a hydrologic model for appl i c a t i o n purposes requires that a balance be maintained between model complexity and data available for model implementation. For example, a detai l e d physical d e s c r i p t i o n of runoff processes may require input data which are often unavailable, while a less complex model which ' operates with more r e a d i l y a v a i l a b l e data may not adequately describe basin response i n a l l cases. The key to successful modelling, therefore, i s often dependent on the model developer's a b i l i t y to simulate a complex process within the l i m i t a t i o n s of a v a i l a b l e data. This process of trade-offs i s i l l u s t r a t e d on sketches presented on Figure 5.1. a) Trade-off Diagram (after b) Modelling Complexity (after Overton and Meadows, 1976) Haan and B a r f i e l d , 1978) Figure 5.1 Perspective on Hydrologic Models - 134 - There are s t i l l many approaches to hydrologic modelling which can be adopted i n addition to those necessitated by trade-offs noted above. Project objectives may di c t a t e both the approach and output requirements of the model, while i n other instances a n a l y t i c a l procedures may be i n - corporated into a model based simply on the personal preference of the developer. General categories of hydrologic models proposed by Clarke (1973) are noted below and b r i e f descriptions are provided to contrast differences i n t h e i r approach. Deterministic vs Stochastic: When variables of a model are s p e c i f i e d by p r o b a b i l i t y d i s t r i b u t i o n s , the model i s stochastic; when each variable i s assigned a single value for a s p e c i f i e d condition, the model i s det e r m i n i s t i c . Physically-based vs Empirical: Physically-based models undertake analysis by rigorous s o l u t i o n of mathe- matical formulas which describe runoff processes; empirical models incor- porate c o e f f i c i e n t s and relationships derived from observation, expe- rience and experiment. Continuous vs Event: Continuous models generate hydrographs over long periods of time and operate through low and high flow seasons; event models are usually implemented only to estimate a single hydrograph for a s p e c i f i e d set of input v a r i a b l e s . - 135 - Lumped vs Di s t r i b u t e d : Lumped models tre a t a watershed as i f i t were homogeneous; i n a d i s t r i b - uted model, input data and basin response c h a r a c t e r i s t i c s are varied across the basin. The goal of th i s study i s to develop a hydrograph procedure capable of simulating rain-on-snow floods i n the coastal mountains of the P a c i f i c Northwest. I t i s intended that the procedures be applicable to extreme flood conditions. Extreme flood i s a subjective c l a s s i f i c a t i o n and i s commonly used i n context with a s p e c i f i c design o b j e c t i v e . For th i s study extreme flood generally r e f e r s to any flood with a return period greater than about 20 years. Specifying that only extreme rain-on-snow floods w i l l be analyzed i s analogous to selecting a s p e c i f i c case from the wide spectrum of runoff events which occur from a basin through the years. Extreme floods are of importance i n many instances of engineering planning and design. Development of hydrograph procedures for rain-on-snow floods i s under- taken with an awareness of data commonly available for engineering design s i t u a t i o n s i n the coastal region of the P a c i f i c Northwest. These data are generally limited to the following (although supplemental s i t e information may also be av a i l a b l e i n some instances): i ) topographic mapping at approximate scale 1:50 000. Even at this - 136 - r e l a t i v e l y large scale a 60 km2 basin, for example, i s only about 12 by 20 cm on a topo map. i i ) estimate of r a i n f a l l over the basin. These design data are usually estimated for the basin of i n t e r e s t based on analysis of r a i n f a l l data recorded at a regional s t a t i o n . Even without s i t e data for confirmation, a design engineer must nevertheless assess the a p p l i - c a b i l i t y of regional data to the basin. In mountainous regions this assessment i s e s p e c i a l l y d i f f i c u l t because p r e c i p i t a t i o n can vary over r e l a t i v e l y short distances both i n plan and e l e v a t i o n . i i i ) estimate of snowmelt over the basin. Procedures for estimating snowmelt include an energy balance approach and empirical tempera- ture-index equations. S u f f i c i e n t c l i m a t o l o g i c a l data for an energy budget approach to snowmelt are generally unavailable at remote mountainous locations and, therefore, temperature-index equations are usually applied. iv) s i t e photos and/or s i t e reconnaissance. Site information i s usually obtained for s p e c i f i c design projects to allow q u a l i t a t i v e assess- ment of such items as land use, f o r e s t cover and drainage network development. - 137 - Based on the objective of this study to analyze only extreme rain-on-snow flood conditions and on l i m i t a t i o n s of data generally a v a i l a b l e , the following guidelines are established for development of hydrograph procedures: i ) rain-on-snow floods w i l l be analyzed as a single event i n response to s p e c i f i e d input r a i n f a l l and snowmelt. i i ) a deterministic approach w i l l be adopted. i i i ) runoff c h a r a c t e r i s t i c s of the basin w i l l be represented by empiri- c a l r e l a t i o n s h i p s . iv) p r o vision for d i s t r i b u t i n g r a i n f a l l and snowmelt across the basin w i l l be incorporated i n the model. - 138 - 5.2 CONTINUOUS FLOW VS EVENT MODELS The e f f e c t on modelling approach "of analyzing only extreme rain-on-snow floods can be i l l u s t r a t e d by examining a fundamental difference between continuous flow and event models. Continous flow models are developed to operate over long periods of time and for a wide range of c l i m a t i c and runoff conditions. Therefore, t h e i r simulation c a p a b i l i t i e s are d i f f e r e n t from those of an event model which operates for a short period i n response to a single set of input conditions. Continuous flow models i n operation i n the P a c i f i c Northwest include the SSARR (Streamflow Synthesis and Reservoir Regulation) Model developed by U.S. Army Corps of Engineers (1972) and the UBC Watershed Model (Quick and Pipes, 1976). The SSARR Model i s applied extensively i n the Columbia River Basin to guide reservoir regulation decisions related to flood protection, navigation and hydro power. The UBC Model i s used by the B.C. Ministry of Environment for annual flood forecasting on the Fraser River and by B.C. Hydro on the Columbia and Peace River systems. One primary requirement of a continuous flow model i s to maintain a water balance over long periods between water inputs i n the form of r a i n and snow and outflow from the basin. This i s generally accomplished by separating p r e c i p i t a t i o n inputs into one of three modes of t r a v e l through the basin: surface runoff, stormflow and groundwater flow. Each of these runoff components has a d i f f e r e n t response c h a r a c t e r i s t i c and therefore occurs downstream as r i v e r flow at d i f f e r e n t times. For example, routing c h a r a c t e r i s t i c s of surface runoff i s important for - 139 - simulating peak flows following periods of high i n t e n s i t y r a i n f a l l , while routing of groundwater flow i s necessary to estimate low flows which occur long a f t e r storm periods. Event models are developed to simulate flood flows which r e s u l t from basin response to a s p e c i f i e d set of input data. These models operate only for that period of runoff dominated by processes with r e l a t i v e l y f a s t response times which contribute to the flood peak. Inputs to groundwater are treated as "losses" during the computation period since t h e i r contribution to streamflow occurs at a l a t e r period than the generated flood hydrograph. In summary, a continuous streamflow model must account for water inputs over long periods by routing runoff components separately. An event model focuses only on those runoff processes which generate a flood hydrograph. Project objectives d i c t a t e the type of information required which, i n turn, establishes a modelling approach to be implemented. The goal of this study i s to develope hydrograph procedures for estimating flood peaks r e s u l t i n g from rain-on-snow. Emphasis, therefore, i s con- centrated on examining the proportion of r a i n and melt inputs which con- tr i b u t e to the flood peak and the routing c h a r a c t e r i s t i c s of r e l a t i v e l y f a s t runoff components. - 140 - 5.3 SELECTION OF MODELLING PROCEDURE Two procedures i n common use for generating flood hydrographs from r a i n - f a l l were examined for possible a p p l i c a t i o n to rain-on-snow flood occur- rences. One method applies a unit-hydrograph concept and the other em- ploys a lag and route technique. Discussion of the conceptual develop- ment and a p p l i c a t i o n of each of these procedures i s a v a i l a b l e in standard hydrology texts both i n the U.S. ( L i n s l e y et a l . , 1982) and Canada (Gray, 1970). Examples of hydrologic models commonly applied in engineering p r a c t i c e using unit-hydrograph procedures include HYMO (Williams and Haan, 1973), SCS-TR20 ( S o i l Conservation Service, 1973) and OTTHYMO (Wisner and PNG, 1982). The lag and route procedure i s incorporated in the HEC-1 Flood Hydrograph Package (U.S. Army Corps of Engineers, 1973). Unit-hydrograph and lag and route techniques are investigated in this study to assess whether empirical r e l a t i o n s h i p s and c o e f f i c i e n t s employed by each method for r a i n f a l l - o n l y could be modified for a p p l i c a t i o n to rain-on-snow i n mountainous regions of the P a c i f i c Northwest. An over- view and i n i t i a l screening of each method i s undertaken i n this section to assess i t s p o t e n t i a l for a p p l i c a t i o n to rain-on-snow floods and i t s merits for more detai l e d examination. Each of the hydrologic models noted above which employ unit-hydrograph concepts u t i l i z e s a d i f f e r e n t procedure to estimate unit-hydrograph shape. However, even with these differences each procedure requires - 141 - that basin lag and recession constant(s) of the unit-hydrograph be determined. When recorded flood hydrographs are unavailable to make th i s assessment for a watershed, basin lag and recession constant can often be estimated from empirical r e l a t i o n s h i p s . A general expression for basin lag i s presented by Watt and Chow (1985). I t i s based on data a v a i l a b l e from throughout the U.S. and from Quebec and southern Ontario i n Canada, but does not include data from coastal B r i t i s h Columbia. Because of t h i s absence of v e r i f i c a t i o n even for r a i n f a l l events i n the coastal mountain regions, i t was concluded that modifications of unit-hydrograph procedures would not be attempted for th i s i n v e s t i g a t i o n of rain-on-snow f l o o d s . A second consideration for discarding conventional unit-hydrograph techniques i s the i r i n a b i l i t y to provide r e a d i l y for s p a t i a l variations i n r a i n f a l l and snowmelt across a basin. Unit-hydrographs require basin averaged conditions to be input to the model. In instances when the v a r i a t i o n i n r a i n f a l l and snowmelt must be accounted for across the basin, unit-hydrograph procedures require that the drainage basin be sub- divided into smaller watershed elements. The lag and route procedure for hydrograph analysis was f i r s t proposed by Clark (1945) and i s based on the p r i n c i p l e that r a i n f a l l onto a basin i s modified by two fa c t o r s : t r a v e l time through the basin and storage c h a r a c t e r i s t i c s of the watershed. Storage i s ac t u a l l y d i s t r i b u t e d across the basin, although i n the lag and route procedure i t i s considered to occur at the basin o u t l e t and i s simulated by a single l i n e a r r e s e r v o i r . - 142 - Travel time of a water p a r t i c l e through a basin can be estimated based on hydraulic p r i n c i p l e s . This feature i s p a r t i c u l a r l y a t t r a c t i v e for ungauged mountainous basins when only topographic maps and s i t e photos are a v a i l a b l e . In t h i s instance, topography can serve as an in d i c a t o r to assess v a r i a t i o n i n tr a v e l time from d i f f e r e n t parts of the basin to the o u t l e t . Lines connecting points of equal t r a v e l time, c a l l e d i s o - chrones, from various segments of the basin to the o u t l e t can then be established. Lag and route procedures simulate storage with one li n e a r reservoir and a single storage c o e f f i c i e n t . In instances when continuous flow simula- t i o n i s required for long periods a single routing c o e f f i c i e n t i s not adequate to simulate runoff (U.S. Army Corps of Engineers, 1972; Quick and Pipes, 1976). However, for analysis of a single flood event, that portion of the runoff hydrograph affected by fa s t response character- i s t i c s can be more r e a d i l y represented by a single routing c o e f f i c i e n t (U.S. Army Corps of Engineers, 1973). Application of lag and route procedures to rain-on-snow events requires, therefore, i n v e s t i g a t i o n to determine i f storage c o e f f i c i e n t s can be derived for snow covered basins i n a s i m i l a r manner as i s applied for r a i n f a l l - o n l y . A second question i s whether storage c o e f f i c i e n t s determined for rain-on-snow events are d i f f e r e n t than those from r a i n f a l l - o n l y on the same ba s i n . Based on preliminary assessment described above of unit hydrographs and lag and route procedures, the lag and route procedure i s selected for - 143 - further i n v e s t i g a t i o n of i t s p o t e n t i a l for ap p l i c a t i o n to rain-on-snow flo o d s . The focus of analysis w i l l be on examining methods for e s t i - mating t r a v e l time and the storage c o e f f i c i e n t for a snow covered water- shed. The lag and route procedure allows for s p a t i a l v a r i a t i o n i n ra i n and snowmelt inputs and basin response, and achieves a balance between model complexity and a v a i l a b l e data. - 144 - 5.4 SOURCES O F RAIN-OH-SHOW DATA Research into the development of hydrograph procedures for extreme r a i n - on-snow floods requires a watershed with the following features and available data for analysis: (i ) high elevation mountainous basin ( i i ) unregulated streamflow ( i i i ) l o c a l gauge which records r a i n f a l l i n t e n s i t y (iv) continuously recording streamflow gauge (v) snow over entire basin (vi) r a i n f a l l occurring over entire snowpack ( v i i ) l o c a l gauge which records a i r temperature ( v i i i ) basin which has experienced and recorded extreme flood event Based on review of available streamflow, p r e c i p i t a t i o n i n t e n s i t y and other c l i m a t o l o g i c a l data, i t was concluded that there i s no suit a b l e watershed i n coastal B r i t i s h Columbia which s a t i s f i e s a l l of the above requirements to a standard needed for research. A l t e r n a t i v e l y , drainage basins were examined i n other segments of the coastal hydrologic region of the P a c i f i c Northwest and suitable watersheds were i d e n t i f i e d i n the Cascade Mountains in Oregon. Two basins i n Oregon were selected for deta i l e d analysis of rain-on-snow floods. One i s the Mann Creek basin which forms part of the Willamette Basin Snow Laboratory (WBSL) established by the U.S. Army Corps of Engi- neers for research studies of snowmelt. Results from this snow labora- tory are incorporated i n the text Snow Hydrology (U.S. Army Corps of Engineers, 1956); a c l i m a t o l o g i c a l summary i s available i n the WBSL Hydrometeorological Log 1949-51 (U.S. Army Corps of of Engineers, 1952); - 145 - and a s p e c i a l research note (U.S. Army Corps of Engineers, 1955) i s available which documented a rain-on-snow event on the basin i n February 1951. The second basin i s Lookout Creek which experienced an extreme rain-on- snow flood i n December 1964. An overview of the areal extent of the flood and damage to the coastal region was presented previously i n Chap- ter 2.3 The U.S. Geological Survey (USGS) documented hourly streamflow data during the December 1964 storm for recording gauges in the coastal region in a s p e c i a l p u b l i c a t i o n (Waananen, et a l , 1971); p r e c i p i t a t i o n and c l i m a t o l o g i c a l data for the region are available from the U.S. Wea- ther Bureau (1965a, 1965b). i n addition to d e t a i l e d analysis of Lookout Creek, hydrographs recorded on s i x other watersheds i n Oregon during the December 1964 storm are also analyzed to assess storage c h a r a c t e r i s t i c s during extreme rain-on-snow floods. Drainage basins analyzed i n this study are described in Table 5.1 and th e i r locations i n Oregon are shown on Figure 5.2. Perspective on the r e l a t i v e magnitude of available rain-on-snow flood data can be gained by comparing unit discharges in Table 5.1 with those for maximum floods on record i n coastal B r i t i s h Columbia shown previously on Figure 2.5. This comparison shows some of the December 1964 flood peaks rate among the highest on record, while the rain-on-snow hydrograph recorded on Mann Creek i s not a very extreme event. TABLE 5.1 SOURCES OF RAIN-ON-SNOW FLOOD DATA Station* Latitude Longitude Drainage Area (km2) Gauge Elev (m) Date of Flood Maximum Discharge (m 3/s) Maximum Unit Discharge (m 3/s)/km 2 1. Nestucca River 45 19 123 25 16.0 552 22 Dec. 1964 24.8 1.6 2. Grave Creek 42 39 123 13 57.3 718 22 Dec. 1964 177 3.1 3. Lookout Creek 44 13 122 15 62.4 376 22 Dec. 1964 189 3.0 4. S. Fork Coquille R. 42 44 124 01 105 570 22 Dec. 1964 340 3.2 5. W. Fork I l l i n o i s R. i 42 03 123 45 110 462 22 Dec. 1964 456 4.1 6. H i l l s Creek 43 41 122 22 137 497 22 Dec. 1964 303 2.2 7. Elk Creek 42 53 122 55 141 390 22 Dec. 1964 251 1 .8 8. Mann Creek 44 18 122 10 13.0 817 7 Feb. 1951 7.3 0.6 Mann Creek (rain only) 1 Nov. 1950 15.6 1 .2 * see Figure 5.2 for station location - 147 - Figure 5.2. Location Map for Oregon Watersheds - 148 - 5.5 APPROACH TO MODEL DEVELOPMENT The primary objective of this i n v e s t i g a t i o n i s to develop a rain-on-snow hydrograph model which can be applied i n a consistent manner to mountain- ous watersheds where recorded h i s t o r i c a l flood data are not a v a i l a b l e for model c a l i b r a t i o n . A lag and route procedure has been selected as a method that i s compatible with l i m i t e d s i t e data which are commonly a v a i l a b l e . An out l i n e of procedures which w i l l be implemented to assess whether a lag and route hydrograph model can be applied for extreme r a i n - on-snow floods i s as follows: ( i ) examine methods for estimating t r a v e l time of a water p a r t i c l e through the basin. ( i i ) tabulate storage c o e f f i c i e n t s derived from analysis of recorded extreme rain-on-snow flood hydrographs. ( i i i ) apply lag and route procedure for a r a i n f a l l - o n l y event on Mann Creek to examine whether the fast runoff contribution to flood peaks i n mountainous regions can be simulated using one storage c o e f f i c i e n t . - 149 - (iv) apply lag and route procedure for rain-on-snow event on Mann Creek to examine whether the model can be adopted for rain-on- snow, and compare the storage c o e f f i c i e n t to that on the same basin for r a i n f a l l - o n l y . (v) apply lag and route procedure for rain-on-snow event on Lookout Creek to undertake a second ap p l i c a t i o n of the model, and to assess storage c o e f f i c i e n t s during more extreme flood events. I t i s generally recognized (e.g. Loague and Freeze, 1985) that hydrograph procedures can ultim a t e l y be modified to reproduce any recorded hydro- graph through a sequence of reassigning parameter values i n the model. However, while such exercises are sometimes c l a s s i f i e d as model c a l i b r a - t i o n , they are more an exercise i n curve f i t t i n g for a single event and r e s u l t s cannot always be extrapolated to other runoff events even on the same basin. A c c e p t a b i l i t y of lag and route procedures to extreme rain-on-snow events w i l l be judged on simulation r e s u l t s of only the i n i t i a l a p p l i c a t i o n of the model. Even though a better f i t between recorded and simulated events could be achieved through a d d i t i o n a l model modifications for each event, such a process i s not possible i n f i e l d a p p l i c a t i o n to ungauged watersheds where recorded data are not available for c a l i b r a t i o n . - 150 - 5.6 LAG AND ROUTE HYDROLOGIC MODEL 5.6.1 Procedures for Computation Implementation of lag and route hydrograph procedures requires the time- area response c h a r a c t e r i s t i c s and the storage c o e f f i c i e n t for the basin. Calculations proceed i n two steps. F i r s t , water inputs onto each sub- area delineated by isochrones are "lagged" to the watershed o u t l e t based on water p a r t i c l e t r a v e l times through the basin. Water inputs can be varied across the basin by specifying d i f f e r e n t amounts for each sub- area. Second, at the basin o u t l e t lagged flows are "routed" through a rese r v o i r whose storage c h a r a c t e r i s t i c s represent those governing the fast component of runoff through the basin. Calculations required for hydrograph development are described i n this section, and procedures for estimating t r a v e l time and storage c h a r a c t e r i s t i c s of the basin are presented i n subsequent sections. Lagged flows at the basin o u t l e t are calculated as follows: B Ji = JiRiAx + Ri_lA2 + ••• + Ri_n+xAn) (5.8) where i ^ = lagged discharge (inflow to reservoir) a f t e r i time i n c r e - ments; B = constant which varies with units; t = time increment for ca l c u l a t i o n s ; Rj_ = water input during i t h time increment; ^ = drainage area of the sub-area into which the basin i s divided by isochrones. For example, for a watershed divided into three sub-areas by isochrones at - 151 - half-hour i n t e r v a l s , a time increment for c a l c u l a t i o n equal to a ha l f - hour, and water inputs i n mm and areas i n km2, Eqn. 5 . 8 would be written as follows: Ii = 0.556(RiAl + Ri_1A2 + Ri_2A3) <5-9> Lagged flows are routed through a reservoir at the basin o u t l e t to pro- duce the simulated flood hydrograph for the watershed. These c a l c u l a - tions are undertaken by combining the c h a r a c t e r i s t i c equation for a li n e a r r e s e r v o i r : S = KQ ( 5 . 1 0 ) and the continuity equation: A S Q ~ ^ ( 5 . 1 1 ) where S = reservoir storage; K = storage c o e f f i c i e n t ; Q = reservoir out- flow; and I = res e r v o i r inflow. Eqns. 5.10 and 5.11 can be combined and rearranged to y i e l d : QM = (/.• + Mutt + Qi {iKTAi) where i and i+1 r e f e r to successive time increments. Eqns. 5.9 and 5.12 are used i n this study to produce "lagged" and "routed" flows, respectively, once the time-area runoff c h a r a c t e r i s t i c s and storage c o e f f i c i e n t are estimated for the basin. - 152 - 5.6.2 Travel Time Travel time of a water p a r t i c l e which contributes to the fa s t component of runoff i s determined by flow v e l o c i t i e s occurring p r i o r to channeliza- t i o n and those occurring a f t e r runoff concentrates s u f f i c i e n t l y to form channels. Examination of topographic maps i n mountainous regions shows channels are evident through a large portion of most drainage basins. Travel time through a basin i s not constant for a l l runoff events. This feature i s referred to as a non-linear c h a r a c t e r i s t i c of basin response, in contrast, unit-hydrograph and lag and route procedures assume l i n e a r basin response. Nevertheless, they have s t i l l been applied s u c c e s s f u l l y i n many instances of engineering planning and design. One explanation for success of l i n e a r hydrologic models i s their a p p l i c a t i o n for a given design condition where basin response can be approximated as l i n e a r over a limited range. A si m i l a r philosophy i s adopted in this study for ap- p l i c a t i o n of lag and route procedures to the sp e c i a l case of extreme rain-on-snow floods. Accordingly, methods proposed and tested i n this i n v e s t i g a t i o n should not be extrapolated for appl i c a t i o n to other runoff conditions. Estimates of v e l o c i t i e s for channelized flow w i l l be based on Manning's equation, and non-channelized v e l o c i t i e s w i l l be based on empirical and t h e o r e t i c a l overland flow v e l o c i t i e s . Travel time of a water p a r t i c l e i n channelized flow can be estimated from Manning's equation: - 153 - V = ± „0.67 ^0.5 (5.1) n and t = L/V ••• (5.2) where V = mean v e l o c i t y ; y = depth; S = slope; n = Manning roughness co- e f f i c i e n t ; t = time; and L = channel length. Solution of Eqn. 5.1 i s shown, graphically on Figure 5.3 for three t y p i c a l mountain slopes and a range of channel depths and Manning's n values. For comparison, overland flow v e l o c i t y estimates proposed by the S o i l Conservation Service (1974) are also included on Figure 5.3. 6 o o _j LU > ~ 4 u CD W S H O R T G R A S S P A S T U R E FOREST WITH GROUND L I T T E R 0 0 . 0 4 SLOPE = 3 ° = 5 % C H A N N E L I Z E D F L O W ( F r o m Manning equation) O V E R L A N D F L O W ( A f t e r so i l conservat ion s e r v i c e , 19 7 4 ) 'cm D E P T H = 0 . 0 5 0 . 0 6 0 . 0 7 0 . 0 8 0 . 0 9 MANNING " n " 0 . 1 0 0 . 1 1 0 . 1 2 Figure 5.3(a) Comparison of Wide Channelized and Overland Flow V e l o c i t i e s 6 5 e > 2 0 S L O P E = 1 0 ° = 1 8 % C H A N N E L I Z E D F L O W ( F r o m Manning equation) O V E R L A N D F L O W ( A f t e r so i l conservat ion s e r v i c e , 1 9 7 4 ) CUflDT t JDACO DAC 1 ri I B F F O R E S T WITH GROW " 1 1 1 u n L •• • • D L I T T E R >JH = 5 cm 0.04 0.05 0.06 0.07 0.08 0.09 0.10 0.11 0.12 MANNING " n " Figure 5.3(b) Comparison of Wide Channelized and Overland Flow V e l o c i t i e s Figure 5 . 3 ( c ) Comparison of Wide Channelized and Overland Flow V e l o c i t i e s - 157 - One method for estimating overland flow v e l o c i t i e s has been developed by the S o i l Conservation Service (1974) for use i n hydrograph a n a l y s i s . V e l o c i t y estimates for a range of h i l l s i d e slopes and for d i f f e r e n t ground covers are' shown g r a p h i c a l l y on Figure 5.4. Examination of Figure 5.4 shows these r e s u l t s represent l i n e a r basin response as velo- c i t y estimates do not vary with magnitude of the r a i n f a l l event. Over- land flow v e l o c i t i e s estimated by the S o i l Conservation are also included with open channel flow curves on Figure 5.3 for comparison. VELOCITY IN FEET PER SECOND Figure 5.4. Overland Flow v e l o c i t i e s ( a f t e r S o i l Conservation Service, 1974) - 158 - Physically-based representations of the overland flow component of basin runoff have been proposed which demonstrate non-linear basin response for variations i n r a i n f a l l i n t e n s i t y (Henderson and Wooding, 1965). The conceptualization of runoff, termed kinematic-overland flow, has been developed for the i d e a l i z e d case of flow over a plane surface. Results of analysis show v e l o c i t y and depth increases i n the downslope d i r e c t i o n where for distance L: where V L = depth at distance L; V L = v e l o c i t y at distance L; V = mean water p a r t i c l e v e l o c i t y over distance L; n = Manning roughness c o e f f i - cient; i = r a i n f a l l i n t e n s i t y ; and S = slope. Based on kinematic-overland flow, mean water p a r t i c l e v e l o c i t y v a r i e s with r a i n f a l l i n t e n s i t y and slope length. For i l l u s t r a t i o n , mean water p a r t i c l e v e l o c i t i e s for slope length = 100 m and r a i n f a l l i n t e n s i t y = 12 mm/hr, estimated as the maximum hourly i n t e n s i t y occurring i n the coastal region for a 24-hour r a i n f a l l of 150 mm, range from 0.04 - 0.08 m/s for a 3° slope, 0.06 - 0.12 m/s for a 10° slope, and 0.08 to 0.15 m/s for an 18° slope. Comparison to results in Figure 5.3 shows mean v e l o c i t i e s calculated for the i d e a l i z e d case of overland flow on a plane surface are less than empirical overland flow v e l o c i t y estimates - 159 - provided by the S o i l Conservation Service. Empirical r e s u l t s presented by the S o i l Conservation Service for overland flow are adopted for this study because these r e s u l t s have received widespread a p p l i c a t i o n in engi- neering studies and because Henderson (1966) cautions the a p p l i c a t i o n of kinematic flow on a watershed scale to r u r a l catchments. Travel time for a p p l i c a t i o n of lag and route procedures to rain-on-snow floods w i l l be estimated from channelized flow v e l o c i t i e s based on Man- ning's equation and on empirical overland flow v e l o c i t i e s from the S o i l Conservation Service. This procedure i s p a r t i c u l a r l y a t t r a c t i v e for un- gauged watersheds because the response of each basin can be estimated based on hydraulic p r i n c i p l e s and empirical results rather than having to rely on equations developed for other basins and regions. As proposed in Chapter 4, this approach to estimating t r a v e l time through a drainage basin considers that an i n t e r n a l drainage network has formed within the snowpack and delay between inputs at the snow surface and transmission to the snowpack base i s minimal. - 160 - 5.6.3 Storage C o e f f i c i e n t The lag and route procedure simulates storage c h a r a c t e r i s t i c s of a water- shed by a single l i n e a r reservoir located at the basin o u t l e t . The s t o r - age c o e f f i c i e n t can be estimated from analysis of recorded flood hydro- graphs where the recession portion of the fast runoff component i s ap- proximated by: where Qt = discharge at time t; Qo = discharge at t=o; and K = storage c o e f f i c i e n t . Taking logarithms of Eqn. 5.6 and rearranging terms y e i l d s : which shows that the storage c o e f f i c i e n t for a basin can be estimated from the slope on a graph p l o t t i n g l n Q versus time. For i l l u s t r a t i o n a flood hydrograph from Lookout Creek in Oregon i s shown on Figure 5.5 on graphs with natural and with semi-log s c a l e s . The hydrograph plotted at natural scales i l l u s t r a t e s the f a s t response and r e l a t i v e l y large magnitude of this extreme flood compared to winter flow preceding the storm. The slope of the recession portion of the hydro- graph on semi-log scales i l l u s t r a t e s storage c h a r a c t e r i s t i c s of the basin. Clark (1945) envisioned that the storage c o e f f i c i e n t be estimated from the recession curve of a hydrograph a f t e r cessation of a pulse of r a i n . While this approximation i s r e l a t i v e l y straightforward in concept, appro- pr i a t e recorded hydrographs may not be a v a i l a b l e . I t i s more l i k e l y that (5.6) (5.7) - 161 - o a CM a 19 20 21 22 23 24 25 26 27 December, 1964 o V [ — — 7 / 19 20 21 22 23 24 25 26 27 December, 1964 Figure 5.5. Rain-On-Snow Flood Hydrograph on Lookout Creek - 162 - a flood peak results from intense r a i n f a l l within a longer duration storm and that lower i n t e n s i t y rain may s t i l l be occurring during hydrograph recession. S i m i l a r l y , for rain-on-snow events snowmelt continues to add water inputs to the snowpack even af t e r r a i n f a l l has ceased. For instances when low i n t e n s i t y r a i n or snowmelt i s s t i l l occurring during hydrograph recession, storage c o e f f i c i e n t s may be overestimated because recession flows would r e s u l t from both water release from stor- age and ad d i t i o n a l water inputs to the basin. Even under these circum- stances, however, storage c o e f f i c i e n t s measured from recorded hydrographs would be more representative of a basin when peak water inputs are much larger than those occurring during hydrograph recession. This i s espe- c i a l l y true for extreme rain-on-snow when peak r a i n f a l l i n t e n s i t i e s are much larger than snowmelt rates. Recession curves for flood hydrographs from seven drainage basins i n Oregon which experienced extreme rain-on-snow floods i n December 1964 and one from Mann Creek i n the Willamette Basin Snow Laboratory for a less extreme event i n February 1951 were analyzed to estimate storage c o e f f i - cients for use i n the lag and route hydrograph procedure. The storage c o e f f i c i e n t for each hydrograph was calculated from the slope of the recession curve plotted on semi-log graph paper. Results for each drain- age basin are summarized i n Table 5.2 - 163 - TABLE 5.2 STORAGE COEFFICIENTS FOR RAIN-ON-SNOW EVENTS Station* Drainage Area (km2) Date of Flood Storage C o e f f i c i e n t * * (h) 1. Nestucca River 16.0 December 22, 1964 13 2. Grave Creek 57.3 December 22, 1964 6 3. Lookout Creek 62.4 December 22, 1964 20 4. S. Fork Coquille R. 105 December 22, 1964 17 5. W. Fork I l l i n o i s R. 110 December 22, 1964 10 6. H i l l s Creek 1 37 December 22, 1964 15 7. Elk Creek 141 December 22, 1964 7 8.a Mann Creek 13.0 February 7, 1951 50 8.b Mann Creek ( r a i n f a l l only) November 1 , 1950 19 S 25 * See Table 5.1 and Figure 5.2 for s t a t i o n l o c a t i o n . * For single l i n e a r r e s e r v o i r : storage = storage c o e f f i c i e n t x discharge Examination of r e s u l t s i n Table 5.2 shows estimated storage c o e f f i c i e n t s are i n a r e l a t i v e l y narrow range for seven drainage basins i n Oregon during an extreme rain-on-snow event i n December 1964. No attempt i s made i n t h i s study to develop f u n c t i o n a l r e l a t i o n s h i p s between storage c o e f f i c i e n t s and basin c h a r a c t e r i s t i c s , land use or geometry. Storage c o e f f i c i e n t s included in Table 5.2 are presented as preliminary estimates for use when lag and route procedures are applied to rain-on-snow floods - 164 - in the P a c i f i c Northwest. A recommended follow-up study to this i n v e s t i - gation i s one which examines storage c o e f f i c i e n t s from recorded rain-on- snow hydrographs throughout the coastal region of Oregon, Washington, B r i t i s h Columbia and Alaska. Comparison of the storage c o e f f i c i e n t estimated for the rain-on-snow flood on Mann Creek with those from Oregon i n December 1964 shows the Mann Creek value i s much la r g e r . One possible explanation for this d i f - ference can be proposed based on physical aspects of water flow through snow. The Mann Creek flood of February 1951 was not an extreme flood and, therefore, an i n t e r n a l snowmelt drainage network may not have been very extensive and water percolation through the snowpack could control much of the runoff process. For the extreme rain-on-snow events i n Oregon an i n t e r n a l drainage network, as described i n Chapter 4.2, may have provided the dominant routing mechanism. In the l a t t e r case, the storage c h a r a c t e r i s t i c s of a basin during extreme rain-on-snow events may approach that for r a i n f a l l when no snowcover i s present. - 165 - 5.7 ANALYSIS OF FLOOD HYDROGRAPHS ON MANN CREEK 5.7.1 Basin Location The Mann Creek basin i s located i n the Cascade Mountains i n Oregon and forms part of the Willamette Basin Snow Laboratory (WBSL) established by the Cooperative Snow investigations program of the Corps of Engineers and Weather Bureau. The basin has a drainage area of 13 km2 and extends from a continuously recording streamflow gauge at elevation 759 m to mountain peaks as high as elevation 1596 m. Basin location and topography are shown on Figure 5.6 at a scale of 1:48 000. Reference numbers included on Figure 5.6 represent locations for hydrometeorological instruments that were established for the WBSL research program. 5.7.2 Rainfall Flood of October 28, 1950 to November 2, 1950 5.7.2.1 Hydrometeorological Data A summary of hydrometeorological data i s available i n the WBSL Hydro- meteorological Log 1949-51 (U.S. Army Corps of Engineers, 1952) for two periods of intense r a i n f a l l from October 28 - November 2, 1950. This r a i n f a l l produced the largest flood recorded on Mann Creek during the two-year period and occurred p r i o r to snow accumulation i n the basin. Hourly r a i n f a l l data recorded at three gauges across the 13 km2 basin are a v a i l a b l e from the WBSL Hydrometeorological Log. A summary of these data i s included i n Table 5.3. - 166 - Figure 5.6. Mann Creek Topography - 167 - TABLE 5.3 MANN CREEK RAINFALL DATA: OCTOBER 28 - NOVEMBER 2, 1950 Station Elevation General Number* (m) Location Daily R a i n f a l l (mm) October November 28 29 30 31 1 2 21 8 817 Basin o u t l e t 994 Near South- eastern Boundary 1409 Northwestern Boundary 107 48 13 9 78 15 102 48 18 13 99 4 100 39 19 23 93 11 See Figure 5.6 for gauge locations. Recorded streamflow on Mann Creek for the storm period i s available i n the Hydrometeorological Log in two-hour increments. Flood hydrographs are shown on Figure 5.7. 0 48 96 144 192 240 288 TIME (HRS) Figure 5.7 Recorded Hydrograph on Mann Creek: Oct. 27-Nov. 6, 1950 - 168 - 5.7.2.2 Travel Time and Storage Coefficient Travel time of a water p a r t i c l e through the watershed i s estimated based on r e s u l t s presented i n Chapter 5.6.2 for channelized and overland flow v e l o c i t i e s . The procedure adopted for t h i s study i s to estimate flow v e l o c i t i e s i n watercourses i d e n t i f i e d on a 1:50 000 scale topography map based on Manning's equation for open channel flow, and across other seg- ments of the basin on estimates for overland flow v e l o c i t i e s . ' One goal i n developing these procedures i s to provide a method which can be ap- p l i e d i n a consistant manner on any ungauged watershed where only a topo- graphic map i s available to guide the a n a l y s i s . Assessment of the time-area runoff c h a r a c t e r i s t i c s of the basin proceeded as follows: ( i ) transects were drawn on the topographic map from the basin o u t l e t to locations along the watershed boundary. ( i i ) sections along each transect were designated as having either channelized or overland flow based on the c r i t e r i o n noted above. ( i i i ) slopes were measured along each transect. (iv) t r a v e l times for overland flow were estimated for measured slopes and v e l o c i t i e s proposed by the S o i l Conservation Service (1974) for forests with ground l i t t e r . - 169 - (v) t r a v e l times for channelized flow in this mountainous stream were estimated for measured slopes and Mannings "n" equal to 0.07. Selection of Manning's n i n upper reaches of mountainous watersheds requires judgement because n varies with r e l a t i v e roughness between the channel bed and banks and the flow depth. Mannings "n" adopted i n this study i s based on values provided by Chow (1959b). This p a r t i c u l a r exercise highlights the impor- tant role that s i t e photos or a s i t e v i s i t can play i n actual a p p l i c a t i o n of flood hydrograph procedures. (vi) points were i d e n t i f i e d along each transect i n half-hour incre- ments and lin e s connecting points of equal t r a v e l time to the basin ou t l e t , c a l l e d isochrones, were drawn. Results for Mann Creek are shown on Figure 5.8 where isochrones i l l u s t r a t e the time-are runoff c h a r a c t e r i s t i c s for the basin. The storage c o e f f i c i e n t for Mann Creek basin during the October 28 - November 2, 1950 r a i n f a l l event was estimated from the slope of recession curves on a semi-log p l o t of the recorded flood hydrographs. This graph i s shown on Figure 5.9 where the recession constant for the fast runoff component on the f i r s t peak i s estimated at 25-hours and on the second peak at 19-hours. - 170 - Figure 5.8. Mann Creek Time-Area Graph - 171 - o 48 96 144 T I M E ( H R S ) 192 240 288 5.9. Semi-Log Plot of Mann Creek Hydrograph, October 27 - November 7, 1950 - 172 - 5.7.2.3 Application of Lag and Route Hydrologic Model The primary purpose of applying the lag and route hydrograph procedure to a r a i n f a l l - o n l y flood event, p r i o r to examining rain-on-snow flood hydrographs, i s to assess whether the fast runoff contribution to flood peaks i n mountainous regions can be simulated using a single storage constant. Even though lag and route procedures are accepted as standard engineering practice for r a i n f a l l events (U.S. Army Corps of Engineers, 1973), a p p l i c a t i o n to a coastal mountain basin was s t i l l undertaken i n t h i s study for confirmation. Application of the lag and route hydro- l o g i c model for the r a i n f a l l - i n d u c e d flood of October 28 - November 2, 1950 on Mann Creek proceeded as follows: i ) hourly r a i n f a l l data were obtained from the Hydrometeorological Log. As shown i n Table 5.3 r a i n f a l l was f a i r l y uniform over the 13 km2 basin for t h i s event. i i ) comparison of r a i n f a l l and recorded streamflow for the f i r s t hydro- graph peak indicated that about 73 percent of r a i n f a l l occurred in the f a s t runoff component. To account for losses, the S o i l Conser- vation Service (1974) curve number approach to estimating d i r e c t runoff was applied to recorded r a i n f a l l for the basin; a curve number of 80 was selected because this value represented the ob- served r a i n f a l l and runoff. i i i ) the lag and route hydrograph model was applied to the Mann Creek basin using estimated e f f e c t i v e r a i n f a l l as input, the time-area graph for runoff response c h a r a c t e r i s t i c s shown on Figure 5.8, and a storage c o e f f i c i e n t of 23 hours. - 173 - Results of the i n i t i a l analysis are shown on Figure 5.10. Preliminary examination of flood hydrographs shows the lag and route procedure simulated the f i r s t runoff peak but underestimated the second. Compari- son of r a i n f a l l and recorded streamflow for the second hydrograph peak indicated that recorded runoff i s greater than r a i n f a l l over the basin. More de t a i l e d review of r a i n f a l l from a l l gauges on or near the basin provided no evidence to suggest errors i n recorded data. A l t e r n a t i v e l y , i t i s reasonable to suppose that published streamflow for this high flow period may be i n error since the flood event was the largest recorded on Mann Creek. Therefore, flow estimates would be based on extrapolation of an ex i s t i n g stage-discharge rating curve for the gauge s i t e . Results of the i n i t i a l a p p l i c a t i o n of a lag and route hydrologic model show that the fa s t runoff component contributing to flood hydrographs for r a i n f a l l events i n mountainous regions can be simulated using a single storage c o e f f i c i e n t and with t r a v e l time for a water p a r t i c l e estimated from channelized and overland flow considerations. Applica- ti o n of lag and route procedures to Mann Creek demonstrates how flood peaks can be estimated with l i m i t e d s i t e information. 03 27 28 29 30 31 1 2 3 4 5 October, 1950 November, 1950 Figure 5.10. Simulated R a i n f a l l Hydrograph on Mann Creek - 175 - 5.7.3 Rain-On-Snow Flood of February 3 - 8 , 1951 5.7.3.1 Hydroaetoerological Data A s p e c i a l research note (U.S. Army Corps of Engineers, 1955) documents and analyzes snowmelt for a rain-on-snow flood on Mann Creek i n Feb- ruary 1951. During the February 3-8, 1951 flood event, a snowpack existed over the entire basin and p r e c i p i t a t i o n occurred as rai n through- out the watershed. The WBSL Hydrometeorological Log contains climate- l o g i c a l data recorded by instruments across the basin. A l i s t i n g of cl i m a t o l o g i c a l stations referenced i n t h i s study and t h e i r general l o c a t i o n i s included i n Table 5.4. TABLE 5.4 MANN CREEK CLIMATOLOGICAL STATIONS Station Elevation General Number* (m) Location C l i m a t o l o g i c a l Data 21,22 817 basin o u t l e t precip., a i r temp., snow 11 902 near southeastern boundary snowcourse 8 994 near southeastern boundary precip., a i r temp. 20B 997 near southeastern boundary snowcourse 32 1125 eastern boundary snowcourse 34 1213 northeastern boundary snowcourse 2,2B 1409 northwestern boundary precip, a i r temp., snow * see Figure 5.6 for gauge locations - 176 - An overview of hydrologic conditions during the rain-on-snow event on Mann Creek i s provided i n Tables 5.5, 5.6 and 5.7 which summarize d a i l y r a i n f a l l , a i r temperature and snowcourse data, r e s p e c t i v e l y . TABLE 5.5 MANN CREEK RAINFALL DATA: FEBRUARY 3-8, 1951 Station Elevation Daily R a i n f a l l (mm) Number (m) 3 4 5 6 7 8 21 817 6 39 17 14 55 0 8 994 8 50 18 - - 3 2 1409 9 43 14 — — 0 TABLE 5.6 MANN CREEK AIR TEMPERATURE DATA: FEBRUARY 3-8, 1951 Station Elevation Mean Dai l y Air Temperature (°C) Number (m) 3 4 5 6 7 8 21 817 0.6 0.6 2.2 1 .1 3.3 3.3 8 994 -0.6 -0.6 0.6 0.0, 2.2 1 .7 2 1409 -1 .7 0.0 0.6 2.2 2.2 3.3 MEAN = (max. + min.) /2 - 177 - TABLE 5.7 MANN CREEK SNOWCOURSE DATA; FEBRUARY, 1951 Station Elevation Snow Depth Water Equivalent Number (m) Date (mm) (mm) 22 817 Feb. 2 762 267 1,1 902 Feb. 2 780 254 20B 997 Feb. 3 820 348 32 1125 Feb. 3 1288 503 34 1213 Feb. 2 2192 800 2B 1409 Feb. 1 2286 782 Estimates for basin averaged snowmelt and r a i n f a l l during the storm period are provided i n the research note published by the U.S. Army Corps of Engineers (1955). A summary of these estimates i s included in Table 5.8. TABLE 5.8 SNOWMELT AND RAINFALL ESTIMATES Length Snowmelt R a i n f a l l From To (hrs) (mm) (mm) Feb. 3 (HR 17) Feb. 5 (HR 18) 50 25 58 Feb. 5 (HR 19) Feb. 6 (HR 18) 24 12 20 Feb. 6 (HR 19) Feb. 7 (HR 24) 30 18 42 Feb. 8 (HR 1 ) Feb. 9 (HR 6) 30 22 0 - 178 - Recorded streamflow on Mann Creek for the storm period i s available i n the Hydrometeorological Log i n two-hour increments. The flood hydro- graph for February 3-8, 1951 i s . shown on Figure 5.11. 0 24 48 72 96 120 144 TIME (HRS) Figure 5.11. Recorded Hydrograph on Mann Creek: February 3-8, 1951 - 179 - 5.7.3.2 Travel Time and Storage Coefficient: Analysis of t r a v e l time of a water p a r t i c l e through the Mann Creek basin for the rain-on-snow event of February 3-8, 1951 considers that an i n t e r n a l network has formed within the snowpack and delay between water inputs at the snow surface and transmission to the snowpack base i s minimal. For t h i s case the time-area runoff c h a r a c t e r i s t i c s for the basin shown previously on Figure 5.8 for r a i n f a l l - o n l y i s adopted for analysis of the rain-on-snow event. The storage c o e f f i c i e n t for Mann Creek basin during the February 3-8, 1951 rain-on-snow event i s estimated from the slope of the recession curve on a semi-log p l o t of the recorded flood hydrograph. This graph i s shown on Figure 5.12 where the recession constant for the fast runoff component i s estimated at 50 hours. - 180 - o i i i i i i • ! 0 24 48 72 96 120 144 T I M E ( H R S ) Figure 5.12. Semi-Log Plot of Rain-on-Snow Hydrograph on Mann Creek - 181 - 5.7.3.3 Application of Lag and Route Hydrograph Model Lag and route hydrograph procedures are applied to Mann Creek for the February 3-8, 1951 flood event to examine whether the model can be adopted for rain-on-snow and to compare basin storage c h a r a c t e r i s t i c s to those for a r a i n f a l l - o n l y runoff event on the same watershed. Applica- t i o n of the lag and route hydrograph model to the rain-on-snow flood of February 3-8, 1951 proceeded as follows: i ) hourly r a i n f a l l data were obtained from the Hydrometeorological Log. As indicated i n Table 5.5 r a i n f a l l was f a i r l y uniform over the basin. i i ) snowmelt estimates provided by the U.S. Army Corps of Engineers (1955) were added to hourly r a i n f a l l data to produce the t o t a l input to the basin. i i i ) d e t a i l e d water balance c a l c u l a t i o n s undertaken by the U.S. Army Corps of Engineers (1955) concluded that approximately a l l rain and snowmelt water inputs to the basin occurred i n the fast component of runoff contributing to the flood hydrograph. Accordingly, no losses to groundwater were extracted from rain and snowmelt water inputs to the lag and route hydrograph model. — iv) a basin storage c o e f f i c i e n t equal to 50 hours was applied for routing basin runoff through a single l i n e a r r e s e r v o i r . - 182 - v) the lag and route hydrograph model was applied with the above input conditions and time-area runoff c h a r a c t e r i s t i c s s i m i l a r to those for r a i n f a l l - o n l y . Results of applying the lag and route hydrological model to the Febru- ary 3-8, 1951 rain-on-snow flood on Mann Creek are shown on Figure 5.13. Results of hydrograph analysis indicate that rain-on-snow floods can be simulated using conventional lag and route procedures with water input to the basin taken as the sum of r a i n f a l l and snowmelt and with no losses to groundwater. Comparison of storage c o e f f i c i e n t s on Mann Creek for the October 1950 r a i n f a l l flood and the February 1951 rain-on-snow event shows the c o e f f i c i e n t s d i f f e r e d by a factor of two. Even though evidence presented i n Chapter 4 suggests basin response c h a r a c t e r i s t i c s for rain-on-snow floods could approach conditions which e x i s t i n the absence of a snow- pack, t h i s does not occur on Mann Creek. Perhaps because the rain-on- snow flood i s not a very extreme event, an i n t e r n a l drainage network did not form s u f f i c i e n t l y to produce more t e r r a i n c ontrolled runoff. Figure 5.13. Simulated Rain-on-Snow Hydrograph on Mann Creek, February 3-8, 1951 - 184 - 5.8 ANALYSIS OF FLOOD HYDROGRAPH ON LOOKOUT CREEK 5.8.1 Basin Location The Lookout Creek basin i s located i n the Cascade Mountains i n Oregon approximately 10 km south of Mann Creek. Lookout Creek has a drainage area of 62.4 km2 and extends from a continuously recording streamflow gauge at elevation 420 m to mountain peaks as high as elevation 1631 m. Basin topography i s shown on Figure 5.14 at a scale of 1:62 500. 5.8.2 Rain-On-Snow Flood of December 21-24, 1964 5.8.2.1 Hydroaeteorological Data An extreme rain-on-snow flood event occurred on Lookout Creek and throughout the Coastal and Cascade Mountains i n Oregon from December 21- 24, 1964. During the flood event on Lookout Creek a snowpack existed over the entire basin and p r e c i p i t a t i o n occurred as ra i n throughout the watershed. Cl i m a t o l o g i c a l data are not measured d i r e c t l y within the drainage basin as was the case for research undertaken on Mann Creek. Therefore, c l i m a t o l o g i c a l data for Lookout Creek must be in f e r r e d from recorded data at regional s t a t i o n s . Figure 5.14. Lookout Creek Topography - 186 - Estimates of r a i n f a l l over the Lookout Creek basin require recorded r a i n f a l l from a l o c a l s t ation and an assessment of r a i n f a l l v a r i a t i o n with e l e v a t i o n . Hourly r a i n f a l l data are available for the storm period from a l o c a l s t a t i o n at McKenzie Bridge located approximately 3.5 km south of the watershed boundary. A summary of d a i l y r a i n f a l l recorded at McKenzie Bridge i s included i n Table 5.9. TABLE 5.9 RAINFALL AT MCKENZIE BRIDGE: DECEMBER 21-24, 1964 Gauge Location Elevation December Pr eci p i ta t i o n (mm) Latitude Longitude (m) 21 22 23 24 44 10 122 10 419 84 95 70 32 Hourly p r e c i p i t a t i o n data are not available for the storm period at any regional stations at higher elevations than McKenzie Bridge and, there- fore, elevation e f f e c t s on storm r a i n f a l l cannot be assessed d i r e c t l y . A l t e r n a t i v e l y , comparison of p r e c i p i t a t i o n recorded for the storm period to that for the entire month of December at lower elevation stations indicates p r e c i p i t a t i o n varied i n a s i m i l a r manner between stations for both storm and longer duration monthly data. This r e s u l t suggests that December monthly p r e c i p i t a t i o n data recorded at stations higher i n eleva- t i o n than McKenzie Bridge could be used as an in d i c a t o r of r a i n f a l l v a r i a t i o n with elevation during the storm period. These results are shown i n Table 5.10 and a p l o t of December p r e c i p i t a t i o n versus elevation i s shown on Figure 5.15. - 187 - TABLE 5.10 RAINFALL NEAR LOOKOUT CREEK BASIN Location Elev. P r e c i p i t a t i o n (mm) Station Latitude Longitude (m) Dec. 21-26 Dec. Storm:Month Marcola 44 10 122 51 162 255 535 0.48 Leaburg 44 06 122 41 206 - 512 - Cascadia State Park 44 24 122 29 259 233 519 0.45 McKenzie Bridge 44 10 122 10 419 323 684 0.47 Belnap Springs 44 18 122 02 656 - 762 - Santiam Pass 44 25 121 52 1448 _ 882 _ 6 0 0 < > _ j U J I 2 0 0 8 0 0 4 0 0 • / / / / / / / /• / / • • / 2 0 0 4 0 0 6 0 0 8 0 0 PRECIPITATION ( mm ) I 0 0 0 Figure 5.15. December 1964 P r e c i p i t a t i o n - 188 - Examination of recorded r a i n f a l l i n t e n s i t y data throughout the coastal region of Oregon (U.S. Army Corps of Engineers, 1966) showed the time d i s t r i b u t i o n of storm r a i n f a l l had a s i m i l a r pattern over large areas. Time d i s t r i b u t i o n s of p r e c i p i t a i t o n recorded at McKenzie Bridge and at Cascadia located approximately 40 km to the northwest at elevation 259 m are shown on Figure 5.16. McKenzie Bra .dge — — / C Cascadia—_ r z: Eo CL. 21 22 23 December, 1964 24 Figure 5.16. Time D i s t r i b u t i o n of R a i n f a l l - 189 - A summary of a i r temperature recorded at McKenzie Bridge and at Santiam Pass located approximately 10 km northwest of Lookout Creek basin i s included i n Table 5.11 for the storm period. These data i l l u s t r a t e the r e l a t i v e l y high temperatures which occurred during days with the largest r a i n f a l l . TABLE 5.11 AIR TEMPERATURE (°C) NEAR LOOKOUT CREEK BASIN McKenzie Bridge (Elev. 419 m) Santiam Pass (Elev . 1448 Day Min. Max. Mean Min. Max. Mean Dec. 20 0 2 1 .1 -4 -3 -3.6 Dec. 21 0 10 5.0 -3 6 1 .1 Dec. 22 4 12 8.3 3 8 5.6 Dec. 23 7 11 8.3 0 6 3.1 Dec. 24 7 9 7.8 1 4 2.2 Dec. 25 0 7 3.3 -2 2 0.0 mean = (min + max)/2 Using a i r temperatures and p r e c i p i t a t i o n for McKenzie Bridge and Santiam Pass, d a i l y snowmelt estimates based on the U.S. Army Corps of Engineers equation for rain-on-snow (Eqn. 4.3) are presented in Table 5.12. - 190 - TABLE 5.12 SNOWMELT ESTIMATES FOR LOOKOUT CREEK Da i l y Snowmelt (mm) Day McKenzie Bridge (Elev. 419 m) Santiam Pass (Elev. 1448 m) Dec. 21 24 7 Dec. 22 39 29 Dec. 23 37 15 Dec. 24 31 10 The depletion of snowpack during the December storm period along the Cascade Range i s evident from snow depth data compiled by the U.S. Army Corps of Engineers (1966) and the U.S. Weather Bureau (1965a). These data are included i n Table 5.13. Corresponding water equivalent for the snow depths are not a v a i l a b l e . TABLE 5.13 SNOW DEPTHS IN CASCADE RANGE: DECEMBER 1964 Station Elevation (m) Dec. 20 Snow 21 Depth 22 (cm) 23 24 25 McKenzie Bridge 419 3 5 0 0 0 0 Belnap Springs 656 39 30 15 0 0 3 Government Camp 1189 140 114 51 15 10 25 Santiam Pass 1448 218 188 127 117 109 122 Odell Lake 1461 163 132 86 71 61 76 Crater Lake 1974 208 229 213 173 168 188 - 191 - Recorded streamflow on Lookout Creek i s available i n a s p e c i a l p u b l i - cation compiled for the storm period by the U.S. Geological Survey (Waananen et a l . , 1971) to document flood_flows throughout the region. The rain-on-snow flood hydrograph for December 21-24, 1964 i s shown on Figure 5.17. 5.8.2.2 Travel Time and Storage Coefficient Travel time of a water p a r t i c l e through the Lookout Creek basin for the extreme rain-on-snow flood of December 21-24, 1964 i s undertaken consid- ering that an i n t e r n a l drainage network has formed within the snowpack. For t h i s case, delay between water inputs at the snow surface and trans- mission to the snowpack base i s minimal. This assessment i s consistent with observations and research studies by snow hydrologists for snowpack response to inputs of l i q u i d water. Travel time of a water p a r t i c l e through the watershed i s based on e s t i - mates for channelized and overland flow v e l o c i t i e s presented in Chapter 5.6.2. Flow v e l o c i t i e s i n watercourses i d e n t i f i e d on a 1:62 500 scale topography map are based on Manning's equation for open channel flow, and across other segments of the basin on estimates for overland flow. Travel time from various points in the Lookout Creek watershed to the basin o u t l e t was determined based on slope, estimated roughness and whether or not flow was considered to be channelized. Details of the procedure are outlined for Mann Creek i n Chapter 5.7.2.2. Results for Lookout Creek are shown on Figure 5.18 where isochrones i l l u s t r a t e the time-area runoff c h a r a c t e r i s t i c s for the basin. o o-J 1 1 1 1 1 1 1 1 1 0 21 48 72 96 120 144 168 192 216 T I M E ( H R S ) i Figure 5.17. Rain-on-Snow Flood Hydrograph on Lookout Creek: December 19-27, 1964 Figure 5.18. Lookout Creek Time-Area Graph - 194 - The storage c o e f f i c i e n t f o r Lookout Creek during the December 21-24, 1964 rain-on-snow event was estimated from the recession curve slope on a semi-log p l o t of the recorded flood hydrograph. This graph i s shown on Figure 5.19 where the recession constant for the fast runoff component i s estimated at 20 hours. 5.8.2.3 Application of Lag and Route Hydrograph Model Lag and route hydrograph procedures are applied to Lookout Creek for the December 21-24, 1964 flood event to undertake a second a p p l i c a t i o n of the model for rain-on-snow, and to analyze a more extreme flood than which occurred on Mann Creek. Application of the lag and route hydrograph model to the rain-on-snow flood of December 21-24, 1964 proceeded as follows: i ) hourly r a i n f a l l across the watershed was estimated based on recorded data at McKenzie Bridge and on the v a r i a t i o n i n r a i n f a l l with eleva- t i o n shown on Figure 5.15. » i i ) d a i l y snowmelt was estimated from the U.S. Army Corps of Engineers equation (Eqn. 4.3) for rain-on-snow using recorded r a i n f a l l and a i r temperature data. Eqn. 4.3 was applied to Lookout Creek because i t was developed from snow research studies in this region and because wind data are not a v a i l a b l e for use in other empirical formulas developed for snowmelt. Hourly d i s t r i b u t i o n of d a i l y snowmelt was  - 196 - simulated by a sine curve as proposed by Colbeck and Davidson (1973) during research studies in the northern Cascade Mountains in Wash- ington state. i i i ) hourly r a i n f a l l and snowmelt were added to produce the t o t a l hourly water input to the basin. iv) no losses to groundwater were considered for water inputs to the Lookout Creek basin. This assessment was based on r e s u l t s of analysis of the rain-on-snow event on Mann Creek. v) the lag and route hydrograph model was applied to the Lookout Creek basin with the above input data, the time-area graph for runoff re- sponse c h a r a c t e r i s t i c s shown on Figure 5.18, and a storage c o e f f i - c i e n t of 20 hours. Results of i n i t i a l analysis are shown on Figure 5.20a. Preliminary exam- inati o n of the simulated flood hydrograph shows the recorded flood peak i s approximately 80 percent greater than that estimated by the model. Further comparison of simulated and flood hydrographs indicates that about 75 mm more runoff occurred on December 22 than was predicted based on recorded r a i n f a l l and snowmelt estimates. - 197 - - 198 - Three possible explanations for the difference between recorded and simu- lated flood hydrographs are as follows: i ) since the December 21-24 flood event was the largest on record on Lookout Creek, streamflow estimates would be based on extrapolation of an e x i s t i n g stage-discharge curve. However, other basins i n the coastal region of Oregon also experienced peak floods with similar unit discharges during t h i s storm event. Therefore, the difference between recorded and simulated flows appears too great to be the r e s u l t of measurement error alone. i i ) snow metamorphism causes an increase i n snow grain s i z e s , and water percolation through coarse grained snow i s faster than through more f i n e l y grained new snow. since new snow f e l l on the basin prior to the December 21-24 flood, snow metamorphism would have occurred during the storm period. However, for t h i s process to y i e l d an a d d i t i o n a l 75 mm of runoff on December 22, water from snowmelt on previous days would have had to be in t r a n s i t through the snowpack. Examination of recorded a i r temperature data p r i o r to the storm suggests melt rates of the required magnitude would not occur. i i i ) f i e l d measurements obtained by Beaudry and Golding (1983) showed that snow trapped by the forest canopy a f f e c t s melt from a forested s i t e . Even though some snow which occurred prior to the extreme r a i n f a l l could have been held by the canopy, i t i s u n l i k e l y that t h i s p o t e n t i a l source of melt could account for an a d d i t i o n a l 75 mm. - 199 - iv) i n i t i a l snowmelt estimates using Eqn. 4.3 may be too low for the case of extreme r a i n f a l l combined with r e l a t i v e l y high temperatures. Examination of snow depth data i n Table 5.13 suggests melt rates were much greater than those predicted by the Corps of Engineers melt equation. Even though water equivalent data are not a v a i l a b l e , very conservative assumptions for snow density y i e l d greater melt rates than those i n i t i a l l y estimated for the basin. Calculations are shown i n Table 5.14 for two reasonable estimates of snowpack density. TABLE 5.14 SNOW DEPTHS AT SANTIAM PASS (Elev. 1448 m) Dec. 18 Dec. 18-20 Dec. 20 Dec. 22 Dec. 20-22 snow water new water water snow water snow depth equiv. snow equiv. equiv. depth equiv. melt ( cm) (mm) ( cm) (mm) (mm) ( cm) (mm) (mm) 137 453 (33%) 81 81 (10%) 534 127 419 (33%) 115 137 548 (40%) 81 162 (20%) 710 127 508 (40%) 202 Available evidence suggests that extreme r a i n f a l l combined with r e l a t i v e - l y high temperatures on Lookout Creek produced greater snowmelt than that predicted by the Corps of Engineers temperature-index equation developed i n this region. Results from the lag and route hydrograph model are shown again on Figure 5.20b with water input on December 22 increased by 75 mm to correspond with recorded runoff. Even though c l i m a t i c data for Lookout Creek are not as extensive as for a f u l l y instrumented research watershed, available regional data indicates an increase i n snowmelt more accurately represents basin conditions during the December f l o o d . - 200 - Results of hydrograph analysis indicate that extreme rain-on-snow floods can be simulated using conventional lag and route procedures with water input to the basin taken as the sum of r a i n f a l l and snowmelt. However, re s u l t s also indicate the importance of c o r r e c t l y estimating input r a i n - f a l l and snowmelt to the hydrograph model. Estimation of input data on ungauged watersheds i s often more d i f f i c u l t than estimating a storage c o e f f i c i e n t and t r a v e l times for basin response. - 201 - 5.9 DISCUSSION OF RESULTS The primary goal of this study i s to develop hydrograph procedures for estimation of extreme floods on ungauged watersheds where data are not a v a i l a b l e for model c a l i b r a t i o n . This goal i s achieved by combining re- s u l t s from each of the Chapters presented in this t h e s i s . Study compo- nents include assessment of flood producing mechanisms in the co a s t a l region; analysis of r a i n f a l l c h a r a c t e r i s t i c s for input to a hydrograph model; examination of the role of a snowpack during extreme floods; and a p p l i c a t i o n of a hydrograph model. To i l l u s t r a t e the continuity between study components, an overview of re s u l t s from previous Chapters i s i n - cluded below with re s u l t s from this Chapter. The i n i t i a l task required in the development of hydrograph procedures i n the coastal region i s to e s t a b l i s h the flood producing mechanism which must be simulated. Floods in the coastal region are generally e i t h e r snowmelt-induced i n spring and summer or r a i n f a l l - i n d u c e d i n f a l l and winter. Rainfall-induced floods can r e s u l t from r a i n f a l l - o n l y or a com- bina t i o n of r a i n and snowmelt. In Chapter 2, h i s t o r i c a l flood records, flood frequency analyses, and atmospheric processes which a f f e c t climate i n the coastal region are examined. It i s shown that extreme floods on most basins i n the coastal region are generated from rain-on-snow events. In Chapter 3, development of hydrograph procedures for extreme rain-on- snow floods i s i n i t i a t e d by analyzing c h a r a c t e r i s t i c s of storm r a i n f a l l - 202 - for input to a model. Estimation of input data to a hydrograph model i s sometimes more d i f f i c u l t on an ungauged watershed than assessment of the response c h a r a c t e r i s t i c s of the basin. Assessment of storm r a i n f a l l i n coastal B.C. i s e s p e c i a l l y d i f f i c u l t because the e x i s t i n g gauge network i s r e l a t i v e l y sparce and there i s d i f f i c u l t y i n transposing data in moun- tainous t e r r a i n because r a i n f a l l can vary over short distances in plan and elevation. Because of the d i f f i c u l t y i n estimating storm r a i n f a l l for hydrograph analysis i n the mountainous coastal region, analysis i s undertaken to i n - vestigate whether regional c h a r a c t e r i s t i c s of storm r a i n f a l l can be iden- t i f i e d even when the magnitude of r a i n f a l l varies between st a t i o n s . Ana- lyses undertaken in Chapter 3 show that regional r a i n f a l l c h a r a c t e r i s t i c s can be i d e n t i f i e d for multi-storm i n t e n s i t y data a v a i l a b l e from Atmos- pheric Environment Service and for single storm d i s t r i b u t i o n s developed as part of this study. In pra c t i c e , these re s u l t s can be used to set l i m i t s on the range of hourly i n t e n s i t i e s that need to be considered by a design engineer i n the absence of s i t e data. The next step in developing hydrograph procedures for rain-on-snow floods i s to assess the role of a snowpack with regard to i t s e f f e c t on runoff response from the basin. This assessment i s undertaken in Chapter 4 to e s t a b l i s h the routing mechanism which must be simulated by a hydrograph model. Examination of available l i t e r a t u r e i n snow hydrology conducted i n this study suggests that development of an i n t e r n a l drainage network - 203 - within the snowpack, not water per c o l a t i o n , i s the dominant routing mech- anism during extreme rain-on-snow floods. Chapter 5 examines the ap p l i c a t i o n of a hydrograph model to extreme r a i n - on-snow floods. Procedures are developed for hydrograph analysis based on the assessment of snowpack routing c h a r a c t e r i s t i c s i n Chapter 4. The assessment of snowpack response to extreme r a i n f a l l i s c r i t i c a l because once the routing mechanism i s established then any model capable of simu- l a t i n g the runoff process can be applied. For reasons discussed i n Sec- tions 5.1 and 5.3, th i s study develops procedures for a p p l i c a t i o n of a lag and route hydrograph model to extreme rain-on-snow floods. Preliminary r e s u l t s from a p p l i c a t i o n of a lag and route model on Mann and Lookout Creeks suggest that this hydrograph procedure can be applied to estimate extreme rain-on-snow floods when the following methodology i s adopted: ( i ) estimate t r a v e l time through the basin based on channelized and overland flow considerations, without any addit i o n a l time i n c r e - ment for water transmission through the snowpack. ( i i ) s e l e c t the storage c o e f f i c i e n t which simulates basin response. 1 - 204 - ( i i i ) specify water inputs to the basin as the sum of r a i n f a l l and snowmelt. (iv) consider there are no water losses to groundwater. I t can be concluded from examination of the above methods that rain-on- snow produces the most extreme flood peaks on a basin because of the r e l a t i v e l y large water inputs a v a i l a b l e for runoff, rather than because of changes i n basin response c h a r a c t e r i s t i c s that can be attributed to a snowpack. Once an i n t e r n a l drainage network forms, the major role of a snowpack i s to contribute snowmelt. Also, during extreme events most r a i n f a l l and snowmelt inputs to the basin occur in the fast component of runoff that produces the flood peak because losses to groundwater are r e l a t i v e l y small i n comparison. One topic for addi t i o n a l research i n the development of lag and route hydrograph procedures i s to examine methods for estimating storage coef- f i c i e n t s for use on ungauged watersheds. Two questions a r i s e for selec- t i o n of a storage c o e f f i c i e n t for extreme rain-on-snow floods. F i r s t , how does the storage c o e f f i c i e n t for rain-on-snow floods compare on the same basin with r a i n f a l l - o n l y floods; and secondly, can storage c o e f f i - cients be estimated from physical c h a r a c t e r i s t i c s of the basin which can be r e a d i l y i d e n t i f i e d on topographic maps. These concerns are discussed below based on snowpack response to inputs of l i q u i d water and results from a p p l i c a t i o n i n t h i s study of the lag and route model, and an outline for a follow-up study to this i n v e s t i g a t i o n i s presented. - 205 - Snow hydrologists have concluded (Colbeck et a l . , 1979) that development of flow channels during snow metamorphism causes a snowcovered watershed to undergo a t r a n s i t i o n from snow-controlled to t e r r a i n - c o n t r o l l e d water movement. This conclusion suggests that during snow metamorphism and channel development, basin response would also undergo a t r a n s i t i o n and approach runoff c h a r a c t e r i s t i c s that e x i s t i n the absence of a snowpack. This occurrence i s not evident for Mann Creek where basin storage c o e f f i - cients were 23 and 50 hours for a r a i n f a l l - o n l y and rain-on-snow flood, r e s p e c t i v e l y . However, the rain-on-snow flood on Mann Creek i s not a very extreme event and perhaps runoff i s s t i l l p a r t l y snow-controlled. It i s worth noting that the basin storage c o e f f i c i e n t of 20 hours on Lookout Creek during the December 1964 flood i s si m i l a r to that on Mann Creek for r a i n f a l l - o n l y . In addition, six other mountainous watersheds i n Oregon ranging in drainage area from 16 to 141 km2 had storage c o e f f i - cients of 6 to 20 hours during the extreme rain-on-snow flood i n December 1964. These storage c o e f f i c e n t values may s i g n i f y that s u f f i c i e n t channel development had occurred during the extreme event such that basin response approached conditions s i m i l a r to r a i n f a l l - o n l y . Further assessment of storage c o e f f i c i e n t s can be undertaken by exam- ining the recession curves of recorded extreme rain-on-snow floods from throughout the coastal hydrologic region i n Oregon, Washington, B r i t i s h Columbia and Alaska. Analysis can be undertaken to develop functional r e l a t i o n s h i p s between storage c o e f f i c i e n t s calculated from recorded hydrographs and basin c h a r a c t e r i s t i c s such as basin length, slope and drainage area. A s i m i l a r approach has been adopted for unit hydrograph - 206 - procedures for r a i n f a l l floods where data from various researchers have been combined (Watt and Chow, 1985) to produce a r e l a t i o n s h i p between basin lag and basin length and slope. U n t i l further research i s under- taken, i t i s recommended that storage c o e f f i c i e n t s calculated i n this study from recorded extreme rain-on-snow floods be adopted for use with lag and route hydrograph procedures. A second topic for further research based on the results of this study i s a re-examination of snowmelt equations. Preliminary evidence from a p p l i - cation of lag and route hydrograph procedures to Lookout Creek suggests that during t h i s extreme rain-on-snow event, snowmelt was much greater than predicted by the Corps of Engineers equation developed in this re- gion for forested areas. I t i s l i k e l y that temperature-index equations, such as developed by the Corps of Engineers, w i l l continue to be applied to ungauged mountainous watersheds because estimates of wind and other c l i m a t i c data are seldom a v a i l a b l e for use in a l t e r n a t i v e melt equations. Since input data to a hydrograph model are very important, snowmelt oc- curring during the s p e c i a l case of extreme rain-on-snow i s highlighted as an important topic for further analysis in the development of procedures for estimating extreme rain-on-snow floods. In conclusion, study components presented in this thesis examine the c h a r a c t e r i s t i c s of extreme floods in the coastal hydrologic region; pro- vide regional c h a r a c t e r i s t i c s of storm r a i n f a l l for estimating input data - 207 - to a hydrograph model; and examine the ap p l i c a t i o n of lag and route pro- cedures to extreme rain-on-snow floods. I t i s hoped that extreme r a i n - on-snow events w i l l be analyzed further in B r i t i s h Columbia as data be- comes a v a i l a b l e . Recently i n s t a l l e d Data C o l l e c t i o n Platforms (DCP's) by B.C. Hydro and the B.C. 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Williams, J.R. and Haan, R.W., (1973), "HYMO, Problem Oriented Computer Language for Hydrologic Modeling, Users Manual", ARS-S-9, A g r i c u l - t u r a l Research Service, U.S. Department of A g r i c u l t u r e . Williams, P., (1948), "The Va r i a t i o n of the Time of Maximum P r e c i p i t a - t i o n along the West Coast of North America", B u l l e t i n of the Ameri- can Meteorological Society, volume 29, Number 4, pages 143-145. Wisner, P. and PNG, C , (1982), "OTTHYMO, A Planning Model for Master Drainage Plans", Proceedings of the F i r s t International Symposium on Urban Drainage Systems, South Hampton. World Meteorological Organization, (1970), "Guide to Hydrometeorological Practices", Technical Paper No. 82, WMO-No. 168, Geneva. World Meteorological Organization, (1973), "Manual for Estimation of Probable Maximum P r e c i p i t a t i o n " , Operational Hydrology Report No. 1, WMO-No. 332, Geneva. - 217 - APPENDIX I MAXIMUM FLOODS ON RECORD IN COASTAL BRITISH COLUMBIA AND SOUTHEAST ALASKA TABLE 1.1 MAXIMUM FLOODS ON RECORD IN COASTAL BRITISH COLUMBIA Maximum Instantaneous Discharge on Record Drainage Flood Regimes Maximum D a l l y Discharge on Record Peak Da l l y S ta t i on Area(A) Spr ing / F a l l / No. of Years Dlscharge(Q) Q/A No. of Years DIscharge(Q p ) Dlscharge(Q) Op Number S ta t i on (sq km) Summer Winter of Record Date mVs ( m 3 / s ) / k m 2 o f Record Date m 3 / s m̂ /s Q 08MH104 Anderson Creek a t the mouth 27.2 X 17 17 Dec 1971 17.2 0.63 NA 08FB006 Atnarko River near the mouth 2430 X X 18 24 Jan 1968 289 0.12 16 29 Jan 1968 340 289 1.18 08DC006 Bear River above B i t t e r Creek 350 X X 16 8 Oct 1974 225 0.64 15 8 Oct 1974 271 225 1.20 08FB007 Be l l a Coola River above Burnt Creek Br idge 3730 X X 18 23 Jan 1968 703 0.19 17 23 Jan 1968 828 703 1.18 08FB002 B e l l a Coola near Hagens&org 4040 X X 21 24 Jan 1968 963 0.24 NA 08HA0I6 Blngs Creek near the mouth 15.5 X 19 14 Jan 1968 14.8 0.95 NA 08HD00I Campbell River a t o u t l e t of Campbelt Lake 1400 X 38 16 Nov 1939 858 0.61 NA 08HB048 Carnat ion Creek a t the mouth 10.1 X 10 26 Dec 1980 21.6 2.14 10 23 Jan 1982 50.0 13.7 3.65 08GA060 Chapman Creek above Sechel t D ive rs ion 64.5 X 13 31 Oct 1981 78.8 1.22 13 31 Oct 1981 148 78.8 1.88 08GA046 Chapman Creek near Mil son Creek 71.5 X 11 13 Oct 1962 193 2.70 NA 086A024 Cheakamus River near Mons 287 X X 23 19 Oct 1940 197 0.69 21 19 Oct 1940 257 197 1.30 08HAOOI Cheakamus River near Westholme 355 X 30 26 Dec 1980 457 1.29 NA 08MH103 C h l l l l w a c k River above SI esse Creek 645 X X 20 26 Dec 1980 262 0.41 20 26 Dec 1980 387 262 1.48 08EG012 Exchamslks River near Terrace 370 X 21 15 Oct 1974 572 1.55 20 1 Nov 1978 864 530 1.63 08CG006 F o r r e s t Kerr Creek above 460 m contour 311 X X 11 a Sep 1981 254 0.82 I I 8 Sep 1981 262 254 1.03 08HB003 Haslam Creek near Cassldy 95.6 X 13 4 Nov 1955 65.1 0.68 5 29 Jan I960 64.6 45.0 1.44 08FF002 Hi rsch Creek Near the mouth 347 X 17 15 Oct 1974 566 1.63 17 15 Oct 1974 807 566 1.43 08CG00I Iskut River below Johnson River 9350 X X 24 !5 Oct 1961 6880 0.74 20 15 Oct 1961 7930 6880 1.15 08CG004 Iskut River above Snlppaker Creek 7230 X X 16 9 Sep 1981 2080 0.29 16 9 Oct 1974 2520 2000 1.26 08MH108 Jacobs Creek above Jacobs Lake 12.2 X 14 19 Jan 1968 19.8 1.62 14 17 Sep 1968 24.6 5.4 4.52 TABLE I . I MAXIMUM FLOODS ON RECORD IN COASTAL BRITISH COLUMBIA Maximum Instantaneous Discharge on Record Drainage Flood Regimes Maximum D a l l y Discharge on Record Peak Da l l y S ta t ion Area(A) Spr ing / F a l l / No. of Years Olscharge(Q) Q/A No. of Years DIscharge(Q p ) Dlscharge(Q) Number S ta t i on (sq km) Summer Winter of Record Date m' /s ( m ^ s ) / k n | 2 of Record Date m^/s m ' /s Q 08MH076 Kanaka Creek near Webster Corners 47.7 X 23 14 Dec 1979 86.2 1.81 22 14 Dec 1979 146 86.2 1.69 08FE003 Kemano River above Powerhouse Ta l l race 583 X 10 15 Oct 1974 646 1.11 10 15 Oct 1974 889 646 1.38 08EB004 K lsp lox River near Hazel ton ' 1870 X X 18 2 Nov 1978 595 0. 32 2 2 Nov 1978 702 595 1.18 08FF001 K l t l m a t River below Hlrsch Creek 1990 X 19 1 Nov 1978 2410 1.21 19 1 Nov 1978 3000 2410 1.24 08EF004 K l tseguec la River near Skeena Crossing 728 X X 12 2 June1964 269 0.37 12 24 Oct 1966 603 229 2.63 08EG006 Kltsumkalum River near Terrace 2180 X X 22 3 June 1936 883 0.41 18 3 June 1936 883 883 1.00 08HF001 Koklsh River a t Beaver Cove 290 X 14 31 Jan 1935 334 1.15 NA 08HF003 Koklsh River below Bonanza Creek 269 X 13 6 Feb 1963 134 0. 50 1 1 5 Oec 1962 164 128 1.28 08HA003 Koks l lah River a t Cowichan S ta t i on 209 X 26 14 Dec 1979 212 1.01 NA 08HB029 L i t t l e QualIcum River near Qua 1icum Beach 237 X 22 27 Dec 1980 166 0.70 21 27 Dec 1980 213 166 1.28 08KB004 L i t t l e QualIcum River a t o u t l e t of Cameron Lake 135 X 32 16 Jan 1961 189 1.40 NA 08FF003 L i t t l e Wedeene River below Bowbyes Creek 188 X 17 1 Nov 1978 274 1.46 16 1 Nov 1978 382 274 1.39 086A06I Mackay Creek a t Montroyal Boulevard 3.63 X 10 31 Oct 1981 9.25 2.55 10 31 Oct 1981 16.2 9. 25 1.75 08MH020 Mahood Creek near S u l l i v a n 34.4 X 24 19 Jan 1968 28.3 0.82 NA 08MH018 Mahood Creek near Newton 18.4 X 22 19 Jan 1968 25.0 1.36 NA 08GA054 Mamquam River above Mashlter Creek 334 X 15 26 Dec 1980 270 0.81 15 26 Dec 1980 369 270 1.37 08GA057 Mashl ter Creek near Squamlsh 38.9 X 10 26 Dec 1980 53.5 1.38 10 3 Nov 1975 121 44.2 2.74 08GD007 Most ay Creek near Dumbel1 • Lake 1550 X 12 1 Sep 1967 254 0.16 7 1 Sep 1967 311 254 1.22 08MHI29 Murray Creek a t 216 St reet Lang ley 26.2 X 14 3 Dec 1982 19.7 1.33 12 23 Jan 1982 49.2 14.0 3.51 08FC002 Nascal 1 River near Ocean Pal Is 383 X 12 25 Oct 1947 886 2.31 5 25 Oct 1947 923 886 1.04 TABLE 1.1 MAXIMUM FLOODS ON RECORD IN COASTAL BRITISH COLUMBIA S t a t i o n Number Drainage Flood Regimes Area(A) Spr ing / F a l l / No. of Years (sq km) Summer Winter of Record Maximum D a l l y Discharge on Record Maximum Instantaneous Discharge on Record Date Discharged)) Q/A No. of Years •Vs ( m 3 / s ) / k m 2 Q f Record Date Peak Da l l y Discharge<Q p ) Discharged)) Qp m3/s m 3 / s 08DB001 08MH105 08HF002 08GA052 08MH058 08MH006 080B002 08GA023 08FB004 08HA010 08HB014 08MH056 08GA064 Nass River above Shumal Creek 19200 X X 45 15 Oct 1961 9460 0.49 16 9 Oct 1974 8920 7670 1.16 Nlcomekl River below Murray Creek 64.5 X 17 19 Jan 1968 28.3 0.44 12 26 Dec 1972 35.4 21.9 1.62 Nlmpklsh River near Eng1ewood 1760 X 11 31 Dec 1926 1270 0.72 NA Noons Creek near Port Moody 6.99 X 15 19 Nov 1962 17.6 2.52 NA N o r r l s h Creek near Dewdney 117 X 21 26 Nov 1963 214 1.83 21 26 Nov 1963 399 214 V 1.86 North A loue t te River a t 232nd S t ree t 37.3 X 25 23 Dec 1963 76.2 2.04 14 26 Dec 1980 118 64.6 1.83 Nusatsum River near Hagensborg 269 X X 17 27 Sep 1973 190 0.71 15 7 Nov 1978 206 120 1.72 P a l l a n t Creek near Queen C h a r l o t t e 76.7 X 11 8 Oct 1974 93.4 1.22 11 15 Oct 1974 126 69.4 1.82 Rubble Creek near Gar iba ld i 74.1 X 11 4 June 1955 48.1 0.65 NA Sal loamt River near Hagensborg 161 X 18 16 Dec 1980 141 0.88 17 16 Dec 1980 241 141 1.71 San Juan River near Port Renfrew 580 X 23 26 Dec 1982 862 1.49 23 26 Dec 1982 1160 862 1.35 S a r l t a River near Bamfield 162 X 31 29 Jan I960 677 4.18 NA SI esse Creek near Vedder Crossing 162 X X 24 29 Apr 1959 72 .2 0.45 22 7 Nov 1978 212 49.6 4.27 Stawamus River Below Ray Creek 40.4 X 11 26 Dec 1980 64.4 1.59 11 26 Dec 1980 113 64.4 1.75 Sumas River near Huntingdon 149 X 30 15 Feb 1982 47.4 0.32 27 15 Feb 1982 49.2 47.4 1.04 to to O TABLE 1.1 MAXIMUM FLOODS ON RECORD IN COASTAL BRITISH COLUMBIA Maximum Instantaneous Discharge on Record Drainage Flood Reqlmes Maximum i D a l l y Dlscharqe on Record Peak Oall y S t a t i o n Area(A) Spr ing / F a l l / No. of Years DI scharge(Q) Q/A No. of Years 0lseharge(Q ) DI scharge(Q) SB. Number S ta t i on (sq km) Summer w in ter of Record Date m 3 / s ( m 3 / s ) / k m 2 o f Record Date m 3 / s m J / s 0 08MH033 S . e l f z e r River a t Cuitus Lake 65.0 X 10 t2 Feb 1951 25.8 0.40 NA 08HB024 Tsable River near Fanny Bay 113 X 23 15 Jan 1964 261 2.31 NA 08HC002 Ucona River a t the Mouth 185 X 23 19 Nov 1962 549 2.97 23 19 Nov 1962 1080 549 1.97 08DD001 Unuk River near Stewart 1480 X X 17 8 Oct 1974 1000 0.68 16 9 Oct 1979 1230 591 2.08 08MH098 Mast Creek near Fort Lang ley 11.4 X 20 23 Dec 1963 12.8 1.12 12 26 Dec 1980 24.1 16.2 1.49 080A002 Yakoun River near Port Clements 477 X 22 28 Nov 1963 612 1.28 15 27 Dec 1979 374 315 1.19 08MH097 Yorkson Creek near Walnut Grove 5.96 X 19 30 Jan 1965 5.10 0.86 NA 08HE006 Zebal los River near Zebal los 181 X 22 13 Nov 1975 728 4.02 18 13 Nov 1975 1 ISO 728 1.62 08EG0I1 Zymagot l tz River near Terrace 376 X X 23 15 Oct 1974 382 1.02 23 15 Oct 1974 549 382 1.44 08EF005 Zymoetz River above O.K. Creek 2980 X X 20 1 Nov 1978 1980 0.66 20 1 Nov 1978 3140 1980 1.59 08EF00J Zymoetz River near Terrace 3080 X X 13 31 Oct 1961 1050 0.34 NA TABLE I.2 MAXIMUM FLOODS ON RECORD IN SOUTHEAST ALASKA Maximum Instantaneous Olscharge on Record Drainage Maximum D a l l y Olscharqe on Record Peak 3 Dall y .S ta t ion AraolA) No. of Years Water Year DI scharge(Q) Q/A No. o t Years Dlscharge(Qp) i Dlscharge(Q) 22. Number S ta t ion (sq km) Of Record ( O c t . - S e p t . ) m3/s ( m J / s ) / k r o 2 of Record Date mVs m3/s 0 15010000 Davis River near Hyder 207 10 1937 295 1.43 10 12 Nov 1936 552 295 1.87 15011500 Red River near Met laka t la 117 15 1977 240 2.05 15 3 Nov 1976 351 240 1.46 15012000 Nins tan ley Creek near Ketchikan 40.1 28 1962 80.1 2.00 30 30 Jan 1962 117 80.1 1.46 15022000 Harding River near WrangelI 175 31 1962 323 1.85 31 14 Oct 1961 425 323 1.32 15026000 Cascade Creek near Petersburg 59.6 38 1920 69.7 1.17 35 11 Sept 1947 92.7 56.6 1.64 1503100 Long River above Long Lake near Juneau 21.5 10 1968 42.5 1.98 10 28 Sept 1968 too 42.5 2.35 15034000 Long River near Juneau 84.2 32 1957 128 1.52 NA 15036000 Speel River near Juneau 585 16 1961 898 1.54 17 27 Sept 1918 1008 566 1.81 15040000 Dorothy Creek near Juneau 39.4 35 1950 47.9 1.22 37 3 Nov 1949 50.4 47.9 1.05 15044000 Car lson Creek near Juneau 62.9 10 1454 98.8 1.57 10 12 Aug 1961 144 82.1 1.75 15048000 Sheep Creek near Juneau 11.8 29 1948 16.0 1.36 30 8 Sept 1948 23.8 16.0 1.49 15050000 Gold Creek a t Juneau 25.3 38 1961 51.8 2.05 39 6 Sept 1981 76.5 27.8 2.75 15052000 Lemon Creek near Juneau 31.3 20 1961 75.3 2.41 22 13 Aug 1961 95.4 75.3 1.27 15052500 Mendenhal1 River near Au ke Bay 220 17 1981 388 1.76 17 8 Sept 1981 481 388 1.24 15052800 Montana Creek near Auke Bay 40.1 10 1970 38.2 0.95 10 23 Aug 1966 54.4 25.5 2.13 15053800 Lake Creek a t Auke Bay 6.5 10 1966 14.0 2.15 10 23 Aug 1966 27.8 14.0 1.99 15054000 Auke Creek a t Auke Bay 10.3 13 1970 5.9 0.57 NA 15056100 Skagway River a t Skagway 376 19 1967 275 0.73 19 7 Sept 1981 464 237 1.96 15056200 West Creek near Skagway 112 15 1967 201 1.79 16 15 Sept 1967 278 201 1.38 15059500 Whipple Creek near Ward Cove 13.7 12 1969 23.9 1.74 12 19 Nov 1968 80 .1 23.9 3.35 15060000 Perseverance Creek near Wacker 7.3 30 1950 13.5 1.85 31 18 Oct 1964 19.3 10. 1 1.91 15068000 Mahoney Creek near Ketchikan 14.8 22 1923 43.0 2.91 22 2 Feb 1954 71.6 30.9 2.32 15070000 F a l l s Creek near Ketchikan 94.5 28 1959 110 1.16 28 1 Nov 1917 158 51.0 3.10 15072000 Fish Creek near Ketchikan 83.1 64 1920 149 1.79 63 15 Oct 1961 153 125 1.22 15074000 E l l a Creek near ketchlkan 51.0 22 1955 41.3 0.81 22 7 Dec 1930 48.7 40.2 1.21 15076000 Manzanlta Creek near Ketchikan 87.8 30 1962 110 1.25 30 14 Oct 1961 165 110 1.50 15078000 Grace Creek near Ketchikan 78.2 16 1965 85.2 1.09 16 4 Sept 1966 113 79.6 1.42 15080000 Orchard Creek near Bel 1 Is land 153 12 1920 164 1.07 11 1 Nov 1917 201 62 .3 3.23 TABLE 1.2 MAXIMUM FLOODS ON RECORD IN SOUTHEAST ALASKA S t a t i o n Number S t a t i o n Drainage Area{A) (sq km) Maximum D a l l y Discharge on Record No. of Years of Record Water Year ( O c t . - S e p t . ) DIscharge(Q) »Vs Q/A (mVs)/km2 Maximum Instantaneous Discharge on Record No. of Years of Record Peak Da l ly DI scharge(Qp) Dlscharge(Q) n>3/s m 3 / s SE. 9 13081500 Stanley Creek near Cralg 15085100 Old Tom Creek near Kasaan 15085600 15085700 15085800 15086600 15088000 15093400 15094000 15098000 15010000 15102000 15106920 15106 940 15106960 15106980 15107000 15108000 15109000 134 15.3 17 32 1973 1952 239 19.0 1.78 1.24 17 32 18 Oct 1964 21 Nov 1979 442 31.4 Indian Creek near H o l l l s 22.8 15 1963 60.3 2.64 13 13 Oct 1961 170 Har r i s River near H o l l l s 74.3 14 1962 145 1.95 15 5 Dec 1959 250 Maybeso Creek a t H o l l l s 39. 1 14 1963 85.0 2.17 14 14 Oct 1961 107 Big Creek near Point Baker 29.0 18 1966 38.5 1.33 18 3 Sept 1966 41.1 Sawmill Creek near S i tka 101 26 1952 141 1.40 20 14 Sept 1952 181 Sashln Creek near Big Por t Walter 9.6 14 1977 36.8 3.83 14 2 Nov 1976 75.0 Deer Lake O u t l e t near Por t Alexander 19.2 16 1963 26.7 1.39 16 14 Dec 1962 31.7 Baranof River a t Baranof 82.9 29 1922 103 1.24 25 6 Oct 1972 255 Takatz Creek near Baranof 45.3 18 1968 45.6 1.01 18 28 Sept 1968 49.6 Hasselborg' Creek near Angoon 146 17 1954 62.9 0.43 17 23 Oct 1953 68.0 Kadashan River above Hook Creek near Tenakee 26.4 12 1973 25.5 0.97 12 15 Sept 1976 52.4 Hook Creek above Tr near Tenakee 11.6 13 1979 20.2 1.74 13 15 Sept 1976 36.5 Hook Creek near Tenakee 20.7 13 1979 25.6 1.24 13 5 Oct 1979 43.0 T o n a l I t e Creek near Tenakee 37.6 14 1979 64.3 1.71 14 9 Oct 1979 102 Kadashan River near Tenakee 97.6 15 1979 127 1.30 NA Paviof River near Tenakee 62.9 24 1979 95.4 1.52 24 30 Oct 1978 131 Fish Creek near Auke Bay 35.2 20 1960 33.1 0.94 20 2 Oct 1961 60.0 143 15.4 46.7 144 61.4 38.5 141 36.8 26.7 70.8 45.6 62.9 16.1 9.1 25.6 40.8 95.4 18.1 3.09 2.04 3.64 1.74 1.74 1.07 1.28 2.04 1.19 3.60 1.09 1.08 3.25 4.01 1.68 2.50 1.37 3.31 - 224 - APPENDIX II DEPTH-DURATION-FREQUENCY DATA FOR THE BRITISH COLUMBIA COASTAL REGION - 225 - TABLE I I . 1 DEPTH-DURATION-FREQUENCY DATA FOR ABBOTSFORD A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 1 0 2 5 50 1 0 0 1 HR 1 2 . 8 1 8 . 7 2 2 . 6 2 7 . 6 31 . 3 3 5 . 0 2 HR 1 8 . 5 2 4 . 8 2 9 . 1 3 4 . 4 3 8 . 4 4 2 . 3 6 HR 3 5 . 6 4 0 . 3 4 3 . 5 4 7 . 5 5 0 . 5 5 3 . 4 12 HR 4 9 . 2 5 8 . 3 6 4 . 3 7 2 . 0 7 7 . 6 8 3 . 3 2 4 HR 61 . 7 7 7 . 8 8 8 . 3 1 0 1 . 8 1 1 1 . 6 1 2 1 . 4 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 o . 2 5 50 1 0 0 1 HR 0 . 2 1 0 . 2 4 0 . 2 6 0 . 2 7 0 . 2 8 0 . 2 9 2 HR 0 . 3 0 0 . 3 2 0 . 3 3 0 . 3 4 0 . 3 4 0 . 3 5 6 HR 0 . 5 8 0 . 5 2 0 . 4 9 0 . 4 7 0 . 4 5 0 . 4 4 12 HR 0 . 8 0 0 . 7 5 0 . 7 3 0 . 7 1 0 . 7 0 0 . 6 9 2 4 HR 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 DEPTH [-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 2 5 50 1 0 0 1 HR 0 . 5 6 0 . 8 3 1 . 0 0 1 . 2 2 1 . 3 8 1 . 5 4 2 HR 0 . 6 4 0 . 8 5 1 . 0 0 1 . 1 8 1 . 3 2 1 . 4 5 6 HR 0 . 8 2 0 . 9 3 1 . 0 0 1 . 0 9 1 . 1 6 1 . 2 3 12 HR 0 . 7 6 0 . 9 1 1 . 0 0 1 . 1 2 1 . 2 1 1 . 2 9 2 4 HR 0 . 7 0 0 . 8 8 1 . 0 0 1 . 1 5 1 . 2 6 1 . 3 7 - 226 - TABLE I I . 2 DEPTH-DURATION-FREQUENCY DATA FOR AGASSIZ CDA RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 10.8 13.1 14.6 16.5 17.9 19.3 2 HR 16.3 18.6 20.2 22. 1 23.5 25.0 6 HR 32.5 36.3 38.8 42.0 44.4 46.7 12 HR 50.0 57.2 62.0 68.2 72.7 77.2 24 HR 73.0 88.6 98.9 111.8 121.4 131.0 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.15 0.15 0.15 0.15 0.15 0.15 2 HR 0.22 0.21 0 . 2 0 0 . 2 0 0.19 0 . 1 9 6 HR 0 . 4 5 0.41 0 . 3 9 0.38 0 . 3 7 0.36 1 2 HR 0.69 0.65 0.63 0.61 0.60 0 . 5 9 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.74 0.90 1 .00 1.13 1 .23 1 .32 2 HR 0.81 0.92 1 .00 1.10 1.17 1 .24 6 HR 0.84 0.94 1 .00 1 .08 1.14 1 .20 12 HR 0.8.1 0.92 1 .00 1.10 1.17 1 .24 24 HR 0.74 0.90 1 .00 1.13 1 .23 1 .33 - 227 - T A B L E I I . 3 D E P T H - D U R A T I O N - F R E Q U E N C Y DATA FOR A L O U E T T E L A K E R A I N F A L L DATA FROM AES RETURN P E R I O D ( Y E A R S ) D U R A T I O N 2 5 10 2 5 50 100 1 HR 1 2 . 3 1 4 . 4 1 5 . 8 1 7 . 6 1 8 . 9 2 0 . 3 2 HR 2 0 . 2 2 4 . 1 2 6 . 7 3 0 . 0 3 2 . 4 3 4 . 9 6 HR 4 4 . 9 4 9 . 1 51 . 8 5 5 . 4 5 8 . 0 6 0 . 5 12 HR 7 0 . 0 81 . 7 8 9 . 6 9 9 . 5 1 0 6 . 8 1 1 4 . 1 2 4 HR 9 7 . 7 1 1 7 . 6 1 3 0 . 8 1 4 7 . 6 1 5 9 . 8 1 7 2 . 1 D E P T H - D U R A T I O N R E L A T I O N S H I P S RETURN P E R I O D ( Y E A R S ) D U R A T I O N 2 5 10 2 5 5 0 100 1. HR 0 . 1 3 0 . 1 2 0 . 1 2 0 . 1 2 0 . 1 2 0 . 1 2 2 HR 0 . 2 1 0 . 2 0 0 . 2 0 0 . 2 0 0 . 2 0 0 . 2 0 6 HR 0 . 4 6 0 . 4 2 0 . 4 0 0 . 3 8 0 . 3 6 0 . 3 5 12 HR 0 . 7 2 0 . 6 9 0 . 6 9 0 . 6 7 0 . 6 7 0 . 6 6 2 4 HR 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 D E P T H - F R E Q U E N C Y R E L A T I O N S H I P S RETURN P E R I O D ( Y E A R S ) D U R A T I O N 2 5 10 2 5 5 0 1 0 0 1 HR 0 . 7 7 0 . 9 1 1 . 0 0 1 . 1 1 1 . 2 0 1 . 2 8 2 HR 0 . 7 6 0 . 9 0 1 . 0 0 1 . 1 2 1 . 2 1 1 . 3 1 6 HR 0 . 8 7 0 . 9 5 1 . 0 0 1 . 0 7 1 . 1 2 1 . 1 7 12 HR 0 . 7 8 0 . 9 1 1 . 0 0 1 . 1 1 1 . 1 9 1 . 2 7 2 4 HR 0 . 7 5 0 . 9 0 1 . 0 0 1 . 1 3 1 . 2 2 1 . 3 2 - 228 - TABLE I I . 4 DEPTH--DURATION--FREQUENCY DATA FOR ALTA LAKE RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 7.0 8.2 9.0 10.0 10.8 11.5 2 HR 11.0 12.6 13.6 14.9 15.9 16.8 6 HR 20.0 . 23.4 25.7 28.6 30.7 32.8 12 HR 29.5 36.5 41.0 46.9 51 .2 55.6 24 HR 43.0 56.2 64.8 75.8 84.0 92.2 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.16 0.15 0.14 0.13 0.13 0. 13 2 HR 0.26 0.22 0.21 0.20 0.19 0.18 6 HR 0.47 0.42 0 .40 0.38 0.36 0.36 12 HR 0.69 0.65 0.63 0.62 0.61 0.60 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 . 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 - 25 50 1 00 1 HR 0.78 0.91 1 .00 1.11 1.19 1 .28 2 HR 0.81 0.92 1 .00 1.10 1.17 1 .24 6 HR 0.78 0.91 1 .00 1.11 1.19 1 .28 12 HR 0.72 0.89 1 .00 1.14 1 .25 1 .35 24 HR 0.66 0.87 1 .00 1.17 1 .30 1 .42 - 229 - TABLE I I . 5 DEPTH-DURATION-FREQUENCY DATA FOR BEAR CREEK RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 23 .7 36.4 44.8 55.4 63.3 71.1 2 HR 35 .4 48 . 1 56.6 67.3 75.2 83. 1 6 HR 63 .8 82.0 94. 1 109.3 120.5 131.8 12 HR 88 .9 120.6 141.5 168.0 187.6 207.0 24 HR 141 .8 205.2 247.2 300.2 339.4 378.5 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0. 1 7 0.18 0.18 0.18 0.19 0.19 2 HR 0. 25 0.23 0.23 0.22 0.22 0.22 6 HR 0. 45 0.40 0.38 0.36 0.36 0.35 12 HR 0. 63 0.59 0.57 0.56 0.55 0.55 24 HR 1 . 00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0. 53 0.81 1 .00 1 .24 1 .41 1 .59 2 HR 0. 62 0.85 1 .00 1.19 1 .33 1 .47 6 HR 0. 68 0.87 1 .00 1.16 1 .28 1 .40 12 HR 0. 63 0.85 1 .0.0 1.19 1 .33 1 .46 24 HR 0. 57 0.83 1 .00 1.21 1 .37 1 .53 - 230 - TABLE I I . 6 DEPTH-DURATION-FREQUENCY DATA FOR BELLA COOLA BC HYDRO RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 10.6 13.0 14.6 16.6 18.1 19.6 2 HR 17.4 22.3 25.6 29.7 32.7 35.7 6 HR 36.7 47.7 55.0 64.2 71 .0 77.8 12 HR 59.6 76.8 88.2 102.6 1 13.3 123.8 24 HR 88.3 113.8 130.6 151.9 167.5 183.1 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 0 0 1 HR 0 . 1 2 0 . 1 1 0 . 1 1 0 . 1 1 0 . 1 1 0 . 1 1 2 HR 0 . 2 0 0 . 2 0 0 . 2 0 0 . 2 0 0 . 2 0 0 . 1 9 6 HR 0.42 0.42 0.42 0.42 0.42 0.42 1 2 HR 0 . 6 8 0 . 6 8 0 . 6 8 0 . 6 8 0 . 6 8 0 . 6 8 24 HR *1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.73 0.89 1 .00 1.14 1 .24 1 .34 2 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 6 HR 0.67 0.87 1 .00 1.17 1 .29 1.41 12 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 24 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 - 231 - TABLE I I . 7 DEPTH-DURATION-FREQUENCY DATA FOR BUNTZEN LAKE RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 14.5 1 8 . 6 21 .3 24.8 27.3 29.8 2 HR 23. 1 28.6 32.3 36.9 40.3 43.7 6 HR 50.0 61 .0 68.4 77.6 84.5 91 .3 12 HR 75.4 99. 1 115.0 1 34.8 1 49.5 164.3 24 HR 111.4 151.0 177.4 210.5 235.2 259.7 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0 .13 0.12 0.12 0.12 0.12 0 . 1 1 2 HR 0 .21 0.19 0.18 0.18 0.17 0 . 1 7 6 HR 0 .45 0.40 0.39 0.37 0.36 0 .35 12 HR 0 .68 0.66 0.65 0.64 0.64 0 .63 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0 .68 0.87 1 .00 1.16 1 .28 1 .40 2 HR 0 .72 0.89 1 .00 1.14 1 .25 1 .35 6 HR 0 .73 0.89 1 .00 1.14 1 .24 1 .34 1 2 HR 0 .66 0.86 1 .00 1.17 1 .30 1 .43 24 HR 0 .63 0.85 . 1 .00 1.19 1 .33 1 .46 - 2 3 2 - TABLE II. 8 DEPTH-DURATION-FREQUENCY DATA FOR BURNABY MTN BCHPA RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 11.0 13.3 14.8 16.7 1 8 . 1 19.5 2 HR 17.8 20. 1 21.5 23.4 24.8 26.2 6 HR 37.4 42.7 46.3 50.7 54.0 57.2 12 HR 54.4 63.8 70. 1 77.9 83.8 89.6 24 HR 75. 1 89.8 99.6 111.8 121.0 129.8 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.15 0.15 0.15 0.15 0.15 0.15 2 HR 0.24 0.22 0.22 0.21 0.21 0.20 6 HR 0.50 0.48 0.46 0.45 0.45 0.44 12 HR 0.72 0.71 0.70 0.70 0.69 0.69 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.75 0.90 1 .00 1.13 1 .22 1 .32 2 HR 0.83 0.93 1 .00 1 .09 1.15 1 .22 6 HR 0.81 0.92 1 .00 1.10 1.17 1 .24 12 HR 0.78 0.91 1 .00 1.11 1 .20 1 .28 24 HR 0.75 0.90 1 .00 1.12 1 .21 1 .30 - 233 - TABLE II . 9 DEPTH -DURATION -FREQUENCY DATA FOR CAMPBELL RIVER BCFS RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 10.7 13.9 15.9 18.6 20.6 22.5 2 HR 15.9 19.0 21.2 23.8 25.8 27.8 6 HR 30.8 37.0 41.2 46.3 50.2 54.0 12 HR 41.3 48.5 53.3 59.3 63.8 68.3 24 HR 54.0 65.3 73.0 82.6 89.8 96.7 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0.20 0.21 0.22 0.23 0.23 0.23 2 HR 0.29 0.29 0.29 0.29 0.29 0.29 6 HR 0.57 0.57 0.56 0.56 0.56 0.56 12 HR 0.76 0.74 0.73 0.72 0.71 0.71 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.67 0.87 1 .00 1.17 1 .29 1 .41 2 HR 0.75 0.90 1 .00 1.13 1 .22 1 .31 6 HR 0.75 0.90 1 .00 1.13 1 .22 1 .31 12 HR 0.77 0.91 1 .00 1.11 1 .20 1 .28 24 HR 0.74 0\89 1 .00 1.13 1 .23 1 .33 - 234 - TABLE 11.10 DEPTH -DURATION--FREQUENCY DATA FOR CAMPBELL RIVER BCHPA RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 13.1 18.5 22.0 26.5 29.8 33. 1 2 HR 17.9 22.8 26. 1 30.3 33.4 36.4 6 HR 32.5 37.0 40.0 43.8 46.6 49.4 12 HR 43.9 48.8 52.2 56.4 59.5 62.6 24 HR 60.0 69.8 76.3 84.5 90.7 96.7 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.22 0.26 0.29 0.31 0.33 0.34 2 HR 0.30 0.33 0.34 0.36 0.37 0.38 6 HR 0.54 0.53 0.52 0.52 0.51 0.51 12 HR 0.73 0.70 0.68 0.67 0.66 0.65 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.60 0.84 1 .00 1 .20 1 .35 1 .50 2 HR 0.68 0.87 1 .00 1.16 1 .28 1 .39 6 HR 0.81 0.93 1 .00 1 .09 1.16 1 .24 12 HR 0.84 0.94 1 .00 1 .08 1.14 1 .20 24 HR 0.79 0.92 1 .00 1.11 1.19 1 .27 - 2 3 5 - TABLE 11.11 DEPTH-DURATION-FREQUENCY DATA FOR CARNATION CREEK CDF RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 11.8 14.8 16.7 19.2 21 . 1 22.9 2 HR 19.8 24.5 27.6 31.5 34.4 37.3 6 HR 43.9 56.3 64.7 75.2 82.9 90.7 12 HR 64.7 83.2 95.4 110.8 1 22.3 1 33.7 24 HR 91 .9 119.0 1 37.3 1 59.8 176.9 193.7 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.13 0.12 0.12 0.12 0.12 0.12 2 HR 0.22 0.21 0.20 0.20 0.19 0. 1.9 6 HR 0.48 0.47 0.47 0.47 0.47 0.47 12 HR 0.70 0.70 0.69 0.69 0.69 0.69 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.70 0.88 1 .00 1.15 1 .26 1 .37 2 HR 0.72 0.89 1 .00 1.14 1 .25 1 .35 6 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 12 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 24 HR 0.67 0.87 1 .00 1.16 1 .29 1.41 - 2 3 6 - TABLE 11.12 DEPTH-DURATION-FREQUENCY DATA FOR*CHILLIWACK MICROWAVE RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 10.0 12.8 14.6 17.0 1 8 . 7 20.4 2 HR 14.0 16.8 1 8 . 6 21 .0 22.7 24.4 6 HR 27. 1 31 .7 34.9 38.8 41 .7 44.6 1 2 HR 39.8 46.8 51.5 57.4 61 .7 66.0 2 4 HR 55.2 66.5 73.9 83.5 90.5 97.4 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0 . 1 8 0.19 0.20 0.20 0.21 0.21 2 HR 0.25 0.25 0.25 0.25 0.25 0.25 6 HR 0.49 0.48 0.47 0.46 0.46 0.46 12 HR 0.72 0.70 0.70 0.69 0.68 0.68 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.68 0.87 1 .00 1.16 1 .28 1 .39 2 HR 0.75 0.90 1 .00 1.13 1 .22 1.31 6 HR 0.78 0.91 1 .00 1.11 1 .20 1 .28 12 HR 0.77 0.91 1 .00 1.11 1 .20 1 .28 24 HR 0.75 0.90 1 .00 1.13 1 .22 1 .32 - 237 - TABLE 11.13 DEPTH-DURATION-FREQUENCY DATA FOR CLOWHOM FALLS RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 10.5 13.2 15.0 17.3 18.9 20.6 2 HR 15.9 18.9 21.0 23.5 25.4 27.3 6 HR 33.1 39.2 43.3 48.5 52.3 56.0 12 HR 52.3 60.2 65.5 72. 1 77.0 82.0 24 HR 78.0 92.9 1 02.7 115.2 124.3 133.4 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.13 0.14 0.15 0.15 0.15 0.15 2 HR 0.20 0.20 0.20 0.20 0.20 0.20 6 HR 0.42 0.42 0.42 0.42 0.42 0.42 12 HR 0.67 0.65 0.64 0.63 0.62 0.61 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.70 0.88 1 .00 1.15 1 .26 1 .38 2 HR 0.76 0.90 1 .00 1.12 1 .21 1 .30 6 HR 0.76 0.91 1 .00 1.12 1 .21 1 .29 12 HR 0.80 0.92 1 .00 1.10 1.18 1 .25 24 HR 0.76 0.90 1 .00 1.12 1.21 1 .30 - 238 - TABLE 11.14 DEPTH-DURATION-FREQUENCY DATA FOR COMOX A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 9.8 12.4 14.1 16.3 17.9 19.6 2 HR 14.3 17.5 1 9 . 7 22.4 24.4 26.4 6 HR 28. 1 33. 1 36.2 40.3 43.4 46.4 12 HR 41.2 48.2 52.9 58.8 63.2 67.6 24 HR 58. 1 69.4 77.0 86.4 93.6 100.6 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.17 0.18 0.18 0.19 0.19 0.19 2 HR 0.25 0.25 0.26 0.26 0.26 0.26 6 HR 0.48 0.48 0.47 0.47 0.46 0.46 12 HR 0.71 0.70 0.69 0.68 0.68 0.67 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1.00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.69 0.88 1 .00 1.15 1 .27 1 .38 2 HR 0.73 0.89 1 .00 1.14 1 .24 1 .34 6 HR 0.78 0.91 1 .00 1.11 1 .20 1 .28 12 HR 0.78 0 . 9 1 1 .00 1.11 1 .20 1 .28 24 HR 0.75 0.90 1 .00 1.12 1.21 1.31 - 239 - TABLE 11.15 DEPTH-DURATION-FREQUENCY DATA FOR COQUITLAM LAKE RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 14.5 16.6 17.9 19.6 20.9 22. 1 2 HR 22.7 25.3 ' 27. 1 29.3 30.9 32.5 6 HR 55.2 63.5 69. 1 76. 1 81.4 86.5 12 HR 92.9 110.8 122.6 137.5 1 48.6 1 59.6 24 HR 1 43.8 174.5 194.6 220.3 239.3 258.2 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.10 0.09 0.09 0.09 0.09 0.09 2 HR 0.16 0.15 0.14 0.13 0.13 0.13 6 HR 0.38 0.36 0.36 0.35 0.34 0.34 12 HR 0.65 0.63 0.63 0.62 0.62 0.62 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 - 1 HR 0.81 0.93 1 .00 1 .09 1.16 1 .23 2 HR 0.84 0.94 1 .00 1 .08 1.14 1 .20 6 HR 0.80 0.92 1 .00 1.10 1.18 1 .25 12 HR 0.76 0.90 1 .00 1.12 1 .21 1 .30 24 HR 0.74 0.90 1 .00 1.13 1 .23 1 .33 - 240 - T A B L E 1 1 . 1 6 D E P T H - D U R A T I O N - F R E Q U E N C Y D A T A F O R C O U R T E N A Y P U N T L E D G E R A I N F A L L D A T A F R O M A E S R E T U R N P E R I O D ( Y E A R S ) D U R A T I O N 2 5 10 25 50 100 1 H R 9.6 12.1 13.7 15.9 17.4 19.0 2 H R 14.5 17.3 1 9 . 3 21.7 23.5 25.3 6 H R 30.6 36.6 40.6 45.6 49.3 53.0 12 H R 45.5 56.3 63.4 72.4 79. 1 85.7 24 H R 66.2 84.5 96.7 112.1 123.4 1 34.6 D E P T H - D U R A T I O N R E L A T I O N S H I P S R E T U R N P E R I O D ( Y E A R S ) D U R A T I O N 2 5 10 25 50 1 00 1 H R 0.14 0.14 0.14 0.14 0.14 0.14 2 H R 0.22 0.21 0.20 0.19 0.19 0.19 6 H R 0.46 0.43 0.42 0.41 0.40 0.39 12 H R 0.69 0.67 0.66 0.65 0.64 0.64 24- H R 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 D E P T H - F R E Q U E N C Y R E L A T I O N S H I P S R E T U R N P E R I O D ( Y E A R S ) D U R A T I O N 2 5 10 25 50 100 1 H R 0.70 0.88 1 .00 1.15 1 .27 1 .38 2 H R 0.75 0.90 1 .00 1.13 1 .22 1.31 6 H R 0.75 0.90 1 .00 1.12 1.21 1.31 12 H R 0.72 0.89 1 .00 1.14 1 .25 1 .35 24 H R 0.68 0.87 1 .00 1.16 1 .28 1 .39 - 241 - TABLE 11.17 DEPTH-DURATION-FREQUENCY DATA FOR DAISY LAKE DAM RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 9.9 14.0 16.6 20. 1 22.6 25. 1 2 HR 15.5 20.6 23.9 28. 1 31.3 34.4 6 HR 32. 1 39.5 44.4 50.6 55.2 59.8 1 2 HR 47.5 55.3 60.6 67. 1 72.0 76.8 24 HR 66.2 78.7 87. 1 97.7 1 05.4 113.0 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.15 0.18 0.19 0.21 0.21 0.22 2 HR 0.23 0.26 0.27 0.29 0.30 0.30 6 HR 0.48 0.50 0.51 0.52 0.52 0.53 12 HR 0.72 0.70 0.70 0.69 0.68 0.68 24 HR 1 .00 1 .00 1 .00 ' 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.60 0.84 1 .00 1 .20 1 .36 1 .51 2 HR 0.65 0.86 1 .00 1 .18 1.31 1 .44 6 HR 0.72 0.89 1 .00 1.14 1 .24 1 .35 12 HR 0.78 0.91 1 .00 1.11 1.19 1 .27 24 HR 0.76 0.90 1 .00 1.12 1.21 1 .30 - 242 - TABLE 11.18 DEPTH-DURATION-FREQUENCY DATA FOR ESTEVAN POINT RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 18.3 22.4 25. 1 28.5 31.0 33.5 2 HR 28.0 35.6 40.7 47.0 51.8 56.4 6 HR 61.6 73.9 82.0 92.3 99.9 107.5 12 HR 90. 1 1 06.4 117.2 130.8 140.9 1 50.8 24 HR 131.0 168.2 193.0 223.9 247.2 270.0 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0 . 1 4 0.13 0.13 0.13 0.13 0 . 1 2 2 HR 0 .21 0.21 0.21 0.21 0.21 0 .21 6 HR 0 .47 0.44 0.43 0.41 0.40 0 .40 12 HR 0 .69 0.63 0.61 0.58 0.57 0 .56 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0 .73 0.89 1 .00 1.14 1 .24 1 .34 2 HR 0 .69 0.88 1 .00 1.16 1 .27 1 .39 6 HR 0 .75 0.90 1 .00 1.13 1 .22 1 .31 12 HR 0 .77 0.91 1 .00 1 .12 1 .20 1 .29 24 HR 0 .68 0.87 1 .00 1.16 1 .28 1 .40 - 243 - TABLE 11.19 DEPTH-DURATION-FREQUENCY DATA FOR HANEY MICROWAVE RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 12.6 15.6 17.5 19.9 21 .8 23.6 2 HR 1 9 . 0 22.8 25.4 28.7 31.1 33.5 6 HR 37.2 42.6 46.2 50.7 54.0 57.4 1 2 HR 56.4 67.7 75. 1 84.5 91 .6 98.4 24 HR 78.7 97.0 109.0 124.3 1 35.6 1 46.9 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.16 0.16 0.16 0.16 0.16 0.16 2 HR 0.24 0.24 0.23 0.23 0.23 0.23 6 HR 0.47 0.44 0.42 0.41 0.40 0.39 12 HR 0.72 0.70 0.69 0.68 0.68 0.67 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.72 0.89 1 .00 1.14 1 .24 1 .35 2 HR 0.75 0.90 1 .00 1.13 1 .22 1 .32 6 HR 0.81 0.92 1 .00 1.10 1.17 1 .24 12 HR 0.75 0.90 1 .00 1.12 1 .22 1.31 24 HR 0.72 0.89 1 .00 1.14 1 .24 1 .35 - 244 - TABLE 11.20 DEPTH-DURATION-FREQUENCY DATA FOR HANEY UBC RF ADMIN RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 11.8 14.3 16.0 18.1 19.6 21.2 2 HR 19.3 22.4 24.5 27. 1 29.0 30.9 6 HR 39.2 45. 1 49. 1 54. 1 57.8 61 .5 12 HR 59.5 71.0 78.6 88.3 95.4 1 02.6 24 HR 89.3 111.8 1 26.7 145.4 1 59.4 173.3 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.13 0.13 0.13 0.12 0.12 0.12 2 HR 0.22 0.20 0.19 0.19 0.18 0.18 6 HR 0.44 0.40 0.39 0.37 0.36 0.35 12 HR 0.67 0.64 0.62 0.61 0.60 0.59 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.74 0.90 1 .00 1.13 1 .23 1 .32 2 HR 0.79 0.92 1 .00 1.11 1.19 1 .26 6 HR 0.80 0.92 1 .00 1.10 1.18 1 .25 12 HR 0.76 0.90 1 .00 1.12 1 .21 1.31 24 HR 0.70 0.88 1 .00 1.15 1 .26 1 .37 - 245 - TABLE 11.21 DEPTH-DURATION-FREQUENCY DATA FOR JORDAN RIVER DIVERSI RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 22.8 32.7 39.3 47.6 53.7 59.9 2 HR 35.0. 44.2 50.3 58.0 63.6 69.3 6 HR 71.5 92.9 107.2 125.2 1 38.4 151.7 12 HR 1 04.4 142.9 168.5 200.6 224.5 248.3 24 HR 1 50.0 205.2 241 .9 288.2 322.6 356.6 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.15 0.16 0.16 0.17 0.17 0.17 2 HR 0.23 0.22 0.21 0.20 0.20 0.19 6 HR 0.48 0.45 0.44 0.43 0.43 0.43 12 HR 0.70 0.70 0.70 0.70 0.70 0.70 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.58 0.83 1 .00 1 .21 1 .37 1 .52 2 HR 0.70 0.88 1 .00 1.15 1 .27 1 .38 6 HR 0.67 0.87 1 .00 1.17 1 .29 1 .41 12 HR 0.62 0.85 1 .00 1.19 1 .33 1 .47 24 HR 0.62 0.85 1 .00 1.19 1 .33 1 .47 - 246 - TABLE II.22 DEPTH-DURATION-FREQUENCY DATA FOR JORDAN RIVER GEN STA RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 10.6 12.3 13.4 14.8 15.9 16.9 2 HR 17.6 20.3 22.2 24.4 26. 1 27.8 6 HR 37.4 44.6 49.3 55.4 59.8 64.3 12 HR 55.9 68.8 77.3 88. 1 96. 1 104.0 24 HR 75.4 99. 1 1 15.0 1 34.9 1 49.8 1 64. 4 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.14 0.12 0.12 0.11 0.11 0.10 2 HR 0.23 0.21 0.19 0 . 1 8 0.17 0.17 6 HR 0.50 0.45 0.43 0.41 0.40 0.39 12 HR 0.74 0.69 0.67 0.65 0.64 0.63 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 2 5 50 1 0 0 1 HR 0 . 7 9 0 . 9 2 1 . 0 0 1 . 1 1 1 . 1 8 1 . 2 6 2 HR 0 . 8 0 0 . 9 2 1 . 0 0 1 . 1 0 1 . 1 8 1 . 2 6 6 HR 0 . 7 6 0 . 9 0 1 . 0 0 1 . 1 2 1 . 2 1 1 . 3 0 12 HR 0 . 7 2 0 . 8 9 1 . 0 0 1 . 1 4 1 . 2 4 1 . 3 5 24 HR 0 . 6 6 0 . 8 6 r o o 1 . 1 7 1 . 3 0 1 . 4 3 - 247 - TABLE I I . 2 3 DEPTH-DURATION-FREQUENCY DATA FOR KITIMAT 2 RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 11.9 14.5 16.2 18.4 20.0 21.6 2 HR 19.7 24. 1 27.0 30.7 33.4 36. 1 6 HR 44.5 57.5 66.2 77.2 85.3 93.4 12 HR 65.5 85.4 98.6 115.3 1 27.7 139.9 24 HR 88.8 109.7 123.6 141.1 1 54.3 167.0 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0.13 0.13 0.13 0.13 0.13 0.13 2 HR 0.22 0.22 0.22 0.22 0.22 0.22 6 HR 0.50 0.52 0.54 0.55 0.55 0.56 12 HR 0.74 0.78 0.80 0.82 0.83 0.84 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.73 0.89 1 .00 1.13 1 .23 1 .33 2 HR 0.73 0.89 1 .00 1.14 1 .24 1 .34 6 HR 0.67 0.87 1 .00 1.16 1 .29 1.41 12 HR 0.66 0.87 1 .00 1.17 1 .29 1 .42 24 HR 0.72 0.89 1 .00 1.14 1 .25 1 .35 - 248 - TABLE 11.24 DEPTH--DURATION--FREQUENCY DATA FOR LADNER BCHPA RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 8.1 9.8 10.9 12.4 13.5 14.5 2 HR 12.5 14.0 15.0 16.2 17.1 18.1 6 HR 22.3 24.5 26.0 27.8 29.2 30.5 12 HR 31.6 38.3 42.7 48.2 52.4 56.5 24 HR 43.2 56.2 64.6 75.6 83.5 91 .4 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.19 0.17 0.17 0.16 0.16 0 . 1 6 2 HR 0.29 0.25 0.23 0.21 0.21 0.20 6 HR 0.52 0.44 0.40 0.37 0.35 0.33 12 HR 0.73 0.68 0.66 0.64 0.63 0.62 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.74 0.90 1 .00 1.13 1 .23 1 .33 2 HR 0.83 0.93 1 .00 1 .08 1.15 1.21 6 HR 0.86 0.94 1 .00 1 .07 1.12 1.18 12 HR 0.74 0.90 1 .00 1.13 1 .23 1 .32 24 HR 0.67 0.87 1 .00 1.17 1 .29 1 .42 - 249 - TABLE 11.25 DEPTH-DURATION-FREQUENCY DATA FOR LANGLEY LOCHIEL RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 11.4 14.6 16.7 19.4 21 .4 23.4 2 HR 16.9 19.5 21.2 23.3 24.9 26.5 6 HR 31.4 37.3 41.2 46. 1 49.8 53.4 12 HR 46.4 55.2 61 .0 68.2 73.6 79.0 24 HR 61 .2 75.6 85.2 97.2 1 06.3 115.2 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.19 0.19 0.20 0.20 0.20 0.20 2 HR 0.28 0.26 0.25 0.24 0.23 0.23 6 HR 0.51 0.49 0.48 0.47 0.47 0.46 12 HR 0.76 0.73 0.72 0.70 0.69 0.69 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 2 HR 0.80 0.92 1 .00 1.10 1.18 1 .25 6 HR 0.76 0.91 1 .00 1.12 1 .21 1 .30 12 HR 0.76 0.91 1 .00 1.12 1.21 1 .30 24 HR 0.72 0.89 1 .00 1.14 1 .25 1 .35 - 250 - TABLE 11.26 DEPTH-DURATION-FREQUENCY DATA FOR MISSION WEST ABBEY RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 13.7 17.8 20.6 24. 1 26.6 29.2 2 HR 19.7 24.5 27.7 31.8 34.8 ' 37.7 6 HR 35.3 40.6 44. 1 48.5 51 .7 55.0 12 HR 51.1 59.2 64.4 71.2 76. 1 81.1 24 HR 72.5 85.4 94. 1 105. 1 113.0 121.2 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0 . 1 9 0.21 0.22 0.23 0.24 0.24 2 HR 0.27 0.29 0.29 0.30 0.31 0.31 6 HR 0.49 0.48 0.47 0.46 0.46 0.45 12 HR 0.71 0.69 0.68 0.68 0.67 0.67 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.67 0.87 1 .00 1.17 1 .29 1 .42 2 HR 0.71 0.88 1 .00 1.15 1 .25 1 .36 6 HR 0.80 0.92 1 .00 1.10 1.17 1 .25 12 HR 0.79 0.92 1 .00 1.10 1 . 1 8 1 .26 24 HR 0.77 0.91 1 .00 1.12 1 .20 1 .29 - 251 - TABLE I I . 2 7 DEPTH--DURATION--FREQUENCY DATA FOR NANAIMO DEPARTURE BA RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 9.4 15.4 19.5 24.5 28.3 32.0 2 HR 13.2 20.0 24.6 30.3 34.6 38.9 6 HR 23.9 29.6 33.3 38.0 41 .6 45. 1 12 HR 33.6 40. 1 44.4 49.8 53.9 57.8 24 HR 41 .3 50.4 56.4 64. 1 69.8 75.4 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 • 1 HR 0.23 0.31 0.35 0.38 0.41 0.43 2 HR 0.32 0.40 0.44 0.47 0.50 0.52 6 HR 0.58 0.59 0.59 0.59 0.60 0.60 12 HR 0.81 0.80 0.79 0.78 0.77 0.77 24 HR 1 .00 1 .00 • 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.48 0.79 1 .00 1 .26 1 .45 1 .65 2 HR 0.54 0.81 1 .00 1 .23 1 .41 1 .58 6 HR 0.72 0.89 1 .00 1.14 1 .25 1 .35 12 HR 0.76 0.90 1 .00 1.12 1.21 1 .30 24 HR 0.73 0.89 1 .00 1.14 1 .24 1 .34 - 252 - TABLE I I . 2 8 DEPTH-DURATION-FREQUENCY DATA FOR N VANCOUVER LYNN CRE RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 14.4 17.5 19.6 22.2 24. 1 26.0 "• 2 HR 23.2 28.2 31.5 35.6 38.7 41.8 6 HR 51.7 64.9 73.5 84.5 92.6 100.7 12 HR 80.8 103.8 1 19.0 1 38.4 152.6 166.9 24 HR 1 20.2 1 56.5 180.5 211.0 233.5 255.8 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.12 0.11 0.11 0.11 0.10 0.10 2 HR 0.19 0 . 1 8 0.17 0.17 0.17 0.16 6 HR 0.43 0.41 0.41 0.40 0.40 0.39 12 HR 0.67 0.66 0.66 0.66 0.65 0.65 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10- 25 50 100 1 HR 0.74 0.90 1 .00 1.13 1 .23 1 .33 2 HR 0.74 0.90 1 .00 1.13 1 .23 1 .33 6 HR 0.70 0.88 1 .00 1.15 1 .26 1 .37 12 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 24 HR 0.67 0.87 1 .00 1.17 1 .29 1 .42 - 253 - TABLE I I . 2 9 DEPTH-DURATION-FREQUENCY DATA FOR PITT MEADOWS STP RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 12.5 17.5 20.9 25. 1 28.2 31.3 2 HR 18.0 24.9 29.5 35.3 39.6 43.9 6 HR 38.0 45.6 50.7 57. 1 61.8 66.5 12 HR 53.0 65.0 73. 1 83.2 90.7 98.2 24 HR 67.7 85.7 97.4 112.6 123.8 134.9 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.19 0.20 0.21 0.22 0.23 0.23 2 HR 0.27 0.29 0.30 0.31 0.32 0.33 6 HR 0.56 0.53 0.52 0.51 0.50 0.49 12 HR 0.78 0.76 0.75 0.74 0.73 0.73 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.60 0.84 1 .00 1 .20 1 .35 1 .50 2 HR 0.61 0.84 1 .00 1 .20 1 .34 1 .49 6 HR 0.75 0.90 1 .00 1.13 1 .22 1 .31 12 HR 0.73 0.89 1 .00 1.14 1 .24 1 .34 24 HR 0.69 0.88 1 .00 1.16 1 .27 1 .38 - 254 - TABLE 11.30 DEPTH-DURATION-FREQUENCY DATA FOR PITT POLDER RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 1 2 .4 14.7 16.2 18.1 19.5 20.9 2 HR 18 .9 22.9 25.4 28.7 31.2 33.6 6 HR 42 .7 51 .7 57.7 65.3 70.9 76.4 12 HR 67 .3 80.3 88.9 99.8 107.9 115.9 24 HR 98 .9 1 19.0 1 32.2 1 49.0 161.5 173.8 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0. 1 3 0.12 0.12 0.12 0.12 0.12 2 HR 0. 1 9 0.19 0.19 0.19 0.19 0.19 6 HR 0. 43 0.43 0.44 0.44 0.44 0.44 12 HR 0. 68 0.67 0.67 0.67 0.67 0.67 24 HR 1 . 00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0. 77 0.91 1 .00 1.12 1 .21 1 .29 2 HR 0. 74 0.90 1 .00 1.13 1 .22 1 .32 6 HR 0. 74 0.90 1 .00 1.13 1 .23 1 .32 12 HR 0. 76 0.90 1 .00 1.12 1.21 1 .30 24 HR 0. 75 0.90 1 .00 1.13 1 .22 1.31 - 255 - TABLE 11.31 DEPTH-DURATION-FREQUENCY DATA FOR PORT ALBERNI A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 1 1 .2 15.2 17.9 21.2 23.7 26.2 2 HR 18 .0 21.7 24. 1 27.2 29.5 31.8 6 HR 37 .9 42.4 45.3 49.0 51 .8 54.5 12 HR 59 .4 71.6 79.8 90.0 97.6 105.1 24 HR 87 . 1 108.5 122.9 1 40.9 154.3 167.5 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0. 1 3 0.14 0.15 0.15 0.15 0.16 2 HR 0. 21 0.20 0.20 0.19 0.19 0.19 6 HR 0. 44 0.39 0.37 0.35 0.34 0.33 12 HR 0. 68 0.66 0.65 0.64 0.63 0.63 24 HR 1 . 00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0. 63 0.85 1 .00 1.19 1 .33 1 .47 2 HR 0. 75 0.90 1 .00 1.13 1 .22 1 .32 6 HR 0. 84 0.94 1 .00 1 .08 1.14 1 .20 12 HR 0. 74 0.90 1 .00 1.13 1 .22 1 .32 24 HR 0. 71 0.88 1 .00 1.15 1 .26 1 .36 - 256 - TABLE 11.32 DEPTH-DURATION-FREQUENCY DATA FOR PORT COQUITLAM CITY RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 11.0 13.4 15.0 17.0 18.4 19.9 2 HR 17.1 1.8.7 19.7 21.0 22.0 23.0 6 HR 36.9 42.4 46.0 50.6 54.0 57.4 12 HR 56.3 65.9 72.4 80.4 86.4 92.4 24 HR 81.1 96.7 107.0 1 20.0 129.6 139.2 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0.14 0.14 0.14 0.14 0.14 0.14 2 HR 0.21 0.19 0.18 0.18 0.17 0.17 6 HR 0.45 0.44 0.43 0.42 0.42 0.41 12 HR 0.69 0.68 0.68 0.67 0.67 0.66 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.74 0.90 1 .00 1.13 1 .23 1 .33 2 HR 0.87 0.95 1 .00 1 .07 1.12 1.17 6 HR 0.80 0.92 1 .00 1.10 1.17 1 .25 12 HR 0.78 0.91 1 .00 1.11 1.19 1 .28 24 HR 0.76 0.90 1 .00 1.12 1.21 1 .30 - 257 - TABLE 11.33 DEPTH-DURATION-FREQUENCY DATA FOR PORT HARDY A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 10.0 11.2 12.1 13.1 13.8 14.6 2 HR 16.3 19.1 20.9 23.3 25.0 26.7 6 HR 37.2 44.2 49.0 54.8 59.2 63.6 12 HR 61.0 73.3 81.4 91 .7 99.2 1 06.8 24 HR 89.5 116.6 134.6 157.4 174.2 190.8 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.11 0.10 0.09 0.08 0.08 0.08 2 HR 0.18 0.16 0.16 0.15 0.14 0.14 6 HR 0.42 0.38 0.36 0.35 0.34 0.33 12 HR 0.68 0.63 0.60 0.58 0.57 0.56 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.83 0.93 1 .00 1 .09 1.15 1.21 2 HR 0.78 0.91 1 .00 1.11 1.19 1 .28 6 HR 0.76 0.90 1 .00 1 .12 1.21 1 .30 12 HR 0.75 0.90 1 .00 1.13 1 .22 1.31 24 HR 0.66 0.87 1 .00 1.17 1 .29 1 .42 - 258 - TABLE I I . 3 4 DEPTH-DURATION-FREQUENCY DATA FOR PORT MELLON RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 ' 50 100 1 HR 18.5 20.9 22.5 24.5 25.9 27.4 2 HR 30.4 33.9 36.1 39.0 41.1 43.2 6 HR 65.0 73.6 79.3 86.5 91 .8 97. 1 12 HR 98.9 119.2 1 32.6 149.5 1 62. 1 1 74.7 24 HR 1 42.6 176.6 199.4 228.0 249.4 270.2 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.13 0.12 0.11 0.11 0.10 0.10 2 HR 0.21 0.19 0.18 0.17 0.16 0.16 6 HR 0.46 0.42 0.40 0.38 0.37 0.36 12 HR 0.69 0.67 0.66 0.66 0.65 0.65 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.82 0.93 1 .00 1 .09 1.16 1 .22 2 HR 0.84 0.94 1 .00 1 .08 1.14 1 .20 6 HR 0.82 0.93 1 .00 1 .09 1.16 1 .22 12 HR 0.75 0.90 1 .00 1.13 1 .22 1 .32 24 HR 0.71 0.89 1 .00 1.14 1 .25 1 .35 - 259 - TABLE 11.35 DEPTH-DURATION-FREQUENCY DATA FOR PORT MOODY GULF OIL RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 5 0 1 0 0 1 HR 1 0 . 6 1 3 . 2 1 5 . 0 1 7 . 2 1 8 . 8 2 0 . 4 2 HR 1 7 . 0 1 9 . 6 2 1 . 4 2 3 . 6 2 5 . 2 2 6 . 9 6 HR 3 8 . 3 4 4 . 3 4 8 . 4 5 3 . 5 5 7 . 2 61 . 0 12 HR 5 8 . 3 7 0 . 3 7 8 . 4 8 8 . 4 9 5 . 9 1 0 3 . 2 2 4 HR 8 4 . 5 1 0 5 . 4 1 1 9 . 0 1 3 6 . 3 1 4 9 . 0 1 6 1 . 8 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 2 5 5 0 1 0 0 1 HR 0 . 1 3 0 . 1 3 0 . 1 3 0 . 1 3 0 . 1 3 0 . 1 3 2 HR 0 . 2 0 0 . 1 9 0 . 1 8 0 . 1 7 0 . 1 7 0 . 1 7 6 HR 0 . 4 5 0 . 4 2 0 . 4 1 0 . 3 9 0 . 3 8 0 . 3 8 12 HR 0 . 6 9 0 . 6 7 0 . 6 6 0 . 6 5 0 . 6 4 0 . 6 4 2 4 HR 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 1 . 0 0 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2. 5 10 25 5 0 1 0 0 1 HR 0 . 7 1 0 . 8 8 1 . 0 0 1 .15 1 . 2 6 1 . 3 6 2 HR 0 . 7 9 0 . 9 2 1 . 0 0 1 . 1 0 1 . 1 8 1 . 2 6 6 HR 0 . 7 9 0 . 9 2 1 . 0 0 1 . 1 1 1 . 1 8 1 . 2 6 12 HR 0 . 7 4 0 . 9 0 1 . 0 0 1 . 1 3 1 . 2 2 1 . 3 2 2 4 HR 0 . 7 1 0 . 8 9 1 . 0 0 I T 1 5 1 . 2 5 1 . 3 6 - 260 - TABLE 11.36 DEPTH-DURATION-FREQUENCY DATA FOR PORT RENFREW BCFP RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 20.6 28.5 33.7 40.4 45.3 50. 1 2 HR 36.5 45.2 50.9 58.2 63.5 68.9 6 HR 80.0 96.0 106.7 120. 1 130.0 139.9 12 HR 1 22.3 142.6 1 56.0 172.9 185.5 198.0 24 HR 168.0 197.3 217.0 241 .4 259.7 277.9 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0 . 1 2 0.14 0. 16 0.17 0.17 0 .18 2 HR 0 .22 0.23 0.23 0.24 0.24 0 .25 6 HR 0 .48 0.49 0.49 0.50 0.50 0 .50 12 HR 0 .73 0.72 0.72 0.72 0.71 0 .71 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0 .61 0.84 1 .00 1 .20 1 .34 1 .49 2 HR 0 .72 0.89 1 .00 1.14 1 .25 1 .35 6 HR 0 .75 0.90 1 .00 1.13 1 .22 1 .31 12 HR 0 .78 0.91 1 .00 1.11 1.19 1 .27 24 HR 0 .77 0.91 1 .00 1.11 1 .20 1 .28 - 261 - TABLE 11.37 DEPTH -DURATION -FREQUENCY DATA FOR PRINCE RUPERT A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 12.3 14.3 15.7 17.4 1 8 . 6 19.8 2 HR 19.0 21 .8 23.7 26.1 27.8 29.6 6 HR 39. 1 49.3 56.0 64.6 70.9 77.2 12 HR 59.9 76.8 88. 1 1 02.2 1 12.8 123.2 24 HR 89.5 112.3 127.7 1 46.6 161.0 175.0 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.14 0.13 0.12 0.12 0.12 0.11 2 HR 0.21 0 . 1 9 0.19 0.18 0.17 0.17 6 HR 0.44 0.44 0.44 0.44 0.44 0.44 12 HR 0.67 0.68 0.69 0.70 0.70 0.70 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.79 0.92 1 .00 1.11 1 . 1 9 1 .27 2 HR 0.80 0.92 1 .00 1.10 1.17 1 .25 6 HR 0.70 0.88 1 .00 1.15 1 .27 1 .38 12 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 24 HR 0.70 0.88 1 .00 1.15 1 .26 1 .37 - 262 - TABLE I I . 3 8 DEPTH--DURATION--FREQUENCY DATA FOR SAANICH DENSMORE RAINFALL DATA FROM AES - RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 7.6 8.8 9.6 10.7 11.5 12.2 2 HR 12.5 14.3 15.5 17.0 18.2 19.3 6 HR 26.4 30.4 33.0 36.4 38.8 41 .2 12 HR 38.3 47.6 53.9 61 .8 67.6 73.4 24 HR 49.4 67.0 78.5 93. 1 1 03.9 115.0 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0 .15 0.13 0.12 0.11 0.11 0 . 11 2 HR 0 .25 0.21 0.20 0.18 0.17 0 . 1 7 6 HR 0 .53 0.45 0.42 0.39 0.37 0 .36 12 HR 0 .77 0.71 0.69 0.66 0.65 0 .64 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 • 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0 .78 0.91 1 .00 1.11 1.19 1 .27 2 HR 0 .80 0.92 1 .00 1.10 1.17 1 .24 6 HR 0 .80 0.92 1 .00 1.10 1.18 1 .25 12 HR. 0 .71 0.88 1 .00 1.15 1 .25 1 .36 24 HR 0 .63 0.85 1 .00 1.19 1 .32 1 .46 - 263 - TABLE II.39 DEPTH--DURATION -FREQUENCY DATA FOR SANDSPIT A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 10.4 12.3 13.5 15.0 16.2 17.3 2 HR 16.2 19.0 20.9 23.2 25.0 26.8 6 HR 30.8 36.5 40.2 44.9 48.5 52.0 12 HR 40.0 46.4 50.8 56.2 60. 1 64.2 24 HR 52. 1 59.8 65.0 71.3 76. 1 80.9 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.20 0.21 0.21 0.21 0.21 0.21 2 HR 0.31 0.32 0.32 0.33 0.33 0.33 6 HR 0.59 0.61 0.62 0.63 0.64 0.64 12 HR 0.77 0.78 0.78 0.79 0.79 0.79 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 Of) 1 HR 0.77 0.91 1 .00 1.11 1 .20 1 .28 2 HR 0.77 0.91 1 .00 1.11 1 .20 1 .28 6 HR 0.77 0.91 1 .00 1.12 1.21 1 .29 12 HR 0.79 0.91 1 .00 1.11 1.18 1 .26 24 HR 0.80 0.92 1 .00 1.10 1.17 1 .24 - 264 - TABLE II.40 DEPTH-DURATION-FREQUENCY DATA FOR SPRING ISLAND RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 15.4 18.7 20.9 23.7 25.7 27.8 2 HR 24. 1 27.8 30.3 33.4 35.7 38.0 6 HR 51 .5 61 .5 68.0 76.4 82.5 88.6 12 HR 81.4 1 04.8 120.4 140.0 1 54.6 1 69. 1 24 HR 121.7 156.7 179.8 209.0 230.9 252.5 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.13 0.12 0.12 0.11 0.11 0.11 2 HR 0.20 0.18 0.17 0.16 0.15 0.15 6 HR 0.42 0.39 0.38 0.37 0.36 0.35 12 HR 0.67 0.67 0.67 0.67 0.67 0.67 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.74 0.89 1 .00 1.13 1 .23 1 .33 2 HR 0.80 0.92 1 .00 1.10 1.18 1 .25 6 HR 0.76 0.90 1 .00 1.12 1.21 1 .30 12 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 24 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 - 265 - TABLE 11.41 DEPTH-DURATION-FREQUENCY DATA FOR STAVE FALLS RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 12.0 13.6 14.7 16.0 17.0 18.0 2 HR 18.4 21.4 23.4 25.8 27.7 29.5 6 HR 40.4 49.6 55.7 63.5 69.2 74.9 12 HR 62.3 78.8 89.8 103.6 113.9 1 24. 1 24 HR 83.8 106.3 121.4 1 40.4 1 54.3 168.2 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.14 0.13 0.12 0.11 0.11 0.11 2 HR 0.22 0.20 0.19 0.18 0 .18 0.18 6 HR 0.48 0.47 0.46 0.45 0.45 0.45 12 HR 0.74 0.74 0.74 0.74 0.74 0.74 2 4 HR 1 .00 1,00 1 .00 ' 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.82 0.93 1 .00 1 .09 1.16 1 .23 2 HR 0.79 0.92 1 .00 1.11 1.18 1 .26 6 HR 0.72 0.89 1 .00 1.14 1 .24 1 .34 12 HR 0.69 0.88 1 .00 1.15 1 .27 1 .38 24 HR 0.69 0.88 1 .00 1.16 1 .27 1 .39 - 266 - TABLE 11.42 DEPTH-DURATION-FREQUENCY DATA FOR STRATHCONA DAM RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 12.1 17.5 21 . 1 25.6 28.9 32.3 2 HR 17.1 21 .9 25.0 29.0 32.0 34.9 6 HR 31.4 38.6 43.5 49.6 54. 1 58.5 12 HR 45. 1 60.4 70.4 83.2 92.6 102.0 24 HR 61.7 88. 1 1 05.4 1 27.4 143.8 1 60. 1 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0 .20 0.20 0.20 0.20 0.20 0 .20 2 HR 0 .28 0.25 0.24 0.23 0.22 0 .22 6 HR 0 .51 0.44 0.41 0.39 0.38 0 .37 12 HR 0 .73 0.69 0.67 0.65 0.64 0 .64 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH [-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0 .57 0.83 1 .00 1.21 1 .37 1 .53 2 HR 0 .68 0.87 1 .00 1.16 1 .28 1 .39 6 HR 0 .72 0.89 1 .00 1.14 1 .24 1 .34 12 HR 0 .64 0.86 1 .00 1.18 1 .32 1 .45 24 HR 0 .59 0.84 1 .00 1 .21 1 .36 1 .52 - 2 6 7 - TABLE II.43 DEPTH-DURATION-FREQUENCY DATA FOR SURREY KWANTLEN PARK RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 11.1 14.5 16.8 19.7 21.8 24.0 2 HR 16.6 20.8 23.7 27.2 29.8 32.4 6 HR 32.6 39. 1 43.4 48.8 52.8 56.8 12 HR 48.7 60. 1 67.7 77.2 84.2 91.3 24 HR 67.9 89.3 1 03.4 121.4 1 34.9 148. 1 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.16 0.16 0.16 0.16 0.16 0.16 2 HR 0.24 0.23 0.23 0.22 0.22 0.22 6 HR 0.48 0.44 0.42 0.40 0.39 0.38 12 HR 0.72 0.67 0.65 0.64 0.62 0.62 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.66 0.86 1 .00 1.17 1 .30 1 .43 2 HR 0.70 0.88 1 .00 1.15 1 .26 1 .37 6 HR 0.75 0.90 1 .00 1.12 1 .22 1.31 12 HR 0.72 0.89 1 .00 1.14 1 .24 1 .35 24 HR 0.66 0.86 1 .00 1.17 1 .30 1 .43 - 268 - TABLE 11.44 DEPTH--DURATION -FREQUENCY DATA FOR SURREY MUNICIPAL HAL RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 9.3 12.4 14.5 17.2 19.1 21.1 2 HR 13.6 17.2 19.6 22.6 24.8 27.0 6 HR 27.5 33.7 37.9 43.1 47.0 50.8 12 HR 40.3 49.6 55.7 63.4 69. 1 74.8 24 HR 55.4 68.4 77.0 87.8 96.0 103.9 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0 .17 0.18 0.19 0.20 0.20 0 .20 2 HR 0 .25 0.25 0.25 0.26 0.26 0 .26 6 HR 0 .50 0.49 0.49 0.49 0.49 0 .49 12 HR 0 .73 0.72 0.72 0.72 0.72 0 .72 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0 .64 0.86 1 .00 1.18 1 .32 1 .45 2 HR 0 .70 0.88 1 .00 1.15 1 .27 1 .38 6 HR 0 .73 0.89 1 .00 1.14 1 .24 1 .34 12 HR 0 .72 0.89 1 .00 1.14 1 .24 1 .34 24 HR 0 .72 0.89 1 .00 1.14 1 .25 1 .35 - 269 - TABLE 11.45 DEPTH-DURATION-FREQUENCY DATA FOR TERRACE A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 10.1 13.6 1 5 . 8 18.7 20.8 22.9 2 HR 14.1 16.9 18.7 21.1 22.8 24.6 6 HR 25.8 32.6 37. 1 42.8 47.0 51.1 12 HR 38.8 52.3 61 .3 72.7 8 1 . 1 89.4 24 HR 55.7 79.4 95.3 115.2 129.8 144.5 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.18 0.17 0.17 0.16 0.16 0.16 2 HR 0.25 0.21 0.20 0.18 0.18 0.17 6 HR 0.46 0.41 0.39 0.37 0.36 0.35 12 HR 0.70 0.66 0.64 0.63 0.62 0.62 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.64 0.86 1 .00 1 .18 1 .31 1 .45 2 HR 0.75 0.90 1 .00 1.13 1 .22 1.31 6 HR 0.70 0.88 1 .00 1.15 1 .27 1 .38 12 HR 0.63 0.85 1 .00 1.19 1 .32 1 .46 24 HR 0.58 0.83 1 .00 1.21 1 .36 1 .52 - 270 - TABLE II.46 DEPTH-•DURATION -FREQUENCY DATA FOR TERRACE PCC RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 7.7 12.0 14.9 18.5 21.2 23.9 2 HR 12.1 18.7 23.2 28.8 32.9 37.0 6 HR 22.3 31.6 37.7 45.5 51 .4 57. 1 12 HR 33.4 45.2 53.0 62.9 70.2 77.5 24 HR 43.9 58.8 68.6 81.1 90.2 99.4 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.18 0.20 0.22 0.23 0.24 0.24 2 HR 0.28 0.32 0.34 0.35 0.36 0.37 6 HR 0.51 0.54 0.55 0.56 0.57 0.57 1 2 HR 0.76 0.77 0.77 0.78 0.78 0.78 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.52 0.81 1 .00 1 .24 1 .42 1 .60 2 HR 0.52 0.81 1 .00 1 .24 1 .42 1 .60 6 HR 0.59 0.84 1 .00 1.21 1 .36 1 .51 12 HR 0.63 0.85 1 .00 1.19 1 .32 1 .46 24 HR 0.64 0.86 1 .00 1.18 1.31 1 .45 - 271 - TABLE 11.47 DEPTH-DURATION-FREQUENCY DATA FOR TOFINO A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 17.8 20.7 22.7 25. 1 26.9 28.7 2 HR 28.0 31.5 33.8 36.8 38.9 41.1 6 HR 60.4 69.5 75.5 83.2 88.8 94.4 12 HR 87.0 99.7 1 08. 1 118.8 1 26.7 1 34.5 24 HR 128.2 1 57.0 176.4 200.6 218.6 236.6 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.14 0.13 0.13 0.13 0.12 0.12 2 HR 0.22 0.20 0.19 0.18 0.18 0.17 6 HR 0.47 0.44 0.43 0.41 0.41 0.40 12 HR 0.68 0.64 0.61 0.59 0.58 0 .57 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.79 0.91 1 .00 1.11 1.19 1 .27 2 HR 0.83 0.93 1 .00 1 .09 1.15 1 .22 6 HR 0.80 0.92 1 .00 1.10 1.18 1 .25 12 HR 0.80 0.92 1 .00 1.10 1.17 1 .24 24 HR 0.73 0.89 1 .00 1.14 1 .24 1 .34 - 272 - TABLE II.48 DEPTH-DURATION-FREQUENCY DATA FOR VANCOUVER HARBOUR RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 12.6 17.5 20.7 24.8 27.8 30.8 2 HR 17.5 22.7 26. 1 30. 5 33.7 36.9 6 HR 31.7 35.9 38.6 42. 1 44.6 47.2 12 HR 45.0 51 .6 56.0 61 .6 65.6 69.7 24 HR 62.2 75.8 85.0 96.2 104.6 113.3 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.20 0.23 0.24 0.26 0.27 0.27 2 HR 0.28 0.30 .0.31 0.32 0.32 0.33 6 HR 0.51 0.47 0.45 0.44 0.43 0.42 12 HR 0.72 0.68 0.66 0.64 0.63 0.62 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH [-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.61 0.84 1 .00 1 .20 1 .34 1 .49 2 HR 0.67 0.87 1 .00 1.17 1 .29 1.41 6 HR 0.82 0.93 1 .00 1 .09 1.16 1 .22 12 HR 0.80 0.92 1 .00 1.10 1.17 1 .24 24 HR 0.73 0.89 1 .00 1.13 1723 1 .33 - 273 - TABLE 11.49 DEPTH -DURATION--FREQUENCY DATA FOR VANCOUVER INT'L A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 10.1 12.7 14.4 16.6 18.3 19.9 2 HR 13.7 17.2 19.5 22.5 24.6 26.8 6 HR 25.7 30.4 33.5 37.4 40.3 43.3 12 HR 39.4 47.9 53.4 60.5 65.8 70.9 24 HR 52.8 66.5 75.4 86.6 95.0 103.4 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.19 0.19 0.19 0.19 0.19 0.19 2 HR 0.26 0.26 0.26 0.26 0.26 0.26 6 HR 0.49 0.46 0.45 0.43 0.42 0.42 12 HR 0.75 0.72 0.71 0.70 0.69 0.69 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.70 0.88 1 .00 1.15 1 .26 1 .38 2 HR 0.70 0.88 1 .00 1.15 1 .26 1 .37 6 HR 0.77 0.91 1 .00 1.12 1 .20 1 .29 12 HR 0.74 0.90 1 .00 1.13 1 .23 1 .33 24 HR 0.70 0.88 1 .00 1.15 1 .26 1 .37 - 2 7 4 - TABLE 11.50 DEPTH-DURATION-FREQUENCY DATA FOR VANCOUVER KITSILANO RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 9.7 11.8 13.3 15.1 16.4 17.8 2 HR 14.3 17.3 19.3 21.7 23.6 25.4 6 HR 30. 1 36.2 40.4 45.6 49.4 53.3 12 HR 45.6 55.3 61 .7 69.7 75.7 81 .6 24 HR 60.2 76.3 86.9 1 00. 1 110.2 1 20.0 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 1 00 1 HR 0.16 0.16 0.15 0.15 0.15 0.15 2 HR 0.24 0.23 0.22 0.22 0.21 0.21 6 HR 0.50 0.47 0.46 0.46 0.45 0.44 1 2 HR 0.76 0.72 0.71 0.70 0.69 0.68 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0.73 0.89 1 .00 1.14 1 .24 1 .34 2 HR 0.74 0.90 1 .00 1.13 1 .22 1 .32 6 HR 0.74 0.90 1 .00 1.13 1 .22 1 .32 12 HR 0.74 0.90 1 .00 1.13 1 .23 1 .32 24 HR 0.69 0.88 1 .00 1.15 1 .27 1 .38 - 275 - TABLE 11.51 DEPTH-DURATION-FREQUENCY DATA FOR VANCOUVER PMO RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 9.9 11.6 12.8 14.2 15.3 16.4 2 HR 15.6 18.1 19.7 21 .8 23.3 24.8 6 HR 33.2 39.4 43.6 48.8 52.6 56.5 12 HR 50.0 62.6 71 .0 81 .6 89.4 97.2 24 HR 68.6 94.3 111.6 133.0 1 49.0 164.9 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.14 0.12 0.11 0.11 0.10 0. 10 2 HR 0.23 0.19 0.18 0.1 6 0.16 0.15 6 HR 0.48 0.42 0.39 0.37 0.35 0.34 12 HR 0.73 0.66 0.64 0.61 0.60 0.59 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.77 0.91 1 .00 1.11 1 .20 1 .28 2 HR 0.79 0.92 1 .00 1.11 1.18 1 .26 6 HR 0.76 0.90 1 .00 1.12 1 .21 1 .30 12 HR 0.70 0.88 1 .00 1.15 1 .26 1 .37 24 HR 0.62 0.85 1 .00 1.19 1 .34 1 .48 - 276 - TABLE II.52 DEPTH-DURATION-FREQUENCY DATA FOR VANCOUVER UBC RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 10.0 13.1 15.1 17.6 19.5 21 .4 2 HR 14.0 16.8 18.7 21.1 22.8 24.6 6 HR 26.7 31.7 34.9 39. 1 42.2 45.2 12 HR 42. 1 52.0 58.4 66.7 72.8 79.0 24 HR 57.8 74.2 85.2 98.9 109.0 119.0 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.17 0.18 0.18 0.18 0.18 0.18 2 HR 0.24 0.23 0.22 0.21 0.21 0.21 6 HR 0.46 0.43 0.41 0.40 0.39 0.38 12 HR 0.73 0.70 0.69 0.67 0.67 0.66 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 * DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 4-0 25 50 100 1 HR 0.66 0.87 1 .00 1.17 1 .29 1 .42 2 HR 0.75 0.90 1 .00 1.13 1 .22 1.31 6 HR 0.76 0.91 1 .00 1.12 1.21 1 .30 12 HR 0.72 0.89 1 .00 1.14 1 .25 1 .35 24 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 - 277 - TABLE 11.53 DEPTH-DURATION-FREQUENCY DATA FOR VICTORIA GONZALES HT RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 7.2 9.1 10.4 12.0 13.2 14.4 2 HR 11.4 14.8 17.0 19.9 22.0 24. 1 6 HR 22.9 30.4 35.3 41 .6 46.2 50.8 1 2 HR 34.0 45.8 53.6 63.6 71 .0 78.4 24 HR 45. 1 63.8 76.3 91 .9 103.7 115.2 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.16 0.14 0.14 0.13 0.13 0.13 2 HR 0.25 0.23 0.22 0.22 0.21 0.21 6 HR 0.51 0.48 0.46 0.45 0.45 0.44 12 HR 0.75 0.72 0.70 0.69 0.69 0.68 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.69 0.88 1 .00 1.15 1 .27 1 .38 2 HR 0.67 0.87 1 .00 1.17 1 .29 1 .42 6 HR 0.65 0.86 1 .00 1 .18 1.31 1 .44 12 HR 0.63 0.85 1 .00 1.19 1 .32 1 .46 24 HR 0.59 0.84 1 .00 1 .20 1 .36 1 .51 - 278 - TABLE II.54 DEPTH--DURATION--FREQUENCY DATA FOR VICTORIA INT'L A RAINFALL DATA FROM AES RETURN PERIOD (YEARS) ' DURATION 2 5 10 25 50 100 1 HR 8.2 9.8 10.9 12.2 13.1 14.1 2 HR 12.6 14.7 16.1 17.9 19.2 20.5 6 HR 25.6 31.0 34.6 39.2 42.6 46.0 12 HR 38.0 47.0 53.0 60.6 66.2 71 .8 24 HR 49.4 63. 1 72.2 83.8 92.4 101.0 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.17 0.16 0.15 0.15 0.14 0.14 2 HR 0.25 0.23 0.22 0.21 0.21 0.20 6 HR 0.52 0.49 0.48 0.47 0.46 0.46 12 HR 0.77 0.75 0.73 0.72 0.72 0.71 24 HR ' 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0.76 0.90 1 .00 1.12 1 .21 1 .30 2 HR 0.78 0.91 1 .00 1.11 1.19 1 .27 6 HR 0.74 0.90 1 .00 1.13 1 .23 1 .33 12 HR 0.72 0.89 1 .00 1.14 1 .25 1 .35 24 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 - 279 - TABLE II.55 DEPTH-DURATION-FREQUENCY DATA FOR VICTORIA MARINE RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 9.4 11.8 13.4 15.4 16.9 18.4 2 HR 15.4 18.8 21.1 24.0 26. 1 28.2 6 HR 31.0 37.6 41.9 47.5 51.5 55.6 12 HR 45.8 55.6 62.0 70.3 76.3 82.4 24 HR 64.8 84.5 97.4 114.0 1 26.2 1 38.5 DEPTH-DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0 . 1 4 0.14 0.14 0.14 0.13 0 . 1 3 2 HR 0 .24 0.22 0.22 0.21 0.21 0 .20 6 HR 0 .48 0.44 0.43 0.42 0.41 0 .40 12 HR 0 .71 0.66 0.64 0.62 0.60 0 .60 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH-FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 0 .70 0.88 1 .00 1.15 1 .26 1 .37 2 HR 0 .73 0.89 1 .00 1.14 1 .24 1 .34 6 HR 0 .74 0.90 1 .00 1.13 1 .23 1 .33 12 HR 0 .74 0.90 1 .00 1.13 1 .23 1 .33 24 HR 0 .67 0.87 1 .00 1.17 1 .30 1 .42 - 280 - TABLE I I . 5 6 DEPTH-DURATION-FREQUENCY DATA FOR VICTORIA SHELBOURNE RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 8.0 9.6 10.8 12.2 13.2 14.2 2 HR 1 1 .7 13.7 15.0 16.6 17.8 19.0 6 HR 23.8 27.3 29.6 32.6 34.8 37.0 12 HR 33.4 42.8 49. 1 57.0 62.9 68.8 2 4 HR 44.9 61.7 73.0 87. 1 97.4 108.0 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.18 0.16 0.15 0.14 0.14 0.13 2 HR 0.26 0.22 0.21 0.19 0.18 0.18 6 HR 0.53 0.44 0.41 0.37 0.36 0.34 12 HR 0.74 0.69 0.67 0.65 0.65 0.64 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.74 0.90 1 .00 1.13 1 .23 1 .32 2 HR 0.78 0 .91 1 .00 1.11 1.19 1 .27 6 HR 0.80 0.92 1 .00 1.10 1.17 1 .25 12 HR 0.68 0.87 1 .00 1.16 1 .28 1 .40 24 HR 0.62 0.85 1 .00 1.19 1 .34 1 .48 - 281 - TABLE 11.57 DEPTH-DURATION-FREQUENCY DATA FOR VICTORIA U VIC RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 8.0 9.4 10.3 11.5 12.4 13.3 2 HR 12.4 14.5 15.9 17.6 18.9 20. 1 6 HR 26.6 33.3 37.7 43.3 47.4 51 .5 12 HR 40.0 49.8 56.3 64.6 70.7 76.7 24 HR 49.9 67.7 79.7 94.6 1 05.6 116.6 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.16 0.14 0.13 0.12 0.12 0.11 2 HR 0.25 0.21 0.20 0.19 0.18 0.17 6 HR 0.53 0.49 0.47 0.46 0.45 0.44 12 HR 0.80 0.74 0.71 0.68 0.67 0.66 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.77 0.91 1 .00 1.12 1 .20 1 .29 2 HR 0.78 0.91 1 .00 1.11 1.19 1 .27 6 HR 0.71 0.88 1 .00 1.15 1 .26 1 .37 12 HR 0.71 0.88 1 .00 1.15 1 .26 1 .36 24 HR 0.63 0.85 1 .00 1.19 1 .33 1 .46 - 282 - TABLE I I . 5 8 DEPTH-DURATION-FREQUENCY DATA FOR WHITE ROCK STP RAINFALL DATA FROM AES RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 100 1 HR 11.7 19.6 24.9 31 .5 36.4 41.2 2 HR 16.0 24.0 29.3 36.0 41.0 45.9 6 HR 27.8 35.3 40.3 46.6 51 .2 55.9 12 HR 36.6 46.6 53.0 61.4 67.6 73.7 24 HR 50.4 64.8 74.4 86.6 95.5 104.6 DEPTH -DURATION RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 10 25 50 1 00 1 HR 0.23 0.30 0.33 0.36 0.38 0.39 2 HR 0.32 0.37 0.39 0.42 0.43 0.44 6 HR 0.55 0.55 0.54 0.54 0.54 0.53 12 HR 0.73 0.72 0.71 0.71 0.71 0.70 24 HR 1 .00 1 .00 1 .00 1 .00 1 .00 1 .00 DEPTH -FREQUENCY RELATIONSHIPS RETURN PERIOD (YEARS) DURATION 2 5 1 0 25 50 100 1 HR 0.47 0.79 1 .00 1 .27 1 .46 1 .66 2 HR 0.55 0.82 1 .00 1 .23 1 .40 1 .57 6 HR 0.69 0.88 1 .00 1.15 1 .27 1 .39 12 HR 0.69 0.88 1 .00 1.16 1 .27 1 .39 24 HR 0.68 0.87 1 .00 1.16 1 .28 1 .41 - 283 - APPENDIX III MAXIMUM 24-HOUR RAINFALL ON RECORD AT BRITISH COLUMBIA COASTAL STATIONS - 284 - T A B L E I I I . 1 T I M E . D I S T R I B U T I O N OF R A I N F A L L ABBOTSFORD A MAXIMUM 2 4 - H O U R R A I N F A L L ON RECORD HOURLY CUM. PERCENT DATE HOUR R A I N R A I N OF 2 4 - H O U R Y R - M - D (MM) (MM) R A I N F A L L 79 12 16 19 2 . 3 2 . 3 2 . 79 12 16 2 0 2 . 7 5 . 0 5 . 79 12 16 21 1 . 7 6 . 7 7 . 79 12 16 2 2 1 . 1 7 . 8 8 . 7 9 12 16 2 3 1 . 7 9 . 5 1 0 . 79 12 16 2 4 2 . 3 1 1 . 8 1 2 . 79 12 17 1 2 . 9 1 4 . 7 1 5 . 7 9 12 17 2 5 . 7 2 0 . 4 21 . 79 12 17 3 7 . 5 2 7 . 9 2 8 . 79 12 17 4 8 . 0 3 5 . 9 3 6 . 79 12 17 5 6 . 8 4 2 . 7 4 3 . 7 9 12 17 6 6 . 8 4 9 . 5 5 0 . 7 9 12 17 7 1 0 . 3 5 9 . 8 61 . 7 9 12 17 8 7 . 1 6 6 . 9 6 8 . 7 9 12 17 9 2 . 6 6 9 . 5 7 0 . 7 9 12 17 1 0 8 . 5 7 8 . 0 7 9 . 7 9 12 17 1 1 5 . 9 8 3 . 9 8 5 . 79 12 17 1 2 1 . 4 8 5 . 3 8 6 . 79 12 17 1 3 3 . 3 8 8 . 6 9 0 . 79 12 17 1 4 1 . 2 8 9 . 8 91 . 79 12 17 1 5 1 . 6 9 1 . 4 9 3 . 7 9 12 17 1 6 2 . 4 9 3 . 8 9 5 . 79 12 17 1 7 3 . 1 9 6 . 9 9 8 . 79 12 17 18 1 . 9 9 8 . 8 1 0 0 . D U R A T I O N FOR I N D I C A T E D D U R A T I O N : MAX OCCURRING W I T H I N MAXIMUM DATE MAX 2 4 - H R R A I N F A L L ON RECORD % OF Y R - M - D (HOURS) (MM) 2 4 - H R (MM) 1 1 0 . 3 1 0 . 1 8 . 1 7 9 8 17 2 1 7 . 4 1 8 . 3 4 . 5 79 8 17 3 2 4 . 2 2 4 . 3 5 . 7 7 9 8 17 4 31 . 9 3 2 . 3 6 . 2 79 8 17 6 4 6 . 5 4 7 . 4 6 . 5 79 12 17 8 5 7 . 6 5 8 . 5 7 . 6 7 9 12 17 1 2 7 4 . 4 7 5 . 7 4 . 4 7 9 12 16 2 4 9 8 . 8 1 0 0 . 9 8 . 8 79 12 16 - 285 - TABLE I I I . 2 TIME DISTRIBUTION OF RAINFALL AGASSIZ CDA MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 75 12 1 21 1 .8 1 .8 2. 75 12 1 22 2.3 . 4.1 3. 75 12 1 23 6.4 10.5 9. 75 12 1 24 4.3 14.8 12. 75 12 2 1 3.6 18.4 15. 75 12 2 2 9.1 27.5 23. 75 12 2 3 6.6 34.1 29. 75 12 2 4 5.3 39.4 33. 75 12 2 5 4.8 44.2 37. 75 12 2 6 3.6 47.8 40. 75 12 2 7 4.6 52.4 44. 75 1 2 ~ 2 8 4.3 56.7 48. 75 12 2 9 5.1 61 .8 52. 75 12 2 10 4. 1 65.9 55. 75 12 2 1 1 3.8 69.7 58. 75 12 2 12 4.6 74.3 62. 75 12 2 13 3.3 77.6 65. 75 12 2 1 4 3.8 81.4 68. 75 12 2 1 5 4.6 86.0 72. 75 12 2 1 6 8.9 94.9 80. 75 12 2 1 7 8.9 103.8 87. 75 12 2 18 6.1 109.9 92. 75 12 2 19 5.6 115.5 97. 75 12 2 20 3.8 119.3 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 9.1 8. 16.6 79 10 27 2 17.8 15. 20.2 79 10 27 3 23.9 20. 24.4 62 2 3 4 29.5 25. 32.0 83 6 10 6 37.9 32. 39.6 79 12 9 8 45.8 38. 49.8 79 12 9 12 63. 1 53. 63.8 80 12 25 24 1 19.3 100. 119.3 75 12 1 - 286 - TABLE I I I . 3 TIME DISTRIBUTION OF RAINFALL ALOUETTE LAKE MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 81 10 30 23 4.8 4.8 3. 81 10 30 24 10.8 15.6 1 1 . 81 10 31 1 2.0 17.6 13. 81 10 31 2 0.2 17.8 13. 81 10 31 3 0.2 18.0 13. 81 10 31 4 0.4 18.4 13. 81 10 31 5 10.0 28.4 20. 81 10 31 6 15.2 43.6 31 . 81 10 31 7 9.6 53.2 38. 81 10 31 8 3.6 56.8 41 . 81 10 31 9 2.6 59.4 43. 81 10 31 10 4.0 63.4 45. 81 10 31 1 1 2.8 66.2 47. 81 10 31 12 3.2 69.4 50. 81 10 31 1 3 8.8 78.2 56. 81 10 31 14 7.2 85.4 61 . 81 10 31 1 5 10.8 96.2 69. 81 10 31 16 7.2 1 03.4 74. 81 10 31 1 7 5.2 1 08.6 78. 81 10 31 18 6.0 114.6 82. 81 10 31 19 5.6 1 20.2 86. 81 10 31 20 6.8 127.0 91 . 81 10 31 21 6.4 1 33.4 96. 81 10 31 22 6.0 139.4 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 15.2 1 1 . 18.4 79 9 4 2 25.2 18. 27.3 79 9 3 3 34.8 25. 34.8 81 10 31 4 38.4 28. 38.8 82 12 3 6 45.2 32. 54.3 80 12 25 8 57.6 41 . 67.2 80 12 25 1 2 85.0 61 . 96.6 80 12 25 24 139.4 100. 139.4 81 10 30 - 287 - TABLE I I I . 4 TIME DISTRIBUTION OF RAINFALL ALTA LAKE MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 75 10 1 6 17 2.8 2.8 3. 75 10 1 6 18 3.3 6.1 8. 75 10 16 19 4.1 10.2 13. 75 10 16 20 3.8 14.0 17. 75 10 1 6 21 4.6 18.6 23. 75 10 1 6 22 4.6 23.2 29. 75 10 16 23 4.6 27.8 35. 75 10 1 6 24 3.6 31 .4 39. 75 10 1 7 1 3.3 34.7 43. 75 10 1 7 2 4.1 38.8 48. 75 10 1 7 3 5.8 44.6 55. 75 10 17 4 4.6 49.2 61 . 75 10 1 7 5 3.0 52.2 65. 75 10 1 7 6 3.0 55.2 69. 75 10 1 7 7 2.8 58.0 72. 75 10 1 7 8 3.0 61 .0 76. 75 10 1 7 9 0.8 61 .8 77. 75 10 1 7 1 0 2.5 64.3 80. 75 10 1 7 1 1 3.0 67.3 84. 75 10 1 7 12 2.8 70. 1 87. 75 10 1 7 1 3 2.5 72.6 90. 75 10 1 7 14 2.8 75.4 94. 75 10 1 7 1 5 2.8 78.2 97. 75 10 1 7 16 2.3 80.5 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 5.8 7. 9.7 79 9 7 2 10.4 13. 15.7 78 7 26 3 14.5 18. 18.6 78 7 26 4 17.8 22. 24. 1 78 7 26 6 26.0 32. 27.8 81 10 31 8 35.2 44. 35.2 75 10 16 1 2 49.4 61 . 49.4 75 10 16 24 80.5 100. 80.5 75 10 16 - 288 - TABLE I I I . 5 TIME DISTRIBUTION OF RAINFALL BEAR CREEK MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 68 1 18 1 1 10.7 10.7 4. 68 1 18 1 2 12.2 22.9 8. 68 1 18 1 3 16.8 39.7 13. 68 1 18 1 4 12.4 52. 1 17. 68 1 18 1 5 14.2 66.3 22. 68 1 18 1 6 11.7 78.0 26. 68 1 18 1 7 11.7 89.7 30. 68 1 18 18 11.9 101.6 34. 68 1 18 19 22.9 124.5 41 . 68 1 18 20 10.7 135.2 45. 68 1 18 21 10.2 145.4 48. 68 1 18 22 20.8 166.2 55. 68 1 18 23 10.2 176.4 59. 68 1 18 24 7.4 183.8 61 . 68 1 19 1 13.2 197.0 66. 68 1 19 2 18.0 215.0 72. 68 1 19 3 12.7 227.7 76. 68 1 19 4 8.6 236.3 79. 68 1 19 5 9.9 246.2 82. 68 1 19 6 11.2 257.4 86. 68 1 19 7 11.9 269.3 90. 68 1 19 8 8.9 278.2 93. 68 1 19 9 11.4 289.6 96. 68 1 19 1 0 10.9 300.5 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF .YR-M-D (HOURS) (MM) 24-HR (MM) 1 22.9 8. 48.8 66 12 11 2 34.8 12. 56.4 67 12 10 3 46.5 15. 76.7 67 12 10 4 64.6 21 . 90.4 67 12 10 6 88.2 29. 98.3 67 12 10 8 114.1 38. 114.1 68 1 1 8 12 166.2 55. 1 66.2 68 1 1 8 24 300.5 1 00. 300.5 68 1 1 8 - 289 - TABLE I I I . 6 TIME DISTRIBUTION OF RAINFALL BELLA COOLA HYDRO MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 75 10 30 24 9.1 9.1 7. 75 10 31 1 6.4 15.5 12. 75 10 31 2 3w0 18.5 14. 75 10 31 3 4. 1 22.6 17. 75 10 31 4 2.5 25. 1 19. 75 10 31 5 1 .0 26. 1 20. 75 10 31 6 4.3 30.4 23. 75 10 31 7 4.6 35.0 27. 75 10 31 8 4.3 39.3 30. 75 10 31 9 0.0 39.3 30. 75 10 31 10 4. 1 43.4 33. 75 10 31 1 1 4.1 47.5 36. 75 10 31 12 2.8 50.3 38. 75 10 31 13 3.8 54. 1 41 . 75 10 31 1 4 3.6 57.7 44. 75 10 31 1 5 4.6 62.3 47. 75 10 31 1 6 5.6 67.9 52. 75 10 31 17 5.8 73.7 56. 75 10 31 18 5.8 79.5 61 . 75 10 31 19 10.2 89.7 68. 75 10 31 20 13.2 102.9 78. 75 10 31 21 14.0 116.9 89. 75 10 31 22 7.6 124.5 95. 75 10 31 23 6.9 131.4 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 14.0 1 1 . 15.0 76 10 27 2 27.2 21 . 27.2 75 10 31 3 37.4 28. 37.4 75 10 31 4 45.0 34. 45.0 75 10 31 6 57.7 44. 57.7 75 10 31 8 69. 1 53. 69. 1 75 10 31 12 83.9 64. 91 .3 71 11 18 24 131 .4 1 00. 131.4 75 10 30 - 290 - TABLE I I I . 7 TIME DISTRIBUTION OF B U N T Z E N L A K E RAINFALL MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 81 10 30 21 7.7 7.7 3. 81 10 30 22 9.0 16.7 6. 81 10 30 23 0.5 17.2 7. 81 10 30 24 0.2 17.4 7. 81 10 31 1 2.4 19.8 8. 81 10 31 2 10.1 29.9 12. 81 10 31 3 15.3 45.2 17. 81 10 31 4 11.0 56.2 22. 81 10 31 5 8.8 65.0 25. 81 10 31 6 15.3 80.3 31 . 81 10 31 7 13.1 93.4 36. 81 10 31 8 17.5 110.9 43. 81 10 31 9 13.1 124.0 48. 81 10 31 10 13.4 137.4 53. 81 10 31 1 1 1 1 .7 149. 1 58. 81 10 31 1 2 13.9 1 63.0 63. 81 10 31 1 3 13.0 176.0 68. 81 10 31 1 4 15.2 191.2 74. 81 10 31 1 5 11.7 202.9 78. 81 10 31 16 11.7 214.6 83. 81 10 31 1 7 11.9 226.5 87. 81 10 31 18 8.3 234.8 91 . 81 10 31 19 15.8 250.6 97. 81 10 31 20 8.5 259. 1 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 17.5 7. 27.7 79 6 30 2 30.6 12. 33.5 76 11 17 3 45.9 18. 45.9 81 10 31 4 59.0 23. 59.0 81 10 31 6 84. 1 32. 84. 1 81 10 31 8 111.0 43. 111.0 81 10 31 12 161.5 62. 161.5 81 10 31 24 259. 1 100. 259. 1 81 10 30 291 TABLE I I I . 8 TIME DISTRIBUTION OF RAINFALL BURNABY MTN BCHPA MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 79 12 1 6 23 1 .4 1 .4 1 . 79 12 16 24 1 .6 3.0 2. 79 12 1 7 1 2.8 5.8 5. 79 12 17 2 2.4 8.2 7. 79 12 1 7 3 4.0 12.2 10. 79 12 17 4 4.8 17.0 14. 79 12 17 5 5.2 22.2 18. 79 12 17 6 4.8 27.0 22. 79 12 1 7 7 5.6 32.6 27. 79 12 17 8 6.0 38.6 32. 79 12 17 9 9.4 48.0 39. 79 12 17 10 7.6 55.6 45. 79 12 1 7 - 1 1 9.4 65.0 53. 79 12 1 7 1 2 9.6 74.6 61 . 79 12 17 1 3 8.0 82.6 68. 79 12 17 1 4 6.8 89.4 73. 79 12 1 7 1 5 4.0 93.4 76. 79 12 1 7 1 6 8.8 1 02.2 84. 79 12 17 1 7 4.0 106.2 87. 79 12 17 18 3.4 109.6 90. 79 12 17 19 3.0 112.6 92. 79 12 17 20 3.6 116.2 95. 79 12 1 7 21 2.0 118.2 97. 79 12 17 22 4.0 122.2 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 9.6 8. 15.0 78 6 13 2 19.0 16. 21 .9 77 11 28 3 27.0 22. 27.2 77 11 28 4 36.0 29. 36.0 79 12 17 6 50.8 . 42. 50.8 79 12 17 8 63.6 52. 63.6 79 12 17 12 85.2 70. 85.2 79 12 17 24 122.2 100. 122.2 79 12 16 - 292 - TABLE I I I . 9 TIME DISTRIBUTION OF RAINFALL CAMPBELL RIVER BCFS MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOU] YR-M- D (MM) (MM) RAINFALL 69 1 1 6 20 0.3 0.3 0. 69 1 1 6 21 1 .3 1 .6 2. 69 1 1 6 22 3.3 4.9 6. 69 1 1 6 23 7.9 12.8 17. 69 1 1 6 24 3.0 15.8 21 . 69 1 1 7 1 6.6 22.4 30. 69 1 1 7 2 7.4 29.8 40. 69 1 1 7 3 10.9 40.7 54. 69 1 1 7 4 9.7 50.4 67. 69 1 1 7 ' 5 6.1 56.5 75. 69 1 1 7 6 3.6 60. 1 80. 69 1 1 7 7 3.3 63.4 84. 69 1 1 7 8 2.0 65.4 87. 69 1 1 7 9 1 .3 66.7 88. 69 1 1 7 10 1.3 68.0 90. 69 1 1 7 1 1 2.3 70.3 93. 69 1 1 7 1 2 2.0 72.3 96. 69 1 1 7 1 3 1 .5 73.8 98. 69 1 1 7 1 4 0.5 74.3 99. 69 1 1 7 1 5 0.8 75. 1 100. 69 1 1 7 16 0.3 75.4 1 00. 69 1 1 7 1 7 0.0 75.4 1 00. 69 1 1 7 18 0.0 75.4 1 00. 69 1 1 7 19 0.0 75.4 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 10.9 14. 12.7 75 8 26 2 20.6 27. 20.6 75 8 26 3 28.0 37. 28.5 75 11 14 4 34.6 46. 34.6 69 1 1 7 6 45.5 60. 45.5 69 1 1 6 8 55.2 73. 55.2 69 1 1 6 12 65. 1 86. 65. 1 69 1 1 6 24 75.4 100. 75.4 69 1 1 6 - 293 - TABLE 111 . 1 0 TIME DISTRIBUTION OF RAINFALL CAMPBELL RIVER BCHPA MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR--M-D (MM) (MM) RAINFALL 79 2 24 6 4.0 4.0 5. 79 2 24 . 7 4.4 8.4 1 1 . 79 2 24 8 5.2 13.6 18. 79 2 24 9 4.5 18.1 23. 79 2 24 10 4.5 22.6 29. 79 2 24 1 1 2.7 25.3 33. 79 2 24 1 2 3.6 28.9 37. 79 2 24 1 3 4.5 33.4 43. 79 2 24 1 4 3.3 36.7 47. 79 2 24 1 5 4.5 41.2 53. 79 2 24 16 2.2 43.4 56. 79 2 24 17 4.1 47.5 61 . 79 2 24 18 4.1 51 .6 67. 79 2 24 19 3.6 55.2 71 . 79 2 24 20 5.8 61 .0 79. 79 2 24 21 5.0 66.0 85. 79 2 24 22 3.1 69. 1 89. 79 2 24 23 1 .6 70.7 91 . 79 2 24 24 0.3 71 .0 92. 79 2 25 1 1 . 1 72. 1 93. 79 2 25 2 0.5 72.6 94. 79 2 25 3 1 .6 74.2 96. 79 2 25 4 2.0 76.2 99. 79 2 25 5 1 . 1 77.3 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM 'DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 5.8 8. 23. 1 79 9 1 2 10.8 14. 25.9 77 12 12 3 14.4 19. 34.0 77 12 12 4 18.6 24. 35.8 77 12 12 6 25.7 33. 41 .2 80 3 13 8 33.4 43. 47.0 80 3 13 12 47.9 62. 59.3 77 10 31 24 77.3 100. 77.3 79 2 24 - 294 - TABLE 111 . 1 1 TIME DISTRIBUTION OF RAINFALL CARNATION CREEK MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 8 1 1 0 30 1 2 8.1 8.1 5. 8 1 1 0 30 13 9.0 17.1 1 1 . 8 1 10 30 1 4 15.5 32.6 20. 8 1 1 0 30 1 5 4.5 37. 1 23. 8 1 1 0 30 1 6 6.4 43.5 27. 8 1 10 30 17 6.2 49.7 31 . 8 1 10 30 18 5.4 55. 1 34. 8 1 10 30 19 4.9 60.0 37. 8 1 10 30 20 5.1 65. 1 40. 8 1 10 30 21 1 .5 66.6 41 . 8 1 10 30 22 1 .3 67.9 42. 8 1 10 30 23 2.4 70.3 43. 8 1 10 30 24 8.1 78.4 48. 8 1 10 31 1 4.7 83. 1 51 . 8 1 10 31 2 4.5 87.6 54. 8 1 1 0 31 3 1 .9 89.5 55. 8 1 1 0 31 4 4.5 94.0 58. 8 1 10 31 5 10.7 104.7 64. 8 1 10 31 6 15.4 1 20. 1 74. 8 1 10 31 7 9.4 1 29.5 80. 8 1 10 31 8 8.8 1 38.3 85. 8 1 10 31 9 7.9 1 46.2 90. 8 1 10 31 1 0 7.9 1 54. 1 95. 8 1 10 31 1 1 8.3 162.4 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 15.5 10. 16.0 80 1 1 1 2 26. 1 16. 26.4 77 2 11 3 35.5 22. 38.6 77 2 11 4 44.3 27. 50.8 77 2 11 6 60. 1 37. 67.4 77 2 11 8 72.9 45. 83.9 77 2 11 12 92.1 57. 97.8 77 2 11 24 1 62.4 100. 1 62.4 81 10 30 - 295 - TABLE 111 . 1 2 TIME DISTRIBUTION OF RAINFALL CHILLIWACK MICROWAVE MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 66 10 18 21 3.0 3.0 4. 66 10 18 22 3.6 6.6 8. 66 10 18 23 3.3 9.9 12. 66 10 18 24 3.8 13.7 17. 66 10 19 1 3.0 16.7 20. 66 10 19 2 3.6 20.3 25. 66 10 19 3 3.0 2 3". 3 29. 66 10 19 4 3.8 27. 1 33. 66 10 19 5 3.8 30.9 38. 66 10 19 6 3.0 33.9 42. 66 10 19 7 3.3 37.2 46. 66 10 19 8 4. 1 41 .3 51 . 66 10 19 9 5.1 46.4 57. 66 10 19 10 3.6 50.0 61 . 66 10 19 1 1 4.3 54.3 67. 66 10 19 1 2 3.0 57.3 70. 66 10 1 9 1 3 2.8 60. 1 74. 66 10 19 1 4 2.5 62.6 77. 66 10 19 1 5 2.8 65.4 80. 66 10 19 16 2.5 67.9 83. 66 10 19 17 3.3 71.2 87. 66 10 19 18 2.8 74.0 91 . 66 10 19 19 3.3 77.3 95. 66 10 19 20 4.3 81.6 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF- YR-M-D (HOURS) (MM) 24-HR (MM) 1 5.1 6. 18.0 66 1 6 2 9.2 1 1 . 21 .3 69 1 1 4 3 13.0 16. 24.9 63 10 21 4 17.1 21 . 29.0 63 10 21 6 23.4 29. 39.4 63 10 21 8 31.0. 38. 44. 1 69 1 1 4 12 44.4 54. 58.0 69 1 1 4 24 81 .6 100. 81 .6 66 10 18 - 296 - TABLE I I I . 1 3 TIME DISTRIBUTION OF RAINFALL CLOWHAM FALLS MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR--M-D (MM) (MM) RAINFALL 79 2 24 1 2 3.0 3.0 3. 79 2 24 13 3.8 6.8 6. 79 2 24 1 4 3.2 10.0 9. 79 2 24 1 5 5.1 15.1 14. 79 2 24 1 6 5.1 20.2 18. 79 2 24 17 5.7 25.9 23. 79 2 24 18 6.2 32. 1 29. 79 2 24 19 4.4 36.5 33. 79 2 24 20 3.8 40.3 36. 79 2 24 21 4.9 45.2 41 . 79 2 24 22 6.8 52.0 47. 79 2 24 23 5.4 57.4 51 . 79 2 24 24 6.8 64.2 58. 79 2 25 1 4.3 68.5 61 . 79 2 25 2 4.2 72.7 65. 79 2 25 3 4.3 77.0 69. 79 2 25 4 3.0 80.0 72. 79 2 25 5 4.0 84.0 75. 79 2 25 6 4.3 88.3 79. 79 2 25 7 4.0 92.3 83. 79 2 25 8 3.5 95.8 86. 79 2 25 9 5.2 101.0 91 . 79 2 25 10 5.8 106.8 96. 79 2 25 1 1 4.7 111.5 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 6.8 6. 20. 1 75 8 28 2 12.2 1 1 . 26.0 80 11 6 3 19.0 17. 32.2 78 1 1 7 4 23.9 21 . 42.7 78 1 1 7 6 32.4 29. 54.2 78 1 1 7 8 44.0 39. 64.8 78 1 1 7 12 62.7 56. 72.3 78 1 1 7 24 111.5 100. 111.5 79 2 24 - 297 - TABLE I IT . 1 4 TIME DISTRIBUTION OF RAINFALL COMOX A MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN" OF 24-HOU1 YR-•M-D (MM) (MM) RAINFALL 83 2 1 0 18 1 .2 1 .2 1 . 83 2 1 0 19 3.2 4.4 5. 83 2 1 0 20 2.6 7.0 8. 83 2 1 0 21 2.5 9.5 1 1 . 83 2 1 0 22 3.7 13.2 15. 83 2 1 0 23 1 .6 14.8 17. 83 2 1 0 24 3.2 18.0 20. 83 2 1 1 1 7.3 25.3 28. 83 2 1 1 2 6.9 32.2 36. 83 2 1 1 3 5.9 38. 1 43. 83 2 1 1 4 4.4 42.5 47. 83 2 1 1 5 5.7 48.2 54. 83 2 1 1 6 5.9 54. 1 60. 83 2 1 1 7 5. 1 59.2 66. 83 2 1 1 8 4.8 64.0 71 . 83 2 1 1 9 3.0 67.0 75. 83 2 1 1 10 4.2 71.2 79. 83 2 1 1 1 1 3.8 75.0 84. 83 2 1 1 1 2 3.2 78.2 87. 83 2 1 1 1 3 3.0 81.2 91 . 83 2 1 1 1 4 2.4 83.6 93. 83 2 1 1 1 5 2.2 85.8 96. 83 2 1 1 1 6 1 .6 87.4 98. 83 2 1 1 17 2.2 89.6 100. • DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24 -HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 7. 3 8. 16.8 83 11 3 2 14. 2 16. 24.8 83 1 1 3 3 20. 1 22. 27.0 62 6 1 4 24. 5 27. 32.0 81 11 14 6 36. 1 40. 37.2 81 11 14 8 46. 0 51 . 46.0 83 2 11 12 60. 2 67. 60.2 83 2 11 24 89. 6 100. 89.6 83 2 10 - 298 - TABLE 111 . 1 5 TIME DISTRIBUTION OF COQUITLAM LAKE RAINFALL MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 81 -10 30 21 7.6 7.6 3. 81 10 30 22 10.1 17.7 8. 81 10 30 23 0.8 18.5 8. 81 10 30 24 1 . 1 19.6 9. 81 10 31 1 1 .3 20.9 9. 81 10 31 2 10.6 31.5 14. 81 10 31 3 10.6 42. 1 19. 81 10 31 4 16.9 59.0 26. 81 10 31 5 10.6 69.6 31 . 81 10 31 6 10.6 80.2 35. 81 10 31 7 11.8 92.0 41 . 81 10 31 8 10.1 102. 1 45. 81 10 31 9 11.7 113.8 50. 81 10 31 10 10.7 124.5 55. 81 10 31 1 1 10.7 1 35.2 60. 81 10 31 1 2 9.3 1 44.5 64. 81 10 31 13 9.8 154.3 68. 81 10 31 14 11.5 165.8 73. 81 10 31 1 5 9.8 175.6 77. 81 10 31 1 6 11.9 187.5 83. 81 10 31 17 9.3 196.8 87. 81 10 31 18 10.7 207.5 92. 81 10 31 1 9 10.6 218.1 96. 81 10 31 20 8.5 226.6 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 16.9 7. 16.9 81 10 31 2 27.5 12. 28.0 73 10 13 3 38. 1 17. 39. 1 77 11 13 4 49.9 22. 51.1 73 10 13 6 71 .7 32. 71.7 81 10 31 8 93. 1 41 . 93. 1 81 10 31 1 2 1 34.3 59. 1 34.3 81 10 31 24 226.6 100. 226.6 81 10 30 - 299 - TABLE 111 . 1 6 TIME DISTRIBUTION OF RAINFALL COURTNEY PUNTLEDGE MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 68 1 17 17 3.8 3.8 3. 68 1 17 18 5. 1 8.9 7. 68 1 17 19 3.6 12.5 10. 68 1 17 20 2.3 14.8 12. 68 1 17 21 5.3 20. 1 16. 68 1 1 7 22 5.8 25.9 21 . 68 1 1 7 23 5.6 31.5 25. 68 1 1 7 24 4.8 36.3 29. 68 1 18 1 4.8 41.1 33. 68 1 18 2 7.9 49.0 39. 68 1 18 3 5.6 54.6 44. 68 1 18 4 6.9 61.5 49. 68 1 18 5 6.4 67.9 55. 68 1 18 6 7.6 75.5 61 . 68 1 18 7 5. 1 80.6 65. 68 1 18 8 3.6 84.2 68. 68 1 18 9 3.6 87.8 71 . 68 1 18 10 6.6 94.4 76. 68 1 18 1 1 5.6 100.0 80. 68 1 18 12 4.1 1 04. 1 84. 68 1 18 1 3 3.0 107. 1 86. 68 1 18 1 4 5.1 112.2 90. 68 1 18 1 5 4.6 116.8 94. 68 1 18 16 7.6 124.4 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 7.9 6. 14.0 80 7 1 0 2 14.0 1 1 . 23.8 80 7 10 3 20.9 17. 29.2 83 9 10 4 26.8 22. 36.4 83 9 10 6 39.5 32. 53.6 83 11 14 8 49.6 40. 64.0 83 11 14 12 69.4 56. 86.8 83 11 14 24 124.4 100. 124.4 68 1 1 7 - 300 - TABLE 111 . 1 7 TIME DISTRIBUTION OF RAINFALL DAISY LAKE DAM MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 75 10 1 6 12 3.6 3.6 4. 75 10 1 6 1 3 2.8 6.4 7. 75 10 1 6 1 4 3.0 9.4 ' 10. 75 10 1 6 1 5 3.6 13.0 13. 75 10 16 1 6 3.0 16.0 16. 75 10 16 1 7 3.6 19.6 20. 75 10 16 18 4.1 23.7 24. 75 10 16 19 4.3 28.0 29. 75 10 1 6 20 4.3 32.3 33. 75 10 1 6 21 5.1 37.4 38. 75 10 1 6 22 6.1 43.5 44. 75 10 16 23 4.6 48. 1 49. 75 10 1 6 24 3.6 51 .7 53. 75 10 1 7 1 4.1 55.8 57. 75 10 1 7 2 4. 1 59.9 61 . 75 10 1 7 3 7.1 67.0 69. 75 10 1 7 4 6.4 73.4 75. 75 10 1 7 5 4.8 78.2 80. 75 10 1 7 6 3.6 81 .8 84. 75 10 1 7 7 3.6 85.4 87. 75 10 1 7 8 3.8 89.2 91 . 75 10 1 7 9 3.3 92.5 95. 75 10 1 7 1 0 2.3 94.8 97. 75 10 1 7 1 1 3.0 97.8 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 7.1 7. 18.8 68 10 29 2 13.5 14. 27.9 69 4 4 3 18.3 19. 41 . 1 69 4 4 4 22.4 23. 46.2 69 4 4 6 30. 1 31 . 53.8 69 4 4 8 41 . 1 42. 57.3 69 4 4 12 58.6 60. 60.9 69 4 4 24 97.8 100. 97.8 75 10 16 - 301 - TABLE 111 . 1 8 TIME DISTRIBUTION OF RAINFALL ESTAVAN POINT MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 78 1 1 6 2 3.4 3.4 2. 78 1 1 6 3 4.4 7.8 4. 78 1 1 6 4 7.2 15.0 7. 78 1 1 6 5 8.2 23.2 1 1 . 78 1 1 6 6 7.2 30.4 14. 78 1 1 6 7 7.7 38. 1 18. 78 1 1 6 8 11.4 49.5 24. 78 1 1 6 9 14.7 64.2 31 . 78 1 1 6 10 13.9 78. 1 37. 78 1 1 6 1 T 13.7 91 .8 44. 78 1 1 6 12 8.7 1 00.5 48. 78 1 1 6 13 3.7 1 04.2 50. 78 1 1 6 1 4 1 .4 1 05.6 50. 78 1 1 6 15 3.9 1 09.5 52. 78 1 1 6 16 3.7 113.2 54. 78 1 1 6 17 12.1 1 25.3 60. 78 1 1 6 18 15.9 141.2 67. 78 1 1 6 19 11.9 1 53. 1 73. 78 1 1 6 20 12.4 1 65.5 79. 78 1 1 6 21 8.2 173.7 83. 78 1 1 6 22 15.9 189.6 90. 78 1 1 6 23 9.7 199.3 95. 78 1 1 6 24 6.9 206.2 98. 78 1 1 7 1 4.1 210.3 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 15.9 8. 27.7 7 1 1 1 2 2 28.6 14. 43.7 7 1 1 1 2 3 42.3 20. 56.7 7 1 1 1 2 4 53.7 26. 68. 1 7 1 1 1 2 6 76.4 36. 80. 1 7 1 1 1 2 8 93.0 44. 98.5 69 11 19 12 116.0 55. . 131.3 69 11 19 24 210.3 100. 210.3 78 11 6 - 302 - TABLE I I I . 1 9 TIME DISTRIBUTION OF RAINFALL HANEY MICROWAVE MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 68 1 18 14 4.8 4.8 3. 68 1 18 15 6.6 11.4 8. 68 1 18 16 5.6 17.0 12. 68 1 18 1 7 5.8 22.8 16. 68 1 18 18 4.3 27. 1 19. 68 1 18 19 3.6 30.7 22. 68 1 18 20 4.6 35.3 25. 68 1 18 21 4.6 39.9 28. 68 1 18 22 3.8 43.7 31 . 68 1 18 23 3.0 46.7 33. 68 1 18 24 4. 1 50.8 36. 68 1 19 1 4.6 55.4 39. 68 1 19 2 4.3 - 59.7 42. 68 1 19 3 6.6 66.3 47. 68 1 19 4 10.2 76.5 54. 68 1 19 5 8.9 85.4 60. 68 1 19 6 6.9 92.3 65. 68 1 19 7 6.9 99.2 70. 68 1 19 8 8.6 107.8 76. 68 1 1 9 9 6.6 114.4 80. 68 1 1 9 10 4.8 119.2 84. 68 1 1 9 1 1 9.4 1 28.6 90. 68 1 19 12 8.1 1 36.7 96. 68 1 19 1 3 5.6 1 42.3 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 10.2 7. 19.3 65 1 1 3 2 19.1 13. 27.9 69 1 1 3 26.0 18. 35.6 64 11 29 4 32.9 23. 40.7 64 11 29 6 48. 1 34. 50.6 80 12 25 8 62.3 44. 63.0 80 12 25 1 2 86.9 61 . 87.4 80 12 25 24 142.3 100. 142.3 68 1 18 - 303 - TABLE I I I . 2 0 TIME DISTRIBUTION OF RAINFALL HANEY UBC MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT. DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 68 1 18 1 4 5.8 5.8 4. 68 1 18 15 5.6 11.4 8. 68 1 18 1 6 5.3 16.7 12. 68 1 18 1 7 5.3 22.0 16. 68 1 18 18 5.8 27.8 20. 68 1 18 19 3.6 31.4 23. 68 1 18 20 3.0 34.4 25. 68 1 18 21 4. 1 38.5 28. 68 1 18 22 4. 1 42.6 31 . 68 1 18 23 5.6 48.2 35. 68 1 18 24 5.6 53.8 39. 68 1 19 1 5.6 59.4 43. 68 1 19 2 5.1 64.5 47. 68 1 19 3 6.9 71 .4 52. 68 1 19 4 7.9 79.3 58. 68 1 19 5 5.8 85. 1 62. 68 1 19 6 5.3 90.4 66. 68 1 19 7 6.9 97.3 71 . 68 1 19 8 6.1 1 03.4 75. 68 1 19 9 6.9 110.3 80. 68 1 19 10 7.1 117.4 85. 68 1 19 1 1 6.4 1 23.8 90. 68 1 19 12 6.9 130.7 95. 68 1 19 1 3 7.1 1 37.8 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 7.9 6. 17.3 73 6 24 2 14.8 1 1 . 29. 1 83 7 11 3 20.6 15. 34.9 83 7 1 1 4 27.5 20. 40.7 83 7 11 6 40.5 29. 53.2 68 9 16 8 52.9 38. 63.3 68 9 16 12 78.4 57. 84. 1 79 12 17 24 137.8 100. 137.8 68 1 18 - 304 - TABLE 111.21 TIME DISTRIBUTION OF RAINFALL JORDAN RIVER DIVERSION MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 79 12 13 16 17.3 17.3 5. 79 12 13 17 11.2 28.5 8. 79 12 13 18 5.6 34. 1 10. 79 12 1 3 19 6.6 40.7 1 2 . 79 12 13 20 9.1 49.8 15. 79 12 13 21 11.7 61 .5 18. 79 12 13 22 10.2 71.7 21 . 79 12 1 3 23 11.7 83.4 24. 79 12 1 3 24 10.2 93.6 27. 79 12 14 1 13.5 107. 1 31 . 79 12 1 4 2 12.4 119.5 35. 79 12 1 4 3 18.0 1 37.5 40. 79 12 1 4 4 17.3 154.8 45. 79 12 14 5 15.7 170.5 50. 79 12 1 4 6 19.8 190.3 56. 79 12 1 4 7 18.5 208.8 61 . 79 12 1 4 8 7.9 216.7 64. 79 12 14 9 14.7 231 .4 68. 79 12 14 10 10.2 241 .6 71 . 79 12 1 4 1 1 15.2 256.8 75. 79 12 1 4 12 13.2 270.0 79. 79 12 1 4 1 3 21.3 291 .3 85. 79 12 14 1 4 21 .8 313.1 92. 79 12 14 15 27.9 341 .0 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 27.9 8. 48.3 76 1 1 4 2 49.7 15. 55.8 80 1 1 2 3 71 .0 21 . 73.2 80 12 26 4 84.2 25. 97.6 80 12 26 6 109.6 32. 136.3 80 12 26 8 132.2 39. 172.9 80 12 26 12 203.5 60. 219.6 80 12 25 24 341 .0 100. 341 .0 79 12 13 - 305 - TABLE 111. 22 TIME DISTRIBUTION OF RAINFALL JORDAN RIVER GENERATING MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 72 12 25 4 1 .3 1 .3 1 . 72 12 25 5 3.0 4.3 2. 72 12 25 6 4. 1 8.4 5. 72 12 25 7 5.1 13.5 8. 72 12 25 8 15.0 28.5 16. 72 12 25 9 9.1 37.6 21 . 72 12 25 10 8.4 46.0 26. 72 12 25 1 1 6.6 52.6 29. 72 12 25 1 2 5.1 57.7 32. 72 12 25 1 3 4.3 62.0 34. 72 12 25 1 4 6.9 68.9 38. 72 12 25 1 5 6.1 75.0 42. 72 12 25 16 3.6 78.6 44. 72 12 25 1 7 7.6 86.2 48. 72 12 25 18 15.7 101.9 57. 72 12 25 19 15.0 116.9 65. 72 12 25 20 13.5 130.4 72. 72 12 25 21 7.6 1 38.0 77. 72 12 25 22 9.9 147.9 82. 72 12 25 23 7.9 1 55.8 87. 72 12 25 24 12.2 168.0 93. 72 12 26 1 6. 1 174.1 97. 72 12 26 2 3.3 177.4 99. 72 12 26 3 2.5 179.9 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 15.7 9. 15.7 72 12 25 2 30.7 17. 30.7 72 12 25 3 44.2 25. 44.2 72 12 25 4 51 .8 29. 51 .8 72 12 25 6 69.6 39. 69.6 72 12 25 8 89.4 50. 89.4 72 12 25 12 112.1 62. 112.1 72 12 25 24 179.9 100. 179.9 72 12 25 - 306 - TABLE 111. 23 TIME DISTRIBUTION OF RAINFALL KITIMAT MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 74 10 1 4 16 3.0 3.0 2. 74 10 1 4 1 7 2.5 5.5 4. 74 10 1 4 18 4.8 10.3 7. 74 10 1 4 19 5.6 15.9 1 1 . 74 10 1 4 20 4.8 20.7 15. 74 10 1 4 21 4.6 25.3 18. 74 10 1 4 22 5.1 30.4 22. 74 10 1 4 23 5.8 36.2 26. 74 10 1 4 24 5.3 41.5 30. 74 10 1 5 1 7.4 48.9 35. 74 10 15 2 6.6 55.5 40. 74 10 1 5 3 7.6 63. 1 46. 74 10 15 4 8.9 72.0 52. 74 10 15 5 10.2 82.2 59. 74 10 15 6 9.4 91 .6 66. 74 10 15 7 7.9 99.5 72. 74 10 15 8 8.4 107.9 78. 74 10 1 5 9 7.6 115.5 83. 74 10 15 10 4.8 120.3 87. 74 10 1 5 1 1 3.0 123.3 89. 74 10 1 5 1 2 5.8 1 29. 1 93. 74 10 15 13 3.6 1 32.7 96. 74 10 1 5 1 4 3.3 136.0 98. 74 10 15 1 5 2.5 1 38.5 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF - YR-M-D (HOURS) (MM) 24-HR (MM) 1 10.2 7. 18.5 66 10 23 2 19.6 14. 33.5 66 10 23 3 28.5 21 . 47.5 66 10 23 4 36.4 26. 60.5 66 10 23 6 52.4 38. 82.6 66 10 23 8 66.6 48. 1 02.7 66 10 23 1 2 90.2 65. 120.4 66 10 23 24 138.5 100. 138.5 74 10 14 - 307 - TABLE I I I . 2 4 TIME DISTRIBUTION OF RAINFALL LADNER BCHPA MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOU] YR--M-D (MM) (MM) RAINFALL 67 10 6 24 0.5 0.5 1 . 67 10 7 1 1 .8 2.3 4. 67 10 7 2 3.8 6.1 9. 67 1 0 7 3 5.6 11.7 18. 67 10 7 4 4.6 16.3 25. 67 10 7 5 3.8 20. 1 31 . 67 10 7 6 3.3 23.4 36. 67 10 7 7 3.6 27.0 41 . 67 10 7 8 2.5 29.5 45. 67 1 0 7 9 4.6 34. 1 52. 67 10 7 1 0 2.8 36.9 57. 67 10 7 1 1 3.0 39.9 61 . 67 10 7 1 2 0.8 40.7 62. 67 10 7 1 3 2.8 43. 5 67. 67 10 7 1 4 3.8 47.3 72. 67 10 .7 1 5 4.8 52. 1 80. 67 10 7 1 6 3.8 55.9 86. 67 1 0 7 1 7 1 .5 57.4 88. 67 10 7 18 3.0 60.4 92. 67 10 7 1 9 1 .3 61.7 94. 67 1 0 7 20 2.3 64.0 98. 67 1 0 7 21 1 .0 65.0 100. 67 10 7 22 0.3 65.3 100. 67 10 7 23 0.0 65.3 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 5.6 9. 12.7 69 4 17 2 10.2 16. 15.6 78 7 10 3 14.0 21 . 19.1 78 7 10 4 17.8 27. 22.4 64 11 29 6 24.7 38. 27.0 64 11 29 8 31 .8 49. 32.2 63 12 23 1 2 41.2 63. 41 .2 67 10 7 24 65.3 100. 65.3 67 10 6 - 308 - TABLE 111.25 TIME DISTRIBUTION OF RAINFALL LANGLEY LOCHIEL MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 72 12 25 5 1 .3 1 .3 1 . 72 12 25 6 3.0 4.3 4. 72 12 25 7 3.0 7.3 7. 72 12 25 8 3.0 10.3 10. 72 12 25 9 2.0 12.3 12. 72 12 25 10 3.8 16.1 16. 72 12 25 -1 1 5.1 21.2 21 . 72 12 25 1 2 5. 1 26.3 26. 72 12 25 13 3.3 29.6 29. 72 12 25 1 4 3.0 32.6 32. 72 12 25 15 2.8 35.4 35. 72 12 25 1 6 6.4 41.8 41 . 72 12 25 1 7 7.6 49.4 49. 72 12 25 18 9.9 59.3 58. 72 12 25 19 9.4 68.7 68. 72 12 25 20 7.4 76. 1 75. 72 12 25 21 4.3 80.4 79. 72 12 25 22 4.6 85.0 84. 72 12 25 23 3.3 88.3 87. 72 12 25 24 4.8 93. 1 92. 72 12 26 1 3.0 96. 1 95. 72 12 26 2 2.8 98.9 98. 72 12 26 3 1 .5 100.4 99. 72 12 26 4 1 .0 101.4 1 00. • DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 9.9 10. 17.3 73 10 6 2 19.3 19. 21 .4 73 10 6 3 26.9 27. 26.9 72 12 25 4 34.3 34. 34.3 72 12 25 6 45.0 44. 45.0 72 12 25 8 52.9 52. . 53.2 79 12 17 12 68.9 68. 68.9 72 12 25 24 101.4 100. 101 .4 72 12 25 - 309 - TABLE I I I . 2 6 TIME DISTRIBUTION OF RAINFALL MISSION WEST ABBEY MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 68 1 18 1 1 3.8 3.8 4. 68 1 18 1 2 3.6 7.4 7. 68 1 18 13 3.6 11.0 1 1 . 68 1 18 1 4 4.3 15.3 15. 68 1 18 15 4.6 19.9 19. 68 1 18 16 4.8 24.7 24. 68 1 18 17 5.3 30.0 29. . 68 1 18 18 4.8 34.8 34. 68 1 18 19 3.8 38.6 37. 68 1 18 20 3.6 42.2 41 . 68 1 18 21 4.6 46.8 45. 68 1 18 22 3.0 49.8 48. 68 1 18 23 4. 1 53.9 52. 68 1 18 24 5.3 59.2 57. 68 1 19 1 3.8 63.0 61 . 68 1 19 2 3.3 66.3 64. 68 1 1 9 3 5.8 72. 1 69. 68 1 1 9 4 5.8 77.9 75. 68 1 19 5 5.1 83.0 80. 68 1 19 6 3.8 86.8 84. 68 1 19 7 5.8 92.6 89. 68 1 19 8 4.6 97.2 94. 68 1 19 9 3.3 100.5 97. 68 1 19 10 3.3 1 03.8 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 5.8 6. 24.4 70 11 23 2 11.6 1 1 . 31 .6 81 9 27 3 16.7 16. 37.0 81 9 27 4 20.5 20. 40.6 81 9 27 6 30.9 30. 50.2 80 12 25 8 38.7 37. 66.8 80 12 25 12 55.0 53. 85.4 80 12 25 24 103.8 100. 103.8 68 1 1 8 - 310 - TABLE I I I . 2 7 TIME DISTRIBUTION OF RAINFALL NANAIMO DEPARTURE BAY MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR--M-D (MM) (MM) RAINFALL 83 2 10 21 3.6 3.6 5. 83 2 10 22 1 .2 4.8 7. 83 2 10 23 4.0 8.8 13. 83 2 10 24 5.9 14.7 21 . 83 2 1 1 1 5.1 19.8 29. 83 2 1 1 2 4.8 24.6 36. 83 2 1 1 3 4.0 28.6 41 . 83 2 1 1 4 4.0 32.6 47. 83 2 1 1 5 4.0 36.6 53. 83 2 1 1 6 5.9 42.5 62. 83 2 1 1 7 5.9 48.4 70. 83 2 1 1 8 1 .8 50.2 73. 83 2 1 1 9 0.4 50.6 73. 83 2 1 1 10 1 .4 52.0 75. 83 2 1 1 1 1 0.4 52.4 76. 83 2 1 1 12 1.0 53.4 77. 83 2 1 1 1 3 1 .0 54.4 79. 83 2 1 1 1 4 1 .0 55.4 80. 83 2 1 1 15 1 .6 57.0 83. 83 2 1 1 16 3.2 60.2 87. 83 2 1 1 17 2.4 62.6 91 . 83 2 1 1 18 2.0 64.6 94. 83 2 1 1 19 2.4 67.0 97. 83 2 1 1 20 2.0 69.0 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 5.9 9. 28.4 72 8 21 2 1 1 .8 17. 37.0 72 8 21 3 15.8 23. 37.0 72 8 21 4 19.8 29. 37.0 72 8 21 6 28.6 41 . 37.6 75 2 12 8 39.6 57. 44.2 75 2 12 12 50.2 73. 50.4 83 2 10 24 69.0 100. 69.0 83 2 10 - 311 - TABLE 111. 28 TIME DISTRIBUTION OF RAINFALL -NORTH VANC. LYNN CREEK MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 8 1 1 0 31 1 6.8 6.8 3. 81 10 31 2 13.6 20.4 8. 81 10 31 3 12.3 32.7 13. 81 10 31 4 10.0 42.7 17. 81 10 31 5 19.6 62.3 25. 81 10 31 6 13.8 76. 1 30. 81 10 31 7 17.9 94.0 37. 81 10 31 8 16.8 110.8 44. 81 10 31 9 12.8 123.6 49. 81 10 31 10 14.0 137.6 55. 81 10 31 1 1 14.7 152.3 61 . 81 10 31 12 11.5 163.8 65. 81 10 31 13 10.8 174.6 69. 81 10 31 14 10.0 184.6 73. 81 10 31 1 5 11.9 196.5 78. 81 10 31 16 9.1 205.6 82. 81 10 31 17 8.9 214.5 85. 81 10 31 18 5.7 220.2 88. 81 10 31 19 5.3 225.5 90. 81 10 31 20 2. 1 227.6 91 . 81 10 31 21 4.2 231 .8 92. 81 10 31 22 6. 1 237.9 95. 81 10 31 23 8.7 246.6 98. 81 10 31 24 4.7 251 .3 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 19.6 8. 19.6 81 10 31 2 34.7 14. 34.7 81 10 31 3 51.3 20. 51 .3 81 10 31 4 68. 1 27. 68. 1 81 10 31 6 94.9 38. 94.9 81 10 31 8 121.1 48. 121.1 81 10 31 12 167.8 67. 167.8 81 10 31 24 251 .3 100. 251 .3 81 10 31 - 312 - TABLE 111. 29 TIME DISTRIBUTION OF RAINFALL PITT MEADOWS STP MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 79 12 16 20 2.9 2.9 2. 79 12 16 21 1.5 4.4 4. 79 12 16 22 1 .0 5.4 5. 79 12 1 6 23 2.1 7.5 6. 79 12 1 6 24 2.7 10.2 9. 79 12 1 7 1 2.1 12.3 10. 79 12 1 7 2 3.2 15.5 13. 79 12 17 3 5.1 20.6 18. 79 12 17 4 6.3 26.9 23. 79 12 17 5 6.3 33.2 28. 79 12 17 6 7.0 40.2 34. 79 12 17 7 8.0 48.2 41 . 79 12 17 8 9.3 57.5 49. 79 12 1 7 9 7.8 65.3 56. 79 12 1 7 1 0 7.8 73. 1 62. 79 12 1 7 1 1 9.3 82.4 70. 79 12 17 1 2 9.5 91 .9 78. 79 12 1 7 1 3 8.3 1 00.2 85. 79 12 1 7 1 4 2.0 102.2 87. 79 12 1 7 1 5 4.1 106.3 91 . 79 12 17 1 6 2.9 109.2 93. 79 12 1 7 1 7 2.5 111.7 95. 79 12 17 18 3.1 114.8 98. 79 12 17 19 2.5 117.3 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 9.5 8. 24.6 74 7 11 2 18.8 16. 37.6 74 7 11 3 27. 1 23. 45.2 74 7 11 4 34.9 30. 51 .0 74 7 11 6 52.0 44. 52.8 74 7 11 8 67.0 57. 67.0 79 12 17 12 87.9 75. 87.9 79 12 17 24 1 1 7 ."3 100. 117.3 79 12 16 - 313 - TABLE I I I . 3 0 TIME DISTRIBUTION OF RAINFALL PITT POLDER MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 68 1 18 1 5 5.8 5.8 4. 68 1 18 16 6.1 11.9 8. 68 1 18 17 5.6 17.5 12. 68 1 18 18 5.6 23. 1 16. 68 1 18 19 3.3 26.4 18. 68 1 18 20 3.6 30.0 21 . 68 1 18 21 3.8 33.8 23. 68 1 18 22 5.1 38.9 27. 68 1 18 23 5.6 44.5 31 . 68 1 18 24 6.1 50.6 35. 68 1 19 1 5.8 56.4 39. 68 1 19 2 5.8 62.2 43. 68 1 19 3 6.9 69. 1 48. 68 1 19 4 8.1 77.2 53. 68 1 19 5 6.9 84. 1 58. 68 1 19 6 5.6 89.7 62. 68 1 19 7 7.1 96.8 67. 68 1 19 8 8.1 1 04.9 73. 68 1 19 9 6.9 111.8 77. 68 1 19 10 8.1 1 19.9 83. 68 1 19 1 1 6.6 1 26.5 88. 68 1 19 1 2 6.9 133.4 92. 68 1 19 1 3 5.8 1 39.2 96. 68 1 19 1 4 5.1 1 44.3 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 8.1 6. 16.8 68 9 17 2 15.2 1 1 . 30.3 68 9 17 3 23. 1 16. 42.5 68 9 17 4 30.2 21 . 51.9 68 9 16 6 43.7 30. 66.6 68 9 16 8 57.7 40. 78.6 68 9 16 12 82.8 57. 94.9 68 9 16 24 144.3 100. 144.3 68 1 1 8 - 314 - TABLE 111.31 TIME DISTRIBUTION OF RAINFALL PORT ALBERNI A MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR--M-D (MM) (MM) RAINFALL 83 2 10 1 7 2.4 2.4 2. 83 2 10 18 4.8 7.2 5. 83 2 10 19 4.2 11.4 8. 83 2 10 20 4.4 15.8 1 1 . 83 2 10 21 2.8 18.6 13. 83 2 10 22 5.0 23.6 16. 83 2 10 23 4.0 27.6 19. 83 2 10 24 5.7 33.3 23. 83 2 1 1 1 7.0 40.3 28. 83 2 1 1 2 8.8 49.1 34. 83 2 1 1 3 10.3 59.4 42. 83 2 1 1 4 6.6 66.0 46. 83 2 1 1 5 6.2 72.2 50. 83 2 1 1 6 6.4 78.6 55. 83 2 1 1 7 6.8 85.4 60. 83 2 1 1 8 7.2 92.6 65. 83 2 1 1 9 7.2 99.8 70. 83 2 1 1 10 8.9 108.7 76. 83 2 1 1 1 1 7.6 116.3 81 . 83 2 1 1 1 2 6.3 122.6 86. 83 2 1 1 1 3 6.9 129.5 90. 83 2 1 1 1 4 5.0 134.5 94. 83 2 1 1 1 5 4.8 139.3 97. 83 2 1 1 1 6 3.8 143.1 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 10.3 7. 16.7 78 5 24 2 19. 1 13. 29. 1 78 5 24 3 26. 1 18. 30.0 78 5 24 4 32.7 23. 35.3 73 10 27 6 45.3 32. 46.8 75 11 13 8 59.6 42. 60.2 79 12 17 1 2 89.3 62. 89.3 83 2 11 24 143. 1 100. 143. 1 83 2 10 - 315 - TABLE 111. 32 TIME DISTRIBUTION OF RAINFALL PORT COQUITLAM CITY YARD MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 72 12 25 3 0.5 0.5 0. 72 12 25 4 3.0 3.5 3. 72 12 25 5 5.1 8.6 7. 72 12 25 6 5.1 13.7 1 1 . 72 12 25 7 5.3 19.0 15. 72 12 25 8 6.1 25. 1 20. 72 12 25 9 7.6 32.7 26. 72 12 25 10 7.4 40. 1 32. 72 12 25 1 1 6.6 46.7 38. 72 12 25 1 2 5.8 52.5 42. 72 12 25 13 5.3 57.8 46. 72 12 25 14 6.9 64.7 52. 72 12 25 - 15 6.1 70.8 57. 72 12 25 16 6.1 76.9 62. 72 12 25 17 6.1 83.0 67. 72 12 25 18 6.9 89.9 72. 72 12 25 19 5.6 95.5 77. 72 12 25 20 8.4 103.9 83. 72 12 25 21 8.9 112.8 91 . 72 12 25 22 5.8 118.6 95. 72 12 25 23 3.3 121 .9 98. 72 12 25 24 1 .8 123.7 99. 72 12 26 1 0.5 1 24.2 100. 72 12 26 2 0.3 1 24.5 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 8.9 7. 12.0 82 7 3 2 17.3 14. 19.8 77 11 25 3 23. 1 19. 28. 1 79 12 17 4 29.8 24. 34.8 79 12 17 6 42.0 34. 49.8 79 12 17 8 55.0 44. 58.0 79 12 17 12 80. 1 64. 80. 1 72 12 25 24 124.5 100. 124.5 72 12 25 - 316 - TABLE I I I . 3 3 TIME DISTRIBUTION OF RAINFALL PORT HARDY MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY _ CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 80 12 9 19 5.0 5.0 3. 80 12 9 20 6.1 11.1 7. 80 12 9 21 6.3 17.4 1 1 . 80 12 9 22 6.5 23.9 14. 80 12 9 23 6.1 30.0 18. 80 12 9 24 7.2 37.2 23. 80 12 10 1 7.0 44.2 27. 80 12 10 2 8.6 52.8 32. 80 12 10 3 7.7 60.5 37. 80 12 10 4 7.7 68.2 41 . 80 12 10 5 6.5 74.7 45. 80 12 10 6 5.7 80.4 49. 80 12 10 7 5.0 85.4 52. 80 12 10 8 4.5 89.9 54. 80 12 10 9 4.3 94.2 57. 80 12 10 1 0 5.4 99.6 60. 80 12 10 1 1 4.9 1 04.5 63. 80 12 10 1 2 5.4 1 09.9 67. 80 12 10 1 3 6.3 116.2 70. 80 12 10 14 7.0 123.2 75. 80 12 10 1 5 9.9 1 33. 1 81 . 80 12 10 16 11.9 1 45.0 88. 80 12 10 1 7 12.8 157.8 96. 80 12 10 18 7.2 1 65.0 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 12.8 8. 12.9 80 6 8 2 24.7 15. 24.7 80 12 10 3 34.6 21 . 34.6 80 12 10 4 41 .8 25. 41 .8 80 12 10 6 55. 1 33. 55. 1 80 12 10 8 65.4 40. 65.4 80 12 10 12 84.6 51 . 85.4 80 12 10 24 165.0 100. 165.0 80 12 9 - 317 - TABLE I I I . 34 TIME DISTRIBUTION OF RAINFALL PORT MELLON MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 72 12 25 1 0.5 0.5 0. 72 12 25 2 3.0 3.5 1 . 72 12 25 3 10.4 13.9 6. 72 12 25 4 12.2 26. 1 1 1 . 72 12 25 5 15.0 41.1 17. 72 12 25 6 16.8 57.9 25. 72 12 25 7 11.9 69.8 30. 72 12 25 8 6.6 76.4 32. 72 12 25 9 11.9 88.3 37. 72 12 25 10 20.8 109. 1 46. 72 12 25 1 1 11.2 120.3 51 . 72 12 25 12 7.4 127.7 54. 72 12 25 13 8.9 1 36.6 58. 72 12 25 14 12.2 1 48.8 63. 72 12 25 15 9.4 158.2 67. 72 12 25 16 7.1 165.3 70. 72 12 25 17 10.2 175.5 74. 72 12 25 18 9.9 185.4 79. 72 12 25 19 11.4 196.8 83. 72 12 25 20 16.0 212.8 90. 72 12 25 21 12.2 225.0 95. 72 12 25 22 5.3 230.3 98. 72 12 25 23 3.0 233.3 99. 72 12 25 24 2.5 235.8 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 20.8 9. 20.8 72 12 25 2 32.7 14. 34.8 70 4 9 3 44.0 19. 44.7 70 4 9 4 55.9 24. 55.9 72 12 25 6 83.0 35. 83.0 72 12 25 8 106.4 45. 1 06.4 72 12 25 1 2 1 45.3 62. 145.3 72 12 25 24 235.8 100. 235.8 72 12 25 - 318 - TABLE 111. 35 TIME DISTRIBUTION OF RAINFALL PORT MOODY GULF OIL REF. MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 72 12 25 3 1 .8 1.8 1 . 72 12 25 4 4.3 6.1 4. 72 12 25 5 7.1 13.2 9. 72 12 25 6 7.4 20.6 14. 72 12 25 7 5.6 26.2 18. 72 12 25 8 6.6 32.8 23. 72 12 25 9 9.7 42.5 29. 72 12 25 10 8.4 50.9 35. 72 12 25 1 1 6.4 57.3 39. 72 12 25 12 7.9 65.2 45. 72 12 25 1 3 5.1 70.3 48. 72 12 25 1 4 5.1 75.4 52. 72 12 25 15 5.6 81.0 56. 72 12 25 1 6 7.9 88.9 61 . 72 12 25 1 7 7.1 96.0 66. 72 12 25 18 6.4 102.4 71 . 72 12 25 19 6.9 109.3 75. 72 12 25 20 9.4 118.7 82. 72 12 25 21 10.9 129.6 89. 72 12 25 22 8.4 138.0 95. 72 12 25 23 3.8 141.8 98. 72 12 25 24 2.3 144.1 99. 72 12 26 1 0.5 144.6 100. 72 12 26 2 0.5 145.1 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 10.9 8. 19.7 78 2 2 2 20.3 14. 21 .8 78 2 2 3 28.7 20. 29.2 81 10 31 4 35.6 25. 36.3 71 10 25 6 49. 1 34. 51.6 72 7 12 8 62.6 43. 67.3 72 7 12 1 2 87. 1 60. 92.3 72 7 12 24 145. 1 100. 1 45. 1 72 12 25 - 319 - TABLE 111. 36 TIME DISTRIBUTION OF RAINFALL PORT RENFREW BCFS MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 79 12 1 3 5 13.6 13.6 5. 79 12 13 6 11.3 24.9 10. 79 12 1 3 7 10.4 35.3 14. 79 12 13 8 12.8 48. 1 19. 79 12 13 9 14.0 62. 1 25. 79 12 13 10 16.0 78. 1 31 . 79 12 13 1 1 14.8 92.9 37. 79 12 1 3 12 9.2 102. 1 41 . 79 12 13 1 3 5.2 107.3 43. 79 12 13 1 4 3.8 111.1 45. 79 12 13 1 5 4.2 115.3 46. 79 12 13 16 3.8 119.1 48. 79 12 13 1 7 6.2 125.3 50. 79 12 13 18 8.4 133.7 54. 79 12 13 19 10.8 144.5 58. 79 12 13 20 10.0 154.5 62. 79 12 13 21 12.0 1 66.5 67. 79 12 1 3 22 9.6 176. 1 71 . 79 12 13 23 12.0 188. 1 76. 79 12 13 24 14.0 202. 1 81 . 79 12 14 1 14.0 216. 1 87. 79 12 1 4 2 14.0 230. 1 93. 79 12 14 3 8.0 238. 1 96. 79 12 14 4 10.2 248.3 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % -OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 16.0 6. 43.7 77 1 1 1 2 30.8 12. 56.3 78 1 1 7 3 44.8 18. 78. 1 78 1 1 7 4 57.6 23. 99.7 78 1 1 7 6 79.3 32. 117.6 78 1 1 7 8 102. 1 41 . 1 25.2 78 1 1 7 12 129.2 52. 153.6 75 10 16 24 248.3 100. 248.3 79 12 13 - 320 - TABLE 111. 37 TIME DISTRIBUTION OF RAINFALL PRINCE RUPERT A MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 74 10 8 4 6.6 6.6 - 5. 74 10 8 5 8.9 15.5 1 1 . 74 10 8 6 8.1 23.6 17. 74 10 8 7 5.3 28.9 21 . 74 10 8 8 3.8 32.7 24. 74 10 8 9 2.5 35.2 26. 74 10 8 10 4.8 40.0 29. 74 10 8 1 1 3.0 43.0 32. 74 10 8 1 2 3.3 46.3 34. 74 10 8 1 3 4.6 50.9 37. 74 10 8 1 4 3.6 54.5 40. 74 10 8 1 5 2.8 57.3 42. 74 10 8 1 6 9.4 66.7 49. 74 10 8 1 7 4.8 71.5 53. 74 10 8 18 1 .8 73.3 54. 74 10 8 19 1 .8 75. 1 55. 74 10 8 20 6.9 82.0 60. 74 10 8 21 7.1 89. 1 66. 74 10 8 22 5.8 94.9 70. 74 10 8 23 8.4 103.3 76. 74 10 8 24 12.2 115.5 85. 74 10 9 1 9.9 125.4 92. 74 10 9 2 6.1 131.5 97. 74 10 9 3 4.3 135.8 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 12.2 9. 16.2 83 9 25 2 22. 1 16. 24.7 76 10 26 3 30.5 22. 33.8 76 10 26 4 36.6 27. 42.8 76 10 26 6 50.3 . 37. 63.4 76 10 26 8 60.7 45. 80.6 76 10 26 12 78.5 58. 98.4 72 10 23 24 135.8 100. 135.8 74 10 8 - 321 - TABLE I I I .38 TIME DISTRIBUTION OF RAINFALL SAANICH DENSMORE MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOU1 YR-M- D (MM) (MM) RAINFALL 72 12 25 3 1 .3 1 .3 1 . 72 12 25 4 3.0 4.3 4. 72 12 25 5 4.1 8.4 9. 72 12 25 6 3.3 11.7 12. 72 12 25 7 4.6 16.3 17. 72 12 25 8 3.8 20. 1 20. 72 12 25 9 2.5 22.6 23. 72 12 25 10 3.6 26.2 27. 72 12 25 1 1 3.8 30.0 30. 72 12 25 1 2 3.0 33.0 33. 72 12 25 1 3 3.8 36.8 37. 72 12 25 1 4 4. 1 40.9 41 . 72 12 25 15 4.6 45.5 46. 72 12 25 16 7.1 52.6 53. 72 12 25 17 4.8 57.4 58. 72 12 25 18 4.1 61 .5 62. 72 12 25 19 4.3 65.8 67. 72 12 25 20 4.6 70.4 71 . 72 12 25 21 4.3 74.7 76. 72 12 25 22 6.6 81.3 82. 72 12 25 23 7.6 88.9 90. 72 12 25 24 4.8 93.7 95. 72 12 26 1 3.6 97.3 99. 72 12 26 2 1 .3 98.6 1 00. * DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 7.6 8. 9.4 72 2 16 2 14.2 14. 15.2 72 2 16 3 19.0 19. 20.9 65 10 5 4 23.3 24. 25. 1 67 1 2 6 32.2 33. 32.2 64 9 30 8 43.4 44. 43.4 72 12 25 12 60.7 62. 60.7 72 12 25 24 98.6 100. 98.6 72 12 25 - 322 - TABLE I I I .39 TIME DISTRIBUTION OF RAINFALL SANDSPIT A MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 78 10 31 1 6 2.9 2.9 4. 78 10 31 17 3.2 6.1 8. 78 10 31 18 1 .7 7.8 10. 78 10 31 19 2.3 10.1 13. 78 10 31 20 1 .5 11.6 15. 78 10 31 21 1 .2 12.8 17. 78 10 31 22 2.1 14.9 20. 78 10 31 23 4.3 19.2 26. 78 10 31 24 4.3 23.5 31 . 78 1 1 1 1 3.8 27.3 36. 78 1 1 1 2 1 . 6 28.9 39. 78 1 1 1 3 1 . 6 30.5 41 . 78 1 1 1 4 3.1 33.6 45. 78 1 1 1 5 3.1 36.7 49. 78 1 1 1 6 2.6 39.3 52. 78 1 1 1 7 3.5 42.8 57. 78 1 1 1 8 4.3 47. 1 63. 78 1 1 1 9 3.5 50.6 67. 78 1 1 1 10 3.1 53.7 72. 78 1 1 1 1 1 3.8 57.5 77. 78 1 1 1 1 2 3.5 61 .0 81 . 78 1 1 1 1 3 5.4 66.4 89. 78 1 1 1 1 4 4.8 71.2 95. 78 1 1 1 1 5 3.8 75.0 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 5.4 7. 12.7 75 10 5 2 10.2 14. 22.0 80 10 31 3 14.0 19. 32.3 80 10 31 4 17.5 23. 38.8 80 10 31 6 24.4 33. 42.8 80 10 31 8 32.2 43. 47.0 75 10 4 12 44.5 59. 51.8 79 11 20 24 75.0 100. 75.0 78 10 31 - 323 - T A B L E 1 1 1 . 4 0 T I M E D I S T R I B U T I O N OF R A I N F A L L SPRING ISLAND MAXIMUM 2 4 - H O U R R A I N F A L L ON RECORD HOURLY CUM. PERCENT DATE HOUR R A I N R A I N OF 2 4 - H O U R Y R - M - D (MM) (MM) R A I N F A L L 7 8 1 1 6 2 3 . 8 3 . 8 2 . 7 8 1 1 6 3 7 . 8 1 1 . 6 6 . 7 8 1 1 6 4 0 . 4 1 2 . 0 6 . 7 8 1 1 6 5 8 . 6 2 0 . 6 1 0 . 7 8 1 1 6 6 8 . 8 2 9 . 4 1 5 . 7 8 1 1 6 7 1 2 . 2 4 1 . 6 21 . 7 8 1 1 6 8 1 2 . 8 5 4 . 4 2 7 . 7 8 1 1 6 9 1 2 . 8 6 7 . 2 3 3 . 7 8 1 1 6 10 1 3 . 8 8 1 . 0 4 0 . 7 8 1 1 6 1 1 1 2 . 0 9 3 . 0 4 6 . 78 1 1 6 1 2 1 3 . 6 1 0 6 . 6 5 3 . 78 1 1 6 1 3 1 0 . 8 1 1 7 . 4 5 8 . 7 8 1 1 6 1 4 8 . 6 1 2 6 . 0 6 3 . 7 8 1 1 6 15 1 3 . 6 1 3 9 . 6 6 9 . 7 8 1 1 6 16 1 2 . 0 1 5 1 . 6 7 5 . 7 8 1 1 6 1 7 1 1 . 2 1 6 2 . 8 81 . 7 8 1 1 6 18 1 2 . 6 1 7 5 . 4 8 7 . 78 1 1 6 19 2 . 4 1 7 7 . 8 8 8 . 7 8 1 1 6 20 4 . 0 1 8 1 . 8 9 0 . 7 8 1 1 6 21 4 . 8 1 8 6 . 6 9 3 . 7 8 1 1 6 2 2 3 . 4 1 9 0 . 0 9 4 . 7 8 1 1 6 23 4 . 8 1 9 4 . 8 9 7 . 7 8 1 1 6 2 4 2 . 4 1 9 7 . 2 9 8 . 7 8 1 1 7 1 4 . 4 2 0 1 . 6 1 0 0 . D U R A T I O N FOR I N D I C A T E D D U R A T I O N : MAX OCCURRING W I T H I N MAXIMUM DATE MAX 2 4 - H R R A I N F A L L ON RECORD % OF Y R - M - D (HOURS) (MM) 2 4 - H R (MM) 1 1 3 . 8 7 . 2 2 . 6 7 9 9 4 2 2 6 . 6 1 3 . 3 0 . 4 75 11 2 5 3 3 9 . 4 2 0 . 4 0 . 6 78 10 2 2 4 5 2 . 2 2 6 . 5 2 . 2 78 1 1 6 6 7 7 . 2 3 8 . 7 7 . 2 78 11 6 8 9 8 . 0 4 9 . 9 8 . 0 78 1 1 6 12 1 4 6 . 0 7 2 . 1 4 6 . 0 78 1 1 6 2 4 2 0 1 . 6 1 0 0 . 2 0 1 . 6 78 1 1 6 - 324 - TABLE 111.41 TIME DISTRIBUTION OF RAINFALL STAVE FALLS MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 80 12 25 1 1 3.5 3.5 3. 80 12 25 1 2 0.6 4. 1 3. 80 12 25 13 0.0 4.1 3. 80 12 25 1 4 0.0 4.1 3. 80 12 25 1 5 0.0 4.1 3. 80 12 25 16 0.0 4.1 3. 80 12 25 1 7 2.0 6.1 5. 80 12 25 18 1 .4 7.5 6. 80 12 25 19 2.9 10.4 8. 80 12 25 20 3.1 13.5 10. 80 12 25 21 4.7 18.2 14. 80 12 25 22 7.1 25.3 19. 80 12 25 23 6.1 31.4 24. 80 12 25 24 10.6 42.0 32. 80 12 26 1 11.0 53.0 40. 80 12 26 2 10.8 63.8 48. 80 12 26 3 8.9 72.7 55. 80 12 26 4 12.8 85.5 64. 80 12 26 5 8.5 94.0 71 . 80 12 26 6 7.7 101.7 77. 80 12 26 7 10.6 1 12.3 85. 80 12 26 8 9.7 122.0 92. 80 12 26 9 5.5 1 27.5 96. 80 12 26 1 0 5.3 1 32.8 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 12.8 10. 14.6 81 9 27 2 21 .8 16. 25.5 81 9 27 3 32.5 24. 32.5 80 12 26 4 43.5 33. 43.5 80 12 26 6 62.6 47. 62.6 80 12 25 8 80.9 61 . 80.9 80 12 25 12 109.3 82. 109.3 80 12 25 24 132.8 100. 132.8 80 12 25 - 325 - TABLE III .42 TIME DISTRIBUTION OF RAINFALL STRATHCONA DAM MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 68 1 17 1 7 5.1 5.1 3. 68 1 1 7 18 9.7 14.8 10. 68 1 1 7 19 7.6 22.4 14. 68 1 1 7 20 6.6 29.0 19. 68 1 1 7 21 12.7 41 .7 27 . 68 1 1 7 22 7.6 49.3 32. 68 1 1 7 23 9.1 58.4 38. 68 1 1 7 24 8.6 67.0 43. 68 1 18 1 8.1 75. 1 48. 68 1 18 2 10.2 85.3 55. 68 1 18 3 7.1 92.4 60. 68 1 18 4 8.1 100.5 65. 68 1 18 5 4.1 104.6 67. 68 1 18 6 10.2 114.8 74. 68 1 18 7 3.6 118.4 76. 68 1 18 8 5.6 1 24.0 80. 68 1 18 9 4. 1 128. 1 83. 68 1 18 10 5.1 1 33.2 86. 68 1 18 1 1 4. 1 1 37.3 88. 68 1 18 1 2 4.1 141.4 91 . 68 1 18 13 1 .8 1 43.2 92. 68 1 18 1 4 3.3 146.5 94. 68 1 18 1 5 3.6 150.1 97. 68 1 18 1 6 5.1 1 55.2 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 12.7 8. 23.9 69 2 7 2 20.3 13. 26.0 73 7 7 3 29.4 19. 29.4 68 117 4 38.0 24. 38.0 68 117 6 56.3 36. 56.3 68 117 8 71 .5 46. 71 .5 68 117 12 100.5 65. 100.5 68 1 17 24 1 55.2 1007 155.2 68 117 - 326 - TABLE III.43 TIME DISTRIBUTION OF RAINFALL SURREY KWANTLEN PARK MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 68 1 18 1 3 4.6 4.6 3. 68 1 18 1 4 6.1 10.7 8. 68 1 18 15 4.8 15.5 1 1 . 68 1 18 16 7.9 23.4 17. 68 1 18 17 5.6 29.0 21 . 68 1 18 18 7.4 36.4 26. 68 1 18 19 5.6 42.0 30. 68 1 18 20 5.6 47.6 34. 68 1 18 21 4.8 52.4 37. 68 1 18 22 5.1 57.5 41 . 68 1 18 23 5.3 62.8 45. 68 1 18 24 3.8 66.6 48. 68 1 19 1 6.4 73.0 52. 68 1 19 2 4.6 77.6 55. 68 1 19 3 7.6 85.2 61 . 68 1 19 4 10.4 95.6 68. 68 1 19 5 10.7 106.3 76. 68 1 19 6 5.8 112.1 80. 68 1 19 7 5.6 117.7 84. 68 1 19 8 6.4 124. 1 89. 68 1 19 9 2.0 1 26. 1 90. 68 1 19 10 4.6 130.7 93. 68 1 19 1 1 4.6 135.3 97. 68 1 19 12 4.8 140. 1 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR -M-D (HOURS) (MM) 24-HR (MM) 1 10.7 8. 18.2 83 9 1 2 21.1 15. 28.2 62 2 3 3 28.7 20. 35. 1 62 2 2 4 34.5 25. 37. 1 62 2 2 6 46.5 33. 46.5 68 1 19 8 57.5 41 . 57.5 68 1 19 1 2 77.3 55. 81 .3 72 12 25 24 1 40. 1 100. 1 40.1 68 1 18 - 327 - TABLE III.44 TIME DISTRIBUTION OF RAINFALL SURREY MUNICIPAL HALL MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOU1 YR-M- D (MM) (MM) RAINFALL 72 12 25 3 0.5 0.5 1 . 72 12 25 4 0.8 1 .3 1 . 72 12 25 5 2.5 3.8 4. 72 12 25 6 4.3 8.1 9. 72 12 25 7 3.0 11.1 12. 72 12 25 8 3.3 14.4 15. 72 12 25 9 3.6 18.0 19. 72 12 25 10 4.3 22.3 23. 72 12 25 1 1 3.8 26. 1 27. 72 12 25 12 3.8 29.9 31 . 72 12 25 13 4. 1 34.0 36. 72 12 25 14 4. 1 38. 1 40. 72 12 25 15 4.3 42.4 45. 72 12 25 16 5.6 48.0 51 . 72 12 25 1 7 6.4 54.4 57. 72 12 25 18 8.4 62.8 66. 72 12 25 19 8.1 70.9 75. 72 12 25 20 7.1 78.0 82. 72 12 25 21 4.6 82.6 87. 72 12 25 22 3.3 85.9 90. 72 12 25 23 3.6 89.5 94. 72 12 25 24 3.0 92.5 97. 72 12 26 1 1 .5 94.0 99. 72 12 26 2 1 .0 95.0 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 8.4 9. 17.5 68 8 27 2 16.5 17. 21.5 81 7 29 3 23.6 25. 26.0 80 7 11 4 30.0 32. 31.6 80 7 11 6 40.2 42". 44.9 67 12 22 8 48.6 51 . 56.6 67 12 21 1 2 64.6 68. 64.6 72 12 25 24 95.0 100. 95.0 72 12 25 - 328 - TABLE III .45 TIME DISTRIBUTION OF RAINFALL TERRACE A MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 2 4-HOU] YR-M- D (MM) (MM) RAINFALL 78 10 30 22 2.3 2.3 2. 78 10 30 23 3.6 5.9 5. 78 10 30 24 4.3 10.2 9. 78 10 31 1 5.3 15.5 13. 78 10 31 2 4.5 20. 0 17. 78 10 31 3 5.5 25.5 22. 78 10 31 4 6.3 31.8 28. 78 10 31 5 8.1 39.9 35. 78 10 31 6 7.3 47.2 41 . 78 10 31 7 7.5 54.7 47. 78 10 31 8 9.7 64.4 56. 78 10 31 9 6.5 70.9 61 . 78 10 31 10 5.1 76.0 66. 78 10 31 1 1 4.1 80. 1 69. 78 10 31 12 4.7 84.8 73. 78 10 31 1 3 4.7 89.5 77. 78 10 31 14 4.0 93.5 81 . 78 10 31 15 3.8 97.3 84. 78 10 31 1 6 3.4 100.7 87. 78 10 31 17 3.4 104. 1 90. 78 10 31 18 3.2 107.3 93. 78 10 31 19 2.8 110.1 95. 78 10 31 20 2.6 112.7 98. 78 10 31 21 2.8 115.5 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24 -HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 9. 7 8. 16.6 80 7 27 2 17. 2 15. 19.2 80 7 27 3 24. 5 21 . 24.5 78 10 31 4 32. 6 28. 32.6 78 10 31 6 45. 4 39. 45.4 78 10 31 8 56. 0 48. 56.0 78 10 31 12 74. 6 65. 74.6 78 10 31 24 115. 5 100. 115.5 78 10 30 - 3 2 9 - TABLE III .46 TIME DISTRIBUTION OF RAINFALL " TERRACE PCC MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 74 10 1 4 1 5 2.0 2.0 2. 74 10 1 4 16 0.5 2.5 3. 74 10 1 4 17 0.8 3.3 4. 74 10 1 4 18 2.0 5.3 6. 74 10 1 4 1 9 1 .3 7. 1 9. 74 10 1 4 20 1 .8 8.9 1 1 . 74 10 1 4 21 2.8 1 1 .7 14. 74 10 1 4 22 2.5 14.2 17. 74 10 1 4 23 3.3 17.5 21 . 74 10 1 4 24 3.8 21.3 26. 74 10 1 5 1 3.8 25. 1 30. 74 10 1 5 2 3.6 28.7 35. 74 10 1 5 3 4.6 33.3 40. 74 10 1 5 4 5.6 38.9 47. 74 10 1 5 5 5.1 44.0 53. 74 10 1 5 6 5. 1 49. 1 59. . 74 10 15 7 6.6 55.7 67. 74 10 1 5 8 8.1 63.8 77. 74 10 1 5 9 5. 1 68.9 83. 74 10 1 5 10 2.0 70.9 86. 74 10 1 5 1 1 4.8 75.7 91 . 74 10 1 5 12 2.3 78.0 94. 74 10 1 5 1 3 2.5 80.5 97. 74 10 1 5 1 4 2.3 82.8 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 8.1 10. 19.1 80 9 19 2 14.7 18. 30.5 68 11 19 •3 19.8 24. 41.9 68 11 19 4 24.9 30. 45.7 68 11 19 6 35.6 43. 50.2 68 11 19 8 43.8 53. 54.3 68 11 19 12 58.2 70. 62.9 68 11 19 24 82.8 100. 82.8 74 10 14 - 330 - TABLE III.47 TIME DISTRIBUTION OF RAINFALL TOFINO A MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 80 12 9 18 7.5 7.5 4. 80 12 9 19 8.9 16.4 8. 80 12 9 20 9.5 25.9 12. 80 12 9 21 7.4 33.3 16. 80 12 9 22 9.7 43.0 21 . 80 12 9 23 12.8 55.8 27. 80 12 9 24 10.8 66.6 32. 80 12 10 1 4.7 71.3 34. 80 12 10 2 1 .6 72.9 35. 80 12 10 3 1 .9 74.8 36. 80 12 10 4 3.1 77.9 37. 80 12 10 5 5.6 83.5 40. 80 12 10 6 6.0 89.5 43. 80 12 1 0 7 4.7 94.2 45. 80 12 10 8 7.9 102. 1 49. 80 12 10 9 13.9 116.0 56. 80 12 10 10 15.1 131.1 63. 80 12 10 1 1 16.4 147.5 71 . 80 12 10 12 17.3 164.8 79. 80 12 1 0 13 14.6 179.4 86. 80 12 10 14 5.8 185.2 89. 80 12 10 15 7.9 1 93. 1 93. 80 12 10 16 8.3 201 .4 97. 80 12 10 17 7.2 208.6 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF — YR-M-D (HOURS) (MM) 24-HR (MM) 1 17.3 8. 21.4 77 10 25 2 33.7 16. 35.3 7111 2 3 48.8 23. 48.8 80 12 10 4 63.4 30. 63.4 80 12 10 6 85.2 41 . 85.2 80 12 10 8 99.3 48. 99.3 80 12 10 12 125. 1 60. 1 25. 1 80 12 10 24 208.6 100. . 208.6 80 12 9 - 331 - TABLE 111.48 TIME DISTRIBUTION OF RAINFALL VANCOUVER A MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 79 12 16 20 3.2 3.2 3. 79 12 16 21 2.6 5.8 5. 79 12 16 22 1 .8 7.6 6. 79 12 16 23 3.6 11.2 9. 79 12 16 24 3.6 14.8 12. 79 12 17 1 3.0 17.8 15. 79 12 17 2 5.2 23.0 19. 79 12 17 3 5.4 28.4 23. 79 12 17 4 5.7 34. 1 28. 79 12 17 5 6.5 40.6 33. 79 12 17 6 5.9 46.5 38. 79 12 17 7 6.7 53.2 44. 79 12 17 8 9.9 63. 1 52. 79 12 17 9 10.9 74.0 61 . 79 12 17 1 0 13.1 87. 1 72. 79 12. 17 1 1 9.4 96.5 79. 79 12 17 1 2 3.2 99.7 82. 79 12 17 1 3 2.3 1 02.0 84. 79 12 17 1 4 2.5 104.5 86. 79 12 17 15 4.7 109.2 90. 79 12 17 1 6 3.5 112.7 93. 79 12 17 1 7 3.2 115.9 95. 79 12 17 1 8 1 .8 117.7 97. 79 12 17 19 3.7 121.4 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 13.1 1 1 . 23. 1 81 6 13 2 24.0 20. 29.5 81 6 13 3 33.9 28. 34.0 81 6 13 4 43.3 36. 43.3 79 12 17 6 55.9 46. 55.9 79 12 17 8 68. 1 56. 68. 1 79 12 17 12 85.3 70. 85.3 79 12 16 24 121.4 100. 121.4 79 12 16 - 332 - TABLE 111. 49 TIME DISTRIBUTION OF RAINFALL VANCOUVER HARBOUR MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 72 12 25 2 0.5 0.5 1 . 72 12 25 3 0.8 1 .3 1 . 72 12 25 4 3.3 4.6 5. 72 12 25 5 4.8 9.4 10. 72 12 25 6 4.6 14.0 15. 72 12 25 7 4.3 18.3 20. 72 12 25 8 4. 1 22.4 24. 72 12 25 9 3.6 26.0 28. 72 12 25 1 0 4.3 30.3 33. 72 12 25 1 1 3.0 33.3 36. 72 12 25 12 3.0 36.3 39. 72 12 25 1 3 4.8 41.1 44. 72 12 25 1 4 4.6 45.7 49. 72 12 25 15 4.3 50.0 54. 72 12 25 1 6 6.4 56.4 61 . 72 12 25 1 7 5.1 61 .5 66. 72 12 25 18 7.4 68.9 74. 72 12 25 19 6.1 75.0 81 . 72 12 25 20 5.3 80.3 86. 72 12 25 21 4.3 84.6 91 . 72 12 25 22 4.3 88.9 96. 72 12 25 23 2.5 91.4 98. 72 12 25 24 1 .5 92.9 1 00. 72 12 26 1 0.0 92.9 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 7.4 8. 15.0 62 2 3 2 13.5 15. 26.9 62 2 3 3 18.9 20. 32.2 62 2 2 4 25.0 27. 32.2 62 2 2 6 34.6 37. 40.3 79 12 17 8 44.0 47. 53.4 79 12 17 12 58.6 63. 67.4 79 12 16 24 92.9 100. 92.9 72 12 25 - 333 - TABLE 111.50 TIME DISTRIBUTION OF RAINFALL VANCOUVER PMO MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 72 12 25 3 0.3 0.3 0. 72 12 25 4 2.0 2.3 2. 72 12 25 5 4.8 7.1 5. 72 12 25 6 9.7 16.8 12. 72 12 25 7 7.6 24.4 17. 72 12 25 8 5.3 29.7 21 . 72 12 25 9 5. 1 34.8 25. 72 12 25 10 8.4 43.2 31 . 72 12 25 1 1 7.6 50.8 36. 72 12 25 12 5.6 56.4 40. 72 12 25 1 3 7.1 63.5 45. 72 12 25 14 5.6 69. 1 49. 72 12 25 15 4.6 73.7 52. 72 12 25 16 5.3 79.0 56. 72 12 25 17 6.9 85.9 61 . 72 12 25 18 8.1 94.0 66. 72 12 25 19 7.6 101.6 72. 72 12 25 20 6. 1 1 07.7 76. 72 12 25 21 8.4 116.1 82. 72 12 25 22 9.4 125.5 89. 72 12 25 23 7.6 1 33. 1 94. 72 12 25 24 6. 1 1 39.2 98. 72 12 26 1 2.3 141.5 1 00. 72 12 26 2 0.0 141.5 100. DURATION FOR INDICATED. DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 9.7 7. 13.2 71 10 25 2 17.8 13. 20.6 71 10 25 3 25.4 18. 28.0 71 10 25 4 31 .5 22. 35.4 71 10 25 6 47.2 33. 47.2 72 12 25 8 60.2 43. 60.2 72 12 25 1 2 82.8 59. 82.8 72 12 25 24 141.5 100. 141.5 72 12 25 - 334 - TABLE 111.51 TIME DISTRIBUTION OF VANCOUVER UBC RAINFALL MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 72 12 25 2 0.3 0.3 0. 72 12 25 3 1 .3 1 .6 1 . 72 12 25 4 4. 1 5.7 5. 72 12 25 5 5.8 11.5 10. 72 12 25 6 6.9 18.4 16. 72 12 25 7 6.4 24.8 22. 72 12 25 8 4. 1 28.9 25. 72 12 25 9 4.3 33.2 29. 72 12 25 10 5.1 38.3 34. 72 12 25 1 1 4.3 42.6 37. 72 12 25 1 2 4.1 46.7 41 . 72 12 25 1 3 3.8 50.5 44. 72 12 25 1 4 5.1 55.6 49. 72 12 25 15 6.9 62.5 55. 72 12 25 16 8.1 70.6 62. 72 12 25 1 7 8.6 79.2 69. 72 12 25 18 7.4 86.6 76. 72 12 25 19 5.8 92.4 81 . 72 12 25 20 5.8 98.2 86. 72 12 25 21 7.4 105.6 93. 72 12 25 22 5.6 111.2 98. 72 12 25 23 1 .8 113.0 99. 72 12 25 24 1 .0 114.0 100. 72 12 26 1 0.0 114.0 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 8.6 . 8. 16.4 79 9 8 2 16.7 15. 21 .4 81 7 7 3 24. 1 21 . 27.3 81 7 6 4 31.0 27. 31.0 72 12 25 6 43. 1 38. 43. 1 72 12 25 8 55.6 49. 55.6 . 72 12 25 12 72.9 64. 72.9 72 12 25 24 114.0 100. 1 14.0 72 12 25 - 335 - TABLE I I I . 5 2 TIME DISTRIBUTION OF RAINFALL -VICTORIA GONZALES HEIGHTS MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M- D (MM) (MM) RAINFALL 79 12 1 3 8 3.4 3.4 3. 79 12 1 3 9 5.0 8.4 8. 79 12 1 3 10 8.3 16.7 17. 79 12 1 3 1 1 6.9 23.6 24. 79 12 13 1 2 6.1 29.7 30. 79 12 1 3 1 3 4.5 34.2 34. 79 12 13 14 4.5 38.7 39. 79 12 1 3 15 3.8 42.5 42. 79 12 1 3 16 4.8 47.3 47. 79 12 13 17 4.0 51 .3 51 . 79 12 13 18 5.9 57.2 57. 79 12 13 19 6.1 63.3 63. 79 12 1 3 20 6.4 69.7 70. 79 12 13 21 5.6 75.3 75. 79 12 1 3 22 4.8 80. 1 80. 79 12 1 3 23 5.4 85.5 85. 79 12 1 3 24 4.3 89.8 90. 79 12 1 4 1 1 .6 91 .4 91 . 79 12 1 4 2 1 .0 92.4 92. 79 12 1 4 3 0.3 92.7 93. 79 12 1 4 4 0.0 92.7 93. 79 12 1 4 5 0.3 93.0 93. 79 12 1 4 6 0.3 93.3 93. 79 12 1 4 7 6.9 1 00.2 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 8.3 8. 9.5 82 12 3 2 15.2 15. 18.6 82 12 3 3 21 .3 21 . 26.9 82 12 3 4 26.3 26. 35.0 82 12 3 6 35.3 35. 47.8 82 12 3 8 43.9 44. 58.9 82 12 3 12 66.9 67. 72.7 82 12 3 24 100.2 100. 100.2 79 12 13 - 336 - TABLE I I I . 5 3 TIME DISTRIBUTION OF RAINFALL VICTORIA INT. A MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 72 12 25 2 1 .3 1 .3 1 . 72 12 25 3 1.8 3.1 3. 72 12 25 4 3.6 6.7 7. 72 12 25 5 3.6 10.3 12. 72 12 25 6 2.3 12.6 14. 72 12 25 7 3.3 15.9 18. 72 12 25 8 1 .3 17.2 19. 72 12 25 9 1 .3 18.5 21 . 72 12 25 10 1.8 20.3 23. 72 12 25 1 1 3.6 23.9 27. 72 12 25 1 2 2.0 25.9 29. 72 12 25 1 3 2.3 28.2 32. 72 12 25 1 4 2.5 30.7 34. 72 12 25 1 5 1 .8 32.5 36. 72 12 25 1 6 5.6 38. 1 43. 72 12 25 1 7 7.6 45.7 51 . 72 12 25 18 4.6 50.3 56. 72 12 25 19 4.8 55. 1 62. 72 12 25 20 5.1 60.2 67. 72 12 25 21 5.8 66.0 74. 72 12 25 22 6.9 72.9 82. 72 12 25 23 9.9 82.8 93. 72 12 25 24 4.3 87. 1 97. 72 12 26 1 2.3 89.4 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 9.9 1 1 . 1 1.9 74 11 9 2 16.8 19. 18.5 74 1 1 9 3 22.6 25. 23.3 74 1 1 9 4 27.7 31 . 27.7 72 12 25 6 37. 1 41 . 38.2 80 11 21 8 50.3 56. 50.3 72 12 25 12 61 .2 68. 61.2 72 12 25 24 89.4 100. 89.4 72 12 25 - 337 - TABLE 111.54 TIME DISTRIBUTION OF RAINFALL VICTORIA MARINE RADIO MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR--M-D (MM) (MM) RAINFALL 72 3 4 22 1 .0 1 .0 1 . 72 3 4 23 1 .5 2.5 2. 72 3 4 24 3.6 6. 1 6. 72 3 5 1 3.3 9.4 9. 72 3 5 2 5.3 14.7 14. 72 3 5 3 4.3 19.0 18. 72 3 5 4 6.9 25.9 24. 72 3 5 5 8.4 34.3 32. 72 3 5 6 6.9 41 .2 38. 72 3 5 7 6.6 47.8 44. 72 3 5 8 10.7 58.5 54. 72 3 5 9 2.3 60.8 56. 72 3 5 10 1 .3 62. 1 58. 72 3 5 1 1 1 .5 63.6 59. 72 3 5 12 5.1 68.7 64. 72 3 5 13 6.6 75.3 70. 72 3 5 1 4 4.8 80. 1 74. 72 3 5 1 5 5.8 85.9 80. 72 3 5 1 6 6.6 92.5 86. 72 3 5 1 7 5.8 98.3 91 . 72 3 5 18 5.3 103.6 96. 72 3 5 1 9 1 .8 1 05.4 98. 72 3 5 20 1 .0 1 06.4 99. 72 3 5 21 1 .3 107.7 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 10.7 10. 17.8 78 1 21 2 17.3 16. 24.8 78 1 21 3 24.2 22. 29.0 82 1 23 4 32.6 30. 35.7 82 1 23 6 43.8 41 . 45. 1 82 1 23 8 52.4 49. 53.3 82 1 23 1 2 66.9 62. 69.4 72 12 25 24 107.7 100. 1 07.7 72 3 4 - 338 - TABLE 111.55 TIME DISTRIBUTION OF RAINFALL VICTORIA SHELBOURNE MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOUR YR-M-D (MM) (MM) RAINFALL 72 12 25 4 1 .3 1 .3 1 . 72 12 25 5 3.3 4.6 5. 72 12 25 6 1 .5 6.1 7. 72 12 25 7 2.8 8.9 10. 72 12 25 8 3.6 12.5 14. 72 12 25 9 4. 1 16.6 19. 72 12 25 10 3.3 19.9 23. 72 12 25 1 1 3.3 23.2 27. 72 12 25 1 2 2.8 26.0 30. 72 12 25 13 4.8 30.8 36. 72 12 25 1 4 3.8 34.6 40. 72 12 25 1 5 5.1 39.7 46. 72 12 25 1 6 5.6 45.3 52. 72 12 25 17 3.8 49.1 57. 72 12 25 18 2.8 51.9 60. 72 12 25 19 3.8 55.7 64. 72 12 25 20 4.6 60.3 70. 72 12 25 21 4.3 64.6 75. 72 12 25 22 5.6 70.2 81 . 72 12 25 23 7.4 77.6 90. 72 12 25 24 5.3 82.9 96. 72 12 26 1 2.5 85.4 99. 72 12 26 2 0.8 86.2 99. 72 12 26 3 0.5 86.7 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 7.4 9. 1 1 .7 71 2 12 2 13.0 15. 14.7 71 2 12 3 18.3 21 . 18.3 72 12 25 4 22.6 26. 22.6 72 12 25 6 31 .0 36. 31 .0 72 12 25 8 37.9 44. 37.9 72 12 25 12 56.9 66. 56.9 72 12 25 24 86.7 100. 86.7 72 12 25 - 339 - TABLE I I I . 5 6 TIME DISTRIBUTION OF RAINFALL VICTORIA U. OF VICT. MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOU1 YR-M- D (MM) (MM) RAINFALL 79 12 13 4 0.8 0.8 1 . 79 12 13 5 1 .4 2.2 2. 79 12 1 3 6 2.4 4.6 5. 79 12 1 3 7 3.9 8.5 9. 79 12 1 3 8 3.3 11.8 13. 79 12 13 9 3.0 14.8 16. 79 12 13 10 6.4 21.2 23. 79 12 1 3 1 1 5.6 26.8 30. 79 12 13 12 5.0 31 .8 35. 79 12 13 1 3 5.2 37.0 41 . 79 12 1 3 1 4 3.8 40.8 45. 79 12 13 1 5 3.2 44.0 49. 79 12 V2 16 4.2 48.2 53. 79 12 1 3 17 3.2 51.4 57. 79 12 13 18 4.8 56.2 62. 79 12 13 19 5.6 61 .8 68. 79 12 1 3 20 5.6 67.4 74. 79 12 1 3 21 4.8 72.2 80. 79 12 1 3 22 4.6 76.8 85. 79 12 13 23 4.2 81 .0 89. 79 12 13 24 4.0 85.0 94. 79 12 1 4 1 1 .8 86.8 96. 79 12 1 4 2 3.0 89.8 99. 79 12 1 4 3 0.8 90.6 100. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 6.4 7. 9.9 81 11 14 2 12.0 13. 15.8 83 1 8 3 17.0 19. 21.4 74 12 20 4 22.2 25. 25.4 82 1 23 6 29.6 33. 37.8 82 1 23 8 37.0 41.. 47.4 82 1 23 12 57.4 63. 57.4 79 12 13 24 90.6 100. 90.6 79 12 13 - 340 - TABLE 111.57 TIME DISTRIBUTION OF RAINFALL WHITE ROCK STP MAXIMUM 24-HOUR RAINFALL ON RECORD HOURLY CUM. PERCENT DATE HOUR RAIN RAIN OF 24-HOU1 YR-M- D (MM) (MM) RAINFALL 71 1 1 2 21 0.5 0.5 ! . 71 11 2 22 0.3 0.8 ] . 71 11 2 23 0.0 0.8 1 . 71 11 2 24 0.3 1 . 1 1 . 71 11 3 1 0.0 1 . 1 1 . 71 11 3 2 0.0 1 . 1 1 . 71 11 3 3 2.3 3.4 4. 71 11 3 4 3.3 6.7 8. 71 11 3 5 4.6 11.3 13. 71 11 3- 6 4.8 16.1 19. 71 11 3 7 4.6 20.7 25. 71 11 3 8 4.8 25.5 30. 71 11 3 9 5.3 30.8 37. 71 11 3 10 4.8 35.6 42. 71 11 3 1 1 4.8 40.4 48. 71 11 3 12 6.4 46.8 56. 71 11 3 1 3 6.1 52.9 63. 71 11 3 14 7.6 60.5 72. 71 11 3 1 5 7.1 67.6 80. 71 11 3 16 6.4 74.0 88. 71 11 3 1 7 5. 1 79. 1 94. 71 11 3 18 2.3 81.4 97. 71 11 3 19 1 .8 83.2 99. 71 11 3 20 1 .0 84.2 1 00. DURATION FOR INDICATED DURATION: MAX OCCURRING WITHIN MAXIMUM DATE MAX 24-HR RAINFALL ON RECORD % OF YR-M-D (HOURS) (MM) 24-HR (MM) 1 7.6 9. 15.2 72 7 9 2 14.7 17. 24.3 72 7 9 3 21 . 1 25. 25.8 72 7 9 4 27.2 32. 27.7 78 1 1 3 6 38.7 46. 38.7 7 1 1 1 3 8 48.5 58. 48.5 7 1 1 1 3 12 67.8 81 . 67.8 7 1 1 1 3 24 84.2 100. 84.2 7 1 1 1 2 - 341 - APPENDIX IV WATER PERCOLATION THROUGH SNOW - 342 - APPENDIX IV WATER PERCOLATION THROUGH SNOW IV.1 VERTICAL UNSATURATED FLOW A phys i c a l model for v e r t i c a l percolation of water through a homogeneous ri p e snowpack has been developed by Colbeck (1971, 1972). Relationships derived by Colbeck and re s u l t s of f i e l d studies undertaken to v e r i f y t h e o r e t i c a l r e s u l t s are summarized i n t h i s section. Development of the theory requires the following r e l a t i o n s h i p s between snowpack permeability and water saturation: k, = kuSz ( i v . . i ) where S = saturation of the snowpack; k u = i n t r i n s i c permeability i n the unsaturated zone; and k s = snowpack permeability at some value of S. Also, <t>e = 4>{1 ~ Si) (IV.2) where 0 = t o t a l porosity of the snowpack; Si = i r r e d u c i b l e saturation; and 0 e = e f f e c t i v e snowpack porosity. Colbeck shows the v e r t i c a l rate of movement of a water input, m, at the snow surface i s given by: (IV.3) - 343 - where ( d z / d t ) m = speed of propagation of a wave with input rate m; p = density of water; g = acceleration due to gravity; and p. = v i s c o s i t y of water. Experiments have been conducted by Colbeck and Davidson (1973) on a small v a l l e y g l a c i e r i n the Cascade Mountains i n Washington State to compare t h e o r e t i c a l and measured f i e l d r e s u l t s . Five holes were bored and flow measuring devices were i n s t a l l e d at -depths ranging from 1 m to 5 m. Results of the study are summarized on Figure IV.1 and show experi- mental data agreed with r e s u l t s derived from t h e o r e t i c a l considerations. S i m i l a r r e s u l t s were also obtained by Colbeck and Anderson (1982) from experimental studies i n Vermont and C a l i f o r n i a , and by Jordan (1978) i n B r i t i s h Columbia. V o l u m e F l u x , m/sec Figure IV. 1 Wave Speed vs Influx Rate (after Colbeck and Davidson, 1973) - 344 - Eqn. IV.3 shows the rate of penetration of any input value i s determined by the input rate at which i t was generated. Therefore, a large input can overtake preceding smaller ones and form a shock f r o n t . Colbeck (1973) demonstrated a n a l y t i c a l procedures for c a l c u l a t i o n of fl u x rates as a function of time at any snow depth for s p e c i f i e d water inputs at the snow surface. This procedure was applied by Dunne et a l . (1976) to pre- d i c t snowmelt percolation c h a r a c t e r i s t i c s i n a subarctic snowpack. Tucker and Colbeck (1977) developed a computer program to calculate flow at any depth for any input at the snow surface and snow properties. Plotted r e s u l t s are av a i l a b l e for three snow surface input shapes over a 12-hour duration: double-peaked, sinusoidal and skewed. Flows at snow depths ranging from 1 m to 8 m are shown on Figure IV.2. Common features of the results of t h e o r e t i c a l c a l c u l a t i o n s for each surface input include the following: i ) a shock wave formed such that an instantaneous increase i n flow occurred at depth. i i ) peak flow rates decreased with depth. i i i ) differences i n the shape of surface inputs disappears with increas- ing depth. - 345 - Figure IV.2. Water Percolation Through Snow (aft e r Tucker and Colbeck, 1977) - 346 - In instances when a shock wave forms during v e r t i c a l percolation, a rapid increase i n snowpack outflow can be observed. However, i t i s important to also recognize that even in these cases, the peak outflow rate i s less than the peak rate input at the snow surface. Eqn. IV.3 can be s i m p l i f i e d further by s u b s t i t u t i n g a = ^ — ^ = 5.46 x 10 6 nT 1s~ 1 fdz\ 3m °- 6 7 (afeJ °- 3 3 (iv.4) 'dz\ _ 3 m 0 6 7 (5.46 x I O 6 ) 0 3 3 jfej-33 (S - (IV.4a) m r e (IV.4b) 5 2 9 m 0 6 7 A £ 3 3 <t>e Colbeck and Anderson (1982) found the grouping ku^ 1/3) 0e~ 1 to be more e a s i l y measured i n experimental studies than either ku^ 1/3) o r p e - 1 alone, and a value for this grouping to be adequate for solution of Eqn. IV.4. Snowmelt analysis undertaken for undisturbed snow in C a l i - f o r n i a and Vermont yielded values for ku v 1/3) 0 e - 1 i n a narrow band ranging from 0.00239 to 0.00301 with a mean value of 0.00270. Applying the mean value to Eqn. IV.4 y i e l d s the following r e l a t i o n s h i p between percolation and water input rates to a ripe snowpack: 1.43m0-67 (iv.5) - 347 Integration of Eqn. IV.5 with z=0 at the snow surface produces an equa- tio n describing depth of penetration into a snowpack with time for a constant input rate: z= 1.43m0-67* { I V ' 6 ) Solution of Eqn. IV.6 i s shown g r a p h i c a l l y on Figure IV.3 for a range of water inputs to a snowpack from r a i n f a l l and snowmelt. For i l l u s t r a t i o n , consider snow depths of 1 to 2 m and t y p i c a l rain and snowmelt inputs for the coastal region ranging from 5 to 20 mm/hr. Results presented on Figure IV.3 show that v e r t i c a l percolation alone can add about 0.5 to 3 hours to the t r a v e l time of a water p a r t i c l e through the basin. - 348 - Figure IV.3. Percolation Rates for V e r t i c a l Unsaturated Flow - 349 - IV.2 BASAL SATURATED FLOW The governing equations for water flow through saturated snow at the base of a snowpack have been developed by Colbeck (1974a). Colbeck envisioned a two-layer model for water flow through snow co n s i s t i n g of v e r t i c a l flow through an unsaturated layer and downslope flow along a boundary i n a saturated layer. For constant slope and u n i t width, Colbeck expresses the con t i n u i t y equation f o r the saturated layer as: where ks = i n t r i n s i c permeability of the saturated zone; 9 = slope; h = saturated layer thickness; x = distance; t = time; and I = net input to saturated zone. By considering a new coordinate system (x', t 1 ) which moves downslope at the wave speed i n the saturated layer, Eqn. IV.7 can be s i m p l i f i e d and solved d i r e c t l y to y i e l d : where to' and tjj' are time l i m i t s f o r the period during which a parcel of water entering the saturated layer at the top of a slope moves to the base, and q (0,t L') i s the un i t discharge at the base of the slope. Eqn. IV.8 states that the outflow from the base of the slope i s equiva- l e n t to the input to the saturated layer integrated over a preceding period equal to the time taken for water to t r a v e l along the slope length. (IV.7) (IV.8) - 350 - Experimental v e r i f i c a t i o n of equations developed for basal saturated flow has not been as extensive as that for v e r t i c a l unsaturated flow. One study was undertaken by Dunne et a l . (1976) i n the Canadian sub- a r c t i c . Snowmelt runoff was measured at seven h i l l s l o p e plots ranging i n area from 1 335 to 2810 m2 with downslope lengths between 37 m and 85 m. Comparison was made between measured snowmelt runoff and values e s t i - mated using Eqn. IV.8. For example, a t h e o r e t i c a l outflow hydrograph was calculated as follows: the input hydrograph to the saturated zone was estimated from surface melt and v e r t i c a l unsaturated flow considera- tions; t r a v e l time along the h i l l s l o p e was estimated; and outflow was calc u l a t e d as the sum of inputs to the saturated zone f or time i n c r e - ments equal to h i l l s l o p e t r a v e l time. Dunne et a l . concluded "the pre d i c t i o n of peak runoff was generally excellent" and "the pr e d i c t i o n of the timing of runoff hydrographs was les s s a t i s f a c t o r y though s t i l l remarkably good". A summary of r e s u l t s f o r 20 hydrographs analyzed at these h i l l s l o p e p l o t s i s shown on Figure IV.4. Observed Peak Dlschorge (cm/hr) Observed Lag to Peak (hrs) Figure IV. 4 Comparison of Predicted and Observed Outflow Hydrographs (a f t e r Dunne et a l . , 1976) - 351 - Closer examination of results on Figure IV.4 for lag times shows i n every instance observed lag times are less than predicted. These r e s u l t s c o n t r a d i c t the conclusion that experimental re s u l t s are e s p e c i a l l y good, p a r t i c u l a r l y when one considers that data are from r e l a t i v e l y small h i l l s l o p e p l o t s ranging in length from only 37 to 85 m. Extrapolation of r e s u l t s to a larger watershed scale suggests observed lag times would be much less than predicted by water percolcation theory. This observa- t i o n supports further the concept of an i n t e r n a l drainage network as the dominant routing mechanism for hydrograph analysis on a watershed scale. Examination of the s p e c i a l case of steady flow provides an opportunity to i l l u s t r a t e response c h a r a c t e r i s t i c s that would occur i f basal saturated flow existed under a snowpack. Colbeck (1974a) showed for steady flow: flow depth h = Ix(ctk,9)-1 <IV-9> unit discharge q = aks6h (iv.10) travel time t = 4>ex{ak90)-1 (iv.11) Even though the a p p l i c a b i l i t y of a steady-state so l u t i o n to actual water runoff problems i s li m i t e d , Eqn. IV.11 nevertheless allows for q u a l i t a - t i v e assessment of the time frame for basin response. Colbeck (1974a) and Dunne et a l . (1976) estimated ks i s approximately equal to 5.1 x 10~9 m2 for saturated flow through snow with grain sizes ranging from 1 mm to 2 mm. Experimental data presented by Colbeck and - 352 - Anderson (1982) indicates 0e i s equal to about 0.46. Substituting these values i n t o Eqn. IV.11 y i e l d s : _ 16.5x (IV.12) 9 Solution of Eqn. IV. 12 i s shown g r a p h i c a l l y on Figure IV.5 for mild and steep mountain slopes. Also included on Figure IV.5 for comparison are corresponding t r a v e l times estimated for overland flow through forests with heavy ground l i t t e r ( S o i l Conservation Service, 1974). Comparison of t r a v e l times shows basin response would be more rapid i n instances where water i s routed through the basin by overland or channelized flow than by saturated water percolation flow through snow. For i l l u s t r a t i o n , consider saturataed flow along the base of a snowpack for a distance of 600 m and h i l l s i d e slopes of 5 and 15 degrees. Results included on Figure IV. 5 show that for basal percolation the t r a v e l time of a water p a r t i c l e ranges from about 10 to 30 hours. This time increment i s i n addition to the time required for v e r t i c a l percolation through the snowpack. - 353 - 6 0 5 0 ~ 4 0 JZ U J IS rz 3 0 U J > < cr I - 2 0 B A S A L S A T U R A T E D F L O W ( FROM E Q N . IV. 12 ) O V E R L A N D F L O W ( A F T E R SOIL C O N S E R V A T I O N SERVICE, 1974) o <n / V S L O P E = 5 °N^SLOPE 2 0 0 4 0 0 6 0 0 8 0 0 D I S T A N C E ( m ) 1000 Figure IV.5. Travel Times for Basal Saturated Flow

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