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Debris supply to torrent-prone channels on the east side of Howe Sound, British Columbia Dagg, Bruce Ronald 1987

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DEBRIS SUPPLY TO TORRENT-PRONE CHANNELS ON THE EAST SIDE OF HOWE SOUND, BRITISH COLUMBIA By BRUCE RONALD DAGG .A.Sc., The U n i v e r s i t y of B r i t i s h Columbia, 1981 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES (Department of Geography) We accept t h i s t h e s i s as conforming to the r e q u i r e d standard THE UNIVERSITY OF BRITISH A p r i l 1987 Bruce Ronald Dagg, COLUMBIA 1987 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 the requirements f o r an advanced degree at the U n i v e r s i t y of B r i t i s h Columbia, I agree t h a t the 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 study. I f u r t h e r agree 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 copying of t h i s t h e s i s f o r s c h o l a r l y purposes may be granted by the head o f my department or by h i s or her r e p r e s e n t a t i v e s . I t i s understood t h a t copying or p u b l i c a t i o n o f t h i s t h e s i s f o r f i n a n c i a l gain s h a l l not be allowed without my w r i t t e n p e r m i s s i o n . Department of Geography The U n i v e r s i t y of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date 30 April 1987 ABSTRACT Deb r i s t o r r e n t s ( c h a n n e l i z e d d e b r i s flows) are a geomorpho-l o g i c a l process only r e l a t i v e l y r e c e n t l y recognised i n southwest B r i t i s h Columbia. They are of i n t e r e s t both because of the tremendous amount of geomorphic work they do, and because of the hazards they pose to e n g i n e e r i n g works and r e s i d e n t i a l develop-ments. Fourteen t o r r e n t s on the east side of Howe Sound, near Vancouver, s i n c e October 1981, have claimed twelve l i v e s . D e b r is t o r r e n t s d i f f e r from water f l o o d s i n that they i n v o l v e l a r g e amounts of coarse organic and i n o r g a n i c d e b r i s . T h e r e f o r e , a major requirement f o r t o r r e n t occurrence i n a given channel i s a supply of m o b i l i z a b l e d e b r i s . T h i s t h e s i s examines d e b r i s supply mechanisms and r a t e s . of d e b r i s supply i n four small watersheds along Howe Sound, near the v i l l a g e of L i o n s Bay. An inv e n t o r y of major d e b r i s sources has been compiled, and s e l e c t e d t y p i c a l s i t e s are examined i n d e t a i l . Study methods i n c l u d e a i r p h o t o i n t e r p r e t a t i o n , ground su r v e y i n g and recon n a i s -sance, f i e l d i n s t r u m e n t a t i o n and s i t e m onitoring, dendrochronol-ogy, and m a t e r i a l s sampling and t e s t i n g . D e b r i s supply i s c o n t r o l l e d by n a t u r a l f a c t o r s such as the nature and d i s t r i b u t i o n of the bedrock and s u r f i c i a l m a t e r i a l s , topographic g r a d i e n t , v e g e t a t i o n , weather, and sur f a c e and groundwater hydrology, and by human a c t i v i t i e s such as log g i n g and road c o n s t r u c t i o n . A wide v a r i e t y of d e b r i s supply mechan-isms operate i n the study area, i n c l u d i n g r o c k f a l l and r o c k s l i d e , t a l u s s h i f t , d e b r i s s l i d e , s o i l wedge f a i l u r e , r a v e l l i n g , and i i snow avalanche. In a d d i t i o n to d e l i v e r i n g d e b r i s to channel systems, some of these processes are capable of t r i g g e r i n g d e b r i s t o r r e n t s . D e b ris r e d i s t r i b u t i o n i n channels occurs through d e b r i s t o r r e n t s which do not reach the fans, f l u v i a l processes (bedload t r a n s p o r t ) , and snow avalanches. A c t i v e d e b r i s removal from main supply p o i n t s , and storage elsewhere i n the channel system, can decrease the frequency but i n c r e a s e the magnitude of t o r r e n t events i n the b a s i n . The wide v a r i e t y of d e b r i s supply, d e b r i s r e d i s t r i b u t i o n , and t o r r e n t t r i g g e r i n g mechanisms a c t i n g i n t h i s r e l a t i v e l y small area p o i n t s to a need f o r c a r e f u l study of i n d i v i d u a l b a sins i f the t o r r e n t p o t e n t i a l i n an area i s to be understood. R e g i o n a l l y - b a s e d c l i m a t o l o g i c a l or h y d r o l o g i c a l models of t o r r e n t occurrence should be employed f o r p r e l i m i n a r y hazard assessment only. TABLE OF CONTENTS Page ABSTRACT i i TABLE OF CONTENTS i v LIST OF TABLES v i i LIST OF FIGURES v i i i LIST OF PLATES X ACKNOWLEDGEMENTS x i CHAPTER 1: INTRODUCTION 1 1.1 INTRODUCTION TO DEBRIS TORRENTS 1 1.1.1 Requirements f o r Torre n t Occurrence 3 1.1.2 De b r i s T o r r e n t Mechanisms 5 1.1.2.1 I n i t i a t i o n 6 1.1.2.2 T r a n s p o r t a t i o n and E r o s i o n 6 1.1.2.3 D e p o s i t i o n 9 1.2 REVIEW OF DEBRIS TORRENT WORK IN SOUTHWEST BRITISH COLUMBIA 11 1.3 SCOPE AND OBJECTIVES OF THESIS 13 CHAPTER 2: EAST HOWE SOUND AREA 15 2.1 LOCATION AND TOPOGRAPHY 15 2.2 BEDROCK GEOLOGY 17 2.3 LATE GLACIAL HISTORY AND SURFICIAL GEOLOGY 21 2.4 WEATHER AND CLIMATE 23 2.5 NATURAL VEGETATION 28 2.6 LAND USE 29 2.7 HYDROLOGY 30 2.8 HISTORY OF DEBRIS TORRENT ACTIVITY ALONG HOWE SOUND 36 2.9 STUDY AREA 37 2.9.1 L o c a t i o n and Topography 37 2.9.2 S u r f i c i a l Geology and M a t e r i a l P r o p e r t i e s 42 2.9.3 D e s c r i p t i o n of Study Catchments 53 2.9.3.1 A l b e r t a Creek 56 2.9.3.2 Magnesia Creek 61 2.9.3.3 M Creek 71 2.9.3.4 Loggers Creek 77 iv Page CHAPTER 3: STUDY METHODS 82 3.1 HISTORICAL RECORDS 82 3.2 AIRPHOTO INTERPRETATION 82 3.3 GROUND SURVEYING AND RECONNAISSANCE 84 3.4 FIELD INSTALLATION AND SITE MONITORING 87 3.5 DENDROCHRONOLOGY 88 3.6 MATERIALS SAMPLING AND TESTING 89 CHAPTER 4: DEBRIS SUPPLY TO CHANNELS 91 4.1 INTRODUCTION 91 4.2 DISCRETE VS CONTINUOUS SUPPLY 92 4.3 MATERIAL SUPPLY FROM ROCK SLOPES AND TALUS SLOPES 93 4.3.1 R o c k f a l l s and R o c k s l i d e s 95 4.3.1.1 Mechanisms 97 4.3.1.2 Type L o c a t i o n - S i t e M2 98 4.3.1.3 Type L o c a t i o n - S i t e M1 114 4.3.1.4 Type L o c a t i o n - Loggers Creek T r i b u t a r y 119 4.3.2 Talus S h i f t 122 4.3.2.1 Mechanisms 122 4.3.2.2 Type L o c a t i o n - S i t e M3 124 4.4 MATERIAL SUPPLY FROM COLLUVIAL SLOPES 129 4.4.1 Shallow Debris S l i d e s and Slumps 130 4.4.1.1 Mechanisms 130 4.4.1.2 Type L o c a t i o n - S i t e Mg3 133 4.4.2 S o i l Wedge F a i l u r e 139 4.4.2.1 Mechanisms 140 4.4.2.2 Type L o c a t i o n - S i t e M1 140 4.4.2.3 Type L o c a t i o n - S i t e Mg1 143 4.4.3 R a v e l l i n g From C o l l u v i a l Slopes 150 4.4.3.1 Mechanisms 150 4.4.3.2 Type L o c a t i o n - M Creek C r o s s i n g 154 4.4.3.3 Type L o c a t i o n - S i t e A1 158 4.5 OTHER DEBRIS SOURCES 161 4.5.1 Wet Snow Avalanches 161 4.5.1.1 Mechanisms 162 4.5.1.2 Type L o c a t i o n - S i t e Mg2 165 4.5.2 Timber H a r v e s t i n g 172 4.6 DISCUSSION 175 CHAPTER 5 DEBRIS REDISTRIBUTION IN CHANNELS 179 5.1 INTRODUCTION 179 5.2 DEBRIS TORRENT PROCESSES 181 5.3 FLUVIAL PROCESSES 184 5.4 WET SNOW AVALANCHES 190 5.5 DISCUSSION 191 v CHAPTER 6 TRIGGERING OF DEBRIS TORRENTS Page 193 6.1 INTRODUCTION 193 6.2 TRIGGERING MECHANISMS IN THE STUDY AREA 197 6.2.1 F l u i d Shear Due to Heavy Rain or Rain Plus Snowmelt 198 6.2.2 F l u i d Shear Due to Breaching of Temporary Channel Blockage 203 6.2.3 Impulsive Loading From Slope F a i l u r e s 203 6.2.4 Impulsive Loading From Snow Avalanches 209 6.2.5 F l u i d i z a t i o n of H i l l s l o p e F a i l u r e s 210 6.3 DISCUSSION 210 CHAPTER 7 CONCLUSIONS 212 7.1 MAJOR FINDINGS OF THESIS 212 7.2 OUTSTANDING PROBLEMS 214 7.3 IMPLICATIONS OF FINDINGS 215 7.3.1 L o c a l I m p l i c a t i o n s 215 7.3.2 Wider I m p l i c a t i o n s 218 LIST OF REFERENCES 219 APPENDIX 1: LABORATORY TESTING OF SOILS 224 DESCRIPTION OF SAMPLES 224 APPENDIX 2: FIELD MEASUREMENT OF HYDRAULIC CONDUCTIVITY 229 DESIGN AND USE OF THE FIELD PERMEAMETER 229 TEST OF THE ACCURACY OF THE FIELD PERMEAMETER 231 FIELD MEASUREMENTS OF HYDRAULIC CONDUCTIVITY 234 APPENDIX 3: DERIVATION OF SOME OF THE DEBRIS TORRENT TRIG-GERING EQUATIONS IN CHAPTER 6 239 FLUID SHEAR EQUATION (5.1) AND (6.2) 239 IMPULSIVE LOADING EQUATIONS (6.6), (6.7), AND (6.8) 240 v i LIST OF TABLES Page TABLE 2. 1 : Some C h a r a c t e r i s t i c s of Drainage Ba s i n s , East Howe Sound Area 19 TABLE 2. 2: P r e c i p i t a t i o n T o t a l s (Rain Plus Snow Water E q u i v a l e n t ) f o r Two Storms i n Autumn 1984 27 TABLE 2. 3: Debris T o r r e n t s and Floods i n East Howe Sound Area, 1960 - 1984 34 TABLE 2. 4: Some C h a r a c t e r i s t i c s of the Study Area Basins 41 TABLE 3. 1 : Government A i r Photographs Used i n Study 83 TABLE 4. 1 : Summary of Tree Core I n t e r p r e t a t i o n s , S i t e M2 1 03 TABLE 4. 2: Movement Observed at P a i n t L i n e s on Tal u s Slope, S i t e M3 125 TABLE 4. 3: P r e c i p i t a t i o n and Temperature at H o l l y b u r n Ridge, 10-28 February 1986 157 TABLE 4. 4: E r o s i o n P in G r i d R e s u l t s , S i t e A1 160 TABLE 6. 1 : Examples of S t a b i l i t y of Debris i n M Creek at Toe of S i t e M2, Under Impulsive Loading from M2 208 TABLE A1 . 1 : Summary of R e s u l t s of Laboratory T e s t i n g of S o i l s 227 TABLE A2 . 1 : H y d r a u l i c C o n d u c t i v i t y T e s t s on Quadra Sand 232 TABLE A2 .2: F i e l d Permeameter T e s t s at S i t e Mg1 234 TABLE A2 .3: F i e l d Permeameter T e s t s a t S i t e Mg3 235 v i i LIST OF FIGURES Page FIGURE 1.1: P r o f i l e of a T y p i c a l D e bris T o r r e n t , C o a s t a l B r i t i s h Columbia 5 FIGURE 2.1: Map of Howe Sound 16 FIGURE 2.2: Topography of East Howe Sound Area 18 FIGURE 2.3: S i m p l i f i e d Bedrock Geology of East Howe Sound Area 20 FIGURE 2.4: Monthly Mean P r e c i p i t a t i o n i n Howe Sound Area 24 FIGURE 2.5: Return Perio d s f o r 24-Hour P r e c i p i t a t i o n T o t a l s i n Howe Sound Area 25 FIGURE 2.6: T o t a l Runoff Per Year, 1914 - 1984, Ca p i l a n o River 31 FIGURE 2.7: Seasonal Runoff D i s t r i b u t i o n , 1972 - 1984, Stawamus R i v e r 33 FIGURE 2.8: D i s t r i b u t i o n of De b r i s T o r r e n t s and Floods i n East Howe Sound Area 35 FIGURE 2.9: Contour Map of Study Area 38 FIGURE 2.10: P r o f i l e s of the Four Study Area Streams 40 FIGURE 2.11: S i m p l i f i e d Bedrock Geology 43 FIGURE 2.12: F o r e s t Cover and Land Use i n the Study Area 44 FIGURE 2.13: S u r f i c i a l Geology i n the Study Area 45 FIGURE 2.14: Gr a i n S i z e D i s t r i b u t i o n s f o r Surface M a t e r i a l s at S i t e Mg3 47 FIGURE 2.15: The U n i f i e d S o i l C l a s s i f i c a t i o n System 48 FIGURE 2.16: S i t e Mg3 Samples: Texture, P l a s t i c i t y , U n i f i e d S o i l C l a s s i f i c a t i o n 49 FIGURE 2.17: Gr a i n S i z e D i s t r i b u t i o n s f o r Surface M a t e r i a l s at S i t e Mg1 51 FIGURE 2.18: S i t e Mg1 Samples: Texture, P l a s t i c i t y , U n i f i e d S o i l C l a s s i f i c a t i o n 52 FIGURE 2.19: Gr a i n S i z e D i s t r i b u t i o n s f o r M a t e r i a l s F i l l i n g the Head of the G u l l y , S i t e M1 54 FIGURE 2.20: Gr a i n S i z e D i s t r i b u t i o n s f o r Surface M a t e r i a l s at S i t e s A1 and Mg2 55 FIGURE 2.21: P r o f i l e of A l b e r t a Creek 57 FIGURE 2.22: P r o f i l e of Magnesia Creek 64 FIGURE 2.23: Map and P r o f i l e of Magnesia Creek near Logging Road 68 FIGURE 2.24: Debris Levee, Stream Confluences, Middle Reaches of Magnesia Creek 69 FIGURE 2.25: P r o f i l e of M Creek 72 FIGURE 2.26: P r o f i l e of Loggers Creek 78 FIGURE 3.1: L o c a t i o n s of Main Study S i t e s 85 FIGURE 4.1: Main Debris Sources i n Study Area 94 FIGURE 4.2: Large R o c k f a l l and R o c k s l i d e S i t e s 96 FIGURE 4.3: Upper P o r t i o n s of S i t e s M2 and M3 99 v i i i Page FIGURE 4.4: Contour Map and P r o f i l e s of S i t e s M2 and M3 105 FIGURE 4.5: S t e r e o p l o t of Poles to F r a c t u r e s at the headscarps of S i t e s M2 and M3 108 FIGURE 4.6: De n s i t y Contours f o r the Poles P l o t t e d on F i g u r e 4.5 ( S i t e s M2 and M3) 109 FIGURE 4.7a: C o n d i t i o n f o r I n t e r l a y e r S l i p 111 FIGURE 4.7b: O r i e n t a t i o n s of J o i n t Planes at Headscarp of S i t e M2 111 FIGURE 4.8: S t e r e o p l o t of Poles to F r a c t u r e s at S i t e s M2 and M3 116 FIGURE 4.9: Schematic D e p i c t i o n s of Plane F a i l u r e and Wedge F a i l u r e 117 FIGURE 4.10: S t e r e o p l o t of Dip V e c t o r s of F r a c t u r e s at S i t e M1, and Average L i n e s of I n t e r s e c t i o n Between J o i n t Sets 118 FIGURE 4.11: Rock R a v e l l i n g and Tal u s S h i f t S i t e s 120 FIGURE 4.12: P r o f i l e of S i t e M3 125 FIGURE 4.13: Shallow D e b r i s S l i d e s and Slumps, S o i l Wedge F a i l u r e s 131 FIGURE 4.14: Contour Map of S i t e Mg3 De b r i s S l i d e s 134 FIGURE 4.15: Water L e v e l s i n Standpipes at S i t e Mg3, With P r e c i p i t a i o n at two Nearby S t a t i o n s 138 FIGURE 4.16: Schematic Drawing of Water C i r c u l a t i o n Near Top of G u l l y , S i t e M1 142 FIGURE 4.17: Contour Map of S i t e Mg1 144 FIGURE 4.18: Upper Part of S i t e Mgl 145 FIGURE 4.19: Water L e v e l s i n Standpipes at S i t e Mgl, With P r e c i p i t a i o n at two Nearby S t a t i o n s 148 FIGURE 4.20: Schematic Drawing of the Scar Caused by a S o i l Wedge Debris F a i l u r e 151 FIGURE 4.21: C o l l u v i a l R a v e l l i n g S i t e s 152 FIGURE 4.22: E r o s i o n Pin G r i d Layout, S i t e A1 160 FIGURE 4.23: Snow Avalanche S i t e s 163 FIGURE 4.24: P r o f i l e of S i t e Mg2 (Lower Part of Magnesia Creek T r i b u t a r y L2) 168 FIGURE 5.1: P r o f i l e of Part of Magnesia Creek 186 FIGURE 6.1: Stream Bed C o n d i t i o n Under F l u i d Shear 199 FIGURE 6.2: Geometry of Impulsive Loading of Creek Bed M a t e r i a l s By H i l l s l o p e F a i l u r e s 206 FIGURE A2.1: The F i e l d Permeameter 230 FIGURE A2.2: Determination of Steady State I n f i l t r a t i o n 230 FIGURE A2.3: F i e l d Permeameter Te s t s at S i t e Mg1 236 FIGURE A2.4: F i e l d Permeameter Te s t s at S i t e Mg3 237 ix LIST OF PLATES Page PLATE 2.1: Study Area Watersheds, Viewed From Howe Sound 39 PLATE 2.2: A l b e r t a Creek Watershed 58 PLATE 2.3: De b r i s Dam on A l b e r t a Creek 62 PLATE 2.4: Magnesia Creek Watershed 63 PLATE 2.5: M Creek Watershed 73 PLATE 2.6: D e b r i s Sources, Mid-reaches of M Creek 73 PLATE 2.7: Loggers Creek Watershed 79 PLATE 4.1: R o c k s l i d e , S i t e M2 101 PLATE 4.2: D i s t u r b e d Trees, Large Tension Crack at Top of S i t e M2 102 PLATE 4.3: A n t i s l o p e Scarp at S i t e M2 106 PLATE 4.4: De b r i s Chutes, Loggers Creek Watershed 121 PLATE 4.5: Log A c t i n g as De b r i s B u t t r e s s , S i t e M3 123 PLATE '4.6: Paint L i n e s , S i t e M3, i n 1986 127 PLATE 4.7: Debris S l i d e s , S i t e Mg3 135 PLATE 4.8: De b r i s i n Magnesia Creek at Toe of S o i l Wedge, S i t e Mg1 146 PLATE 4.9: Colluvium Exposed i n Road Cut at M Creek, E l e v a t i o n 690 m. 155 PLATE 4.10: S i t e of E r o s i o n P in G r i d , S i t e A1 159 PLATE 4.11: Snow Avalanche Track, S i t e Mg2 166 PLATE 4.12: Pai n t L i n e s , S i t e Mg2, 1986 171 PLATE 4.13: Log Debris i n Magnesia Creek T r i b u t a r y L3 174 PLATE 5.1: Debris Accumulations i n Magnesia Creek 187 x ACKNOWLEDGEMENTS I am deeply indebted to Dr. Michael Bovis f o r h i s guidance, encouragement, o c c a s i o n a l prodding, and l o g i s t i c a l support throughout t h i s p r o j e c t . I a l s o thank Drs. S. 0 . R u s s e l l and M. Church f o r t h e i r suggestions. Dr. 0 . Hungr reviewed a r e l a t e d paper, co-authored with M. B o v i s , and some of h i s comments have been i n c o r p o r a t e d i n t h i s t h e s i s . B i l l G r a i n g e r , Doug Johnson, and Trevor Gale a s s i s t e d i n the f i e l d and l a b o r a t o r y i n three s u c c e s s i v e summers. Sandy Lapsky, Penny Jones, Don Jones, and Derek Kaye a l s o p r o v i d e d o c c a s i o n a l f i e l d a s s i s t a n c e . Thanks a l s o to s e v e r a l members of the geomorpholgical community at U.B.C., p a r t i c u l a r l y Peter Buchanan, Dan Hogan, John Wolcott, Joe Desloges, and J e f f Schmok, for many u s e f u l d i s c u s s i o n s and other a s s i s t a n c e . I thank the c o u n c i l of the V i l l a g e of L i o n s Bay f o r a l l o w i n g me v e h i c l e access to the upper watersheds. The B. C. M i n i s t r y of T r a n s p o r t a t i o n and Highways s u p p l i e d data from t h e i r remote weather s t a t i o n s i n the Howe Sound Area. I am a l s o indebted to my brother, B i l l , f o r hundreds of hours of f r e e computer time, and f o r a r r a n g i n g h i s overseas v a c a t i o n to c o i n c i d e with the f i n a l four months of t h e s i s w r i t i n g . F i n a l l y , I thank Sandy Lapsky f o r her encouragement and moral support, and f o r a s s i s t i n g with the f i n a l p r e p a r a t i o n of t h i s t h e s i s . x i Chapter 1 INTRODUCTION 1.1 INTRODUCTION TO DEBRIS TORRENTS The term " d e b r i s flow" i s w e l l known i n the geomorphic l i t e r a t u r e , but " d e b r i s t o r r e n t " (Swanston and Swanson, 1976) i s a r e l a t i v e l y new and c o n t r o v e r s i a l term which seems to be p e c u l i a r to the P a c i f i c Northwest r e g i o n . A d e b r i s flow can be d e f i n e d as the f a i l u r e of s a t u r a t e d m a t e r i a l which deforms c o n t i n u o u s l y as a more or l e s s v i s c o u s s l u r r y . A d e b r i s t o r r e n t i s a s p e c i a l c l a s s of d e b r i s flow, which occurs i n a steep pre-e x i s t i n g stream channel, and thus can a t t a i n a very h i g h water content and move very r a p i d l y . Cohesive m a t e r i a l s are r e l a t i v e l y unimportant i n the water-debris mixture (Miles and K e l l e r h a l s , 1981). Debris t o r r e n t s are of i n t e r e s t from a geomorphic p o i n t of view because of the tremendous amount of geomorphic work they do, and from an e n g i n e e r i n g p o i n t of view because of the hazards they pose to human a c t i v i t i e s . They have long been known i n mountain-ous regions of Japan and Europe, where high p o p u l a t i o n d e n s i t i e s have f o r c e d people to l i v e where t o r r e n t s are common. Only r e l a t i v e l y r e c e n t l y have people i n B r i t i s h Columbia begun to l i v e i n s i m i l a r p o t e n t i a l l y hazardous areas. As a r e s u l t , i t i s only in the l a s t 10-15 years that t h i s phenomenon has been recognised and s t u d i e d l o c a l l y . Two communities i n southwest B r i t i s h Columbia have r e c e n t l y been d i r e c t l y a f f e c t e d by d e b r i s t o r r e n t s : Port A l i c e (Nasmith and Mercer, 1979), and L i o n s Bay (Thurber C o n s u l t a n t s , 1983). In a d d i t i o n , major t r a n s p o r t a t i o n c o r r i d o r s 1 i n the r e g i o n , such as those used by the Squamish Highway north of Vancouver and the Trans-Canada Highway west of Hope, have r e c e n t l y been d i s r u p t e d by d e b r i s t o r r e n t a c t i v i t y . These events have l e d to i n c r e a s e d awareness of the p o t e n t i a l hazard in B r i t i s h Columbia, and have spurred a number of recent s t u d i e s ( M i l e s and K e l l e r h a l s , 1981; Thurber C o n s u l t a n t s , 1983; VanDine, 1985a). Among the e a r l i e s t users of the term " d e b r i s t o r r e n t " were Swanston and Swanson (1976), who gave the f o l l o w i n g d e s c r i p t i o n : "Debris t o r r e n t s t y p i c a l l y occur i n steep i n t e r m i t t e n t f i r s t - and second-order channels. These events are t r i g g e r e d d u r i n g extreme d i s c h a r g e events by s l i d e s from adjacent h i l l s l o p e s , which enter a channel and move d i r e c t l y downstream, or by the breakup and m o b i l i z a t i o n of d e b r i s accumulations in the channel. The i n i t i a l s l u r r y of water and organic d e b r i s commonly e n t r a i n s l a r g e q u a n t i t i e s of a d d i t i o n a l i n o r g a n i c and l i v i n g and dead organic m a t e r i a l from the stream bed and banks. Some t o r r e n t s are t r i g g e r e d by d e b r i s avalanches of l e s s than 100 m3, but u l t i m a t e l y i n v o l v e 10,000 m3 of d e b r i s e n t r a i n e d along the t r a c k of the t o r r e n t . As the t o r r e n t moves downstream, hundreds of meters of channel may be scoured to bedrock. When a t o r r e n t l o s e s momentum, there i s d e p o s i t i o n of a t a n g l e d mass of l a r g e organic d e b r i s in a matrix of sediment and f i n e organic m a t e r i a l c o v e r i n g areas up to s e v e r a l h e c t a r e s " (p. 213). M i l e s and K e l l e r h a l s (1981) noted t h a t : "There i s o b v i o u s l y a complete, gradual t r a n s i t i o n from f a s t d e b r i s t o r r e n t s on e x c e e d i n g l y steep s l o p e s , moving mainly boulders and f o r e s t d e b r i s , to much slower-moving d e b r i s or mud flows which c o n t a i n a higher percentage of cohesive m a t e r i a l s and can occur on remarkably f l a t s l o p e s " (p. 398). I t seems u s e f u l , t h e r e f o r e , to r e t a i n the term "d e b r i s t o r r e n t " to d i f f e r e n t i a t e between the type of event d e s c r i b e d above, which i s p r e v a l e n t i n the P a c i f i c Northwest and elsewhere, and the slower-moving d e b r i s flows d e s c r i b e d by r e s e a r c h e r s such 2 as Johnson and Rodine (1984), Costa (1984), and o t h e r s . The most important d i f f e r e n c e between a d e b r i s t o r r e n t and a water f l o o d i s the tremendous amount of m a t e r i a l other than water which the t o r r e n t can t r a n s p o r t . T h i s makes t o r r e n t s much l e s s p r e d i c t a b l e and p o t e n t i a l l y much more d e s t r u c t i v e than f l o o d s . The h y d r o l o g i c a l r e l a t i o n s normally used to p r e d i c t f l o o d occurrence and magnitude cannot be a p p l i e d to d e b r i s t o r r e n t s , and e n g i n e e r i n g works such as b r i d g e s , c u l v e r t s , and b u i l d i n g s must be designed d i f f e r e n t l y ( M i l e s and K e l l e r h a l s , 1981) . 1.1.1 Requirements for T o r r e n t Occurrence Before a d e b r i s t o r r e n t can occur, the f o l l o w i n g four c o n d i t i o n s must be s a t i s f i e d : ( i ) there must be a s u f f i c i e n t l y steep and c o n f i n e d channel; ( i i ) there must be a s u f f i c i e n t l y l a r g e runoff event; ( i i i ) there must be a supply of m o b i l i z a b l e d e b r i s i n the channel; and ( i v ) there must be a t r i g g e r i n g event. Items ( i i i ) and ( i v ) above are the p r i n c i p a l f o c i of t h i s t h e s i s , and are d i s c u s s e d i n Chapters 4 and 6 r e s p e c t i v e l y . Items ( i ) and ( i i ) are d i s c u s s e d below. Channel g r a d i e n t c o n t r o l s the s t a b i l i t y of the m a t e r i a l s . As Takahashi (1981) has shown, d e b r i s i n the channel w i l l be m o b i l i z e d when the downslope component of the weight of the d e b r i s , p l u s the h y d r a u l i c drag from stream flow, exceed the boundary f r i c t i o n and i n t e r n a l shearing r e s i s t a n c e of the 3 m a t e r i a l . I t i s d i f f i c u l t to d e f i n e a " s u f f i c i e n t l y " steep slope angle, but i t has been found that most d e b r i s t o r r e n t s i n i t i a t e on slopes of at l e a s t 25° ( u s u a l l y over 30°), and commence d e p o s i t i o n on slopes of 10°-15° (Thurber C o n s u l t a n t s , 1983). Channel confinement c o n t r o l s the c o n c e n t r a t i o n of both water and d e b r i s i n the channel, and the h y d r a u l i c r a d i u s , and hence the " e f f i c i e n c y " of the flow. A g r e a t e r t h i c k n e s s of d e b r i s and water can be expected i n a narrow channel than i n a wider one, which i n c r e a s e s the l i k e l i h o o d of d e b r i s t o r r e n t i n i t i a t i o n . These concepts are examined more f u l l y i n Chapter 6. It i s a l s o very d i f f i c u l t to determine what c o n s t i t u t e s a " s u f f i c i e n t l y " l a r g e r u n o f f event. I t has been found t h a t , under the r i g h t c o n d i t i o n s , a r a i n s t o r m of only moderate i n t e n -s i t y and d u r a t i o n can t r i g g e r a d e b r i s t o r r e n t , while under d i f f e r e n t c o n d i t i o n s a heavy storm at the same l o c a t i o n might have no e f f e c t (Miles and K e l l e r h a l s , 1979; Thurber C o n s u l t a n t s , 1983; VanDine, 1985a). Thurber C o n s u l t a n t s (1983) found that v i r t u a l l y a l l of the known t o r r e n t s i n the Howe Sound area were a s s o c i a t e d with storms with r e t u r n p e r i o d s of only two years or l e s s at the nearest weather s t a t i o n . The . i n f l u e n c e of snowmelt i s a l s o very d i f f i c u l t to q u a n t i f y . As a minimum, i t appears that p r e c i p i t a t i o n p l u s snowmelt must be enough to s a t u r a t e the stream bed and banks, and stream d i s c h a r g e must be s u f f i c i e n t to s u s t a i n the t o r r e n t pore p r e s s u r e s . The i n f l u e n c e of streamflow on d e b r i s t o r r e n t s i s d i s c u s s e d i n d e t a i l i n Chapter 6. 4 1.1.2 Debris Torrent Mechanisms Much of our knowledge of d e b r i s t o r r e n t mechanisms comes from Japanese work (e.g. Okuda, 1978; Takahashi, 1981) though r e c e n t l y there have been s i g n i f i c a n t c o n t r i b u t i o n s from l o c a l i n v e s t i g a t o r s such as Hungr et a l . (1984) and VanDine (1985a). While the causes of d e b r i s t o r r e n t s are not f u l l y understood, o b s e r v a t i o n s i n Japan and elsewhere have i n c r e a s e d our under-standing of some aspects of the phenomenon. I t i s u s e f u l to d i v i d e d e b r i s t o r r e n t channels i n t o three zones: i n i t i a t i o n , t r a n s p o r t and e r o s i o n , and d e p o s i t i o n (Figure 1.1). Although the boundaries are u s u a l l y t r a n s i t i o n a l and d i f f i c u l t to l o c a t e , the t o r r e n t mechanisms a c t i n g i n these zones are somewhat d i f f e r e n t , as d i s c u s s e d below. F i g u r e 1.1: P r o f i l e of a T y p i c a l D e b r i s T o r r e n t , C o a s t a l B r i t i s h  Columbia ( a f t e r Thurber C o n s u l t a n t s , 1983; Nasmith and Mercer, 1979T 5 1.1.2.1 I n i t i a t i o n Because more energy i s r e q u i r e d to m o b i l i z e d e b r i s than to t r a n s p o r t i t , the i n i t i a t i o n zone i s g e n e r a l l y much steeper than the t r a n s p o r t and e r o s i o n zone ( F i g u r e 1.1). As noted above, i t has been found that most t o r r e n t i n i t i a t i o n zones have g r a d i e n t s of at l e a s t 25°, and u s u a l l y over 30° (Thurber C o n s u l t a n t s , 1983). Some s o r t of t r i g g e r i s a l s o r e q u i r e d to m o b i l i z e the d e b r i s , three common ones being: ( i ) high r u n o f f ( f l u i d s hear); ( i i ) h i l l s l o p e f a i l u r e or snow avalanche i n t o channels (impulsive l o a d i n g ) ; and ( i i i ) ground v i b r a t i o n . A d e t a i l e d d i s c u s s i o n of d e b r i s t o r r e n t t r i g g e r s i s given i n Chapter 6. 1.1.2.2 T r a n s p o r t a t i o n and E r o s i o n Once i n i t i a t e d , a t o r r e n t w i l l continue moving pr o v i d e d the channel remains steep and c o n f i n e d enough to t r a n s p o r t the debris-water mixture. The c r i t i c a l s l o p e , below which t r a n s p o r t w i l l cease, i s d i f f i c u l t to d e f i n e , as i t depends on a v a r i e t y of f a c t o r s , such as the nature of the d e b r i s and i t s water content. Takahashi (1981) suggested that t o r r e n t s g e n e r a l l y maintain motion i n mountain canyons steeper than 15°, while Thurber Con s u l t a n t s (1983) s t a t e d that s l o p e s g r e a t e r than 10° are r e -q u i r e d . T h i s p o i n t i s d i s c u s s e d f u r t h e r i n the context of d e b r i s d e p o s i t i o n ( S e c t i o n 1.1.2.3). Almost a l l i n v e s t i g a t o r s r e p o r t that c o n s i d e r a b l e e r o s i o n of the channel bed and banks occurs downstream from the p o i n t of t o r r e n t i n i t i a t i o n (e.g. Swanston and Swanson, 1976). E n t r a i n -ment of a d d i t i o n a l m a t e r i a l can occur by e r o s i o n and undercut-6 t i n g of oversteepened banks, or by scour of bed m a t e r i a l . Innes (1983) s t a t e d that i f the d e b r i s c o n c e n t r a t i o n a t the f r o n t of the flow i s l e s s than the maximum p o s s i b l e , d e b r i s from the bed w i l l be i n c o r p o r a t e d . Takahashi (1981) showed that t h i s process of growth conti n u e s u n t i l a steady s t a t e v o l u m e t r i c g r a i n c o n c e n t r a t i o n , C, .., i s a c h i e v e d : ' d 0 0 <a~ (<5 - P ) (tan* - tane) v 1 * 1 ; where: p = f l u i d d e n s i t y d = g r a i n d e n s i t y 6 = slope angle of channel bed $ = angle of shearing r e s i s t a n c e of the bed m a t e r i a l . S e v e r a l models have been proposed to d e s c r i b e d e b r i s flow motion and growth. Johnson (1970) and Johnson and Rodine (1984) suggested a Coulomb-viscous model: that i s , a model i n c o r p o r a t i n g a Coulomb s t r e n g t h and a Newtonian v i s c o u s term. Takahashi (1981) favoured a more p u r e l y Newtonian model (Innes, 1983). Innes (1983) reviewed both t h e o r i e s , and suggested that e i t h e r may be a p p r o p r i a t e , depending upon the amount of c l a y p r e s e n t . Where c l a y contents are low, Newtonian models should be more a p p r o p r i a t e (Innes, 1983). Thus, Takahashi's model seems to be the best f o r the bouldery d e b r i s t o r r e n t s of the P a c i f i c North-west r e g i o n . T h i s i m p l i e s that r e s i s t a n c e to shear r e s i d e s s o l e l y i n some analogue of f l u i d v i s c o s i t y . As a t o r r e n t moves down the channel, an i n t e r n a l s o r t i n g process occurs, f o r c i n g the l a r g e r boulders and cobbles to the top of the f l o w i n g mass. S e v e r a l r e s e a r c h e r s have c i t e d Bagnold's (1954) d i s p e r s i v e s t r e s s as a l i k e l y cause of t h i s 7 phenomenon. Since the hig h e s t v e l o c i t i e s are found at the top of the fl o w i n g mass, the l a r g e s t m a t e r i a l i s f o r c e d to the f r o n t of the flow (Takahashi, 1981), r e s u l t i n g i n well-documented bouldery f r o n t s (Okuda, 1978; Innes, 1983; VanDine, 1985a). The f r o n t i t s e l f has l i t t l e e r o s i v e power, and may flow over roads without even damaging the pavement (Takahashi, 1981), but the more t u r b u l e n t and more l i q u i d mixture behind the f r o n t may cause c o n s i d e r a b l e e r o s i o n (M. Church, p e r s . comm., 1983). The bouldery f r o n t has a c o n s i d e r a b l e i n f l u e n c e on the nature of the d e b r i s t o r r e n t as a whole. The f r o n t can become s t a l l e d i f the channel suddenly narrows, f l a t t e n s , or tu r n s , forming a temporary dam. T h i s r e s u l t s from the high f r i c t i o n a l s t r e n g t h of t h i s rubble phase. When the dam i s breached, the t o r r e n t resumes with renewed v i g o u r , s i n c e flow depth i s now g r e a t e r . If t h i s happens at s e v e r a l p o i n t s along the channel, the t o r r e n t w i l l appear to p u l s a t e as i t passes an o b s e r v a t i o n p o i n t downstream. T h i s type of behaviour i s w e l l documented (M i l e s and K e l l e r h a l s , 1981; R u s s e l l , 1972; Rapp and Nyberg, 1980). F r i c t i o n at the boundary of the flow causes the edges of the t o r r e n t f r o n t to be d e p o s i t e d as coarse levees on e i t h e r s i d e of the stream channel. Costa (1983) c i t e d a lack of damage to small t r e e s surrounded by d e b r i s flow levees as evidence that v e l o c i t i e s are very low at the flow margins. Levees may extend a l l the way to the d e b r i s fan, and many t o r r e n t s may d e p o s i t most of the t r a n s p o r t e d m a t e r i a l as levees (Costa, 1983). The bouldery f r o n t may generate very high impact pressures on u p r i g h t o b s t a c l e s , s i n c e most of the flow momentum i s concen-8 t r a t e d t h e r e . Bridges at the mouth of the c o n f i n e d p a r t of the channel o f t e n bear the brunt of the t o r r e n t as the f r o n t , along with logs and other l a r g e d e b r i s , surges out of the g u l l y . T h i s i s e s p e c i a l l y common along the Squamish Highway north of Vancou-ver, as seen by the recent d e s t r u c t i o n of the highway b r i d g e s on M Creek and A l b e r t a Creek (Thurber C o n s u l t a n t s , 1983). In other cases, b u i l d i n g s are the f i r s t o b s t a c l e s encountered, and can be s e v e r e l y damaged, as at Port A l i c e (Nasmith and Mercer, 1979) and L i o n s Bay (Thurber C o n s u l t a n t s , 1983). In some cases, s t r o n g ground v i b r a t i o n s have been observed to accompany the passage of a d e b r i s t o r r e n t (Okuda, 1978; Rapp and Nyberg, 1980). Okuda (1978) suggested that the v i b r a t i o n s c o u l d h e l p to f l u i d i s e the d e b r i s by d e c r e a s i n g i t s s h e a r i n g r e s i s t a n c e . Rapp and Nyberg (1980) added that the v i b r a t i o n s c o u l d help to d e s t a b i l i s e temporary dams which have formed i n the channel. More r e s e a r c h i s r e q u i r e d on t h i s aspect of d e b r i s t o r r e n t motion. 1.1.2.3 Deposi t ion When the c o n d i t i o n s necessary f o r d e b r i s t r a n s p o r t are no longer s a t i s f i e d , the t o r r e n t w i l l slow, and d e b r i s w i l l be d e p o s i t e d . T h i s normally occurs at the end of the steep, c o n f i n e d channel, where d e b r i s fans o f t e n form in the d e p o s i t i o n zones. These fans are s i m i l a r to the more f a m i l i a r a l l u v i a l or f l u v i a l fans, but normally c o n t a i n c o a r s e r m a t e r i a l and have steeper g r a d i e n t s (Ryder, 1971). They are o f t e n convex upwards in p r o f i l e , and may e x h i b i t lobes or bulges, whereas f l u v i a l fans tend to be concave upwards (Thurber C o n s u l t a n t s , 1983). 9 Thurber C o n s u l t a n t s (1983) found that most d e b r i s fans along Howe Sound have g r a d i e n t s between 10° and 16°. The c r i t i c a l s lope depends upon the d e b r i s c h a r a c t e r i s t i c s , and some fans can have c o n s i d e r a b l y lower g r a d i e n t s . Fans i n the Howe Sound area f l a t t e r than 4° do not appear to have been b u i l t by d e b r i s t o r r e n t a c t i v i t y (Thurber C o n s u l t a n t s , 1983). Takahashi (1981) analysed the d e p o s i t i o n of d e b r i s where the slope of the stream channel decreases a b r u p t l y , without an accompanying i n c r e a s e i n channel width, and showed that d e b r i s would stop moving i f the slope angle, 6 , i n the downstream s e c t i o n were such t h a t : 4 - „ r , a ^ (o - p )Cdu tana 0 v tane < T~Z \-= (1.2) ( 0 - p ;Cdu + p where C^u i s the g r a i n c o n c e n t r a t i o n i n the steeper channel upstream, a i s the dynamic f r i c t i o n angle of the p a r t i c l e s i n the flow, and a l l other parameters are as i n equation (1.1). If C^ u s a t i s f i e s equation (1.1), equation (1.2) can be s i m p l i f i e d t o : tane < J f ^ T t a n e u (1.3) tan <j) where 6 « i s the slope angle i n the upstream s e c t i o n . Since tana i s g e n e r a l l y smaller than tan<|>, the flow can reach a f l a t t e r angle than a (Takahashi, 1981). Complete developments of these equations are given i n Takahashi (1981) and Innes (1983), and an example of the use of (1.3) i s given i n S e c t i o n 5.1. A remarkable aspect of the d e p o s i t i o n zones of d e b r i s t o r r e n t s i s the r a p i d l o s s of energy which occurs once the flow moves out of the channel and onto the f a n . Th i s i s probably r e l a t e d to r a p i d drainage of the unconfined, c o a r s e - g r a i n e d 10 d e b r i s , and the attendant i n c r e a s e i n f r i c t i o n a l s t r e n g t h . There are s e v e r a l r e p o r t s of c a r s or b u i l d i n g s being almost completely b u r i e d by d e b r i s , yet s u s t a i n i n g l i t t l e or no damage (Nasmith and Mercer, 1979; VanDine, 1985a). The d e b r i s appears to flow as a pseudoviscous mass once most of the energy i s d i s s i p a t e d . Although damage to s t r u c t u r e s on the lower p a r t s of fans i s u s u a l l y minimal from the d e b r i s i t s e l f , the a s s o c i a t e d f l o o d i n g can be severe. As d e b r i s s e t t l e s and blocks the channel, the runoff which f o l l o w s the t o r r e n t may be d i v e r t e d a c r o s s the fan, causing c o n s i d e r a b l e p r o p e r t y damage (Nasmith and Mercer, 1979; E i s b a c h e r and Clague, 1981). F l o o d water can a l s o rework the d e p o s i t e d d e b r i s , causing f u r t h e r damage (Eisbacher and Clague, 1981). 1.2 REVIEW OF DEBRIS TORRENT WORK IN SOUTHWEST BRITISH COLUMBIA One of the f i r s t d i s c u s s i o n s of d e b r i s t o r r e n t s i n the P a c i f i c Northwest was i n a paper by Swanston and Swanson (1976). Although d e b r i s t o r r e n t s were only a small p a r t of t h i s paper, t h e i r d e s c r i p t i o n of the phenomenon remains one of the best and most o f t e n c i t e d i n the l o c a l l i t e r a t u r e . T h i s was a l s o one of the f i r s t papers to a c t u a l l y use the term " d e b r i s t o r r e n t " i n s t e a d of simply " d e b r i s flow". At about the same time, B r i t i s h Columbians f i r s t became aware of the p o t e n t i a l hazard posed by d e b r i s t o r r e n t s . R u s s e l l (1972) had d e s c r i b e d some "severe f l o o d s " (almost c e r t a i n l y d e b r i s t o r r e n t s ) i n 1969 on the east s i d e of Howe Sound, but the problem remained l a r g e l y unknown u n t i l 1973. The community of 1 1 Port A l i c e , on northern Vancouver I s l a n d , had been b u i l t on a d e b r i s fan i n 1965. In 1973 and again i n 1975, t h i s community " s u f f e r e d s u b s t a n t i a l damage from d e b r i s flows, f o r t u n a t e l y with no l o s s of l i f e . These events focussed a t t e n t i o n on t h i s hazard . (Nasmith and Mercer, 1979). Nasmith and Mercer (1979) d e s c r i b e d these two d e b r i s t o r r e n t s , and the dykes which were subsequently b u i l t to p r o t e c t the town. In December 1979 the community of Port Moody, near Vancou-ver, was s t r u c k by a d e b r i s t o r r e n t which caused c o n s i d e r a b l e damage, but a g a i n , f o r t u n a t e l y , no l o s s of l i f e (Eisbacher and Clague, 1981). L i k e Port A l i c e , Port Moody i s l a r g e l y b u i l t on d e b r i s f a n s . Included i n E i s b a c h e r and Clague's (1979) paper was a l i s t of some h i s t o r i c a l d e b r i s t o r r e n t s i n c o a s t a l B r i t i s h Columbia, i n c l u d i n g a c a t a s t r o p h i c event in 1921 at B r i t a n n i a Beach (which may not have been a d e b r i s t o r r e n t ) i n which 37 l i v e s were l o s t . M i l e s and K e l l e r h a l s (1981) d e s c r i b e d s e v e r a l d e b r i s t o r r e n t s i n the area of Hope in December 1980, and i n c l u d e d a s e c t i o n on e n g i n e e r i n g i m p l i c a t i o n s , i n c l u d i n g f l o o d p l a i n zoning and land use. The general p u b l i c , and government, however, remained l a r g e l y unaware of the hazard. Roads and a few homes had been damaged, but no l i v e s had been l o s t from d e b r i s t o r r e n t s s i n c e 1964, when three people were k i l l e d i n an i s o l a t e d l o g g i n g camp at Ramsay Arm, 200 km northwest of Vancouver (Eisbacher and Clague, 1981). That a l l changed on the night of 28 October 1981, when a d e b r i s t o r r e n t swept down M Creek, d e s t r o y i n g the Squamish Highway b r i d g e . In the darkness and c o n f u s i o n , nine people l o s t 12 t h e i r l i v e s . S e c t i o n 2.9.3.3 i n c l u d e s a d e t a i l e d d e s c r i p t i o n of t h i s event. The M Creek d i s a s t e r spurred the B r i t i s h Columbia M i n i s t r y of T r a n s p o r t a t i o n and Highways to commission Thurber Consultants to study the extent of the hazard along the Squamish Highway. The Thurber C o n s u l t a n t s ' (1983) r e p o r t was one of the most e x t e n s i v e s i t e - s p e c i f i c s t u d i e s of d e b r i s t o r r e n t s ever p e r f o r -med. Twenty-six small creeks along Howe Sound between Horseshoe Bay and B r i t a n n i a Beach were examined i n terms of t h e i r f l o o d and t o r r e n t p o t e n t i a l , and the l i k e l y maximum s i z e of such events. D e b r i s fans, many of which are p a r t l y developed, were zoned a c c o r d i n g to the degree of the hazard. D e t a i l s of t h i s area and i t s d e b r i s t o r r e n t h i s t o r y are given i n Chapter 2. Two papers stemming from Thurber C o n s u l t a n t s ' work are Hungr et a l . (1984) and VanDine (1985a). The former suggested ways of a n a l y s i n g d e b r i s t o r r e n t hazards to design remedial measures. VanDine (1985a) reviewed the s t a t e of knowledge of d e b r i s t o r r e n t s i n the southern Canadian C o r d i l l e r a , i n terms of c o n t r o l l i n g f a c t o r s , t o r r e n t hazard, and hazard m i t i g a t i o n . He a l s o drew on Japanese and European experience, and f u r t h e r developed some of Takahashi's (1981) c r i t e r i a f o r d e b r i s t o r r e n t i n i t i a t i o n (but see a l s o Bovis et a l . , 1985 and VanDine, 1985b). 1.3 SCOPE AND OBJECTIVES OF THESIS One of the four requirements f o r t o r r e n t occurrence l i s t e d i n S e c t i o n 1.1.1 i s a supply of m o b i l i z a b l e d e b r i s i n the stream channel. Without t h i s , even the most severe runoff event can 1 3 only produce a f l o o d , a phenomenon which i s much l e s s d e s t r u c t i v e and much b e t t e r understood than a d e b r i s t o r r e n t . A n a t u r a l s t a r t i n g p o i n t f o r understanding d e b r i s t o r r e n t s t h e r e f o r e i s the d e b r i s supply system. In t h i s t h e s i s , d e b r i s supply mechanisms and r a t e s of d e b r i s supply are examined i n four s m a l l watersheds on the east s i d e of Howe Sound, B r i t i s h Columbia. Chapter 2 d e a l s with the r e g i o n a l s e t t i n g i n terms of topography, geology, c l i m a t e , v e g e t a t i o n , land use, and h y d r o l -ogy. D e t a i l e d d e s c r i p t i o n s of the four study catchments are g i v e n , along with t h e i r known and i n f e r r e d h i s t o r i e s of t o r r e n t a c t i v i t y . Chapter 3 o u t l i n e s the f i e l d and l a b o r a t o r y study methods. Chapter 4 d e s c r i b e s the v a r i o u s d e b r i s supply mechan-isms which are a c t i v e i n t h i s area, and i n d i c a t e s t h e i r s p a t i a l d i s t r i b u t i o n . D e t a i l e d analyses of s p e c i f i c s i t e s are g i v e n . Chapter 5 d e a l s with d e b r i s r e d i s t r i b u t i o n i n the stream chan-n e l s . Chapter 6 examines d e b r i s t o r r e n t t r i g g e r i n g mechanisms i n the study a r e a . T h i s chapter i s i n c l u d e d because many of the mechanisms which supply d e b r i s to channels may themselves be capable of t r i g g e r i n g d e b r i s t o r r e n t s . In such cases, the d e b r i s supply and t o r r e n t t r i g g e r i n g mechanisms may be i n d i s t i n g u i s h -a b l e . The c o n c l u s i o n s of the t h e s i s are given in Chapter 7. 14 Chapter 2 EAST HOWE SOUND AREA Recently, the east shore of Howe Sound, n o r t h of Vancouver (Figure 2.1), has become n o t o r i o u s as one of the most d e b r i s t o r r e n t - p r o n e areas i n B r i t i s h Columbia. The amount of a t t e n t i o n t h i s area has r e c e i v e d i s a r e f l e c t i o n of i t s importance as a t r a n s p o r t a t i o n c o r r i d o r , i t s pr o x i m i t y to Vancouver, and i t s i n c r e a s i n g use as a r e s i d e n t i a l area, but a s i g n i f i c a n t t o r r e n t hazard does e x i s t t h e r e . While t h i s study focusses on only a small p o r t i o n of the east Howe Sound area, i t i s u s e f u l to look f i r s t at the general c h a r a c t e r i s t i c s of the e n t i r e area, because much of the e a r l i e r work has been in t h i s broader, r e g i o n a l c o n t e x t . 2.1 LOCATION AND TOPOGRAPHY Howe Sound i s a t y p i c a l f j o r d on the southwest co a s t of B r i t i s h Columbia. I t s mouth i s about 10 km northwest of Vancou-ver, and i t runs north from there f o r about 40 km to Squamish (Fi g u r e 2.1). P h y s i o g r a p h i c a l l y , the re g i o n l i e s w i t h i n the P a c i f i c Ranges of the Coast Mountains. The mountain w a l l on the east s i d e of Howe Sound r i s e s s t e e p l y from the s h o r e l i n e to a r i d g e s e p a r a t i n g the small c o a s t a l watersheds from those of the Cap i l a n o River and other r i v e r s to the east (Figure 2.1). T y p i c a l peaks on t h i s r i d g e are 1600 m or more i n e l e v a t i o n ; these are l o c a t e d 2 to 5 km from the s h o r e l i n e south of Brunswick P o i n t , and as much as 10 km i n l a n d f u r t h e r n o r t h . Twenty-six streams having watersheds v a r y i n g in 15 area from 0.3 to 54 km (but only three l a r g e r than 7.0 km ) flow west from the d i v i d e to the sea over a s h o r e l i n e d i s t a n c e of 29 km between Horseshoe Bay and B r i t a n n i a Creek (Figure 2.2). Stream g r a d i e n t s and watershed areas are given i n Table 2.1. (In t h i s t a b l e , b a s i n l e n g t h i s taken as the h o r i z o n t a l d i s t a n c e from the stream mouth to the r i d g e c r e s t , measured along the longest t r i b u t a r y ; and average g r a d i e n t i s d e f i n e d as the angle whose tangent i s equal to the basin r e l i e f d i v i d e d by the b a s i n l e n g t h ) . The unusual steepness of the topography i s i l l u s t r a t e d by Table 2.1, and by the contour l i n e s on F i g u r e 2.2. 2.2 BEDROCK GEOLOGY F i g u r e 2.3 shows the major g e o l o g i c a l u n i t s i n the east Howe Sound area. The most abundant are the p l u t o n i c rocks ( d i o r i t e , q u artz d i o r i t e , and g r a n o d i o r i t e ) which were i n t r u d e d mainly i n the l a t e Cretaceous p e r i o d (Roddick and Woodsworth, 1979). In the study area these tend to be predominantly quartz d i o r i t e . Most of the other rocks are part of the Gambier Group (mainly v o l c a n i c , with some sedimentary and metamorphic r o c k s ) , which are Lower Cretaceous i n age. These rocks are found mainly at lower e l e v a t i o n s , and in the upper reaches of some of the watersheds. V o l c a n i c rocks ( a n d e s i t e to r h y o d a c i t e ) dominate the Gambier Group in the study area, but shales and conglomerates are common at lower e l e v a t i o n s . In many of the watersheds a prominent topographic break i n slope occurs at the c o n t a c t between these two rock types. The p l u t o n i c rocks are r e l a t i v e l y r e s i s t a n t to weathering, 1 7 F i g u r e 2 . 2 : Topography o f Eas t Howe Sound Area B r i t a n n i a Creek B r i t a n n i a Beach T h i s t l e Creek Da i sy Creek Unnamed Creek No F u r r y Creek Unnamed Creek No Unnamed Creek No K a l l a h n e Creek Bertram Creek Brunswick P o i n t Cree Brunswick P o i n t Deeks Creek Loggers Creek M Creek Magnesia Creek A l b e r t a Bay A l b e r t a Creek Harvey Creek -L i o n s Bay Rundle Creek Lone Tree Creek Newman Creek T u r p i n Creek C h a r l e s Creek S t r i p Creek Mont izambert Creek S c l u f i e l d Creek Unnamed Creek No Disbrow Creek Horseshoe Bay - k i l o m e t r e s contour i n t e r v a l : 200 m 18 Table 2 .1 : Some C h a r a c t e r i s t i c s o f Drainage B a s i n s , Ea s t Howe Sound Area Creek B a s i n C h a r a c t e r i s t i c s Avg Creek P o r t i o n Years Area Length R e l i e f Average G r a d i e n t o f B a s i n o f G r a d i e n t Above fan Logged Logg ing (km 2) (km) Cm) (degrees) ( d e g r e e s ) a W a a B r i t a n n i a 28.5 10 .6 2025 10 8 . 3 - 11 13 '55 - '62 T h i s t l e . 1 .53 3-80 1326 19 2 • 19 0 Daisy- 2 . 3 I 3 . 6 5 1326 20 0 21 0 -Unnamed No. 9 2 .28 3-35 1195 19 6 13 0 -F u r r y 5^-3 12.4 1680 7 7 3 - 12 18 •50 - '77 Unnamed No. 8 1.20 2 .55 1006 21 5 14 0 -Unnamed No. 7 1. 14 2 . 3 8 1378 30 0 27 9 '68 - '79 K a l l a h n e 3 . 6 0 4 . 7 5 I652 19 2 20 16 p r i o r to Bertram 1.91 3 . 5 0 1646 25 2 21 0 0 0 Brunswick P o i n t 0 . 3 4 1. 20 503 22 7 29 5 •54 - '55 £ Deeks 11.5 8 . 0 0 I760 12 4 14 11 '54 - '55 Loggers 2.91 3 . 10 I650 28 0 27 41 '52 - ' 5 7 , '60 - '65 M 3 . 2 5 3 . 2 5 1720 27 9 28 38 '60 - '65 Magnesia 4 . 7 8 4 . 55 1760 21 1 19 32 p r i o r to h D < > '60 - ' 6 5 A l b e r t a 1.31 2. 70 I38O 27 1 27 o D '57? Harvey 6 .71 4 . 90 1646 18 6 14 22 '66 - '70 Rundle 0 . 3 0 1.30 600 24 8 23 0 -Lone Tree 1.44 3 .15 I520 25 8 30 0 -Newman 2 . 0 0 2 .40 1440 31 0 30 16 '66 T u r p i n 0 . 5 9 1.60 1140 35 5 35 25 •66 - '68 C h a r l e s 1.70 2 .55 1365 28 2 27 1 ? S t r i p 0.54 1.72 1120 33 . 1 30 0 '68 - ' 6 9 Montizambert 3-92 3-90 1425 20 . 1 22- 8 S c l u f i e l d 0 . 3 8 1.52 925 31 • 3 30 0 -Unnamed No. 1 0 . 5 6 1.75 1195 34 • 3 31 0 -Disbrow 1.33 2 . 6 0 1230 25 • 3 32 0 -Notes : a - data from Thurber C o n s u l t a n t s , I983 b - minor l o g g i n g d u r i n g road c o n s t r u c t i o n F i g u r e 2.3 : S i m p l i f i e d Bedrock Geology o f Ea s t Howe Sound Area Sky P i l o t Mounta in (2025 m) B r i t a n n i a Creek F u r r y Creek Brunswick P o i n t Boundary o f Study A r e a Brunswick Mounta in (1760 m) Harvey Creek s c a l e - k i l o m e t r e s contour i n t e r v a l : 500 m Horseshoe Bay P l u t o n i c r o c k s : g r a n o d i o r i t e and q u a r t z d i o r i t e , a l s o t o n a l i t e , minor gabbro . M a i n l y l a t e C r e t a c e o u s . Lower C r e t a -ceous Gambier Group: ande-s i t e to r h y o -d a c i t e f l ows , g reens tone , a r g i l l i t e , conglomerate , l i m e s t o n e , and s c h i s t . P a l e o z o i c (?) g n e i s s , s c h i s t , and a m p h i b o l i t e . (Geology from Roddick and Woodsworth, 1979) and tend to form high c l i f f s . In the study area the most impressive of these i s the northwest face of Mt. Harvey, which r i s e s 500 metres i n a d i s t a n c e of 250-300 m, an average slope of about 60°. E x t e n s i v e r o c k f a l l d e p o s i t s are found at the base of d i o r i t e c l i f f s throughout the study area. In some areas, such as i n the upper basins of C h a r l e s and M Creeks, these rocks are h i g h l y f r a c t u r e d and produce a c t i v e rock f a l l s or rock avalan-ches, which can act as major p o i n t sources of coarse d e b r i s i n the stream channels, and i n some cases may be d i r e c t l y respons-i b l e f o r the i n i t i a t i o n of d e b r i s t o r r e n t s . Church and Desloges (1984) noted t h a t : "Between Horseshoe Bay and Brunswick Point the l o c a t i o n and o r i e n t a t i o n of t o r r e n t - p r o n e d e b r i s channels and of major bedrock c l i f f s i s c o n t r o l l e d mainly by f r a c t u r e zones t r e n d i n g north of east and north, and by n o r t h -west t r e n d i n g metamorphic f o l i a t i o n " (p. 14). The l e s s competent v o l c a n i c rocks r a v e l more e a s i l y and more c o n t i n u o u s l y over time than the d i o r i t i c r ocks. A c t i v e l y r a v e l l i n g s l o p e s may c o n t r i b u t e c o n s i d e r a b l e amounts of d e b r i s to the stream channels over time, but t h i s m a t e r i a l tends to be much f i n e r and more e a s i l y broken down than the d i o r i t i c r ocks. Consequently, the p o t e n t i a l f o r l a r g e d e b r i s t o r r e n t s tends to be lower i n watersheds dominated by Gambier Group roc k s . 2.3 LATE GLACIAL HISTORY AND SURFICIAL GEOLOGY In some of the study area watersheds, s u r f i c i a l d e p o s i t s c o n t r i b u t e more d e b r i s to channels than do bedrock s l o p e s . For t h i s reason, a d e t a i l e d examination of the c h a r a c t e r and d i s t r i -b u tion of s u r f i c i a l m a t e r i a l s i n the study area i s r e q u i r e d i f 21 the d e b r i s t o r r e n t p o t e n t i a l i s to be assessed. The Howe Sound area was g l a c i a t e d s e v e r a l times i n the P l e i s t o c e n e Epoch. The l a s t of these episodes, the F r a s e r G l a c i a t i o n , began between 20,000 and 15,000 years ago i n t h i s area (Clague, 1981). During t h i s and e a r l i e r g l a c i a t i o n s the l a r g e v a l l e y now occupied by Howe Sound was deeply scoured by g l a c i a l i c e . Consequently, d e p o s i t s p r e - d a t i n g the l a s t g l a c i a -t i o n were l a r g e l y removed. The v a l l e y became "U-shaped", with steep w a l l s and a r e l a t i v e l y f l a t bottom. G l a c i e r s i n t r i b u t a r y v a l l e y s formed s e v e r a l c i r q u e s along the r i d g e s between Howe Sound and the v a l l e y s to the east and west. V a r y i n g t h i c k n e s s e s of t i l l were d e p o s i t e d on the steep mountain sl o p e s and i n the v a l l e y s . As the i c e receded, r e l a t i v e l y loose d e p o s i t s of a b l a t i o n t i l l were l e f t in some a r e a s . The area became i c e - f r e e between 12,500 and 10,000 years ago (Clague, 1981). As a r e s u l t of i s o s t a t i c d e p r e s s i o n d u r i n g and immediately a f t e r d e g l a c i a t i o n , e a r l y p o s t g l a c i a l r e l a t i v e sea l e v e l s were up to 200 m higher than present i n areas such as the F r a s e r Lowland, near Vancouver (Clague et a l . , 1982). Although t h i s p e r i o d was r e l a t i v e l y s h o r t - l i v e d , e x t e n s i v e fans of f l u v i a l and g l a c i o -marine m a t e r i a l s formed along the east s i d e of Howe Sound. The la r g e s i z e of these fans can be e x p l a i n e d by Clague's (1981) o b s e r v a t i o n t h a t : " I t i s probable that the bulk of p o s t g l a c i a l d e p o s i t s , except i n lake basins and at the mouths of major r i v e r s , were l a i d down w i t h i n a few hundred years f o l l o w i n g d e g l a c i a t i o n " (p. 19). As r e l a t i v e sea l e v e l f e l l due to r a p i d i s o s t a t i c recovery, these now e l e v a t e d fans were d i s s e c t e d (Church and Desloges, 1984). 22 These i n c i s i o n s continue to supply l a r g e amounts of m a t e r i a l to the lower reaches of the stream channels. The d i s t r i b u t i o n of s u r f i c i a l m a t e r i a l s i n the east Howe Sound area i s ra t h e r d i s c o n t i n u o u s . There are l a r g e areas, p a r t i c u l a r l y at higher l e v e l s , where s u r f i c i a l d e p o s i t s are absent and bare rock i s exposed. In areas where the bedrock i s not exposed, i t i s normally o v e r l a i n by a t h i n l a y e r of hard b a s a l t i l l from the F r a s e r G l a c i a t i o n . T h i s i n turn may be o v e r l a i n by s e v e r a l metres of l o o s e r a b l a t i o n t i l l or c o l l u v i u m , o f t e n c o n t a i n i n g l a r g e rock fragments or bo u l d e r s . Talus slopes are common i n the upper p a r t s of many of the catchments. 2.4 WEATHER AND CLIMATE The r e g i o n experiences a t y p i c a l west-coast c l i m a t e , with g e n e r a l l y m i l d temperatures and a strong winter maximum of p r e c i p i t a t i o n . Monthly mean p r e c i p i t a t i o n v a l u e s are shown i n F i g u r e 2.4, and re t u r n p e r i o d s f o r 24 hour p r e c i p i t a t i o n t o t a l s i n F i g u r e 2.5. The winter p r e c i p i t a t i o n maximum i s evident at a l l s t a t i o n s ( F i g u r e 2.4). The l o c a t i o n s of these s t a t i o n s are shown i n F i g u r e 2.1. D a i l y mean sea l e v e l temperatures range from 3°C i n January to 17°C i n J u l y (Church and Desloges, 1984). The h e a v i e s t p r e c i p i t a t i o n i s a s s o c i a t e d with moisture-laden c y c l o n e s which approach from the southwest i n l a t e autumn and e a r l y w i n t e r . I t i s not uncommon f o r a s e r i e s of these systems to pass through in a matter of a few days. As storms s t r i k e the mountain w a l l on the east s i d e of Howe Sound there i s c o n s i d e r -able o r o g r a p h i c enhancement of p r e c i p i t a t i o n ( F i g u r e 2.4). 23 Figure 2 .4: Monthly Mean P r e c i p i t a t i o n i n Howe Sound Area ( a f t e r Thurber Consultants, 1983) 50 0, J F M A M J J A S O N D Hollyburn Ridge: 951 m Mean annual snow water equivalent (mm) Mean annual r a i n (mm) Mean annual p r e c i p . (mm) Data from .Environment Canada (1982). Snow plus r a i n may not equal t o t a l due to v a r y i n g lengths of record. • Snow water equivalent Rain o •H -P d p •H ft •H O CD SH 400, 300-39 1211 1255 200-ZZZ2 100-0 J , F M A M J J A S . 0 N D Point Atkinson: 9 m 400, o •H p P •rH ft •H o 0 rH PH 300 75 1807 1897 200 100 ol J F M A M J J A S O N D Gambler Harbour: 61 m 400 3 300. o P 200-) nJ P •H o QJ rH PH CI zzz. 92 2073 2165 J F M A M J J A S O N D B r i t a n n i a Beach: 4-9 m 40O, 300. o P 200 cd p •rH .S1 I O O - I o co rH PH 1 177 2110 2247 J F M A M J J A S 0 N Squamish: 2 m D 24 H o l l y b u r n Ridge ( e l e v a t i o n 951 m) r e c e i v e s over twice the p r e c i p i t a t i o n at Point Atkinson ( e l e v a t i o n 9 m). Orographic enhancement i s most pronounced i n the area south of Brunswick P o i n t , probably because low l e v e l a i r f l o w s approach the c o a s t l i n e at a more o b l i q u e angle n o r t h of that p o i n t (Church and Desloges, 1984). There has been no success i n t r y i n g to r e l a t e p r e c i p i t a i o n events to d e b r i s t o r r e n t occurrences i n t h i s a r ea. I t was noted i n Chapter 1 that most d e b r i s t o r r e n t s i n the Howe Sound area have been a s s o c i a t e d with p r e c i p i t a t i o n events having r e t u r n p e r i o d s of two years or l e s s , while a number of more severe storms have f a i l e d to t r i g g e r t o r r e n t s (Thurber C o n s u l t a n t s , 1983). It w i l l be argued l a t e r t h a t t h i s i s l a r g e l y a conse-quence of d e b r i s supply and the a v a i l a b i l i t y of d e b r i s t o r r e n t t r i g g e r s , but the e x i s t e n c e of c e l l s of very high p r e c i p i t a t i o n w i t h i n f r o n t a l storms must a l s o be c o n s i d e r e d . T h i s type of a c t i v i t y has been detected i n the lower F r a s e r V a l l e y by Bonser (1982) through radar measurements. The Atmospheric Environment S e r v i c e (A. E. S.) m e t e o r o l o g i c a l s t a t i o n network i n the Howe Sound area i s too sparse to document t h i s e f f e c t , but some supplementary remote s t a t i o n s , operated i n t e r m i t t e n t l y i n the past two years by the p r o v i n c i a l M i n i s t r y of T r a n s p o r t a t i o n and Highways, pr o v i d e some f u r t h e r i n s i g h t . Data from two of these s t a t i o n s , p l u s four nearby A. E. S. s t a t i o n s , are given in Table 2.2 f o r two storms i n l a t e 1984. S t a t i o n l o c a t i o n s are shown in Fi g u r e 2.1. The v a r i a t i o n i n p r e c i p i t a t i o n from s t a t i o n to s t a t i o n and from day to day i s w e l l i l l u s t r a t e d . For example, 26 Table 2.2: P r e c i p i t a t i o n Totals (Rain Plus Snow Water Equivalent) f o r Two Storms i n Autumn 1984 : S t a t i o n 7 Day T o t a l (mm) 24 Hour Tota l s (mm) 10 Hour T o t a l s (mm) 2-8/10/84 7/10/84 8/10/84 9/10/84 7/10/84 8/10/84 9/10/84 Brunswick P i t > l 0 5 a 63 36 35 32 26 25 Mt. Harvey 216 119 63 69 68 40 61 Gambier Harbour 112.1 58.8 3 0 .1 39-0 * H o l l y b u r n Ridge 166.8 58.0 30 .6 48.0 * * P o i n t Atkinson 73-8 18.8 30.O 20 .6 * •* Squamish 166.7 85.O 35-4 59-8 * o 7-13/12/84 12/12/84 13/12/84 14/12/84 b 12/12/84 13/12/84° 14/12/84 Brunswick P i t >75 d 3 16 28 3 19 25 Mt. Harvey # 2 * •X- 2 Gambier Harbour 146.6 1.6 25. 2 5.0 •a-Hollyburn Ridge 1 5 4 .2 e Trace 26.0 3-6 * P o i n t Atkinson 111.2 0.0 40.0 14.2 •X- * Squamish 140.2 0.0 53-6 2 1 . 5 * * Notes a b c d e * Data not a v a i l a b l e 2-5 October. Other s t a t i o n s r e c e i v e d 5-2 - 25.4 mm on these days A debris t o r r e n t occured i n S c l u f i e l d Creek on t h i s day. Ten hour period from 16:15 on 13 December to 0 2 : 1 5 on 14 December. Data not a v a i l a b l e 10 December. Other s t a t i o n s r e c e i v e d 2 . 5 - 8.2 mm on t h i s day. Includes 84.2 mm snow water equivalent (84.2 cm snow). Indicates data missing or not a v a i l a b l e . a l l but one of the s t a t i o n s recorded between 30.1 and 36.0 mm of p r e c i p i t a t i o n on 8 October 1984, but p r e c i p i t a t i o n at these same s t a t i o n s ranged between 18.8 and 85.0 mm on 7 October, and between 20.6 and 59.8 mm on 9 October. The December storm set o f f a d e b r i s t o r r e n t on S c l u f i e l d Creek, but p r e c i p i t a t i o n i n the region was not e x c e s s i v e by l o c a l standards: r e t u r n p e r i o d s f o r 24 hour p r e c i p i t a t i o n t o t a l s ranged between 1.0 years at H o l l y -burn Ridge and 1.4 years at Point A t kinson (see F i g u r e 2.5). I f , as o f t e n happens, a r a p i d r i s e i n f r e e z i n g l e v e l accompanies heavy f r o n t a l p r e c i p i t a t i o n , a l a r g e snowmelt component may be added to the r e s u l t i n g r u n o f f , and t h i s hampers the a n a l y s i s of p r e c i p i t a t i o n as a d e b r i s t o r r e n t t r i g g e r . For example, snowmelt may w e l l have been a f a c t o r i n the S c l u f i e l d Creek d e b r i s t o r r e n t of 14 December 1984 s i n c e H o l l y b u r n Ridge r e c e i v e d snow e q u i v a l e n t to 84.2 mm of r a i n i n the week p r i o r to t h i s event (Table 2.2). Temperatures at H o l l y b u r n Ridge never exceeded 0°C dur i n g the p e r i o d 8 December - 12 December, and were always below -1.4°C f o r the l a s t four days of that p e r i o d , but reached 1.5°C and 1.0°C r e s p e c t i v e l y on 13 and 14 December. 2.5 NATURAL VEGETATION The f o r e s t cover in the Howe Sound area, along with the C a p i l a n o and Seymour River drainages to the e a s t , was d e s c r i b e d by O'Loughlin (1972a): "Most slopes support a dense cover of c o n i f e r o u s f o r e s t except on the most p r e c i p i t o u s b l u f f and bedrock areas. Western red cedar (Thuja p l i c a t a Donn), western hemlock (Tsuga h e t e r o p h y l l a (Ra f n.) Sarg.) and Douglas f i r (Pseudotsuga m e n z i e z i i (Mayr.) Franco) are the dominant canopy formers on the lower 28 s l o p e s . ... At approximately 900 m the i n c r e a s i n g importance of mountain hemlock (Tsuqa mertensiana (Bong.) Carr) yellow cedar (Chaemaecyparis n o o t k a t e n s i s (D. Don) Spach) and a m a b i l i s f i r (Abies a m a b i l i s (Dougl.) F o r b . ) , accompanied by a r e d u c t i o n i n o c c u r r -ence of western red cedar and western hemlock and the e l i m i n a t i o n of Douglas f i r , mark the lower s l o p e s of the 'Mountain Hemlock Zone' ( K r a j i n a 1965)" (p. 9). In the study area, the t r a n s i t i o n between the two f o r e s t types may occur s l i g h t l y higher than noted above; there i s an ext e n s i v e stand of Douglas f i r near 1100 m on the nort h s i d e of M Creek, near the top - of a l a r g e r o c k s l i d e ( S i t e M2) d e s c r i b e d i n S e c t i o n 4.3.1.2. Red a l d e r (Alnus rubra Bong.) i s very common along abandoned roads, and i n v a l l e y bottoms, avalanche t r a c k s , and other c l e a r e d areas. Shrubs such as salmonberry and fireweed grow i n avalanche t r a c k s , g u l l i e s and minor stream channels, and on recent l a n d s l i d e s c a r s . 2.6 LAND USE The lower slopes along Howe Sound are t r a v e r s e d by the B r i t i s h Columbia Railway (completed i n 1956) and by B r i t i s h Columbia Highway 99 (the Squamish Highway), which was completed in 1958. Since the completion of the highway, there has been c o n s i d e r a b l e r e s i d e n t i a l development near the s h o r e l i n e , p a r t i c u -l a r l y on fans at the mouths of cr e e k s . Other important land-use a c t i v i t i e s i n c l u d e l o g g i n g , mining, and e x t r a c t i o n of sand and g r a v e l . Copper was mined i n the northern areas i n the past, but there are no a c t i v e mines at prese n t . Many of the e l e v a t e d f l u v i a l fans have been mined f o r sand and g r a v e l , p a r t i c u l a r l y the one at the mouth of Fur r y Creek. Fourteen of the 26 Howe Sound watersheds have been logged 29 to some degree, mainly i n the 1950's and 1960's (see Table 2.1). The amount of timber h a r v e s t i n g i n these basins ranges from 1% in Cha r l e s Creek to 41% i n Loggers Creek. There i s a l a r g e network of abandoned l o g g i n g roads i n the r e g i o n , and many of these have d e t e r i o r a t e d c o n s i d e r a b l y . The i n f l u e n c e of l o g g i n g and road c o n s t r u c t i o n on d e b r i s supply w i l l be d i s c u s s e d i n d e t a i l i n Chapter 4. 2.7 HYDROLOGY The hydrology of the east Howe Sound area i s d i f f i c u l t to asse s s , because there are no gauged streams on the east s i d e of Howe Sound except Stawamus R i v e r , which ente r s the sea j u s t south of Squamish (Figure 2.1). Thurber C o n s u l t a n t s (1983) gained some i n s i g h t i n t o the hydrology of the reg i o n by examining the records of Stawamus River and Capilano R i v e r , the l a t t e r d r a i n i n g much of the area j u s t east of the Howe Sound watersheds. A c o m p l i c a t i n g f a c t o r i s that the Capilano and Stawamus watersheds have areas of 2 2 172 km and 40 km r e s p e c t i v e l y above t h e i r gauging s t a t i o n s , much l a r g e r than most of the Howe Sound watersheds (see Table 2.1). The sm a l l e r , steeper Howe Sound streams are apt to e x h i b i t more r a p i d runoff response to p r e c i p i t a t i o n events than the l a r g e r gauged r i v e r s , so peak flows here are l i k e l y to be higher than an a n a l y s i s of Capilano and Stawamus Ri v e r hydrographs alone would p r e d i c t . The C a p i l a n o River r e c o r d shows c o n s i d e r a b l e year-to-year f l u c t u a t i o n , but no obvious long-term trends ( F i g u r e 2.6). In f a c t , the average discharge f o r the l a s t ten-year p e r i o d i s 30 Figure 2 . 6 : T o t a l Runoff Per Year, 1914 - 1984, Capilano River 900 800 O ; 700 o §637 cd ^ 6 0 0 o EH 500 400 1910 Y e a r l y r u n o f f t o t a l 10 year running mean Mean of 64 y e a r l y t o t a l s 2 Drainage area: 172 km (Data from Environment Canada 1985) 1920 1930 1940 1950 31 I960 1970 1980 equal to the average f o r the e n t i r e p e r i o d of r e c o r d . Perhaps more r e l e v a n t to the d e b r i s t o r r e n t problem i s the seasonal runoff d i s t r i b u t i o n of Stawamus River (Figure 2.7) which shows two discharge peaks: one due to snowmelt in May and June, and one due to heavy r a i n s i n l a t e autumn and e a r l y winter. The Howe Sound watersheds almost c e r t a i n l y show s i m i l a r d i s t r i b u t i o n s , with the r a i n f a l l - r e l a t e d peak flows l i k e l y being higher i n some yea r s . Because the heavy winter r a i n s are o f t e n accompanied by a r a p i d r i s e i n f r e e z i n g l e v e l s , runoff may c o n t a i n a s i g n i f i c a n t snowmelt component. In the Howe Sound area these winter storms seem to be more important than the s p r i n g snowmelt i n t r i g g e r i n g d e b r i s t o r r e n t s : of the nineteen t o r r e n t s which have occurred s i n c e 1960, seventeen have been in the months October - February (see Table 2.3 and F i g u r e 2.8b). A l l of the f a c t o r s d i s c u s s e d i n e a r l i e r s e c t i o n s of t h i s chapter (topography, geology, c l i m a t e , v e g e t a t i o n , and land use) i n f l u e n c e the h y d r o l o g i c regime of the area. The steepness of the topography, combined with the p a u c i t y of s u r f i c i a l m a t e r i a l s i n many areas, leads to f a i r l y r a p i d runoff from r a i n f a l l events. D e f o r e s t a t i o n through timber h a r v e s t i n g or f i r e may a l s o i n c r e a s e peak flows (Swanston and Swanson, 1976), although i t i s d o u b t f u l that the t o t a l runoff would be i n c r e a s e d (Thurber C o n s u l t a n t s , 1983). Logging roads can a l s o i n t e r r u p t s u r f a c e drainage, and can even d i v e r t small streams from one watershed i n t o another (Thurber C o n s u l t a n t s , 1983). 32 Figure 2. ?; Seasonal Runoff D i s t r i b u t i o n , 1972 - 1984,  Stawamus River G Mean monthly values f o r p e r i o d of record & <s> Highest and lowest monthly values i n p e r i o d of record Drainage area: 40.4 km (Data from Environment Canada, 1985) 20 15 10 ra \ S5.0 CD ixD U ^ O CO • H Q >> £ 2 . 0 -p ci CD S 1 .0 0 . 5 J \7 \7 J J _ \37 \7 J L J L A M J J A S 0 N 33 -p-t-9 >TJ p I I I CD I—' C CO 4 0 4 H- CL H-cn o p CD 2 r-1 0 -p- !\> D MD JO h-1 -0 ro h-1 > O -P" r-1 00 CD MD V_r\ MD 00 ro ro P c+ O 0 o O 00 •>o 2 2 -0 CD u ^ CD 0 CD O O . CD ON 3 O 0 0 ro CO a 0 c ra 0 0 CD O c+ 0 O O \ P CD <! <! CD CD 0 3 •• 0 <! - • • C + c+ c+ O • • *d O c+ " " " • - • MD ^} " MD - O • c+ • • !-»• MD )-»• MD MD MD r->- )->• MD MD }->• MD MD }->• ON H » MD MD MD CO 00 CO MD MD MD MD ~o MD MD O MD CO CD CO ro CO 00 -0 N j ro ro MD ON ON CO ON •P" !->• 00 ON ro O c+ O CD CD P MD P TI TI B r i t a n n i a T h i s t l e Daisy-Unnamed No . 9 F u r r y Unnamed No. 8 Unnamed No. 7 K a l l a h n e Bertram Brunswick P o i i Deeks Loggers M Magnesia A l b e r t a Harvey Rundle Lone Tree Newman T u r p i n C h a r l e s S t r i p Mont izambert S c l u f i e l d Unnamed No. 1 Disbrow r9 P a1 CD ro O >—' a NO CD C ON cr O <-i O H" 1 CO P I-* i-3 NO O a 00 •P-a CD CD ,—s CO P c+ M CO O C + [ft CD P CD ra O-- rr -r] M 0 MD O i-i • CO cr O CD -P- 0 CD ^ ra 0 0 rS CO P r—1 ra c+ c+ P rc c+ 0 ra - CD 00 NO O 03 r: ~* a* P hi CD P Figure 2.8: D i s t r i b u t i o n of Debris Torrents and Floods i n East Howe Sound Area ( a f t e r Thurber Consultants, 1983: Church and Desloges, 1984) ro p CD > (D T3 CD TJ U O o CD u O ° n 8 4 O - Flood VA — Debris Torrent i r i i I 1 ' 1 ' | 1 1965 1970 T - ^ T j I I I I I 1975 1980 1* 7? Y///7A f 4 •A i 9 6 0 year a) Yearly d i s t r i b u t i o n , i 9 6 0 - 1984. ~ 1 I I I I I I 1 I l I r J F M A M J J A S O N D month 1 - includes one f l o o d p r i o r to i960 2 - includes one f l o o d and one. event of un c e r t a i n o r i g i n p r i o r to i960 b) Monthly d i s t r i b u t i o n of known events, I906 - 1984. 35 2.8 HISTORY OF DEBRIS TORRENT ACTIVITY ALONG HOWE SOUND Table 2.3 l i s t s nineteen d e b r i s t o r r e n t s known to have oc c u r r e d on the east s i d e of Howe Sound s i n c e 1960. I t i s i n t e r e s t i n g to note that f o u r t e e n of these have occurred s i n c e Autumn 1981. Not much i s known about t o r r e n t a c t i v i t y i n the p e r i o d p r i o r to the completion of the highway i n 1958, but a i r p h o t o i n t e r p r e t a t i o n i n d i c a t e s that t o r r e n t s o c c u r r e d i n the 1930*s i n Magnesia, A l b e r t a , and Harvey Creeks, and a l s o p o s s i b l y in Newman and Unnamed #1 creeks (Thurber C o n s u l t a n t s , 1983). A l l d e b r i s t o r r e n t s known, or thought, to have o c c u r r e d i n A l b e r t a , Magnesia, and M Creeks are d i s c u s s e d i n d e t a i l i n S e c t i o n 2.9. Of the creeks i n the Howe Sound area known to have been a f f e c t e d by d e b r i s t o r r e n t s , a l l except Kallahne Creek are south of Brunswick Point (Figure 2.2). Fi g u r e 2.8 i l l u s t r a t e s the d i s t r i b u t i o n of d e b r i s t o r r e n t s between 1960 and 1984. The events tend to be grouped, followed by long p e r i o d s of r e l a t i v e quiescence ( F i g u r e 2.8a), duri n g which people tend to be unmindful of the hazard. T h i s problem i s t y p i f i e d by Newman Creek, on whose fan a marina was c o n s t r u c t e d some time a f t e r the 1969 event. The marina was damaged i n both 1981 and 1983 (Church and Desloges, 1984). F i g u r e 2.8b i l l u s -t r a t e s the tendency f o r d e b r i s t o r r e n t s to occur i n f a l l and winter, as noted i n S e c t i o n 2.7. There have been twelve deaths d i r e c t l y a t t r i b u t a b l e to d e b r i s t o r r e n t a c t i v i t y i n t h i s area s i n c e Autumn 1981. Nine o c c u r r e d d u r i n g the October 1981 event on M Creek, when t r a v e l -l e r s on the highway at night f a i l e d to n o t i c e that the bridge had 36 been des t r o y e d . One person was k i l l e d attempting to c r o s s C h a r l e s Creek dur i n g the December 1981 event. The February 1983 t o r r e n t on A l b e r t a Creek claimed two l i v e s when a t r a i l e r i n which the v i c t i m s were s l e e p i n g was destroyed by d e b r i s . As i n d i c a t e d by Table 2.3, the most a c t i v e creeks i n the area i n terms of d e b r i s t o r r e n t occurrence are C h a r l e s , Newman, A l b e r t a , and Magnesia. T h i s may be somewhat m i s l e a d i n g , as the recor d i s very s h o r t , but i t does give some i n d i c a t i o n of the p o t e n t i a l hazard i n t h i s a r ea. 2.9 STUDY AREA 2.9.1 L o c a t i o n and Topography As noted e a r l i e r , t h i s study focusses on only a small p o r t i o n of the east Howe Sound area, i n p a r t i c u l a r the watersheds of A l b e r t a , Magnesia, M, and Loggers Creeks. These four streams enter Howe Sound i n a 4.5 km s t r e t c h north of Lions Bay (Fi g u r e 2.2). The area i s shown i n d e t a i l i n F i g u r e 2.9, and i s p i c t u r e d i n P l a t e 2.1. These contiguous catchments were s e l e c t e d f o r study because of t h e i r broad range of c h a r a c t e r i s t i c s , the f a c t that two of the creeks are among the most t o r r e n t - p r o n e i n the area, and the good access to the streams' upper reaches. A l s o , Loggers, M, and Magnesia Creeks are the most e x t e n s i v e l y logged of a l l Howe Sound b a s i n s , while A l b e r t a Creek has had e s s e n t i a l l y no l o g g i n g (see Table 2.1), so an o p p o r t u n i t y e x i s t s to assess the i n f l u e n c e of lo g g i n g on d e b r i s supply. The steepness of the t e r r a i n can be a p p r e c i a t e d from F i g u r e 2.9 and from P l a t e 2.1. The hig h e s t p o i n t s are Mount Harvey 37 F i g u r e 2.9: Contour Map o f Study A r e a ( s c a l e - 1:20,000) Mount Harvey (1625 m) B r u n s w i c k Mtn. (1760 m) Watershed Bdy. Squamish Hwy. Othe r Roads L o g g i n g Roads B. C. R a i l w a y H O W E S O U N D sc a l e - metres c o n t o u r i n t e r v a l 2 0 m P l a t e 2.1; Study Area Watersheds, Viewed From Howe Sound Brunswick Mountain M Creek Mt. Harvey Harvey Creek Loggers Creek I (1625 m) and Brunswick Mountain (1760 m), only 3.25 km and 3.5 km r e s p e c t i v e l y from the s h o r e l i n e . P r o f i l e s of the four creeks and t h e i r main t r i b u t a r i e s are shown i n F i g u r e 2.10. The compara-t i v e l y low and v a r i a b l e g r a d i e n t of Magnesia Creek has important i m p l i c a t i o n s f o r d e b r i s t r a n s p o r t : i t w i l l be shown l a t e r t h a t at l e a s t one d e b r i s t o r r e n t has come to r e s t i n the middle reach of the creek. Some ge n e r a l c h a r a c t e r i s t i c s of the four catchments are summarised i n Table 2.4. The two morphometric parameters 39 F i g u r e 2 .10: P r o f i l e s o f the four Study Area Streams LOGGERS CREEK D i s t a n c e (metres) ( V e r t i c a l s c a l e same as H o r i z o n t a l s c a l e ) 40 Table 2.4; Some C h a r a c t e r i s t i c s o f the Study A r e a B a s i n s Creek B a s i n Area (km 2) B a s i n R e l i e f (m) B a s i n Length (km) Hypsometric I n t e g r a l G eometric Shape F a c t o r Mean , E l e v a t i o n (m) A l b e r t a 1.31 I38O 2.70 0.359 5-57 420 Magnesia 4.78 1760 4.55 0.551 4.33 994 M 3.25 1720 3.25 0.525 3.26 860 Loggers 2.91 I65O 3.10 0.556 3 O 0 974 Notes •  p a - Geometric shape f a c t o r = ( B a s i n Length) . See Smart (I978) B a s i n Area b - Mean e l e v a t i o n i s the e l e v a t i o n above which 50% o f the b a s i n a r e a (measured i n the h o r i z o n t a l p l a n e ) i s l o c a t e d . See a l s o Table 2.1. "hypsometric i n t e g r a l " (Mark, 1975) and "geometric shape f a c t o r " (Smart, 1978) are presented here f o r d e s c r i p t i v e purposes o n l y . The geometric shape f a c t o r g i v e s an i n d i c a t i o n of how elongated a bas i n i s : i t s value would be 1.000 f o r a square b a s i n and 1.273 (4 / ir) f o r a round b a s i n . More elongated b a s i n s , such as A l b e r t a Creek, have higher values of t h i s parameter. F i g u r e s 2.11 and 2.12 show the d i s t r i b u t i o n of bedrock geology, and v e g e t a t i o n and land use i n the study area. The extent of timber h a r v e s t i n g can be judged by the areas l a b e l l e d " r e p l a n t e d " and "not s a t i s f a c t o r i l y r e p l a n t e d " on F i g u r e 2.12. Geology, v e g e t a t i o n , and land use are t r e a t e d i n more d e t a i l below, f o r each of the four study catchments. 2.9.2 S u r f i c i a l Geology and M a t e r i a l P r o p e r t i e s F i g u r e 2.13 shows the d i s t r i b u t i o n of s u r f i c i a l m a t e r i a l s i n the study area, as d e p i c t e d by Thurber C o n s u l t a n t s (1983). The main u n i t s are a Hard, dense b a s a l t i l l , and a much l o o s e r c o l l u v i u m or a b l a t i o n t i l l . Both c o n t a i n a complete range of p a r t i c l e s i z e s , from c l a y to co b b l e s , and the c o l l u v i u m c o n t a i n s abundant l a r g e b o u l d e r s . Where present, the b a s a l t i l l d i r e c t l y o v e r l i e s the bedrock, and the t i l l i s normally o v e r l a i n by c o l l u v i u m . In some p l a c e s , c o l l u v i a l m a t e r i a l r e s t s d i r e c t l y on bedrock. To assess the s o i l p r o p e r t i e s , an e x t e n s i v e sampling and t e s t i n g programme was c a r r i e d out at s e v e r a l s i t e s , p a r t i c u l a r l y in Magnesia Creek watershed. Sampling s i t e s are shown on F i g u r e 2.13. Most of the samples are from sites.Mg1 and Mg3, and these are d i s c u s s e d below. More d e t a i l i s given i n Appendix 1. 42 F i g u r e 2• 11: S i m p l i f i e d Bedrock Geology ^ Approximate l o c a t i o n s o f g e o l o g i c a l n + „ „ t c , ("adjusted a c c o r d i n g t"-~ COnxacXfa |_ f i e ( d o b s e r v a t i o n s Lower Cretaceous Gambier Group: mainly IkG i n t e r m e d i a t e to a c i d i c v o l c a n i c s , some sediments and metasediments P l u t o n i c rocks (mainly d i o r i t e and _P quar tz d i o r i t e ) i n t r u d e d i n t o IkG i n l a t e Cretaceous Geology based on Roddick and Woodsworth (1979) V C r e e k . C o a s t l i n e D D e e k s C r e e k L L o g g e r s C r e e k M M C r e e k Mg M a g n e s i a C r e e k A A l b e r t a Crock H H a r v e y C r e e k H NounC H a r v e y B * ( e l e v . 1625 o) B r u n s w i c k Men. ( d e v . 1 760 ra) W a t e r s h e d Bdy. S q u a r i s h H v y . O t h e r R o a d s L o g g i n g R o a d s R. C. R a i l w a y l'owu r T r a n s -m i s s i o n L i n e H O W E S O U N D s c a l e - n o t r e s c o n t o u r 1 n t e r v a l : 1 00m -p--p-F i g u r e 2.12: F o r e s t Cover and Land Use i n the  Study Area 1 Not S a t i s f a c t o r i l y Replanted 2 C l e a r e d Zone 3 Bare Rock 4 A l p i n e (Based on Thurber C o n s u l t a n t s , 1983) \ Creek, C o a s t l i n e D Deeks Creek L Loggers Creek M M Creek Mg Magnesia Creek A A l b e r t a Creek H Harvey Creek H a Mount: Harvey B & ( e l e v . 1625 IT.) Brunswick Men. ( e l e v . 1760 n) Watershed Bdy. Squamish Hwy. Other Roads Logging Roads B. C. Railway Power Trans-m i s s i o n Line T R E E DN H O W E S O U N D s c a l e - n c t r t s contour i n t e r v a l : 100m F i g u r e 2 .13 : S u r f i c i a l Geology i n the Study Area 1 T i l l over r o c k , minor f l u v i a l g r a v e l 2 G l a c i o - f l u v i a l sand, g r a v e l 3 Fan : sand, g r a v e l , bou lder s 4 Bou ldery fan A1 Sampl ing s i t e (see t ex t ) (Based on Thurber C o n s u l t a n t s , 1983) D L M Mg A H H * B a Creek, C o a s t l i n e Decks Creek Loggers Creek M Creek Magnesia Creek A l b e r t a Creek Harvey Creek Mount Harvey ( e l e v . 1625 m) Brunswick Mtn. ( e l e v . 1760 n) Watershed Bdy. Squamish Hwy. Other Roads Logging Roads B. C. Railway Power Tr a n s -m i s s i o n Line H O W E S O U N D s c a l e - metres contour i n t e r v a l : 100m F i g u r e 2.14 g i v e s t y p i c a l g r a i n s i z e d i s t r i b u t i o n s ( t r u n -cated at 2 mm or -1 <j> ) f o r the b a s a l t i l l and the o v e r l y i n g m a t e r i a l at s i t e Mg3. Both u n i t s are dominated by sand - s i z e d m a t e r i a l , although t i l l samples g e n e r a l l y have l e s s sand and more c l a y (the s a n d / s i l t boundary i s assumed here to be 0.074 mm, and the s i l t / c l a y boundary 0.002 mm). None of the samples t e s t e d c o n t a i n more than 15% c l a y by weight. The 11 c o l l u v i u m / a b l a t i o n t i l l samples average 65% sand, 28% s i l t , and 7% c l a y , and the three t i l l samples average 54% sand, 34% s i l t , and 12% c l a y . The hypothesis t h a t these two m a t e r i a l types can be d i f f e r e n t i a t e d on the b a s i s of t h e i r g r a i n s i z e d i s t r i b u t i o n s was t e s t e d s t a t i s t i c -a l l y by comparing the average percent f i n e r values at 0.074 mm and 0.002 mm, using a Student's t t e s t (Walpole and Meyers, 1978). In both cases, the mean value s are s i g n i f i c a n t l y higher in the t i l l at the ct =0.10 l e v e l of s i g n i f i c a n c e , but not at the a = 0.05 l e v e l . T h i s i n d i c a t e s that i t would be d i f f i c u l t to e x p l a i n the d i f f e r e n c e s between the b a s a l t i l l and the c o l l u v i u m on the b a s i s of g r a i n s i z e a l o n e . The p r o p e r t i e s of the two s o i l l a y e r s become b e t t e r d i f f e r -e n t i a t e d i f each sample i s c l a s s i f i e d by the U n i f i e d S o i l C l a s s i f i c a t i o n (U. S. C.) system ( F i g u r e 2.15). T h i s i n c o r p o r -ates both g r a i n s i z e and p l a s t i c i t y data, the l a t t e r being obtained from A t t e r b e r g l i m i t t e s t s . F i g u r e 2.16 summarises the A t t e r b e r g l i m i t s and the r e l a t i v e p r o p o r t i o n s of sand, s i l t , and c l a y f o r s e v e r a l samples from S i t e Mg3: samples of the upper l a y e r are c l a s s i f i e d as s i l t y sand (one sample i s c l a y e y sand), while the b a s a l t i l l samples are c l a y s of low p l a s t i c i t y or 46 F i g u r e 2.14: G r a i n S i z e D i s t r i b u t i o n s f o r S u r f a c e M a t e r i a l s a t S i t e Mg3 2.0 A b l a t i o n t i l l / c o l l u v i u m samples ~~ B a s a l t i l l samples i i 1 r 3 5 7 G r a i n S i z e ( P h i U n i t s ) i i — 1.0 0.5 T I I 1 0.2 0.1 0.05 0.02 0.01 0.005 0.002 G r a i n S i z e (mm) SAND I SILT CLAY 47 Figure 2.15: The U n i f i e d S o i l C l a s s i f i c a t i o n System ( a f t e r C r a i g , 1978) . '. ' Group symbols Laboratory criteria Description Fines (%) Grading Plasticity Notes Coarse grained (More than 50% larger than 63 fjin US sieve size) Gravels (More than 507o of coarse fracl ion of gravel size) Well-graded gravels, sandy gravels, with little or no fines GW 0-5 C t y > 4 I < Cc < 3 Dual symbols if 5-12% fines. Dual symbols if above 'A' line and 4 <!'} < 7 Poorly-Graded gravels, sandy gravels, with little or no fines GP 0-5 Nol satisfying GWrequiremenis Silty gravels, silty sandy gravels C M > 12 Below 'A' line or PI < A Clayey gravels, clayey sandy gravels C C > 12 Above "A ' line and PI > 7 Sands (More than 50% of coarse fraction of sand size) Well-graded sands, gravelly sands, with lit lie or no fines SW 0-5 Cu>6 1 < C c < 3 Poorly-graded sands, gravelly sands, with little or no Tines SP 0-5 Nol satisfying SW requirements Silty sands SM > i : Below 'A' line or PI < 4 Clayey sands SC > 12 Above 'A' line and Pi> 7 Pine grained (More than 50% smaller than 63 /ini BS gievc size) Silts and clays (Liquid limit less than 50) Inorganic sills, silty or clayey fine sands, with slight plasticity M L Use Plasticity Chart Inorganic clays, silly clays, sandy clays of low plasticity C L Use Plasticity Chart Organic sills and organic silty clays of low plasticity O L Use Plasticity Chart Silts and clays (Liquid limit greater than 50) Inorganic silts of high plasticity MM Use Plasticity Chart Inorganic clays of high plasticity C H Use Plasticity Chart Organic clays of high plasticity O i l Use Plasticity Chart Highly organic soils Peal and other highly organic soils Pt Cu _ D 60 ~ D 10 Cc _ D 3 0 D 6 0 D 1 0 PLASTICITY CHART C 30-Q- 20 • 10 CL . :CL-MLI / ML or OL CH MH or OH 40 50 60 Liquid limit 80 90 100 4 8 F i g u r e 2.16: S i t e M g 3 Samples : T e x t u r e , P l a s t i c i t y , U n i f i e d  S o i l C l a s s i f i c a t i o n o P e r c e n t C l a y P L A S T I C I T Y C H A R T C H c l a y e y sands. The above r e s u l t s may be m i s l e a d i n g , because the samples have been t r u n c a t e d at 2 mm (the g r a v e l - s a n d boundary) i n the a n a l y s i s , s i n c e i t i s d i f f i c u l t to o b t a i n r e p r e s e n t a t i v e samples of m a t e r i a l l a r g e r than 2 mm. However, almost a l l of the samples c o n t a i n e d s i g n i f i c a n t amounts of m a t e r i a l i n the g r a v e l s i z e range (Appendix 1). I f the complete samples are c l a s s i f i e d a c c o r d i n g to the U. S. C. system, the ba s a l t i l l samples become c l a y e y sandy g r a v e l s or s i l t y sandy g r a v e l s , while the a b l a t i o n t i l l / c o l l u v i u m samples a l l become s i l t y sandy g r a v e l s (Figure 2.15). Test r e s u l t s from S i t e Mg1 are summarized i n F i g u r e s 2.17 and 2.18. As at S i t e Mg3, there i s some o v e r l a p between the g r a i n s i z e d i s t r i b u t i o n s of the two main m a t e r i a l types, but the b a s a l t i l l tends to c o n t a i n l e s s sand and more c l a y . The 11 c o l l u v i u m / a b l a t i o n t i l l samples average 64% sand, 26% s i l t , and 10% c l a y , and the three b a s a l t i l l samples average 51% sand, 35% s i l t , and 14% c l a y . A s t a t i s t i c a l t e s t s i m i l a r to that performed on the S i t e Mg3 samples had s i m i l a r r e s u l t s : the average percent f i n e r than 0.074 mm i s s i g n i f i c a n t l y higher i n the t i l l at the a = 0.10 l e v e l of s i g n i f i c a n c e but not at the a = 0.05 l e v e l , and the percent c l a y i n the t i l l i s s i g n i -f i c a n t l y higher at the a = 0.05 l e v e l , but not at the a = 0.025 l e v e l . The two samples i n d i c a t e d on F i g u r e 2.17 as stream-bottom samples have anomalously low amounts of f i n e s , probably due to the a c t i o n of running water. They are not i n c l u d e d i n the above a n a l y s e s . 50 F i g u r e 2.17: G r a i n S i z e D i s t r i b u t i o n s f o r Sur face M a t e r i a l s a t  S i t e Mgl 100 2.0 A b l a t i o n t i l l / c o l l u v i u m samples B a s a l t i l l samples Stream bottom samples i i r r 5 7 G r a i n S i ze (Phi U n i t s ) I i 1.0 0.5 T l 1 I I 1 0.2 0.1 0.05 0.02 0.01 0.005 0.002 G r a i n S i z e (mm) . SAND I SILT CLAY 51 F i g u r e 2 .18 : S i t e Mgl Samples: T e x t u r e , P l a s t i c i t y , U n i f i e d  . S o i l C l a s s i f i c a t i o n P e r c e n t C l a y F i g u r e 2.18 gives t e x t u r e and p l a s t i c i t y data, as w e l l as U. S. C. c l a s s i f i c a t i o n s , f o r s e v e r a l samples from Mg1. The a b l a t i o n t i l l / c o l l u v i u m samples ( s u r f a c e m a t e r i a l ) are a l l c l a s s i f i e d as s i l t y sands or s i l t s of low p l a s t i c i t y , while the ba s a l t i l l samples are c l a y e y sands or low p l a s t i c i t y c l a y s . As at S i t e Mg3, a l l samples become c l a s s i f i e d as g r a v e l s i f not tr u n c a t e d at the gravel-sand boundary (2 mm). More d e t a i l s are given i n Appendix 1. T e x t u r a l analyses were a l s o performed on c o l l u v i u m from S i t e s M1 (four samples), Mg2 (four samples), and A1 (two sam-p l e s ) . F i g u r e s 2.19 and 2.20 show that the r e s u l t s here are s i m i l a r to those obtained at Mg1 and Mg3. A t t e r b e r g l i m i t t e s t s were not performed on these samples, but they probably have s i m i l a r p l a s t i c i t i e s to the other two s i t e s . The s i m i l a r i t y i n the g r a i n s i z e d i s t r i b u t i o n s of the b a s a l t i l l and the o v e r l y i n g m a t e r i a l noted above was a l s o noted by O'Loughlin (1972a), who i n t e r p r e t e d t h i s as "suggesting that the s o i l B h o r i z o n s are d e r i v e d from the weathering of a b l a t i o n t i l l which i s more permeable and of lower u n i t weight than the bas a l t i l l " (p. 9). He a l s o measured the u n i t weights of these m a t e r i a l s and estim-ated that the s o i l B h o r i z o n had a dry u n i t weight (Y^) of 3 3 1110 kg/m and a s a t u r a t e d u n i t weight (Y ^) of 1660 kg/m . Corresponding estimates f o r the unweathered b a s a l t i l l were Y . = 2170 kg/m3 and Y „ T = 2347 kg/m3 (O'Loughlin, 1972a). 0 S a l 2.9.3 D e s c r i p t i o n of Study Catchments The f o l l o w i n g s u b s e c t i o n s give d e t a i l e d d e s c r i p t i o n s of each of the four study area catchments. The general t o p i c s 53 Figure 2 . 1 9 : Grain Size D i s t r i b u t i o n s f o r M a t e r i a l s F i l l i n g the Head of the G u l l y , S i t e Ml Range of values from S i t e Mgl (11 samples) Range of values from S i t e Mg3 (11 samples) S i t e Ml samples 3 5 7 Grain S i z e (Phi U n i t s ) 2.0 1.0 0 . 5 0 . 2 0 . 1 0 . 0 5 0 . 0 2 0 . 0 1 0 . 0 0 5 0 . 0 0 2 Grain Size (mm) SAND SILT CLAY 54 F i g u r e 2 .20: G r a i n S i z e D i s t r i b u t i o n s f o r Surface M a t e r i a l s at S i t e s A l and Mg2 100 80 _ Range o f va lue s from S i t e Mgl (11 samples) Range o f va lue s from S i t e Mg3 (11 samples) p x: •rH 0) 6o rQ rH CD C •rH EH - 40 o> o JH CD P H 20 S i t e A l samples S i t e Mg2 samples i 1 1 r 3 5 7 G r a i n S i z e (Phi U n i t s ) 2.0 1.0 0 .5 0.2 0.1 0.05 0.02 0.01 0.005 G r a i n S i z e (mm) SAND SILT 0 .002 CLAY 55 covered are catchment physiography, bedrock and s u r f i c i a l geology, land use, d e b r i s t o r r e n t h i s t o r y , and present channel c o n d i t i o n , i n c l u d i n g major d e b r i s sources. 2.9.3.1 A l b e r t a Creek A l b e r t a Creek i s the steepest of the four i n the study 2 area, and has the s m a l l e s t catchment (1.3 km ). I t s watershed occupies a very narrow s t r i p ( l e s s than 500 metres wide) on the west s i d e of Mount Harvey, below the 1380 m e l e v a t i o n . T h i s watershed i s bounded on the nort h and south by the much l a r g e r catchments of Magnesia and Harvey creeks r e s p e c t i v e l y . F i g u r e 2.21 i s a p r o f i l e of A l b e r t a Creek. The catchments of A l b e r t a and Harvey Creeks can be seen i n P l a t e 2.2. Bedrock i n A l b e r t a Creek watershed i s predominantly v o l c a n -i c s and sediments of the Gambier Group, although p l u t o n i c rocks are present above about 700 m (Fi g u r e 2.11). There are no prominent f r a c t u r e d bedrock c l i f f s . S u r f i c i a l d e p o s i t s are absent, or r e l a t i v e l y t h i n , throughout the watershed above the 250 m e l e v a t i o n ( F i g u r e 2.13). Minor amounts of t i l l and c o l l u -vium mantle the steep rock slopes i n the middle and upper reaches of the watershed, but i n many areas the creek flows i n a bedrock canyon, as w i l l be noted below. An o l d fan, c o n s i s t i n g of g l a c i o f l u v i a l sand and g r a v e l , e x i s t s between about the 100 m and 250 m l e v e l s , above a recent fan of sand, g r a v e l , and boulders. The recent fan covers 9100 m (Thurber C o n s u l t a n t s , 1983). The fans of A l b e r t a Creek are h e a v i l y developed, as part of the community of Li o n s Bay i s l o c a t e d t h e r e . In p l a c e s on the fans the stream channel has been a r t i f i c i a l l y c h a n n e l i z e d and 56 Figure 2.21: P r o f i l e o f A l b e r t a Creek F - f o r e s t r y ( logg ing ) road c r o s s i n g H - Highway 99 c r o s s i n g R - B r i t i s h Columbia Ra i lway c r o s s i n g S - s u b d i v i s i o n road c r o s s i n g D - new d e b r i s r e t e n t i o n dam 57 P l a t e 2.2; A l b e r t a Creek Watershed i n Centre, Harvey Creek on Right d i v e r t e d through the housing development. P r i o r to the t o r r e n t events of 1983 ( d e s c r i b e d below) there were s i x c r o s s i n g s of the lower reaches of the creek by roads ( f i v e c u l v e r t s and one bridge) and one c r o s s i n g by the railway (a b r i d g e ) . O f f i c i a l l y there has been no l o g g i n g i n A l b e r t a Creek watershed, but there i s strong evidence that t h i s i s not the case. The l o g g i n g road which g i v e s access to Harvey Creek watershed immediately to the south c r o s s e s A l b e r t a Creek at about the 660 m l e v e l . By 1957 i t had been extended i n t o 58 A l b e r t a Creek watershed, although i t had not yet reached the creek. Church and Desloges (1984) noted that t h e r e are unhealed s k i d t r a c k s l e a d i n g from a l a n d i n g above the r i g h t bank down to the creek ( c l e a r l y v i s i b l e i n 1957 a i r p h o t o s ) , and that a number of cut l o g s were present above the r i g h t bank near the 500 m l e v e l . Most of these were removed by h e l i c o p t e r i n November 1985. There have been at l e a s t three d e b r i s t o r r e n t s i n A l b e r t a Creek s i n c e 1932. A comparison of 1932 and 1939 a i r photos r e v e a l s that at some po i n t i n the i n t e r v e n i n g p e r i o d , a d e b r i s t o r r e n t , t r i g g e r e d by a d e b r i s s l i d e at the 1060 m l e v e l , ran a l l the way to the sea (Church and Desloges, 1984). T o r r e n t s a l s o seem to have occu r r e d at the same time i n Magnesia Creek, Harvey Creek, and a small stream between Harvey and A l b e r t a Creeks, but a l l of these stopped before r e a c h i n g t i d e w a t e r . A small d e b r i s t o r r e n t o c c u r r e d on 3 December 1982. Thurber C o n s u l t a n t s (1983) gave the f o l l o w i n g d e s c r i p t i o n : "The t o r r e n t began at an unknown e l e v a t i o n along the creek above the f o r e s t r y road, scoured much of the channel to the w a t e r f a l l above the f o r e s t r y road, d e p o s i t e d 350-450 m of angular rock and l o g d e b r i s behind, and p a r t i a l l y removed, f o r e s t r y b r i d g e / f i l l s t r u c t u r e . Some d e b r i s [was] a l s o observed immediately upstream of the water intake s t r u c t u r e at e l e v 280 m. 100 m of rock and l o g d e b r i s was d e p o s i t e d at the base of w a t e r f a l l [ e l e v . 300 m]." T h i s event was r e l a t i v e l y minor, and the d e b r i s d e p o s i t s occurred at p l a c e s where the g r a d i e n t f l a t t e n s l o c a l l y (Church and Desloges, 1984). The major recent d e b r i s t o r r e n t i n A l b e r t a Creek occurred on 11 February 1983. Heavy r a i n had been recorded throughout 59 the region i n the p r e v i o u s few days, with s n o w f a l l at higher e l e v a t i o n s . There was heavy (but not extreme) r a i n f a l l i n the area on the night of 10-11 February, accompanied by a r i s e i n the f r e e z i n g l e v e l to above 3000 m (Church and Desloges, 1984). Evidence at the f o r e s t r y road c r o s s i n g at the 660 m l e v e l i n d i c a t e d that a snow avalanche (undoubtably t r i g g e r e d by the warm r a i n s ) had surged over the w a t e r f a l l j u s t upstream of the c r o s s i n g and landed on and below the road (Thurber C o n s u l t a n t s , 1983; Church and Desloges, 1984). I t appears that the impulsive lo a d of the avalanche was s u f f i c i e n t to m o b i l i z e d e b r i s i n the channel j u s t below the road, and thus t r i g g e r the t o r r e n t (Church and Desloges, 1984). As the t o r r e n t moved down the creek, the channel was scoured to bedrock i n many p l a c e s . The t o r r e n t moved through the v i l l a g e of L i o n s Bay in s i x surges between 3:30 and 6:30 a. m. (Thurber C o n s u l t a n t s , 1983). A l l of the creek c r o s s i n g s were destroyed or s e v e r e l y damaged except f o r the concrete r a i l w a y b r i d g e , which was b u r i e d by d e b r i s . Three houses were destroyed and another was damaged, and two l i v e s were l o s t . More d e t a i l e d d e s c r i p t i o n s of t h i s event are given by Thurber C o n s u l t a n t s (1983) and Church and Desloges (1984). Today A l b e r t a Creek downstream of the road flows d i r e c t l y on bedrock i n many areas. In the f i r s t 200-300 metres below the road (the main source area f o r the 1983 t o r r e n t ) there are steep c o l l u v i a l s l o p e s on both s i d e s , which continue to c o l l a p s e , but below t h i s the stream e n t e r s a V-shaped bedrock g u l l y , with many w a t e r f a l l s . Minor d e b r i s accumulations occur at l o c a l l y f l a t 60 areas, such as at the bases of w a t e r f a l l s , but, f o r the most p a r t , the channel i s r e l a t i v e l y f r e e of loose d e b r i s u n t i l i t becomes i n c i s e d i n t o u n c o n s o l i d a t e d s u r f i c i a l m a t e r i a l at about the 270 metre l e v e l , j u s t above the s u b d i v i s i o n . Upstream of the f o r e s t r y road c r o s s i n g the creek flows mainly on bedrock, with some boulders and some log d e b r i s (Thurber C o n s u l t a n t s , 1983). In November and December of 1985 a concrete d e b r i s dam was c o n s t r u c t e d i n a bedrock gorge at about the 700 m l e v e l , a short d i s t a n c e upstream from the w a t e r f a l l near the road c r o s s i n g ( P l a t e 2.3). The dam has a box c u l v e r t i n i t s base to allow the passage of water and small d e b r i s , and three s m a l l e r c u l v e r t s near the top. Immediately upstream of the dam a l a r g e number of l o g s have been anchored down to the stream bed p a r a l l e l to the d i r e c t i o n of flow, to s t a b i l i s e the stream bed at that p o i n t . Thurber C o n s u l t a n t s (1983) r a t e d a l l 26 of the creeks between Horseshoe Bay and B r i t a n n i a Beach on t h e i r r i s k of producing d e b r i s t o r r e n t s , on a s c a l e of zero to f o u r . They r a t e d A l b e r t a Creek as three, i n d i c a t i n g that there i s s t i l l a high p r o b a b i l i t y of t o r r e n t occurrence i n t h i s creek. 2.9.3.2 Magnesia Creek Magnesia Creek i s the l e a s t steep of the four study b a s i n s , (Figure 2.10), and has the l a r g e s t catchment a r e a . I t d r a i n s a 2 4.8 km area between Mount Harvey and Brunswick Mountain. The upper p a r t of the basin i s a well-formed g l a c i a l c i r q u e on the r i d g e between these two peaks ( F i g u r e 2.9). The h i g h e s t p o i n t s i n the b a s i n are Brunswick Mountain (1760 m) and Mount Harvey 61 P l a t e 2 . 3 : D e b r i s Dam on A l b e r t a C r e e k . L e f t : L o o k i n g Ups t ream. Wood c r i b b i n g was l a t e r removed. R i g h t : L o o k i n g Downstream. Note l o g s anchored to s tream bed i n f o r e -ground . (1625 m). A p r o f i l e of the creek and i t s main t r i b u t a r i e s i s shown i n F i g u r e 2.22, and a photograph of the catchment i s shown in P l a t e 2.4. Both v o l c a n i c and p l u t o n i c rocks are common i n Magnesia Creek watershed (Figure 2.11). An impressive d i o r i t e c l i f f occurs on the northwest f l a n k of Mount Harvey, and there are ex t e n s i v e r o c k f a l l d e p o s i t s at i t s base. R o c k f a l l from d i o r i t e c l i f f s a l s o occurs near the two small l a k e s at 1300 m. S u r f i c i a l d e p o s i t s are widespread (Figure 2.13), except i n the h i g h e s t areas, where bare rock forms the w a l l s of a g l a c i a l c i r q u e . In most areas, a hard l a y e r of ba s a l t i l l o v e r l i e s the bedrock, and t h i s i s o f t e n o v e r l a i n by up to s e v e r a l metres of l o o s e r a b l a t i o n t i l l or c o l l u v i u m . Large amounts of u n c o n s o l i d a t e d m a t e r i a l are P l a t e 2.4: Magnesia Creek Watershed. Brunswick Mountain on  l e f t , Mount Harvey on r i g h t . 6 3 F i g u r e 2 .22: P r o f i l e o f Magnes ia Creek F - f o r e s t r y ( logg ing) road c r o s s i n g H - Highway 99 c r o s s i n g R - B r i t i s h Columbia Rai lway c r o s s i n g D - new d e b r i s r e t e n t i o n dam L - l ake 2000 r 1500 ON o 1000 •H > CD 500 T r i b u t a r y L l T r i b u t a r y L 3 . L 1 T r i b u t a r y L2 F 0.5 i.O 1.5 2.0 2.5 3-0 Dis tance Upstream From Mouth, Main Stem (km) 3-5 4.0 s t o r e d i n the creek channel i t s e l f . An o l d fan of g l a c i o f l u v i a l sand and g r a v e l e x i s t s above and to the south of the recent fan, and i s being mined by the M i n i s t r y of T r a n s p o r t a t i o n and High-ways. The recent fan i s d e s c r i b e d as bouldery, and covers an 2 area of about 63,600 m (Thurber C o n s u l t a n t s , 1983). There are a t o t a l of 18 r e s i d e n c e s on the recent fan, f i f t e e n of which are along the s h o r e l i n e below the r a i l w a y . Two road b r i d g e s and one r a i l w a y b r i d g e c r o s s the creek on the fan, and there i s a f o r e s t r y road b r i d g e at the 620 m l e v e l . Some time p r i o r to 1932 a f o r e s t f i r e burned much of the middle and lower p a r t s of Magnesia Creek catchment. The f i r e reached but a p p a r e n t l y d i d not go south of the creek. By 1957 l o g g i n g had begun south of the creek at e l e v a t i o n s between 200 m and 550 m, p r i m a r i l y along the creek. A l o g g i n g road system from the mouth of the creek extended as f a r as the present l o c a t i o n of the bridge at 620 m, and i n t o A l b e r t a Creek water-shed. By 1966 there had been c o n s i d e r a b l e l o g g i n g i n the upper part of the b a s i n , and a l l l o g g i n g roads shown on F i g u r e 2.12 had been c o n s t r u c t e d . By 1968 the timber h a r v e s t i n g was complete: 32 % of the b a s i n had been logged (Thurber C o n s u l t a n t s , 1983). Three d e b r i s t o r r e n t s are known or i n f e r r e d to have occ u r r e d s i n c e 1932. A i r p h o t o a n a l y s i s shows that between 1932 and 1939, and probably at the same time as the event i n A l b e r t a Creek d e s c r i b e d e a r l i e r , a d e b r i s t o r r e n t moved down the main channel of the creek from near the 1200 m l e v e l and came to a stop i n the r e l a t i v e l y f l a t area below the c o n f l u e n c e of the main channel and t r i b u t a r i e s L2 and L3, at about 750 m. T h i s event w i l l be 65 d i s c u s s e d f u r t h e r i n S e c t i o n 5.1. On 12 October 1962 Hurricane Freda h i t the south coast of B r i t i s h Columbia. R a i n f a l l a s s o c i a t e d with the storm was heavy but not extreme i n the Howe Sound ar e a , d e s p i t e newspaper r e p o r t s to the c o n t r a r y . S t a t i o n s near sea l e v e l at Squamish and B r i t a n n i a Beach recorded 53.6 and 49.8 mm of p r e c i p i t a t i o n r e s p e c t i v e l y on 12 October. The Squamish t o t a l has a r e t u r n p e r i o d of onl y 1.05 years (Figure 2.5). The h u r r i c a n e t r i g g e r r e d a t o r r e n t on Magnesia Creek which destroyed both the r a i l w a y and highway b r i d g e s (Thurber C o n s u l t a n t s , 1983). "Trees swept i n t o the creek by the g a l e s j o i n e d with logged timber from a l o g g i n g camp on the mountain slope to form a jam. The creek, fed by mel t i n g snow and some of the he a v i e s t r a i n on r e c o r d , pushed t r e e s , g r a v e l and mud ahead of i t , p i l e d up tons of d e b r i s i n f r o n t of some houses and pushed one house o f f i t s f o u n d a t i o n " . (The Vancouver Pr o v i n c e , 15 October 1962). A r e s i d e n t i n t e r -viewed by Thurber C o n s u l t a n t s (1983) r e c a l l e d t h a t the storm had only l a s t e d four hours, and that the water i n the creek had r i s e n and f a l l e n very r a p i d l y before and a f t e r the event. Thurber C o n s u l t a n t s (1983) d e s c r i b e a small d e b r i s t o r r e n t on 28 October 1981 which blocked the r a i l w a y bridge and caused some damage to a property on the north s i d e of the creek at i t s mouth. The event was very s m a l l , and i s perhaps b e t t e r c l a s s i -f i e d as a f l o o d (Church and Desloges, 1984). In any event, there has been no d i s t u r b a n c e of the creek between the 250 m and 750 m l e v e l s f o r many yea r s , as shown by the o b s e r v a t i o n s noted i n the f o l l o w i n g paragraphs. U n l i k e A l b e r t a Creek, Magnesia Creek i s choked with rock 66 and l o g d e b r i s throughout i t s lower reaches. Between the apex of the fan and the confluence of the main creek and t r i b u t a r y L1, at about 475 m, the creek flows mainly i n a bedrock canyon with a s e r i e s of w a t e r f a l l s . There are l a r g e rock and l o g accumulations at s e v e r a l p o i n t s . Many of these are moss-covered and appear to have been s t a b l e f o r many y e a r s . Large a l d e r t r e e s grow on these d e b r i s accumulations and at the edges of the creek i n many l o c a t i o n s . Upstream of the c o n f l u e n c e the channel widens and becomes f i l l e d with rock and l o g d e b r i s . I t has a s t e p - p o o l p r o f i l e , t y p i c a l of t o r r e n t - p r o n e channels which have remained undisturbed f o r some time (Church and Desloges, 1984). F i g u r e 2.23 i l l u s t -r a t e s t h i s f o r a s e c t i o n near the f o r e s t r y road c r o s s i n g at the 620 m e l e v a t i o n . Church and Desloges (1984) estimate that 3 3 between 20 m and 30 m of d e b r i s per metre of channel l e n g t h i s s t o r e d here. The value i s probably at l e a s t h a l f the higher f i g u r e f o r the e n t i r e l e n g t h of the channel between the 500 and 770 m l e v e l s , a d i s t a n c e of c l o s e to 1 km. In t h i s reach the channel o f t e n b i f u r c a t e s , and l a r g e (30 cm diameter) a l d e r s and the o c c a s s i o n a l cedar grow on the c e n t r a l bars or d e b r i s l o b e s . Tree cores e x t r a c t e d i n February 1987 from f i v e of the l a r g e s t a l d e r s downstream of the b r i d g e showed that these t r e s s were between 15 and 21 years o l d (see S e c t i o n 5.2). A d e b r i s levee about 120 metres long occupies the v a l l e y bottom immediately downstream of the confluence of the main channel and t r i b u t a r y L3 ( F i g u r e s 2.9, 2.24). Thurber C o n s u l t -ants (1983) d e s c r i b e t h i s as a snow avalanche d e b r i s levee 67 s c a l e - metres A p r o f i l e s ca l e same as map s c a l e F i g u r e 2.24; D e b r i s Levee, Stream C o n f l u e n c e s , M i d d l e Reaches o f Magnesia Creek s c a l e - metres Contour i n t e r v a l : 1 m c o n t r i b u t e d by t r i b u t a r y L2, but i t seems l i k e l y that a l l three of the streams which come together here may have helped to b u i l d t h i s f e a t u r e . I t i s 3 to 4 metres high and i s f i l l e d with abundant f a l l e n logs and b o u l d e r s . Near the downstream end of the levee there i s abundant f r e s h rock and organic d e b r i s c o n t r i b u t e d by a s o i l wedge f a i l u r e from a t r i b u t a r y g u l l y ( S i t e Mgl). T h i s reach of Magnesia Creek i s d i s c u s s e d f u r t h e r i n S e c t i o n s 4.4.2.3, 4.5.1.2, and 5.2. Upstream of the levee the main channel c o n t i n u e s to have a s t e p - p o o l p r o f i l e f o r about 500 metres, u n t i l a d e b r i s chute e n t e r s from the the l e f t s i d e . Above t h i s p o i n t the creek flows i n c r e a s i n g l y on bedrock, and any d e b r i s accumulations appear o l d and s t a b l e . T r i b u t a r y L3, which has a l a r g e r d i s c h a r g e than the main channel, continues to be choked with l o g s and rock d e b r i s to a p o i n t about 600 metres upstream from the c o n f l u e n c e , the upper l i m i t of the logged t e r r a i n . 'Above t h i s p o i n t i t flows through o l d r o c k f a l l d e p o s i t s on a f a i r l y g e n t l e g r a d i e n t . S e v e r a l snow-avalanche t r a c k s descend the south s i d e of Brunswick Mountain and reach the creek from the r i g h t i n t h i s a r ea. T r i b u t a r y L2 ( S i t e Mg2) i s d i s c u s s e d i n d e t a i l i n S e c t i o n 4.6.1.2. Desp i t e having the lowest g r a d i e n t of the four creeks i n the study area, Magnesia Creek i s c o n s i d e r e d to be the most l i k e l y to produce a l a r g e d e b r i s t o r r e n t , because of the tremen-dous amount of m o b i l i z a b l e d e b r i s s t o r e d i n the channel. Thurber C o n s u l t a n t s (1983) give i t a hazard r a t i n g of 4, i n d i c a t i n g a very high p r o b a b i l i t y of t o r r e n t occurrence. 70 2.9.3.3 M Creek M Creek i s comparable in steepness to A l b e r t a Creek ( F i g u r e 2 2.10), and has a catchment area of 3.3 km , somewhat sm a l l e r than Magnesia Creek. I t d r a i n s the area south of Brunswick Mountain and a r i d g e extending between that p o i n t and an unnamed peak i n f o r m a l l y known as Hat Mountain (Figure 2.9). The h i g h e s t p o i n t i n the watershed i s a p o i n t about 300 m south of the peak of Brunswick Mountain, at the 1720 m l e v e l . F i g u r e 2.25 i s a p r o f i l e of the creek and i t s t r i b u t a r i e s . P l a t e 2.5 shows the catchment of M Creek. Gambier Group rocks dominate M Creek b a s i n ( F i g u r e 2.11), but the four l a r g e s t d e b r i s sources o r i g i n a t e i n f r a c t u r e d d i o r i t e , i n a narrow band between 900 m and 1200 m. These s i t e s are i d e n t i f i e d on P l a t e 2.6. Compared to Magnesia Creek, there i s c o n s i d e r a b l y more exposed bedrock and l e s s s u r f i c i a l m a t e r i a l i n the M Creek watershed ( F i g u r e 2.13). Rock c l i f f s are widespread i n the upper p a r t of the b a s i n , and f o r much of i t s l e n g t h the creek flows i n a bedrock canyon. The stream channel i t s e l f i s d e s c r i b e d in d e t a i l at the end of t h i s s e c t i o n . Where present, the s u r f i c i a l d e p o s i t s u s u a l l y c o n s i s t of t h i n l a y e r s of b a s a l t i l l o v e r l a i n by loose c o l l u v i u m or a b l a t i o n t i l l . D e p o s its of t a l u s and coarse r o c k f a l l d e b r i s are common at higher l e v e l s , p a r t i c u l a r l y south of the creek. There i s no o l d fan, 2 and the present bouldery fan i s only 14,100 m i n extent (Thurber C o n s u l t a n t s , 1983), l e s s than one quarter the s i z e of the Magnesia Creek fan. U n l i k e A l b e r t a and Magnesia Creeks, the fan of M Creek has 71 F i g u r e 2.2S: P r o f i l e o f M C r e e k F - f o r e s t r y ( l o g g i n g ) r o a d c r o s s i n g H - H i g h w a y 99 c r o s s i n g R - B r i t i s h C o l u m b i a R a i l w a y c r o s s i n g D i s t a n c e U p s t r e a m F r o m M o u t h , M a i n S t e m ( k m ) P l a t e 2 . 5 ? M Creek Watershed. Prominent scar i n mid-reaches i s S i t e M2. not been e x t e n s i v e l y developed. Only two homes, one of which was destroyed i n 1981, are l o c a t e d t h e r e . There are only two creek c r o s s i n g s i n t h i s a rea: a new highway bri d g e and a r a i l w a y b r i d g e . In a d d i t i o n , there i s a f o r e s t r y road c r o s s i n g at 690 m. The f o r e s t f i r e which burned much of Magnesia Creek p r i o r to 1932 a l s o a f f e c t e d the lower p a r t of M Creek b a s i n . The middle p a r t of the bas i n was logged some time a f t e r 1957, when the road from Magnesia Creek was extended n o r t h . By 1966 there had been widespread l o g g i n g on both s i d e s of the creek, and by 1968 timber h a r v e s t i n g was complete, with 38 % of the watershed logged (Thurber C o n s u l t a n t s , 1983). A major d e b r i s t o r r e n t swept down M Creek i n the e a r l y hours of 28 October 1981. R a i n f a l l that night was high but not e x c e p t i o n a l , and there had been very l i t t l e snow i n October, 3 even at higher l e v e l s . An estimated 20,000 m of rock and l o g d e b r i s emerged from the narrow bedrock canyon j u s t upstream of the highway and knocked out the c e n t r a l , t r e s t l e - s u p p o r t e d span of the highway b r i d g e . In the darkness, heavy r a i n , and r e s u l t -ing c o n f u s i o n that n i g h t , nine people l o s t t h e i r l i v e s as t h e i r v e h i c l e s plunged i n t o the creek. I t i s unclear e x a c t l y where or how the t o r r e n t was t r i g g e r e d , but two major d e b r i s sources enter the creek near 850 m, j u s t downstream of a 45 m high w a t e r f a l l . The l a r g e r of these, S i t e M2, i s a rock and d e b r i s s l i d e on the r i g h t bank, i n unlogged t e r r a i n ( P l a t e 2.6). The age of t h i s s l i d e i s u n c e r t a i n , (see the d i s c u s s i o n in S e c t i o n 4.3.1.2), but i t i s d e f i n i t e l y not v i s i b l e i n 1968 a i r p h o t o s , and appears much smaller than present i n 1979. While the s l i d e may not have 74 a c t u a l l y t r i g g e r e d the 1981 event, i t undoubtably c o n t r i b u t e d c o n s i d e r a b l e amounts of d e b r i s to the creek before and probably d u r i n g the t o r r e n t . A smaller d e b r i s source i s S i t e M1, a steep bedrock g u l l y on the l e f t s i d e of the creek about 50 m upstream of M2 ( P l a t e 2.6). The upper p a r t of t h i s g u l l y i s f i l l e d with a b l a t i o n t i l l and/or c o l l u v i u m , and c o n s i d e r a b l e seepage occurs at the base of t h i s m a t e r i a l . The lower reaches of t h i s g u l l y are completely scoured to bedrock. A f a i l u r e of the m a t e r i a l i n t h i s g u l l y may have t r i g g e r e d the 1981 d e b r i s t o r r e n t (Church and Desloges, 1984). These two d e b r i s sources are d i s c u s s e d f u r t h e r in S e c t i o n 4.3.1. It i s apparent that the road c r o s s i n g at 690 m served as a s u b s t a n t i a l d e b r i s b a r r i e r (M. J . Bovis, p e r s . comm.), and i t i s p o s s i b l e that the creek was t e m p o r a r i l y blocked at t h i s p o i n t . If t h i s was the case, the t o r r e n t would have had i n c r e a s e d energy when the temporary dam broke. There i s some evidence of channel blockages at one or two other l o c a t i o n s between the f o r e s t r y road and the source area. The p a r t of M Creek between the highway and the f o r e s t r y road i s d i f f i c u l t to t r a v e r s e , but i t appears to be r e l a t i v e l y f r e e of d e b r i s , having been scoured by the 1981 event. For the f i r s t s e v e r a l hundred metres above the highway the creek flows mainly i n a deep bedrock gorge, with o c c a s i o n a l pockets of boulders and l o g d e b r i s (Thurber C o n s u l t a n t s , 1983). Near the f o r e s t r y road i t i s i n c i s e d i n c o l l u v i u m and t a l u s , and d e b r i s i s a c t i v e l y c o n t r i b u t e d from road c u t s . Above the road the creek channel i s i n bedrock, with i n c r e a s i n g amounts of boulders and 75 organic m a t e r i a l . Near the toe of the l a r g e r o c k s l i d e ( s i t e M2) l a r g e amounts of rock d e b r i s are present i n the channel, and the stream runs subsurface or i s f o r c e d to the extreme l e f t s i d e of the g u l l y bottom. Above the w a t e r f a l l , which i s j u s t upstream of s i t e s M1 and M2, the boulders i n the creek become l a r g e r and appear more s t a b l e . A d e b r i s chute ( s i t e M5) e n t e r s on the l e f t bank at about 1200 metres, near the base of another high w a t e r f a l l . The creek has a much lower g r a d i e n t upstream of t h i s w a t e r f a l l , and the stream bed seems to have been s t a b l e f o r a long time. There i s r e l a t i v e l y l i t t l e loose d e b r i s i n the channel i n t h i s upper reach. T r i b u t a r y R1 j o i n s M Creek about 50 m upstream of the f o r e s t r y road c r o s s i n g . In i t s lower reaches the t r i b u t a r y i s overgrown and has not been d i s t u r b e d f o r some time, but a s m a l l , a c t i v e r o c k s l i d e and d e b r i s chute ( s i t e M3) feed m a t e r i a l d i r e c t l y i n t o the channel at about 1050 m, and extend downstream fo r some 280 metres, to a p o i n t about 250 m upstream of the c o n f l u e n c e . In most of t h i s reach the creek flows subsurface, through coarse r o c k f a l l d e b r i s . A minor d e b r i s t o r r e n t which occu r r e d i n a t r i b u t a r y channel i n the winter of 1985-1986 has covered the r o c k f a l l d e b r i s with f i n e organic and i n o r g a n i c m a t e r i a l , and a few logs and l a r g e r rocks, at a p o i n t 275 m upstream of the c o n f l u e n c e . Although M Creek downstream from the f o r e s t r y road was e s s e n t i a l l y scoured by the 1981 event, there remains enough d e b r i s near the toe of s i t e M2 (which i s s t i l l accumulating), 76 and i n t r i b u t a t y R1, to g i v e the creek a high r i s k of t o r r e n t i n g . Thurber C o n s u l t a n t s (1983) g i v e i t a hazard r a t i n g of 3, i n d i c a t -ing a high p r o b a b i l i t y of occurrence. 2.9.3.4 Loggers Creek Loggers Creek has a gr a d i e n t s i m i l a r to M Creek and A l b e r t a 2 Creek ( F i g u r e 2.10), and a catchment area of 2.9 km . I t d r a i n s p a r t of the area west of a rid g e extending n o r t h from "Hat Mountain" (Figure 2.9). T h i s peak (1650 m) i s the hig h e s t p o i n t in the watershed. A p r o f i l e of the creek and i t s one t r i b u t a r y i s shown i n Fig u r e 2.26. Loggers Creek catchment i s shown i n P l a t e 2.7. The catchment i s dominated by Gambier Group rocks, except above about 1000 m, where d i o r i t e s are most common (Fi g u r e 2.11). Prominent c l i f f s occur i n both rock types, and act as important d e b r i s sources. S u r f i c i a l m a t e r i a l s are even l e s s abundant than i n M Creek (Figure 2.13). There are l a r g e areas of bare rock i n the middle and upper p o r t i o n s of the catchment, and the t i l l and c o l l u v i u m are very t h i n where pr e s e n t . Very coarse r o c k f a l l d e b r i s i s common at higher l e v e l s and i n the creek i t s e l f . There i s no g l a c i o f l u v i a l f an, and the recent fan i s very small (4300 m 2). There i s no housing or other development on the small f an. The r a i l w a y c r o s s e s the creek by means of a c u l v e r t , and the highway uses a b r i d g e . A f o r e s t r y road c r o s s e s at 960 m; the c u l v e r t which was here has c o l l a p s e d and the creek flows over the road s u r f a c e . Timber h a r v e s t i n g i n Loggers Creek began somewhat e a r l i e r 77 F i g u r e 2 . 2 6 : P r o f i l e o f Loggers Creek F - f o r e s t r y ( logg ing ) road c r o s s i n g H - Highway 99 c r o s s i n g R - B r i t i s h Columbia Ra i lway c r o s s i n g 78 P l a t e 2.7; Loggers Creek Watershed than i n the other three b a s i n s . Between 1952 and 1954 a road was c o n s t r u c t e d up from the water on the nort h s i d e of the creek, and some lo g g i n g was c a r r i e d out th e r e . By 1957 there were roads and lo g g i n g on both s i d e s of the creek, but o n l y below 500 m. Logging i n the middle p a r t of the b a s i n began i n the e a r l y 1960's, when the l o g g i n g road system was extended no r t h from M Creek, and by 1968 the road and the timber h a r v e s t i n g a c t i v i t i e s had been extended i n t o the bas i n of t r i b u t a r y R1. In t o t a l , 41 % of Loggers Creek basin was logged. 79 There i s no h i s t o r y of d e b r i s t o r r e n t s i n Loggers Creek. L i k e M Creek, i t s lower reaches are i n a very deep bedrock canyon which i s d i f f i c u l t to t r a v e r s e . Thurber Consultants (1983) d e s c r i b e the v a l l e y s i d e s as s l o p i n g 35° - 45°, being predominantly bedrock, and w e l l - v e g e t a t e d . Abundant boulders and l o g d e b r i s are p r e s e n t . The only s i g n i f i c a n t t r i b u t a r y e n t e r s on the r i g h t bank, at 530 m. Above t h i s p o i n t the creek channel i s completely choked with l o g s and l a r g e boulders, and the stream flows subsurface o n l y . Surface water appears near the f o r e s t r y road c r o s s i n g , but the channel remains d e b r i s - f i l l e d . Many of the rocks and log s i n the channel upstream of the road are moss-covered and look s t a b l e , but in p l a c e s f r e s h - l o o k i n g boulders r e s t on top of l o g s , so there must have been some recent movement t h e r e . The t r i b u t a r y channel i s f i l l e d with s m a l l e r and l e s s s t a b l e rock d e b r i s , and appears to be an a c t i v e c o n t r i b u t o r of m a t e r i a l to the main creek. Rocks d i s l o d g e d from the road cut were observed to r o l l down the dry channel f o r s e v e r a l hundred metres, m o b i l i z i n g other m a t e r i a l as they went. S e v e r a l a c t i v e d e b r i s s l i d e s , o r i g i n a t i n g at the road, supply d e b r i s d i r e c t l y to the main creek and the t r i b u t a r y . I t i s d i f f i c u l t to imagine the m a t e r i a l i n the main creek channel between the f o r e s t r y road and the t r i b u t a r y ever becoming m o b i l i z e d , on account of i t s very coarse nature, but m o b i l i z a t i o n of a l a r g e amount of d e b r i s i n the t r i b u t a r y channel c o u l d c o n c e i v a b l y t r i g g e r a moderate d e b r i s t o r r e n t i n Loggers Creek. Thurber C o n s u l t a n t s (1983) gave t h i s creek a hazard r a t i n g of 3, 80 i n d i c a t i n g a h i g h p r o b a b i l i t y of occurrence. 81 Chapter 3 STUDY METHODS 3.1 HISTORICAL RECORDS H i s t o r i c a l records p e r t a i n i n g to the d e b r i s t o r r e n t problem in the Howe Sound area are a v a i l a b l e from a number of sources. Newspaper a r t i c l e s , as w e l l as records of the M i n i s t r y of Highways and the B r i t i s h Columbia R a i l r o a d , p r o v i d e accounts of past d e b r i s t o r r e n t events. M i n i s t r y of F o r e s t s p u b l i c a t i o n s d e s c r i b e the f o r e s t cover and l o g g i n g h i s t o r y , and p u b l i c a t i o n s of the f e d e r a l M i n i s t r y of Environment (Environment Canada) give c l i m a t o l o g i c a l and h y d r o l o g i c a l records f o r the a r e a . Thurber C o n s u l t a n t s (1983) have compiled a l l r e l e v a n t i n f o r m a t i o n from these and other sources, and there seemed l i t t l e p o i n t i n d u p l i c a t i n g t h e i r e f f o r t s here. . T h e r e f o r e , t h i s t h e s i s depends l a r g e l y upon Thurber C o n s u l t a n t s ' (1983) report f o r h i s t o r i c a l i n f o r m a t i o n , although some of the Environment Canada r e p o r t s a l s o have been used. 3.2 AIRPHOTO INTERPRETATION A i r photographs (both F e d e r a l and P r o v i n c i a l ) of the study area were obtained f o r s e v e r a l years between 1932 and 1982. Some of the photo s e r i e s covered only narrow areas along the c o a s t l i n e and d i d not show the upper p a r t s of the study b a s i n s . However, complete coverage i s a v a i l a b l e f o r the years 1939, 1946, 1952, 1957, 1968, 1979, and 1982. Table 3.1 l i s t s a l l the a i r p h o t o s used i n t h i s study. The a i r p h o t o s were used to examine f i r e and l o g g i n g a c t i v i t y 82 Table 3.1: Government A i r Photographs Used i n Study Year F e d e r a l / P r o v i n c i a l Photo Numbers Approximate Scale Extent of Coverage 1932 F A4441- 71-75 1: ! 1 5 , 0 0 0 p a r t i a l 1939 P BC BC I 3 4 - 82-84 143- 80,81 1 : : 3 l , 0 0 0 complete 1946 F A10398- 9 1 - 9 3 . 114-116 1 ! 131 ,000 complete 1952 P BC I 6 3 4 - 58-61 1: : 3 8 , 0 0 0 complete 1954 P BC 1682- 56-60 1: : 15,840 p a r t i a l 3 . 1957 P BC BC 2348- 81-85 2349- 18-21 1: :15,840 complete 1966 P BC 5175- 106-108 1: : 3 l , 0 0 0 complete* 3 1968 P BC BC 7116- 101-104 7117- 215-218 1: :15,840 complete 1976 P BC 5715- ^5-50 1; : 1 2 , 0 0 0 p a r t i a l 0 1979 P BC 79 194- 6 0 - 6 2 , 8 0 - 8 2 1: : 2 0 , 0 0 0 complete 1982 P BC BC 82 0 5 8 - 6 9 - 7 3 82 0 6 0 - 124-126 1: : 2 0 , 0 0 0 complete Notes: a - coverage of lower h a l f of basins only b - except the highest parts of Magnesia Creek basin, i n the v i c i n i t y o f the two small lakes c - coverage of a narrow s t r i p along the c o a s t l i n e only 83 and d e b r i s t o r r e n t occurrence i n the study basins over the past 50 y e a r s . Some of the f i n d i n g s of t h i s p a r t of the study have a l r e a d y been noted i n Chapter 2. A i r p h o t o i n t e r p r e t a t i o n was a l s o c a r r i e d out to develop an i n v e n t o r y of major d e b r i s sources in the study area, and to i d e n t i f y areas where d e t a i l e d f i e l d study would be most p r o f i t a b l e . The most recent set of a i r p h o t o s (the 1982 s e r i e s ) was used to develop a more d e t a i l e d p l a n i m e t r i c base map of the area than the standard NTS 1:50,000 map sheets. Mapping was done at a s c a l e of 1:20,000 with the a i d of a WILD A6 s t e r e o p l o t t e r at the Department of Geography, U. B. C. I t was not p o s s i b l e to p l o t contour l i n e s with t h i s instrument because there are not enough p o i n t s of known e l e v a t i o n c l e a r l y v i s i b l e on the a i r p h o t o s . T h e r e f o r e , the topographic maps used i n t h i s study were c o n s t r u c -ted by t r a n s f e r r i n g contour l i n e s from the standard NTS sheets to the p l a n i m e t r i c base map, and a d j u s t i n g these s l i g h t l y as regui red. 3.3 GROUND SURVEYING AND RECONNAISSANCE F i e l d work was c a r r i e d out between January 1984 and February 1987, with the main work i n the summers of 1984 and 1985. The f i r s t few weeks of the main 1984 f i e l d season were devoted to walking up the four creeks where p o s s i b l e , to assess channel c o n d i t i o n s and i d e n t i f y d e b r i s sources not obvious on the a i r p h o t o s , and to making p r e l i m i n a r y v i s i t s to some of the major s i t e s i d e n t i f i e d on a i r p h o t o s . F i g u r e 3.1 shows a l l s i t e s in the study area examined i n any d e t a i l : i . e . those s i t e s where 84 Figure 3-1: Locations of Main Study S i t e s 00 © A1 O R G - 3 • Named s i t e s Storage r a i n gauges Magnesia Creek debris levee S i t e of p a i n t l i n e s and cored t r e e s , lower Magnesia Creek V Creek, C o a s t l i n e D Deeks Creek L Loggers Creek M M Creek Mg Magnesia Creek A A l b e r t a Creek H Harvey Creek H * Mount Harvey ( e l e v . 1625 B Brunswick Men. ( e l e v . 1760 Watershed Bdy. Squamish Hwy. Other Roads Logging Roads B. C. Railway Power Tr a n s -m i s s i o n Line H O W E S O U N D s c a l e - metres contour i n t e r v a l : 100m s u r v e y i n g , f i e l d t e s t i n g , m a t e r i a l s sampling, or monitoring of r a i n f a l l , groundwater l e v e l s , or slope movement were performed. During a i r p h o t o a n a l y s i s and p r e l i m i n a r y f i e l d r e connais-sance, s e v e r a l d i f f e r e n t d e b r i s supply mechanisms were found to be a c t i v e i n the study area. Surveying was c a r r i e d out at s i t e s p r o v i d i n g good examples of some of these mechanisms, i n order to q u a n t i f y slope and channel morphology. For example, the middle reaches of Magnesia and M Creeks, and t h e i r major t r i b u t a r i e s , were surveyed with a Brunton Compass and a 30 m tape, i n order to develop d e t a i l e d stream p r o f i l e s and l o c a t e major d e b r i s sources and accumulations. Topographic s u r v e y i n g with a t h e o d o l i t e and 30 m tape was c a r r i e d out at three s i t e s : the f o r e s t r y road c r o s s i n g of Magnesia Creek; s i t e Mg1, a s o i l wedge f a i l u r e i n the middle reaches of Magnesia Creek; and s i t e Mg3, a s e r i e s of shallow d e b r i s s l i d e s on the r i g h t bank of a major t r i b u t a r y of Magnesia Creek (see F i g u r e 3.1). The headscarp area of s i t e Mg1 was surveyed i n three s u c c e s s i v e summers, i n order to q u a n t i f y s c a r p r e t r e a t . In a d d i t i o n , topographic mapping with a theodo-l i t e and e l e c t r o n i c d i s t a n c e meter (E.D.M.) was c a r r i e d out at the headscarp of a major r o c k s l i d e on M Creek ( s i t e M2) and on a l a r g e d e b r i s levee i n Magnesia Creek. A l l of the surveys were t i e d i n to p o i n t s which c o u l d be a c c u r a t e l y l o c a t e d on the 1:20,000 s c a l e base map, such as major stream confluences or f o r e s t r y road c r o s s i n g s of main channels. The surveys were then a d j u s t e d to an a r b i t r a r y c o o r d i n a t e system on the base map, using standard plane surveying techniques. 86 3.4 FIELD INSTRUMENTATION AND SITE MONITORING Mon i t o r i n g and f i e l d t e s t i n g were c a r r i e d out at s e v e r a l s i t e s to measure p r e c i p i t a t i o n , groundwater l e v e l s , s o i l move-ment, and s o i l p r o p e r t i e s . Three storage r a i n gauges were i n s t a l l e d between 600 m and 1000 m i n the watersheds of Magnesia and M Creeks ( F i g u r e 3.1). I t was planned to use data from these gauges, along with the M i n i s t r y of Highways weather s t a t i o n records d e s c r i b e d e a r l i e r , to assess r a i n f a l l c h a r a c t e r i s t i c s in the study area, but i t proved i m p r a c t i c a l to v i s i t the storage gauges f r e q u e n t l y enough f o r the records to be of much v a l u e . Shallow standpipe piezometers were i n s t a l l e d at two s i t e s (Mg1 and Mg3) where c o n s i d e r a b l e amounts of seepage had been noted. A l l were l e s s than two metres long, and only penetrated to the top of the bas a l t i l l l a y e r . Since they were not s e a l e d , they c o u l d only give i n f o r m a t i o n about the groundwater t a b l e , r a t h e r than p i e z o m e t r i c l e v e l : thus, they might more a c c u r a t e l y be c a l l e d o b s e r v a t i o n w e l l s . The standpipes were used to monitor changes i n the p o s i t i o n of the groundwater t a b l e over the s p r i n g , summer, and f a l l . At three s i t e s (Mg2, M3, and Magnesia Creek near 580 m), l i n e s were p a i n t e d on rocks i n the stream channel or d e b r i s channel. The p a i n t l i n e s were observed over a two year p e r i o d to d e t e c t downstream or downslope movement. Two other l o c a t i o n s , immediately upstream and downstream of the lo g g i n g road bridge on Magnesia Creek, were photographed s e v e r a l times from the same p o i n t s , with the same o b j e c t i v e . Two g r i d s of f i f t e e n e r o s i o n p i n s each were p l a c e d on 87 s l o p e s where slow, continuous e r o s i o n was thought to be t a k i n g p l a c e ( s i t e A1 and part of s i t e M2). The pi n s were p i e c e s of s t e e l r e i n f o r c i n g rod ( r e b a r ) , 10 cm long, p a i n t e d i n 1 cm h o r i z o n t a l bands. They were d r i v e n i n t o the ground u n t i l only the top centimetre was exposed. The changing exposure of the p i n s was observed over a two year p e r i o d . The r e s u l t s from s i t e M2 were d i f f i c u l t to i n t e r p r e t (few of the pins c o u l d be l o c a t e d the f o l l o w i n g y e a r ) , and w i l l not be presented here, but the s i t e A1 r e s u l t s are d i s c u s s e d i n S e c t i o n 4.4.3.3. At s i t e s Mg1 and Mg3 (Figure 3.1), estimates of the hyd-r a u l i c c o n d u c t i v i t y of the s u r f a c e m a t e r i a l were made with a f i e l d permeameter. T h i s instrument i s d e s c r i b e d i n d e t a i l i n Appendix 2. I t i s f e l t that i t g i v e s reasonable, order-of-magni-tude estimates of the s a t u r a t e d h y d r a u l i c c o n d u c t i v i t y of the su r f a c e s o i l s . 3.5 DENDROCHRONOLOGY A 26 cm t r e e corer was used to c o l l e c t 4 mm diameter t r e e cores at three s i t e s : the l a r g e r o c k s l i d e on the r i g h t s i d e of M Creek ( s i t e M2); the l a r g e d e b r i s levee i n Magnesia Creek near the confluence of the main stem and t r i b u t a r i e s L2 and L3; and a s e r i e s of d e b r i s lobes i n Magnesia Creek, downstream of the lo g g i n g road c r o s s i n g ( F i g u r e 3.1). In the l a t t e r two cases, cores were used s o l e l y to estimate the minimum ages of the levee and d e b r i s lobe s u r f a c e s . At s i t e M2, two d i f f e r e n t types of cores were taken. On the margins of the s l i d e , roughly one t h i r d of the way down from the 88 headscarp, mature t r e e s have been e x t e n s i v e l y s c a r r e d by rocks bounding downslope from the headscarp. Many of the s c a r s are now p a r t l y or completely healed. S e v e r a l of these t r e e s were cored, both i n s c a r r e d and unscarred areas, to estimate the timing of impacts. At the top of the M2 s l i d e there i s a l a r g e t e n s i o n crack between the headscarp and the a c t i v e l y r a v e l l i n g s c a r p . Trees on the block between the two scarps are t i l t e d i n a v a r i e t y of d i r e c t i o n s , i n d i c a t i n g c o n s i d e r a b l e movement. S e v e r a l t r e e s here were cored on t h e i r upslope and downslope s i d e s to detect r e a c t i o n wood which would i n d i c a t e the t i m i n g of the movement. The t r e e cores were glued to sheets of wood and then sanded u n t i l t h e i r tops were f l a t and smooth. They were examined in the l a b o r a t o r y by eye and under a b i n o c u l a r microscope at low magnif-i c a t i o n . 3.6 MATERIALS SAMPLING AND TESTING As p a r t of the g e n e r a l reconnaissance program d i s c u s s e d e a r l i e r , o b s e r v a t i o n s were made of bedrock and s u r f i c i a l mater-i a l s p r e s e n t . T h i s was done to develop reasonably accurate g e o l o g i c a l maps of the area ( F i g u r e s 2.11 and 2.13), and to h e l p i d e n t i f y major d e b r i s source areas. E x t e n s i v e sampling of s o i l s was c a r r i e d out at s e v e r a l s i t e s (see S e c t i o n 2.9.2). The s o i l s were brought back to the l a b o r a t o r i e s i n the Department of Geography, U. B. C , f o r t e s t i n g . T e s t s i n c l u d e d f i e l d moisture content, t e x t u r e , organic content, and A t t e r b e r g L i m i t s . A l l t e s t s were performed accord-89 ing to ASTM procedures. In a d d i t i o n , a few samples were shear t e s t e d u s ing a vacuum t r i a x i a l t e s t i n the C i v i l E n g i n e e r i n g l a b o r a t o r i e s at U. B. C. The vacuum t r i a x i a l t e s t s were plagued with u n c e r t a i n t y , due to problems with the c a l i b r a t i o n of a load c e l l , and the r e s u l t s have not been presented i n t h i s t h e s i s . A l l other s o i l t e s t i n g i s summarized i n S e c t i o n 2.9.2 and i n Appendix 1. 90 Chapter 4 DEBRIS SUPPLY TO CHANNELS 4.1 INTRODUCTION One of the requirements f o r d e b r i s t o r r e n t occurrence o u t l i n e d i n Chapter 1 i s a supply of m o b i l i z a b l e d e b r i s i n the stream channel. T h i s chapter examines the v a r i o u s means by which d e b r i s i s t r a n s p o r t e d from h i l l s l o p e s to stream channels, and g i v e s d e t a i l e d analyses of processes at s e l e c t e d s i t e s . The terminology proposed by Varnes (1978) i s used, with the a d d i t i o n -a l terms " d e b r i s t o r r e n t " , which has been d e f i n e d i n Chapter 1, and " s o i l wedge", p r e v i o u s l y d e f i n e d by D i e t r i c h and Dunne (1978). It i s not e s s e n t i a l that d e b r i s be present i n the stream channel p r i o r to the i n i t i a t i o n of a d e b r i s t o r r e n t , s i n c e slope f a i l u r e s can be converted d i r e c t l y i n t o t o r r e n t s i f there i s s u f f i c i e n t water i n the stream and i f the channel i s s u f f i c i e n t l y steep to t r a n s p o r t s a t u r a t e d d e b r i s . In such cases, the same slope f a i l u r e a c t s as both a d e b r i s supply mechanism and a t o r r e n t t r i g g e r , simultaneously s a t i s f y i n g requirements ( i i i ) and ( i v ) i n S e c t i o n 1.1.1. T h i s l i n k between d e b r i s supply mechan-isms and d e b r i s t o r r e n t t r i g g e r s i s d i s c u s s e d i n Chapter 6. Even i f they do s t a r t as slope f a i l u r e s i n t o a stream channel, d e b r i s t o r r e n t s almost always e n t r a i n a d d i t i o n a l m a t e r i a l from the bed and banks as they move downstream. Swanston and Swanson (1976) noted t h a t : "Some t o r r e n t s are t r i g g e r e d by d e b r i s avalanches of l e s s than 100 m3, but u l t i m a t e l y i n v o l v e 10,000 m3 of d e b r i s e n t r a i n e d along the t r a c k of the t o r r e n t " (p. 213). 91 Thus, i n most cases, the bulk of the d e b r i s moved by a t o r r e n t e x i s t s i n the stream channel p r i o r to the event. In the l i t e r -a t ure s u r p r i s i n g l y l i t t l e has been s a i d about d e b r i s supply mechanisms, except i n the context of t o r r e n t i n i t i a t i o n by l a n d s l i d i n g . A notable e x c e p t i o n i s VanDine (1985a), who notes t h a t : "(The) volume of d e b r i s i n a creek bed i s dependent upon the c h a r a c t e r of the creek banks and adjacent v a l l e y w a l l s . Important c h a r a c t e r i s t i c s i n c l u d e s l o p e , type and d i s t r i b u t i o n of bedrock and overburden, v e g e t a t i o n , and land use, both adjacent to the creek and i n the drainage b a s i n . " (p. 47). A l s o , some of the f o r e s t r y l i t e r a t u r e (O'Loughlin, 1972a, 1972b; Swanston and Swanson, 1976) d i s c u s s e s slope f a i l u r e s i n t o stream channels. Hungr et a l . (1984) note that most of the d e b r i s i n v o l v e d i n a t o r r e n t comes d i r e c t l y from the stream channel and i t s immediate v i c i n i t y , but say l i t t l e about how the d e b r i s i s i n i t i a l l y d e l i v e r e d to the channel. T h e i r approach i s examined in S e c t i o n 4.7. S t i l l , most of the work on d e b r i s flows has c o n c e n t r a t e d on mechanisms of i n i t i a t i o n , t r a n s p o r t , and depos-i t i o n , r a t h e r than d e b r i s supply. I t i s hoped that t h i s t h e s i s w i l l c o n t r i b u t e to f i l l i n g t h i s gap. 4.2 DISCRETE VS CONTINUOUS DEBRIS SUPPLY One of the i n i t i a l q u e s t i o n s that t h i s t h e s i s set out to i n v e s t i g a t e was whether d e b r i s was s u p p l i e d to stream channels more or l e s s c o n t i n u o u s l y along the l e n g t h of the creeks, or at r e l a t i v e l y few d i s c r e t e p o i n t s . A c c o r d i n g l y , an i n v e n t o r y of a l l major d e b r i s sources was compiled. A i r p h o t o i n t e r p r e t a t i o n was used to i d e n t i f y p o t e n t i a l d e b r i s sources and make p r e l i m i n a r y 92 assessments of the dominant processes at each s i t e . F i e l d checking was then used to r e f i n e the i n i t i a l c l a s s i f i c a t i o n s of these s i t e s , and, i n a few cases, e l i m i n a t e them as major d e b r i s sources. In a d d i t i o n , the upper p a r t s of the creeks were t r a v e r s e d wherever p o s s i b l e , as a f i n a l check. S e v e r a l important d e b r i s sources, not obvious on the a i r p h o t o s , were i d e n t i f i e d i n t h i s way. F i g u r e 4.1 shows the major d e b r i s sources and poten-t i a l d e b r i s sources i n the four study catchments, c l a s s i f i e d a c c o r d i n g t o the dominant process at each l o c a t i o n . S e c t i o n s 4.3 - 4.5 d i s c u s s the v a r i o u s d e b r i s supply mechanisms i d e n t i f i e d on F i g u r e 4.1, and give d e t a i l e d a n a l y s e s of s e v e r a l t y p i c a l examples. A second reason f o r t r a v e r s i n g the creeks was to assess bank s t a b i l i t y i n areas not c o n s i d e r e d major d e b r i s sources, as a check on the hypothesis of continuous d e b r i s supply. A f i n a l d i s c u s s i o n of the qu e s t i o n of d i s c r e t e versus continuous d e b r i s supply i s d e f e r r e d u n t i l S e c t i o n 4.6, a f t e r the major d e b r i s supply mechanisms have been d e s c r i b e d . 4.3 MATERIAL SUPPLY FROM ROCK SLOPES AND TALUS SLOPES M a t e r i a l loosened from rock s l o p e s by r o c k f a l l or rock-s l i d i n g may be d e l i v e r e d d i r e c t l y to creek channels, or i t may be s t o r e d as t a l u s or r o c k f a l l d e p o s i t s . In the former case, the d e b r i s source areas must be a t , or very c l o s e t o , the channel s i d e s , and the i n t e r v e n i n g slopes steep enough to t r a n s p o r t the m a t e r i a l . T a l u s can be d e l i v e r e d to creek channels a l s o , through t a l u s s h i f t . While the source areas may be the same, 93 M3 -p-F i g u r e 4 . 1 : Main D e b r i s Sources i n Study Area rTT""~rn R o c k f a l l and R o c k s l i d e S i t e s A c t i v e T a l u s , R o c k f a l l , . and R o c k s l i d e D e b r i s I n a c t i v e or S t ab l e Ta lus D e b r i s S l i d e s , S o i l Wedge F a i l u r e s Snow Avalanche S i t e s G u l l i e s , D e b r i s Tracks Uns tab le \ Creek, A s C o a s t l i n e D Decks Creek L Loggers Creek M M Creek Mg Magnesia Creek A AlbecLu Creek H Harvey Creek H a Mount Harvey B * ( e l e v . 1625 m) Brunswick Mtn. ( e l e v . 1760 m) Watershed Bdy. Squami sh Hwy. Other Roads Logging Roads B. C. Railway Power Trans-m i s s i o n Line H O W E U N D s c a l e - metres contour i n t e r v a l : lOOin m a t e r i a l supply from t a l u s s l o p e s d i f f e r s g r e a t l y from m a t e r i a l supply from rock slopes i n terms of the l e n g t h of time the d e b r i s spends i n t r a n s i t between the source area and the stream channel. A l s o , as i n d i c a t e d below, there are d i f f e r e n c e s between the mechanisms of r o c k f a l l and r o c k s l i d e and those of t a l u s s h i f t . In some cases, t a l u s may be d e l i v e r e d to creek channels by wet snow avalanches. T h i s i s c o n s i d e r e d i n S e c t i o n 4.5.1. 4.3.1 R o c k f a l l s and R o c k s l i d e s There are s e v e r a l l a r g e r o c k f a l l s and r o c k s l i d e s i n the study area ( F i g u r e 4.2). Most of these are o l d and r e l a t i v e l y s t a b l e , but there are three important a c t i v e s i t e s at about the 1000 m l e v e l i n M Creek watershed, and a l s o i n road c u t s in the catchments of A l b e r t a and Loggers creeks . A l l of these a c t i v e s i t e s , except some in Loggers Creek watershed, have the p o t e n t i a l to c o n t r i b u t e c o n s i d e r a b l e amounts of d e b r i s d i r e c t l y to stream channels. There are a l s o s e v e r a l s i t e s where rock s l o p e s are g r a d u a l -l y d i s i n t e g r a t i n g and s u p p l y i n g d e b r i s to channels. The process has been termed " r a v e l l i n g " here, although the main d i f f e r e n c e between i t and l a r g e r o c k s l i d e s and r o c k f a l l s i s one of s c a l e . The d e b r i s tends to be much f i n e r , and the a c t u a l process by which the m a t e r i a l i s removed from the rock face may be d i f f i c u l t to determine. R a v e l l i n g i s g e n e r a l l y more continuous and l e s s c a t a s t r o p h i c than r o c k f a l l or r o c k s l i d e . F i g u r e 4.11 shows the s i t e s i n the study area where r a v e l l i n g of rock sl o p e s seems to be important. R a v e l l i n g s u p p l i e s m a t e r i a l d i r e c t l y to Loggers Creek and i t s t r i b u t a r y at s e v e r a l p o i n t s , and to Magnesia Creek 95 ON F i g u r e 4.2: Large R o c k f a l l and R o c k s l i d e S i t e s r T T T T 1 , Major Rock Faces , Unstab le Road Cuts =»- D e b r i s Tracks ----- G u l l i e s , Debr i s Chutes -Stable or I n a c t i v e Ta lus Recent R o c k f a l l s and R o c k s l i d e s A c t i v e Ta lus M3 S p e c i f i c S i t e s Referenced i n the t ex t Coast 1 i no. D D«-'eks Creek L L o w e r s Creek M M C r u e k Mg Magnesia Creek A A l b e r t a C r i > n k H Harvey Creek H a Mount H a r v c y ( e l e v . 1625 m) B ^  Brunswick M t n . ( d e v . 1760 n>) Wa cc r s h e d Bdy. — . - S q u a r i s h Wvy. O t h e r Roads Logging Ro a d s B. C. Railway I'owi.' r Trans-m i s s i o n Lino H 0 W E U N D sea 1 c - .Tictros c o n t o n r i n t c r v n 1 : I 00m t r i b u t a r y L1 at at l e a s t one l o c a t i o n . 4.3.1.1 Mechanisms Large r o c k f a l l s and r o c k s l i d e s i n the study area almost always occur i n w e l l - j o i n t e d d i o r i t e c l i f f s i n the upper p a r t s of the watersheds. The only known cases of r o c k f a l l i n the v o l c a n i c rocks are where slopes have been oversteepened from l o g g i n g road c o n s t r u c t i o n . In many cases, d i o r i t e c l i f f s occur near the c o n t a c t between the p l u t o n i c rocks and the l e s s competent v o l c a n i c s of the Gambier Group. As noted i n S e c t i o n 2.2, the o r i e n t a t i o n s of most of the major c l i f f s and stream channels are c o n t r o l l e d by n o r t h e a s t - and n o r t h - t r e n d i n g f r a c t u r e zones, and northwest-trending f o l i a t i o n (Church and Desloges, 1984). Slope f a i l u r e s t h e r e f o r e are c o n t r o l l e d by a combination of f r a c t u r e o r i e n t a t i o n and c l i f f o r i e n t a t i o n . S e c t i o n 4.3.1.2 i n c l u d e s a d e t a i l e d a n a l y s i s of the i n f l u e n c e of these two parameters on one p a r t i c u l a r r o c k s l i d e - S i t e M2. R a v e l l i n g occurs i n the more e r o d i b l e rocks, such as the r h y o l i t e s and other v o l c a n i c rocks of the Gambier Group, r a t h e r than the blocky, more competent, p l u t o n i c rocks. Because of t h i s , the rock fragments r e l e a s e d from the s l o p e s tend to be s m a l l e r , and the s l o p e s themselves tend to be l e s s steep. F o l l o w i n g the i n i t i a l r o c k f a l l or s l i d e (removal of m a t e r i a l from the c l i f f f a c e ) , there are v a r i o u s means by which d e b r i s i s d e l i v e r e d to stream channels. For example, where an unstable c l i f f i s s i t u a t e d c l o s e to a channel, rocks loosened from the c l i f f face are d e l i v e r e d q u i c k l y to the creek. Conversely, below i t s headscarp, S i t e M3 i s r e a l l y more of an example of t a l u s 97 s h i f t than of r o c k f a l l d i r e c t l y i n t o the stream channel. In other cases, such as i n much of Magnesia Creek watershed, r o c k f a l l serves only to b u i l d up t a l u s or c o l l u v i u m i n areas away from the stream channels. As noted above, there are only a few s i t e s where d e b r i s i s d e l i v e r e d d i r e c t l y from rock faces to creek channels (see F i g u r e 4.2). The f o l l o w i n g s e c t i o n s d e s c r i b e three of these s i t e s . 4.3.1.2 Type L o c a t i o n - S i t e M2 S i t e M2 i s an a c t i v e r o c k s l i d e which s u p p l i e s d e b r i s d i r e c t l y to M Creek at 850 m (Figure 4.2). The s l i d e o r i g i n a t e s at 1100 m, i n unstable, unlogged t e r r a i n on the north s i d e of the creek. An adjacent s l i d e ( S i t e M3) a l s o s t a r t s here ( P l a t e 2.6). F i g u r e 4.3 shows the upper p a r t s of these two f e a t u r e s . The main s l i d e (M2) i s about 45 m wide at the top and 20 m wide where i t e n t e r s the creek, some 370 m downslope. The average g r a d i e n t i s o . 4 3 41 . An estimated 2.5 x 10 m of m a t e r i a l has been removed from 3 3 the slope by t h i s s l i d e , and about 3.5 x 10 m of t h i s remains i n M Creek channel near 850 m. The apparent d i s c r e p e n c y between these two f i g u r e s i s d i s c u s s e d l a t e r i n t h i s s e c t i o n . The headscarp i s developed i n w e l l - j o i n t e d d i o r i t i c rocks. 3 A 10,000 - 15,000 m block here appears to have r o t a t e d and s l i d s l i g h t l y downslope (Figure 4.3). The t e n s i o n crack behind t h i s block i s one to two metres deep, and can be t r a c e d f o r about 90 metres. The scarp of the c u r r e n t l y a c t i v e p a r t of the s l i d e i s on the downslope sid e of t h i s p o t e n t i a l l y unstable b l o c k . Here r o t a t i o n has been s u f f i c i e n t to allow i n d i v i d u a l rocks to become loosened from the face, probably by s l i d i n g along a j o i n t set 98 F i g u r e k.J: Upper P o r t i o n s o f S i t e s M2 and M3 99 orthogonal to the master s e t , which d i p s s t e e p l y i n t o the f a c e . A rock mechanics a n a l y s i s of the headscarp area of t h i s s l i d e i s given l a t e r i n t h i s s e c t i o n . The age of t h i s l a n d s l i d e i s u n c e r t a i n , but a i r p h o t o s show that i t was not present i n 1968, and was much smaller than present i n 1979. The second o b s e r v a t i o n i s c o n s i s t e n t with statements by the owners of a small cabin c o n s t r u c t e d near the s l i d e i n 1975. While there have probably been small r o c k f a l l events here f o r s e v e r a l years, a l a r g e p o r t i o n of the s l i d e must have o c c u r r e d as a s i n g l e high-energy f a i l u r e , as i n d i c a t e d by the d e b r i s accumulations behind l a r g e t r e e s about halfway down the s l i d e and the e x t e n s i v e damage to t r e e s downslope of the headscarp ( P l a t e 4.1). An attempt to date the s l i d e by dendrochronology was incon-c l u s i v e . Trees on the unstable block at the top of the s l i d e are t i l t e d i n a v a r i e t y of d i r e c t i o n s ( P l a t e 4.2), i n d i c a t i n g a c o n s i d e r a b l e amount of movement. Tree cores were e x t r a c t e d from the upslope and downslope s i d e s of 10 of these t r e e s , to d e t e c t r e a c t i o n wood which would i n d i c a t e the timing of the movement. The l o c a t i o n s of these t r e e s are shown in F i g u r e 4.3. As shown in Table 4.1, h a l f of these t r e e s showed no unusual growth p a t t e r n s at a l l , while the other f i v e y i e l d c o n f l i c t i n g i n t e r -p r e t a t i o n s . There i s evidence of p o s s i b l e d i s t u r b a n c e i n the e a r l y 1970's, but t h i s i s e r r a t i c and not very c o n v i n c i n g . There are a number of mature Douglas f i r t r e e s on the .margins of the s l i d e , about one t h i r d of the way down from the headscarp, which have been damaged by rocks bounding down the 100 P l a t e 4.1: R o c k s l i d e . S i t e M2. Note damage to t r e e s on s l i d e  m a r g i n s , l a r g e d e b r i s a c c u m u l a t i o n s "behind t r e e s . 101 P l a t e 4 . 2 ; D i s t u r b e d T r e e s . Large T e n s i o n C r a c k a t Top o f S i t e M2 Table 4 . 1 : Summary o f Tree Core I n t e r p r e t a t i o n s , S i t e M2 Tree No. E s t i m a t e d age i n 1985 ( y e a r s ) O b s e r v a t i o n s , Comments S l i d e M a r gins o 1 2 = 3 4 5 6 7 8 9 10 Headscarp A r e a 226 :267 2 4 5 ? 163 264 7 ? 270 One r e c e n t Damaged i n Damaged i n One r e c e n t Damaged i n Two r e c e n t s i n c e I963 One r e c e n t Two r e c e n t Damaged i n One r e c e n t s c a r , date unknown 1966, a l s o p o s s i b l y i n I857 1983, 1968, p o s s i b l y i n 1979 and I765 s c a r , date unknown 1964 s c a r s , dates unknown but one s i n c e 1956. s c a r , date unknown s c a r s , d a t e s unknown 1983, p o s s i b l y i n 1904 and I858 s c a r , date unknown one 11 ^170 I n c r e a s e d growth on downslope s i d e s i n c e 1945 12 - 2 3 1 I n c r e a s e d growth on downslope s i d e s i n c e 1952 13 -242 Major i n c r e a s e i n growth on downslope s i d e i n 1973, i n c r e a s e i n growth on both s i d e s I 9 5 3 - I 9 6 I 14 251 No obv i o u s changes i n growth p a t t e r n s 15 336 No obv i o u s changes i n growth p a t t e r n s 16 250 No obv i o u s changes i n growth p a t t e r n s 17 >254 I n c r e a s e d growth on downslope s i d e s i n c e 1963> t r e e p o s s i b l y damaged i n 1972 18 2 230 No o b v i o u s changes i n growth p a t t e r n s 19 2= 220 No o b v i o u s changes i n growth p a t t e r n s 20 268 Decrease i n growth on both s i d e s s i n c e 1955 A n t i s l o p e Scarp Between M2 and M3 21 >412 S l i g h t i n c r e a s e i n growth on downslope s i d e i n 1965 i n c r e a s e on u p s l o p e s i d e i n 1889» s l o p e . Impact s c a r s have been observed as high as s i x metres above the present ground s u r f a c e . Many of the s c a r s now are p a r t l y or completely covered by new wood advancing i n from the s i d e s . Cores were taken from s c a r r e d and unscarred p o r t i o n s of s e v e r a l of these t r e e s to date the impacts. As with the t r e e s from the headscarp area, the dates y i e l d e d by these damaged t r e e s are q u i t e v a r i a b l e (Table 4.1), but there seems to have been e r r a t i c r o c k f a l l a c t i v i t y here s i n c e at l e a s t the e a r l y 1960's. In a d d i t i o n , one 8 cm diameter t r e e in t h i s area was f e l l e d a f t e r the c o r e r broke duri n g an attempt to core a s c a r r e d p o r t i o n of the t r e e . A d i s k from t h i s t r e e showed that a major impact occu r r e d i n 1976, and e i g h t d i f f e r e n t sequences of r e a c t i o n wood were noted i n the past two c e n t u r i e s . F i g u r e 4.4 shows the extent of s l i d e s M2 and M3, and the rid g e between them. An important f e a t u r e of t h i s c e n t r a l r i d g e i s the presence of at l e a s t seven u p h i l l - f a c i n g or a n t i s l o p e s c a r p s . P l a t e 4.3a shows one of these, which i s developed in s u r f i c i a l m a t e r i a l a t 975 m. Features s i m i l a r to t h i s are f a i r l y common i n the southern C o r d i l l e r a (M. J . Bov i s , p e r s . comm., 1986). Bovis (1982) d e s c r i b e d a set of a n t i s l o p e scarps at A f f l i c t i o n Creek, 150 km north of Howe Sound. He i n t e r p r e t e d them as a response to downslope r o t a t i o n of quartz monzonite rocks, due to unloading at the toe of the sl o p e , f o l l o w i n g downwastage of a l a r g e g l a c i e r . R o t a t i o n was c o n t r o l l e d by a master j o i n t set s t r i k i n g p a r a l l e l to-, and d i p p i n g s t e e p l y i n t o , the s l o p e . For t h i s to be p o s s i b l e , a motion r e f e r r e d to as " f l e x u r a l s l i p " by Goodman and Bray (1976) would have to occur 1 04 F i g u r e 4. 4: Contour Map and P r o f i l e s o f S i t e s M2 and M3 P l a t e 4 . 3 ; A n t i s l o p e Scarps a t S i t e M2  ( a ) ; Scarp i n s u r f i c i a l m a t e r i a l s a t 975 m. 106 between the block columns. Bovis (1982) performed a kinematic t e s t f o r t h i s u s ing a s t e r e o p l o t of j o i n t p o l e s , and found that f l e x u r a l s l i p was p o s s i b l e at t h i s s i t e . The a n t i s l o p e scarps at M2 may have a s i m i l a r o r i g i n . I t i s l i k e l y that the steep w a l l s of Howe Sound are s t i l l a d j u s t i n g to d e g l a c i a t i o n , and that the a n t i s l o p e scarps near M Creek are a r e s u l t of t h i s . S i m i l a r f e a t u r e s have not been d i s c o v e r e d elsewhere i n the study area, but a l a r g e mountain-top crack at the top of C h a r l e s Creek probably has a s i m i l a r o r i g i n . Whatever t h e i r cause, the presence of scarps suggests s t r o n g l y that a l a r g e amount of downslope r o t a t i o n i s o c c u r r i n g i n the area of s i t e s M2 and M3. The most impressive of the scarps occurs at the same e l e v a t i o n as the a c t i v e scarp of the main s l i d e , and the t e n s i o n crack behind i t seems to be connected to the t e n s i o n crack behind the unstable block d e s c r i b e d e a r l i e r ( F i g u r e 4.3). T h i s f e a t u r e i s shown i n P l a t e 4.3b. A kinematic a n a l y s i s s i m i l a r to that performed by Bovis (1982) has been c a r r i e d out f o r the S i t e M2 - M3 headscarp a r e a . F i g u r e 4.5 shows the data from a survey of a c c e s s i b l e rock j o i n t s at t h i s l o c a t i o n . The poles to the planes of a l l j o i n t s surveyed are p l o t t e d on a lower hemisphere, equal area s t e r e o n e t (Schmidt n e t ) . Density contours, drawn a c c o r d i n g to the method o u t l i n e d by Hoek and Bray (1977, p. 52) are shown on F i g u r e 4.6. In r e a l i t y , there may not be enough data to draw these contours with much co n f i d e n c e , but i t was f e l t to be j u s t i f i e d here i n that i t p r o v i d e s some v i s u a l evidence that there are three dominant j o i n t s e t s . The master set s t r i k e s p a r a l l e l to the slope (average 107 F i g u r e 4.5: S t e r e o p l o t o f P o l e s to F r a c t u r e s a t the Headscarps o f S i t e s M2 and M3 N ± T S © S i t e M2: headscarp & S i t e M3: headscarp 0 S i t e M2: a c t i v e scarp ( f r o n t o f u n s t a b l e b l o c k ) 2 i n d i c a t e s two p o l e s h a v i n g the same O r i e n t a t i o n 108 F i g u r e 4.6: D e n s i t y Contours f o r the P o le s P l o t t e d on F i g u r e 4.5  ( S i t e s M2 and M3) azimuth 158 ) and di p s s t e e p l y i n t o or out of i t , as i n d i c a t e d by the pole c o n c e n t r a t i o n s i n the northeast and southwest quadrants of F i g u r e 4.6. A second j o i n t set s t r i k e s normal to the master set (046°), and d i p s s t e e p l y (70°) to the southeast. There are only l i m i t e d data on the t h i r d s e t , which s t r i k e s p a r a l l e l to the master s e t , and d i p s i n the op p o s i t e d i r e c t i o n , roughly orthogon-a l to the d i p of the master s e t . T h i s s i t u a t i o n i s very s i m i l a r to that analysed by Bovis (1982) at A f f l i c t i o n Creek. The c o n d i t i o n f o r f l e x u r a l s l i p i s i l l u s t r a t e d i n F i g u r e 4.7a. For the block columns to be ab l e to s l i d e past one another while r o t a t i n g downslope i n a t o p p l i n g motion, the angle, a , between the j o i n t plane normal and the h o r i z o n t a l must be such t h a t : a < 6 - <f> (4.1) where: 0 = the slope angle of the rock f a c e ; and <|> = the angle of f r i c t i o n on the j o i n t plane, (Goodman and Bray, 1976). T h i s assumes that the major p r i n c i p a l s t r e s s , o"! , i s p a r a l l e l to the rock face c l o s e to the s u r f a c e . The rock face at the top of S i t e s M2 and M3 has mean and maximum slope s of 41° and 44°, i n the azimuth d i r e c t i o n 234°. The great c i r c l e s r e p r e s e n t i n g t h i s face are i n c l u d e d on F i g u r e 4.5. Great c i r c l e s are a l s o p l o t t e d f o r slop e s of ( 9 - <t>), with <f> = 30° and 35°. These values of <)> were used because 30° i s probably a t y p i c a l lower bound f o r cut s u r f a c e s i n f e l s i c igneous rock, and s m a l l - s c a l e a s p e r i t i e s on the j o i n t planes c o u l d add as much as 5° to t h i s v a l u e . Since the kinematic a n a l y s i s presented here i s intended only as an i l l u s t r a t i o n , i t was f e l t that rock shear 1 10 F i g u r e 4.7a: C o n d i t i o n f o r I n t e r l a y e r S l i p ( a f t e r Goodman and Bray , 1976; E o v i s , 1982) Requirement f o r i n t e r l a y e r s l i p : e > <j>+cc n = normal d i r e c t i o n to j o i n t p lane a, = major p r i n c i p a l s t r e s s d i r e c t i o n crn = normal s t r e s s on j o i n t p l ane cr= shear s t r e s s on j o i n t p l ane <$> = angle o f s h e a r i n g r e s i s t a n c e a l o n g j o i n t p l ane oc = angle o f i n c l i n a t i o n o f j o i n t p lane normal F i g u r e 4. 7b: O r i e n t a t i o n s o f J o i n t P lanes a t Headscarp o f  S i t e M2 Headscarp (behind A c t i v e scarp ( f r o n t o f u n s t a b l e b l o c k ) u n s t a b l e b l o c k ) 6 <<t>< © 6- 4" © 111 s t r e n g t h t e s t i n g , to o b t a i n a more p r e c i s e value, was not j u s t i f i e d . A c c o r d i n g to i n e q u a l i t y (4.1), i n t e r l a y e r s l i p i s p o s s i b l e on any j o i n t plane whose pole p l o t s o u t s i d e of a great c i r c l e r e p r e s e n t i n g ( 8 - $ )° . Small c i r c l e s r e p r e s e n t i n g d e v i a t i o n s of 10° and 20° from the slope d i r e c t i o n are a l s o shown on F i g u r e 4.5: p o l e s p l o t t i n g o u t s i d e of these bounds represent planes whose s t r i k e d e v i a t e s too much from that of the rockface to permit i n t e r l a y e r s l i p . F i g u r e 4.5 i n d i c a t e s that there are a number of planes which meet these requirements and thus w i l l allow f l e x u r a l s l i p . T h i s evidence, combined with the presence of the a n t i s l o p e scarps noted e a r l i e r , argues s t r o n g l y i n favour of f l e x u r a l s l i p at S i t e s M2 and M3. The a c t u a l f a i l u r e mechanism at the f r o n t of the unstable block at S i t e M2 i n v o l v e s s l i d i n g along j o i n t s orthogonal to the master s e t . As i l l u s t r a t e d i n F i g u r e 4.7b, the r o t a t i o n of t h i s block has i n c r e a s e d the angle of i n c l i n a t i o n , <3, of these c r o s s j o i n t s from w e l l below <|> to c l o s e to <j>. T h i s assumes that the c r o s s j o i n t s i n t e r s e c t the master set at roughly 90° ( i . e . 6 - a ) . As noted above, there are only l i m i t e d data on the o r i e n t a t i o n of t h i s c r o s s j o i n t s e t , but o b s e r v a t i o n s i n the f i e l d support the above assumption. 3 It was noted e a r l i e r that some 3500 m of s l i d e d e b r i s remains i n M Creek channel at the toe of the s l i d e . I f the reasonable assumption i s made that the October 1981 d e b r i s t o r r e n t scoured the channel at t h i s p o i n t , the r a t e of d e b r i s 3 supply from M2 would be about 700 m per year. T h i s value, combined with the e a r l i e r estimate of the t o t a l s l i d e volume 1 1 2 (2.5 x 10 m ), i m p l i e s that some 2.1 x 10 m of d e b r i s must have been d e l i v e r e d to the creek p r i o r to the 1981 event. However, Hungr (pe r s . comm., 1987) found t h a t , while the upper pa r t of M2 was very a c t i v e , very l i t t l e coarse d e b r i s seemed to have reached the creek by October 1981. The " c o r r i d o r " between the toe of the s l i d e was f l o o r e d by f o r e s t d e b r i s and looked s u r p r i s i n g l y u n d i s t u r b e d compared with the a c t i v e t a l u s accumula-t i o n which e x i s t s there today. What became of the m i s s i n g 4 3 2.1 x 10 m of d e b r i s ? I t i s known from the t r e e r i n g r e c o r d that i n d i v i d u a l boulders have been bounding down t h i s slope from the headscarp f o r many y e a r s . Presumably these c o u l d have reached the creek without having a notable long-term e f f e c t on the f o r e s t f l o o r , and thus would not be d e t e c t e d by Hungr's o b s e r v a t i o n . Perhaps 10% of the t o t a l s l i d e volume can be accounted f o r i n t h i s way. Another 10% may be s t o r e d on the slope above the creek, i n d e b r i s accumulations behind t r e e s , as 3 shown i n P l a t e 4.1b. T h i s s t i l l means that 20,000 m of d e b r i s would have reached the creek s i n c e October 1981, yet there are 3 3 only 3500 m there now. Even i f h a l f of the 20,000 m were f i n e enough to have been s e l e c t i v e l y removed by the creek i t s e l f , 3 3 there i s s t i l l some 6.5 x 10 m of m a t e r i a l unaccounted f o r . Thus, Hungr's o b s e r v a t i o n s notwithstanding, and a l l o w i n g f o r some e r r o r i n the above volume estimates, i t i s d i f f i c u l t to escape 3 the c o n c l u s i o n that there must have been at l e a s t 5000 m of loose m a t e r i a l (one q u a r t e r of the t o t a l t o r r e n t volume) present at the toe of the s l i d e when the t o r r e n t o c c u r r e d . From the d i s c u s s i o n above, i t i s unclear whether or not 1 13 t h i s r o c k s l i d e played a major r o l e i n the 1981 d e b r i s t o r r e n t , although i t c e r t a i n l y s u p p l i e d some d e b r i s . Both Church and Desloges (1984) and Hungr (pers. comm., 1987) favour a slope f a i l u r e at s i t e M1, some 50 m upstream from s i t e M2 (Figure 4.2) as the t o r r e n t t r i g g e r . Even i f S i t e M2 played only a very small r o l e i n the 1981 t o r r e n t , there i s now a l a r g e amount of coarse d e b r i s i n M Creek at the toe of the s l i d e , and there i s the p o s s i b i l i t y of a major s l o p e f a i l u r e here. T h i s s i t e c l e a r l y has the p o t e n t i a l to play a major r o l e i n any f u t u r e d e b r i s t o r r e n t s i n M Creek. 4.3.1.3 Type L o c a t i o n - S i t e M1 S i t e M1 i s a steep (30°), V-shaped bedrock chute e n t e r i n g M Creek from the south s i d e about 50 m upstream of the toe of S i t e M2 ( F i g u r e 4.2, P l a t e 2.6). The p o s i t i o n of t h i s g u l l y seeems to be c o n t r o l l e d by a f a u l t , which continues upslope. At i t s top, some 160 m upslope from the creek, the g u l l y i s f i l l e d with c o l l u v i u m , but f o r most of i t s l e n g t h i t i s completely scoured. I t has been suggested t h a t a f a i l u r e of the d e b r i s i n t h i s g u l l y on 28 October 1981 t r i g g e r r e d the major t o r r e n t i n M Creek (Church and Desloges, 1984). T h i s p o s s i b i l i t y i s d i s c u s s e d f u r t h e r i n S e c t i o n 4.5.2.2. A h i g h l y f r a c t u r e d c l i f f s u p p l i e s r o c k f a l l d e b r i s d i r e c t l y to the top of the g u l l y . The rocks here are mainly quartz d i o r i t e , with c o n s i d e r a b l e s u l p h i d e m i n e r a l i z a t i o n , suggesting t h a t the c o n t a c t with the Gambier Group may be nearby (see F i g u r e 2.11). F r a c t u r e o r i e n t a t i o n data from t h i s rock face and from s i t e M5, a s i m i l a r s i t e about 500 m upstream ( F i g u r e 4.2) are 1 1 4 p l o t t e d on F i g u r e 4.8. There i s more s c a t t e r i n t h i s p l o t than in F i g u r e 4.5, but there seems to be a dominant set at S i t e M1 d i p p i n g s t e e p l y to the northwest ( s t r i k e / d i p = 042/74N), while the dominant set at S i t e M5 s t r i k e s north and i s almost v e r t i c a l (008/86E). There are not enough data to allow a contour d e n s i t y p l o t of the poles of the f r a c t u r e s at S i t e s M1 and M5 to be drawn. The a c t i v e rock face at S i t e M1 has a slope of 60° - 80°, i n the azimuth d i r e c t i o n 220°. Great c i r c l e s r e p r e s e n t i n g t h i s f a c e , and f o r slopes of (0 -<(» ) , with <f> = 30° and 35°, have been i n c l u d e d on F i g u r e 4.8, i n a s i m i l a r manner to F i g u r e 4.5. I t can be seen from t h i s that none of the mapped f r a c t u r e s would allow f l e x u r a l s l i p , a requirement f o r t o p p l i n g . I t should be noted that the steepest and most a c t i v e part of t h i s c l i f f i s not a c c e s s i b l e , and that the f r a c t u r e o r i e n t a t i o n data f o r t h i s area are taken from an adjacent rock exposure. On the b a s i s of t h i s l i m i t e d i n f o r m a t i o n , t o p p l i n g cannot be e l i m i n a t e d as a p o s s i b l e f a i l u r e mechanism at t h i s s i t e . Two other p o s s i b l e f a i l u r e modes i n v o l v e b l o c k s s l i d i n g along s i n g l e p l a n a r f r a c t u r e s (plane f a i l u r e s ) , or along the l i n e s of i n t e r s e c t i o n between two planes (wedge f a i l u r e s ) , which crop out i n the rock face (Figure 4.9). For these to be kinema-t i c a l l y p o s s i b l e , the f a i l u r e plane or l i n e of i n t e r s e c t i o n must be l e s s steep than the rock face, but steeper than the angle of shearing r e s i s t a n c e of the m a t e r i a l , $ . Dip v e c t o r s of planes or l i n e s s u s c e p t i b l e to these types of f a i l u r e must f a l l o u t s i d e of a great c i r c l e r e p r e s e n t i n g the slope face and i n s i d e 1 1 5 Figure 4.8: Stereoplot of Poles to Fractures at S i t e s Ml and M5 a S i t e Ml: f r a c t u r e surfaces • S i t e M5: f a u l t o S i t e M5: f r a c t u r e surfaces 2 i n d i c a t e s two poles having the same o r i e n t a t i o n 1 1 6 F i g u r e 4.9: Schematic D e p i c t i o n s of Plane F a i l u r e ( l e f t ) and  Wedge F a i l u r e ( r i g h t ) i n F r a c t u r e d Rock ( a f t e r  Hoek and Bray, 1977) of a small c i r c l e of r a d i u s (90° - 9 ) on a s t e r e o p l o t . F i g u r e 4.10 shows that none of the mapped f r a c t u r e s are s u s c e p t i b l e to plana r f a i l u r e s , but that wedge f a i l u r e s c o u l d occur along l i n e s of i n t e r s e c t i o n between j o i n t s e t s 1 and 2. The o r i e n t a -t i o n s of the three s e t s are 042/74N, 099/87S, and 165/45E. I t must be emphasized that sets 2 and 3 are d e f i n e d by only two measurements each, so t h i s a n a l y s i s must be c o n s i d e r e d extremely t e n t a t i v e . On the b a s i s of t h i s very l i m i t e d amount of data, i t appears that rocks are being loosened from the face at S i t e M1 through wedge f a i l u r e s , but n e i t h e r t o p p l i n g nor plane f a i l u r e s can be e l i m i n a t e d as p o s s i b i l i t i e s . More d e t a i l e d analyses, t a k i n g i n t o account the weight of the bl o c k s and pore water 117 Figure 4.10: S t e r e o p l o t of Dip Vectors of Fractures at S i t e Ml. and Average Lines of I n t e r s e c t i o n Between Fracture Sets & Dip vectors of i n d i v i d u a l f r a c t u r e s o Vectors r e p r e s e n t i n g average trend and plunge of 2 - 3 l i n e s of i n t e r s e c t i o n between f r a c t u r e sets 118 p r e s s u r e s i n the j o i n t s , are beyond the scope of t h i s t h e s i s . S i t e M1 continues to be an a c t i v e c o n t r i b u t o r of coarse d e b r i s to M Creek, s i n c e rocks which f a l l from the f r a c t u r e d face i n t o the steep chute are d e l i v e r e d d i r e c t l y to the creek channel. T h i s i s a more continuous d e b r i s supply mechanism than the b r i e f but l a r g e r d e b r i s f a i l u r e which o c c u r r e d here e a r l i e r . Another important d i f f e r e n c e between these two processes i s t h a t , while the momentum of the 1981 d e b r i s s l i d e may have been s u f f i c i e n t to t r i g g e r a major t o r r e n t i n M Creek, the i n d i v i d u a l rocks now being d e l i v e r e d to the creek through r o c k f a l l are accumulating at the toe of the chute, and c o u l d be m o b i l i z e d and i n c l u d e d i n a f u t u r e d e b r i s t o r r e n t . 4.3.1.4 Type L o c a t i o n - Loggers Creek T r i b u t a r y F i g u r e 4.11 shows the main s i t e s i n the study area where minor r o c k f a l l and r o c k s l i d e ( r a v e l l i n g ) i s o c c u r r i n g . The most e x t e n s i v e s i t e where r a v e l l i n g s u p p l i e s d e b r i s d i r e c t l y to a creek channel i s i n the t r i b u t a r y of Loggers Creek. No q u a n t i -t a t i v e work has been done here, but some i n t e r e s t i n g o b s e r v a t i o n s were made d u r i n g two v i s i t s to t h i s l o c a t i o n . E x t e n s i v e v o l c a n i c c l i f f s are present at and below the l o g g i n g road cut at 1000 m, and these are undergoing a c o n s i d e r a b l e amount of r a v e l l i n g . S e v e r a l steep d e b r i s chutes descend from the road cut down to the the creek bed, which appears to be dry f o r most of the year. ( P l a t e 4.4 shows some d e b r i s chutes t y p i c a l of the area, although these p a r t i c u l a r ones feed Loggers Creek i t s e l f , r a t h e r than the t r i b u t a r y ) . In the e n t i r e reach between the road cut and the c o n f l u e n c e with the main stem of Loggers Creek, the t r i b u t a r y 119 O F i g u r e 4.11: Rock R a v e l l i n g and Ta lus S h i f t S i t e s f T ~ T T 1 R a v e l l i n g (Minor R o c k f a l l ) S i t e s Ta lus S h i f t S i t e s ffies^ A c t i v e R o c k s l i d e s Feed ing Ta lus S lopes C h u t e s , D e b r i s Tracks M3 S p e c i f i c S i t e s Referenced i n the Text M i n o r D e b r i s T o r r e n t Creek, As Coastline D Decks Creek L Loggers Creek M M Creek Mg Magnesia Creek A Alberta Creek H Harvey Creek H a Mount Harvey (elev. 1625 B a Brunswick Mtn. (elev. 1760 Watershed Bdy. Squamish Hwy. Other Roads ~-—-, _ Logging Roads B. C. Railway Power Trans-mission Line H 0 W E U N D seme - met res contour in ce rva. 1: 1 OOni P l a t e 4.4: D e b r i s Chutes, Loggers Creek Watershed. Dust above  r i d g e i n middle d i s t a n c e i s from one of the d e l i b e r - a t e l y induced "cobble flows" d e s c r i b e d i n the t e x t . channel bottom i s f i l l e d with c o b b l e - s i z e d rubble r e s t i n g at i t s angle of repose. The above o b s e r v a t i o n s suggest that the t r i b u t a r y i s capable of t r a n s p o r t i n g rock fragments, loosened from the h i l l s l o p e s by r a v e l l i n g , i n t o the main channel of Loggers Creek. To t e s t t h i s h y p othesis, i n d i v i d u a l rocks were d i s l o d g e d from the road cut and observed as they r o l l e d down the d e b r i s chutes and i n t o the dry t r i b u t a r y channel. In almost a l l cases, these rocks m o b i l i z e d l a r g e amounts of loose m a t e r i a l from the stream bed. The r e s u l t i n g "cobble flows" were observed to t r a v e l s e v e r a l hundred metres downstream, towards the c o n f l u e n c e . These crude t e s t s s t r o n g l y suggest t h a t , over the long term, r a v e l l i n g of the v o l c a n i c rocks i n Loggers Creek T r i b u t a r y sub-catchment can 121 supply c o n s i d e r a b l e amounts of d e b r i s to Loggers Creek at the c o n f l u e n c e . 4.3.2 T a l u s S h i f t T h i s i s p o t e n t i a l l y an important d e b r i s supply mechanism i n the study area, but the only case of t a l u s f e e d i n g rocks d i r e c t l y i n t o the main stream channels i s the p r e v i o u s l y mentioned Loggers Creek T r i b u t a r y ( S e c t i o n 4.3.1.4). There are s e v e r a l other s i t e s where a c t i v e downslope s h i f t of m a t e r i a l i s o c c u r r i n g , but these a l l end i n s t a b l e accumulations on slopes of lower g r a d i e n t s . F i g u r e 4.11 shows a l l s i t e s where a c t i v e t a l u s s h i f t i s occur-r i n g . In many cases, these n e c e s s a r i l y c o i n c i d e with the r o c k f a l l and r o c k s l i d e s i t e s d i s c u s s e d i n S e c t i o n 4.3. The main s i t e s are Loggers Creek T r i b u t a r y , s i t e M3, and the lower part of s i t e M2. 4.3.2.1 Mechanisms In the study area, slopes e x p e r i e n c i n g t a l u s s h i f t u s u a l l y c o n s i s t of d i o r i t e b l o c ks r e s t i n g at t h e i r angle of repose i n a d i l a t e d s t a t e . Since the d e b r i s i s very unstable at t h i s angle, any added lo a d i s s u f f i c i e n t to m o b i l i z e some of the m a t e r i a l and cause i t to r o l l . T y p i c a l sources of t h i s added lo a d are rocks f a l l i n g from the c l i f f above, and animals such as bears and deer c r o s s i n g the upper p a r t of the s l o p e . I t i s apparent that g r a v i t y i s the main f a c t o r d r i v i n g t a l u s s h i f t . Pore water pressure i s not important, as the m a t e r i a l has a high p o r o s i t y , so l a r g e amounts of water can r e a d i l y flow through i t . I t i s p o s s i b l e t h a t , under very high runoff c o n d i -t i o n s , h y d r a u l i c drag f o r c e s c o u l d become important at a few 122 s i t e s where t a l u s occupies ephemeral stream channels, but t h i s would occur very r a r e l y . Logs and l a r g e boulders can a c t as s i g n i f i c a n t d e b r i s b u t t r e s s e s , a l l o w i n g the smaller m a t e r i a l to accumulate at steeper angles than would otherwise be p o s s i b l e (see P l a t e 4.5). T h i s can allow s i g n i f i c a n t t r a n s i e n t accumulations of m a t e r i a l to form i n the upper p a r t s of t a l u s s l o p e s . I f these b u t t r e s s e s move, or, i n the case of l o g s , e v e n t u a l l y r o t , the r e s u l t i n g l a r g e downslope flows of m a t e r i a l c o u l d d e s t a b i l i z e l a r g e p o r t i o n s of the e n t i r e t a l u s accumulation, d e l i v e r i n g l a r g e volumes of m a t e r i a l to the toe of the s l o p e . P l a t e 4.5: Log A c t i n g as Debris B u t t r e s s , S i t e M3 123 4.3.2.2 Type L o c a t i o n - S i t e M3 The headscarp of S i t e M3 has been d i s c u s s e d , along with the adjacent S i t e M2, i n S e c t i o n 4.3.1.2. For a d i s t a n c e of some 360 m below the headscarp, the channel of M Creek T r i b u t a r y R1 i s f i l l e d with coarse r o c k f a l l d e b r i s , at an average g r a d i e n t of 36°. Below t h i s p o i n t , the stream g r a d i e n t averages 29° f o r 325 m, to i t s c o n f l u e n c e with M Creek. The d e b r i s accumulation has 3 3 an average width of 13 m; an estimated 3 - 4 x 1 0 m of m a t e r i a l i s s t o r e d here. F i g u r e 4.12 i s a p r o f i l e of t h i s s i t e , along the l i n e i n d i c a t e d i n F i g u r e 4.4. The age of t h i s r o c k f a l l and t a l u s accumulation i s uncer-t a i n , but i t i s much o l d e r than the l a r g e r rock s l i d e ( S i t e M2) 100 m to the south. Whereas the l a t t e r i s not even v i s i b l e on a i r photographs from 1968 or e a r l i e r , the former s i t e does not appear to have changed much s i n c e 1939, the date of the e a r l i e s t a v a i l a b l e photos. Timber h a r v e s t i n g in the e a r l y 1960's removed any t r e e s which may have been of use i n d a t i n g the s l i d e , but i t d e f i n i t e l y has been present f o r at l e a s t f i f t y y e a r s . The d i o r i t e blocks on t h i s t a l u s slope r e s t in a d i l a t e d s t a t e , c l o s e to t h e i r angle of repose. I t i s i m p o s s i b l e to walk on the upper p a r t of t h i s slope without d i s l o d g i n g rocks at every step. The slope becomes more s t a b l e and l e s s steep towards i t s toe (Figure 4.12). Near 840 m, the stream emerges from the d e b r i s , and rocks and logs i n the channel are moss-covered and look f a i r l y s t a b l e . The d i l a t e d s t a t e of the d e b r i s on t h i s slope can be a p p r e c i a t e d from P l a t e s 4.5 and 4.6. To o b t a i n an i n d i c a t i o n of the amount of d e b r i s s h i f t 124 F i g u r e 4 .12 : P r o f i l e o f S i t e M3 800 I i i i i | | I I I I I I I I I I I I I I 0 100 200 300 400 D i s t a n c e Upslope from Datum (m) Table 4 . 2 : Movement Observed at P a i n t L i n e s on Ta lus S l o p e ,  S i t e M3 L i n e - E l e v a t i o n S lope G r a d i e n t Movement Movement C o l o u r ' (met re s ) (degrees) 1984-1985 1985-1986 Red Y e l l o w Blue Orange 875 917 948 1014 32.5 36 35-5 37 none minor c o n s i d e r a b l e f moderate none a c o n s i d e r a b l e c o n s i d e r a b l e * c o n s i d e r a b l e * Notes : a -b -c -d -e -f -minor d e b r i s t o r r e n t c ro s se s l i n e 5 m n o r t h o f edge o f t a l u s s lope some downslope movement o f log s on r i g h t hand s i d e many l o g s and some rocks have moved downslope two new l o g s (one 70 cm d i a . ) on l e f t hand s i d e , many r o c k s have moved downslope downslope movement o f rocks and s m a l l l og s some downslope movement o f rocks and s m a l l logs 125 at t h i s s i t e , four l i n e s were p a i n t e d a c r o s s the slope i n J u l y 1984 (see F i g u r e 4.12 f o r l o c a t i o n s ) . Table 4.2 summarizes the e l e v a t i o n s of the p a i n t l i n e s , the average g r a d i e n t s of the t a l u s slope there, and the amount of movement over a two-year p e r i o d . P l a t e 4.6 shows three of the l i n e s as they appeared i n 1986. The upper three l i n e s have experienced c o n s i d e r a b l e movement, while the lowest one i s f a i r l y s t a b l e (Table 4.2). From t h i s i t can be i n f e r r e d that <f> , the constant volume f r i c t i o n angle, must be cv r about 37° f o r t h i s m a t e r i a l . Where the slope angle, 8, of the t a l u s accumulation i s equal or c l o s e to t h i s v a l u e , downslope s h i f t of t a l u s w i l l occur i f any a d d i t i o n a l l o a d i s a p p l i e d . As 8 decreases towards the toe of the t a l u s , the d e b r i s becomes more s t a b l e . Since the average g r a d i e n t of the stream channel below 870 m i s only 29°, i t i s c l e a r that t h i s t r i b u t a r y i s incapable of t r a n s p o r t i n g m a t e r i a l from S i t e M3 to M Creek under dry c o n d i t i o n s . However, i n f i n i t e slope theory s t a t e s that f u l l y s a t u r a t e d d e b r i s w i l l be unstable on s l o p e s where: tan 6 ^ (tan <j>)/2 (4.2) I f <t> = 37°, the c r i t i c a l slope i s 8 - 20.6°, a c o n d i t i o n which i s s a t i s f i e d f o r the e n t i r e l e n g t h of T r i b u t a r y R1. Above 870 m the d e b r i s i s so coarse and so porous that f u l l s a t u r a t i o n i s u n l i k e l y ; so, while a c t i v e t a l u s s h i f t i s o c c u r r i n g at t h i s s i t e , i t i s not, at present, a major s u p p l i e r of d e b r i s to M Creek. Coarse d e b r i s i s g r a d u a l l y accumulating near the break in slope, at about 870 m. Below 870 m, the channel of T r i b u t a r y R1 i s f i l l e d with moss-covered logs and b o u l d e r s , i n a f i n e r matrix; While t h i s 1 26 P l a t e 4 . 6 : P a i n t L i n e s , S i t e M3, i n 1986. ( a ) : Red p a i n t l i n e , u n d i s t u r b e d . ( b ) i Yellow  p a i n t l i n e . Arrows i n d i c a t e rocks which have moved downslope. Note r e c e n t l y scoured channel from minor d e b r i s t o r r e n t behind red l i n e . P l a t e 4 . 6 , continued: ( c ) : Blue p a i n t l i n e . Arrow i n d i c a t e s p a r t o f l i n e , pack i s on l i n e . Person i s standing on p a i n t e d rock which has moved downslope.  (d): Debris f r o m recent minor d e b r i s t o r r e n t , downslope of red l i n e . reach appears f a i r l y s t a b l e , and i s not steep enough to promote t a l u s s h i f t , the presence of the f i n e s suggests that s i g n i f i c a n t pore water pre s s u r e s c o u l d develop here under high d i s c h a r g e c o n d i t i o n s . Thus, i t i s c o n c e i v a b l e that a small d e b r i s t o r r e n t c o u l d s t a r t here, i n the manner proposed by Takahashi (1981) ( S e c t i o n 6.1.1). Such a t o r r e n t c o u l d d e l i v e r l a r g e amounts of m a t e r i a l , most of which came i n i t i a l l y from the headscarp of S i t e M3, to the main channel of M Creek. In f a c t , a small t o r r e n t d i d occur here, probably i n s p r i n g 1986. I t s t a r t e d at 1020 m, where a l o g g i n g road c r o s s e s a small creek which j o i n s t r i b u t a r y R1 near 860 m ( F i g u r e 4.11). P r i o r to t h i s event, the channel i n which the t o r r e n t occurred was so s m a l l and w e l l - v e g e t a t e d that i t s presence was not obvious, even from a few metres away, but i t i s now scoured out ( P l a t e 4.6a). The t o r r e n t was not powerful enough to d e s t a b i l i z e the bed of T r i b u t a r y R1, but i t d e p o s i t e d a c o n s i d e r a b l e amount of f i n e s and o r g a n i c s i n t h i s channel ( P l a t e 4.6d). 4.4 MATERIAL SUPPLY FROM COLLUVIAL SLOPES In t h i s t h e s i s , " c o l l u v i a l s l o p e s " are c l a s s e d as s l o p e s mantled by r e l a t i v e l y f i n e g r a i n e d s u r f i c i a l d e p o s i t s , and slopes mantled by mixtures of coarse blocky m a t e r i a l i n a f i n e r matrix ( c o l l u v i u m ) . A l s o i n c l u d e d are stream channels f i l l e d with these m a t e r i a l s , and s i d e c a s t m a t e r i a l s below l o g g i n g roads. In r e a l i t y , t a l u s i s a l s o a form of c o l l u v i u m , but i t was c o n s i d e r e d s e p a r a t e l y because the absence of f i n e s g i v e s t a l u s rather d i f f e r e n t p r o p e r t i e s than matrix-supported c o l l u v i u m . 1 29 M a t e r i a l from c o l l u v i a l slopes i s d e l i v e r e d to stream channels by shallow d e b r i s s l i d e s and slumps, minor d e b r i s t o r r e n t s or s o i l wedge f a i l u r e s i n t r i b u t a r y channels, r a v e l l i n g , and slope wash. 4.4.1 Shallow Debris S l i d e s and Slumps Shallow d e b r i s s l i d e s and slumps are important d e b r i s sources i n areas where abundant s u r f i c i a l d e p o s i t s e x i s t near stream channels. These d e p o s i t s are most common in Magnesia Creek watershed (Figure 2.13), although i s o l a t e d s u r f i c i a l d e p o s i t s e x i s t i n the lower p a r t s of a l l the study area water-sheds. F i g u r e 4.13 shows that most shallow d e b r i s s l i d e s , which a f f e c t creek channels d i r e c t l y , occur i n a small area along Magnesia Creek T r i b u t a r y L3 ( S i t e Mg3). I s o l a t e d small bank f a i l u r e s (slumps) e x i s t along a l l of the c r e e k s . 4.4.1.1 Mechanisms Shallow d e b r i s s l i d e s and slumps u s u a l l y occur on slopes oversteepened by stream e r o s i o n . As noted in S e c t i o n 2.3, the loose s u r f i c i a l d e p o s i t s are u n d e r l a i n by much harder and r e l a t i v e l y impermeable bedrock or b a s a l t i l l . The f a i l u r e s u r f a c e i s u s u a l l y d e f i n e d by the c o n t a c t between these m a t e r i a l s and the o v e r l y i n g l o o s e r m a t e r i a l s . C o n s i d e r a b l e seepage has been observed at many of these f a i l u r e s . During wet p e r i o d s , the groundwater t a b l e must r i s e i f the i n f i l t r a t i o n r a t e exceeds the ra t e of l a t e r a l seepage. As the water t a b l e r i s e s , the pressure head i n c r e a s e s , d e c r e a s i n g the shear s t r e n g t h , a c c o r d i n g to the f o l l o w i n g equation: 1 30 F i g u r e 4.13: Shal low D e b r i s S l i d e s and Slumps,  S o i l Wedge F a i l u r e s EZ3 M 1 Sha l low D e b r i s S l i d e s , Slumps S o i l Wedge F a i l u r e s S p e c i f i c S i t e s Referenced i n Text Prominent G u l l y Upslope o f S i t e Mgl D L M Mg A H H a Creek, Coast 1ine Deeks Creek Loggers Creek M Creek Magnesia Creek Alberta Creek Harvey Creek Mount Harvey (elev. 1625 m) Brunswick Mtn. (elev. 1760 zi) Watershed Bdy. Squamish Hwy. Other Roads Logg ing Roads 5. C. Railway Power Trans-mission Line H O W E S O U N D scale - me t r c s contour Interval: 100m S = c' + (<5 - u) tan*' (4.3) where: S = shear s t r e n g t h of the m a t e r i a l along any f a i l u r e s u r f a c e c = cohesion of the m a t e r i a l <5 = t o t a l s t r e s s normal to the f a i l u r e s u r f a c e u = pore water pr e s s u r e at the f a i l u r e s u r f a c e (f)' = angle of s h e a r i n g r e s i s t a n c e of the m a t e r i a l A s l i d i n g f a i l u r e roughly p a r a l l e l to the slope s u r f a c e w i l l occur i f S decreases to such an extent that the f a c t o r of s a f e t y a g a i n s t f a i l u r e , Fs, has a value l e s s than u n i t y i n equation (4.4). The equations governing t h i s c o n d i t i o n a r e : Fs = | (4.4) T = [(1 - m ) y , + m y ] z sine cose (4.5) d sat 2 <5 = [ (1 - m )y , + m Y J z cos e (4.6) d sat 2 u = m z Y cos e (4.7) w where: T = shear s t r e s s along the f a i l u r e s u r f a c e z = depth to the f a i l u r e s u r f a c e , measured v e r t i c a l l y m = r a t i o of the height of the groundwater t a b l e above the f a i l u r e s u r f a c e to the t o t a l depth, z (m = 1 i f the water t a b l e i s at the ground surface) = dry u n i t weight of the m a t e r i a l above the f a i l u r e s u r f a c e Y „ = + = s a t u r a t e d u n i t weight of the m a t e r i a l above the sax. f a i l u r e s u r f a c e Y = u n i t weight of water 1 32 6 = angle of i n c l i n a t i o n of the slope and the f a i l u r e s u r f a c e , r e l a t i v e to the h o r i z o n t a l . The above equations i n d i c a t e that a simple r i s e i n the groundwater t a b l e may be s u f f i c i e n t to i n i t i a t e a shallow d e b r i s s l i d e . While the r e l a t i o n s h i p s are more complex, the same p r i n c i p l e a p p l i e s to the i n i t i a t i o n of r o t a t i o n a l slumps. E i t h e r type of f a i l u r e i s more l i k e l y i f the support at the toe i s removed by stream e r o s i o n , s i n c e t h i s i n c r e a s e s shear s t r e s s by i n c r e a s i n g 8. T h i s e f f e c t i s common i n the study a r e a . 4.4.1.2 Type L o c a t i o n - S i t e Mg3 S i t e Mg3 i s a s e r i e s of e i g h t shallow d e b r i s s l i d e s on the banks of Magnesia Creek T r i b u t a r y L3, 275 to 500 metres upstream of i t s c o n f l u e n c e with the main stem (Figure 4.13). F i g u r e 4.14 shows these s l i d e s , and g i v e s d e t a i l s of the l a r g e s t one (No. 8). Two of the d e b r i s s l i d e s are shown i n P l a t e 4.7. Most occur on the r i g h t bank, which r i s e s 10 - 15 metres, at an average of 35°, to a f l a t t e r t e r r a c e - l i k e f e a t u r e ( F i g u r e 4.14). The s u r f a c e of t h i s upper area c o n s i s t s p r i m a r i l y of l a r g e boulders which have f a l l e n from c l i f f s on the south s i d e of Mt. Brunswick, and of decaying stumps and l o g s . F i n e m i n e r a l s o i l i s s c a r c e . The depth to b a s a l t i l l i s u n c e r t a i n on the t e r r a c e - l i k e f e a t u r e , but on the stream bank i t i s l e s s than one metre, the t i l l being o v e r l a i n by a l a y e r of loose a b l a t i o n t i l l or c o l l u v i u m . An e x t e n s i v e s o i l sampling and t e s t i n g program was c a r r i e d out at t h i s s i t e . The r e s u l t s were d e s c r i b e d i n S e c t i o n 2.9.2, and are summarized in F i g u r e s 2.14 and 2.16. While the g r a i n s i z e d i s t r i b u t i o n s of the t i l l and the c o l l u v i u m are s i m i l a r , 133 F i g u r e 4.14: Contour Map o f S i t e Mg3 D e b r i s S l i d e s j^t-mGS S l i d e Headscarp, S l i d e Number S l i d e Margins A R G o ' o St a n d p i p e " P i e z o m e t e r s " , Storage R a i n Gauge P l a t e 4 . 7 ; D e b r i s S l i d e s , S i t e Mg.3. ( a ) : S l i d e No. 8. Note abundant l o g d e b r i s i n channe l and above bank. the t i l l s tend to c o n t a i n somewhat l e s s sand and more c l a y , and are somewhat more p l a s t i c . The presence of seepage at s e v e r a l slope f a i l u r e s i n t h i s area suggests that groundwater p l a y s an important r o l e , f o r the reasons o u t l i n e d i n S e c t i o n 4.4.1.1. I t seems reasonable that a perched water t a b l e would e x i s t at the cont a c t between the r e l a t i v e l y impermeable b a s a l t i l l and the o v e r l y i n g m a t e r i a l . During wet p e r i o d s , t h i s water t a b l e would r i s e towards the s u r f a c e , i n c r e a s i n g the p o s s i b i l i t y of slope f a i l u r e s . O'Lough-l i n (1972b) emphasized the importance of t h i s n e a r - s u r f a c e t i l l l a y e r i n the occurrence of d e b r i s s l i d e s i n the Howe Sound, Ca p i l a n o R i v e r , and Indian River areas. He estimated that the shallow o v e r l y i n g s o i l s would become completely s a t u r a t e d i f the 24 hour r a i n f a l l t o t a l exceeded about 15 cm, which happens about once every seven years at the H o l l y b u r n Ridge s t a t i o n (see F i g u r e 2.5). A perched water t a b l e would e x i s t only i f there i s a marked c o n t r a s t between the h y d r a u l i c c o n d u c t i v i t i e s of the two s o i l l a y e r s . To t e s t t h i s , three estimates of the c o n d u c t i v i t y i n the upper l a y e r were made with a f i e l d permeameter (Appendix 2), -3 y i e l d i n g an average h y d r a u l i c c o n d u c t i v i t y of 1.4 x 10 cm/s, _ 3 with a standard d e v i a t i o n of 1.0 x 10 cm/s. I t was not p o s s i b l e to estimate the h y d r a u l i c c o n d u c t i v i t y of the very hard ba s a l t i l l with t h i s device because the auger co u l d not penetrate the l a y e r . T y p i c a l v a l u e s f o r g l a c i a l t i l l range between 10 4 cm/s and 10 1 0 cm/s (Freeze and Cherry, 1979, p. 29), so there i s a s i g n i f i c a n t c o n d u c t i v i t y c o n t r a s t at the s t r a t i g r a p h i c 136 c o n t a c t . O'Loughlin (1972a) measured the c o n d u c t i v i t y of these p a r t i c u l a r u n i t s i n the l a b o r a t o r y . While he d i d not i d e n t i f y the sampling l o c a t i o n s , h i s d e s c r i p t i o n s of the s o i l s leave l i t t l e doubt that the m a t e r i a l s sampled were very s i m i l a r to those at s i t e Mg3 and elsewhere i n the study a r e a . He repo r t e d an average h y d r a u l i c c o n d u c t i v i t y of 3.9 x 10 cm/s (standard _ 3 d e v i a t i o n = 2.4 x 10 cm/s) f o r e i g h t samples of the B ho r i z o n _ c s o i l s , and a value of 2 x 10 cm/s f o r one sample of unweath-ered, compacted b a s a l t i l l . C o n d u c t i v i t y values f o r the organic m a t e r i a l on the g e n t l y s l o p i n g area behind the stream bank were too high to measure with the f i e l d permeameter. Almost a l l of the r a i n f a l l i n g here must q u i c k l y p e r c o l a t e down to the u n d e r l y i n g s o i l l a y e r s , and thence seep out on the bank; there i s no evidence of s u r f a c e r u n o f f . Water t a b l e l e v e l s were monitored by three shallow standpipe "piezometers" i n s t a l l e d at d e b r i s s l i d e No. 8 (F i g u r e 4.14). These penetrate only to the top of the b a s a l t i l l l a y e r , at depths of 0.30, 0.49, and 0.42 metres. Since they are open along t h e i r e n t i r e l e n g t h , the standpipes measure the height of the perched water t a b l e , rather than p i e z o m e t r i c head. F i g u r e 4.15 shows water t a b l e p o s i t i o n s i n the three standpipes over three f i e l d seasons, along with the cumulative p r e c i p i t a t i o n recorded at two nearby s t a t i o n s (see F i g u r e 2.1 f o r l o c a t i o n s ) . F i g u r e 4.15 shows that water l e v e l s are r e l a t i v e l y high toward the end of the wet season, but then f a l l r a p i d l y through the dry summer months. In the l a t e summer and e a r l y f a l l the standpipes are o f t e n completely dry, except f o r some moist s o i l at t h e i r bases. 1 37 Water L e v e l Above S tandpipe Base (m) o o o o o o u\ vO CO > CO o \ o > L. i 1 \ i \ CO c+ P ft t i p -t i CD Q co tr1 K i-i c+ P - o O P O H Cl US US I—1 3 ft tn d P W o P CD ra CD S3 CD fttl p - bd C O t i P CD cf C 3 M CD P < P - M s: sa p i-S P -c+ CD ft CD O {Jt} t l f P -p CD c+ O P - P -< CD c+ O GO P -C+ P c+ P -O 3 l->- t\J V/\ O W\ O O O O O O . O o o Cumulat ive P r e c i p i t a t i o n S ince 1 Jan (mm) (V) V7\ o o Fig ure -p-CO S, c+ P P c+ c+ CD p -O Ul CD < CDm 1—1 CD CO 3 p - p -i 3 o CO e+ c+ P £f h-1 ft < - d p -Ci t i >"S CD CD m O P - P Ci c+ co c+ p -o cr c+ CD P h-1 ro TO While these o b s e r v a t i o n s are not d e t a i l e d enough to i n d i c a t e a r e l a t i o n s h i p between s o i l s a t u r a t i o n and i n d i v i d u a l p r e c i p i t a t i o n events, i t i s c l e a r that the perched water t a b l e r i s e s and f a l l s i n response to seasonal p r e c i p i t a t i o n . T h i s o b s e r v a t i o n , combined with the i n f e r e n c e s of O'Loughlin (1972b), suggests that d e b r i s s l i d e s here most l i k e l y occur i n l a t e f a l l , winter, or spr i n g . A f i n a l f a c t o r which may have c o n t r i b u t e d to the d e b r i s s l i d e s at t h i s s i t e i s c l e a r - c u t l o g g i n g i n the e a r l y and mid 1960's. I t i s w e l l known that l o g g i n g can i n c r e a s e the suscept-i b i l i t y of steep f o r e s t s l o p e s to l a n d s l i d i n g (eg. O'Loughlin, 1972a, 1972b; Swanston and Swanson, 1976), by d e c r e a s i n g the root s t r e n g t h of the s o i l over time, and by d i s r u p t i n g the h i l l s o p e hydrology by d e c r e a s i n g e v a p o t r a n s p i r a t i o n . O'Loughlin (1972a, 1972b) found that the frequency of l a n d s l i d e s was c o n s i d e r a b l y higher on c l e a r - c u t t e r r a i n than on unlogged t e r r a i n i n the Howe Sound area, but that most new l a n d s l i d e s were d i r e c t l y a t t r i b u t -able to l o g g i n g road c o n s t r u c t i o n . None of the e i g h t s l i d e s at S i t e Mg3 can be c o n s i d e r e d r o a d - r e l a t e d , but i t i s d i f f i c u l t to b e l i e v e that l o s s of root s t r e n g t h was not a major f a c t o r i n many of them. An a n a l y s i s of a i r p h o t o s i n d i c a t e s t h a t most of the d e b r i s s l i d e s were not present p r i o r to l o g g i n g . 4.4.2 S o i l Wedge F a i l u r e S o i l wedge f a i l u r e ( d e b r i s s l i d e s from c o l l u v i u m - f i l l e d hollows) i s among the most important d e b r i s supply processes to main stem channels in the study area, i n that l a r g e amounts of d e b r i s are d e l i v e r e d r a p i d l y to s p e c i f i c p o i n t s . I f l a r g e 1 39 amounts of water are i n v o l v e d , these f a i l u r e s may resemble d e b r i s t o r r e n t s , but g e n e r a l l y are much sm a l l e r , o f t e n o c c u r r i n g i n " z e r o - o r d e r " basins p r e v i o u s l y f i l l e d with c o l l u v i u m or g l a c i a l d r i f t . They have been noted i n both Magnesia and M Creek watersheds; the main s i t e s are shown i n F i g u r e 4.13. 4.4.2.1 Mechanisms S o i l wedge f a i l u r e s occur i n channels, or depressions i n bedrock or b a s a l t i l l , which have become f i l l e d by loose d e b r i s over long p e r i o d s of time. These b u r i e d channels, o f t e n c a l l e d " s o i l wedges" ( D i e t r i c h and Dunne, 1978) are q u i t e common i n the P a c i f i c Northwest and other mountainous r e g i o n s , such as Japan and New Zealand (Humphrey, 1982). Water may continue to flow through the d e b r i s , while i n some cases there may be no s u r f a c e e x p r e s s i o n of the b u r i e d channel at a l l (Humphrey, 1982). During a l a r g e runoff event, pore p r e s s u r e s may i n c r e a s e , i n i t i a t i n g a d e b r i s avalanche or d e b r i s t o r r e n t (Takahashi, 1981; Humphrey, 1982). The b u r i e d channel becomes scoured below the p o i n t of i n i t i a t i o n , and a l a r g e amount of rock, l o g s , and f i n e organic and i n o r g a n i c m a t e r i a l i s d e p o s i t e d i n t o the main stem channel. If the main channel at t h i s p o i n t i s steep enough and narrow enough, the i n i t i a l flow may t r i g g e r a major d e b r i s t o r r e n t . I t i s thought that the 1981 event i n M Creek s t a r t e d in t h i s way, as a r e l a t i v e l y small s o i l wedge f a i l u r e from S i t e M1. 4.4.2.2 Type L o c a t i o n - S i t e M1 T h i s s i t e has been introduced e a r l i e r , i n S e c t i o n 4.3.1.3. A V-shaped, f a u l t - c o n t r o l l e d , bedrock chute descends about 160 m, at a slope of 30°, to a p o i n t i n M Creek some 50 m upstream of 140 S i t e M2, the l a r g e r o c k s l i d e ( F i g u r e s 4.2, 4.13). The head of the g u l l y i s f i l l e d with a b l a t i o n t i l l and/or c o l l u v i u m . C o n s i d e r a b l e seepage i s evident at the c o n t a c t between the rock and s o i l l a y e r s . Since the g u l l y and the a s s o c i a t e d f a u l t can be t r a c e d as a small s u r f a c e d e p r e s s i o n f o r some d i s t a n c e upslope, i t seems l i k e l y that subsurface flow i s c o n c e n t r a t e d here (Figure 4.16). Thus, the s i t e seems to be a good example of the s o i l wedge f a i l u r e s d e s c r i b e d by Humphrey (1982) and noted above. Apparently the m a t e r i a l f i l l i n g the lower p a r t of t h i s g u l l y f a i l e d i n the e a r l y hours of 28 October 1981, t r i g g e r i n g the M Creek d e b r i s t o r r e n t (Church and Desloges, 1984). No estimate has been made of the volume of m a t e r i a l i n v o l v e d , but i t was 4 3 probably o n l y a small p o r t i o n of the estimated 2 x 10 m of d e b r i s d e p o s i t e d on M Creek fan. by the t o r r e n t . Since the channel of M Creek i s steep (26°) and narrow here, the d e b r i s c o u l d e a s i l y have continued downstream from the toe of S i t e M1, e n t r a i n i n g a d d i t i o n a l m a t e r i a l as i t went, becoming a f u l l - s c a l e d e b r i s t o r r e n t . No s t r e n g t h or p l a s t i c i t y t e s t s were performed on the s o i l at the head of the g u l l y , but the g r a i n s i z e d i s t r i b u t i o n s of four samples of t h i s m a t e r i a l are not much d i f f e r e n t from those determined f o r the c o l l u v i a l m a t e r i a l s at S i t e s Mg1 and Mg3, although they tend to be somewhat c o a r s e r (Figure 2.19). The two samples with over 80% sand and very l i t t l e c l a y may a c t u a l l y be products of i n s i t u weathering of the a l t e r e d rock near the c o n t a c t between the v o l c a n i c s and the p l u t o n i c r o c k s . There are not enough data on the geometry of the s i t e , or the groundwater 141 F i g u r e 4.16: Schematic Drawing o f Water C i r c u l a t i o n Near Top o f G u l l y , S i t e Ml G u l l y c o n t i n u e s downslope to M Creek ( s k e t c h p a r t l y by P. Buchanan) 142 c o n d i t i o n s , to allow meaningful slope s t a b i l i t y a n a l y s i s , but the q u a l i t a t i v e o b s e r v a t i o n s above suggest that t h i s was a s o i l wedge f a i l u r e s i m i l a r to those analysed by Humphrey (1982). 4.4.2.3 Type L o c a t i o n - S i t e Mq1 S i t e Mg1 i s a small d e b r i s t o r r e n t or s o i l wedge f a i l u r e which e n t e r s the l e f t bank of Magnesia Creek near the toe of the l a r g e d e b r i s l e v e e , at 693 m (Figure 4.13). The wedge i s 205 m long, averages 7.5 m i n width, and has an average gr a d i e n t of 28.5° ( F i g u r e 4.17). I t o r i g i n a t e s i n a small bowl near 800 m, where c o n s i d e r a b l e seepage occurs throughout much of the year (Figure 4.18). In the d r i e s t p a r t s of the year, seepage sources i n the bowl dry up, but water continues to flow from s e v e r a l p o i n t s on the l i p of the bowl, at 797 m. The groundwater which seeps out here almost c e r t a i n l y comes from a g u l l y which descends the northwest f l a n k of Mt. Harvey and disappears near 920 m, a short d i s t a n c e above the abandoned lo g g i n g road ( F i g u r e 4.13). T h i s s i t e looked very f r e s h when f i r s t seen by the author i n June 1984. A l a r g e accumulation of logs and f i n e d e b r i s was present i n the channel of Magnesia Creek at the toe of the d e b r i s t r a c k ( P l a t e 4.8). The d e b r i s d e p o s i t looked s i m i l a r to those observed by the author s h o r t l y a f t e r the J u l y 1983 d e b r i s t o r r e n t s near Hope and Revelstoke (Evans and L i s t e r , 1984). D e b r i s was found on top of a 1.5 m diameter l o g on the r i g h t side of the d e b r i s t r a c k , some 5 m above the channel. A sample of t h i s m a t e r i a l , and two samples from the fan, have g r a i n s i z e d i s t r i b u t i o n s s i m i l a r to s o i l samples from the headscarp area ( S e c t i o n 2.9.2), but have very high organic c o n t e n t s . These 143 F i g u r e 4. 17: Contour Map o f S i t e Mgl cont inued at lower l e f t F i g u r e 4 .18 : Upper P a r t o f S i t e Mgl LEGEND Seepage P o i n t s I n t e r m i t t e n t Streams Standpipe "P iezometers Main Survey S t a t i o n s Other Surveyed _ G P o i n t s S l i d e Margins rrrrr^ Headscarp P o s i t i o n i n 1985 nr,^ Headscarp P o s i t i o n i n ^ 1984 (where d i f f e r e n t from 1985) Headscarp P o s i t i o n i n \ 1986 (where d i f f e r e n t from 1985) • s c a l e - metres contour i n t e r v a l : 0.5 u Inset Map A b o v e 145 P l a t e 4.8: D e b r i s i n Magnesia Creek at Toe of S o i l Wedge,  S i t e Mql three samples c o n t a i n 36%, 31%, and 41% o r g a n i c m a t e r i a l by weight, while no other samples at t h i s s i t e have more than 10.5%, and the average i s l e s s than 5%. These h i g h v a l u e s may be p a r t l y due to the f a c t that wood f l o a t s , and so tends to be prominent i n l a t e r a l d e b r i s t o r r e n t d e p o s i t s , but two of these samples were c o l l e c t e d approximately i n the c e n t r e of the d e b r i s fan. A l s o , high organic contents are t y p i c a l of d e b r i s t o r r e n t d e p o s i t s i n t h i s r e g ion (Church and Desloges, 1984; VanDine, 1985a). By the f o l l o w i n g s p r i n g , much of the f i n e m a t e r i a l had been washed away, and the d e p o s i t s looked l e s s f r e s h . Thurber Consultants (1983) make no mention of t h i s s i t e , and i t i s not v i s i b l e on the 1982 a i r p h o t o s . Thus, while a small t r i b u t a r y has 1 46 probably e x i s t e d here f o r some time, at l e a s t below 760 m (Figure 4.17), i t i s apparent that a recent d e b r i s f a i l u r e resembling a small t o r r e n t caused c o n s i d e r a b l e s c o u r i n g of t h i s channel, probably i n the winter of 1983 - 1984. U n l i k e M1 , the t o r r e n t t r a c k at Mg1 i s developed almost e n t i r e l y i n s u r f i c i a l m a t e r i a l s . The only exposure of bedrock i s near the toe of the channel, at 710 m, where an outcrop of Gambier Group v o l c a n i c rock e x i s t s . The channel appears to l i e on the upper s u r f a c e of the b a s a l t i l l l a y e r , although t h i s m a t e r i a l i s r a r e l y exposed, and i t s top few c e n t i m e t r e s have been weathered by running water. The s i d e s of the g u l l y c o n s i s t of loose and o f t e n s a t u r a t e d c o l l u v i u m , which continues to slough i n t o the stream. An e x t e n s i v e s o i l sampling and t e s t i n g programme was c a r r i e d out at t h i s s i t e . D e t a i l s are given i n S e c t i o n 2.9.2, and some of the r e s u l t s are summarized i n F i g u r e s 2.17 and 2.18. As at S i t e Mg3, there i s some o v e r l a p between the g r a i n s i z e d i s t r i b u -t i o n s of the two main m a t e r i a l types, but the b a s a l t i l l tends to c o n t a i n l e s s sand and more c l a y . The c o n s i d e r a b l e amount of groundwater seepage at t h i s s i t e has been noted above. The main seepage p o i n t s are i n d i c a t e d on F i g u r e 4.18. F i v e shallow standpipe "piezometers", s i m i l a r to the ones at s i t e Mg3, were i n s t a l l e d here i n J u l y 1984 (see F i g u r e 4.18 f o r l o c a t i o n s ) , to monitor groundwater l e v e l s . The standpipes penetrate to the c o l l u v i u m / b a s a l t i l l c o n t a c t , at depths of 0.35 m, 0.21 m, 0.52 m, 0.44 m, and 0.32 m. F i g u r e 4.19 shows water t a b l e p o s i t i o n s i n the standpipes over three 1 47 Water L e v e l Above S tandpipe Base (m) Q oo o pj 3 fx & 'ri ra >d > i -1 tr1 K H- O O H ra <•<: (vj ^ 3 CD >-+> S ^) p P ^ H-o c+ CD CD CD O 0q H- CD *~i CD r - 1 H - "rl I—1 CD c+ ^ P < P CD c+ CD <rr O H H- H-< m o W ro 3 H-<H-H- P o c+ H-m o c+ P t i CD a1 P ra CD 00 c+ P c+ H-O •3 ra ra CD 3 O I—1 <; CD O c+ O c+ P I—1 Cfl Vyv O o o o o o o Cumula t ive P r e c i p i t a t i o n S ince 1 Jan. o o o (mm) f i e l d seasons, along with the cumulative p r e c i p i t a t i o n recorded at two nearby s t a t i o n s (see F i g u r e 2.1 f o r l o c a t i o n s ) . Standpipe B was destroyed i n the autumn of 1984 by a small f a i l u r e on the headscarp of the upper bowl, and by the f o l l o w i n g May the top of Standpipe A had been d e f l e c t e d downslope by loose m a t e r i a l on the s u r f a c e . Standpipe A was a c t u a l l y f l o w i n g the f i r s t times i t was observed i n May 1985 and May 1986, but was dry the r e s t of the s p r i n g and summer i n both y e a r s . Standpipe C was destroyed by slope movement i n the winter of 1985-1986. As at S i t e Mg3, water t a b l e l e v e l s i n the standpipes dropped r a p i d l y from l a t e s p r i n g each year, and were o f t e n c l o s e to zero i n l a t e summer and e a r l y autumn (F i g u r e 4 . 1 9 ) . Water l e v e l s at t h i s s i t e c l e a r l y f l u c t u -ate i n response to seasonal p r e c i p i t a t i o n t r e n d s . As at s i t e Mg3, i t i s reasonable to expect a s i g n i f i c a n t h y d r a u l i c c o n d u c t i v i t y c o n t r a s t between the bas a l t i l l and the o v e r l y i n g c o l l u v i u m at s i t e Mg1. To t e s t t h i s , s i x f i e l d permeameter t e s t s were made i n the c o l l u v i u m (Appendix 2). A mean c o n d u c t i v i t y of 1.1 x 10 cm/s was found f o r t h i s m a t e r i a l , _ 3 with a standard d e v i a t i o n of 1.2 x 10 cm/s. T h i s r e s u l t i s s i m i l a r to that obtained at s i t e Mg3, and a l s o to that determined by O'Loughlin (1972a) (see S e c t i o n 4.4.1.2). Since t y p i c a l values f o r b a s a l t i l l range between 10 4 cm/s and 10 1 ^ cm/s (Freeze and Cherry, 1979, p. 29), there i s a s i g n i f i c a n t conduct-i v i t y c o n t r a s t here. The e x i s t e n c e of the h y d r a u l i c c o n d u c t i v i t y c o n t r a s t , the year-round seepage around the upper bowl, the r i s e of the perched water t a b l e in the wet season, and the loose c o l l u v i u m 1 49 o v e r l y i n g the hard b a s a l t i l l , a l l suggest that t h i s s i t e i s an example of the s o i l wedge f a i l u r e s d e s c r i b e d by Humphrey (1982). Humphrey's (1982) F i g u r e 1.1, reproduced here as F i g u r e 4.20, i l l u s t r a t e s t h i s s i t u a t i o n s c h e m a t i c a l l y , except that i n the present case the b a s a l l a y e r i s t i l l r a t h e r than bedrock. Apparently, a pore pressure r i s e i n the s o i l wedge t r i g g e r e d a r o t a t i o n a l slump near 800 m, and t h i s d e s t a b i l i z e d the e n t i r e wedge below. The steep headwall of the upper bowl i s a c t i v e l y r e c e d i n g , as i n d i c a t e d by the d e s t r u c t i o n of standpipes A, B, and C ( F i g u r e 4.18) over a two year p e r i o d . F i g u r e 4.18 a l s o shows the p o s i t i o n of the headscarp i n the summers of 1984, 1985, and 1986, as determined by d e t a i l e d t h e o d o l i t e and tape surveys. I t i s c l e a r from t h i s evidence that the m a t e r i a l i n v o l v e d i n the main f a i l u r e i s unstable on s l o p e s oversteepened by toe e r o s i o n . 4.4.3 R a v e l l i n g From C o l l u v i a l Slopes As noted above ( S e c t i o n 4.3.1), r a v e l l i n g tends to be more continuous and l e s s c a t a s t r o p h i c than most other slope processes in the study area. T h i s s e c t i o n d e s c r i b e s r a v e l l i n g from oversteepened c o l l u v i a l s l o p e s , l o g g i n g road f i l l s , and s i m i l a r l o c a t i o n s . The main s i t e s are shown i n F i g u r e 4.21. 4.4.3.1 Mechanisms As s t a t e d i n S e c t i o n 4.3.1, r a v e l l i n g e n t a i l s the gradual d i s i n t e g r a t i o n of s l o p e s , mainly by mechanical weathering p r o c e s s e s , and the subsequent downslope t r a n s p o r t of r e l e a s e d m a t e r i a l . T h i s t r a n s p o r t can be i n the form of r o l l i n g , s l i d i n g , or entrainment by running water. In the case of c o l l u v i a l s l o p e s , t h i s i s most conspicuous where the slope has been 150 F i g u r e 4-. 20: Schematic Drawing o f the Scar Caused by a S o i l Wedge D e b r i s F a i l u r e ( a f t e r Humphrey, 1982) F i g u r e 4 . 2 1 : C o l l u v i a l R a v e l l i n g -Sites C o l l u v i a l S lopes S u s c e p t i b l e to R a v e l l i n g A1 S p e c i f i c S i t e s Referenced i n the Text V Creek, Conscline D Deeks Creek L Loggers Creek M M Creek Mg Magnesia Creek A Alberta Creek H Harvey Creek H * Mount Harvey (elev. 1625 m) B * Brunswick Men. (elev. 1760 m) Watershed Bdy. Squamish Hwy. Other Roads Logging Roads B. C. Railway Power Trans-mission Line H O W E S O U N D scale - mc c res contour interval; 100m oversteepened, u s u a l l y by stream a c t i o n , or by log g i n g road c o n s t r u c t i o n . Where they are not undercut, most c o l l u v i a l s lopes are w e l l vegetated and appear f a i r l y s t a b l e . Such s l o p e s , i f steeper than about 40°, probably supply d e b r i s to streams g r a d u a l l y through inconspicuous processes such as s o i l creep, minor slumping, uprooting of t r e e s , e t c . (O. Hungr, pers. comm., 1987). T h i s w i l l be d i s c u s s e d f u r t h e r i n S e c t i o n 4.6. As i n the case of rock r a v e l l i n g , c o l l u v i a l r a v e l l i n g may not d i r e c t l y supply d e b r i s to nearby stream channels, but i n some cases the r e l e a s e d m a t e r i a l may simply accumulate on lo g g i n g roads or other r e l a t i v e l y f l a t a reas. Since r a v e l l i n g i n v o l v e s the r e l e a s e of i n d i v i d u a l p a r t i c l e s or very small amounts of m a t e r i a l from the s l o p e , the momentum of t h i s d e b r i s i s much lower than i n some other slope processes such as r o c k f a l l or d e b r i s s l i d e s . I f c o l l u v i a l r a v e l l i n g i s to d e l i v e r d e b r i s to a stream channel the r a v e l l i n g slope must be c l o s e to the creek, and the i n t e r v e n i n g slope must be steep enough to t r a n s -port the m a t e r i a l . In some cases, steep chutes extend from the toe of the a c t i v e slope down to nearby stream channels. In some cases, the p a r t i c l e s r e l e a s e d from the slope are so sm a l l , and the process i s so slow, that i t cannot e a s i l y be observed i n the f i e l d . However, i t sometimes can be det e c t e d through repeated o b s e r v a t i o n s of an e r o s i o n p i n network. While r a v e l l i n g may supply s i g n i f i c a n t amounts of d e b r i s to stream channels at a few l o c a t i o n s over a p e r i o d of s e v e r a l years, the process w i l l not be important from a d e b r i s t o r r e n t p o i n t of view i f most of the m a t e r i a l i n v o l v e d i s f i n e enough to be r e a d i l y 1 53 t r a n s p o r t e d by the creeks without accumulating as bedload. However, even r a v e l l i n g of very f i n e m a t e r i a l may g r a d u a l l y undermine i n d i v i d u a l c o b b l e s , boulders, and logs i n a c o l l u v i a l s l o p e , and cause t h i s c o a r s e r d e b r i s to r o l l and accumulate at the toe of the s l o p e . 4.4.3.2 Type L o c a t i o n - M Creek C r o s s i n g An abandoned l o g g i n g road c r o s s e s M Creek at the 690 m e l e v a t i o n ( F i g u r e 4.21). On the l e f t s i d e of the creek, the road cut has exposed a steep (40-45°) c o l l u v i a l face some 15 m long and 6-8 m high ( P l a t e 4.9). Fine m a t e r i a l and the o c c a s i o n a l cobble or boulder are loosened from t h i s face, mainly through f r o s t a c t i o n or seepage i n the sl o p e , and tumble down onto the road, where a d e b r i s apron has developed ( P l a t e 4.9a). The road p r o v i d e s access to a small rock quarry between M and Loggers Creeks, and was s t i l l d r i v e a b l e by four wheel d r i v e v e h i c l e i n the summer of 1985. A photo taken on 2 March 1986 ( P l a t e 4.9b) g i v e s an i n d i c a t i o n of the amount of d e b r i s accumulated on the road i n j u s t a few months. S e v e r a l obvious d e b r i s t r a c k s descend s t e e p l y from the road to the stream channel, suggesting that t h i s slope can supply d e b r i s d i r e c t l y to M Creek. The slope f a i l u r e s here normally i n v o l v e i n d i v i d u a l rocks and very small amounts of 3 s o i l , probably l e s s than 0.5 m at a time. A s i n g l e 30 cm diameter boulder was observed tumbling down the slope and onto the road i n January 1984, when the temperature was probably only s l i g h t l y above f r e e z i n g ; i t seems l i k e l y that f r o s t a c t i o n was r e s p o n s i b l e f o r l o o s e n i n g t h i s rock from the s l o p e . 3 A l a r g e r f a i l u r e , c o n s i s t i n g of 15-20 m of rock, s o i l , 154 P l a t e 4.9: Colluvium Exposed i n Road Cut at M Creek E l e v a t i o n 690 m~ ' • Cbli 2 March 1986. Note f r e s h debris on road. and small t r e e s , o c c u r r e d here i n the winter of 1 985 - 1986. The o b s e r v a t i o n s and weather data presented below i n d i c a t e that t h i s f a i l u r e probably o c c u r r e d on 23 or 24 February 1986 ( P l a t e 4.9b). When the s i t e was v i s i t e d on 2 March, the d e b r i s on the road looked very f r e s h , as mud had not yet been washed o f f the l a r g e r rocks by r a i n . There was evidence of continued slope e r o s i o n , i n the form of m i n i a t u r e d e b r i s flows, f o l l o w i n g the i n i t i a l f a i l u r e . Apparently, some of the m a t e r i a l i n v o l v e d in t h i s f a i l u r e was able to c r o s s the road and c o n t i n u e down the steep slope to the creek. T h i s event was l i k e l y caused by high pore water pre s s u r e s i n the slope, due to s a t u r a t i o n from heavy r a i n and snowmelt. Table 4.3 shows that the weather c o n d i t i o n s i n the region were i d e a l f o r g e n e r a t i n g such a s i t u a t i o n on the dates noted above. The nearest A. E. S. c l i m a t e s t a t i o n , H o l l y b u r n Ridge (see F i g u r e 2.1 f o r l o c a t i o n ) , had r e c e i v e d 85.2 cm of snow in the p r e v i o u s two weeks, with temperatures r a r e l y exceeding f r e e z i n g , while the temperature a t two sea l e v e l s t a t i o n s (Squamish and Port Mellon) d i d not exceed 2.5 ° C a f t e r 14 February. While the M Creek l o g g i n g road c r o s s i n g ( e l e v a t i o n 690 m) i s somewhat lower than H o l l y b u r n Ridge (951 m), and some snowmelt would have been p o s s i b l e , there would c e r t a i n l y have been a c o n s i d e r a b l e amount of snow there on 22 February. I t was noted i n S e c t i o n 2.4 that heavy winter r a i n s are f r e q u e n t l y accompanied by r a p i d warming i n t h i s r e g i o n . Such a storm o c c u r r e d on 23 February, as H o l l y b u r n Ridge recorded 94.4 mm of r a i n , and an average temperature i n c r e a s e of 4.5 ° C over the p r e v i o u s day (Table 4.3). T h i s heavy r a i n (estimated to have a 1 56 Table 4 . 3: P r e c i p i t a t i o n and Temperature at Hollyburn Ridge , 10-28 February 1986 Date P r e c i p i t a t i o n Rain Snow (mm) T o t a l a A i r Max. Temp. Min. (°C) Mean Max. D a i l y Port M e l l o n 0 Temp. (°C) Squamish d 10 10 1.0 2.0 - 6 .0 -2 .0 8 .0 9-0 11 -1 .0 - 4 . 5 -2 .8 6 •5 10 .0 12 0.5 - 9 - 5 -4 •5 9 .0 8.5 13 0.0 - 7 - 5 - 3 .8 8 .0 7.0 14 280 28.0 3-5 -10 .0 -3 •3 6 .0 5-5 15 216 21.6 -1 .0 - 6 .0 - 3 •5 0 • 5 2.0 16 56 5-6 -5 .0 - 9 - 5 -7 •3 1 • 5 2.0 17 16 1.6 -7 .0 -12 .0 - 9 •5 2 .0 1.0 18 -7 .0 -13 .0 -10 . 0 1 • 5 0 . 0 19 Trace Trace - 2 . 5 -18.0 -10 .3 1 .0 - 1 . 0 20 10 1.0 - 2 . 5 -10 .5 -6 • 5 1 • 5 2.0 21 130 13.0 -2 .0 -13 .5 -7 .8 2 • 5 1-5 22 134 13.4 1.0 - 6 . 5 -2 .8 2 .0 2.0 23 94.4 20 96.4 5-5 - 2 . 0 1 .8 4 • 5 2-5 24 37-0 37-0 6.0 0.0 3 .0 9 .0 5.0 25 6.0 2.0 4 .0 14 .0 9-0 26 9-5 - 1 .0 4 •3 12 . 0 12.5 27 1.2 1.2 10.5 0.5 5 •5 13 . 0 15.0 28 8.5 1.0 4 .8 13 .0 15.0 Notes: a - 10 mm of snow assumed equivalent to 1.0 mm of r a i n b - e l e v a t i o n 951 m c - e l e v a t i o n 8 m d - e l e v a t i o n 31 m recurrence i n t e r v a l of 1.6 y e a r s : see F i g u r e 2.5), combined with a l a r g e amount of snowmelt, would probably have been enough to s a t u r a t e the slope and induce f a i l u r e . The above i s p u r e l y s p e c u l a t i v e , but the c i r c u m s t a n t i a l evidence i n support of t h i s argument seems q u i t e s t r o n g . Small f a i l u r e s on oversteepened slopes probably are f a i r l y common throughout the study area i n winter and s p r i n g , when the s l o p e s are s a t u r a t e d . Debris from a small r o c k f a l l s i m i l a r to the c o l l u v i a l f a i l u r e d e s c r i b e d above was observed near the l o g g i n g road c r o s s i n g of A l b e r t a Creek on 2 March 1986. T h i s f a i l u r e probably o c c u r r e d d u r i n g the same storm. 4.4.3.3. Type L o c a t i o n - S i t e A1 S i t e A1 i s an open slope below a l o g g i n g road cut on the north s i d e of A l b e r t a Creek, near the 660 m e l e v a t i o n (Figure 4.21). Church and Desloges (1984) note that c o n d i t i o n s here have been aggravated by road c o n s t r u c t i o n and l o g s k i d d i n g i n the l a t e 1950's or e a r l y 1960!s, although p a r t s of the slope were a c t i v e 3 p r i o r to that time. They estimate that as much as 10,000 m of 3 m a t e r i a l , i n c l u d i n g at l e a s t 1000 m of coarse d e b r i s , c o u l d have been d e l i v e r e d to the creek from t h i s slope i n the p e r i o d 1966 - 1982. T h i s assumes an average s u r f a c e lowering of 0.5 m 2 (3.3 cm/year), over a 22,000 m area. T h i s i s merely an order-of-magnitude estimate, as e r o s i o n tends to be c o n c e n t r a t e d along drainage l i n e s , but i t does suggest that a s u b s t a n t i a l p o r t i o n of the m a t e r i a l i n v o l v e d i n the February 1983 d e b r i s t o r r e n t may have o r i g i n a t e d at t h i s s i t e (Church and Desloges, 1984). The g r a i n s i z e d i s t r i b u t i o n s of two samples of the s o i l on 1 58 t h i s slope are s i m i l a r to the c o l l u v i u m at s i t e s Mg1 and Mg3, with g r a v e l dominating the t o t a l sample, and sand dominating the sub-2mm sample ( F i g u r e 2.20). A t t e r b e r g l i m i t t e s t s were not performed f o r t h i s m a t e r i a l , but the p l a s t i c i t y i s undoubtably very low. T h i s s o i l probably has very s i m i l a r p r o p e r t i e s to the c o l l u v i u m i n Magnesia Creek b a s i n . In an e f f o r t to assess the amount of m a t e r i a l being removed from a small p a r t of t h i s slope, an e r o s i o n p i n g r i d was e s t a b -l i s h e d here i n l a t e August 1984, at a p o i n t where the h i l l s l o p e g r a d i e n t i s about 45° ( P l a t e 4.10). The p i n s c o n s i s t e d of e i g h t - i n c h s e c t i o n s of h a l f - i n c h diameter s t e e l r e i n f o r c i n g rod, p a i n t e d i n 1 cm wide bands. F i f t e e n p i n s were d r i v e n v e r t i c a l l y i n t o the ground, on a 4 m by 4 m g r i d ( F i g u r e 4.22), u n t i l only P l a t e 4.10: S i t e of E r o s i o n Pin G r i d , S i t e A l . G r i d i s  between f a l l e n t r e e i n foreground and l a r g e  Douglas f i r i n background. Note l a r g e angular  rocks i n slope behind t r e e . I 159 Table 4.4: Erosion P i n Grid R e s u l t s , S i t e A l P i n Number Erosion (E) or Deposition (D) , (c: 28 May 1985 a 30 J u l y 1986 1 E 1.0 E: 1 5 2 0.0 E: 1 5 3 E 0.5 E: 1 0 4 E 0.2 E: 2 5 c E: 1 o a 6 D 2.0 d 7 D 3.0 d 8 E 0.2 d 9 D 2.0 E: 2 0 10 c E: 0 5 a 11 D 9-0 d 12 D 6.5 d 13 D 6.0 d 14 D 1.0 d 15 c E: 3 • 5 a Notes: a: E r o s i o n / d e p o s i t i o n since 29 August 1984. b: E r o s i o n / d e p o s i t i o n since 28 May 1985-c: P i n not l o c a t e d i n 1985-d: P i n not l o c a t e d i n 1986. Abundant loose s o i l accumulated i n t h i s corner of the g r i d . Figure 4. 22: Erosion P i n Gr i d l a y o u t , S i t e A l V 10 E W 1 5 o o 8 14 13 o o 4m-O a o a 3 12 11 o 160 the top centimetre was exposed. The p o s i t i o n of the ground s u r f a c e r e l a t i v e to the top of each p i n was recorded i n May 1985 and J u l y 1986, to assess the amount of e r o s i o n (Table 4.4). Apparently the f a l l e n t r e e , v i s i b l e i n the foreground i n P l a t e 4.10, promoted accumulation i n the lower r i g h t h a l f of the g r i d of m a t e r i a l eroded from upslope areas, while e r o s i o n occurred i n the upper l e f t h a l f of the g r i d . I t i s evident that m a t e r i a l i s a c t i v e l y moving down t h i s s l o p e , and the m a t e r i a l p r e s e n t l y s t o r e d behind the t r e e c e r t a i n l y would have been d e l i v e r e d to the creek i f not f o r t h i s o b s t a c l e . While the data on Table 4.4 are somewhat v a r i a b l e , they suggest that the estimate of slope e r o s i o n of 3.3 cm/year proposed by Church and Desloges (1984) i s q u i t e reasonable. As noted in S e c t i o n 4.4.3.1, most of the m a t e r i a l removed from t h i s c o l l u v i a l slope by s u r f a c e e r o s i o n would be too f i n e - g r a i n e d to be of importance i n d e b r i s t o r r e n t g e n e r a t i o n , s i n c e normal stream flow would prevent i t s accumul-a t i o n ; however, l a r g e r p a r t i c l e s are d e l i v e r e d o c c a s i o n a l l y , due to undermining of coarse d e b r i s i n the slope ( P l a t e 4.10). As noted above, Church and Desloges (1984) f e e l t h a t at l e a s t 10% of the m a t e r i a l d e l i v e r e d from t h i s slope s i n c e 1966 would have been coarse enough to accumulate i n the creek p r i o r to the 1983 d e b r i s t o r r e n t . 4.5 OTHER DEBRIS SOURCES 4.5.1 Wet Snow Avalanches Wet snow avalanches are common on the southern fl a n k of Brunswick Mountain, and in steep chutes or stream channels i n 161 the upper p a r t s of a l l of the basins (Figure 4.23) . While i n some cases avalanches may be important s u p p l i e r s of coarse d e b r i s to t o r r e n t - p r o n e channels, t h e i r importance i s l i m i t e d by two f a c t o r s . F i r s t , most avalanche t r a c k s occur i n Magnesia Creek t r i b u t a r y L3 i n a r e l a t i v e l y low-gradient s e c t i o n above the most probable t o r r e n t i n i t i a t i o n p o i n t ( t h i s t r i b u t a r y has an average o g r a d i e n t of only 17 between the 1000 m and 1300 m e l e v a t i o n s , F i g u r e s 4.23, 2.22). The second l i m i t a t i o n concerns the a b i l i t y of avalanches i n t h i s r e g i o n to d e l i v e r much m a t e r i a l other than snow to the creek channels. T h i s i s d i s c u s s e d i n the f o l l o w i n g s e c t i o n . 4.5.1.1 Mechanisms There i s very l i t t l e p u b l i s h e d i n f o r m a t i o n on the geomorphic e f f e c t s of snow avalanches i n the Coast Mountains, but Gardner (1970, 1983) and Luckman (1978) have made some important obser-v a t i o n s i n the Rocky Mountains. These papers d e a l p r i m a r i l y with e f f e c t s above the t r e e l i n e ; but there i s a l s o some suggestion t h a t wet snow avalanches may a i d d e b r i s t r a n s p o r t on t a l u s slopes or i n c o n f i n e d chutes i n f o r e s t e d areas. In the study area, there appear to be three p o s s i b l e d e b r i s t r a n s p o r t mechan-isms: (i) scour at the base of an avalanche moving over a bare ground s u r f a c e ; ( i i ) t r a n s p o r t of d e b r i s p r e v i o u s l y i n c o r p o r a t e d i n t o the snowpack; and ( i i i ) scour at the edges of an avalanche t r a c k i n c i s e d i n t o u n c o n s o l i d a t e d s u r f i c i a l m a t e r i a l s . The e f f e c t s of avalanches moving over bare ground s u r f a c e s have been d e s c r i b e d i n a l l three of the above papers. Gardner (1970) noted t h a t s p r i n g avalanches are more e f f e c t i v e e r o s i v e 162 ON F i g u r e 4 .23 : Snow Avalanche S i t e s i:::::!) Open s lope avalanche t r a c k s G u l l i e s p o t e n t i a l l y , . s u s c e p t i b l e to snow avalanche a c t i v i t y Mg2 S p e c i f i c s i t e s r e f e r e n c e d i n the t e x t Creek. \s Coast 1i ne D Decks Creek L L o i ; > ; o r s Creek M M Creek Mg Magnesia Creek A Alberta Creek H Harvey Creek H * Mount Harvey (elev. 1625 B a Brunswick Mtn. (elev. 1760 Vaco r shed Bdy. Squarish Mvy, Othc f Roads Logging Roads B. C. Railway Power Trans-miss: on Line H O W E S O U N D scale - rr.*.: t r e s con ton r interval: 100m agents than winter avalanches, because they are apt to i n v o l v e h i g h d e n s i t y snow, moving a c r o s s thawed ground r a t h e r than an o l d snow s u r f a c e . Sometimes a veneer of snow d e p o s i t e d by the avalanche i t s e l f w i l l p r o t e c t the ground s u r f a c e from e r o s i o n (Gardner, 1983), but i n the absence of t h i s , b l o c k s as l a r g e as 1 m can be e n t r a i n e d and c a r r i e d (Gardner, 1970). Wet snow avalanches i s s u i n g from bedrock gorges ( c o u l o i r s ) and moving a c r o s s snow-free loose rock d e b r i s are p a r t i c u l a r l y e f f e c t i v e d e b r i s t r a n s p o r t mechanisms (Gardner, 1970). Small t r e e s may o c c a s i o n a l l y be uprooted and moved downslope, along with i n o r -ganic m a t e r i a l trapped i n the t r e e r o o t s , by avalanches with u n u s u a l l y long runout d i s t a n c e s . Avalanches w i l l r e a d i l y t r a n s p o r t d e b r i s which has been p r e v i o u s l y i n c o r p o r a t e d i n t o the snowpack by processes such as r o c k f a l l and slumping from c o l l u v i a l s l o p e s . Since these processes o f t e n are a s s o c i a t e d with freeze-thaw a c t i v i t y i n winter and s p r i n g , there c o u l d be abundant m a t e r i a l a l r e a d y on or in the snow when an avalanche begins. T h i s type of d e b r i s t r a n s p o r t can o b v i o u s l y be accomplished by snow-on-snow a v a l -anches, which are probably more common than those moving over bare, e r o d i b l e ground. Even in the absence of snow avalanches, the presence of a hard snow l a y e r i n an area of r o c k f a l l w i l l s h i f t the r o c k f a l l d e p o s i t i o n area downslope, due to simple s l i d i n g on the snow s u r f a c e (Gardner, 1970). A t h i r d p o s s i b l e d e b r i s t r a n s p o r t mechanism i s scour at the edges of snow-on-snow avalanches i n c o l l u v i a l . g u l l i e s . There are few p u b l i s h e d accounts of t h i s , but i t seems p o s s i b l e that 164 r a p i d l y moving snow c o u l d erode the bases of steep c o l l u v i a l s l o p e s and t r a n s p o r t the loosened m a t e r i a l downslope to the a b l a t i o n zone. Potter (1969) noted damage to t r e e s caused by snow-on-snow avalanches i n Wyoming. Very few of the avalanche s i t e s i n d i c a t e d on F i g u r e 4.23 appear to be important from a d e b r i s supply p o i n t of view. The open-slope avalanche t r a c k s on the south f l a n k of Brunswick Mountain are w e l l - v e g e t a t e d , so avalanches here would be unable to i n c o r p o r a t e much coarse d e b r i s . In any event, the stream g r a d i e n t here probably i s too low to generate d e b r i s t o r r e n t s , as noted i n the previous s e c t i o n . The steep bedrock gorges i n A l b e r t a and M Creeks c e r t a i n l y experience avalanches, but lack e n t r a i n a b l e d e b r i s which c o u l d be d e l i v e r e d t o lower p a r t s of the c r e e k s . However, avalanches here may be very important in i n i t i a t i n g d e b r i s t o r r e n t s or r e d i s t r i b u t i n g d e b r i s i n the mid-reaches of the creeks," as w i l l be d i s c u s s e d in S e c t i o n s 5.3 and 6.2.5. The lower h a l f of Magnesia Creek T r i b u t a r y L2, c a l l e d S i t e Mg2 here, i s the main l o c a t i o n i n the study area where snow avalanching c o u l d be an important d e b r i s supply mechanism. 4.5.1.2 Type L o c a t i o n - S i t e Mg2 S i t e Mg2 i s the lower h a l f of Magnesia Creek T r i b u t a r y L2, where the channel i s deeply i n c i s e d i n u n c o n s o l i d a t e d c o l l u v i a l m a t e r i a l s ( P l a t e 4.11a). The c o l l u v i u m here i s s i m i l a r to that elsewhere i n the Magnesia Creek watershed, as i n d i c a t e d by the g r a i n s i z e d i s t r i b u t i o n s of four samples (Figure 2.20). The channel v a r i e s i n width between about 5 and 12 metres, with 165 P l a t e 4 . 1 1 : Snow Avalanche Track, S i t e Mg2 (a): Unstable c o l l u v i a l  slopes on edge of  t r a c k , upslope of  l o g g i n g road  (person i s standing  on road). uu.*-iiL» ^ it-(b): Near toe of g u l l y . Large boulders e i t h e r were transported  by snow avalanches, or s l i d on top of hard snow. 1 166 g r a d i e n t s i n c r e a s i n g from 21° near Magnesia Creek to 37.5° i n the lower p a r t of a steep bedrock gorge at the top of the c o l l u v i a l slope ( F i g u r e 4.24). The channel s i d e s slope at about 45°, to h e i g h t s of up to 10 m. There i s s t r o n g evidence of avalanche d e p o s i t i o n of d e b r i s near the toe of the s l o p e . A d e b r i s levee which has been d e s c r i b e d e a r l i e r ( S e c t i o n 2.9.2, F i g u r e 2.24) runs between the main stem and the t r i b u t a r y f o r some 120 metres upstream of the c o n f l u e n c e . Thurber C o n s u l t a n t s (1983) d e s c r i b e t h i s f e a t u r e as "snow avalanche d e b r i s levees c o n t r i b u t e d by t r i b u t a r y L2, 3-4 m high f i l l e d with abundant f a l l e n l o g s and b o u l d e r s " (Magnesia Creek F o l i o , page 3). I t i s not obvious to the author that the levee was n e c e s s a r i l y c o n s t r u c t e d by snow avalanching ( d e b r i s t o r r e n t s i n the main stem channel c o u l d be more important), but t h i s i s c e r t a i n l y p o s s i b l e . The shape of the f e a t u r e ( i n p l a n view) does suggest that t r i b u t a r y L2 i s the source of much of the m a t e r i a l (see F i g u r e 2.24). The levee i s densely vegetated with a l d e r t r e e s and shrubs such as salmonberry, but there are no l a r g e c o n i f e r s and only two l a r g e stumps th e r e . The l a r g e s t stump, l o c a t e d at the toe of the l e v e e , has r o t t e d too much to allow a complete t r e e r i n g count to be made, but the t r e e was a metre in diameter and at l e a s t 75 years o l d when c u t . The smaller stump, on the c r e s t of the levee and roughly h a l f way along i t , i s about 50 cm in diameter. A 12 cm diameter hemlock adjacent to t h i s second stump was cut down in 1984, and was found to be 13 years o l d . I t had been damaged on the upstream s i d e , perhaps by a rock, two years e a r l i e r . F i v e t r e e s i n the area were cored i n 1985, with 26 cm 167 Figure 4.24: P r o f i l e of S i t e Mg2 (lower p a r t of Magnesia Creek T r i b u t a r y L2) 120Q-1100-1000-o •rH - P a > a> 900-i—I Orange Pa i n t Line Blue Paint Line Yellow Paint-l i n e Average gradient above t h i s point = 4 7 . 5 ° End of ground survey, mouth of bedrock gorge 30 m upslope 800. Debris Levee on r i g h t 700 n Abandoned l o g g i n g road, avalanche debris Toe of main avalanche d e p o s i t i o n area Large accumulation of rocks and logs (avalanche debris) J L J 1 1 I I I I I J I L I I 1 J I 1 L 100 200 300 4oo 500 600 700 Distance Upstream from Confluence with Main Stem (m) 800 t r e e c o r e r . The approximate l o c a t i o n s of these t r e e s are shown on F i g u r e 2.24. Only No. Mg1-4, a s m a l l , 16-year-old cedar, i s l o c a t e d on the levee i t s e l f . Tree No. Mg2-1, a small spruce near the top of the levee, was 25 years o l d . Trees Mg1-1, Mg1-2 and Mg1-3, a l l f i r s , y i e l d e d poor q u a l i t y c o r e s , but seemed to be between 14 and 32 years o l d . One of these had much of i t s bark s t r i p p e d o f f the upstream s i d e , p o s s i b l y by a recent snow avalanche. The lac k of o l d t r e e s or even stumps on t h i s levee i s supported by a i r p h o t o a n a l y s i s . Throughout the p e r i o d 1939 to p r e s e n t , the c o n i f e r o u s t r e e cover here was c o n s p i c u o u s l y sparse i n comparison with adjacent areas, which suggests that the area i s inundated by e i t h e r snow or water f a i r l y r e g u l a r l y . Upstream of the l e v e e , there are numerous small pockets of coarse d e b r i s and l o g s , probably d e r i v e d from a b l a t i o n of d e b r i s - l a d e n avalanche snow. Major accumulation areas are shown on F i g u r e 4.24, and P l a t e 4.11b shows some d e b r i s i n f e r r e d to be a v a l a n c h e - d e p o s i t e d . The above evidence i n d i c a t e s that avalanches occur here, but i t i s unclear which of the three d e b r i s t r a n s p o r t mechanisms (scour at the base of the avalanche, t r a n s p o r t of d e b r i s p r e v i -o u s l y i n c o r p o r a t e d i n the snow, and scour at the avalanche edges) i s dominant, as a l l are p o s s i b l e at t h i s s i t e . The s i t e c e r t a i n -l y meets the c o n d i t i o n s d e s c r i b e d by Gardner (1970) a p p l i c a b l e to the f i r s t mechanism, as a steep bedrock gorge d i s c h a r g e s i n t o a g u l l y f l o o r e d by loose rocky d e b r i s . I t i s easy to v i s u a l i z e a wet s p r i n g avalanche e n t r a i n i n g m a t e r i a l from the snow-free g u l l y 169 f l o o r and t r a n s p o r t i n g i t downslope. Since the oversteepened c o l l u v i a l w a l l s of the g u l l y are u n s t a b l e , they c o u l d c e r t a i n l y y i e l d d e b r i s to the top of the snowpack, as r e q u i r e d i n mechanism ( i i ) above. There are s e v e r a l small a c t i v e d e b r i s s l i d e s from the banks, both above and below the l o g g i n g road, and a l a r g e stump and root mass, i n c l u d i n g a small l i v i n g f i r t r e e , was found r e s t i n g on rocks i n the c e n t r e of the channel i n J u l y 1985. The i n c i d e n c e of d e b r i s s l i d e s here has been aggravated by extensive timber h a r v e s t i n g i n the 1960's, and the attendant l o s s of root s t r e n g t h (O'Loughlin, 1972a). F i n a l l y , the g u l l y w a l l s may be e r o d i b l e by snow at the edges of a passing avalanche, as in the t h i r d mechansim. In an attempt to assess the s t a b i l i t y of the m a t e r i a l on the g u l l y f l o o r , three l i n e s were p a i n t e d a c r o s s the channel i n J u l y 1984 (Figure 4.24), i n a manner s i m i l a r to at S i t e M3 ( S e c t i o n 4.4.1.2). P l a t e 4.12 shows the l i n e s i n 1986. The lowest (yellow) l i n e has undergone n e g l i g i b l e d i s t u r b a n c e over two y e a r s , while the other two show only very s l i g h t downslope movement and some accumulation of new m a t e r i a l from upslope. While i t i s impossible to s t a t e with c e r t a i n t y that t h i s movement of m a t e r i a l has been avalanche-induced, the manner in which new rocks and logs have been p l a c e d beside and on top of undisturbed m a t e r i a l p o i n t s to snow a b l a t i o n as the probable mode of depos-i t i o n ( P l a t e 4.12). Gardner (1970) c i t e d s i m i l a r evidence, n o t i n g that "balanced b o u l d e r s " were probably avalanche d e p o s i t e d . In c o n c l u s i o n , there i s l i t t l e doubt that wet snow ava l a n -170 P l a t e 4.12: P a i n t L i n e s , S i t e Mg2, 1986. ( a ) : Yellow p a i n t l i n e . (b) : Blue p a i n t l i n e . Arrows i n d i c a t e p a i n t e d rocks s t i l l i n p l a c e . Large boulder i n d i c a t e d by- black cross has s h i f t e d downslope s i n c e 1984. I n both photos, person i s  standing beside rocks which have moved downslope. P l a t e 4.12, c o n t i n u e d : ( c ) : Red P a i n t L i n e . Person i s s t a n d i n g beside rock which has moved downslope. Pack i n background i s on l i n e . ches are common at S i t e Mg2. I t i s apparent t h a t some of these are capable of t r a n s p o r t i n g coarse d e b r i s , although the exact mechanism by which t h i s occurs i s u n c e r t a i n . D e b r i s - l a d e n avalanches must o c c a s i o n a l l y reach Magnesia Creek i t s e l f , so t h i s s i t e must be considered a l o c a t i o n where snow avalanches are d e l i v e r i n g m a t e r i a l to a t o r r e n t - p r o n e stream channel. 4.5.2 Timber H a r v e s t i n g Timber h a r v e s t i n g can i n f l u e n c e the d e b r i s t o r r e n t p o t e n t i a l of a watershed i n s e v e r a l ways. I n d i r e c t e f f e c t s 172 i n c l u d e changes i n the basin hydrology through such a c t i v i t i e s as d i v e r s i o n of s u r f a c e drainage by roads and changes in evapo-t r a n s p i r a t i o n through changes in the f o r e s t cover. These e f f e c t s have been d i s c u s s e d i n S e c t i o n 2.6. The tendency f o r d e b r i s s l i d e a c t i v i t y to i n c r e a s e f o l l o w i n g l o g g i n g , due to l o s s of root s t r e n g t h , i s well-documented (e.g. O'Loughlin, 1972a; Swanston and Swanson, 1976), and has been d i s c u s s e d above ( S e c t i o n s 4.5.1.2, 4.6.1.2). An i n c r e a s e i n d e b r i s s l i d e a c t i v i t y i m p l i e s an i n c r e a s e in the amount of d e b r i s in the creek channels. There are two other ways i n which timber h a r v e s t i n g has d i r e c t l y i n troduced l a r g e amounts of p o t e n t i a l l y m o b i l i z a b l e d e b r i s i n t o channels i n the study a r e a . The f i r s t i s through l o g g i n g road c o n s t r u c t i o n , where l a r g e amounts of m a t e r i a l have been dumped on oversteepened sl o p e s below road c u t s (road s i d e c a s t ) , or where road f i l l s have been c o n s t r u c t e d by p l a c i n g m a t e r i a l on the v a l l e y s i d e or bottoms. The i n f l u e n c e of road c u t s i n generating slope f a i l u r e s such as r o c k f a l l , rock r a v e l -l i n g , and c o l l u v i a l r a v e l l i n g has been d e s c r i b e d i n e a r l i e r s e c t i o n s . Among the areas where road s i d e c a s t has found i t s way i n t o the creek channels are the A l b e r t a Creek road c r o s s i n g at 650 m e l e v a t i o n , the c r o s s i n g s of M Creek and two of i t s t r i b u -t a r i e s near 680 m, and the c r o s s i n g of Loggers Creek at 1080 m, ( F i g u r e 4.1). Another type of l o g g i n g - r e l a t e d m a t e r i a l i s l a r g e organic d e b r i s , such as cut logs and stumps, which have been l e f t i n or near the creek channels f o l l o w i n g l o g g i n g . There are l a r g e 1 73 areas, p a r t i c u l a r l y i n Magnesia Creek and Loggers Creek b a s i n s , where t r e e s appear to have been f e l l e d and simply l e f t to r o t . A l s o , Church and Desloges (1984) noted the presence of s e v e r a l cut logs above the r i g h t bank of A l b e r t a Creek near the 500 m l e v e l , d e s p i t e the f a c t t h at o f f i c i a l l y the b a s i n has never been logged. Most of these were removed by h e l i c o p t e r i n l a t e 1985. P l a t e 4.7a shows a l a r g e log-strewn area near S i t e Mg3 (Figure 4.1). Much of t h i s l a r g e organic d e b r i s may e v e n t u a l l y f i n d i t s way i n t o the channels, where i t may become an important p a r t of P l a t e 4.13: Log Debris i n Magnesia Creek T r i b u t a r y L3 (photo by  D. Kaye) 174 the channel p r o f i l e , as seen i n P l a t e 4.13. The strong c o n t r o l exerted by l a r g e organic d e b r i s on stream channel morphology i n the Queen C h a r l o t t e I s l a n d s has been d e s c r i b e d by Hogan (1985). In time, l o g s i n the channel must e i t h e r r o t , f r e e i n g the m a t e r i a l s t o r e d behind them to be m o b i l i z e d i n d e b r i s t o r r e n t s , or themselves become i n v o l v e d i n t o r r e n t s . Indeed, some of the worst damage to s t r u c t u r e s on d e b r i s fans i n the area has been a t t r i b u t e d to the impact of l a r g e logs and t r e e s c a r r i e d by d e b r i s t o r r e n t s . 4.6 DISCUSSION T h i s chapter has d e s c r i b e d the major d e b r i s supply mechan-isms to steep channels in the study area. The most important of these appear to be r o c k f a l l , r o c k s l i d e , d e b r i s s l i d e , and s o i l wedge f a i l u r e , but a l l of the processes d e s c r i b e d here may be l o c a l l y important. For example, while r o c k f a l l i s widespread in Loggers Creek, much of the channel i s choked with very coarse r o c k f a l l and l o g d e b r i s , which would be very d i f f i c u l t to m o b i l i z e , even under extreme d i s c h a r g e c o n d i t i o n s . For t h i s reason, rock r a v e l l i n g i n the main t r i b u t a r y b a s i n i s the process most l i k e l y t o c o n t r i b u t e to d e b r i s t o r r e n t s i n t h i s creek, and r o c k f a l l i s probably l e s s important here. On the other hand, an u n u s u a l l y l a r g e r o c k f a l l event here c o u l d p o t e n t i a l l y d e s t a b i l i z e the channel m a t e r i a l through impulsive l o a d i n g . T h i s theory i s d i s c u s s e d i n S e c t i o n 6.2.3. At the beginning of t h i s chapter ( S e c t i o n 4.2), the q u e s t i o n of d i s c r e t e versus continuous d e b r i s supply was r a i s e d . T h i s 175 chapter has shown that a wide v a r i e t y of d i s c r e t e d e b r i s sources (p o i n t sources) e x i s t i n the four study watersheds, but l i t t l e has been s a i d about the stream banks i n areas between these p o i n t sources. In f a c t , some of the minor d e b r i s supply proces-ses which have been d e s c r i b e d here, such as bank slumping, rock r a v e l l i n g , and c o l l u v i a l r a v e l l i n g , as w e l l as s o i l creep and t r e e throw, may be more e x t e n s i v e and more important than i n i t i a l l y apparent, e s p e c i a l l y where s i d e s l o p e s are steeper than about 40° (0. Hungr, pe r s . comm., 1987). D e b r i s supply r a t e s from these processes are l i k e l y lower than from some of the more conspicuous processes d e s c r i b e d i n d e t a i l i n t h i s chapter, but, i n the absence of major p o i n t sources, these minor inconspicous processes may dominate over the long term. A l s o , the i n f l u e n c e of processes such as c o l l u v i a l r a v e l l i n g i s d i f f i c u l t to assess, because, while most of the m a t e r i a l r e l e a s e d i s so f i n e that i t i s q u i c k l y removed from the channel system by stream a c t i o n , very coarse m a t e r i a l may be undermined and d e l i v e r e d to the creek. Since both p o i n t sources and " l i n e sources" seem to e x i s t i n the study area, the q u e s t i o n of whether d i s c r e t e or continuous d e b r i s supply i s dominant i s probably impossible to r e s o l v e without r e s o r t i n g to a sediment budget a n a l y s i s . Such an approach would l i k e l y y i e l d u s e f u l r e s u l t s , but i s beyond the scope of t h i s t h e s i s . Hungr et a l . (1984) took a somewhat d i f f e r e n t approach to the d e b r i s supply problem i n t h e i r e s t i m a t i o n s of p o t e n t i a l d e b r i s t o r r e n t volume. Recognizing that most of the d e b r i s i n a t o r r e n t comes d i r e c t l y from the channel and i t s immediate 176 v i c i n i t y , they suggested e s t i m a t i n g the average "channel d e b r i s 3 y i e l d r a t e " (m /m of channel) f o r t o r r e n t - p r o n e channels. T h i s parameter c o u l d i d e a l l y be estimated from a channel c l a s s i f i -c a t i o n system t a k i n g i n t o account such f a c t o r s as "the g r a d i e n t , type of bed m a t e r i a l , height and steepness of s i d e s l o p e s i f made of e r o d i b l e m a t e r i a l , and the g e n e r a l s t a b i l i t y c o n d i t i o n of the channel as r e f l e c t e d by v e g e t a t i o n " (p. 666). F u r t h e r , they noted that "Point sources of d e b r i s , such as i n d i v i d u a l l a n d s l i d e s in creek banks, may add 10% or more to the t o t a l magnitude. T h e i r p o t e n t i a l may, however, be d i f f i c u l t to i d e n t i f y , i n which case they should be accounted f o r by a c o n s e r v a t i v e l y s e l e c t e d y i e l d r a t e " (p. 666). The approach of Hungr et a l . (1984) c l e a r l y emphasizes l i n e sources or continous d e b r i s supply over the more obvious but p o s s i b l y l e s s important p o i n t sources. Thurber Consultants (1983) had taken a s i m i l a r approach, p r o v i d i n g the f o l l o w i n g estimates of creek bank s t a b i l i t y above the fan as a f i r s t step ( i t should be noted that the study team for t h i s r e p o r t c o n s i s t e d of the three authors of Hungr et a l . , 1984, and D. F. VanDine): A l b e r t a Creek: 30% u n s t a b l e , 40% p o t e n t i a l l y u n s t a b l e ; Magnesia Creek, main stem: 30% u n s t a b l e , 50% p o t e n t i a l l y u n stable; t r i b u t a r i e s : 40% u n s t a b l e , 30% p o t e n t i a l l y u n stable; M Creek, main stem: 30% u n s t a b l e , 40% p o t e n t i a l l y u n s t a b l e ; t r i b u t a r i e s : 30% u n s t a b l e , 40% p o t e n t i a l l y u n s t a b l e ; Loggers Creek, main stem: 30% u n s t a b l e , 50% p o t e n t i a l l y u n s t a b l e ; t r i b u t a r y : 40% u n s t a b l e , 30% p o t e n t i a l l y u n s t a b l e . While e s t i m a t i n g channel s t a b i l i t y in t h i s manner i s l o g i c a l when c o n s i d e r i n g the e n g i n e e r i n g problem of e s t i m a t i n g the magnitude of the "design" d e b r i s t o r r e n t , i t does not address the geomorphic problem of understanding d e b r i s supply mechanisms, 1 77 and so has not been adopted in t h i s t h e s i s . One of the most important c o n c l u s i o n s to be drawn from a l l the evidence presented i n t h i s chapter i s the d i f f e r e n c e s between the dominant d e b r i s supply processes i n v a r i o u s p a r t s of the study area. T h i s p o i n t s to the need to look i n d e t a i l at i n d i v i d u a l basins when a s s e s s i n g d e b r i s t o r r e n t p o t e n t i a l . Such a need i s a l s o inherent i n the channel d e b r i s y i e l d r a t e approach of Hungr et a l . (1984). The d i v e r s i t y of d e b r i s supply mechan-isms and r a t e s , even w i t h i n t h i s very small a r e a , s e v e r e l y hampers the a p p l i c a t i o n of r e g i o n a l r a i n f a l l - or runoff-based models to the d e b r i s t o r r e n t problem. Yet t h i s i s o n l y p a r t of the s t o r y . The p i c t u r e i s f u r t h e r complicated by d e b r i s r e d i s t -r i b u t i o n i n stream channels, and by the p o t e n t i a l of c e r t a i n h i l l s l o p e processes to t r i g g e r d e b r i s t o r r e n t s i n a d d i t i o n to s u p p l y i n g d e b r i s to channels. These t o p i c s are the f o c i of Chapters 5 and 6 r e s p e c t i v e l y . 178 Chapter 5 DEBRIS REDISTRIBUTION IN CHANNELS 5.1 INTRODUCTION In the previous chapter, a number of processes were des-c r i b e d which act to supply d e b r i s to channels at s p e c i f i c p o i n t s , such as at the toe of a r o c k s l i d e , the mouth of a g u l l y or d e b r i s chute, and the confluence of a t r i b u t a r y with the main stem. Since a l l of the c o n d i t i o n s necessary to generate a d e b r i s t o r r e n t i n a given channel are s i m u l t a n e o u s l y s a t i s f i e d only r a r e l y , i t i s important to understand what happens to the m a t e r i a l between the time i t i s d e l i v e r e d to a channel and the t r i g g e r i n g of a d e b r i s t o r r e n t . Since most of the m a t e r i a l i n v o l v e d i n a t o r r e n t i s e n t r a i n e d from the stream bed and banks (Swanston and Swanson, 1976; Hungr et a l . , 1984), the c h a r a c t e r and d i s t r i b u t i o n of e n t r a i n a b l e d e b r i s i n a channel are e q u a l l y as important as the amount of m a t e r i a l s t o r e d i n the channel. T h i s chapter examines the evidence f o r three p o s s i b l e d e b r i s r e d i s t r i b u t i o n mechanisms i n the study a r e a : d e b r i s t o r r e n t s which become s t a l l e d i n the mid-reaches of a channel system; normal f l u v i a l processes; and wet snow avalanches i n t o the creek channels. Debris r e d i s t r i b u t i o n i n stream channels has important i m p l i c a t i o n s f o r the magnitude and frequency of d e b r i s t o r r e n t s . In the absence of r e d i s t r i b u t i o n , m a t e r i a l must accumulate at s p e c i f i c d i s c r e t e p o i n t s i n a channel system as the d e b r i s supply processes d e s c r i b e d i n Chapter 4 continue to operate, r e s u l t i n g i n a l o c a l i n c r e a s e i n d e b r i s t h i c k n e s s or channel g r a d i e n t , or 1 79 both. Both of these c o n d i t i o n s enhance the p o s s i b i l i t y of d e b r i s t o r r e n t s i n i t i a t i n g at the p o i n t where the d e b r i s has been accumulating. Takahashi (1981) and VanDine (1985a) have shown that i f creek bed m a t e r i a l i s to be m o b i l i z e d , the t h i c k n e s s of the m a t e r i a l i n v o l v e d must exceed the mean p a r t i c l e diameter, d. T h i s c o n d i t i o n w i l l be met more e a s i l y by a t h i c k d e b r i s accumu-l a t i o n than by a t h i n n e r one. The i n f l u e n c e of channel g r a d i e n t , 6 , i n d e b r i s m o b i l i z a t i o n i s expressed in equation (5.1) below. The channel m a t e r i a l w i l l be m o b i l i z e d by running water in the channel i f : ( ^ s a t + d V t a n 9 = ( ^ s a t " V tan*' (5'1) where: Y + = the u n i t weight of the s a t u r a t e d d e b r i s s a T Y = the u n i t weight of water h = the depth of water i n the channel, measured normal to the channel bed = the angle of s h e a r i n g r e s i s t a n c e of the d e b r i s 6 and d are as d e f i n e d above. T h i s equation, from Bovis et a l . (1985), i s a s l i g h t l y m o d i f i e d form of Takahashi's (1981) equation ( 3 ) . I t shows t h a t , as the channel g r a d i e n t i n c r e a s e s , l e s s water i s r e q u i r e d to m o b i l i z e the d e b r i s , and thus the d e b r i s t o r r e n t p o t e n t i a l i s i n c r e a s e d . T h i s equation w i l l be d i s c u s s e d more f u l l y i n S e c t i o n 6.1.1. A l t e r n a t i v e l y , i f much of the d e b r i s i s removed from i t s i n i t i a l p o i n t of d e p o s i t i o n i n the channel by some means, p o i n t accumulations of d e b r i s w i l l be, on average, t h i n n e r and l e s s steep, and thus more s t a b l e . However, i f d e b r i s r e d i s t r i b u t i o n r e s u l t s i n r e l a t i v e l y uniform accumulations of m a t e r i a l along 180 much of the channel, the t o t a l amount of m o b i l i z a b l e d e b r i s i n the channel can u l t i m a t e l y become much g r e a t e r . The r e s u l t i s a decrease in t o r r e n t frequency i n the creek, but an i n c r e a s e i n p o t e n t i a l t o r r e n t magnitude. Torrent-prone channels e x p e r i e n c i n g a c t i v e d e b r i s r e d i s t r i b u t i o n between t o r r e n t events may be much more hazardous from a human and e n g i n e e r i n g p o i n t of view, both because of the l a r g e r volumes of d e b r i s p o t e n t i a l l y i n v o l v e d , and because the lower frequency may l e a d to a decrease in p u b l i c awareness of the p o t e n t i a l hazard. Magnesia Creek appears to be the most a c t i v e of the four i n the study area with respect to d e b r i s r e d i s t r i b u t i o n . As noted in S e c t i o n 2.7.2, there i s a reach of t h i s creek c l o s e to 1 km 3 long, between 500 m and 770 m, where at l e a s t 15m of l o o s e d e b r i s per metre of channel l e n g t h i s s t o r e d . The v a l l e y s i d e s are not p a r t i c u l a r l y steep here, and there are no obvious p o i n t sources of d e b r i s downstream of S i t e Mg1, at the 700 m l e v e l (see S e c t i o n 4.5.2.3), so i t must be concluded that most of the d e b r i s i n the channel has been t r a n s p o r t e d from upstream d e b r i s sources by some means. 5.2 DEBRIS TORRENT PROCESSES F u l l - s c a l e d e b r i s t o r r e n t s o b v i o u s l y are a very e f f e c t i v e means of t r a n s p o r t i n g l a r g e amounts of d e b r i s downstream. The steepness of most of the Howe Sound creeks i s s u f f i c i e n t to allow the m a j o r i t y of t o r r e n t s in t h i s r e gion to continue at l e a s t to the apex of the d e b r i s fans, and i n many cases r i g h t to the sea. However, some of the l a r g e r creeks i n the area, such as Harvey 181 and Magnesia, have long reaches of r e l a t i v e l y low g r a d i e n t , where t o r r e n t s may p o s s i b l y come to r e s t . In f a c t , there i s evidence that t h i s happened i n both these creeks, i n the 1930's. Church and Desloges (1984) examined a i r phototgraphs of the region taken i n 1932 and 1939, and determined that sometime i n the i n t e r v e n i n g p e r i o d a t o r r e n t moved down Harvey Creek, stopping near the 500 m l e v e l , where the g r a d i e n t i s about 12°. As noted i n S e c t i o n 2.7.2, there i s s i m i l a r evidence f o r a t o r r e n t o r i g i n a t i n g near the 1200 m l e v e l on the main stem of Magnesia Creek, and stopping near 750 m, where the g r a d i e n t i s about 16.6°. The l i k e l i h o o d of d e b r i s t o r r e n t s stopping i n t h i s reach of Magnesia Creek can be examined i n more d e t a i l . The 16.6° g r a d i e n t here seems to be on the upper l i m i t f o r t o r r e n t depos-i t i o n , given the o b s e r v a t i o n s of Thurber C o n s u l t a n t s (1983) that most d e b r i s fans i n the r e g i o n have g r a d i e n t s between 10° and 16°. Equation (1.3) i n S e c t i o n 1.1.2.3 (from Takahashi, 1981) s t a t e s that moving d e b r i s w i l l stop where there i s an abrupt decrease in channel g r a d i e n t , i f the g r a d i e n t of the downstream reach, e, i s such that tane < tane u (5.2) tan<j) where 9 W i s the g r a d i e n t of the upstream reach, $ i s the angle of s h e a r i n g r e s i s t a n c e of the bed m a t e r i a l , and a i s a k i n e t i c angle of f r i c t i o n f o r the f l o w i n g d e b r i s . These l a s t two parameters are d i f f i c u l t to determine in the f i e l d , but Hungr et a l . (1984) suggest that a = 30° i s a reasonable value f o r well-graded c o h e s i o n l e s s m a t e r i a l . VanDine (1985a) assumed a value of 182 <i> = 37.5 i n h i s F i g u r e 6, and t h i s conforms c l o s e l y to the f i n d i n g i n the present study that <f>cv = 37° at S i t e M3 ( S e c t i o n 4.4.1.2). The average g r a d i e n t of Magnesia Creek between 1000 m and 770 m i s 23.3°. The r i g h t hand sid e of i n e q u a l i t y (5.2) y i e l d s a value of 0.32 when a = 30°, <t> = 37.5°, and = 23.3°. Since tan(16.6°) = 0.30, the i n e q u a l i t y does seem to be s a t i s f i e d at t h i s l o c a t i o n , so a t o r r e n t c o u l d c o n c e i v a b l y come to a stop here. Equation (5.2) assumes no change i n channel width, which may not be t r u e at t h i s l o c a t i o n , s i n c e T r i b u t a r y L3, which i s a c t u a l l y l a r g e r than the main stem, e n t e r s here. However, the d e b r i s levee which has been d e s c r i b e d e a r l i e r ( S e c t i o n s 2.7.3 and 4.5 .1) does provide a c o n s i d e r a b l e degree of confinement between the 770 m and 725 m e l e v a t i o n s (Figure 2.24). In any event, a widening of the channel would promote r a t h e r than i n h i b i t d e b r i s d e p o s i t i o n . The above arguments, combined with the a i r p h o t o evidence, s t r o n g l y suggest that at l e a s t one d e b r i s t o r r e n t has come to r e s t i n t h i s mid-reach p o r t i o n of Magnesia Creek. In f a c t , small t o r r e n t events in t h i s creek may be rather more common than the h i s t o r i c a l r ecord would i n d i c a t e . A t o r r e n t which occurred d u r i n g a high runoff event but stopped w e l l above the populated fan c o u l d e a s i l y escape the n o t i c e of observers at the highway or below, who would only see a f l o o d with perhaps an u n u s u a l l y high amount of f i n e m a t e r i a l being t r a n s p o r t e d . I t i s c e r t a i n l y p o s s i b l e that much of the m a t e r i a l p r e s e n t l y s t o r e d i n Magnesia Creek channel has been d e p o s i t e d by such events. The above d i s c u s s i o n r a i s e s the q u e s t i o n as to why the 1962 183 d e b r i s t o r r e n t i n Magnesia Creek d i d not s t a l l i n t h i s mid-reach area, but ra t h e r continued down to the fan, d e s t r o y i n g both the highway and rai l w a y bridges (Thurber C o n s u l t a n t s , 1983). One can only speculate t h a t perhaps t h i s event was more f l u i d than some (a lower value of the parameter a ) , and thus was ab l e to continue through the lower g r a d i e n t p o r t i o n of the channel. C e r t a i n l y t o r r e n t s can maintain motion on slop e s as low as 10 i f they c o n t a i n l a r g e amounts of f i n e s or organic mulch (Thurber C o n s u l t a n t s , 1983), and the g r a d i e n t of Magnesia Creek i s r a r e l y l e s s than 13 upstream of the fan. While t o r r e n t s i n t h i s region r a r e l y have l a r g e p r o p o r t i o n s of f i n e m a t e r i a l , they o f t e n comprise c o n s i d e r a b l e amounts of smal l organic d e b r i s (Thurber C o n s u l t a n t s , 1983; Church and Desloges, 1984). Thus, the apparent v a r y i n g t o r r e n t h i s t o r y of t h i s creek can probably be e x p l a i n e d s o l e l y by v a r i a t i o n s i n the c h a r a c t e r of the d e b r i s between s u c c e s s i v e events. 5.3 FLUVIAL PROCESSES A second means of d e b r i s r e d i s t r i b u t i o n i n channels i s through normal bedload t r a n s p o r t . D e b r i s i n t r o d u c e d i n t o channels at a s p e c i f i c p o i n t , by some means such as a d e b r i s s l i d e , s o i l wedge f a i l u r e , r o c k s l i d e , e t c . , i n i t i a l l y may c o n t a i n a wide range of p a r t i c l e s i z e s , but over time stream a c t i o n can s e l e c t i v e l y remove the f i n e r m a t e r i a l . The r e s u l t i s a poten-t i a l l y unstable c o a r s e - g r a i n e d l a g d e p o s i t i n the channel. Moderately high r u n o f f events, while not s u f f i c i e n t to d e s t a b i l -i z e the bed and generate d e b r i s t o r r e n t s , may be capable of 184 t r a n s p o r t i n g some of t h i s coarse d e b r i s as bedload f o r short d i s t a n c e s downstream. D e p o s i t i o n occurs where there i s a decrease i n v e l o c i t y , due to a decrease i n g r a d i e n t , an i n c r e a s e i n channel width, or channel blockage by some o b s t a c l e such as a l o g or f a l l e n t r e e . Over time, the stream may develop a s t e p - p o o l p r o f i l e , with l a r g e amounts of coarse m a t e r i a l s t o r e d in the channel. Church and Desloges (1984) note that t h i s type of p r o f i l e i s t y p i c a l of steep mountain streams, i f they have remained undisturbed f o r some time. F i g u r e s 2.23 and 5.1 give some i n d i c a t i o n of t h i s i n a p o r t i o n of Magnesia Creek. Debris accumulations at two l o c a t i o n s i n the low-gradient reach of Magnesia Creek between 500 m and 770 m have been monitored f o r 2 - 3 years, to assess the amount of d e b r i s r e d i s t r i b u t i o n o c c u r r i n g here. The most e x t e n s i v e l y s t u d i e d of these two s i t e s i s at the f o r e s t r y road b r i d g e , at 620 m. A map of t h i s part of the channel i s shown i n F i g u r e 2.23; the average g r a d i e n t here i s 14.8 (along p r o f i l e A-B). The d e b r i s lobe immediately upstream of the b r i d g e was photographed e i g h t times between 16 February 1984 and 22 February 1987. There has been very l i t t l e change in the c o n f i g u r a t i o n of the rocks or even small p i e c e s of organic m a t e r i a l at t h i s p o i n t over a three year p e r i o d . Conversely, the d e b r i s lobe immediately downstream of the bridge (photographed four times between 25 May 1984 and 22 February 1987) has experienced n o t i c e a b l e r e d i s t r i b u t i o n of organic d e b r i s and small r o c k s . The f i r s t and l a s t of the four photos are shown in P l a t e 5.1. The second monitored set of d e b r i s accumulations i s about 135 m downstream of the b r i d g e . 185 F i g u r e 5 - 1 ' P r o f i l e o f P a r t o f Magnes ia Creek ^ - > u 0 5 0 T O O T50 20U D i s t a n c e Upstream from Datum (m) P l a t e 5.1; Debris accumulations i n Magnesia Creek. (a): Looking downstream  from bridge, May  1 9 8 ^ i ^ l ' - Looking downstream from bridge, February 1987- Arrows i n d i c a t e l o c a t i o n s where rocks or organic d e b r i s have moved. P l a t e 5-1, continued: ( c ) : Yellow p a i n t l i n e , November 1984. (d); Yellow p a i n t l i n e , Feb- ruary 1987- There are s e v e r a l new rocks i n the v i c i n i t y o f the green  and brown pack. Three l i n e s were p a i n t e d a c r o s s the creek channel i n November 1984, at e l e v a t i o n s of 579 m, 584 m, and 586 m (Figure 5.1). These l i n e s were checked f o r evidence of movement i n May 1985 and again i n February 1987. Only the yellow l i n e (the middle one) shows any s i g n i f i c a n t d i s t u r b a n c e : a new accumulation of small rocks which can be seen i n the 1987 photo ( P l a t e 5.1). The o b s e r v a t i o n s at the bridge and at the p a i n t l i n e s suggest that there i s some d e b r i s r e d i s t r i b u t i o n by f l u v i a l a c t i o n i n t h i s reach of Magnesia Creek, but that i t i s a f a i r l y slow process. V a l l e y - b o t t o m v e g e t a t i o n can be used to f u r t h e r assess the s t a b i l i t y of the d e b r i s i n t h i s reach of Magnesia Creek. There are no c o n i f e r o u s t r e e s on the v a l l e y bottom, but cedars are common r i g h t at the edge of the creek, at the toes of the v a l l e y s i d e s l o p e s . Red a l d e r s grow on some of the l a r g e r d e b r i s accumulations i n the middle of the channel area, i n p l a c e s where the creek e i t h e r b i f u r c a t e s or i s f o r c e d to one s i d e of the v a l l e y . Cores e x t r a c t e d i n February 1987 from the three l a r g e s t of these t r e e s near the red p a i n t l i n e y i e l d e d age estimates of 15, 17, and 19 ye a r s . Two a l d e r s i n the c e n t r e of the channel area, about 35 m downstream of the bridge, are each estimated to be 21 years o l d (these t r e e s are v i s i b l e i n the middle d i s t a n c e i n P l a t e s 5.1a and 5.1b). T h i s evidence suggests some sporadic d i s t u r b a n c e of the creek channel m a t e r i a l s , at l e a s t i n the f i r s t few years a f t e r the 1962 d e b r i s t o r r e n t . Assuming that the t o r r e n t destroyed a l l p r e - e x i s t i n g a l d e r s , but not t h e i r root systems, new t r e e s l i k e l y would have sprouted from these roots i n the f o l l o w i n g s p r i n g (M. North, p e r s . comm., 1987), and should 189 have been 24 years o l d i n 1987. The two t r e e s near the bridge are only s l i g h t l y younger than t h i s (21 y e a r s ) , but the t r e e s near the p a i n t l i n e s are younger and more v a r i e d i n age. There must be some d e b r i s r e d i s t r i b u t i o n o c c u r r i n g here. 5.4 WET SNOW AVALANCHES A t h i r d p o s s i b l e d e b r i s r e d i s t r i b u t i o n mechanism i s wet snow avalanches i n t o the creek channels. As noted i n S e c t i o n 4.6.1, avalanches on open slopes are capable of e n t r a i n i n g and t r a n s -p o r t i n g coarse d e b r i s under c e r t a i n circumstances, and i t seems reasonable to expect that t h i s c o u l d happen i n stream channels as w e l l . The most obvious processes i n v o l v e snow e i t h e r simply pushing rocks along the channel, or e n t r a i n i n g m a t e r i a l and r e d e p o s i t i n g ; i t i n the avalanche d e p o s i t i o n zone downstream. In a d d i t i o n , two i n d i r e c t e f f e c t s of snow avalanches i n t o creeks c o u l d be r e s p o n s i b l e f o r r e d i s t r i b u t i n g channel m a t e r i a l . I f , as o f t e n happens, an avalanche occurs d u r i n g a heavy rainstorm accompanied by a r a p i d i n c r e a s e i n temperature, the snow might melt r a p i d l y upon e n t e r i n g a creek channel. The r e s u l t i n g i n c r e a s e i n stream d i s c h a r g e c o u l d enable the creek to t r a n s p o r t l a r g e r m a t e r i a l than would otherwise be p o s s i b l e , thus r e s u l t i n g i n some r e d i s t r i b u t i o n of coarse d e b r i s . A second i n d i r e c t e f f e c t c o u l d be a temporary blockage of the channel by avalanche-d e p o s i t e d snow. The surge of water r e s u l t i n g from the breaching of such a temporary dam would allow the entrainment and t r a n s -p o r t , a t . l e a s t f o r short d i s t a n c e s , of normally s t a b l e d e b r i s . Snow avalanches should be a more e f f e c t i v e d e b r i s r e d i s -190 t r i b u t i o n mechanism i n steeper, more c o n f i n e d channels. F i g u r e 4.23 shows the channel segments i n the study area thought to be most s u s c e p t i b l e to snow a v a l a n c h i n g . While the d e b r i s - f i l l e d reach of Magnesia Creek d e s c r i b e d above i s u n l i k e l y to be a f f e c t e d by snow av a l a n c h i n g except near the d e b r i s levee, and then only r a r e l y , l a r g e p o r t i o n s of the main stems of A l b e r t a and M Creeks may be a f f e c t e d f a i r l y f r e q u e n t l y . In f a c t , i t i s known that the February 1983 d e b r i s t o r r e n t on A l b e r t a Creek was t r i g g e r e d when an avalanche from upstream d e s t a b i l i z e d channel m a t e r i a l j u s t downstream of the lo g g i n g road c r o s s i n g at 660 m (Church and Desloges, 1984). A concrete d e b r i s dam c o n s t r u c t e d at the 700 m e l e v a t i o n i n l a t e 1985 ( S e c t i o n 2.7.1) probably has e l i m i n a t e d t h i s hazard i n A l b e r t a Creek. 5.5 DISCUSSION Deb r i s r e d i s t r i b u t i o n has an important i n f l u e n c e on d e b r i s t o r r e n t magnitude. I f m a t e r i a l d e p o s i t e d i n a stream channel at r e l a t i v e l y few d i s c r e t e p o i n t s can be t r a n s p o r t e d and re d e p o s i t e d downstream, the t o t a l amount of m o b i l i z a b l e d e b r i s s t o r e d i n a channel can be inc r e a s e d s i g n i f i c a n t l y . In other words, the channel d e b r i s y i e l d r a t e (Hungr et a l . , 1984; see a l s o S e c t i o n 4.6), and t h e r e f o r e the maximum p o t e n t i a l t o r r e n t magnitude, can be much h i g h e r . Debris r e d i s t r i b u t i o n a l s o tends to reduce t o r r e n t frequency, by i n c r e a s i n g the amount of m a t e r i a l which must be d e l i v e r e d to a creek before a c r i t i c a l d e b r i s t h i c k n e s s or.channel g r a d i e n t i s a t t a i n e d at a p o i n t i n the channel. T h i s combination of higher magnitude and lower frequency can make 191 streams e x p e r i e n c i n g a c t i v e d e b r i s r e d i s t r i b u t i o n much more hazardous from a d e b r i s t o r r e n t p o i n t of view. Three d i f f e r e n t d e b r i s r e d i s t r i b u t i o n mechanisms can operate i n the study a r e a . There i s evidence i n the middle reaches of Magnesia Creek of d e b r i s t o r r e n t s t r a n s p o r t i n g l a r g e amounts of m a t e r i a l from upstream and d e p o s i t i n g i t where the g r a d i e n t decreases, and of bedload t r a n s p o r t r e d i s t r i b u t i n g coarse m a t e r i a l . P a r t s of both M Creek and A l b e r t a Creek are suscept-i b l e to m a t e r i a l t r a n s p o r t and d e p o s i t i o n by wet snow avalanches. While both A l b e r t a and M creeks have r e c e n t l y experienced major d e b r i s t o r r e n t s , Magnesia has not, and t h i s i s at l e a s t p a r t l y due to i t s a c t i v e d e b r i s r e d i s t r i b u t i o n . Magnesia Creek would appear to be "primed" f o r a very l a r g e d e b r i s t o r r e n t . 192 Chapter 6 TRIGGERING OF DEBRIS TORRENTS 6.1 INTRODUCTION I t has been s t a t e d s e v e r a l times i n the p r e c e d i n g chapters t h a t , i n a d d i t i o n to s u p p l y i n g d e b r i s to stream channels, some h i l l s l o p e events are capable of a c t u a l l y t r i g g e r i n g d e b r i s t o r r e n t s . For example, the October 1981 event i n M Creek appears to have been t r i g g e r e d by a s o i l wedge f a i l u r e at S i t e M1, and the February 1983 t o r r e n t i n A l b e r t a Creek has been a t t r i b u t e d to a wet snow avalanche. Thus, while the main focus of t h i s t h e s i s i s d e b r i s supply (the t o p i c of Chapter 4), i t was f e l t necessary to i n c l u d e a chapter on t o r r e n t t r i g g e r i n g . S e c t i o n 6.1 b r i e f l y reviews the major t o r r e n t t r i g g e r i n g mechanisms which have been proposed i n the l i t e r a t u r e . Not a l l of these are a c t i v e i n the present study area. F o l l o w i n g t h i s review, S e c t i o n 6.2 d e s c r i b e s i n more d e t a i l those mechanisms which seem to be a c t i v e or p o s s i b l e i n the study area, and d i s c u s s e s some examples in d e t a i l . I t was noted i n Chapter 1 t h a t , even with a favourable channel c o n f i g u r a t i o n , a high runoff event, and a supply of m o b i l i z a b l e d e b r i s , there was s t i l l a need fo r some s o r t of t r i g g e r before channel d e b r i s c o u l d be m o b i l i z e d as a t o r r e n t . A v a r i e t y of t r i g g e r mechanisms have been proposed, but they can a l l be grouped i n t o three broad c l a s s e s : f l u i d shear, h i l l s l o p e f a i l u r e s i n t o channels, and ground v i b r a t i o n s . Each of these are d e s c r i b e d i n turn below. F l u i d shear seems to be the most o f t e n c i t e d d e b r i s t o r r e n t 193 t r i g g e r i n g mechanism. The process has been d e s c r i b e d i n d e t a i l by Takahashi (1981), and subsequently by a number of others (e.g. Innes, 1983; VanDine, 1985a). The p h y s i c s of the process w i l l be d i s c u s s e d i n S e c t i o n 6.2.1, but the e s s e n t i a l p o i n t i s t h a t , under the r i g h t c o n d i t i o n s , the appearance of an unusually high flow of water in a stream channel can cause the bed m a t e r i a l to become m o b i l i z e d at a s p e c i f i c p o i n t . Once s t a r t e d , t h i s process i s s e l f - s u s t a i n i n g , as the a d d i t i o n a l l o a d due to m o b i l i z e d bed m a t e r i a l from upstream w i l l cause d e s t a b i l i z a t i o n of m a t e r i a l downstream of the i n i t i a t i o n p o i n t . There are s e v e r a l p o s s i b l e causes of u n u s u a l l y h i g h stream d i s c h a r g e . The simplest of these i s j u s t very i n t e n s e r a i n , p o s s i b l y combined with r a p i d snowmelt. In the s m a l l , steep b a s i n s most prone to d e b r i s t o r r e n t a c t i v i t y , r u n o f f response to heavy r a i n can be very r a p i d , e s p e c i a l l y i f the ground i s a l r e a d y s a t u r a t e d from p r e v i o u s r a i n f a l l events. R a i n f a l l and snowmelt t r i g g e r i n g of t o r r e n t s has been c i t e d by s e v e r a l r e s e a r c h e r s (e.g. M i l e s and K e l l e r h a l s , 1979; Caine, 1980; Takahashi, 1981; Innes, 1983; VanDine, 1985a). A second set of p o s s i b l e causes of very high stream d i s -charge i n c l u d e s c a t a s t r o p h i c g l a c i a l lake drainages ( j o k u l h l a u p s ) and other outburst f l o o d s . Jackson (1979) d e s c r i b e s a s e r i e s of j o k u l h l a u p - g e n e r a t e d d e b r i s t o r r e n t s which severed the Canadian P a c i f i c Railway and the Trans-Canada Highway, at the S p i r a l Tunnels near F i e l d , in the Rocky Mountains of B r i t i s h Columbia. Clague et a l . (1985) d e s c r i b e a s e r i e s of d e b r i s flows on K l a t t a s i n e Creek, i n the southern Coast Mountains of B r i t i s h 194 Columbia, t r i g g e r e d by the c a t a s t r o p h i c d r a i n i n g of a moraine-dammed l a k e . Moraine-dammed lakes such as the one at K l a t t a s i n e Creek are common i n the Coast Mountains of B r i t i s h Columbia, and many have d r a i n e d c a t a s t r o p h i c a l l y i n the past (Clague et a l . , 1985). A t h i r d mechanism f o r producing u n u s u a l l y high stream d i s c h a r g e i s the breaching of a temporary channel blockage, due to a l o g jam or h i l l s l o p e f a i l u r e i n t o a creek channel. There are r e l a t i v e l y few r e f e r e n c e s to t h i s process i n the l i t e r a t u r e , but Takahashi (1981) makes a p a s s i n g r e f e r e n c e to i t . The r e s u l t i s s i m i l a r to the outburst f l o o d s d e s c r i b e d above: very high d i s c h a r g e downstream of the f a i l e d dam. H i l l s l o p e f a i l u r e s , such as d e b r i s avalanches, d e b r i s s l i d e s , rock s l i d e s , and s o i l wedge f a i l u r e s , i n t o creek channels, a l s o t r i g g e r t o r r e n t s . Temporary damming of the channel by l a n d s l i d e d e b r i s can l e a d i n d i r e c t l y to a d e b r i s t o r r e n t when the dam f a i l s , as noted above. Two more d i r e c t mechanisms are f l u i d i z a t i o n of s l i d e d e b r i s immediately upon e n t e r i n g a stream channel, and d e s t a b i l i z a t i o n of p r e - e x i s t i n g bed m a t e r i a l through impulsive l o a d i n g from l a n d s l i d e d e b r i s . Impulsive l o a d i n g w i l l be d i s c u s s e d i n d e t a i l i n S e c t i o n 6.2.3. There are many r e f e r e n c e s i n the l i t e r a t u r e to t o r r e n t i n i t i a t i o n through h i l l s l o p e f a i l u r e s (e.g. Swanston and Swanson, 1976; Nasmith and Mercer, 1979; Rapp and Nyberg, 1980; Eisbacher and Clague, 1981; Johnson and Rodine, 1984; Costa, 1984; Evans and L i s t e r , 1984). Although not always s t a t e d , the i m p l i c a t i o n i s t h a t most of these are due to r a p i d f l u i d i z a t i o n of slope 195 f a i l u r e s , p a r t i c u l a r l y d e b r i s s l i d e s and d e b r i s avalanches. As the i n i t i a l slope f a i l u r e e n t e r s a stream channel having abundant d i s c h a r g e , the d e b r i s q u i c k l y becomes s a t u r a t e d , and continues to flow downstream. Debris flows i n i t i a t e d i n t h i s way tend to grow c o n s i d e r a b l y i n s i z e as they move downstream, and of t e n the i n i t i a l slope f a i l u r e c o n s t i t u t e s only a small p r o p o r t i o n of the f i n a l flow volume (Swanston and Swanson, 1976). Another p o s s i b l e t o r r e n t t r i g g e r i n g mechanism i s d e b r i s d e s t a b i l i z a t i o n due to ground v i b r a t i o n s . There are few d e t a i l e d r e f e r e n c e s to t h i s mechanism i n the l i t e r a t u r e , but Eisbacher (1982) notes that some d e b r i s flows have been a t t r i b u t e d to earthquakes. I t has a l s o been suggested t h a t v i b r a t i o n s from thunder c o u l d cause d e b r i s flows (Winder, 1965 and S c o t t , 1972, both c i t e d i n Innes, 1983). Okuda (1978) and Rapp and Nyberg (1980) have noted that s t r o n g v i b r a t i o n s o f t e n accompany the passage of d e b r i s flows down a channel, and t h a t these v i b r a t i o n s c o u l d s t a r t f u r t h e r flows or d e s t a b i l i z e temporary dams which may have formed i n the channel. While a l l of these processes c e r t a i n l y seem to be p h y s i c a l l y p o s s i b l e , t o r r e n t t r i g g e r i n g through g r o u n d - v i b r a t i o n s i s probably r e l a t i v e l y r a r e compared to f l u i d shear and h i l l s l o p e f a i l u r e s i n t o channels. Often i t i s p o s s i b l e that two or more of the above-mentioned mechanisms may have acted together to t r i g g e r a d e b r i s t o r r e n t , or i t i s unclear a f t e r the f a c t which one of s e v e r a l mechanisms may have been r e s p o n s i b l e . For example, t o r r e n t s o f t e n accompany in t e n s e rainstorms which a l s o produce e x t e n s i v e d e b r i s s l i d e a c t i v i t y i n t o stream channels, as has happened twice i n recent 196 years i n the v i c i n i t y of Hope, B r i t i s h Columbia (M i l e s and K e l l e r h a l s , 1979; Evans and L i s t e r , 1984). In such cases, any of the f o l l o w i n g t o r r e n t t r i g g e r i n g mechanisms, or some combination, would be p o s s i b l e : ( i ) f l u i d shear due to high r u n o f f a s s o c i a t e d with the rai n s t o r m ; ( i i ) f l u i d shear due to the breaching of a temporary l a n d s l i d e dam i n a channel; ( i i i ) d i r e c t f l u i d i z a t i o n of a h i l l s l o p e f a i l u r e i n t o a channel; or ( i v ) i m p u l s i v e l o a d i n g from a h i l l s l o p e f a i l u r e i n t o a channel. D e t a i l e d s i t e i n v e s t -i g a t i o n s a f t e r a t o r r e n t may or may not enable the t r i g g e r to be d e f i n i t i v e l y i d e n t i f i e d . T h i s problem w i l l be i l l u s t r a t e d i n Se c t i o n 6.2, which d e a l s with p o s s i b l e d e b r i s t o r r e n t t r i g g e r i n g mechanisms i n the study area on the east si d e of Howe Sound. 6.2 TRIGGERING MECHANISMS IN THE STUDY AREA Only some of the t o r r e n t t r i g g e r i n g mechanisms d e s c r i b e d above can p o t e n t i a l l y operate i n the study area. There are only two small l a k e s , i n Magnesia Creek catchment, and these are c o n t r o l l e d by bedrock l i p s , so the c a t a s t r o p h i c lake drainage mechanism i s not p o s s i b l e here. Ground v i b r a t i o n s have not been c i t e d as a t r i g g e r f o r any of the recent events i n the Howe Sound area, but i n a s e i s m i c a l l y a c t i v e area such as c o a s t a l B r i t i s h Columbia there i s no reason to di s c o u n t t h i s as a p o s s i b i l i t y . F l u i d shear due to the breaching of a temporary channel blockage, f l u i d i z a t i o n of h i l l s l o p e f a i l u r e s , and impulsive l o a d i n g from snow avalanches a l l are p o s s i b l e i n the study area, but w i l l be d i s c u s s e d only b r i e f l y below, due to shortage of data. F l u i d shear due to high r u n o f f , and impulsive l o a d i n g by h i l l s l o p e 1 97 f a i l u r e s , are given more d e t a i l e d treatment. 6.2.1 F l u i d Shear due to Heavy Rain or Rain Plus Snowmelt The process (here c a l l e d f l u i d shear) by which high d i s -charge i s a b l e to d e s t a b i l i z e a channel bed, has been d e s c r i b e d by Takahashi (1981). As the depth of water f l o w i n g in a channel i n c r e a s e s , a p o i n t i s reached where the bed becomes u n s t a b l e , and a "sediment g r a v i t y flow" occurs (Takahashi, 1981). The l i m i t i n g channel g r a d i e n t , 8, f o r sediment g r a v i t y flow i s d e f i n e d by Takahashi's equation (3): t a n e - c * ( o - cp)VP(! + h / d ) t a n * ( 6 - i ) where: c* = the g r a i n c o n c e n t r a t i o n by volume i n the s t a t i c d e b r i s bed d = the d e n s i t y of the g r a i n s p = the d e n s i t y of the f l u i d a> = the angle of s h e a r i n g r e s i s t a n c e of the bed m a t e r i a l h = the depth of water, measured normal to the bed d = the diameter of the g r a i n s . T h i s i n e q u a l i t y i s e q u i v a l e n t to equation (5.1) i n Chapter 5 of t h i s t h e s i s , s i n c e p g = Y and gc*(<5 - p ) = (Y - Y ) (Bovis et sat w a l . , 1985). Equation (5.1) i s reproduced here as (6.2): h t + I r ) tane = (Y . - Y ) t a n * (6.2) 'sat d'w sat w where . and are the u n i t weights of s a t u r a t e d d e b r i s and sat w 3 water, r e s p e c t i v e l y . A more complete d e r i v a t i o n of (6.2) from (6.1) i s given i n Appendix 3. Takahashi (1981) d e f i n e s a L as the depth below the s u r f a c e where the a p p l i e d t a n g e n t i a l shear s t r e s s i s equal to the shear s t r e n g t h ( F i g u r e 6.1a). He notes t h a t , 198 Figure 6 . 1 : Stream Bed C o n d i t i o n s Under F l u i d Shear T = a p p l i e d t a n g e n t i a l shear s t r e s s T L = i n t e r n a l shear s t r e n g t h b) Case o f p a r t i a l s a t u r a t i o n ( a f t e r VanDine , 1985)• 199 while the c o n d i t i o n f o r sediment g r a v i t y flow i s a^ z d, the c o n d i t i o n f o r a true d e b r i s flow i s a^ - kh, where k i s a numerical c o e f f i c i e n t e q u i v a l e n t to about 0.7. T h i s requirement i s r e l a t e d to the need f o r g r a i n s to be u n i f o r m l y d i s p e r s e d throughout the whole depth of the flow i n a t r u e d e b r i s flow. A p p l y i n g t h i s c o n d i t i o n , he obtained a second l i m i t i n g equation: tane , c * ( 6 -C*)Vp^ + k - i ) tan*' (6.3) A d e b r i s flow w i l l occur i f equations (6.1) and (6.3) are s i m u l t a n e o u s l y s a t i s f i e d . Equation (6.3) i s c l e a r l y independent of the amount of water in the creek (except as i t a f f e c t s c * ) , but (6.1) and t h e r e f o r e (6.2) i n c l u d e t h i s , through the parameter "h". I f (6.2) i s r e w r i t t e n to express the f a c t o r of s a f e t y , Fs, a g a i n s t bed m o b i l i z a t i o n , as: ' [*sat + V d Y W ] tane ( 6 ' 4 ) i t becomes c l e a r that an i n c r e a s e i n the depth of water, h, w i l l l e a d to a decrease i n the s t a b i l i t y of the debris, l a y e r . As noted i n Chapter 5, an i n c r e a s e i n the stream g r a d i e n t , 6 , w i l l a l s o decrease bed s t a b i l i t y . In a drainage b a s i n whose p r o f i l e i s concave upwards (the usual c a s e ) , these two f a c t o r s tend to oppose each other, s i n c e g r a d i e n t w i l l decrease and depth of flow w i l l i n c r e a s e (assuming minimal i n c r e a s e s i n width and v e l o c i t y ) i n a downstream d i r e c t i o n . I t f o l l o w s that the i n i t i a t i o n p o i n t of a t o r r e n t must be a c e r t a i n d i s t a n c e down-stream of the creek source, to allow f o r s u f f i c i e n t c o n c e n t r a t i o n of r u n o f f from r a i n f a l l and/or snowmelt. If the channel g r a d i e n t , 0, i s c l o s e to the angle of 200 shearing r e s i s t a n c e , 4>' , the channel d e b r i s may be m o b i l i z e d under c o n d i t i o n s of l e s s than f u l l s a t u r a t i o n . F i g u r e 6.1b i l l u s t r a t e s t h i s c o n d i t i o n , where the value of the parameter m v a r i e s from 0.0, f o r completely dry d e b r i s , t o 1.0, f o r f u l l s a t u r a t i o n to the s u r f a c e . I f the e f f e c t s of f r i c t i o n on the channel s i d e s are ignored, the s t a b i l i t y of the d e b r i s can be approximately expressed by the well-known i n f i n i t e slope formula, which can be w r i t t e n as: p _ _ [(1 - m ) Y d + m ( Y s at. ~ y w ) ] tan*' l (. ^ F s " [(1 - m ) Y d + m Y s a t ] tane ( b ' b ) where Y ^ i s the dry u n i t weight of the channel d e b r i s , and a l l other parameters are as d e f i n e d above. I t can be shown that t h i s e x p r e s s i o n i s e q u i v a l e n t to Takahashi's (1981) equation (1). As i n the case of (6.4), t h i s equation a l s o shows that s t a b i l i t y w i l l decrease as the amount of water present i n c r e a s e s . Often d e b r i s t o r r e n t s occur simultaneously i n s e v e r a l b a s i n s in a small area, during moderate to heavy ra i n s t o r m s . Table 2.3 in S e c t i o n 2.7 shows s e v e r a l examples of t h i s i n recent years i n the Howe Sound area, the most recent being the storm of 15 November 1983, which was a s s o c i a t e d with d e b r i s t o r r e n t s on Montizambert, C h a r l e s , and Newman creeks. Church and Desloges (1984) estimate that t h i s event had a 3.5 year recurrence i n t e r v a l . When t o r r e n t events are grouped l i k e t h i s , there i s a strong p o s s i b i l i t y that they were t r i g g e r e d by f l u i d shear r e s u l t i n g from high storm r u n o f f . However, i f e x t e n s i v e h i l l -s lope f a i l u r e s are known to have occured, the p o s i b i l i t i e s of slope f a i l u r e f l u i d i z a t i o n , impulsive l o a d i n g , and temporary channel damming a l s o must be c o n s i d e r e d . In any case, i t i s 201 probable that r a i n f a l l i s the r e a l cause, e i t h e r d i r e c t l y or i n d i r e c t l y . As an example of the s t a b i l i t y of channel d e b r i s under f l u i d shear, the d e b r i s accumulation i n M Creek at the toe of S i t e M2 ( S e c t i o n 4.3.1.2) can be examined. Four to f i v e metres of coarse d e b r i s i s s t o r e d here, on a g r a d i e n t of about 33°. The d e b r i s accumulation i s 15 - 20 m wide at the s u r f a c e . If some assumptions are made about the values of some of the parameters,' the s t a b i l i t y of the m a t e r i a l when f u l l y s a t u r a t e d can be assessed with equation (6.4). A reasonable value f o r V o i s about 37.5 , f o r the reasons given i n S e c t i o n 5.1. The s a t u r a t e d u n i t weight of the m a t e r i a l i s d i f f i c u l t to assess without knowing the p o r o s i t y of the d e b r i s accumulation, but as 3 a f i r s t estimate, Y . = 20 kN/m seems reasonable. Hoek and s a t Bray (1977) suggest that t h i s i s a t y p i c a l value f o r broken g r a n i t i c rock. Using these v a l u e s , and knowing that Y = 9.81 3 kN/m , equation (6.4) shows that the d e b r i s w i l l be unstable even i f h =0, r e g a r d l e s s of the s i z e of the m a t e r i a l as expressed by the parameter d. The same r e s u l t i s obtained f o r any reasonable value of Y s a t ' s o * s concluded that t h i s d e b r i s w i l l be uns t a b l e when only p a r t i a l l y s a t u r a t e d . T h e r e f o r e , the approp-r i a t e equation i s (6.5). I f Y^ = 17.0 kN/m (a reasonable value f o r c r y s t a l l i n e igneous rubble, again from Hoek and Bray, 1976), and i f <|> , 6 , Y s a t ' a n c ^ Y w a r e a s a l ) 0 v e ' (6.5) shows that the d e b r i s i s unstable f o r m > 0.28 (see Table 6.1). T h i s means that i f the 4 m t h i c k d e b r i s accumulation here i s s a t u r a t e d to more than about 1 m above i t s base, i t may be u n s t a b l e . Without 202 doing any h y d r o l o g i c m o d e l l i n g , i t i s d i f f i c u l t to estimate what s o r t of r a i n f a l l or r a i n f a l l p l u s snowmelt event would be r e q u i r e d t o produce t h i s degree of bed s a t u r a t i o n . The drainage 2 area of M Creek above t h i s p o i n t i s about 0.6 km . 6.2.2 F l u i d Shear Due to Breaching of Temporary Channel  Blockage T h i s mechanism i s c e r t a i n l y p o s s i b l e i n the study area, given the l a r g e p o r t i o n s of the channel s i d e s c o n s i d e r e d unstable or p o t e n t i a l l y unstable ( S e c t i o n 4.7). In many p l a c e s the channels are l e s s than 10 metres wide and much l e s s than one metre deep, so i t would not take a p a r t i c u l a r l y l a r g e h i l l s l o p e f a i l u r e to form a temporary dam. As the h i l l s l o p e f a i l u r e s i n c r e a s e i n s i z e , the s i z e of the p o t e n t i a l outflow from a breached dam i n c r e a s e s , but so does the p o t e n t i a l f o r bed d e s t a b i l i z a t i o n through impulsive l o a d i n g . In many cases, i f a d e b r i s t o r r e n t has been t r i g g e r e d by a h i l l s l o p e f a i l u r e , i t may be i m p o s s i b l e to t e l l a f t e r the f a c t which of these two mechan-isms was r e s p o n s i b l e . In the present study, no attempts have been made to model t h i s t o r r e n t t r i g g e r i n g process f o r s p e c i f i c channel segments. 6.2.3 Impulsive Loading From Slope F a i l u r e s Impulsive l o a d i n g from l a n d s l i d e s has been c i t e d as a p o s s i b l e d e b r i s flow t r i g g e r i n g mechanism by Innes (1983) and VanDine (1985a). The r a p i d a p p l i c a t i o n of a l a r g e e x t e r n a l load, i n the form of l a n d s l i d e d e b r i s , onto p r e - e x i s t i n g bed m a t e r i a l c o u l d r e s u l t i n a c o n s i d e r a b l e i n c r e a s e i n shear s t r e s s at the base of the channel d e b r i s . If the a d d i t i o n a l load i s t r a n s -203 f e r r e d e n t i r e l y to the pore water, there w i l l be no accompanying in c r e a s e i n shear s t r e n g t h . The r e s u l t i s a short-term decrease i n the f a c t o r of s a f e t y a g a i n s t m o b i l i z a t i o n , u n t i l the excess pore water pressure i s able t o d i s s i p a t e , and t h i s c o u l d lead to d e b r i s t o r r e n t i n i t i a t i o n . T h i s mechanism has been d e s c r i b e d by Hutchinson and Bhandari (1971), f o r mudflow g e n e r a t i o n , although they s t a t e that the r e s u l t s should be a p p l i c a b l e to other p r o c e s s e s . The author i s not aware of any p u b l i s h e d equations d e s c r i b -ing the s t a b i l i t y of channel d e b r i s under impulsive l o a d i n g . The f o l l o w i n g i s a f i r s t attempt to develop such a set of equations. The i n c r e a s e i n shear s t r e s s at the base of the loo s e channel d e b r i s i s not simply due to the weight of the added m a t e r i a l , but ra t h e r to i t s momentum as i t reaches the stream bed. The f o r c e , F, exerted by the l a n d s l i d e d e b r i s at impact can be expressed as: F = M [2Lg (sing - cosfB tan V ) 1 0 ' 5 ( 6 > g ) where: M = the mass of the l a n d s l i d e d e b r i s L = the d i s t a n c e t r a v e l l e d by the d e b r i s , measured along the slope g = the a c c e l e r a t i o n due to g r a v i t y B = the average slope of the d e b r i s path <t>^  = the r o l l i n g f r i c t i o n angle of the d e b r i s t = the time i n t e r v a l over which the loa d i s a p p l i e d to the creek bed. A complete development of t h i s equation i s given i n Appendix 3. <|>P i s used i n p l a c e of the us u a l angle of she a r i n g r e s i s t a n c e , <)>' , because the d e b r i s i s assumed to r o l l and bound down the 204 slope below the headscarp, r a t h e r than s l i d e . The r e s u l t a n t f o r c e can be broken down i n t o three components, one a c t i n g v e r t i c a l l y , one a c t i n g h o r i z o n t a l l y i n a downstream d i r e c t i o n , and one a c t i n g h o r i z o n t a l l y i n a d i r e c t i o n normal to the stream a x i s . While the t h i r d component may w e l l have an i n f l u e n c e on the s t a b i l i t y of the stream bed, i t i s d i f f i c u l t to handle., i n a c o n v e n t i o n a l two-dimensional s t a b i l i t y a n a l y s i s , and w i l l not be con s i d e r e d f u r t h e r here. The e f f e c t of the other two components i s to change the f a c t o r of s a f e t y a g a i n s t stream bed m o b i l i z a t i o n from that d e s c r i b e d by equation (6.5), f o r the s t a t i c case, t o : F s = [(1 - m )Yd + m ( V s a t - Y w ) ] ztan*' ( f i ? ) [(1 - m )y + m Y Jztane + (tanesing + c o s g c o s a ) F / A u S cl X under impulsive l o a d i n g . In t h i s e quation, z, a , and 6 are as d e f i n e d i n F i g u r e 6.2, A i s the area over which the new d e b r i s i s d e p o s i t e d , i n plan view, and a l l other parameters are as i n equations (6.1) through (6.6). Appendix 3 g i v e s a complete d e r i v a t i o n of (6.7). If the h i l l s l o p e f a i l u r e e n t e r s the channel from a d i r e c t i o n normal to the creek a x i s , with no downstream component, a = 90° and (6.7) s i m p l i f i e s t o : P Q _ [(1 - m ) Y d + m(Y«;at. - Y w ) J z tancK , f i R ) {[ (1 - m ) Y d + m Y s a t ] z + F/A sine}tane K b ' 0 > Hutchinson and Bhandari (1971) note t h a t : "The t r a n s f e r e n c e of t h r u s t forward from a loaded area w i l l , of course, be l i m i t e d by the r e s i s t a n c e to pa s s i v e f a i l u r e at any c r o s s s e c t i o n " (p. 357). Th i s has been n e g l e c t e d i n the development of equations (6.7) and (6.8), so they may be s l i g h t l y c o n s e r v a t i v e . As noted above, once s t a r t e d , the flow process tends to be s e l f - s u s t a i n i n g . A f a c t o r which tends to m i t i g a t e a g a i n s t t h i s process of t o r r e n t i n i t i a t i o n i s the g e n e r a l l y coarse nature of 205 F i g u r e 6 . 2 : Geometry o f Impuls ive L o a d i n g o f Creek Bed M a t e r i a l s By H i l l s l o p e F a i l u r e s Note : diagrams not to s c a l e . 206 d e b r i s accumulations in stream channels, p a r t i c u l a r l y where they have been subjected to normal stream flow f o r some time. If the p o r o s i t y of such a d e p o s i t i s high enough to a l l o w pore water p r e s s u r e s to d i s s i p a t e r a p i d l y , the assumption of undrained l o a d i n g may be negated. As an example of the p o s s i b i l i t y of t o r r e n t i n i t i a t i o n by t h i s process, the reach of M Creek which was examined i n S e c t i o n 6.2.1 can be assessed f o r s t a b i l i t y under impulsive l o a d i n g through a major r o c k f a l l from S i t e M2. To perform t h i s a n a l y s i s , the parameters i n equations (6.6) and (6.7) must be measured or assi g n e d assumed v a l u e s . In (6.6), the value s L = 366 m and 3 = 41° can be measured from F i g u r e 4.4. Vario u s v a l u e s of M, t , and <J>R were used i n the a n a l y s i s . In r e a l i t y , <1>R may be r e l a t i v e l y c o n s t a n t , but i s d i f f i c u l t to measure, so value s of 15° and 25° have been used. <f>p would c e r t a i n l y be much lower than . In (6.7), the values z = 4 m, = 37.5 , 6 = 33 , Y , = 20 S a l kN/m3, y = 17 kN/m3, and " Y = 9.81 kN/m3 were used, as i n S e c t i o n 6.2.1. The parameter m was allowed to vary somewhat, but not above 0.28, which was found e a r l i e r to be the l i m i t i n g value f o r s t a b i l t y under s t a t i c c o n d i t i o n s . The angle a i s about 50° at t h i s l o c a t i o n . The area, A, used here was the present area of the main d e b r i s lobe i n M Creek at the toe of S i t e M2, estimated to be 686 m2. Table 6.1 shows the f a c t o r of s a f e t y , Fs, c a l c u l a t e d from equations (6.6) and (6.7) under the above assumptions, f o r v a r i o u s combinations of M, t , and m. The r a t i o n a l e for s e l e c t i n g these values of M and t i s as f o l l o w s . The volume of the 207 Table 6 . 1 : Examples o f S t a b i l i t y o f D e b r i s i n M Creek at Toe o f S i t e M 2 , Under Impuls ive Load ing from M 2 Assumpt ions : - e q u a t i o n s ( 6 . 6 ) and ( 6 . 7 ) app ly - p a s s i v e r e s i s t a n c e to f a i l u r e can be n e g l e c t e d - s i n g l e r o c k s l i d e event i n v o l v e s 10% o f the u n s t a b l e b l o c k at the s l i d e headscarp - measured parameter v a l u e s : L = 366 m; (? = 4 1 ° ; cc = 5 0 ° ; 6 = 3 3 ° ; z = 4 m , - assumed parameter v a l u e s : 0 = q 3 7 - 5 ; A = 686 m ; T d = 17 k N / m J ; T s a t = 20 kN/rn^ mass, M (kg x 1 0 6 ) t i m e , t (seconds) (degrees) r e s u l t a n t f o r c e , F a (kN x 1 0 3 ) m F a c t o r o f S a f e t y , Fs 0 . 0 0 . 0 b 0 . 10 1.115 0 . 0 b 0 .25 1 . 0 1 8 0 . 0 b 0 . 2 8 1 .000 0 . 0 b 0 .30 0 . 9 8 7 3 - 3 9 60 25 2 . 6 4 0 . 125 1 .019 2 . 6 4 0 •15 1 . 0 0 4 2 . 6 4 0 .20 0 . 9 7 5 3 - 3 9 60 15 3 . 2 3 0 . 10 1 . 0 1 8 3 . 2 3 0 . 125 1 .003 3 . 2 3 0 •15 0 . 9 8 9 2 .54 30 25 3 . 9 6 0 •075 1.012 3 - 9 6 0 . 10 0 . 9 9 8 3-96 0 . 125 0 .984 2 .54 30 15 4 . 8 3 0 . 0 5 1 .003 4 . 8 3 0 .075 0 . 9 8 9 4 . 8 3 0 . 10 0 . 9 7 5 Notes a - f o r c e F c a l c u l a t e d u s i n g e q u a t i o n ( 6 . 6 ) b - s t a t i c case d i s c u s s e d i n S e c t i o n 6 . 2 . 1 . ( 6 . 7 ) i s e q u i v a l e n t to ( 6 . 5 ) When F = 0 , 208 p o t e n t i a l l y unstable block at the top of the r o c k s l i d e has been 3 estimated to be 12,565 m (Section 4.3.1.2). If t h i s m a t e r i a l 3 has a u n i t weight of 26.5 kN/m , a reasonable value for i n t a c t g r a n i t i c rock, the mass of the block would be 3.39 x 1 0^ kg. It was i n i t i a l l y assumed that 10% of t h i s might be i n v o l v e d i n a s i n g l e r o c k f a l l event, which might take one minute to be depos-i t e d in the stream channel. Secondly, i t was assumed that 75% of t h i s r o c k f a l l might a r r i v e at the creek channel in only h a l f that time (Table 6.1). The r e s u l t s presented here are o b v i o u s l y s u b j e c t to a c o n s i d e r a b l e amount of e r r o r , because of a l l the assumptions i n v o l v e d , but they do serve to i l l u s t r a t e the f e a s i b i l i t y of d e b r i s t o r r e n t i n i t i a t i o n through impulsive l o a d i n g , under c e r t a i n c o n d i t i o n s , i n the study area. 6.2.4 Impulsive Loading From Snow Avalanches The p r i n c i p l e s governing impulsive l o a d i n g by wet snow avalanches i n t o stream channels are no d i f f e r e n t from those a s s o c i a t e d with l a n d s l i d e d e b r i s l o a d i n g . T h i s mechanism would be p o s s i b l e where steep, c o n f i n e d channels discharge i n t o channels of lower gr a d i e n t , c o n t a i n i n g abundant loose d e b r i s . Such a s i t u a t i o n e x i s t e d at e l e v a t i o n 650 m i n A l b e r t a Creek, where the stream plunges over a b e d r o c k - c o n t r o l l e d w a t e r f a l l , p r i o r to 11 February 1983. On that date, a wet snow avalanche surged over the w a t e r f a l l and t r i g g e r e d a major d e b r i s t o r r e n t j u s t downstream of that p o i n t (Church and Desloges, 1983). Parts of Magnesia and M Creeks a l s o may be s u s c e p t i b l e to t o r r e n t t r i g g e r i n g through snow avalanching (see Figure 4.23). 209 6.2.5 F l u i d i z a t i o n of H i l l s l o p e F a i l u r e s T o r r e n t t r i g g e r i n g through h i l l s l o p e f a i l u r e f l u i d i z a t i o n i s p o s s i b l e i f there i s abundant water i n the stream, but the slope f a i l u r e does not completely block the channel, nor does i t have s u f f i c i e n t momentum to d e s t a b i l i z e the bed m a t e r i a l through impulsive l o a d i n g . I t i s b e l i e v e d that the 1981 event on M Creek began i n t h i s way, from a r e l a t i v e l y small s o i l wedge f a i l u r e at S i t e M1 ( S e c t i o n 4.4.2.2). A small t o r r e n t was recorded on Magnesia Creek at the same time as the M Creek event (Thurber C o n s u l t a n t s , 1983), but the o b s e r v a t i o n s noted i n S e c t i o n s 2.8.3.2 and 5.2 i n d i c a t e that any t o r r e n t here must have s t a r t e d below the 475 m e l e v a t i o n . I f such an event d i d occur, i t may have been due to f l u i d i z a t i o n of d e b r i s from minor bank slumping or d e b r i s s l i d i n g i n the lower reaches of the creek. 6.3 DISCUSSION It i s apparent from the above d i s c u s s i o n that a number of d e b r i s t o r r e n t t r i g g e r i n g mechanisms have the p o t e n t i a l t o operate in the study area. Many of these i n v o l v e both c l i m a t i c and geomorphic f a c t o r s . For example, heavy r a i n f a l l may be capable of t r i g g e r i n g t o r r e n t s d i r e c t l y through f l u i d shear, or i t may cause h i l l s l o p e f a i l u r e s , which i n turn can become f l u i d i z e d and develop i n t o t o r r e n t s , or can t r i g g e r them through impulsive l o a d i n g or temporary channel blockage. Impulsive l o a d i n g by snow avalanches i s a l s o d i r e c t l y c o n t r o l l e d by c l i m a t i c f a c t o r s , although geomorphic f a c t o r s , such as the g r a d i e n t , l e n g t h , and roughness of the avalanche path are a l s o 210 important. Debris t o r r e n t s i n the study area i n v o l v e a complex c o u p l i n g of c l i m a t i c and geomorphic c o n t r o l s . The s t r o n g i n f l u e n c e of h i l l s l o p e f a i l u r e s on many of the t o r r e n t t r i g g e r i n g mechanisms d e s c r i b e d here suggests that much of what has been presented i n Chapter 4 has i m p l i c a t i o n s not only f o r d e b r i s supply (and thus t o r r e n t magnitude), but a l s o for t r i g g e r i n g events (and thus t o r r e n t f r e q u e n c y ) . Torrent magnitude and frequency are c o n t r o l l e d by a complex i n t e r a c t i o n of d e b r i s supply mechanisms (Chapter 4), d e b r i s r e d i s t r i b u t i o n mechanisms (Chapter 5), and t r i g g e r i n g mechanisms (Chapter 6). As has been shown above, some geomorphic events are capable of p l a y i n g any of these three r o l e s , depending on a number of other c o n d i t i o n s . Small wonder that there has been l i t t l e success i n p r e d i c t i n g d e b r i s t o r r e n t s i n t h i s area with models based p u r e l y on c l i m a t i c and h y d r o l o g i c f a c t o r s ! 21 1 Chapter 7 CONCLUSIONS 7.1 MAJOR FINDINGS OF THESIS The o b j e c t i v e s of t h i s t h e s i s , set out i n S e c t i o n 1.3, were to examine d e b r i s supply mechanisms and r a t e s of d e b r i s supply i n the study area, and the i m p l i c a t i o n s of these on d e b r i s t o r r e n t magnitude and frequency. As d e s c r i b e d i n Chapter 4, a wide v a r i e t y of d e b r i s supply mechanisms was found to operate in 2 a r e l a t i v e l y small area (12.3 km ) c o n s i s t i n g of four watersheds. These mechanisms, and the r a t e s at which they operate, are dependent upon a v a r i e t y of n a t u r a l f a c t o r s , such as c l i m a t e , rock type, s u r f i c i a l geology, hydrology, v e g e t a t i o n , and land g r a d i e n t , and man-controlled f a c t o r s , such as timber h a r v e s t i n g and road c o n s t r u c t i o n . As these f a c t o r s vary throughout the study area, so does the r e l a t i v e importance of each d e b r i s supply mechanism. These f a c t s make i t extremely d i f f i c u l t to g e n e r a l i z e about d e b r i s supply to t o r r e n t - p r o n e channels, even i n a r e l a t i v e l y small area. An important sub-topic of d e b r i s supply i s d e b r i s r e d i s t r i -b u t i o n i n channels. As noted in Chapter 5, creeks which are able to t r a n s p o r t channel d e b r i s short d i s t a n c e s downstream from the supply p o i n t s a r e ; : a b l e t o s t o r e much more m a t e r i a l i n m a r g i n a l l y s t a b l e c o n f i g u r a t i o n s than would be p o s s i b l e otherwise. These creeks may experience l e s s frequent but l a r g e r d e b r i s t o r r e n t s than streams not a c t i v e l y r e d i s t r i b u t i n g t h e i r bedloads. Magnesia Creek i s by f a r the most a c t i v e of the four i n the study area i n terms of d e b r i s r e d i s t r i b u t i o n , and i s p r e s e n t l y 212 s t o r i n g l a r g e amounts of m o b i l i z a b l e m a t e r i a l i n i t s mid reaches. I t has not experienced a major t o r r e n t s i n c e 1962. The i n f l u e n c e of d e b r i s r e d i s t r i b u t i o n on t o r r e n t magnitude and frequency has r e c e i v e d l i t t l e a t t e n t i o n i n the l i t e r a t u r e . A t h i r d important t o p i c i s the i n t e r a c t i o n between d e b r i s supply and t o r r e n t t r i g g e r i n g , d i s c u s s e d i n Chapter 6. Some d e b r i s supply mechanisms, such as r o c k f a l l , r o c k s l i d e , d e b r i s s l i d e , s o i l wedge f a i l u r e , and snow avalanche, are capable of t r i g g e r i n g d e b r i s t o r r e n t s through d e b r i s f l u i d i z a t i o n or i m p u l s i v e l o a d i n g , or by forming temporary channel blockages. H i l l s l o p e f a i l u r e s l a r g e enough to a c t as t o r r e n t t r i g g e r s occur s p o r a d i c a l l y , o f t e n d u r i n g rainstorms of only moderate magnitude (2 - 5 year r e t u r n p e r i o d ) . In other cases, l a r g e r rainstorms may cause only minor slope i n s t a b i l i t y . T o r r e n t i n i t i a t i o n i s not c o n t r o l l e d by c l i m a t i c or h y d r o l o g i c f a c t o r s alone, but r a t h e r i n v o l v e s a complex c o u p l i n g of c l i m a t i c and geomorphic i n f l u e n c e s . Timber h a r v e s t i n g and l o g g i n g road c o n s t r u c t i o n has o f t e n been c i t e d as a major cause of d e b r i s t o r r e n t a c t i v i t y , both i n the s c i e n t i f i c l i t e r a t u r e and i n the news media and general p u b l i c . The e x t e n s i v e l o g g i n g i n Loggers, M, and Magnesia c r e e k s , and the lack of l o g g i n g i n A l b e r t a Creek, a f f o r d s an o p p o r t u n i t y to assess the i n f l u e n c e of such a c t i v i t i e s , i n a q u a l i t a t i v e sense, i n the study a r e a . The tendency f o r c e r t a i n types of h i l l s l o p e f a i l u r e , p a r t i c u l a r l y d e b r i s s l i d e , to i n c r e a s e a f t e r l o g g i n g , has been w e l l documented (e.g. O'Lough-l i n , 1972a; Swanston and Swanson, 1976), and i s i l l u s t r a t e d i n 213 the study area at s i t e s such as Mg3. T h i s i s l a r g e l y a conse-quence of the l o s s of t r e e root s t r e n g t h i n s o i l s on m a r g i n a l l y s t a b l e s l o p e s . Road c o n s t r u c t i o n a l s o has a major i n f l u e n c e on d e b r i s supply, as i l l u s t r a t e d by the numerous small f a i l u r e s in both c o l l u v i a l and rock s l o p e s i n road c u t s , and the f a i l u r e s of road f i l l s and s i d e c a s t m a t e r i a l s . T h i s i s important even in A l b e r t a Creek, which has seen e s s e n t i a l l y no l o g g i n g . Church and Desloges (1984) suggest t h a t a s u b s t a n t i a l p o r t i o n of the m a t e r i a l i n v o l v e d i n the 1983 d e b r i s t o r r e n t i n that creek came from a slope below a log g i n g road ( s i t e A1), where the s t a b i l i t y has been decreased by " b o o t l e g " l o g s k i d d i n g . On the other hand, the l a r g e s t and most impressive s i n g l e d e b r i s source i n the e n t i r e area, the r o c k s l i d e at s i t e M2, occurs i n t e r r a i n which has never been logged. A l s o , the most a c t i v e creek i n the Howe Sound area i n terms of recent d e b r i s t o r r e n t s , C h a rles Creek, has seen only minimal l o g g i n g (1% of the b a s i n ) , and no road c o n s t r u c t i o n . Debris t o r r e n t i s a n a t u r a l phenomenon, but there i s l i t t l e doubt that the process can be aggravated by poor l o g g i n g p r a c t i c e s . 7.2 OUTSTANDING PROBLEMS While there i s f a i r l y c o n v i n c i n g evidence f o r most of the processes proposed i n Chapters 4, 5, and 6, many questions remain. The bi g g e s t s i n g l e problem with t h i s work i s a dearth of data to v e r i f y many of mechanisms. Many important parameters, p a r t i c u l a r l y <|>', are very d i f f i c u l t to measure i n the f i e l d , and have been a s s i g n e d "reasonable" values in the a n a l y s e s . There i s 214 l i t t l e doubt i n the author's mind that the v a r i o u s d e b r i s supply mechanisms do operate i n the manner d e s c r i b e d , but the data problem weakens many of the arguments presented here. For example, the rock mechanics analyses at s i t e s M1 and M2 must be c o n s i d e r e d p r e l i m i n a r y , s i n c e the most a c t i v e rock faces at these s i t e s are not r e a d i l y a c c e s s i b l e , making g e o l o g i c data c o l l e c t i o n both d i f f i c u l t and hazardous. D i s c u s s i o n of t r i g g e r i n g mechan-isms i n Chapter 6 a l s o s u f f e r s from a shortage of q u a n t i t a t i v e i n f o r m a t i o n . For example, the f l u i d shear mechanism has been c o n v i n c i n g l y d e s c r i b e d by others (Takahashi, 1981; VanDine, 1985a), but to prove that i t has operated i n the study area one would need to know not only $ , but a l s o h, the depth of water flo w i n g i n the channel at the time of t o r r e n t i n i t i a t i o n . T h i s would r e q u i r e some s o r t of creek stage r e c o r d e r , or, a l t e r n a t i v e -l y , c a r e f u l h y d r o l o g i c m o d e l l i n g . The d i s c u s s i o n of impulsive l o a d i n g i s a l s o u n c e r t a i n , p a r t i c u l a r l y in the values of the parameters <|> and <t>^ , and i n the l e n g t h of time over which the impulsive l o a d i s a p p l i e d . In summary, a shortage of hard data has f o r c e d much of the i n f o r m a t i o n presented i n t h i s t h e s i s to be q u a l i t a t i v e , which means that some of the c o n c l u s i o n s are o n l y p r e l i m i n a r y . 7.3 IMPLICATIONS OF FINDINGS 7.3.1 L o c a l I m p l i c a t i o n s There are a number of i m p l i c a t i o n s of the f i n d i n g s of t h i s t h e s i s f o r the d e b r i s t o r r e n t problem in the study area, but what f o l l o w s must be c o n s i d e r e d t e n t a t i v e , due to the data 215 problem noted above. The f i r s t concerns the p o t e n t i a l f o r Magnesia Creek to produce a major d e b r i s t o r r e n t , due to the l a r g e amounts of d e b r i s p r e s e n t l y s t o r e d i n the channel. T h i s creek appears to be "primed", and i t seems only a matter of time before a l a r g e t o r r e n t occurs here. Indeed, Thurber C o n s u l t a n t s (1983) rate the p r o b a b i l i t y of t o r r e n t occurrence here as "very h i g h " . T h i s has not gone unnoticed by the M i n i s t r y of Highways, who have r e c e n t l y c o n s t r u c t e d a d e b r i s r e t e n t i o n dam at the apex of the d e b r i s fan. The dam, which i s designed to r e t a i n coarse d e b r i s while a l l o w i n g water to pass, presumably w i l l p r o t e c t both the highway and the r e s i d e n t i a l development below i t , p r o v i d i n g that a proper maintenance program i s c a r r i e d out. As f a r as the author i s aware, t h i s s t r u c t u r e has performed s a t i s f a c t o r i l y to date. Perhaps of more concern i s the s i t u a t i o n i n M Creek, i n p a r t i c u l a r the l a r g e r o c k s l i d e at s i t e M2. T h i s s l i d e may or may not have pla y e d an important r o l e in the 1981 t o r r e n t , but i n any case i t has s u p p l i e d a l a r g e amount of coarse d e b r i s to the creek s i n c e that time. In a d d i t i o n , a t the s l i d e headscarp there 3 i s a l a r g e unstable b l o c k , probably exceeding 10,000 m i n volume. If a l a r g e amount of t h i s block were to f a i l at once, i t c o u l d t r i g g e r a major t o r r e n t through impulsive l o a d i n g . Even without a major f a i l u r e , c ontinued d e b r i s supply from t h i s s i t e may u l t i m a t e l y form an unstable d e b r i s accumulation i n the creek, which c o u l d be m o b i l i z e d by some other means, such as f l u i d shear "or snow avalanche. T h i s i s p u r e l y s p e c u l a t i v e , but i t should not be d i s c o u n t e d as a p o s s i b i l i t y , and c e r t a i n l y 216 warrants more d e t a i l e d i n v e s t i g a t i o n . U n l i k e Magnesia Creek, M Creek does not have a d e b r i s r e t e n t i o n dam on i t s fan, but n e i t h e r does i t have e x t e n s i v e r e s i d e n t i a l development. Follow-ing the 1981 event, a new c l e a r - s p a n highway bridge was b u i l t upstream of the destroyed b r i d g e . While t h i s new s t r u c t u r e would appear to have an adequate opening to pass the "design" d e b r i s t o r r e n t (Thurber C o n s u l t a n t s , 1983), i t i s now c l o s e r to a w a t e r f a l l i n a narrow bedrock canyon j u s t upstream. As p o i n t e d out by Church and Desloges (1984): "Torrents descending these d e f i l e s are extremely powerful-'and may be launched over w a t e r f a l l s . I t i s not obvious that the r e b u i l t highway i s e n t i r e l y safe here" (p. 65). Both of the other two. creeks i n the study area, A l b e r t a and Loggers, have the p o t e n t i a l to produce moderately l a r g e d e b r i s t o r r e n t s , and both are c o n s i d e r e d by Thurber C o n s u l t a n t s (1983) to have a high p r o b a b i l i t y of t o r r e n t occurrence. The fan of Loggers Creek i s undeveloped, except f o r the ra i l w a y and highway c r o s s i n g s , both of which would l i k e l y be destroyed by the design d e b r i s t o r r e n t (Thurber C o n s u l t a n t s , 1983). Presumably the highway bridge i s scheduled to be r e p l a c e d i n the next few year s , as p a r t of an ongoing p r o j e c t to upgrade the e n t i r e highway between Horseshoe Bay and Squamish. C o n s t r u c t i o n a s s o c i a t e d with t h i s p r o j e c t i s p r e s e n t l y o c c u r r i n g along the lower reaches of A l b e r t a Creek, i n the v i l l a g e of L i o n s Bay. Whether t h i s w i l l be s u f f i c i e n t to p r o t e c t r e s i d e n c e s from f u t u r e t o r r e n t a c t i v i t y here remains to be seen, but the new d e b r i s dam at the 700 m l e v e l w i l l c e r t a i n l y h e l p to s t a b i l i z e the upper reaches of the creek, and a f f o r d some p r o t e c t i o n to 217 the v i l l a g e below. 7.3.2 Wider I m p l i c a t i o n s In a broader sense, the most important f i n d i n g of t h i s t h e s i s i s the tremendous range of d e b r i s supply processes and t o r r e n t t r i g g e r i n g mechanisms which seem to operate i n a very small a r e a . The importance of these processes i s h i g h l y v a r i -a b l e , i n both space and time. T h i s p o i n t s to a need f o r c a r e f u l examination of each i n d i v i d u a l b a s i n i n an area i f the d e b r i s t o r r e n t p o t e n t i a l i s to be understood. Such a study should i n c l u d e m o nitoring or repeated o b s e r v a t i o n s of important s i t e s , as i l l u s t r a t e d by the r a p i d development of the r o c k s l i d e at s i t e M2. There i s doubt as to how important a r o l e t h i s s l i d e played in the 1981 t o r r e n t on M Creek, but i t i s now unquestionably the l a r g e s t d e b r i s source i n the watershed. There seems l i t t l e p rospect of employing r e g i o n a l l y - b a s e d c l i m a t o l o g i c a l or h y d r o l -ogic models f o r anything more than p r e l i m i n a r y hazard assessment. D e t a i l e d b a s i n s t u d i e s should be made whenever new development i s contemplated i n mountainous t e r r a i n . T h i s approach has now been adopted i n the Howe Sound area (Thurber C o n s u l t a n t s , 1983), but i t must become more widespread i n B r i t i s h Columbia and elsewhere i n North America as mountain development expands. 218 LIST OF REFERENCES BAGNOLD, R. 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F. 1985a. D e b r i s t o r r e n t s and d e b r i s flows in the Southern Canadian C o r d i l l e r a . Canadian G e o t e c h n i c a l J o u r n a l , 22, pp. 44-68. VANDINE, D. F. 1985b. D e b r i s t o r r e n t s and d e b r i s flows i n the Southern Canadian C o r d i l l e r a : Reply. Canadian G e o t e c h n i c a l J o u r n a l , 22, p. 609. VARNES, D. J . 1 978. Slope movement types and proce s s e s . I_n L a n d s l i d e s , A n a l y s i s and C o n t r o l . E d i t e d by R. L. Schuster and R. J . K r i z e k . N a t i o n a l Academy of S c i e n c e s , T r a n s p o r t a t i o n Research Board, S p e c i a l Report 176, pp. 11-33. WALPOLE, R. E. and MYERS, R. H. 1978. P r o b a b i l i t y and S t a t i s t i c s f o r Engineers and S c i e n t i s t s . Second e d i t i o n . MacMillan P u b l i s h i n g Co., Inc., New York. 223 APPENDIX 1 LABORATORY TESTING OF SOILS T h i s appendix g i v e s d e s c r i p t i o n s of a l l s o i l samples c o l l e c t e d d u r i n g the study, and p r e s e n t s r e s u l t s of a l l l a b o r a -t o r y t e s t i n g of s o i l s . Some of the samples c o l l e c t e d were l a t e r deemed to be not p a r t i c u l a r l y u s e f u l , and t h e r e f o r e were not t e s t e d . A l l t e s t r e s u l t s are summarized i n Table A1.1 at the end of t h i s appendix. None of the r e s u l t s are p l o t t e d here: a l l p e r t i n e n t graphs are i n c l u d e d i n S e c t i o n 2.8.2. B r i e f d e s c r i p t i o n s of the s o i l samples are given below. Each sample number i s p r e f i x e d with the name of the s i t e where i t was c o l l e c t e d , and a l l s i t e s are shown on F i g u r e 2.13. Where i t has been determined, the U n i f i e d S o i l C l a s s i f i c a t i o n (U.S.C.) code i s i n d i c a t e d i n p a r e n t h e s i s a f t e r the sample number (see F i g u r e 2.15 f o r a d e s c r i p t i o n of the U.S.C. system). The U.S.C. code f o r the e n t i r e sample i s given f i r s t , f o l l o w e d by the code f o r the p o r t i o n of the sample f i n e r than 2 mm. A [C] or [T] i n d i c a t e s that t h i s sample i s one of the c o l l u v i u m or b a s a l t i l l samples analysed i n S e c t i o n 2.8.2. DESCRIPTION OF SAMPLES S i t e Mg1 Mg1-1: Apparently impermeable matrix m a t e r i a l with seepage above, from r i g h t bank near toe of s o i l wedge stream channel. Mg1-2 [ C ] : Matrix of 1 m high v e r t i c a l undercut stream bank, near Mg1 -1. Mg1-3: S o i l wedge f a i l u r e d e b r i s near edge of Magnesia Creek. Mg1-3A: De b r i s from same l o c a t i o n , but 30 cm below s u r f a c e . Mg1-4: Stream bed m a t e r i a l , s o i l wedge channel. Mg1-5: S o i l wedge f a i l u r e d e b r i s found on top of l o g approx-imately 5 m above stream channel. 224 -6 (GC/SC) [ T ] : T i l l on stream bed about 1/4 of the way up the s o i l wedge. Not t y p i c a l l y exposed. -7 [ C ] : T y p i c a l matrix m a t e r i a l of s o i l wedge stream bottom. (Framework here i s cobbles and b o u l d e r s ) . -8 [ C ] : Channel bottom m a t e r i a l adjacent to Mg1-7, but not submerged. -9 (GM-GW/SM) [ T ] : Impermeable l a y e r at a seepage p o i n t . -10 [ C ] : Layer above seepage p o i n t at Mg1-9. -11 [ C ] : In excavation i n upper bowl, headscarp area. Layer j u s t above highest seepage p o i n t , 35 cm below s u r f a c e . -12 (GC/CL) [ T ] : Same e x c a v a t i o n , t i l l i n seepage zone. -13 (GL/ML) [ C ] : Surface m a t e r i a l near e x c a v a t i o n . -14 [ C ] : Surface m a t e r i a l from upper bowl. -15: Undisturbed s o i l from headscarp, 15 cm below s u r f a c e . -16: 1-3 cm t h i c k l a y e r of burnt o r g a n i c s exposed in headscarp, 25 cm below s u r f a c e . -17: 10 cm t h i c k c o b b l e - f r e e l a y e r exposed i n headscarp, 30 cm below s u r f a c e . - 1 8 [ C ] : 80 cm t h i c k l a y e r c o n t a i n i n g r o o t s and cob b l e s . Exposed i n headscarp, 70 cm below s u r f a c e . -19 (GP/-): 30 cm t h i c k lense w i t h i n Mg1-18 m a t e r i a l . -20: Small zone of burnt o r g a n i c s . -21 (GP/-): Layer j u s t above t i l l , exposed i n headscarp. -22 (GM-GP/SM) [ T ] : Impermeable t i l l at base of headscarp, with c o n s i d e r a b l e seepage above. -23: 1 m behind headscarp. Depth to t i l l here i s 1.25 m. No sample. -24A (GM/SM) [C ] : Large sample of su r f a c e m a t e r i a l , same l o c a t i o n as Mg1—11. -24B (GM/SM) [C ] : Large sample of su r f a c e m a t e r i a l , same l o c a t i o n as Mg1-11. -25: Sandy s o i l c o n t a i n i n g p i p i n g h o l e s , from headscarp. -26 (GM/SM) [C ] : Large sample of u n f a i l e d m a t e r i a l from headscarp. Mgl-27 (GM-GW/SM) [ C ] : Large sample of u n f a i l e d m a t e r i a l from headscarp. S i t e Mg2 Mg2-1 [ C ] : Surface m a t e r i a l , top of l e f t bank, near blue p a i n t l i n e . Mg2-2 [ C ] : Surface m a t e r i a l , h a l f way up l e f t bank, near blue p a i n t l i n e . Mg2-3 [ C ] : Surface m a t e r i a l , top of r i g h t bank, near blue p a i n t l i n e . Mg2-4 [ C ] : Surface m a t e r i a l , h a l f way up r i g h t bank, near blue p a i n t l i n e . S i t e Mg3 Mg3-1 [ C ] : Surface m a t e r i a l , top of d e b r i s s l i d e No.4. Mg3-2 (GC-GM/SC-SM) [ T ] : Very hard t i l l forming s l i d e plane, d e b r i s s l i d e No.4. Mg3-3 (GM/SM) [ C ] : Surface l a y e r , "dry zone", d e b r i s s l i d e No.4. 225 Mg3-4 [ C ] : Surface l a y e r , "moist zone", d e b r i s s l i d e No. 4. Mg3-5 [C ] : From excavation adjacent to top of d e b r i s s l i d e No.4. 22 cm t h i c k zone o v e r l y i n g hard b a s a l t i l l . Mg3~6: 15 cm t h i c k s u r f a c e o r g a n i c l a y e r , same excavation as Mg3-5. [C]: From headscarp of d e b r i s s l i d e No.6. (GC/CL) [ T ] : Impermeable t i l l , seepage above. S l i d e No.8. (GC-GP/SC) [ C ] : Saturated loose s u r f a c e m a t e r i a l , down-slope of seepage p o i n t , s l i d e No.8. Mg3-10 (GM/SM) [C ] : Unsaturated l o o s e s u r f a c e of seepage p o i n t , s l i d e No.8. Mg3-11A: Large sample of org a n i c s u r f a c e of u n s u c c e s s f u l Mg3-Mg3-Mg3-7 8 9 1-  m a t e r i a l , upslope Mg3-1IB: Mg3 Mg3 Mg3 Mg3 Mg3 Mg3 Mg3 1 2A: gauge t e s t s , Large gauge t e s t s Large RG-2, s i t e m a t e r i a l near r a i n f i e l d permeameter sample RG-2, sample of s i t e o r g a n i c s u r f a c e of u n s u c c e s s f u l m a t e r i a l near r a i n f i e l d permeameter of s u r f a c e m a t e r i a l from slope adjacent to s l i d e No.8, s i t e of f i e l d permeameter t e s t No.2. Large sample of s u r f a c e m a t e r i a l from slope adjacent to s l i d e No.8, s i t e of f i e l d permeameter t e s t No.2. (GM/SM) [C]: Large sample of s u r f a c e m a t e r i a l , same l o c a t i o n as Mg3-10. -13B [ C ] : Large sample of s u r f a c e m a t e r i a l , same l o c a t i o n as Mg3-10. (GM/SM) [ C ] : U n f a i l e d s u r f a c e m a t e r i a l from s l i d e No.8, upslope of standpipes A and C. (GM/SM) [C ] : U n f a i l e d s u r f a c e m a t e r i a l i n co n t a c t with hard t i l l , s l i d e No.8, downslope of standpipe B. (GC/SC) [ T ] : Hard b a s a l t i l l , same s i t e as Mg3-15. -1 2B: -1 3A -14 -15 -16 S i t e A1 A1-1 (GM or GC / SM or SC) [ C ] : Saturated s o i l o v e r l y i n g bedrock, near toe of 46 s l o p e . A1-2 (GM or GC / SM or SC) [ C ] : Surface s o i l at e r o s i o n p i n g r i d . S i t e M1 M1-1 [ C ] : Soil. ( w e a t h e r e d rock) from headscarp. M1-2 [CJ: Permeable s o i l i n g u l l y . M1-3 [CJ: S o i l (weathered rock) on 45 slope at edge of g u l l y . M1-4 [ c j : Saturated s o i l from head of g u l l y , below M1-2. Near S i t e M2 M2-6: Sandy t i l l (?) from a n t i s l o p e scarp at s i t e s M2 and M3 (see F i g u r e 4.4). 975 m, between 226 Table A l . 1 : Summary of Results of Laboratory Testing of S o i l s Sample S i z e F r a c t i o n s (fo ) Percent F i e l d A t t e r b e r g U. S.C No . G r a v e l Sand S i l t C l a y Organics M o i s t - L i m i t s C l a s s i f i c a -and by wt.' ure L i q u i d P l a s t i c t i o n l a r g e r Con-t e n t E n t i r e Sample - 2 mm only Mgl - 1 16 46 Mgl - 2 71 61 24 3 0 77 22 3 Mgl - 3 62 92 28 18 8 49 0 41 41 4 Mgl-3A 85 08 12 40 2 43 0 09 30 .6 MgU4 62 20 32 85 4 35 0 60 4 6 Mgl -5 75 64 17 56 6 53 0 27 35 5 89 9 26 Mgl - 6 60 20 21 21 13 61 4 98 12 4 32 0 4 GC SC Mgl - 7 63 60 34 18 1 60 0 62 1 3 Mgl - 8 74 36 13 84 8 41 3 39 24 6 43 4 Mgl - 9 82 40 10 33 4 79 2 48 38 5 9 32 GM-GW SM Mgl -10 64 69 23 55 10 59 1 17 10 5 47 4 Mgl -11 35 44 . 41 26 18 59 4 71 5 5 37 0 36 Mgl-12 56 07 17 70 19 38 6 85 40 6 2 GC CL Mgl -13 51 48 23 58 16 69 8 25 21 9 38 7 26 8 GL ML Mgl-14 56 99 23 70 13 72 5 59 18 0 Mgl -15 51 87 28 34 16 32 3 47 10 1 33 5 Mgl -16 Mgl -1? ^7 74 31 88 17 71 2 6? 6 7 33 . 1 Mgl-18 72 53 19 3^ 5-63 2 50 13 8 Mgl -19 97 95 40 4 GP Mgl-20 GP Mgl-21 98 03 36 31 0 Mgl -22 58 20 31 56 7 2 88 31 •3 32 3 26 •5 GM-GP SM Mgl-24A 55 53 26 19 11 03 7 25 20 •5 34 9 28 .6 GM SM Mgl-24B 59 72 22 80 10 23 7 25 23 . 2 34 4 28 .6 GM SM Mgl -25 50 58 31 78 16 80 0 84 10 0 41 8 Mgl -26 44 09 40 26 13 25 2 40 23 9 31 3 25 .8 GM SM Mgl -27 63 29 26 91 7 82 1 98 22 •3 3? 1 25 • 7 GM-GW SM Mg2-1 46 62 30 10 20 18 3 10 8 8 36 .8 Mg2-2 71 49 18 93 6 36 3 22 11 8 5 4 Mg2-3 51 04 34 18 13 95 0 83 11 8 15 0 Mg2-4 32 97 40 69 24 60 1 74 8 6 14 .4 Mg3-1 65 35 23 42 9 29 1 94 29 6 Mg3-2 65 95 17 98 11 68 4 39 7 4 24 2 19 9 GC-GM SC-SM Mg3-3 39 56 34 88 19 52 6 04 7 1 24 3 24 .8 GM SM Mg3-4 33 5^ 38 95 21 53 5 98 33 .2 Mg3-5 44 78 30 81 19 11 5 30 Mg3-6 Mg3-7 37 67 39 89 18 08 4 36 8 7 28 9 Mg3-8 44 30 25 85 21 61 8 24 15 9 28 4 18 .8 GC CL Mg3-9 67 50 23 17 7 12 2 21 25 5 29 7 19 • 3 GC-GP SC Mg3-10 36 74 36 18 21 70 5 38 9 3 35 2 44 9 33 .6 GM SM Mg3-U 227 Table A l . l , continued Sample Size Fractions {%) Percent F i e l d Atterberg U . S.C. No . Gravel Sand S i l t Clay Organics Moist- Limits C l a s s i f i c a -and by wt. ure Li q u i d P l a s t i c tion l a r g e r Con-tent Entire Sample -2 mm only Mg3-12 Mg3-13A 48.06 Mg3-13B 51-74 Mg3-14 Mg3-15 Mg3-16 A l - 1 A l - 2 M l - 1 M l - 2 M l - 3 M l - 4 49-25 59-40 66.83 48.23 37.22 42.00 40.54 24.83 30.68 29.98 66.8? 37 66 10 80 3 48 21 9 22 8 20 2 GM SM 35 23 10 18 2 85 18 7 38 47 10 81 1 47 18 7 26 4 25 4 GM SM 27 08 11 33 2 19 24 6 26 7 24 0 GM SM 21 00 9 95 2 22 13 0 24 5 17 3 GC SC 64.42 52.83 57-91 24.22 12.79 15.14 10.37 14.76 11.48 7-39 1.76 2.32 O.38 1-73 0 .63 1.52 12.7 12.7 7-6 3-8 6.8 6.7 34.6 4.9 12.8 48.8 7-9 30.8 GMorGC SMorSC GMorGC SMorSC M2-6 80.06 13.81 5.33 0.80 6.0 2.3 228 APPENDIX 2 FIELD MEASUREMENT OF HYDRAULIC CONDUCTIVITY F i e l d measurements of h y d r a u l i c c o n d u c t i v i t y were made using a f i e l d permeameter borrowed from Peter Buchanan, Depart-ment of G e o l o g i c a l S c i e n c e s , U. B. C. T h i s appendix d e s c r i b e s the design and use of the permeameter, o u t l i n e s a t e s t of i t s accuracy by Buchanan (1983), and g i v e s the r e s u l t s of the f i e l d c o n d u c t i v i t y measurements. DESIGN AND USE OF THE FIELD PERMEAMETER The permeameter i s s i m i l a r to that d e s c r i b e d by Talsma and Hallam (1980). I t s main f e a t u r e s are shown i n F i g u r e A2.1. It comprises two c o n c e n t r i c a c r y l i c tubes, supported by an a d j u s t a b l e base. The inner ( a i r i n l e t ) tube passes through a rubber stopper i n the outer tube. The t e s t . procedure f i r s t r e q u i r e s a hole to be augered of diameter s l i g h t l y l a r g e r than that of the outer tube. The permeameter i s i n v e r t e d and f i l l e d with water, and then p l a c e d i n the hole i n an u p r i g h t p o s i t i o n . As water i n f i l t r a t e s i n t o the ground, the r a t e at which the water l e v e l i n the outer tube drops i s observed, u n t i l a steady rate i s achieved. A graph of r e s e r v o i r l e v e l versus elapsed time i s p l o t t e d ( F i g u r e A2.2). The steady s t a t e d i s c h a r g e , Q, i s determined by m u l t i p l y i n g the ra t e of i n f i l t r a t i o n , i n cm/s, by the f a c t o r 29.694 cm /cm, which i s simply the c r o s s - s e c t i o n a l area of the water r e s e r v o i r minus the outer c r o s s - s e c t i o n a l area of the a i r i n l e t tube. Thus: 229 F i g u r e A2 . 1 : The F i e l d Permeameter 6. 8 2a 1 2 3 4 7 9 1. Rubber s topper 2. A i r i n l e t tube ( O . D . * 1.59 cm) 3 . Water r e s e r v o i r ( O . D . = 6 . 9 8 cm, I . D . = 6 . 3 5 cm, l e n g t h = 160 cm) 4. Graduated s c a l e (cm) w i t h zero a t top 5 . A d j u s t a b l e base 6 . Water l e v e l i n r e s e r v o i r 7- Auger h o l e (2a = 7-62 cm) 8 . A i r bubbles 9. Water l e v e l i n auger h o l e , bottom o f r e s e r v o i r (diagram based on Talsma and H a l l a m , 1980, F i g u r e 1) F i g u r e A2.2: D e t e r m i n a t i o n o f Steady S t a t e I n f i l t r a t i o n o rH CD > CD •H O > CD CD CC CD -P rH O CQ T r a n s i e n t i n f i l t r a t i o n Steady s t a t e i n f i l t r a t i o n E l ap sed time (seconds) - dh (cm) -Q ( c m - V s ) = d t ( s ) x 2 9 . 6 9 4 ( c m V c m ) 230 Q = |£ x 29.694 (A2.1) 3 where: Q = di s c h a r g e i n cm /s dh = drop i n water l e v e l (cm) i n dt seconds. The problem of three dimensional flow from a c y l i n d r i c a l h o l e , in a homogeneous and i s o t r o p i c medium, above the water t a b l e , has been s o l v e d by Glover (1953). In i t s most ge n e r a l form, the equation i s : K = 2?|p [H s i n h " 1 -j* - ( a 2 + H 2 ) 0 - 5 + a] (A2.2) where: K = h y d r a u l i c c o n d u c t i v i t y i n cm/s 3 Q = steady s t a t e d i s c h a r g e i n cm /s H and a are as d e f i n e d i n F i g u r e A2.1. If H > 10a, t h i s equation s i m p l i f i e s t o : K = [ s i n h - 1 ^ - 1 ] (A2.3) If there i s an impermeable boundary at or near the bottom of the hole, flow w i l l be two-dimensional r a t h e r than t h r e e -d i m e n s i o n a l . In t h i s case, the a p p r o p r i a t e equation i s : „ _ 3Q ln(H/a) = TTH (3H + 2S> ( A 2 * 4 ) where S i s the d i s t a n c e from the bottom of the hole to the impermeable boundary (Talsma and Hallam, 1980). TEST OF THE ACCURACY OF THE FIELD PERMEAMETER Buchanan (1983) made ten dete r m i n a t i o n s of the h y d r a u l i c c o n d u c t i v i t y of the Quadra Sands using the f i e l d permeameter. Mi l n e (1977) had made 100 c o n d u c t i v i t y measurements at the same l o c a t i o n by t a k i n g samples and us i n g a l a b o r a t o r y f a l l i n g head t e s t . Table A2.1 summarises the r e s u l t s of these two t e s t i n g programs. The hypothesis that the two samples come from the same 231 normally d i s t r i b u t e d p o p u l a t i o n i s t e s t e d s t a t i s t i c a l l y below. The s t a t i s t i c a l t e s t s used are d e s c r i b e d i n Walpole and Meyers (1978). I n i t i a l l y i t i s assumed that the samples come from two d i f f e r e n t p o p u l a t i o n s . I f i t can be shown that the two popula-t i o n s have the same mean, ju, and v a r i a n c e , o"2 , i t can be conclud-ed that the f i e l d permeameter g i v e s good estimates of the h y d r a u l i c c o n d u c t i v i t y of the Quadra Sands (assuming that Milne's (1977) t e s t s g i v e good r e s u l t s ) . The n u l l h y p o t h e s i s , H Q , that the two p o p u l a t i o n s have the same v a r i a n c e , i s t e s t e d using the F t e s t at the a = 0.05 l e v e l of s i g n i f i c a n c e : ^ 0 : °io = ° 2 o a_ : 0.05 C r i t i c a l Region: F < f l _ a / 2 (u x ,u 2 ) and F > f a / 2 ( u j ,u 2) where Uj = nl - 1 = 99 and u 2 = n 2 - 1 = 9 C. R.: F < 0.44 and F > 3.34 C a l c u l a t i o n s : 2 ( 0 . 0 2 1 7 ) 2 F " it2 " (0.0254) 2 ~ ° * 7 3 C o n c l u s i o n s : Since the c a l c u l a t e d value of F i s not w i t h i n the c r i t i c a l r e g i o n , the n u l l h y p othesis i s accepted, and i t i s concluded that d i 2 = d 2 2 . The n u l l h y p o t h e s i s , H Q , that the two p o p u l a t i o n s have the same mean, can now be t e s t e d , a l s o at the a = 0.05 l e v e l of s i g n i f i c a n c e . Since i t i s known that the two p o p u l a t i o n standard d e v i a t i o n s are equal, the a p p r o p r i a t e t e s t i s the Student's t TABLE A2.1: H y d r a u l i c C o n d u c t i v i t y T e s t s on Quadra Sand Milne Buchanan Sample S i z e : Sample Mean (cm/s): Sample Standard D e v i a t i o n : ni = 100 n 2 = 10 5ci = 0.0444 x"2 = 0.0521 s1 = 0.0217 s 2 = 0.0254 232 test, with a pooled standard deviation, s : H Q: jUj - p 2 = 0 H : p - ;u * 0 a : 0 i 0 5 C r i t i c a l Region; Calculations: s P = Conclusions: t < - t a / 2 and t > t a / where u = n^ + n 2 - 2 = 108 C. R.: t < -1.960 and t > 1.960 (n. - 1 ) S 7 2 + (n, - 1)So 2 n l + n 2 ~ 2 O R (100 - 1)(0.0217)2 + X10 - 1)(0.0254)2 u - ° = 0.220 x i ~ x 2 S P 100 + 1 0 - 2 0.5 1 n. ^ 0.0444 ~ 0.0521 0.0220 = -1.0553 1_ n 2 1 100 1_" 10 0.5 Since the calculated value of t i s not within , the c r i t i c a l region, the n u l l hypothesis i s accepted, and i t i s concluded that Pi = ju2 . The above results suggest that, at least for sand, the f i e l d permeameter should give reasonable estimates of hydraulic conductivity. However, stochastic variation in the measured conductivity values indicates that a number of measurements should be made at each s i t e (Buchanan, 1983). Also, since the Quadra Sand i s probably not a tr u l y homogeneous and iso t r o p i c medium, the results must be treated with caution. In spite of these concerns, the f i e l d permeameter should give good order of magnitude estimates of hydraulic conductivity of near-surface s o i l s i f a number of measurements are made at each s i t e . 233 FIELD MEASUREMENTS OF HYDRAULIC CONDUCTIVITY The f i e l d permeameter was used to estimate the h y d r a u l i c c o n d u c t i v i t i e s of s o i l s at s i t e s Mg1 and Mg3. T h i s s e c t i o n of Appendix 2 g i v e s d e t a i l s of the t e s t r e s u l t s and computations. S i t e Mg1 Seven f i e l d permeameter t e s t s were attempted i n or near the upper bowl at s i t e Mg1. T e s t s 1, 2, and 3 were i n loose f a i l e d m a t e r i a l i n the bowl i t s e l f , and t e s t s 4 through 7 were in u n f a i l e d s o i l adjacent to t h i s bowl. No t e s t s were attempted in the hard b a s a l t i l l , as the auger c o u l d not penetrate t h i s l a y e r . Test no. 5 was aborted a f t e r 12 minutes, when no a p p r e c i -able drop i n the water l e v e l had o c c u r r e d . Table A2.2 g i v e s the r e s u l t s of these t e s t s . For ease of c a l c u l a t i o n , equation (A2.2) TABLE A2.2: F i e l d Permeameter Te s t s at S i t e Mq1 Ma t e r i a 1 Test dh dt Q D T C(X10 4 ) K(x10 4 type no (cm) (min) (cmVs) (cm) (cm) (cm Z ) (cm/s) f a i l e d 1 18.9 15 0.623 28 13 9.208 5.74 2 9.4 12 0.388 40 20 6.096 2.36 3 20.9 6 1 .72 30 15 n/a 3 33.3 average K = 1.38X10~ 3 cm/s, standard d e v i a t i o n = 1 .70x10 u n f a i l e d 4 10.3 10 0.510 30 15 9.208 4.69 5 0.0 12 n/a b II 6 21.2 7 1 .50 30 15 9.208 13.8 7 8.6 12 0.355 27 15 1 2.58 4.46 average K = 7.65X10~ 4 cm/s, standard d e v i a t i o n = 5.33x10 -3 /e y r\ / osxiu a a a O . J J X I O 4 Impermeable boundary at bottom of h o l e . Equation . (A2.4) used, with S = 0. Test aborted a f t e r 12 minutes Average K f o r a l l s i x t e s t s = 1.07x10_ 3 cm/s Standard d e v i a t i o n f o r a l l s i x t e s t s = 1.17x10 234 can be r e w r i t t e n as: K = QC (A2.5) where: C = [H s i n h " 1 - - (H 2 + a 2 ) 0 , 5 + aJ/2TrH 3 (A2.6) Since a i s f i x e d (a = 3.81 cm f o r the auger used i n t h i s t e s t i n g program), C i s a f u n c t i o n of H alone. From F i g u r e A2.1, H = D - T. The parameters dh and dt are determined from F i g u r e A2.3. The p o s s i b i l i t y that the f a i l e d s o i l i n the bowl has a d i f f e r e n t h y d r a u l i c c o n d u c t i v i t y from the u n f a i l e d s o i l adjacent to the bowl was t e s t e d using the same s t a t i s t i c a l t e s t s as i n the pr e v i o u s s e c t i o n . With the l i m i t e d data a v a i l a b l e , i t was not p o s s i b l e to conclude that the c o n d u c t i v i t i e s of the two s o i l s were any d i f f e r e n t . The average h y d r a u l i c c o n d u c t i v i t y f o r the s i x t e s t s i s 1.07x10 cm/s, with a standard d e v i a t i o n of 1.17x10~ 3. S i t e Mg3 Three t e s t s were performed at s i t e Mg3, on an u n f a i l e d slope adjacent to d e b r i s s l i d e No. 8 i n Fi g u r e 4.14. The r e s u l t s of these t e s t s are i n F i g u r e A2.4 and Table A2.3. In TABLE A2.3: F i e l d Permeameter Test s at S i t e Mq3 Test no dh (cm) dt (cm) Q (cm 3/s) D (cm) T (cm) C ( X 1 0 ~ 4 ) (cm ) K ( X 1 0 " 4 (cm/s) 1 2 3 17.3 34.5 5.7 7 8 5 1 .22 2.13 0.564 24 33 41 15.2 15.2 20.0 18.94 7.235 5.683 23.2 15.4 3.21 average K = = 1.39x10 3 cm/s, -3 standard d e v i a t i o n = 1.01x10 235 F i g u r e A 2 . 3 : F i e l d P e r m e a m e t e r T e s t s a t S i t e M g l E l a p s e d T i m e ( m i n u t e s ) a d d i t i o n , two t e s t s were attempted in the r e l a t i v e l y f l a t area behind the top of the creek bank, near r a i n gauge RG-2 (see F i g u r e 4.14). The s u r f a c e here c o n s i s t s of coarse r o c k f a l l d e b r i s ( t y p i c a l l y > 50 cm diameter) o v e r l a i n by a t h i n l a y e r of r o t t i n g logs and other organic d e b r i s . In both cases, the water in the permeameter d r a i n e d away i n s t a n t l y . F i n a l Check T e x t u r a l analyses of the s o i l s at both s i t e s Mg1 and Mg3 have shown them to be predominantly i n the sand s i z e . The permeameter t e s t s y i e l d e d mean h y d r a u l i c c o n d u c t i v i t y values of - 3 _ 3 1.07x10 cm/s and 1.39x10 cm/s r e s p e c t i v e l y f o r the two s i t e s . Freeze and Cherry (1979, p. 29) i n d i c a t e that values on _ 3 the order of 10 cm/s are very reasonable f o r s i l t y sand. A l s o , as noted i n S e c t i o n 4.4.1.2, O'Loughlin (1972a) performed l a b o r a t o r y t e s t s on e i g h t samples of s o i l s i m i l a r to s u r f a c e m a t e r i a l at s i t e s Mg1 and Mg3, and found an average h y d r a u l i c _ 3 c o n d u c t i v i t y of 3.9x10 cm/s, with a standard d e v i a t i o n of - 3 2.4x10 cm/s. These two f a c t s lend a d d i t i o n a l c r e d i b i l i t y to the f i e l d permeameter t e s t r e s u l t s . 238 APPENDIX 3 DERIVATION OF SOME OF THE DEBRIS TORRENT TRIGGERING EQUATIONS IN CHAPTER 6 FLUID SHEAR EQUATION (5.1) AND (6.2) Equation (5.1) and (6.2) (the same equation) was presented i n Bovis et a l . (1985), although no d e r i v a t i o n was g i v e n . It was s t a t e d there that t h i s equation was e q u i v a l e n t to Takahashi's (1981) equation (3). A proof of t h i s i s presented here. Takahashi's equation (3), here c a l l e d (6.1), i s a statement of l i m i t e q u i l i b r i u m f o r the s t a b i l i t y of s a t u r a t e d channel d e b r i s submerged under water of depth h (measured normal to the s u r f a c e ) : [ c * ( c - P ) + P(1 + h/d)]tan8 = c*(<5 - P).tah<(>/ (6.1) where: c* = the g r a i n c o n c e n t r a t i o n by volume i n the s t a t i c d e b r i s bed o" = the d e n s i t y of the g r a i n s p = the d e n s i t y of the f l u i d (j/ = the angle of shearing r e s i s t a n c e of the bed m a t e r i a l 0 = the channel g r a d i e n t d = the diameter of the g r a i n s . Since the d e b r i s i s s a t u r a t e d , the g r a i n c o n c e n t r a t i o n can be s t a t e d as: c* = Vs Vw + Vs where Vs i s the volume of the s o l i d s ( g r a i n s ) and Vw i s the volume of the water. S i m i l a r l y , p = Mw/Vw and 0 = Ms/Vs, where Mw i s the mass of water i n v o l v e d , and Ms i s the mass of the 239 g r a i n s . I t f o l l o w s t h a t : */ _ » Vs Ms _ Vs Mw y p ; (Vw + Vs)Vs (Vw + Vs)Vw Vw Ms _ Vs Mw _ Vw Ms ~ Vs Mw (Vw + Vs)Vw (Vw + Vs)Vw (Vw + Vs)Vw The u n i t weights of water and s a t u r a t e d m a t e r i a l can be expressed as: v „ Mw , n Ms + Mw Yw = 9 W a n d Y s a t = 9 Vs + Vw where g i s the a c c e l e r a t i o n due to g r a v i t y . T h e r e f o r e : Y Y = [Vw(Ms + Mw) ~ Mw(Vs + Vw)] = (Vw Ms - Vs Mw) s a t w g (Vw + Vs)Vw g (Vw + Vs)Vw Comparing t h i s e x p r e s s i o n with the one obtained above shows t h a t : c*«5 - P ) = ( Y S A T - Y w ) / g S u b s t i t u t i n g t h i s i n t o (6.1), and n o t i n g that p = Y../g, y i e l d s : w — Yt — Y s a t . ^ Tw + + Y J J L tane = Y s a t Iw_ tan* o r : ( Y s a t " Yw"|)tane = ( Y s a t - Yw)tan<|> (6.2) IMPULSIVE LOADING EQUATIONS (6.6), (6.7) and (6.8) Consider an a c t i v e l a n d s l i d e i n c l i n e d at an angle to the h o r i z o n t a l . Assume that a l l f a i l u r e s are at the headscarp, a d i s t a n c e L from the toe of the s l i d e , measured along the s l i d e s c a r . T h i s s i t u a t i o n i s i l l u s t r a t e d i n F i g u r e 6.2. M a t e r i a l r e l e a s e d from the headscarp would have an i n i t i a l v e l o c i t y of V q = 0, and a f i n a l v e l o c i t y , V^, given by: 2 2 V * = V c + 2aL t o or: V f = ( 2 a L ) 0 - 5 where a i s the a c c e l e r a t i o n . The a c c e l e r a t i o n w i l l be a f u n c t i o n of g r a v i t y , the slope g r a d i e n t , 3 , and an angle of f r i c t i o n 240 a g a i n s t r o l l i n g d e b r i s , <}>_,: a = (Mg sine - Mg cose tarxjO/M = g(sine - cose tan<f> ) where M i s the mass of the r o l l i n g or s l i d i n g d e b r i s . The value of <(>_ i s d i f f i c u l t to as s e s s , but i t w i l l be l e s s than <(> , the K angle of shearing r e s i s t a n c e . The momentum, P, of s l i d e d e b r i s of mass M as i t reaches the toe of the slope w i l l be given by: P = MVf = M ( 2 a L ) 0 , 5 = M [ 2Lg (sine - cose tan<f> ) ] 0 * 5 The t o t a l f o r c e , F, a p p l i e d to the stream bed at the toe of the s l i d e by the s l i d e d e b r i s having t h i s momentum i s j u s t P/t, where t i s the time i n t e r v a l over which the s l i d e d e b r i s i s a p p l i e d to the creek bed. Ther e f o r e , F i s given by: 0 5 p _ M [2Lg (sine - c o s e t a n ^ ) ] ' ( 6 ^ The f o r c e , F, can be r e s o l v e d i n t o three components: F v , a c t i n g v e r t i c a l l y , F p , a c t i n g h o r i z o n t a l l y and i n a downstream d i r e c t i o n , and F T , a c t i n g h o r i z o n t a l l y and i n a d i r e c t i o n t r a n s v e r s e to the stream a x i s . If a i s the angle between the s l i d e a x i s and the stream a x i s , measured i n the h o r i z o n t a l plane ( F i g u r e 6.2), the three components a r e : F v = Fs i n g Fp = Fcosecosa F T = Fc o s e s i n a F T has only a very small e f f e c t on the s t a b i l i t y of the channel d e b r i s , and w i l l not be co n s i d e r e d f u r t h e r here. In the absence of impulsive l o a d i n g , the s t a b i l i t y of the channel d e b r i s can be expressed by the standard " i n f i n i t e s l o p e " s t a b i l i t y equation, i f the e f f e c t s of f r i c t i o n on the channel 241 s i d e s are n e g l e c t e d : p - _ [(1 - m)Vd + m ( Y Q A T - Y W ) ] t a n ^ , f i ^ F S " [(1 - m)Y D + m Y S A T ] tane ( 6 * 5 ' where Y s a t , Y ^ , and Y^ are, r e s p e c t i v e l y , the u n i t weights of s a t u r a t e d d e b r i s , dry d e b r i s , and water, <f>' i s the angle of s h e a r i n g r e s i s t a n c e of the m a t e r i a l , 6 i s the slope of the channel, and m i s the r a t i o of the t h i c k n e s s of s a t u r a t e d d e b r i s to the t o t a l d e b r i s t h i c k n e s s ( F i g u r e 6.1b). If the numerator 2 and denominator of t h i s e x p r e s s i o n are m u l t i p l i e d by zcos e, where z i s the t h i c k n e s s of the d e b r i s , measured v e r t i c a l l y , (6.5) can be w r i t t e n as Fs = S/T, where S i s the shear s t r e n g t h at the base of the d e b r i s , and T i s the shear s t r e s s at that l o c a t i o n . S and T are thus given by: S = 0 tan<f>' = [(1 - m)^ + M ( Y - Y ) ] z C o s 2 e tand/ d sat w T = [(1 - m)Y + m Y ] z sine cose d sat where d , the e f f e c t i v e s t r e s s , i s equal to 0 - u, i n which 6 i s the t o t a l s t r e s s and u i s the pore water p r e s s u r e . I t i s d e s i r e d to f i n d the e f f e c t s of the f o r c e components F v and F p on S and T. Looking f i r s t at S, i t can be assumed that the i n c r e a s e i n t o t a l s t r e s s , AO", w i l l i n i t i a l l y be borne e n t i r e l y by the pore water p r e s s u r e , u. T h e r e f o r e AO = Au, and a = o + AO" - (u + A U ) = 6 - u as b e f o r e . In other words, i n i t i a l l y there w i l l be no change i n the shear s t r e n g t h , S. In time, the excess pore water pressure w i l l d i s s i p a t e , and S w i l l i n c r e a s e . If the l o a d i s a p p l i e d r a p i d l y , and the pressure d i s s i p a t i o n i s r e l a t i v e l y slow, the l o a d i n g can be assumed to be undrained, and there w i l l be a short-term r e d u c t i o n i n Fs, due to an i n c r e a s e i n T. 242 The shear s t r e s s , T, i s a f f e c t e d by both F v and F p . These f o r c e s must be d i v i d e d by the area over which they are a p p l i e d , to convert them to s t r e s s components. I f A i s the area, measured i n the h o r i z o n t a l plane, the true area i s A/coso. The e f f e c t s of F v and F p on T are : F v sine/(A/cose) and F p cose/(A/cose) R e c a l l i n g the expre s s i o n s f o r F v and F p , the t o t a l e f f e c t on T i s : F F 2 AT = si n e cose sin3 + cos 6 cosot cosB F 2 = -r cos 6 (tanesinB + cosacosg) A Now, the f a c t o r of s a f e t y a g a i n s t m o b i l i z a t i o n under impulsive l o a d i n g i s given by: F s = — S s T + AT [(1 - m)Vd + m(Ysat. ~ > ) ] z c o s 2 e t a n r • [(1 - m)Y d + m Y s a t ] z s i n e c o s e + (p cos 8/A)(tanesinB + cosacosB) D i v i d i n g the numerator and denominator of t h i s l a s t e x p r e s s i o n 2 by cos e y i e l d s : F q = [(1 - m)Y d + m ( Y s a t - Y w ) ] ztanj, ( g _» [(1 - m)Yd + m Y $ a t J z t a n e + (tanesing + cosacos3)F/A If the l a n d s l i d e e n t e r s the creek at r i g h t angles, the angle a i s 90°, and (6.7) can be s i m p l i f i e d t o : v '_ [(1 - m)Y H + m ( Y ^ t - Y w)]ztan<D / , 8 ) b s ~ {[(1 - m)Y d + mY .] z + sinB F/A}tane ( b' a> S cl 243 

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