Open Collections

UBC Theses and Dissertations

UBC Theses Logo

UBC Theses and Dissertations

Debris avalanche and debris torrent initiation, Whatcom County, Washington, U.S.A. Buchanan, Peter 1988

Your browser doesn't seem to have a PDF viewer, please download the PDF to view this item.

Item Metadata

Download

Media
831-UBC_1988_A6_7 B82.pdf [ 11.34MB ]
Metadata
JSON: 831-1.0302653.json
JSON-LD: 831-1.0302653-ld.json
RDF/XML (Pretty): 831-1.0302653-rdf.xml
RDF/JSON: 831-1.0302653-rdf.json
Turtle: 831-1.0302653-turtle.txt
N-Triples: 831-1.0302653-rdf-ntriples.txt
Original Record: 831-1.0302653-source.json
Full Text
831-1.0302653-fulltext.txt
Citation
831-1.0302653.ris

Full Text

DEBRIS AVALANCHE AND DEBRIS TORRENT INITIATION, WHATCOM COUNTY, WASHINGTON, U.S.A. By, PETER BUCHANAN B . S c , M c G i l l U n i v e r s i t y , 1981 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n THE FACULTY OF GRADUATE STUDIES (Department of G e o l o g i c a l Sciences) We accept t h i s t h e s i s as conforming t o the r e q u i r e d standard THE UNIVERSITY OF BRITISH COLUMBIA June, 1988 © Peter Buchanan, 1988 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department of Geological Sciences The University of British Columbia Vancouver, Canada Date June 6, 1988 DE-6 (2/88) ABSTRACT Heavy r a i n f a l l on the evening of January 9 and morning of January 10, 1983 triggered debris avalanches and debris torrents at Smith Creek, western Whatcom County, Washington, USA. Nine debris avalanches are back analyzed i n d e t a i l . Conclusions are drawn concerning, 1) clim a t i c controls on debris avalanches and debris torrents; 2) debris avalanche c h a r a c t e r i s t i c s ; 3) h i l l s l o p e hydrology; 4) slope s t a b i l i t y . R a i n f a l l data show that the January 9-10, 1983 storm had a 71-year recurrence i n t e r v a l i n the 12-hour duration, with less than 6-year recurrence intervals i n 1, 2, and 3-hour durations. In contrast, r a i n f a l l during a torrent event on January 29-30, 1971 had recurrence inte r v a l s of less than 2 years i n a l l durations, but snowmelt was a contributing factor. The types of debris torrents produced by these contrasting storms are discussed. Four d i s t i n c t f a i l u r e geometries are defined, based on avalanche descriptions: 1) wedges; 2) drainage depressions; 3) logging roads; 4) discontinuity surfaces. Three scour zones are also distinguished, based on slope segment types observed. To model storm water table le v e l s a one-dimensional, v e r t i c a l , transient, saturated-unsaturated f i n i t e difference i n f i l t r a t i o n program i s linked to a kinematic wave equation. R a i n f a l l duration and intensity, i n i t i a l conditions, s o i l hydraulic conductivity, and s o i l depth are factors c o n t r o l l i n g v e r t i c a l s o i l discharge rates. January, 1983 discharges are c l e a r l y distinguishable from comparison storm discharges at a l l avalanches. Kinematic wave results help d i f f e r e n t i a t e Coulomb shear and washout type f a i l u r e s , and provide pore pressures for s t a b i l i t y analyses. The modified Mohr-Coulomb strength equation i s used to outline factors c o n t r o l l i n g debris avalanche i n i t i a t i o n . The factors are: 1) slope angle; 2) s o i l depth; 3) s o i l density; 4) vegetative cover; 5) bedrock surface c h a r a c t e r i s t i c s ; 6) snow. These factors are quantitatively assessed. I n f i n i t e slope analyses show l i m i t i n g slope angles of 29.7° for Group I vegetation, and 24.6° for Group III vegetation. Vegetative cover and s o i l depth are the two c o n t r o l l i n g factors that change s i g n i f i c a n t l y over the short term. A root cohesion parameter, C r, i s used to assess the shear strength provided by vegetation. Four vegetative covers are distinguished, three of which were logged between 1918 and 1950: Group I - r e l a t i v e l y weak understory vegetation (C r range: 1.6 -2.0 kPa); Group II - understory plus stunted trees (C r range: 2.3 - 2.6 kPa); Group III - understory plus mixed, regenerating forest (C r range: 2.6 - 3.0 kPa); Group IV - old-growth forest of higher root strength. TABLE OF CONTENTS Page A b s t r a c t i i Table of Contents i i i L i s t of T a b l e s v i i L i s t o f F i g u r e s v i i i L i s t o f Symbols and A b b r e v i a t i o n s x Acknowledgements x i i CHAPTER 1 : INTRODUCTION 1.1 Debris t o r r e n t s i n Whatcom County 1 1.2 Scope of study 2 F i g u r e s 5 CHAPTER 2: PHYSIOGRAPHIC SETTING 2 .1 I n t r o d u c t i o n 7 2 .2 Bedrock geology 8 2 . 3 S e i s m i c i t y 9 2 .4 G l a c i a l d e p o s i t s 10 2 . 5 P o s t - g l a c i a l c l i m a t e 12 2 . 6 S o i l development 12 2 . 7 V e g e t a t i o n 14 2 . 8 Drainage and topography 16 2 . 9 Mass movement 17 F i g u r e 2 0 CHAPTER 3: CLIMATOLOGICAL FACTORS CONTROLLING DEBRIS AVALANCHE AND DEBRIS TORRENT INITIATION 3 .1 I n t r o d u c t i o n 21 3.2 L o c a l m e t e o r o l o g i c a l s t a t i o n s 21 3.3 Storm c h a r a c t e r i s t i c s 22 3.4 Storm frequency analyses 25 3 . 5 Study area p r e c i p i t a t i o n 28 3 .6 Study area snowmelt 3 2 3 .7 R e l a t i o n s h i p between c l i m a t i c data and d e b r i s t o r r e n t c h a r a c t e r i s t i c s and frequency of occurrence 35 T a b l e s 3 7 F i g u r e s 4 2 i i i TABLE OF CONTENTS - Cont'd Page CHAPTER 4: CHARACTERISTICS OF DEBRIS AVALANCHES 4.1 I n t r o d u c t i o n 4 6 4.2 F a c t o r s c o n t r o l l i n g d e b r i s avalanche i n i t i a t i o n 47 4.3 Surveying procedure 50 4.4 Scarp dimensions and s o i l volumes removed 51 4.5 Wedges 51 4.5.1 W-l 52 4.5.2 W-2 53 4.6 Drainage d e p r e s s i o n s 55 4.6.1 DD-1 56 4.6.2 DD-2 58 4.6.3 DD-3 59 4.7 Logging roads 61 4.7.1 LR-1 62 4.7.2 LR-2 64 4.8 D i s c o n t i n u i t y s u r f a c e s 66 4.8.1 DS-1 67 4.8.2 DS-2 68 4.9 Washouts 69 4.10 Rock s l i d e s 70 4.11 D i s c u s s i o n 71 Table 74 F i g u r e s 7 5 CHAPTER 5: ENGINEERING PROPERTIES OF SOILS AT DEBRIS AVALANCHE HEADSCARPS 5.1 I n t r o d u c t i o n 9 0 5.2 G r a i n s i z e d i s t r i b u t i o n s and c l a s s i f i c a t i o n s 91 5.3 D e n s i t y parameters 91 5.4 Shear s t r e n g t h parameters 95 5.4.1 S o i l 95 5.4.2 Roots 98 5.5 H y d r a u l i c c o n d u c t i v i t i e s 98 5.5.1 S o i l m a t r i x 100 5.5.2 Bulk s o i l 102 5.5.3 Rock and c o l l u v i u m / t i l l 103 5.6 S o i l matrix c h a r a c t e r i s t i c curves 104 5.7 Moi s t u r e contents 108 5.8 D i s c u s s i o n 110 Tab l e s 112 F i g u r e s 117 i v TABLE OF CONTENTS, Cont'd Page CHAPTER 6 : HILLSLOPE HYDROLOGY AT AVALANCHE HEADSCARPS 6.1 I n t r o d u c t i o n 124 6.2 I n f i l t r a t i o n program 124 6.3 Assumptions 126 6.4 Input parameters 127 6.5 Drainage c o n d i t i o n t e s t s 128 6.5.1 K s a t - c o n t r o l l e d drainage 129 6.5.2 K ^ ^ - c o n t r o l l e d drainage 131 6.6 E f f e c t of v a r i a t i o n i n input parameters 13 3 6.7.1 R a i n f a l l d u r a t i o n and i n t e n s i t y 13 3 6.7.2 I n i t i a l c o n d i t i o n s 135 6.7.3 S o i l depth 135 6.7.4 H y d r a u l i c c o n d u c t i v i t y 13 6 6.7 Comparison storm d i s c h a r g e r a t e s 13 6 6.8 Kinematic wave equation 138 6.9 Wave v e l o c i t y assessment 141 6.10 Water t a b l e s 14 3 6.10.1 P l a n a r s l o p e water t a b l e p r o f i l e s 144 6.10.2 Drainage d e p r e s s i o n d i s c h a r g e p r o f i l e s 14 6 6.10.3 Drainage d e p r e s s i o n water t a b l e s 148 6.11 S o i l development and storm r u n o f f i m p l i c a t i o n s 150 6.12 C o n c l u s i o n 151 F i g u r e s 153 CHAPTER 7: SLOPE STABILITY ANALYSES 7.1 I n t r o d u c t i o n 170 7.2 Slope s t a b i l i t y program 170 7.3 Assumptions 171 7.4 Input parameters 17 2 7.5 Back analyzed r o o t cohesion 174 7.5.1 R e s u l t s 174 7.5.2 C o n c l u s i o n s and comparison wi t h p u b l i s h e d values..176 7.6 F a i l u r e s u r f a c e geometries 177 7.7 E f f e c t s of f a c t o r s c o n t r o l l i n g i n i t i a t i o n 179 7.7.1 Slope angle 179 7.7.2 S o i l d e n s i t y 180 7.7.3 S o i l depth 180 7.7.4 Root cohesion 181 7.7.5 D i s c o n t i n u i t y s u r f a c e s 182 7.8 Slope s t a b i l i t y on January 9-10, 1983 183 7.9 Slope s t a b i l i t y on December 13-14, 1979 184 7.10 I m p l i c a t i o n s f o r the January 29-30, 1971 storm 185 7.11 D i s c u s s i o n 186 T a b l e s 188 F i g u r e s 19 0 v TABLE OF CONTENTS, Cont'd CHAPTER 8 : CONCLUSIONS AND RECOMMENDATIONS Page 8 . 1 C o n c l u s i o n s 2 0 3 8 . 2 Recommendations 2 0 8 REFERENCES 2 1 1 APPENDICES Appendix I Table 3 . 4 i n Imperial U n i t s 2 1 9 Appendix I I De n s i t y parameter and moisture content e q u a t i o n s . 2 2 0 Appendix I I I Parameter d i s t r i b u t i o n s 2 2 1 1 . Evidence f o r normal d i s t r i b u t i o n i n d e n s i t y parameters 2 2 1 2 . P r o b a b i l i t y p l o t s f o r e and G m 2 2 1 Appendix IV Shear s t r e n g t h parameters 2 3 0 1 . V o i d r a t i o - f r i c t i o n angle r e l a t i o n s 2 3 0 2 . S o i l c ohesion d e t e r m i n a t i o n a t DD-3 2 3 1 Appendix V H y d r a u l i c c o n d u c t i v i t y parameters 2 3 2 1 . Well permeameter e v a l u a t i o n 2 3 2 2 . Cumulative outflow vs time p l o t 2 3 3 3 . K s a-j- temperature adjustment 2 34 4 . H y d r a u l i c c o n d u c t i v i t y of rock m a t r i x 2 34 5 . Hanging water column device 2 34 6. C h a r a c t e r i s t i c curve parameters: m, n, a 2 34 Appendix VI Hydrology a n a l y s i s 23 5 1 . K j j u i ^ c a l c u l a t i o n s 2 3 5 2 . Maximum s u r f a c e water depths 2 3 6 Appendix VII Slope s t a b i l i t y a n a l y s i s 2 3 7 1 . Pore p r e s s u r e s under i n f i n i t e s l o p e seepage 2 37 2 . Surcharge c a l c u l a t i o n s 2 3 7 v i LIST OF TABLES Ta b l e Page 3.1 C l i m a t i c data r e c o r d i n g s t a t i o n l o c a t i o n s 37 3.2 Comparison storm frequency analyses 37 3.3 Study area m o n i t o r i n g r e s u l t s and r a t i o s t o Nooksack Salmon Hatchery r a i n f a l l 38 3.4 Maximum p r e c i p i t a t i o n i n t e n s i t i e s at Nooksack Salmon Hatchery and study area f o r comparison storms 39 3.5 R e g i o n a l c l i m a t o l o g i c a l data b e f o r e and d u r i n g comparison storms 4 0 3.6 Nooksack Salmon Hatchery d a i l y p r e c i p i t a t i o n b e f o r e and d u r i n g comparison storms 41 3.7 Rocky Creek snow course data f o r comparison storms 41 4.1 Scarp dimensions and s o i l volumes removed 74 5.1 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 and T e x t u r a l C l a s s i f i c a t i o n 112 5.2 S p e c i f i c g r a v i t i e s 113 5.3 V o i d r a t i o s , p o r o s i t i e s , and u n i t weights 114 5.4 S o i l shear s t r e n g t h parameters 115 5.5 S o i l h y d r a u l i c c o n d u c t i v i t i e s 116 5.6 F i e l d c a p a c i t i e s and degrees of s a t u r a t i o n 116 7.1 Root cohesion a t f a i l u r e , January 9-10, 1983 188 7.2 S e l e c t e d Ca v a l u e s 188 7.3 Slope s t a b i l i t y on January 9-10, 1983 189 7.4 Slope s t a b i l i t y on December 13-14, 1979 189 v i i LIST OF FIGURES F i g u r e Page 1.1 Western Whatcom County and the study area 5 1.2 S t r u c t u r e of the back a n a l y s i s 6 2.1 Study area geology 2 0 3.1 C l i m a t o l o g i c a l s t a t i o n l o c a t i o n map 42 3.2 Depth-Duration-Frequency curves developed from Nooksack Salmon Hatchery records, 1966-1985 43 3.3 a,b Comparison of Nooksack Salmon Hatchery and study area p r e c i p i t a t i o n , February 20, 1984 44 c,d,e Estimated comparison storm r a i n f a l l i n the study area - hyetographs 44 3.4 Estimated comparison storm r a i n f a l l i n the study area - cumulative curves 45 4.1 Debris avalanche l o c a t i o n map 7 6 4.2 Wedge headscarp geometries 7 7 4.3 Wedges: W-l 78 4.4 Wedges: W-2 79 4.5 Drainage d e p r e s s i o n headscarp geometries 80 4.6 Drainage d e p r e s s i o n s : DD-1 81 4.7 Drainage depressions: DD-2 82 4.8 Drainage d e p r e s s i o n s : DD-3 83 4.9 Logging roads: LR-1 84 4.10 Logging roads: LR-2 85 4.11 D i s c o n t i n u i t y s u r f a c e s : DS-1 86 4.12 D i s c o n t i n u i t y s u r f a c e s : DS-2 87 4.13 Debris scour zones at W-2 88 4.14 Debris scour zones at DS-1 89 5.1 S o i l g r a i n s i z e d i s t r i b u t i o n s 117 5.2 F r i c t i o n angle vs v o i d r a t i o r e l a t i o n 117 5.3 Weighting scheme used t o determine i n t e g r a t e d mean f r i c t i o n angle f o r avalanche W-l 118 5.4 Well permeameter apparatus 119 5.5 S o i l moisture r e t e n t i o n data and f i t t e d curves 12 0 5.6 C h a r a c t e r i s t i c curves: M o i s t u r e content vs p r e s s u r e head 121 5.7 C h a r a c t e r i s t i c curves: R e l a t i v e c o n d u c t i v i t y vs pressure head 122 5.8 Normal p r o b a b i l i t y p l o t s f o r v o i d r a t i o and moisture content a t avalanche LR-1 123 6.1 Flow equations, b a s a l boundary c o n d i t i o n s , and assumptions f o r both drainage cases 153 6.2 Pressure head p r o f i l e s d u r i n g v e r t i c a l i n f i l t r a t i o n t o an impermeable b a s a l boundary 154 v i i i LIST OF FIGURES, Cont'd F i g u r e Page 6.3 Pressure head p r o f i l e s d u r i n g v e r t i c a l i n f i l t r a t i o n t o a d i s c h a r g i n g b a s a l boundary 155 6.4 P r e s s u r e head p r o f i l e s d u r i n g v e r t i c a l d r a i n a g e from a d i s c h a r g i n g b a s a l boundary 156 6.5 E f f e c t of r a i n f a l l d u r a t i o n on d i s c h a r g e r a t e 157 6.6 E f f e c t of r a i n f a l l i n t e n s i t y and i n i t i a l c o n d i t i o n s on d i s c h a r g e r a t e 158 6.7 E f f e c t of s o i l depth on d i s c h a r g e r a t e 159 6.8 E f f e c t of 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 on d i s c h a r g e r a t e 160 6.9 Discharge r a t e s d u r i n g January 9-10, 1983 storm 161 6.10 Discharge r a t e s d u r i n g December 13-14, 1979 storm 162 6.11 Discharge r a t e s d u r i n g January 29-30, 1971 storm 163 6.12 E f f e c t of Kkui^/n on water t a b l e p r o f i l e s 164 6.13 Water t a b l e p r o f i l e s on p l a n a r s l o p e s , January 9-10, 1983 165 6.14 Water t a b l e p r o f i l e s on p l a n a r s l o p e s , December 13-14, 1979 166 6.15 Drainage d e p r e s s i o n d i s c h a r g e , January 9-10, 1983 167 6.16 Drainage d e p r e s s i o n d i s c h a r g e , December 13-14, 1979 168 6.17 Drainage d e p r e s s i o n c r o s s - s e c t i o n s and water t a b l e s 168 7.1 S t a b i l i t y a n a l y s i s : W-l 190 7.2 S t a b i l i t y a n a l y s i s : W-2 191 7.3 S t a b i l i t y a n a l y s i s : DD-1 192 7.4 S t a b i l i t y a n a l y s i s : DD-2 193 7.5 S t a b i l i t y a n a l y s i s : DD-3 194 7.6 S t a b i l i t y a n a l y s i s : LR-1 195 7.7 S t a b i l i t y a n a l y s i s : LR-2 196 7.8 S t a b i l i t y a n a l y s i s : DS-1 197 7.9 S t a b i l i t y a n a l y s i s : DS-2 198 7.10 I n f i n i t e s l o p e a n a l y s i s : F a c t o r of s a f e t y vs s l o p e angle under Group I and Group I I I v e g e t a t i o n 199 7.11 F a c t o r of s a f e t y vs f r i c t i o n angle a t DD-1 and W-l 200 7.10 S t a b i l i t y a n a l y s i s : DD-2 with t h i n s o i l 201 7.11 S t a b i l i t y a n a l y s i s : DS-1 with t h i n s o i l 202 i x LIST OF SYMBOLS AND ABBREVIATIONS U n i t s : [M] - Mass [L] - Length [t] - time S - Shear s t r e n g t h , [ M ] / [ L H t 2 ] r - Shear s t r e s s , [ M ] / [ L ] [ t 2 ] C r - r o o t cohesion, [ M ] / [ L ] [ t 2 ] C - e f f e c t i v e s o i l cohesion, [ M ] / [ L ] [ t 2 ] a - t o t a l normal s t r e s s , [ M ] / [ L ] [ t 2 ] u - pore p r e s s u r e , [ M ] / [ L ] [ t 2 ] 0' - e f f e c t i v e angle of i n t e r n a l f r i c t i o n , [degrees] 0* r - r e s i d u a l e f f e c t i v e angle of i n t e r n a l f r i c t i o n , [degrees] <jP - f r i c t i o n angle w i t h r e s p e c t to changes i n s o i l m a t r i x s u c t i o n when t o t a l s t r e s s i s h e l d c o n s t a n t , [degrees] G - s l o p e angle, [degrees] e - v o i d r a t i o n - p o r o s i t y b - b u l k d e n s i t y , [M]/[L 3] T - u n i t weight a t f i e l d c a p a c i t y , [ M ] / [ L 2 ] [ t 2 ] r d - dry u n i t weight, [ M ] / [ L 2 ] [ t 2 ] T s - s a t u r a t e d u n i t weight, [ M ] / [ L 2 ] [ t 2 ] 9 m - mass based moisture content 6 V - volume based moisture content ®v-sat ~ s a t u r a t e d , volume based moisture c o n t e n t ®v-res ~ r e s i d u a l , volume based moisture content G s - s o l i d s s p e c i f i c g r a v i t y s - sample standard d e v i a t i o n CV - c o e f f i c i e n t of v a r i a t i o n e c r i t i c a l " c r i t i c a l v o i d r a t i o RD - r e l a t i v e d e n s i t y H - t o t a l h y d r a u l i c head, [L] h - p r e s s u r e head, [L] z - e l e v a t i o n head, [L] 1 - downslope d i s t a n c e , [L] K - h y d r a u l i c c o n d u c t i v i t y , [ L ] / [ t ] K s a t ~ s ° i l matrix h y d r a u l i c c o n d u c t i v i t y ( s a t u r a t e d ) , [ L ] / [ t ] Y - l o g K s a t K(h) - s o i l m a t r i x h y d r a u l i c c o n d u c t i v i t y under n e g a t i v e p r e s s u r e heads, [ L ] / [ t ] K b u l k ~ b u l k h y d r a u l i c c o n d u c t i v i t y ( s a t u r a t e d ) , [ L ] / [ t ] Q - d i s c h a r g e , [ L 3 ] / [ t ] q - s p e c i f i c d i s c h a r g e , [ L ] / r t ] A - c r o s s s e c t i o n a l area, [ L 2 ] Q.f H, r, S - parameters f o r w e l l permeameter method, [ L 3 ] / [ t ] , [ L ] , [ L ] , [L] m ; Rt a - parameters f o r p r e d i c t i v e equations f o r c h a r a c t e r i s t i c curves C(h) - s p e c i f i c moisture c a p a c i t y , [ L - 1 ] Q(t) - v e r t i c a l d i s c h a r g e r a t e , [ L ] / [ t ] R - r a i n f a l l r a t e , [ L ] / [ t ] x LIST OF SYMBOLS AND ABBREVIATIONS, Cont'd S e - e f f e c t i v e s a t u r a t i o n h w - s a t u r a t e d zone t h i c k n e s s , [L] t - time, [t] e - e f f e c t i v e p o r o s i t y x - downslope d i s t a n c e from drainage d i v i d e , [L] I - i n p u t r a t e , [ L ] / [ t ] c - average l i n e a r v e l o c i t y , [ L ] / [ t ] L - a c c e p t a b i l i t y c r i t e r i o n FS - f a c t o r o f s a f e t y R u - pore p r e s s u r e r a t i o D - v e r t i c a l s o i l depth, [L] SR - s t r e n g t h r a t i o x i ACKNOWLEDGEMENTS T h i s study c o u l d not have been completed without the p a t i e n c e , support and f i n a n c i a l backing o f Dr. K. Wayne Savigny, A s s o c i a t e P r o f e s s o r i n G e o l o g i c a l Sciences a t UBC. A number of d i s c u s s i o n s w i t h Gerry Thorsen, a G e o l o g i s t a t the Department of N a t u r a l Resources i n Washington, helped t o focus on the problems t h a t needed t o be i n v e s t i g a t e d . Bruce Dagg's p r e c i s i o n was a p p r e c i a t e d i n the f i e l d s u r v e y i n g p a r t o f the t h e s i s . A l s o h e l p f u l were d i s c u s s i o n s w i t h Bruce as he undertook a s i m i l a r study. Dr. Jan De V r i e s , of the Department of S o i l Science at UBC, p r o v i d e d h i s l a b o r a t o r y f o r the s o i l s t e s t s , and ai d e d i n s o r t i n g out the techniques r e q u i r e d t o complete the a n a l y s i s . Thanks a l s o t o the Groundwater Group a t UBC f o r use of t h e i r l a b f a c i l i t i e s and equipment. F i n a l l y , Bruce James and Mrigesh K s h a t r i y a saved me a l o t of time and e f f o r t by p r o v i d i n g computerized s o l u t i o n s t o l a b o r i o u s a n a l y s e s . S p e c i a l thanks t o the Graduate Students i n G e o l o g i c a l S c i e n c e s a t UBC. Support from a number of i n d i v i d u a l s i n t h i s group, e s p e c i a l l y v a r i o u s o f f i c e - m a t e s , i s a p p r e c i a t e d . F i n a l l y , thanks t o my parents and f a m i l y . The study was p a r t i a l l y funded by UBC-NSERC O p e r a t i n g Grant No. 5-81923, and by UBC-NSERC Equipment Grant No. 5-80071. x i i 1 CHAPTER 1: INTRODUCTION 1.1 De b r i s t o r r e n t s i n Whatcom County Heavy r a i n f a l l on the evening of January 9 and morning of January 10, 1983 t r i g g e r e d d e b r i s t o r r e n t s i n creek s of western Whatcom County, Washington, USA. The Whatcom County area and creeks a f f e c t e d by January, 1983 d e b r i s t o r r e n t s a re shown i n F i g u r e 1.1. Most of the t o r r e n t s o c c u r r e d on creek s t h a t d r a i n from Stewart and Lookout Mountains i n t o Lake Whatcom, which i s about 11 km ESE of Bellingham. T o r r e n t s were most severe at the mouths of Smith and Olsen Creeks, on the n o r t h shore of Lake Whatcom. Smith Creek has a h i s t o r y of d e b r i s t o r r e n t a c t i v i t y . Syverson (1984) d e s c r i b e d events i n December, 1917, November, 1949, and January, 1971. NOAA (1983c) d e s c r i b e d the 1983 t o r r e n t , A f o u r f o o t w a l l o f water and d e b r i s crashed down i n t o Lake Whatcom a f t e r heavy r a i n s caused logjams t o break i n Smith and Olsen Creeks which feed i n t o the l a k e . Approximately f i f t y houses were damaged or destroyed by the f l a s h f l o o d and accompanying mudslides, and about one hundred r e s i d e n t s had t o be evacuated ... Estimates of damage are almost twelve m i l l i o n d o l l a r s . A group of landowners f i l e d a damage s u i t a g a i n s t G e o r g i a -P a c i f i c Corp., S c o t t Paper, and the Washington S t a t e Department of N a t u r a l Resources, c l a i m i n g t h a t a number of environmental r e g u l a t i o n s were v i o l a t e d when the b a s i n was logged. T h i s study was undertaken f i r s t t o e x p l a i n how and why the r a i n f a l l t r i g g e r e d the January, 1983 d e b r i s t o r r e n t s . In a d d i t i o n , q u e s t i o n s arose d u r i n g the c o u r t cases t h a t are addressed. The q u e s t i o n s concerned the s e v e r i t y of the January 9-10, 1983 r a i n s t o r m , and the e f f e c t s of l o g g i n g a c t i v i t y on s l o p e s t a b i l i t y . Answers were sought s o l e l y t o f u r t h e r understanding of these events, not t o i n f l u e n c e the l i t i g a t i o n . 1 . 2 Scope of study S i t e v i s i t s t o the Smith and Olsen Creek b a s i n s i n l a t e January and e a r l y February, 1983 r e v e a l e d widespread evidence of mass movements. These were c h a r a c t e r i z e d by f r e s h headscarps and t r a c k s along which a l l s o i l had been scoured t o bedrock. The t r a c k s l e d i n t o e x i s t i n g creeks. Under V a r n e s 1 (1978) c l a s s i f i c a t i o n scheme these f a i l u r e s are termed d e b r i s avalanches. VanDine (1985) d e s c r i b e s the consequent d e b r i s t o r r e n t s as water charged, predominantly c o a r s e g r a i n e d i n o r g a n i c and o r g a n i c m a t e r i a l f l o w i n g down steep, c o n f i n e d p r e -e x i s t i n g channels. Sediment and o r g a n i c m a t e r i a l from the avalanches s u p p l i e d most of the d e b r i s f o r the t o r r e n t s , hence i t was d e c i d e d t o c o n c e n t r a t e on d e b r i s avalanche i n i t i a t i o n . Nine avalanches were s e l e c t e d f o r d e t a i l e d study, r a t h e r than t a k i n g a broader, l e s s d e t a i l e d look a t the much g r e a t e r number t h a t o c c u r r e d . The d e b r i s avalanches i n v e s t i g a t e d were w i t h i n the study area shown i n F i g u r e 1.1. Seven o c c u r r e d i n the Smith Creek b a s i n , where the d e n s i t y of s l o p e f a i l u r e s was h i g h e s t . Another two were found south of Smith Creek on the shore of Lake Whatcom. The avalanches were chosen t o r e p r e s e n t a number of d i f f e r e n t i n i t i a t i o n mechanisms. 3 A f t e r a review of the p h y s i o g r a p h i c s e t t i n g o f the study area i n Chapter 2, the study progresses through d e t a i l e d back analyses of the nine avalanches. The a n a l y s e s focus on the i n f i l t r a t i o n of r a i n f a l l d u r i n g storms, and the consequent water t a b l e b u i l d - u p i n shallow s o i l p r o f i l e s . Root c o h e s i o n v a l u e s are back c a l c u l a t e d . Conclusions are drawn c o n c e r n i n g , 1) c l i m a t i c c o n t r o l s on d e b r i s avalanches and d e b r i s t o r r e n t s ; 2) c h a r a c t e r i s t i c s of d e b r i s avalanches; 3) h i l l s l o p e hydrology; 4) s l o p e s t a b i l i t y . Chapter 3 reviews c l i m a t o l o g i c a l f a c t o r s c o n t r o l l i n g d e b r i s avalanche and d e b r i s t o r r e n t i n i t i a t i o n . Frequency a n a l y s i s are used t o determine the s e v e r i t y of r a i n f a l l and snowmelt t h a t t r i g g e r e d d e b r i s t o r r e n t s i n January, 1971 and January, 1983. Study area p r e c i p i t a t i o n i s estimated f o r use i n l a t e r i n f i l t r a t i o n a n a l y s e s . Chapter 4 i n c l u d e s d e t a i l e d avalanche d e s c r i p t i o n s . F a c t o r s c o n t r o l l i n g avalanche i n i t i a t i o n are reviewed, and scour paths of m o b i l i z e d d e b r i s are d e s c r i b e d . D i s t i n c t i n i t i a t i o n geometries are d e f i n e d . The d e r i v a t i o n of h y d r o l o g i c and s l o p e s t a b i l i t y parameters i s e x p l a i n e d i n Chapter 5, and r e s u l t s are p r e s e n t e d . Chapter 6 addresses h i l l s l o p e hydrology and the pore p r e s s u r e s t h a t t r i g g e r e d the d e b r i s avalanches. R a i n f a l l r e c o r d s , s o i l parameters, and s o i l depths are e n t e r e d i n an i n f i l t r a t i o n model, which i s used t o develop d i s c h a r g e r a t e s through s o i l p r o f i l e s with time. The d i s c h a r g e r a t e s are 4 entered i n a k i n e m a t i c wave equation t h a t generates water t a b l e p r o f i l e s a t the avalanche headscarps. F a c t o r s c o n t r o l l i n g pore p r e s s u r e i n c r e a s e s are assessed. Pore p r e s s u r e s developed d u r i n g the January 9-10, 1983 storm are d i s t i n g u i s h e d from pore p r e s s u r e s d u r i n g l e s s severe storms. In Chapter 7, s l o p e geometry, s t a b i l i t y parameters, and water t a b l e p r o f i l e s from p r e v i o u s chapters are used t o c a r r y out s l o p e s t a b i l i t y a n a l y s es. Root cohesion v a l u e s are back c a l c u l a t e d , and the q u a n t i t a t i v e e f f e c t s of f a c t o r s c o n t r o l l i n g avalanche i n i t i a t i o n are reviewed. F i g u r e 1.2 i s a flow c h a r t summarizing the s t r u c t u r e of the back a n a l y s i s . Chapter 8 summarizes c o n c l u s i o n s which emerge from the a n a l y s e s . Recommendations f o r f u r t h e r study are a l s o o u t l i n e d . The appendices c o n s i s t mainly of c a l c u l a t i o n s used t o d e r i v e a n a l y s i s parameters i n Chapter 5. 5 F i g u r e 1.1: Western Whatcom County and the study area F i g u r e 1 .2: S t r u c t u r e o f t h e b a c k a n a l y s i s 6 I N F I L T R A T I O N H Y D R A U L I C C O N D U C T I V I T Y AND S O I L C H A R A C T E R I S T I C CURVES (5) R A I N F A L L I N T E N S I T Y AND D U R A T I O N ( 3 ) I N I T I A L C O N D I T I O N S (5) S O I L D E P T H ( 4 ) DOWNSLOPE H Y D R A U L I C C O N D U C T I V I T Y (<?) - I N F I L T R A T I O N PROGRAM - ( S O I L D I S C H A R G E RATE) ( 6 ) S L O P E A N G L E ( 4 ) - K I N E M A T I C WAVE E Q U A T I O N - ^ -(WATER T A B L E P R O F I L E ) ( 6 ) S O I L SHEAR STRENGTH (5) S O I L U N I T WEIGHT (5) BEDROCK GEOMETRY AND S O I L P R O F I L E AT H E A D S C A R P ( 4 ) - • S T A B I L I T Y P R O G R A M - * -C7) ROOT COHESION (7) N u m b e r s i n p a r e n t h e s e s a r e c h a p t e r s w h e r e p a r a m e t e r s a n d t e c h n i q u e s a r e d e v e l o p e d a n d / o r a p p l i e d . 7 CHAPTER 2: PHYSIOGRAPHIC SETTING 2.1 Introduction The study area i s s i t u a t e d on the west s l o p e o f the Cascade Mountains, b o r d e r i n g the Puget Sound lowlands. The Cascades are a p h y s i o g r a p h i c s u b d i v i s i o n of the C o r d i l l e r a n Region. Bedrock c o n s i s t s o f the Eocene Chuckanut Formation o v e r l y i n g a f a u l t e d p r e - T e r t i a r y basement. The rocks were f o l d e d a l o n g n o r t h -northwest t r e n d i n g axes i n the Late Eocene. Late T e r t i a r y e r o s i o n reduced outcrop from a broad area c o v e r i n g the Nooksack-S k a g i t R i v e r f l o o d p l a i n s t o a s m a l l e r outcrop b e l t found today. S e v e r a l g l a c i a l advances inundated the area d u r i n g the P l e i s t o c e n e and scoured much of the b a s i n t o bedrock. G l a c i a l d e p o s i t s i n c l u d e d i s c o n t i n u o u s t i l l and outwash. A humid temperate c l i m a t e p r e v a i l e d d u r i n g p o s t - g l a c i a l landscape development. Weathering, f l u v i a l a c t i v i t y , and mass movement were the dominant geomorphic processes. Weathering of the rock produced r e s i d u a l s o i l h i g h on h i l l s l o p e s . C o l l u v i a l s o i l accumulated low on h i l l s l o p e s . C o n i f e r o u s f o r e s t s were e s t a b l i s h e d on both s o i l types. Running water and mass movement continue t o erode the rock, causing steep h i l l s l o p e s and deep stream channels. These p o s t - g l a c i a l geomorphic p r o c e s s e s continue t o shape the c u r r e n t landscape. In a d d i t i o n , the f o r e s t cover i s b e i n g m o d i f i e d by 2 0 t h c e n t u r y l o g g i n g a c t i v i t y . 8 2.2 Bedrock geology The Chuckanut Formation comprises a n o r t h e a s t t r e n d i n g outcrop b e l t running from the coast near Bellingham t o the North Cascades f o o t h i l l s ( F i g u r e 2.1). The u n i t i n c l u d e s sandstone, conglomerate, mudstone, and minor c o a l d e p o s i t e d i n what was an e x t e n s i v e f l u v i a l system i n western Washington. Johnson (1984) separated the formation i n t o 7 members. The o l d e s t , a 3 000 m (9842•) t h i c k s e c t i o n known as the Bellingham Bay Member ( E a r l y Eocene, 49-55 ma), outcrops throughout Stewart Mountain and the Smith Creek b a s i n . Johnson d e s c r i b e d i t as a mea n d e r i n g - r i v e r and adjacent f l o o d p l a i n d e p o s i t with provenance i n the r a p i d l y u p l i f t e d metamorphic t e r r a n e s i n e a s t e r n Washington. The rocks c o n s i s t o f , " a l t e r n a t i n g coarse g r a i n e d (sandstone and minor conglomerate) and f i n e g r a i n e d (mudstone, f i n e - g r a i n e d sandstone, and minor coal) i n t e r v a l s o r g a n i z e d i n t o f i n i n g upward sequences." Bed t h i c k n e s s e s range from .3 m (mudstones) t o 3 m (sandstones). The sandstones are micaceous arkoses c o n s i s t i n g of 35% m o n o c r y s t a l l i n e quartz, 35% p l a g i o c l a s e f e l d s p a r , 10% potassium f e l d s p a r , and 9% mica. A weak s i l i c a cement binds the g r a i n s together. The Chuckanut Formation i s u n d e r l a i n by p h y l l i t e o f undetermined p r e - T e r t i a r y age. The p h y l l i t e i s carbonaceous, w i t h l e n s e s o f quartz m o b i l i z e d d u r i n g metamorphism. Deformations have produced p e n e t r a t i v e cleavage and numerous r e l a t e d f r a c t u r e s i n the rock. The u n i t o u t c r ops a t the southern t i p of Stewart Mountain and f u r t h e r south on Anderson Mountain ( F i g u r e 2.1). Johnson concluded t h a t both u n i t s were openly f o l d e d about north-northwest t r e n d i n g , north p l u n g i n g axes i n the l a t e Eocene. A northwest t r e n d i n g s y n c l i n e a x i s c r o s s e s Smith Creek near i t s a l l u v i a l fan at Lake Whatcom ( F i g u r e 2.1). These f o l d s were r e f o l d e d g e n t l y about northeast t r e n d i n g axes. F a u l t s with minor o f f s e t s a l s o o c c u r r e d . P o o r l y developed d i s c o n t i n u i t i e s are a s s o c i a t e d w i t h the f o l d i n g and with bedding s u r f a c e s . These d i s c o n t i n u i t i e s are not measurably open. At 55 Ma ± 5 Ma B.P. the Chuckanut Formation covered the Nooksack and S k a g i t R i v e r f l o o d p l a i n s up t o 125 km i n l a n d . By 45 Ma ± 5 Ma B.P. e r o s i o n reduced t h i s outcrop area t o a 100 km X 40 km b e l t c o v e r i n g the northwest t i p of p r e s e n t day Washington S t a t e ( H e l l e r , e t a l . , 1987). P l e i s t o c e n e outwash and d r i f t l a t e r covered more outcrop, e s p e c i a l l y on the Nooksack f l o o d p l a i n . Shallow, e r o s i o n r e l a t e d , s t r e s s r e l e a s e j o i n t s occur p a r a l l e l t o the ground s u r f a c e i n the t h i c k sandstone u n i t s . 2.3 S e i s m i c i t y The study area i s l o c a t e d about 200 km e a s t of the i n t e r f a c e between the North American and P a c i f i c t e c t o n i c p l a t e s . In the Puget Sound area s e i s m i c i t y i s caused by the Juan de Fuca p l a t e being subducted beneath the c o n t i n e n t . Both shallo w (15 - 2 0 km) and deep (40 - 50 km) s e i s m i c i t i e s are p o s s i b l e . A s e i s m i c i t y map shows t h a t t h r e e earthquakes of R i c h t e r magnitude 10 3 - 4 have o c c u r r e d i n western Whatcom County s i n c e 1965. Weichert and Rogers (1987) found a 10% p r o b a b i l i t y t h a t peak h o r i z o n t a l a c c e l e r a t i o n s of .16g to .23g w i l l occur i n 50 y e a r s . T h i s puts Smith Creek i n t o Zone 4 s e v e r i t y on t h e i r peak a c c e l e r a t i o n r i s k maps 1. S e i s m i c i t y has not been c i t e d as a t r i g g e r i n g mechanism i n p r i o r d e b r i s avalanche s t u d i e s (Orme, 1987; Syverson, 1984), and i s not c o n s i d e r e d a t r i g g e r i n g mechanism i n t h i s study. 2.4 G l a c i a l d e p o s i t s The area was g l a c i a t e d r e p e a t e d l y i n the P l e i s t o c e n e Epoch. The l a t e stage F r a s e r g l a c i a t i o n (20,000-10,000 y r s B.P.) i s most s i g n i f i c a n t because i t was the l a s t g l a c i a l event t o inundate the area and i t s d e p o s i t s mantle h i l l s l o p e s i n the study area. The Puget lobe of the C o r d i l l e r a n Ice Sheet covered the North Cascades f o o t h i l l s t o a depth of 1740 m (5700') d u r i n g the maximum i c e extent i n the Vashon Stade, about 15,000 y r s B.P. (Easterbrook and Rahm, 1970). The Smith Creek b a s i n was t h e r e f o r e covered t o i t s maximum e l e v a t i o n of 940 m. A t y p i c a l Vashon b a s a l t i l l c o n s i s t s of unsorted, u n s t r a t i f i e d , rounded, pebbles, cobbles, and b o u l d e r s i n a dense matrix of sand, s i l t , and c l a y . The m a t e r i a l i s h e a v i l y o v e r c o n s o l i d a t e d by g l a c i a l i c e l o a d i n g . E a s t e r b r o o k (1963) found t h a t c l a s t s were composed mainly of g r a n o d i o r i t e , 1 - S e v e r i t i e s range from 0 t o 6. Zone 0 s e v e r i t y encompasses h o r i z o n t a l a c c e l e r a t i o n s from 0 t o .04g; Zone 6 s e v e r i t y i s g r e a t e r than .6g. 11 q u a r t z i t e , and c h e r t from the Coast and Cascade mountains i n B r i t i s h Columbia. The matrix c o n s i s t s mainly of a n g u l a r quartz ground t o sand, s i l t , and c l a y s i z e s . G l a c i a l s c o u r i n g and/or p o s t - g l a c i a l e r o s i o n caused t h i n t i l l s e c t i o n s o n l y .5 m (1.6 1) t h i c k t o remain i n hollows on h i l l s l o p e s . T h i c k e r s e c t i o n s were found on s h a l l o w s l o p e s near Smith Creek, below 190 m (600') e l e v a t i o n . R e s i s t a n t bedrock l a y e r s (e.g. massive sandstone) were o f t e n exposed on h i l l s l o p e s . Small outwash fans were a l s o d e p o s i t e d below 190 m e l e v a t i o n i n the b a s i n near the Smith Creek channel. The d e p o s i t s c o n s i s t of w e l l - s o r t e d sand and f i n e g r a v e l washed from l o c a l h i l l s l o p e s , and d e t a i n e d i n the i c e or debris-dammed drainage courses. S e c t i o n s have been p a r t i a l l y eroded s i n c e g l a c i a t i o n , but 10 m t h i c k exposures s t i l l e x i s t . Slope wash and shallow mass movement eroded t h i n g l a c i a l d e p o s i t s immediately f o l l o w i n g the F r a s e r G l a c i a t i o n . C o l l u v i u m was d e p o s i t e d i n bedrock depressions and near stream channels, where i t commonly o v e r l i e s t i l l . The two are of s i m i l a r t e x t u r e and o f t e n can not be d i s t i n g u i s h e d . In some l o c a t i o n s the c o l l u v i u m showed angular c l a s t s dominated by Chuckanut Formation, as opposed to rounded g r a n o d i o r i t e and q u a r t z i t e c l a s t s found i n t i l l . T hicknesses range from a few c e n t i m e t e r s to a maximum of 1 m near stream channels. Ice l o a d i n g and/or d e s i c c a t i o n caused the c o l l u v i u m t o be o v e r c o n s o l i d a t e d . 12 2.5 P o s t - g l a c i a l c l i m a t e A warming c l i m a t e 14,500 t o 15,000 y r s B.P. promoted the r a p i d r e t r e a t of the Puget Lobe of the C o r d i l l e r a n Ice Sheet. Clague and Luternauer (1982) s t a t e t h a t temperatures f l u c t u a t e d as d e g l a c i a t i o n progressed, but remained w i t h i n a few degrees of p r e s e n t v a l u e s . The p r e s e n t c l i m a t e i n the North Cascades f o o t h i l l s i s humid-temperate. G o l d i n (1984) estimated average annual temperature ranges of 4.4°C (40°F) a t 1067 - 1524 m (3500 -5000') e l e v a t i o n s , t o 10°C (50°F) near s e a - l e v e l . P r e c i p i t a t i o n ranges from 2540 mm (100") a t 1067 - 1524 m, t o 889 mm (35") a t sea l e v e l . Syverson (1984) r e p o r t e d t h a t average annual s n o w f a l l ranges from 127 cm (50") a t 170 - 270 m (500 - 800') to l e s s than 25 cm (10") i n v a l l e y s . The wide range of v a l u e s i s caused by 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 . Syverson (1984) a l s o found t h a t 40% of the p r e c i p i t a t i o n f a l l s d u r i n g the w i n t e r months of November, December, and January. At Smith Creek the annual p r e c i p i t a t i o n i s about 1140 mm (45"), the average a i r temperature i s 8.3°C (47°F), and the average f r o s t f r e e p e r i o d i s 140 days (Goldin, 1984) . Weathering, f l u v i a l e r o s i o n and d e p o s i t i o n , and mass movement are the dominant geomorphic processes under these c l i m a t i c c o n d i t i o n s . 2.6 S o i l development Exposed rock i s converted to t h i n r e s i d u a l s o i l by p h y s i c a l and chemical weathering processes. D e t e r i o r a t i o n b egins as 13 s t r e s s r e l e a s e and h y d r o - f r a c t u r i n g (Selby, 1985) cause s u r f i c i a l f r a c t u r e s . Wetting and d r y i n g , f r o s t a c t i o n , and t r e e r o o t s wedge the f r a c t u r e s open and break the rock i n t o fragments. Some rock fragments are t r a n s p o r t e d downslope as c o l l u v i a l d e p o s i t s , w h i l e others d i s i n t e g r a t e i n t o i n d i v i d u a l g r a i n s when f e l d s p a r s and mica are c h e m i c a l l y weathered by o r g a n i c a c i d s and/or hydrated. Chemical weathering does not, however, produce a s i g n i f i c a n t r e s i d u a l c l a y m i n e r a l content. S o i l p r o f i l e s t y p i c a l l y c o n s i s t of .05 m (2") of l e a v e s and twigs (LFH horizon) o v e r l y i n g .74 m (29") of sand, loamy sand, or sandy loam (A and B h o r i z o n s ) . The combined A and B h o r i z o n t h i n s t o .3 m (12") and t h i c k e n s t o 1.4 m (55"), depending on sl o p e geometry. The h o r i z o n s form a continuous l a y e r which i s i n t e r r u p t e d by o c c a s i o n a l s u r f a c e rock outcrop. G r a v e l - s i z e weathered rock fragments t o 35 - 50% by weight are found throughout the p r o f i l e (Goldin, 1984). Macropores r e s u l t from i n s i t u r o o t decay and subsurface e r o s i o n , o r p i p i n g . The B h o r i z o n i s u n d e r l a i n by a t h i n , d i s c o n t i n u o u s C h o r i z o n of p a r t l y weathered rock fragments. The h o r i z o n c o n s i s t s of g r a v e l - s i z e fragments i n t e r f i n g e r e d w i t h sand, s i l t , c l a y , and o r g a n i c matter. The C h o r i z o n o f t e n o c c u r s as an i n d i s t i n c t , l e s s than .05 m (2") l a y e r , but c o l l u v i a l accumulations t o .5 m (20") can occur i n the axes of drainage d e p r e s s i o n s . H y d r a u l i c a l l y eroded macropores sometimes occur at the base of the h o r i z o n . A c l e a r l y d e f i n e d bedrock boundary i s found a t the base of the s o i l p r o f i l e . 14 P r o f i l e development i s poor on c o l l u v i u m and t i l l . A f t e r g l a c i a t i o n these dense d e p o s i t s were loosened somewhat by w e t t i n g and d r y i n g , t r e e r o o t p e n e t r a t i o n , and f r o s t a c t i o n . However, s o i l development on c o l l u v i u m was i n h i b i t e d by i t s continued downslope movement and a d d i t i o n a l i n p u t from upslope sources. The s i l i c e o u s , cohesive t i l l m a trix a l s o r e s i s t e d chemical breakdown i n t o component g r a i n s . P r o f i l e s t y p i c a l l y c o n s i s t of .05 m (2") o f twigs and l e a v e s o v e r l y i n g .2 m (8") of l o o s e , c o h e s i o n l e s s sandy loam and r o o t s . Below t h i s about .5 m (20") of dense sandy loam w i t h cohesion grades i n t o unweathered c o l l u v i u m or t i l l . G r a v e l - s i z e rock fragments t o 35 - 50% occur throughout the p r o f i l e . Macropores and a w e l l d e f i n e d C h o r i z o n are not as common. 2.7 V e g e t a t i o n V e g e t a t i o n was q u i c k l y e s t a b l i s h e d a f t e r g l a c i a l r e t r e a t . Clague and Luternauer (1982) s t a t e t h a t 12,700 y r s B.P. a r b o r e a l v e g e t a t i o n dominated by Pinus c o n t o r t a (Lodgepole pine) was growing i n the southern Coast Mountains i n B r i t i s h Columbia. Shade i n t o l e r a n t p i n e was r e p l a c e d by Abies ( f i r ) , P i c e a (spruce), and Tsuga (hemlock). In the study area, Tsuga and Thuja (cedar) predominated. At about 10,500 y r s b.p. abundant Pseudotsuga m e n z i e s i i ( D o u g l a s - f i r ) appeared, and o t h e r s p e c i e s decreased i n number. The emergence of Pseudotsuga i n d i c a t e d t h a t c o n d i t i o n s became warmer and somewhat d r i e r than e a r l i e r . Old-growth f o r e s t s today c o n s i s t mainly of Pseudotsuga, with 15 Tsucra h e t e r o p h y l l a (western hemlock) and Thuja p l i c a t a (western r e d c e d a r ) . Trees i n these stands can reach 50 m (165 1) i n he i g h t , w i t h diameters 1.5 m (5') or g r e a t e r . P o l y s t i c h u m  minuitum (common swordfern) makes up the u n d e r s t o r y . F r a s e r (1986) found t h a t 5% of the Smith Creek b a s i n i s covered by o l d growth f o r e s t . Another 18% of the b a s i n has never been logged, but has been d i s t u r b e d by a c a t a s t r o p h i c event such as f i r e . Logging a c t i v i t y has a l t e r e d the f o r e s t cover over 77% of the b a s i n . Most s i g n i f i c a n t i s the p e r i o d 1918-1950, when 67% of the b a s i n was logged (47% - c l e a r c u t , 2 0% - p a r t i a l c u t ; F r a s e r , 1986). About 35% of the b a s i n was logged between 1943-1950. Regenerating f o r e s t s are pioneered by Alnus r u b r a (red a l d e r ) . Alnus found on d e b r i s avalanche headscarps grows a t r a t e s up t o .6 m/yr ( 2 ' / y r ) - The Alnus f o r e s t s g i v e way t o shade t o l e r a n t mixed f o r e s t s of Tsucra, Pseudotsuqa, and Alnus. On some dry, stony s i t e s Arbutus m e n z i e s i i (madrone) has regenerated. Trees are t y p i c a l l y 15 - 30 m (49 - 9 8 ' ) i n he i g h t , w i t h diameters of about .3 m ( l 1 ) . The u n d e r s t o r y c o n s i s t s o f Polystichum. Acer c i r c i n a t u m (vine maple), Vaccinium  p a r v i f o l i u m (red h u c k l e b e r r y ) , and G a u l t h e r i a s h a l l o n ( s a l a l ) . Mixed f o r e s t s cover the area logged from 1918-1950. On some abandoned l o g g i n g roads Alnus f o r e s t s r e s i s t the c o n i f e r s u c c e s s i o n . Between 1951-1979 another 10% of the b a s i n was c l e a r c u t ( F r a s e r , 1986). Most of t h i s area was cut near the summit r i d g e of Stewart Mountain and i s r e g e n e r a t i n g s l o w l y because of i t s exposed l o c a t i o n . Scrub c o n i f e r s , Gramineae ( g r a s s ) , Carex (sedge) and l o g g i n g s l a s h cover the area. 2.8 Drainage and topography F l u v i a l e r o s i o n c o n t r o l l e d study area s l o p e development b e f o r e and a f t e r g l a c i a t i o n . The r e s u l t s are V-shaped v a l l e y s t y p i c a l of downcutting streams i n moist, temperate c l i m a t e s (Morisawa, 1968). Streams are deeply i n c i s e d i n the 13 km2 (5 mi 2) Smith Creek b a s i n , with 30° - 45° s i d e s l o p e s common. E l e v a t i o n d i f f e r e n c e s of 390 m (1280 1) can occur between r i d g e s and v a l l e y bottoms i n the midst of the b a s i n . Deep i n c i s i o n occurs because the Chuckanut Formation i s a low p e r m e a b i l i t y u n i t t h a t encourages shallow h i l l s l o p e r u n o f f r a t h e r than i n f i l t r a t i o n . Runoff c o n c e n t r a t i o n i n streams i n c i s e s channels, e s p e c i a l l y d u r i n g extreme events. Smith Creek's d e n d r i t i c drainage p a t t e r n i n d i c a t e s t h a t bedrock s t r u c t u r e does not c o n t r o l b a s i n s c a l e stream flow. However, t h i c k , massive sandstone and conglomerate beds cause w a t e r f a l l s and c o n t r o l drainage on a l o c a l s c a l e . Study area s l o p e s can be d i v i d e d i n t o t h r e e d i s t i n c t segments. The f i r s t segment i n c l u d e s p l a n a r s l o p e s near drainage d i v i d e s , which are o v e r l a i n by r e s i d u a l s o i l . Moving downslope, "drainage d e p r e s s i o n s " (Swanston, 1970) develop. Drainage d e p r e s s i o n s concentrate shallow groundwater flow and 17 s u r f a c e flow, and are f i l l e d with r e s i d u a l s o i l and c o l l u v i u m . The d e p r e s s i o n a x i s i s d e f i n e d i n t h i s study as the l o c u s of low p o i n t s i n the d e p r e s s i o n . The d e p r e s s i o n s have a l s o been l a b e l e d "zero o r d e r b a s i n s " (Tsukamoto, e t a l . , 1982), "wedges" (Humphrey, 1982), and "unchanneled v a l l e y s " ( D i e t r i c h , e t a l . , 1986). The t h i r d segment i n c l u d e s f i r s t o r d e r channels below the d e p r e s s i o n s , t h a t l e a d t o a l o c a l base l e v e l such as Smith Creek, i t s branches, or Lake Whatcom. The second and/or t h i r d segments may be absent from a given s l o p e . A s i m i l a r scheme was o u t l i n e d by Tsukamoto and Minematsu (1987). 2.9 Mass movement A range of mass movement types m o d i f i e d the p o s t - g l a c i a l landscape. Slope f a i l u r e s i n c l u d e d d e b r i s avalanches, stream washouts, and rock s l i d e s . During s i n g l e r a i n f a l l - s n o w m e l t events, numerous d e b r i s avalanches and stream washouts removed s o i l , c o l l u v i u m , and a l l u v i u m from s l o p e s . C o l l u v i u m from these events continued down channels as d e b r i s t o r r e n t s . D e b r i s e r o s i o n helped i n c i s e the channels, c a u s i n g bank s l o p e angles of 50° or g r e a t e r i n rock. Debris t o r r e n t s were mainly r e s p o n s i b l e f o r the b u i l d i n g of an a l l u v i a l fan i n t o Lake Whatcom. Rock s l i d e s were not as common and probably c o n t r i b u t e d l e s s m a t e r i a l t o the t o r r e n t s . The events of January 9-10, 1983 are t y p i c a l of such mass movements. The process began with Coulomb f a i l u r e of the s o i l and v e g e t a t i o n a t numerous s i t e s . I n i t i a l s c a r p s i z e s were 18 l i m i t e d t o 10 - 20 m (33 - 66') i n l e n g t h by a r e s i s t a n t l a y e r at a shallow depth. Bedrock, unweathered c o l l u v i u m / t i l l , and some coarse C h o r i z o n m a t e r i a l were l e f t a t the headscarps. The f a i l e d m a t e r i a l s m o b i l i z e d q u i c k l y and scoured s o i l and v e g e t a t i o n from rock and c o l l u v i u m / t i l l as i t p r o g r e s s e d downslope. Channel sediments and o r g a n i c d e b r i s were sometimes m o b i l i z e d by shallow groundwater and s u r f a c e water, without Coulomb f a i l u r e o c c u r r i n g upslope. VanDine (1985) found two processes r e s p o n s i b l e f o r sediment movement by water i n a f i r s t -o r d e r or g r e a t e r channel. As the s u r f a c e water l e v e l r i s e s , g r a i n s on the bed are eroded by v i s c o u s drag. F u r t h e r water l e v e l r i s e causes m o b i l i z a t i o n and flow of a l l or p a r t of the creek bed. Takahashi (1981) developed a s e r i e s of equations p r e d i c t i n g creek bed m o b i l i z a t i o n , which he termed a sediment g r a v i t y flow. In t h i s study, e r o s i o n and/or m o b i l i z a t i o n of a stream bed i s c a l l e d a washout. In r a r e r cases rock s l i d e s o c c u r r e d a l o n g d i s c o n t i n u i t i e s or weak l a y e r s such as mudstone or c o a l . Varnes (1978) c l a s s i f i e s them as rock b l o c k s l i d e s i f the mass remains i n t a c t , or rock s l i d e s i f the mass d i s i n t e g r a t e s . One example of each was found i n and near the study area a f t e r the January, 1983 storm, but more may have o c c u r r e d . F a i l e d d e b r i s c o a l e s c e d i n Smith Creek and i t s t r i b u t a r i e s and flowed as d e b r i s t o r r e n t s i n t o Lake Whatcom. Remnant d e b r i s dams i n the Smith Creek channel i n d i c a t e t h a t a number of dams were formed and breached as the d e b r i s surged downstream. Deposits i n the channel and on an a l l u v i a l f a n a t Lake Whatcom were made up of mainly of woody d e b r i s , w i t h s m a l l e r volumes of rock fragments and s o i l . 20 F i g u r e 2.1; Study area geology Chuckanut Formation i s the s t i p p l e d p a t t e r n , from Johnson, 1984. - r Qud Quaternary deposits- « — — — — fault, dashed where inferred undifferentiated , . contact, dashed where inferred Th dotted where covered Huntingdon Formotion p gradational contact Tcbm Bald Mountain Member —J—» anticline axis, showing plunge c o Tew Warrick Member — » syncline axis, showing plunge Formr. Tcm Maple Falls Member | 5km | kanut Tcs Slide Member -Chuc Tcp Padden Member Teg Governors Point Member Tcb Bellingham Bay Member PT pre-Tertiary rock e 21 CHAPTER 3: CLIMATOLOGICAL FACTORS CONTROLLING DEBRIS AVALANCHE AND DEBRIS TORRENT INITIATION 3.1 I n t r o d u c t i o n Major p r e c i p i t a t i o n and snowmelt events t r i g g e r d e b r i s avalanches and t o r r e n t s i n Whatcom County. The main purpose of t h i s chapter i s t o ev a l u a t e p r e c i p i t a t i o n r e c u r r e n c e i n t e r v a l s and snowmelt magnitudes d u r i n g storms t h a t i n i t i a t e d d e b r i s t o r r e n t s i n the study area. The January, 1983 and January, 1971 storms are i n v e s t i g a t e d along with a moderate i n t e n s i t y December, 1979 storm t h a t d i d not i n i t i a t e d e b r i s t o r r e n t s . A second purpose i s t o estimate p r e c i p i t a t i o n i n t e n s i t i e s and snowmelt magnitudes i n the study area d u r i n g t h e s e storms. Estimates must be made because no m e t e o r o l o g i c a l s t a t i o n s occur i n the immediate v i c i n i t y . 3.2 L o c a l m e t e o r o l o g i c a l s t a t i o n s R a i n f a l l a n a l y s e s f o r t h i s study are based on a f i x e d -i n t e r v a l , r e c o r d i n g p r e c i p i t a t i o n gauge a t Nooksack Salmon Hatchery, about 21 km (13 miles) n o r t h - n o r t h e a s t o f the study area, near K e n d a l l , Washington. The gauge e l e v a t i o n o f 134.1 m (410') i s w i t h i n the e l e v a t i o n range of 94 - 515 m (307 - 1690') f o r avalanches i n v e s t i g a t e d i n t h i s study, and i t i s the gauge c l o s e s t t o the study area. S t a t i o n l o c a t i o n data are summarized i n Table 3.1 and F i g u r e 3.1. Records are p u b l i s h e d monthly by the N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n as, Hourly  P r e c i p i t a t i o n Data f o r Washington S t a t e . The gauge went i n t o 22 o p e r a t i o n i n 1964, but complete records were not kept u n t i l 1966. Records f o r the 19-year p e r i o d from 1966-1985 were used i n the p r e c i p i t a t i o n frequency a n a l y s i s . Other c l i m a t o l o g i c a l data i n c l u d i n g d a i l y p r e c i p i t a t i o n , s n o w f a l l , snow on ground, and maximum/minimum temperatures were recorded d a i l y i n Whatcom County. These data are p u b l i s h e d monthly by the N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n as, C l i m a t o l o g i c a l Data f o r Washington S t a t e . Recording s t a t i o n s a t Bellingham, Newhalem, and D i a b l o Dam are r e l e v a n t t o the study area (see Table 3.1, F i g . 3.1). L o c a t i o n data show t h a t the Bellingham s t a t i o n i s c l o s e s t t o the study area but a t a lower e l e v a t i o n . The Newhalem and D i a b l o Dam s i t e s are a t a p p r o p r i a t e e l e v a t i o n s but are not near the study area. The c l i m a t i c data w i l l be used mainly as rough guide f o r temperatures and snow-line e l e v a t i o n s f o r each storm, and f o r comparative purposes between the storms. F i n a l l y , the U.S. S o i l C o n s e r v a t i o n S e r v i c e monitors snow depths and snow-water e q u i v a l e n t s bimonthly i n the North Cascades r e g i o n . Snow mon i t o r i n g r e c o r d s a t Rocky Creek are c l o s e s t i n e l e v a t i o n and l o c a t i o n t o the study area ( l o c a t i o n data i n T a b l e 3.1, F i g . 3.1). Snow m o n i t o r i n g data p r o v i d e rough e s t i m a t e s of snow depths and snow d e n s i t i e s d u r i n g storms i n the study area. The data can a l s o be compared between storms and t o e s t a b l i s h e d mean v a l u e s . 23 3.3 Storm c h a r a c t e r i s t i c s The January 9-10, 1983 storm produced the most severe 12 and 24-hour d u r a t i o n p r e c i p i t a t i o n on r e c o r d a t Nooksack Salmon Hatchery. The maximum 12-hour p r e c i p i t a t i o n depth was 102 mm (4.0"), w h i l e the 24-hour depth was 160 mm (6.3"). Continuous r a i n a t i n t e n s i t i e s of 5.1 - 10.2 mm/hr (.2 - .4 in / h r ) was recorded from 1800, January 9 t o 1200, January 10. Syverson (1984) r e p o r t e d t h a t Smith Creek had f l o o d e d over i t s banks and was c a r r y i n g l a r g e l o g s and other d e b r i s onto the a l l u v i a l fan s h o r t l y a f t e r 0400, January 10. Two a d d i t i o n a l storms are evaluated i n or d e r t o compare t h e i r magnitudes with t h a t of the January, 1983 event. On December 13-14, 1979, a 12-hour maximum of 76 mm (3.0") o f p r e c i p i t a t i o n f e l l a t Nooksack Salmon Hatchery. T h i s r e p r e s e n t s the second most severe 12-hour storm on r e c o r d . The 24-hour maximum was 112 mm (4.4"), and the 2 day storm t o t a l was 142 mm (5.6"). Rain f e l l c o n t i n u o u s l y from 1000, December 13 t o 0000, December 14 a t a r a t e of 2.5 - 10.2 mm/hr (.1 - .4 i n / h r ) . The storm d i d not i n i t i a t e numerous d e b r i s avalanches or d e b r i s t o r r e n t s a t Smith Creek. However, Syverson (1984 p.76) re p o r t e d t h a t Sygotowitz Creek on the east s l o p e o f Stewart Mountain s u f f e r e d d e b r i s t o r r e n t s i n 1979, p o s s i b l y d u r i n g the December 13-14 storm. In a d d i t i o n , d e b r i s avalanches o c c u r r e d i n the Vancouver, B r i t i s h Columbia area t o the n o r t h 1 . 1 - E i s b a c h e r and Clague (1981) r e p o r t e d continuous heavy r a i n f a l l on December 12-14 and December 16-17 near Vancouver, B.C. The P i t t P o l d e r s t a t i o n recorded 144 mm d u r i n g the f i r s t 24 On January 30, 1971 a d e b r i s t o r r e n t was re c o r d e d a t the mouth of Smith Creek. Between January 29 - 31, 107 mm (4.22") of p r e c i p i t a t i o n f e l l . The maximum 12 and 24-hour p r e c i p i t a t i o n depths were 48.5 mm (1.91") and 74.4 mm (2.93"), r e s p e c t i v e l y . I n t e n s i t i e s ranged from .3 mm/hr (.01 in/ h r ) t o 5.8 mm/hr (.23 i n / h r ) . Syverson (1984) r e p o r t e d t h a t the d e b r i s t o r r e n t entered the Smith Creek a l l u v i a l fan a t about 1300, January 30. In a s p e c i a l r e p o r t on the 1971 event the N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n (1971b p.4) concludes t h a t , " F l o o d i n g along the Nooksack (River) on the 3 0th and 31st was caused by m e l t i n g snow i n the mountains and heavy r a i n f a l l near the Canadian border ... Highways and other p r o p e r t y i n numerous l o c a l i t i e s were damaged by mudslides. Some of the most e x t e n s i v e damage t o highways occurred near Bellingham..." The r e p o r t suggests t h a t snowmelt played a more important r o l e i n t r i g g e r i n g these d e b r i s t o r r e n t s . In terms o f t o r r e n t morphology, Syverson (1984 p.44) suggested t h a t channel s c o u r i n g i n Smith Creek was the p r i n c i p a l d i f f e r e n c e between pre-1971 and post-1971 a i r photos. A review of 1970 and 1974 a i r photos by the author l e d t o the same c o n c l u s i o n 2 . Only a s m a l l number of storm and 169 mm d u r i n g the second, f o r a t o t a l o f 313 mm. Maximum storm i n t e n s i t i e s were about 4.4 mm/hr and 8.1 mm/hr, r e s p e c t i v e l y , over 12-18 hours. Debris avalanches s i m i l a r t o the January, 1983 f a i l u r e s at Smith Creek o c c u r r e d near the end of each storm phase. T o t a l storm p r e c i p i t a t i o n was exceeded f o u r times t h i s century, hence the storm was not a unique event. 2 - A i r p h o t o s p r o v i d e d by Washington Department of N a t u r a l Resources, Olympia, Washington. 1969-1970 photos: B/W, 1:12,000, P r o j e c t symbol: NW-69. 1974 photos: B/W, 1:63360, P r o j e c t symbol: NW-H-74. 25 d e b r i s avalanches were l i n k e d t o the event. 3.4 Storm frequency a n a l y s e s To e v a l u a t e storm magnitudes the P r e c i p i t a t i o n - F r e q u e n c y A t l a s of the Western U n i t e d S t a t e s ( N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n , 197 3) and the r e c o r d i n g p r e c i p i t a t i o n gauge r e c o r d s a t Nooksack Salmon Hatchery were u t i l i z e d . In the f i r s t frequency a n a l y s i s , Nooksack Salmon Hatchery storm data are compared t o A t l a s r e c u r r e n c e i n t e r v a l s f o r d i f f e r e n t d e p t h - d u r a t i o n combinations. In the second a n a l y s i s , r e c u r r e n c e i n t e r v a l data are c a l c u l a t e d d i r e c t l y u s i n g a h y d r o l o g i c frequency equation and Nooksack Salmon Hatchery r e c o r d s . I n t e n s i t y - D u r a t i o n - F r e q u e n c y curves are a l s o developed from the r e c o r d s . P r e c i p i t a t i o n maxima f o r d i f f e r e n t d u r a t i o n s have been c a l c u l a t e d by the N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n and p u b l i s h e d with Nooksack Salmon Hatchery r e c o r d s a f t e r 1973. Annual maxima bef o r e 1973 were c a l c u l a t e d by e n t e r i n g h i g h i n t e n s i t y - d u r a t i o n storms t o a FORTRAN program which p i c k e d out the maximum f o r a g i v e n d u r a t i o n . P r e c i p i t a t i o n depths were recorded on the hour and must be f i x e d i n t e r v a l c o r r e c t e d t o a 'true' maximum v a l u e f o r a g i v e n i n t e r v a l . The c o r r e c t i o n s used by the C a l i f o r n i a Department of Water Resources (1976 p.20) were a p p l i e d . The P r e c i p i t a t i o n - F r e q u e n c y A t l a s of the Western U n i t e d S t a t e s ( N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n , 1973) 26 c o n t a i n e d a s e t of i s o p l u v i a l ( p r e c i p i t a t i o n depth) maps showing d i f f e r e n t r e c u r r e n c e i n t e r v a l s f o r 6 and 24-hour d u r a t i o n r a i n f a l l s . M u l t i p l e - r e g r e s s i o n s c r e e n i n g t e c h n i q u e s were a l s o presented f o r d e t e r m i n a t i o n of p r e c i p i t a t i o n depths a t 1, 2, 3, and 12-hour d u r a t i o n s . The re c u r r e n c e i n t e r v a l s p r e s e n t e d were 2, 5, 10, 25, 50, and 100 years. Study area storm data f i t i n t o a r e c u r r e n c e i n t e r v a l range ( i . e . 25 - 50 years) f o r each d u r a t i o n . Nooksack Salmon Hatchery data from 1966-1985 were a l s o used t o c a r r y out a d i r e c t p r e c i p i t a t i o n - f r e q u e n c y a n a l y s i s . The ge n e r a l h y d r o l o g i c frequency a n a l y s i s e q u a t i o n (Chow, 1964) was a p p l i e d , P j i = P i + Kj S i (3.1) where j r e f e r s t o recur r e n c e i n t e r v a l i n y e a r s i r e f e r s t o s p e c i f i c storm d u r a t i o n i n hours P j i = p r e c i p i t a t i o n i n inches f o r r e c u r r e n c e i n t e r v a l j and d u r a t i o n i P i = mean maximum annual storm f o r d u r a t i o n i Kj = frequency f a c t o r ( i n standard d e v i a t i o n s ) f o r re c u r r e n c e i n t e r v a l of j years S i = standard d e v i a t i o n of maximum annual storm f o r d u r a t i o n i The equation i s s o l v e d f o r K j , which i s then used as inp u t , w i t h the c o e f f i c i e n t of skew, g, t o a Pearson's Type I I I p r o b a b i l i t y d i s t r i b u t i o n t a b l e (Harter, 1969), t o determine the re c u r r e n c e i n t e r v a l . Table 3.2 g i v e s r e s u l t s of both a n a l y s e s f o r the t h r e e comparison storms. The January, 1983 and 27 December, 1979 data show i n c r e a s i n g r e c u r r e n c e i n t e r v a l s w i t h i n c r e a s i n g d u r a t i o n s c o n s i d e r e d . General e q u a t i o n data show maximum r e c u r r e n c e i n t e r v a l s a t the 12-hour p r e c i p i t a t i o n depth, w i t h 24-hour d u r a t i o n s being l e s s severe. The p a t t e r n i n d i c a t e s t h a t p r e c i p i t a t i o n was s u s t a i n e d , c o n s i s t e n t , and of moderate i n t e n s i t y . High i n t e n s i t y showers would have caused h i g h e r s h o r t d u r a t i o n r e c u r r e n c e i n t e r v a l s . In some cases t h e r e i s a marked d i s c r e p a n c y between Frequency A t l a s and g e n e r a l equation r e s u l t s . The g e n e r a l equation data are more accurate f o r two reasons. F i r s t , the storm data are being compared d i r e c t l y with p a s t r e c o r d s r a t h e r than an i n t e r p o l a t e d map. Second, i f Nooksack Salmon Hatchery data were used i n map c o m p i l a t i o n , only 7 y e a r s of data would have been a v a i l a b l e a t p r i n t i n g , as opposed t o 19 y e a r s u s i n g s t a t i o n r e c o r d s from 1966-1985. The g e n e r a l equation r e s u l t s f o r the 1983 and 1979 storms show low r e c u r r e n c e i n t e r v a l s (5.3 years or l e s s ) f o r 1, 2, and 3-hour d u r a t i o n s . The January, 1983 data show t h a t the storm had unusual r e c u r r e n c e i n t e r v a l s of 71 and 64 y e a r s i n the 12 and 24-hour d u r a t i o n s . These val u e s are about t e n times g r e a t e r than the 7.8 and 6.9 years recorded f o r December, 1979 i n the same d u r a t i o n s . The data i n d i c a t e t h a t the January, 1983 storm i s the most severe s u s t a i n e d r a i n f a l l on r e c o r d a t the Nooksack Salmon Hatchery. In c o n t r a s t , the January, 1971 p r e c i p i t a t i o n had r e c u r r e n c e i n t e r v a l s of l e s s than two years i n a l l d u r a t i o n s and should be c o n s i d e r e d an o r d i n a r y event. 28 In t e n s i t y - D u r a t i o n - F r e q u e n c y curves are used t o d e p i c t the frequency a n a l y s i s r e s u l t s . The curves were developed by a p p l y i n g the g e n e r a l frequency equation a t known r e c u r r e n c e i n t e r v a l s . The r e s u l t s are p l o t t e d i n F i g u r e 3.2 as an i n t e n s i t y v e r s u s d u r a t i o n graph with l a b e l e d r e c u r r e n c e i n t e r v a l curves. The roughness of the curves r e f l e c t s problems i n de v e l o p i n g r e p r e s e n t a t i v e c o e f f i c i e n t s of skew. The C a l i f o r n i a Department of Water Resources (1976 p.15) s t a t e s t h a t the c o e f f i c i e n t o f skew i s s e n s i t i v e t o l a r g e storm events i n s m a l l s t a t i s t i c a l samples. The curves do, however, se r v e as a f i r s t approximation f o r r e c u r r e n c e i n t e r v a l s of d i f f e r e n t i n t e n s i t i e s and d u r a t i o n s . 3.5 Study a r e a p r e c i p i t a t i o n P r e c i p i t a t i o n i n t e n s i t i e s at the Nooksack Salmon Hatchery are not r e p r e s e n t a t i v e of the study area because of d i f f e r e n c e s i n e l e v a t i o n and o r o g r a p h i c p o s i t i o n between the two l o c a t i o n s . Comparison storm p r e c i p i t a t i o n must t h e r e f o r e be est i m a t e d . D i r e c t m o n i t o r i n g i n the b a s i n was used t o develop a c o r r e l a t i o n between the s i t e s , which was used t o back-estimate study area r a i n f a l l d u r i n g the comparison storms. A S i e r r a t i p p i n g - b u c k e t r e c o r d i n g r a i n gauge was used t o monitor study area r a i n f a l l d u r i n g the p e r i o d s February 8-15, February 18-23, March 18-21, March 22-29, and A p r i l 1-11, 1984. M o n i t o r i n g d u r a t i o n was l i m i t e d by t r a n s p o r t c o s t s t o and from the s i t e . The gauge was l o c a t e d i n a powerline c l e a r c u t about 29 800 m (.5 miles) n o r t h e a s t of the mouth of Smith Creek, a t 200 m (656') e l e v a t i o n (see F i g u r e 4.1). P r e c i p i t a t i o n was recorded c o n t i n u o u s l y i n 1.27 mm (.05") increments and compiled as h o u r l y t o t a l s , s i m i l a r t o the Nooksack Salmon Hatchery r e c o r d s . A l l p r e c i p i t a t i o n was assumed t o have o c c u r r e d as r a i n f a l l because no s n o w f a l l was recorded a t the h i g h e r e l e v a t i o n Newhalem and D i a b l o Dam s i t e s d u r i n g the p e r i o d . R a i n f a l l i n t e n s i t i e s v ersus time d u r i n g a t y p i c a l storm on February 20, 1984 are shown f o r both l o c a t i o n s i n F i g u r e 3.3 a,b. The p l o t s show the c o n s i d e r a b l e v a r i a t i o n i n r a i n f a l l i n t e n s i t y and d u r a t i o n between the s i t e s over the course of a s i n g l e storm. A c o r r e l a t i o n between study area and Nooksack Salmon Hatchery data was developed by averaging r a t i o s between 6, 12, and 24-hour p r e c i p i t a t i o n i n t e n s i t y maxima f o r f i v e p e r i o d s i n which d i s t i n c t storms occurred. Maxima were o b t a i n e d u s i n g the FORTRAN program d e s c r i b e d e a r l i e r . The g i v e n d u r a t i o n s were chosen because the p r e c i p i t a t i o n magnitude a n a l y s i s showed lon g e r d u r a t i o n r a i n f a l l t o be more important i n d e b r i s avalanche i n i t i a t i o n . The monitoring r e s u l t s and r a t i o s i n Table 3.3 show t h a t study area p r e c i p i t a t i o n i s of g r e a t e r i n t e n s i t y than the Nooksack Salmon Hatchery i n the g i v e n d u r a t i o n s . The standard d e v i a t i o n of .50 i n the combined average r a t i o of 1.86 i n d i c a t e s the v a r i a b l e nature of p r e c i p i t a t i o n i n the area and the need f o r a l o n g e r m o n i t o r i n g p e r i o d . The average r a t i o was a p p l i e d to h o u r l y r a i n f a l l t o estimate 30 study area p r e c i p i t a t i o n f o r the t h r e e comparison storms. F i g u r e 3.3 c,d,e shows estimated study area p r e c i p i t a t i o n f o r the t h r e e storms p l o t t e d as hyetographs. Note t h a t the recorded d e b r i s t o r r e n t s o c c u r r e d a t l e a s t midway through the h i g h i n t e n s i t y p r e c i p i t a t i o n p e r i o d d u r i n g the storms. T a b l e 3.4 p r e s e n t s r e c o r d e d and estimated p r e c i p i t a t i o n i n t e n s i t y maxima f o r d i f f e r e n t d u r a t i o n s a t Nooksack Salmon Hatchery and the study area. P r e c i p i t a t i o n i n t e n s i t i e s are used i n h y d r o l o g i c s t u d i e s of the d e b r i s avalanches i n Chapter 6. F i g u r e 3.4 shows study area data f o r the t h r e e storms p l o t t e d as cumulative p r e c i p i t a t i o n curves. The p r e c i p i t a t i o n i s assumed t o have occ u r r e d i n the same time d i s t r i b u t i o n as t h a t a t Nooksack Salmon Hatchery. The s l o p e s of the cumulative p r e c i p i t a t i o n curves show t h a t January, 1983 p r e c i p i t a t i o n was of moderately g r e a t e r i n t e n s i t y than December, 1979 f o r the f i r s t 11 hours of heavy p r e c i p i t a t i o n , and of much g r e a t e r i n t e n s i t y f o r the l a s t 8 hours of the storms. January, 1971 i n t e n s i t i e s were l e s s severe than the other two storms. F i g u r e 3.4 a l s o h i g h l i g h t s the p r e c i p i t a t i o n d i f f e r e n c e between the two areas. A f t e r 19 hours of heavy r a i n f a l l i n January, 1983, 279 mm (11") of r a i n was estimated i n the study area, whereas 150 mm (5.9") was recorded a t Nooksack Salmon Hatchery. The p o s i t i o n of Stewart Mountain and the Smith Creek b a s i n as the f i r s t major orographic b a r r i e r t o e a s t e r l y moving ocea n i c storms i s the p r i n c i p a l cause of h e a v i e r p r e c i p i t a t i o n . The p o s i t i o n of Nooksack Salmon Hatchery on the d r i e r l e e s i d e 31 of Sumas Mountain r e s u l t s i n lower p r e c i p i t a t i o n . Antecedent p r e c i p i t a t i o n and snowmelt are important f a c t o r s i n t h a t they c o n t r o l i n i t i a l s o i l moisture c o n d i t i o n s b e f o r e the onset of storms. Dry antecedent c o n d i t i o n s cause g r e a t e r pore space storage i n the s o i l and w i l l h e l p t o absorb heavy r a i n f a l l . Wetter c o n d i t i o n s cause a v a i l a b l e pore space t o be q u i c k l y f i l l e d . T a b l e 3.5 i s a summary of antecedent p r e c i p i t a t i o n a t Nooksack Salmon Hatchery f o r a t e n day p e r i o d ending one day a f t e r the storms. The data p r o v i d e an i n d i c a t i o n of antecedent p r e c i p i t a t i o n magnitudes and a l l o w comparison between storms. The t a b l e shows t h a t moderate t o heavy p r e c i p i t a t i o n o c c u r r e d b e f o r e each storm, w i t h January, 1983 c o n d i t i o n s b e i n g most severe. F i g u r e 3.3 c,d,e shows t h a t a r e l a t i v e l y dry 24-hour p e r i o d preceded each of the t h r e e comparison storms. Study area p r e c i p i t a t i o n o ccurred mainly as r a i n d u r i n g the January, 1983 and January, 1971 storms. For 1983, Haggard (1985) s t a t e d t h a t on January 9-10 p r e c i p i t a t i o n below 1220 m (4000 1) o c c u r r e d p r i m a r i l y as r a i n . Table 3.1 shows the maximum study area e l e v a t i o n as 910 m (2986 1). Radiosonde o b s e r v a t i o n s 3 near Q u i l l a y u t e , on the west coast of the Olympic p e n i n s u l a , showed t h a t on 0000, January 10, 1983 the f r e e z i n g l e v e l was between 1963 and 2258 m (6440 - 7408'). A l s o , no new s n o w f a l l was recorded at the Newhalem (elev. 160 m) and D i a b l o Dam ( e l e v . J - Obtained from Dr. C l i f f o r d Mass at the U n i v e r s i t y of Washington, S e a t t l e , Washington. 271.6 m) on January 9-10, 1983 or January 29-30, 1971. For 1979, 13 cm of new snow were recorded a t D i a b l o Dam December 13, w h i l e 5 cm were recorded on December 14. S n o w f a l l data are summarized i n Table 3.5. Comparison with F i g u r e 3.3d i n d i c a t e s t h a t most of the snow f e l l b e fore the main r a i n s t o r m on the evening of the 13th. At l e a s t some of the r e c o r d e d p r e c i p i t a t i o n d i d not pene t r a t e the s o i l p r o f i l e because i t f e l l as snow. 3 . 6 Study area snowmelt Snowmelt d u r i n g storms can c o n t r i b u t e s i g n i f i c a n t water volumes t o the study area b a s i n under f a v o r a b l e c l i m a t i c c o n d i t i o n s . Antecedent snowmelt i n f l u e n c e s s o i l m o isture content b e f o r e the onset of the h e a v i e s t r a i n f a l l . R e g i o n a l c l i m a t o l o g i c a l data are used t o assess snowmelt b e f o r e and d u r i n g the t h r e e storms. Table 3.5 i s a t a b u l a t i o n o f d a i l y p r e c i p i t a t i o n , s n o w f a l l , snow on ground, and maximum and minimum temperatures a t the th r e e c l i m a t o l o g i c a l data s t a t i o n s f o r ten day p e r i o d s ending a day a f t e r the storms. T a b l e 3.7 p r e s e n t s recorded snow depths, snow water e q u i v a l e n t s , and snow d e n s i t i e s a t the Rocky Creek snow monitoring s i t e . T ables 3.5 and 3.7 show t h a t the b a s i n snow cover and snowmelt c o n d i t i o n s d u r i n g the January, 1971 storm were more severe than e i t h e r January, 1983 or December, 1979. The January, 1983 data show a t h i n snow cover r e c e d i n g from Newhalem (el e v . 160 m) t o D i a b l o Dam (elev. 272 m) between January 5-8. 33 Given the warm temperatures on the 8th the snow p r o b a b l y receded another 100 m (328') to 370 m (1214') e l e v a t i o n i n the study area by the morning of the 9th. I n c r e a s i n g snow depths o c c u r r e d t o the 800 - 900 m (2625 - 2953') r i d g e t h a t forms the summit of Stewart Mountain. Wooldridge (1984) r e p o r t e d a t l e a s t .31 m (12") of snow covered the r i d g e on January 8, 1983. P o t e n t i a l l y important i s the 8 cm (3") snowmelt under 11°C temperatures a t D i a b l o Dam between January 7 and 8. Rocky Creek snow d e n s i t y data i n d i c a t e t h a t the snowmelt equaled about 27 mm (1.1") of water. T h i s c o n t r i b u t i o n i s s i g n i f i c a n t i n terms of m a i n t a i n i n g antecedent s o i l moisture. Snowmelt c o u l d a l s o become a t h r e a t t o s l o p e s t a b i l i t y when combined with a moderate t o h i g h r e c u r r e n c e i n t e r v a l storm. Snowmelt data on January 9-10, 1983 are not a v a i l a b l e because the snow l i n e was above D i a b l o Dam; t h i s f a c t i s s i g n i f i c a n t because the c o n t r i b u t i n g area was l i m i t e d t o the «370 m (1214*) e l e v a t i o n and above. A l s o , the maximum recorded temperature of 5°C at Newhalem and D i a b l o Dam on the n i g h t of the storm l i m i t e d the magnitude of snowmelt t h a t c o u l d occur. The December, 1979 data are s i m i l a r t o January, 1983 i n t h a t a t h i n snow cover o c c u r r e d near e l e v a t i o n 27 0 m a t the onset of the h e a v i e s t p r e c i p i t a t i o n . However, at h i g h e r e l e v a t i o n s the Rocky Creek snow mon i t o r i n g data i n d i c a t e a t h i n snow pack (39% of average) and lower than average snow d e n s i t i e s on December 27, 1979. Under those circumstances snowmelt Would not have c o n t r i b u t e d s i g n i f i c a n t water to the s o i l b e f o r e the storm. 34 Snow f a l l s recorded a t the beginning of the December, 1979 storm at Newhalem and D i a b l o Dam i n d i c a t e t h a t p r e c i p i t a t i o n began as snow and then changed t o r a i n at those e l e v a t i o n s . The g r e a t e r depth of snow a t Newhalem shows t h a t snow accumulation was s p o r a d i c . Study area snowmelt magnitudes d u r i n g the storm were again l i m i t e d by the small a r e a l extent of the snow and near f r e e z i n g temperatures. The January, 1971 data shows a t h i c k , r i p e n i n g snowpack i n p l a c e f o r nine days b e f o r e the storm. Rocky Creek snow mo n i t o r i n g data show the pack t o be 2.2 times the average depth f o r the be g i n n i n g of February. The .45 g/cm 3 snow d e n s i t y recorded i s 1.3 times the average. In terms of antecedent p r e c i p i t a t i o n , T a b l e s 3.5 and 3.6 show t h a t on January 27-28 l i t t l e p r e c i p i t a t i o n f e l l at Nooksack Salmon Hatchery, but a t o t a l o f 10.2 cm (4") of snow melted a t D i a b l o Dam t o c o n t r i b u t e 46 mm (1.8") of water t o the s o i l . In the study area t h i s snowmelt kept the s o i l moist d u r i n g the dry p e r i o d b e f o r e the storm. During the storm, D i a b l o Dam c l i m a t o l o g i c a l data show 15.2 cm (6") of snowmelt from January 30, 0800 t o January 31, 0800. T h i s t r a n s l a t e s t o 68 mm (2.7") of meltwater u s i n g the Rocky Creek d e n s i t y data. A temperature i n v e r s i o n (maximum 8°C) at the h i g h e r e l e v a t i o n D i a b l o Dam s i t e was p a r t l y r e s p o n s i b l e f o r the r a p i d snowmelt. Snow was s t i l l p r e s e n t i n Newhalem on January 31, i n d i c a t i n g t h a t most of the study area had a m e l t i n g snow cover d u r i n g the d e b r i s t o r r e n t s . The maximum 24-hour p r e c i p i t a t i o n estimated f o r the January, 35 1971 storm i s 139 mm (5.5"). I f snowmelt c o n t r i b u t e d the water e q u i v a l e n t of 68 mm (2.7") d u r i n g t h i s p e r i o d the t o t a l would be 207 mm (8.2"). One t h i r d of the t o t a l would be c o n t r i b u t e d by snowmelt. The new 24-hour moisture i n p u t i s comparable t o the estimated p r e c i p i t a t i o n of 209 mm (8.2") i n 1979 and 300 mm (11.8") i n 1983. These rough estimates of snowmelt magnitude and a r e a l extent suggest t h a t snowmelt made a more s i g n i f i c a n t c o n t r i b u t i o n t o s o i l moisture i n the 1971 event than i n the 1979 and 1983 events. 3.7 R e l a t i o n s h i p between c l i m a t i c data and d e b r i s t o r r e n t c h a r a c t e r i s t i c s and frequency o f o c c u r r e n c e The p r e c i p i t a t i o n magnitude a n a l y s i s shows t h a t a 71-year r e c u r r e n c e i n t e r v a l r a i n f a l l i n a 12-hour d u r a t i o n t r i g g e r e d the numerous d e b r i s avalanches seen i n January, 1983 on study area s l o p e s . The 12-hour estimated i n t e n s i t y of 15.9 mm/hr (.63 i n / h r ) i n the study area i s c r i t i c a l because the d e b r i s t o r r e n t s were recorded w i t h i n 12 hours of the b e g i n n i n g of heavy r a i n f a l l . Snowmelt d i d not appear t o p l a y a c r i t i c a l r o l e i n f a i l u r e i n i t i a t i o n . The f a c t t h a t January, 1983 d e b r i s avalanches were i n i t i a t e d on both snow covered and bare ground a l s o i n d i c a t e s t h a t i n f i l t r a t i n g r a i n f a l l was the p r i n c i p a l t r i g g e r i n g mechanism. The January, 1971 event r e p r e s e n t s a d i f f e r e n t type of d e b r i s t o r r e n t t r i g g e r i n g mechanism. The p r e c i p i t a t i o n magnitude a n a l y s i s confirms a i r p h o t o evidence t h a t r a i n f a l l t r i g g e r e d d e b r i s avalanches were not widespread. The a v a i l a b l e c l i m a t i c data a l s o suggest t h a t snowmelt c o n t r i b u t e d s i g n i f i c a n t l y t o b a s i n r u n o f f volume. The event i s t h e r e f o r e c o n s i d e r e d a combination s n o w m e l t / r a i n f a l l f l o o d t h a t m o b i l i z e d s o i l and a l l u v i u m i n t o a d e b r i s t o r r e n t , without i n i t i a t i n g widespread avalanches. The d i v i d i n g l i n e between January, 1983 and January, 1971 type events i s not d i s t i n c t . A number of Holocene t o r r e n t s should have o c c u r r e d , c o n s i d e r i n g the l e s s than 100-year r e c u r r e n c e i n t e r v a l f o r d e b r i s avalanche i n i t i a t i o n and the p o s s i b i l i t y of numerous snowmelt induced t o r r e n t s . Orme, et a l . (1986) documented evidence of p o s t - g l a c i a l d e b r i s t o r r e n t a c t i v i t y . A t r e e - r i n g a n a l y s i s i n d i c a t e d t h a t c o l l u v i u m f i l l e d hollows were evacuated two or t h r e e times a century f o r the past few hundred y e a r s . Trenching of d i s t r i b u t a r y a l l u v i a l fans showed evidence of f o s s i l f o r e s t s o i l s b u r i e d by d e b r i s . F i n a l l y , lake-bottom cores i n the Smith Creek a l l u v i a l fan confirmed s i x d e p o s i t s of magnitude much g r e a t e r than the January, 1983 event i n the l a s t 3,400 ye a r s . The d e p o s i t s were the r e s u l t of d e b r i s avalanche and t o r r e n t t r i g g e r i n g storms. T h i s evidence r e f u t e s Syverson' (1984) premise t h a t 2 0th century d e b r i s avalanches and d e b r i s t o r r e n t s were caused s o l e l y by l o g g i n g a c t i v i t y . 37 Table 3.1: Climatic data recording station locations Distance from North West Elevation, study area, Observ. Location: Lat.: Long.: m ( f t ) km (miles) time Study =48° 45' =122° 17' 94 - 910 area (307 - 2985) Hourly precipitation data 1: Nooksack 48° 54' 122° 29' 134.1 (410) 21 (13) NNE Hourly Salmon Htchy. Climatological data : Bellingham 48° 47' 122° 09' 42.7 (140) 14 (8.7) WNW 1700 Newhalem 48° 41' 121° 15' 160.0 (525) 68 (42.5) ESE 0700 Diablo Dam 48° 43' 121° 09' 271.6(891) 78 (48.3) ESE 0800 Snow course data : Rocky Creek 48° 41' 121° 48' 640.1 (2100) 36 (22.6) ESE Bi -monthly Sources: 1 - National Oceanic and Atmospheric Administration, 1971a p.16 2 - U.S. Soil Conservation Service, 1987 Table 3.2: Comparison storm frequency analyses Storm Duration, hours Maximum pr e c i p i t a t i o n depth, mm (inches) NOAA A t l a s 1 : recurrence i n t e r v a l range, yrs General equation 2: recurrence i n t e r v a l , yrs January 1 12 (0.46) <2 1.5 9-10, 2 22 (0.86) 5 - 1 0 2.8 1983 3 29 (1.15) 5 - 1 0 5.3 6 55 (2.15) 25 - 50 43 12 103 (4.04) >100 71 24 161 (6.33) >100 64 December 1 12 (0.46) <2 1.5 13-14, 2 16 (0.64) <2 1.3 1979 3 24 (0.94) 2 - 5 1.5 6 44 (1.74) 5 - 1 0 3.9 12 77 (3.03) 25 - 50 7 . 8 24 112 (4.42) 50 - 100 6.9 January 1 7 (0.26) <2 1.1 29-30, 2 12 (0.47) <2 1.0 1971 3 16 (0.62) <2 1.0 6 28 (1.09) <2 1.0 12 49 (1.93) 2 - 5 1.1 24 75 (2.96) 2 - 5 1.7 Sources: 1 - National Oceanic and Atmospheric Administration, 1973 2 - National Oceanic and Atmospheric Administration, 1966-1985 Table 3.3: Study area monitoring r e s u l t s and r a t i o s t o Nooksack Salmon Hatchery r a i n f a l l P r e c i p i t a t i o n maxima i n mm (inches) f o r g i v e n d u r a t i o n s : 38 Dates,1984 February 8-15: 6 h r : 12 hr: 24 hr: February 18-23: 6 hr: 12 h r : 24 h r : March 18-21: Study area 27 (1.05) 28 (1.10) 38 (1.50) 42 (1.65) 71 (2.80) 97 (3.80) Nooksack Salmon H a t c h e r y 1 25 (1.00) 28 (1.10) 32 (1.25) 23 (0.90) 36 (1.40) 51 (2.00) R a t i o (S.A./N.S.H.) 1. 05 1. 00 1.25 1.83 2.00 1.90 6 hr: 12 hr: 24 hr: March 22-29: 6 hr: 12 h r : 24 h r : A p r i l 1-11: 6 h r : 12 h r : 24 hr: 48 (1.90) 32 (1.25) 71 (2.80) 32 (1.25) 52 (2.05) 52 (2.05) 20 (0.80) 27 (1.05) 34 (1.35) 28 (1.10) 15 (0.60) 43 (1.70) 15 (0.60) 20 (0.80) 20 (0.80) 8 (0.30) 13 (0.50) 23 (0.90) 1.73 2 . 08 1.65 2 . 08 2 . 56 2 . 56 2 . 67 2 . 10 1. 50 Average: 1.8 6 Standard d e v i a t i o n : 0.50 Source: 1 - N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n , 1984 39 Table 3.4: Maximum p r e c i p i t a t i o n i n t e n s i t i e s a t Nooksack Salmon Hatchery and study area f o r comparison storms Maximum p r e c i p i t a t i o n i n t e n s i t i e s i n mm/hr (i n / h r ) January 9-10, 1983: Duration, hours Nooksack Salmon Study area H a t c h e r y 1 1 11.7 (0.46) 21.7 (0.86) 2 10.9 (0.43) 20.3 (0.80) 3 9.7 (0.38) 18.1 (0.71) 6 9.1 (0.36) 16.9 (0.67) 12 8.5 (0.34) 15.9 (0.63) 24 6.7 (0.26) 12.5 (0.49) December 13-14, 1979: Duration, hours Nooksack Salmon Study area H a t c h e r y 1 1 11.7 (0.46) 21.7 (0.86) 2 8.1 (0.32) 15.1 (0.60) 3 8.0 (0.31) 14.8 (0.58) 6 7.4 (0.29) 13.7 (0.54) 12 6.4 (0.25) 11.9 (0.47) 24 4.7 (0.18) 8.7 (0.34) January 29-30, 1971: Duration, hours Nooksack Salmon Study area H a t c h e r y 1 1 6.7 (0.26) 12.4 (0.49) 2 6.0 (0.24) 11.1 (0.44) 3 5.3 (0.21) 9.8 (0.39) 6 4.6 (0.18) 8.6 (0.34) 12 4.1 (0.16) 7.6 (0.30) 24 3.1 (0.12) 5.8 (0.23) Source: 1 - N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n , 1971b; 1979b; 1983b 40 Table 3.5; R e g i o n a l c l i m a t o l o g i c a l data b e f o r e and d u r i n g comparison storms. Codes: D = Day of month P = P r e c i p i t a t i o n , i n mm S/SG = Sno w f a l l / Snow on ground, i n cm Tmax/Tmin = Maximum temperature / Minimum temperature, i n degrees C e l s i u s Station: Bellingham Newhalem Diablo Dam (elev. 42.7 m) (elev. 160 m) (elev. 271.6 m) January. 1983: S/ Tmax/ S/ Tmax/ S/ Tmax/ D: P: SG: Tmin: P: SG: Tmin: P: SG: Tmin: 2 9/4 5/1 3 T/ 3/-1 3 8 8/4 13 3/3 3/0 31 5/5 3/-1 4 8 10/4 8 1/3 3/0 15 T/5 3/0 5 41 9/4 36 T/T 3/0 64 3/8 3/-1 6 5 7/1 3 4/-1 8 /5 6/-1 7 5 13/6 8 4/0 20 3/8 6/0 8 23 12/6 46 11/2 79 T/T 11/1 9 5 9/3 10 5/0 15 5/1 10 46 12/7 81 3/0 119 4/2 11 14/4 25 4/1 20 8/2 December, , 1979: S/ Tmax/ s/ Tmax/ S/ Tmax/ D: P: SG: Tmin: P: SG: Tmin: P: SG: Tmin: 5 3 9/4 10 8/4 5 8/3 6 3 10/7 15 6/3 10 7/3 7 3 9/7 5 7/4 3 8/4 8 8 10/6 3 7/4 7/4 9 10 8/6 28 9/1 33 7/4 10 3 7/-2 51 4/-1 53 T/T 11/0 11 6/-1 T 6/-1 T/T 3/-1 12 8/-1 31 4/-1 28 T/T 6/0 13 13 9/0 25 13/* 1 2/* 13 3/3 3/1 14 51 11/4 119 5/* 5/-2 94 4/1 January. 1971: S/ Tmax/ s/ Tmax/ s/ Tmax/ D: P: SG: Tmin: P: SG: Tmin: P: SG: Tmin: 21 8 1/ 5/0 23 /10 4/0 13 8/18 3/-1 22 10 5/0 10 17/28 3/-1 13 19/38 1/-2 23 5 8/4 43 14/41 2/-1 28 /33 3/-2 24 28 8/4 56 /36 3/-1 58 /20 3/1 25 3 4/-1 28 /20 4/-1 25 /18 4/0 26 51 10/2 61 1/20 2/-1 56 10/28 2/0 27 3 11/8 15 /20 2/-1 13 /25 4/1 28 11/2 /20 5/-1 /23 6/1 29 13 11/5 /18 4/-1 /20 6/1 30 48 11/6 48 /13 4/-1 58 /15 7/3 31 13 11/8 18 /8 4/-1 23 /T 8/3 1 - * = Data not recorded Source: N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n , 1971a; 1979a; 1983a Table 3.5 i n I m p e r i a l u n i t s presented i n Appendix I 41 T a b l e 3.6: Nooksack Salmon Hatchery d a i l y p r e c i p i t a t i o n b e f o r e and d u r i n g comparison storms P r e c i p i t a t i o n depths i n mm (inches) January. 1983: December. 1979 January. 1971: 2 25 (1.0) 5 18 (•V) 22 -3 20 (.8) 6 23 (.9) 23 -4 10 (•4) 7 3 (-1) 24 -5 41 (1.6) 8 5 (.2) 25 51 ( 2 . 0 ) 1 6 8 (.3) 9 48 (1.9) 26 46 (1.8) 7 56 (2.2) 10 27 3 (-1) 8 31 (1.2) 11 5 (.2) 28 9 69 (2.7) 12 29 28 (1.1) 10 114 (4.5) 13 99 (3.9) 30 71 (2.8) 11 14 43 (1.7) 31 10 (-4) 1 - Recorded from 1000 to 2400. R a i n f a l l not recorded h o u r l y between 1500 Jan. = 17.5 mm (6.9") 18 and 1000 Jan. 25, Accumulated r a i n f a l l Source: N a t i o n a l Oceanic and Atmospheric A d m i n i s t r a t i o n , 1971b, 1979b, 1983b Table 3.7: Rocky Creek snow course data f o r comparison storms Snow water Snow Snow depth, equiv. Densi Storm Date cm (in) cm (in) g/cm January, 1983: Dec. 27, 1982 81 (32) 28 (11.0) . 343 Jan. 29, 1983 102 (40) 36 (14.0) . 350 December, 1979: Dec. 27, 1979 36 (14) 10 (4.0) .286 January, 1971: Jan. 3, 1981 191 (75) 57 (22.5) .298 Feb. 5, 1971 300 (118) 135 (53.1) .450 Beginning o f January 91 (36) 31 (12.2) .339 month averages: (16 years of record) February 135 (53) 52 (20.5) . 387 (22 years of record) Source: U.S. S o i l C o nservation S e r v i c e , 1987 Figure 3.1: Climatological station location map (locations from National Oceanic and Atmospheric Administration, 1971a) North Cascades D I A B L O D A M 0 : 1 0 0 0 f t contour 0 1 0 kilometers F i g u r e 3.2: I n t e n s i t y - D u r a t i o n - F r e q u e n c y curves developed from Nooksack Salmon Hatchery r e c o r d s , 1966-1985 43 TIME IN H O U R S 44 F i g u r e 3.3 a,b: Comparison of Nooksack Salmon Hatchery and study area p r e c i p i t a t i o n , February 20, 1984 c.d,e; Estimated comparison storm r a i n f a l l i n the study area - hyetographs a.) February 20 , 1984 at Nooksack Sa lmon Hatchery 1—i—i—i—r~i—I—i—i—i—r—|—l—l—i—i—l I—l i I r 12 2 4 HOURS S INCE 0 0 0 0 , FEBRUARY 2 0 , 1 9 8 4 TJ ZD m o "0 3D b.) February 20 , 1984 in study area ~i—i—i—i—r~i—r~i—r 12 24 HOURS SINCE 0000, F E B R U A R Y 20, 1984 a 1.0 2 a.5 -CL U c.) January, 1983 in study area JAN. 8 | JAN. 9 'a JAN. 9 JAN., 10 nnnnn DEBRIS TORRENTS Ul I I I II I I I I I I I I I I I I I I I I I I I I I I I I I I I I ! I I I I I I I II I I I I H I I I I H I I I I I I 12 24 36 48 60 HOURS FROM 0000, JANUARY 8, 1983 M l ll r0 72 -25 20 r 15 i- io E-5 a 1.0 ° 0.5 a. d.) December, 1979 in study area DEC. 12 DEC. 13 DEC. 13 DEC. 14 I i I i i i i i i i i | i i i i i i i i I i I | 0 12 24 ru ro n r 2 5 r 2 0 15 E-10 5 i II I i i | II i i i i i i i i i I I i i i i i i i I I i I i i i i i I i i i i i | 0 36 48 60 72 HOURS FROM 0000, DECEMBER 12, 1979 TJ 70 m o 32 I o X TO •v m o 1 O z: X 70 Qj 1.0 1 0.5 CJ u £ 0.0 e.) January, 1971 in study area JAN. 28 JAN. 29 JAN. 29 JAN. 30 l l l l l l i l l I l | i l M l l ! i l i i | i i i I i ITI i i i | i i i i i i i i i i i | i i i i i i i i i i I | I i i i i i i i i i i | 0 0 12 24 36 48 60 72 HOURS FROM 0000, JANUARY 28, 1971 x F i g u r e 3.4: Estimated comparison storm r a i n f a l l i n the study area - cumulative curves 45 HOURS FROM BEGINNING OF HEAVY PRECIPITATION 46 CHAPTER 4: CHARACTERISTICS OF DEBRIS AVALANCHES 4.1 I n t r o d u c t i o n Chapter 4 begins with a review of the components of shear s t r e n g t h and shear s t r e s s , u s i n g the Mohr-Coulomb equation. R a i n f a l l and consequent pore p r e s s u r e i n c r e a s e s t r i g g e r e d the nine avalanches s t u d i e d , but f i e l d evidence showed t h a t many f a c t o r s c o n t r i b u t e d t o f a i l u r e i n i t i a t i o n . These f a c t o r s are reviewed and r e l a t e d t o the Mohr-Coulomb equation. The nine avalanches chosen f o r back a n a l y s i s are d e s c r i b e d i n d e t a i l . They are grouped i n t o f o u r dominant s l o p e geometries: 1) wedges; 2) drainage d e p r e s s i o n s ; 3) l o g g i n g roads; 4) d i s c o n t i n u i t y s u r f a c e s . Each geometry has d i s t i n c t f a i l u r e mechanics, which are d i s c u s s e d . Important c o n t r i b u t i n g f a c t o r s are assessed a t each headscarp. The q u a n t i t a t i v e e f f e c t s o f the f a c t o r s on s l o p e s t a b i l i t y are d i s c u s s e d i n Chapter 7. The movement of avalanche d e b r i s down the scarps i s a l s o d e s c r i b e d t o e x p l a i n how i t reached l o c a l base l e v e l s . The d i s c u s s i o n i n c l u d e s a c a t e g o r i z a t i o n of scour zones which d e b r i s c r e a t e s . Mass movements t h a t v a r i e d from t y p i c a l d e b r i s avalanches are a l s o reviewed. These i n c l u d e stream washouts, rock s l i d e s , and rock b l o c k s l i d e s . S o i l and r a i n f a l l data i n Chapters 2 and 3 were recorded i n Im p e r i a l u n i t s , which were i n c l u d e d . Analyses i n Chapters 4 - 7 are recorded and presented i n m e t r i c u n i t s (mks) o n l y . 47 4.2 F a c t o r s c o n t r o l l i n g d e b r i s avalanche i n i t i a t i o n Shear s t r e n g t h and shear s t r e s s parameters c o n t r o l the s t a b i l i t y of s l o p e s . A m o d i f i e d Mohr-Coulomb eq u a t i o n expresses r e s i s t a n c e o f s o i l w i t h r o o t s t o f a i l u r e i n shear, and i s used t o d e f i n e the f a c t o r s c o n t r o l l i n g avalanche i n i t i a t i o n . C o n sider a v e r t i c a l column of s o i l o v e r l y i n g a u n i t area of p o t e n t i a l f a i l u r e s u r f a c e ^ . The s u r f a c e i s t i l t e d a t an angle, 8, from the h o r i z o n t a l . The shear s t r e n g t h of the column i n terms of e f f e c t i v e s t r e s s parameters i s , Shear s t r e n g t h , S = ( C r + C ) + ( a - u ) tan 0 ' (4.1) where C r = e f f e c t i v e r o o t cohesion C* = e f f e c t i v e s o i l cohesion a = t o t a l normal s t r e s s = W cose, where W = weight of the s o i l column above the f a i l u r e s u r f a c e u = pore p r e s s u r e 0 ' = e f f e c t i v e angle of i n t e r n a l f r i c t i o n or, f r i c t i o n angle The t o t a l shear s t r e s s on the s o i l column i s , Shear s t r e s s , ~C = W s i n e (4.2) I f the shear s t r e n g t h drops below the shear s t r e s s the s o i l column f a i l s . Pore p r e s s u r e i n c r e a s e s t r i g g e r e d most d e b r i s ^ - Forces a c t i n g on t h i s s u r f a c e are converted t o s t r e s s e s by d i v i d i n g by the u n i t area, i . e . Force i n kN d i v i d e d by area i n m2 equals s t r e s s i n kPa. 48 avalanches on January 9-10, 1983. A s m a l l number may have been t r i g g e r e d by s l o p e u n d e r c u t t i n g , d e b r i s impact from o t h e r f a i l u r e s , o r t r e e t o p p l i n g . Important f a c t o r s c o n t r o l l i n g shear s t r e n g t h and shear s t r e s s are s l o p e angle, s o i l depth, s o i l d e n s i t y , v e g e t a t i v e cover, d i s c o n t i n u i t y s u r f a c e s , and snow. The e f f e c t s o f these f a c t o r s are summarized as f o l l o w s : 1) Slope angle. As h i l l s l o p e s steepen, s t e e p e r f a i l u r e s u r f a c e angles develop i n the s o i l . Eq. 4.1 shows t h a t as the angle i n c r e a s e s , normal s t r e s s decreases and the shear s t r e s s i n c r e a s e s , hence the s l o p e becomes l e s s s t a b l e . In shallow s o i l s the bedrock s l o p e angle c o n t r o l s the s o i l s l o p e angle. At l o g g i n g roads the cut and f i l l s l o p e s are st e e p e r than the bedrock s l o p e . 2) S o i l depth. S o i l depth c o n t r o l s the s o i l column weight, W, i n Eq. 4.1 and Eq. 4.2. An i n f i n i t e s l o p e s t a b i l i t y program, INSLOPE 1, was used t o assess the e f f e c t of s o i l depth on the r a t i o of shear s t r e n g t h t o shear s t r e s s . The r a t i o remained constant w i t h s o i l depth when cohesion was zero. The r a t i o decreased w i t h i n c r e a s e d s o i l depth when co h e s i v e s t r e n g t h was i n v o l v e d . Root cohesion i n f o r e s t s o i l s makes t h i s case a p p l i c a b l e . Increased s o i l depth i s t h e r e f o r e g e n e r a l l y a d e s t a b i l i z i n g f a c t o r . I t c o u l d be a s t a b i l i z i n g f a c t o r i f the toe of a p o t e n t i a l f a i l u r e s u r f a c e was being loaded. 3) S o i l d e n s i t y . Loose s o i l has lower f r i c t i o n a l r e s i s t a n c e 1 - The program was w r i t t e n by D.H. Gray, Department of C i v i l E n g i n e e r i n g - U n i v e r s i t y of Wisconsin, Madison. t o shear, and t h e r e f o r e lower shear s t r e n g t h , than dense s o i l . The s o i l d e n s i t y - f r i c t i o n angle r e l a t i o n w i l l be d i s c u s s e d i n Chapter 5. 4) V e g e t a t i v e cover. The r o o t cohesion, C r, v a r i e s with r o o t s t r e n g t h and r o o t d e n s i t y of the v e g e t a t i v e cover. Eq. 4.1 shows t h a t shear s t r e n g t h i n c r e a s e s with i n c r e a s i n g C r. O'Loughlin (1972) found t h a t understory s p e c i e s have lower C r v a l u e s than t r e e s p e c i e s with s t r o n g e r r o o t s . Older t r e e s have g r e a t e r C r v a l u e s than younger t r e e s because r o o t systems are b e t t e r developed. F i n a l l y , immature t r e e s p e c i e s such as Alnus may have a lower C r than more mature s p e c i e s such as Pseudotsucra. Tsuga, and Thuia. 5) D i s c o n t i n u i t y s u r f a c e s . Smooth d i s c o n t i n u i t y s u r f a c e s w i t h few f r a c t u r e s do not allow p e n e t r a t i o n of r o o t s (Tsukamoto and Kusakabe, 1984), and may have low f r i c t i o n a l r e s i s t a n c e t o shear (Lambe and Whitman, 1979). The r o c k - s o i l c o n t a c t t h e r e f o r e becomes a weak zone i n the p r o f i l e . In some cases the smooth s u r f a c e i s the primary f a c t o r c o n t r o l l i n g i n i t i a t i o n , hence i t i s d i s c u s s e d as a d i s t i n c t f a i l u r e geometry i n S e c t i o n 4.8. 6) Snow. In Chapter 3 the snow l i n e was estimated a t 370 m on January 9-10, 1983, t h e r e f o r e snow cover may have i n f l u e n c e d d e b r i s avalanche i n i t i a t i o n above t h i s e l e v a t i o n . Snow may d e s t a b i l i z e s l o p e s by c o n t r i b u t i n g groundwater t o u n d e r l y i n g s o i l and r e t a r d i n g s u r f a c e r u n o f f from the f o r e s t f l o o r . As a r e s u l t , pore p r e s s u r e s may b u i l d up t o l e v e l s t h a t would not 50 otherwise have been reached. These c o n t r o l l i n g f a c t o r s were observed i n the study area, and t h e i r r e l a t i v e importance was recorded a t each headscarp d u r i n g f i e l d i n v e s t i g a t i o n s . Headscarp r o o t s t r e n g t h e v a l u a t i o n s were a i d e d by l o g g i n g h i s t o r i e s i n F r a s e r (198 6) and Syverson (1984). The i n f l u e n c e of snow c o u l d o n l y be surmised from headscarp e l e v a t i o n s . 4.3 S u r v e y i n g procedure F i g u r e 4.1 d e p i c t s the nine avalanche l o c a t i o n s . To determine s l o p e geometries, d e t a i l e d s u r v e y i n g of the scarps took p l a c e between A p r i l 27 and June 15, 1983. T r a n s i t - s t a d i a methods 2 were used t o o b t a i n p o i n t s from which b l o c k diagrams were developed. The surveyed s u r f a c e s c o n s i s t e d of rock, c o l l u v i u m , or t i l l f o r merly o v e r l a i n by s o i l and v e g e t a t i o n . Block diagrams were produced on an XT compatible microcomputer w i t h the program SURFER 3. The scarps were surveyed from headscarps h i g h on s l o p e s t o l o c a l base l e v e l s such as Smith Creek or i t s branches, or Lake Whatcom. L o n g i t u d i n a l c r o s s - s e c t i o n s were surveyed a t the avalanche headscarps. In the case of drainage d e p r e s s i o n s , s e c t i o n s were a l s o surveyed a c r o s s the headscarps, p e r p e n d i c u l a r t o the d e p r e s s i o n a x i s . 2 - The t r a n s i t was a model 262S by Wm. Ainsworth, Denver, Colorado. 3 - Produced by Golden Software, Inc., Golden, Colorado 51 4.4 Scarp dimensions and s o i l volumes removed Tabl e 4.1 i s a summary of survey r e s u l t s . Included f o r each avalanche are s l o p e t r a v e l d i s t a n c e s , e l e v a t i o n drops, estimates of s o i l volumes i n v o l v e d i n i n i t i a l f a i l u r e , and t o t a l volumes removed. The r e s u l t s show t h a t s m a l l i n i t i a l f a i l u r e s cause removal of much l a r g e r s o i l volumes downslope. The r a t i o o f t o t a l volume removed t o i n i t i a l f a i l u r e volume ranged from 21 -79. Scour over d i s t a n c e s o f hundreds of meters i n d i c a t e s t h a t d e b r i s was d e s t r u c t i v e and h i g h l y mobile a f t e r f a i l u r e i n i t i a t i o n . 4.5 Wedges Wedges are a d i s t i n c t d e b r i s avalanche geometry. Humphrey (1982) d e s c r i b e s wedges as s o i l - f i l l e d shallow bedrock d e p r e s s i o n s on the order of two meters deep. F a i l u r e i s a t t r i b u t e d t o pore p r e s s u r e b u i l d - u p a t the downslope l i p of the wedge. Avalanches W-l and W-2 f i t t h i s d e s c r i p t i o n , but onl y i n l o n g i t u d i n a l s e c t i o n s down the s l o p e s . The wedges are bounded down the f a l l l i n e by outcrops o f massive sandstone d i p p i n g i n t o the s l o p e . They become l e s s prominent along t o p o g r a p h i c contours as rock a t t i t u d e , s l o p e aspect, or s l o p e angle change. Deep s o i l and g e n t l e r s l o p e s c h a r a c t e r i z e the wedge, wh i l e shallow s o i l and steep s l o p e s c h a r a c t e r i z e the massive sandstone. Wedge geometries are d e p i c t e d i n F i g u r e 4.2. 52 4.5.1 W - l Avalanche W-l i s depicted i n Figure 4.3. The complete s o i l p r o f i l e was removed as about 50 m3 of s o i l i n i t i a l l y f a i l e d i n the 13 m long X 5 m wide wedge at 434 - 444 m elevation. Planar slopes ranged from 48° at the headscarp to 31° near the downslope l i p of the wedge. S o i l depths 4 averaged .7 m above a steep rock slab at the head of the scarp, 1 m i n the middle of the wedge, and thinned to .4 m on the downslope l i p . The rock outcrop acted as a barr i e r , behind which deep, loose, well drained s o i l accumulated. Massive sandstone beds dipped 27° into the slope. The area was logged between 1918 and 1943, and regenerated to a mixed forest. Scrubby, mixed Pseudotsuga, Alnus. and Tsuqa to 10 m were found at the headscarp, with a scrub Tsuqa and scattered Polystichum understory. Weak understory vegetation covered the steep rock slab downslope from the wedge. Deep, loose s o i l and steep slopes were the most important factors c o n t r o l l i n g i n i t i a t i o n . F a i l e d debris quickly removed t h i n s o i l and vegetation on the massive sandstone outcrop and moved downslope to a logging road at 42 0 m elevation. The road re s i s t e d f a i l u r e but spread the debris over a wider path. Below the road the debris removed vegetation and a th i n layer of s o i l , leaving part of the s o i l p r o f i l e and underlying colluvium. Some debris accumulated 4 - S o i l depths were measured i n the v e r t i c a l for convenience of input to i n f i l t r a t i o n and slope s t a b i l i t y analyses. 53 behind t h r e e a l d e r t r e e s t h a t r e s i s t e d f a i l u r e . A t about 370 m e l e v a t i o n the d e b r i s path j o i n e d a shallow drainage d e p r e s s i o n . H y d r a u l i c a l l y eroded p i p e s were found i n exposed c o l l u v i u m a t t h i s j u n c t i o n . Downslope, the d e b r i s scoured c o l l u v i u m / a l l u v i u m to bedrock i n what had p r e v i o u s l y been a f i r s t o r d e r channel. Groundwater and s u r f a c e water i n the d e p r e s s i o n helped m o b i l i z e the d e b r i s . The average d e p r e s s i o n a x i s s l o p e was 31.5°. The rock s l o p e angle steepened t o 43° as d e b r i s entered the scoured, Smith Creek channel. 4.5.2 W-2 Avalanche W-2 i s d e p i c t e d i n F i g u r e 4.4. The complete s o i l p r o f i l e was removed as about 110 m3 of s o i l i n i t i a l l y f a i l e d i n the 15 m l o n g X 10 m wide wedge at 441 - 450 m. P l a n a r wedge s l o p e s ranged from 41° a t the headscarp t o 31° near the downslope l i p . Bedding s u r f a c e s i n the massive sandstone dipped 50° i n t o the s l o p e . S o i l depths were 1.2 m at the headscarp, .8 m i n the middle of the scarp, and .3 m a t the downslope l i p of the wedge. The rock again was a b a r r i e r behind which l o o s e s o i l b u i l t up. Orme (1987) r e p o r t e d evidence of channeled and b r a i d e d s u r f a c e flow above the scarp a f t e r the storm. The flow p r o b a b l y r e s u l t e d from e x f i l t r a t i o n of groundwater through t h i n s o i l and r o o t s . The area was logged between 1918-1943, and regenerated t o mixed f o r e s t . Headscarp v e g e t a t i o n c o n s i s t e d of mixed Alnus, Tsuga, and Thuja t o 15 m, with a scrub c o n i f e r and a dense 54 Polystichum u n d e r s t o r y . The dense Polystichum u n d e r s t o r y i n d i c a t e d h i g h antecedent s o i l moisture. O c c a s i o n a l Pseudotsuga t o 40 m were l e f t behind a f t e r l o g g i n g . Deep, l o o s e s o i l and steep s l o p e s were the most important c o n t r o l l i n g f a c t o r s . A t h i n snowpack may have caused excess pore p r e s s u r e b u i l d - u p by r e t a r d i n g s u r f a c e water flow. F a i l e d d e b r i s c u t a 15 m wide swath i n t h i n s o i l and v e g e t a t i o n o v e r l y i n g the massive sandstone. At 427 m e l e v a t i o n the d e b r i s flowed around but c o u l d not uproot a Thuja stump g r e a t e r than 1 m i n diameter. At about 422 m e l e v a t i o n the scarp widened t o 33 m as a secondary headscarp developed on a 34° s l o p e . The s o i l was 1.3 m deep near the headscarp and th i n n e d t o .6 m near the toe. Debris impact from above was probably a t r i g g e r i n g mechanism, while the t h i c k s o i l p r o f i l e a t the base of the steep s l o p e appeared t o be an important c o n t r i b u t i n g f a c t o r . By 390 m e l e v a t i o n drainage d e p r e s s i o n s began t o c h a n n e l i z e the d e b r i s . Debris from the p r i n c i p a l headscarp flowed down a shallow, 28° d e p r e s s i o n on the east edge of the scarp, removing t h i n s o i l w h i l e l e a v i n g t h i c k e r c o l l u v i u m and t i l l . Much of the d e b r i s from the secondary headscarp flowed away from the main sc a r p ^ n t o a deep, northwest t r e n d i n g drainage d e p r e s s i o n s u b p a r a l l e l t o the bedrock s t r i k e . Some of the d e b r i s flowed over a bedrock r i d g e i n t o a shallow d e p r e s s i o n running along the west s c a r p edge. At 325 m e l e v a t i o n the bedrock steepened t o 46° as the shallow d e p r e s s i o n s converged a t a deeply eroded creek f o l l o w i n g a lineament. From 306 - 266 m e l e v a t i o n the channel was scoured t o bedrock 8 - 10 m up i t s 46° s i d e s l o p e s . The channel d i d not c o n t r i b u t e s i g n i f i c a n t sediment t o the d e b r i s , because i t had been scoured between 1970 and 1978 5. F i n a l l y , the d e b r i s dropped down a steep s l o p e t o Powerline Creek, a south f o r k of Smith Creek, where i t was dammed by a l o g g i n g road c r o s s i n g the creek. Water and d e b r i s overtopped the road a f t e r the b a s i n f i l l e d . 4.6 Drainage d e p r e s s i o n s Drainage d e p r e s s i o n s are a second d i s t i n c t geometry. Tsukamoto, e t a l . (1982) surveyed t h r e e d i s t r i c t s i n Japan and found t h a t d e b r i s avalanches are more common on convergent s l o p e s ( i . e . drainage d e p r e s s i o n s ) , as opposed t o p l a n a r or d i v e r g e n t s l o p e s . P i e z o m e t r i c s t u d i e s by Swanston (1970), O'Loughlin (1972), Anderson and Burt (1978) and o t h e r s show t h a t pore p r e s s u r e s are h i g h e r i n drainage d e p r e s s i o n s than the surrounding s l o p e s , hence they are l i k e l y i n i t i a t i o n s i t e s . D i e t r i c h , e t a l . (1986, Eq. 25) p r e s e n t s a steady s t a t e equation b a l a n c i n g p r e c i p i t a t i o n i n put i n the area above a p o t e n t i a l avalanche headscarp with s o i l d i s c h a r g e c a p a b i l i t i e s a t the headscarp, 5 - Evidence f o r s c o u r i n g from a i r photos. Washington S t a t e Department of N a t u r a l Resources (DNR), Olympia, Washington. 1969-1970 photos: B/W, 1:12,000 P r o j e c t symbol: NW-69, 45B-90,91. 1978 photos: B/W, 1:12,000, P r o j e c t symbol NW-78, 58C-22,23. 56 Ro a ( t ) = h ( t ) K s a t s i n 9 cos 9 (4.3) where Ro = r a i n f a l l r a t e a ( t ) c o n t r i b u t i n g area per u n i t contour l e n g t h a t time, t h(t) depth of s a t u r a t i o n a t time, t Ksat 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 0 = s l o p e angle Steady s t a t e i s seldom i f ever reached, but the equation shows t h a t g r e a t e r c o n t r i b u t i n g areas i n drainage d e p r e s s i o n s cause g r e a t e r depths of s a t u r a t i o n , o t h e r f a c t o r s b e i n g c o n s t a n t . DD-1, DD-2, and DD-3 were i n i t i a t e d i n drainage d e p r e s s i o n s . In a l l t h r e e cases the bedding s u r f a c e s t r i k e was p a r a l l e l or s u b p a r a l l e l t o the d e p r e s s i o n a x i s . At DD-1 and DD-3, weathering of mudstone i n t e r b e d s d i p p i n g i n t o the s l o p e c r e a t e d a V-shaped trough. At DD-2, a shallower trough developed on d i s c o n t i n u i t i e s between massive sandstone beds. The d e p r e s s i o n s were f i l l e d i n by s o i l creep, s l o p e wash, and r e s i d u a l s o i l development, as d e p i c t e d i n F i g u r e 4.5. The trough shapes allow deeper p r o f i l e s t o develop. 4.6.1 DD-1 Avalanche DD-1 i s d e p i c t e d i n F i g u r e 4.6. About 29 m3 of s o i l i n i t i a l l y f a i l e d i n a 8 m long X 4 m wide pocket i n the d e p r e s s i o n a x i s a t 3 03 - 3 08 m e l e v a t i o n . S o i l depths ranged from 1 m on a 26° s l o p e a t the headscarp t o .6 m on a 43° s l o p e 57 below the pocket. The s o i l was l o o s e and wet. Two 100 - 150 mm diameter p i p e s , eroded by subsurface flow, o c c u r r e d a t the headscarp. S i m i l a r p i p e s were found a t W-l and a d j a c e n t t o W-2 a t the t r a n s i t i o n between unchanneled and channeled flow i n drainage d e p r e s s i o n s . Before the avalanche the p i p e s probably f e d a channel on the steep s l o p e below the headscarp. The area was logged between 1943-1950 and regenerated t o hardwood f o r e s t . V e g e t a t i o n surrounding the drainage d e p r e s s i o n c o n s i s t e d of Alnus t o 15 m with a Polystichum and scrub Tsuga un d e r s t o r y . Low cohesive s t r e n g t h understory v e g e t a t i o n dominated a 6 m wide swath i n the d e p r e s s i o n a x i s . Deep, l o o s e s o i l and weak v e g e t a t i o n were the most s i g n i f i c a n t c o n t r o l l i n g f a c t o r s . P i e r s o n (1983) found t h a t c o l l a p s e d p i p e s can cause excess pore p r e s s u r e s , hence the p i p e s might have t r i g g e r e d the i n i t i a l f a i l u r e . 0 The d e b r i s flowed down the steep s l o p e t o 298 m e l e v a t i o n , where bedrock s i d e s l o p e s t h a t channeled the d e b r i s became l e s s prominent. The avalanche then changed d i r e c t i o n t o flow more d i r e c t l y down a 27° s l o p e , and spread over a l a r g e r a r ea. V e g e t a t i o n and a t h i n s o i l l a y e r were removed, l e a v i n g up t o 1 m of c o l l u v i u m underneath. Surface water eroded a g u l l y t o bedrock i n the c o l l u v i u m . At 256 m e l e v a t i o n the s l o p e decreased t o 14° as the d e b r i s flowed over a l o g g i n g road. Below the road the d e b r i s skimmed a p l a n a r , 17° p l a t e a u , l e a v i n g > 1 m t h i c k c o l l u v i u m i n p l a c e . The rock s l o p e angle steepened t o 45° as d e b r i s entered the Smith Creek channel. 58 4 . 6 . 2 D D - 2 Avalanche DD-2 i s d e p i c t e d i n F i g u r e 4.7. About 70 m3 of s o i l f a i l e d on a 5 m long X 6 m wide, 48° s l o p e a t 478 - 485 m e l e v a t i o n . J u s t downslope the s l o p e eased t o 31°. S o i l depths ranged from 1.2 - 1.4 m i n the d e p r e s s i o n a x i s t o 0.5 - 0.9 m a t the s c a r p edges. The s o i l was loo s e and dry. F a i l u r e was i n i t i a t e d i n a 1943-1950 p a r t i a l c u t , j u s t downslope from o l d growth f o r e s t . Headscarp v e g e t a t i o n c o n s i s t e d o f Tsuqa. Thuja, and Alnus t o 15 m, wit h a scrub c o n i f e r u n d e r s t o r y . V e g e t a t i o n i n the d e p r e s s i o n a x i s was s i m i l a r t o t h a t i n the surrounding f o r e s t . The steep headscarp and deep, l o o s e s o i l were important f a c t o r s c o n t r o l l i n g i n i t i a l f a i l u r e . D e b r is flowed down a 38° sl o p e p a r a l l e l t o the bedding s u r f a c e s t r i k e . At 439 m e l e v a t i o n the scarp j o i n e d the trunk drainage d e p r e s s i o n f o r the dev e l o p i n g f i r s t o r d e r channel. The d e p r e s s i o n was deeper and wetter (due t o a l a r g e r drainage area) than the feeder d e p r e s s i o n t h a t f a i l e d upslope. Bedding s u r f a c e s c u t a c r o s s the d e p r e s s i o n a t 45° t o the a x i s , c a u s i n g a more U-shaped c r o s s - s e c t i o n . About 24 m3 of l o o s e s o i l f a i l e d i n a secondary headscarp i n the d e p r e s s i o n a x i s , a t 439 - 445 m e l e v a t i o n . S o i l depths ranged from 1.1 m a t the headscarp t o .8 m downslope. V e g e t a t i o n i n the d e p r e s s i o n c o n s i s t e d o f scrub Alnus and Acer with a dense Polystichum and Carex unde r s t o r y . T h i s v e g e t a t i o n had a lower shear s t r e n g t h than the surrounding f o r e s t . Deep, l o o s e s o i l and weak d e p r e s s i o n v e g e t a t i o n were 59 important c o n t r o l l i n g f a c t o r s . D e b ris from the f a i l u r e s scoured the two d e p r e s s i o n s t o t h e i r j u n c t i o n a t 429 m e l e v a t i o n , where a l o g g i n g road c r o s s i n g the s l o p e was removed. Below the road the d e p r e s s i o n became a deep trough s u b p a r a l l e l t o bedding, w i t h 4 - 7 m s i d e w a l l s r i s i n g a t 30° - 40° from the trough a x i s . The channeled d e b r i s scoured the d e p r e s s i o n , l e a v i n g o n l y boulders and g r a v e l i n the a x i s and t h i n c o l l u v i u m on the s i d e w a l l s . The d e b r i s had c o n s i d e r a b l e momentum when i t reached Smith Creek, as evidenced by impact s c o u r i n g t o 30 m up the o p p o s i t e v a l l e y w a l l . 4 . 6 . 3 DD-3 Avalanche DD-3 i s d e p i c t e d i n F i g u r e 4.8. The avalanche was u n l i k e o t h e r s s t u d i e d i n t h a t a number of s m a l l s l o p e f a i l u r e s c o n t r i b u t e d d e b r i s t h a t scoured a deep drainage d e p r e s s i o n . The s c a r p i s l o n g e r and narrower than other avalanche s c a r p s . The h i g h e s t f a i l u r e (above surveyed area) o c c u r r e d a t 610 m e l e v a t i o n on a p l a n a r , 37° slope f e e d i n g the d e p r e s s i o n . About 19 m3 of s o i l f a i l e d , l e a v i n g a 6 m long X 4 m wide sca r p . Shallow o v e r l a n d flow fed the scarp, as evidenced by u n d e r s t o r y and p i n e needles o r i e n t e d p a r a l l e l t o flow i n a 1 m wide swath above i t . Snow probably r e t a r d e d t h i s flow and c o n t r i b u t e d t o pore p r e s s u r e b u i l d - u p . A smooth, 39° bedrock d i s c o n t i n u i t y was exposed beneath the l o o s e , wet s o i l . S o i l depths ranged from .6 m a t the head t o .15 m a t the toe of the f a i l u r e . V e g e t a t i o n c o n s i s t e d of scrub Alnus and Tsuga, with Carex and Polvstichum. 6 0 The v e g e t a t i o n had lower cohesive s t r e n g t h than t h a t o f the surrounding f o r e s t , which was apparent o l d growth. The d i s c o n t i n u i t y s u r f a c e , l o o s e s o i l , weak v e g e t a t i o n , and snow were f a c t o r s c o n t r o l l i n g i n i t i a t i o n . F a i l e d d e b r i s was funneled i n t o the shallow d e p r e s s i o n and scoured a channel i n g l a c i a l t i l l and c o l l u v i u m . At 540 m e l e v a t i o n another 20 m3 of s o i l f a i l e d on a smooth bedrock s l a b , s c o u r i n g the d e p r e s s i o n a x i s t o bedrock. A 1 - 2 m wide swath was c ut i n the t i l l and c o l l u v i u m , exposing a dense r o o t network. At 515 m e l e v a t i o n a t r a n s i t i o n from o l d growth t o a 1943-1950 c l e a r c u t v e g e t a t i o n o c c u r r e d . A i r photos show t h a t between 407 - 515 m e l e v a t i o n near DD-3 had regenerated o n l y t o unders t o r y v e g e t a t i o n by 1970. Alnus and scrub Tsuqa p a r t i a l l y covered the area i n 198 3 6. The scarp widened t o 6 m as about 2 0 m3 of s o i l f a i l e d on a 40° bedrock s l a b i n the d e p r e s s i o n a x i s . T h i s i s the top of the surveyed headscarp i n F i g u r e 4.8. A p r o f i l e o f c o l l u v i u m and t i l l u n d e r l y i n g l o o s e , p o o r l y developed s o i l was exposed. S c o u r i n g t o bedrock at 507 - 500 m e l e v a t i o n undercut the toe o f a V-shaped drainage d e p r e s s i o n on a 30° s i d e s l o p e t o the west, t r i g g e r i n g f a i l u r e of another 20 m3 of s o i l . By 494 m e l e v a t i o n the main d e p r e s s i o n a x i s had developed i n t o the deep trough d e p i c t e d i n F i g u r e 4.5. At 503 - 490 m e l e v a t i o n on the 6 - A i r photos from Washington S t a t e DNR. 1970 photos: P r o j e c t symbol NW-69, 240 45B-90,91. 1983 photos: P r o j e c t symbol NW-C-83, 8-45-048,049. 61 e a s t s i d e of d e p r e s s i o n water eroded t h i n s o i l from a 39° s i d e s l o p e d e p r e s s i o n t o c o n t r i b u t e a d d i t i o n a l sediment t o the trough. From 451 - 405 m e l e v a t i o n the d e b r i s scoured the d e p r e s s i o n t o bedrock on a 37° s l o p e . Steeper w a t e r f a l l s o c c u r r e d where r e s i s t a n t sandstone l a y e r s c r o s s e d the d e p r e s s i o n . At 405 m e l e v a t i o n a l o g g i n g road was d i s l o d g e d and c o n t r i b u t e d about 25 m3 of s o i l t o the d e b r i s . J u s t downslope from the road a 35° s i d e s l o p e was undercut and c o n t r i b u t e d another 20 m3. G r a v e l , c o b b l e s and b o u l d e r s from the upper e l e v a t i o n f a i l u r e s were d e p o s i t e d i n a shallower, 17° reach from 376 - 357 m e l e v a t i o n . F i n e r sediments and v e g e t a t i v e d e b r i s continued down the channel. Below 356 m e l e v a t i o n t h i n a l l u v i u m was scoured from the bedrock channel was eroded and l i t t l e d e b r i s was d e p o s i t e d . F i g u r e 4.8 shows t h a t the channel meandered s l i g h t l y b e f o r e dropping i n t o Smith Creek. 4 . 7 Logging roads Logging road f a i l u r e s are a t h i r d d i s t i n c t f a i l u r e geometry. S i d l e , e t a l . (1985) s t a t e t h a t l o g g i n g roads c o n t r i b u t e t o s l o p e i n s t a b i l i t y by, 1) steepening the s l o p e of c u t and f i l l s u r f a c e s ; 2) adding weight t o the s l o p e i n the s i d e c a s t f i l l ; 3) removing support i n the c u t s l o p e ; 4) r e r o u t i n g and c o n c e n t r a t i n g drainage water. A f i f t h f a c t o r i s the i n a b i l i t y o f s t a b i l i z i n g r o o t s t o completely p e n e t r a t e the t h i c k f i l l 62 p r o f i l e . In the study area numerous f i l l s l o p e f a i l u r e s o c c u r r e d a t l o g g i n g roads. Seepage e r o s i o n g e n e r a l l y o c c u r r e d on c u t s l o p e s . Two f i l l s l o p e f a i l u r e s , LR-1 and LR-2, were i n v e s t i g a t e d . LR-1 f a i l e d a t the i n t e r s e c t i o n of a drainage d e p r e s s i o n and a l o g g i n g road. High pore p r e s s u r e s i n the d e p r e s s i o n and t h i c k , u n s t a b l e s o i l p r o f i l e s on f i l l s l o p e s a t the l o g g i n g road make such l o c a t i o n s prone t o f a i l u r e . Numerous d e b r i s avalanches were i n i t i a t e d a t these i n t e r s e c t i o n s i n January, 1983. LR-2 f a i l e d near a headland d i v i d i n g two drainage d e p r e s s i o n s . Pore p r e s s u r e s are low a t such l o c a t i o n s because groundwater flows away from the d i v i d e toward adjacent d e p r e s s i o n s 7 . The examples show t h a t l o g g i n g road f a i l u r e s can occur anywhere i n a g i v e n b a s i n . 4.7.1 LR-1 Avalanche LR-1 i s d e p i c t e d i n F i g u r e 4.9. The headscarp was fe d by a narrow drainage d e p r e s s i o n 17 m wide by 2 m deep. At 422 m e l e v a t i o n the d e p r e s s i o n d i s c h a r g e d a t a l o g g i n g road covered by Alnus and a dense Polystichum understory, c a u s i n g a 4 m l o n g X 1 m wide slump i n the f i l l s l o p e . Movement ceased a f t e r .5 m displacement. Surface water f l o w i n g i n t o the slumped d e p r e s s i o n , then down a 43°, 18 m long s l o p e . Overland flow on ' - O'Loughlin (1972 p.60) p r o v i d e s a f i e l d example from the Coast Mountains of B r i t i s h Columbia. Pore p r e s s u r e s near a drainage d i v i d e were c o n s i s t e n t l y much lower (h = 100-150 mm) than those i n adjacent depressions (h = 400-900 mm) t h a t s l o p e was evidenced by o r i e n t a t i o n of l i t t e r and unde r s t o r y v e g e t a t i o n p a r a l l e l t o flow. At 411 m e l e v a t i o n a s m a l l e r l o g g i n g road i n t e r c e p t e d the s u r f a c e flow and d i v e r t e d i t 5 m n o r t h from the d e p r e s s i o n . The p r i n c i p a l headscarp o c c u r r e d where flow d i s c h a r g e d from the road t o the f i l l s l o p e . Snow may have r e t a r d e d s u r f a c e flow a t t h i s d i s c h a r g e p o i n t and c o n t r i b u t e d t o the pore p r e s s u r e i n c r e a s e . About 24 m3 of s o i l i n i t i a l l y f a i l e d on the 45° f i l l s l o p e , exposing a bedrock d i s c o n t i n u i t y d i p p i n g 42° downslope a t the headscarp. Bedding d i s c o n t i n u i t y s t r i k e s were p a r a l l e l t o the contours, w i t h the downslope d i p s of 49° being g r e a t e r than the 34° s l o p e angle. The bedrock a t t i t u d e d i s c o u r a g e d the development of deep drainage d e p r e s s i o n s such as a t DD-2. S o i l depths ranged from .8 m on the f o r e s t f l o o r t o about 1.5 m i n the t h i c k e s t p r o f i l e a t the l o g g i n g road. The headscarp area was p a r t i a l l y c ut between 1943-1950. The l o g g i n g roads regenerated t o widely spaced Alnus and dense Polystichum. Downslope from the p r i n c i p a l headscarp the v e g e t a t i o n c o n s i s t e d of mixed Tsuga, Thuia, and Alnus t o 15 m, with a Polystichum understory. Logging road v e g e t a t i o n appeared to have a lower shear s t r e n g t h than the surrounding mixed f o r e s t . Tree r o o t s d i d , however, pen e t r a t e the complete s o i l p r o f i l e . Deep s o i l on steep f i l l s l o p e s , the steep u n d e r l y i n g rock s u r f a c e , and weak roadbed v e g e t a t i o n were f a c t o r s c o n t r o l l i n g f a i l u r e . 64 F a i l e d d e b r i s flowed over a 27° l i p j u s t downslope from the headscarp. At 394 m e l e v a t i o n t h r e e Thuja t r e e s i n the middle of the s c a r p r e s i s t e d f a i l u r e and anchored the s o i l beneath t h e i r r o o t s . The d e b r i s passed over the r o o t s and s t r i p p e d shallow s o i l and v e g e t a t i o n from a 30 m long, 39° bedrock s l a b . The s l a b p r o v i d e d a smooth s u r f a c e f o r s l i d i n g , and d i d not a l l o w r o o t p e n e t r a t i o n . At the base of the s l a b a l o g g i n g road was i n c o r p o r a t e d i n the d e b r i s . Between 353 - 327 m e l e v a t i o n the d e b r i s flowed down a s e r i e s of steep rock s l a b s and shallower s l o p e segments covered by c o l l u v i u m . Feeder drainage d e p r e s s i o n s entered the main channel a t 353 m and 328 m e l e v a t i o n . Below 327 m e l e v a t i o n a shallow, U-shaped channel developed i n massive sandstone. A f t e r removing another l o g g i n g road a t 312 m e l e v a t i o n the d e b r i s emptied i n t o a south f o r k of Smith Creek. 4.7.2 LR-2 Avalanche LR-2 i s d e p i c t e d i n F i g u r e 4.10. About 13 0 m3 of s o i l i n i t i a l l y f a i l e d i n an 11 m wide scarp a t 446 - 453 m e l e v a t i o n , exposing the f i l l s o i l . The i n t a c t f i l l s l o p e angle was 43°, w h i l e the u n d e r l y i n g rock s l o p e was estimated a t 34°. S o i l depths ranged from .8 m i n u n d i s t u r b e d s o i l t o 2.3 m i n the t h i c k e s t s o i l p r o f i l e below the l o g g i n g road. The s c a r p area was c l e a r c u t from 1943-1950. By 1970 the s l o p e regenerated t o deciduous u n d e r s t o r y v e g e t a t i o n w i t h a p a r t i a l Alnus cover. In 1983 the v e g e t a t i o n c o n s i s t e d mainly of 65 Alnus, w i t h a scrub Tsuga and Acer unde r s t o r y . Polystichum and Gramineae were the v e g e t a t i v e cover on the l o g g i n g road grade. The road cover may have had a lower shear s t r e n g t h than the surrounding f o r e s t . Tree r o o t s on the f i l l s l o p e d i d not pe n e t r a t e the f u l l s o i l p r o f i l e at i t s deepest p o i n t . Deep s o i l on steep f i l l s l o p e s , weak roadbed v e g e t a t i o n , and incomplete r o o t p e n e t r a t i o n were c o n t r i b u t i n g f a c t o r s t o f a i l u r e a t t h i s otherwise s t a b l e l o c a t i o n . An o l d e r f a i l u r e o f s i m i l a r geometry o c c u r r e d 30 m t o the no r t h e a s t on the same l o g g i n g road. The avalanche o c c u r r e d between 1970 and 1974 8, p o s s i b l y d u r i n g the 1971 d e b r i s t o r r e n t event. The d e b r i s c ut a 10 m wide swath i n the f o r e s t t o 395 m e l e v a t i o n , but was a r r e s t e d by lower angle s l o p e s and r e s i s t a n t v e g e t a t i o n . C o n t r i b u t i n g f a c t o r s were the same as a t LR-2. The d e b r i s c u t a 9 m wide swath i n the 31° s l o p e . By 43 5 m e l e v a t i o n a shallow drainage d e p r e s s i o n was ch a n n e l i n g the flow. At 395 m e l e v a t i o n , d e b r i s from the e a r l i e r avalanche was r e m o b i l i z e d . The combined d e b r i s mass scoured the d e p r e s s i o n and s p i l l e d out onto a p l a n a r s l o p e t o the ea s t . C o l l u v i a l s o i l , t i l l , and s l a b s of bedrock were exposed. A 10 m hig h , 59° c l i f f l e d t o Smith Creek, a t 327 m e l e v a t i o n . ° - A i r photos from Washington S t a t e DNR. 1970 photos: P r o j e c t symbol NW-69, 240 45B-90,91. 1974 photos: P r o j e c t symbol NWH-74, 4E-74,75. 1983 photos: P r o j e c t symbol NW-C-83, 8-45-048,049. 66 4.8 D i s c o n t i n u i t y s u r f a c e s The l a s t avalanche group c o n s i s t s of f a i l u r e s c o n t r o l l e d by smooth, rock d i s c o n t i n u i t y s u r f a c e s . Tsukamoto and Kusakabe (1984) s t a t e t h a t t r e e r o o t s on such s l o p e s , "... reach the u n d e r l y i n g base rock s u r f a c e but cannot p e n e t r a t e i n t o the rock because i t has no f i s s u r e s . " They found these s l o p e s t o be the l e a s t s t a b l e i n t h e i r c l a s s i f i c a t i o n system. O'Loughlin and Pearce (1976) r e c o g n i z e d t h i s f a i l u r e geometry on l a t e T e r t i a r y sandstones i n New Zealand. The sandstone s u r f a c e s are d e s c r i b e d as, "smooth, p l a n a r , ... and r a r e l y p e n e t r a t e d by t r e e r o o t s . Shear s t r e n g t h a t the r e g o l i t h - s a n d s t o n e i n t e r f a c e i s low." In a d d i t i o n , the smooth s u r f a c e may have low f r i c t i o n a l r e s i s t a n c e t o shear. Lambe and Whitman (1979) suggest use of a r e s i d u a l f r i c t i o n angle f o r f r i c t i o n a l r e s i s t a n c e of sand a g a i n s t a rough p l a n a r s u r f a c e such as c o n c r e t e . In t h i s case, the s u r f a c e i s the sandstone d i s c o n t i n u i t y . The r e s i d u a l f r i c t i o n angle i s o f t e n lower than a peak f r i c t i o n angle f o r shear w i t h i n the s o i l . F r i c t i o n angles are d i s c u s s e d f u r t h e r i n Chapter 5. In the study area, d i s c o n t i n u i t y s u r f a c e s exposed a t headscarps showed l i t t l e or no evidence of f i s s u r e s or sheared r o o t s . In most cases the d i s c o n t i n u i t i e s are bedding s u r f a c e s t h a t d i p downslope, but they can be e x f o l i a t i o n j o i n t s i n massive sandstone beds. Two avalanches on the shore of Lake Whatcom, DS-1 and DS-2, were i n v e s t i g a t e d . 67 4 . 8 . 1 D S - 1 Avalanche DS-1 i s depicted i n Figure 4.11. About 58 m3 of s o i l i n i t i a l l y f a i l e d i n an 8 m long X 8 m wide slump at 202 -209 m. The bedding surface s t r i k e at the headscarp was roughly p a r a l l e l to the slope contours, while the 36° dip was s i m i l a r to the 29° headscarp slope. The headscarp geometry was s i m i l a r to a wedge i n that a pocket of deep s o i l was embedded i n the slope. S o i l depths were 1.2 m i n the middle of the scarp, and thinned to .75 m downslope. The s o i l was dense and poorly drained. The area was probably logged between 1920 and 1940, and regenerated to mixed forest. Headscarp vegetation consisted of Tsuga. Alnus, and Pseudotsuga to 20 m with a Polystichum and S a l a l understory. Occasional Pseudotsuga to 40 m were l e f t behind a f t e r logging. The smooth discontinuity and deep s o i l were the important c o n t r o l l i n g factors. Some slump debris remained at the headscarp, unlike at other debris avalanches. The rest of the debris cleared an 8 m swath down a 34° slope below. The debris path was underlain by a smooth discontinuity surface. Other slumps occurred on the slope that did not develop into debris avalanches. About 14 m northeast of the DS-1 headscarp a p a r t i a l l y f a i l e d slump was found. The slump had a well defined headscarp with 1 m displacement, an 8 m long X 5 m wide main body of disturbed s o i l , and a bulging toe with trees t i l t e d downslope. More slumps were found upslope to the northeast. About 60 m southwest a slump developed i n t o a s h o r t d e b r i s avalanche, which was h a l t e d 30 m downslope by o l d growth c o n i f e r s . The t r e n d of the slumps was s i m i l a r t o the t r e n d of a r i d g e above, t h e r e f o r e the d i s t a n c e s t o the drainage d i v i d e were s i m i l a r a t d i f f e r e n t s c a r p s . A secondary scarp developed i n DS-1 a t 180 m e l e v a t i o n , c a u s i n g the sc a r p width t o expand from 8 t o 24 m. A i m s o i l p r o f i l e f a i l e d on a p l a n a r , 37° s l o p e . F a i l u r e was probably t r i g g e r e d by the impact of d e b r i s from above. Debris avalanched down the steep, p l a n a r slope, exposing smooth bedrock, c o l l u v i u m , and g l a c i a l t i l l . At 148 m e l e v a t i o n the sl o p e steepened t o 45° and a shallow d e p r e s s i o n developed i n the s l o p e . A i r photos from 1978 show t h a t the headscarp of an e a r l i e r d e b r i s avalanche o c c u r r e d a t t h i s e l e v a t i o n 9 . At 102 m e l e v a t i o n the sl o p e eased t o 13°, and some d e b r i s was d e p o s i t e d . The r e s t of the d e b r i s flowed i n t o Lake Whatcom. 4.8.2 DS-2 Avalanche DS-2 i s d e p i c t e d i n F i g u r e 4.12. About 22 m3 of s o i l i n i t i a l l y f a i l e d on a 15 m long X 4 m wide d i s c o n t i n u i t y s u r f a c e d i p p i n g downslope a t 43°, a t 150 - 156 m e l e v a t i o n . A .55 m t h i c k s o i l p r o f i l e t h i c k e n e d t o .8 m as the s l o p e eased t o 31° downslope. The headscarp geometry i s s i m i l a r t o the y - A i r photos from Washington S t a t e DNR. 1978 photos: P r o j e c t symbol NW-78, 57D-16,17. 69 secondary headscarp a t W-2, where drainage from a steep rock outcrop i n i t i a t e d f a i l u r e on shallower s l o p e s and t h i c k e r s o i l below. The area was probably logged between 1920-1940, and regenerated t o mixed f o r e s t . Headscarp v e g e t a t i o n c o n s i s t e d of sparse Tsuga, Arbutus, and Alnus t o 15 m. The Arbutus i s i n d i c a t i v e of rocky s o i l w ith abundant l i g h t . O c c a s i o n a l Pseudotsuga t o 40 m were l e f t behind a f t e r l o g g i n g . A dense un d e r s t o r y c o n s i s t e d of S a l a l , Acer, and scrub Tsuga. The smooth bedrock s l a b and r e l a t i v e l y weak v e g e t a t i o n c o n t r o l l e d i n i t i a l f a i l u r e . About 32 m3 of s o i l f a i l e d i n a secondary headscarp on a steep, p l a n a r s l o p e t o the south, at 132 - 140 m e l e v a t i o n . The sca r p developed i n a .9m t h i c k p r o f i l e o v e r l y i n g a 6 m long X 6 m wide, 46° bedding d i s c o n t i n u i t y . V e g e t a t i o n was s i m i l a r t o t h a t a t the upper headscarp. Scars from the two f a i l u r e s combined a t 132 m e l e v a t i o n t o cut a 15 m swath down the 38° s l o p e . The d e b r i s was channeled by t h r e e shallow drainage d e p r e s s i o n s . A 40 m Pseudotsuga on a r i d g e between de p r e s s i o n s r e s i s t e d f a i l u r e . At 99 m e l e v a t i o n the s l o p e l e s s e n e d t o 18° as some l o g d e b r i s was d e p o s i t e d . Much of the d e b r i s flowed i n t o Lake Whatcom. 4 . 9 Washouts Washouts were observed on low g r a d i e n t creeks without steep s i d e s l o p e s or headwaters. The f i r s t order channels showed 70 sediment scouring and exposed roots, plus disturbed understory vegetation with a few f a l l e n trees on banks. There was no evidence of slope f a i l u r e at the headwaters or on depression sideslopes. The scarp at DD-3 most cl o s e l y resembled a washout. Washouts were the p r i n c i p l e f a i l u r e mode on pre-Tertiary p h y l l i t e s south of the study area. 4.10 Rock s l i d e s A li m i t e d number of rock s l i d e s were triggered by the January 9-10, 1983 storm. Figure 4.1 shows the location of a rock s l i d e on the F i r s t South Fork of Smith Creek. The s l i d e headscarp was fed by surface water diverted by a logging road. The road captured runoff from a number of small drainage depressions as i t traversed the slope toward 165° azimuth. Above the scarp the road switchbacked toward 315° azimuth, while the runoff continued downslope. Channel dimensions and slope indicate that the maximum discharge was about .3 m3/s. The topography before f a i l u r e steepened from 25° on a plateau above the channel to 43° where the channel sidewalls were eroded. The rock consisted of heavily fractured, weathered mudstone and fine sandstone. The apparent dip of bedding surfaces was 42° down the f a l l l i n e . About 5,000 m3 of s o i l and rock disintegrated at f a i l u r e or on impact downslope. Much of the coarse rock debris remained below the scarp i n the channel. Much larger f a i l u r e s also occurred. Thorsen (1987) reported a rock block s l i d e on the southwest side of Stewart Mountain, near the p r e - T e r t i a r y p h y l l i t e c o n t a c t . F a i l u r e o c c u r r e d i n an e r o s i o n a l l y undercut carbonaceous s h a l e bed d i p p i n g i n t o a canyon. The b l o c k s l i d as a more or l e s s i n t a c t mass, d i s t u r b i n g an area of a t l e a s t 20,000 m2 . Thorsen suggested t h a t f a i l u r e developed s l o w l y as the canyon was eroded, and the January, 1983 p r e c i p i t a t i o n t r i g g e r e d the s l i d e . Both s l i d e s were t r i g g e r e d by e x t r a o r d i n a r y water p r e s s u r e s along bedding d i s c o n t i n u i t i e s . C o n t r i b u t i n g f a c t o r s i n c l u d e d downslope bedding d i p s , s l o p e oversteepening by e r o s i o n , and a f r a c t u r e d and weathered f i n e g r a i n e d rock mass. 4.11 D i s c u s s i o n Wedges, drainage d e p r e s s i o n s , l o g g i n g roads, and d i s c o n t i n u i t y s u r f a c e s are d i s t i n g u i s h e d as d i s t i n c t headscarp geometries at the avalanches s t u d i e d . The s i g n i f i c a n t f a c t o r s c o n t r o l l i n g i n i t i a t i o n are o u t l i n e d f o r each geometry. These f a c t o r s i n c l u d e s l o p e angle, s o i l depth, s o i l d e n s i t y , r o o t cohesion, d i s c o n t i n u i t y s u r f a c e s , and snow. Washouts and rock d e b r i s avalanches are v a r i a t i o n s on t y p i c a l d e b r i s avalanche forms. Washouts are the s c o u r i n g of drainage d e p r e s s i o n s without Coulomb f a i l u r e . Rock s l i d e s and rock b l o c k s l i d e s i n v o l v e f a i l u r e of f r a c t u r e d , weathered mudstone along d i s c o n t i n u i t i e s . Avalanche d e s c r i p t i o n s i n d i c a t e t h r e e scour zones, which correspond t o the t h r e e s l o p e segment types o u t l i n e d i n Chapter 2. An i d e a l i z e d d e b r i s avalanche would s t a r t i n Zone 1 and have 72 the f o l l o w i n g e f f e c t s as i t passes through each scour zone: Zone 1: P l a n a r s l o p e s above drainage d e p r e s s i o n s Complete r e s i d u a l s o i l p r o f i l e removal a t the avalanche headscarp and steep, p l a n a r s l o p e s below. Bedrock exposed. P a r t i a l s o i l p r o f i l e removal ("skimming") on g e n t l e r , p l a n a r s l o p e s below headscarp. Colluvium and/or t i l l c over exposed. Scarp width i n c r e a s e s or remains constant as d e b r i s moves downslope. Zone 2: Drainage d e p r e s s i o n s P a r t i a l o r complete removal of r e s i d u a l s o i l and c o l l u v i u m i n the drainage d e p r e s s i o n . M o b i l i z a t i o n a i d e d by c o n c e n t r a t i o n of water i n the d e p r e s s i o n , l o o s e s o i l , and weak v e g e t a t i o n . D e b r i s channeled i n the d e p r e s s i o n . Zone 3: F i r s t order channels Complete removal of t h i n a l l u v i u m and c o l l u v i u m i n the channel, exposing a bedrock trough t h a t channels the d e b r i s . Most of the v e g e t a t i o n i n the path of any avalanche i s removed, f i r m l y anchored t r e e s with h i g h shear s t r e n g t h r o o t s b e i n g a n o t a b l e e x c e p t i o n . Avalanches W-l and W-2 were i n i t i a t e d i n Zone 1 and had the d e s c r i b e d e f f e c t s i n a l l t h r e e zones. F i g u r e 4.13 d e p i c t s these zones a t W-2. Avalanche DD-1 was i n i t i a t e d i n Zone 2 near the t r a n s i t i o n t o a f i r s t order channel. Zone 3 became p o o r l y developed below the headscarp, so the d e b r i s was not c h a n n e l i z e d . F a i l u r e s o c c u r r e d i n a l l t h r e e zones t o c o n t r i b u t e sediment t o DD-3. DD-2 and LR-1 were i n i t i a t e d h i g h i n Zone 2 and moved i n a w e l l d e f i n e d Zone 3 channel. LR-2, DS-1, and DS-2 were a l l i n i t i a t e d i n Zone 1 and moved downslope i n t o p o o r l y d e f i n e d Zone 2 drainage d e p r e s s i o n s . F i g u r e 4.14 shows how d e p r e s s i o n development was discouraged by a l a c k of 73 d i s c o n t i n u i t y i n the u n d e r l y i n g rock. Washouts were i n i t i a t e d i n Zone 3 or Zone 2. The d i s c u s s i o n makes c l e a r the numerous paths by which d e b r i s reaches l o c a l base l e v e l s . 74 T a b l e 4.1: Scarp dimensions and volumes removed Volume of T r a v e l E l e v a t i o n i n i t i a l T o t a l volume Avalanche: d i s t a n c e , drop, f a i l u r e 1 , mass wasted 2, m m m3 m3 W-l 285 159 50 1805 W-2 377 184 110 8695 DD-1 150 89 29 1400 DD-2 305 198 70 3855 DD-3 420 224 1 9 3 3135 LR-1 220 142 24 2570 LR-2 205 125 130 2690 DS-1 180 113 58 2685 DS-2 160 65 22 3 1600 Average 256 144 57 3160 1 - I n i t i a l f a i l u r e volumes determined by e s t i m a t i n g the i n i t i a l source areas, i n d i c a t e d by complete s o i l p r o f i l e removal and a l a c k of s p l a y e d m a t e r i a l a t the scarp edges. 2 - T o t a l volume estimated by m u l t i p l y i n g scarp area by an average depth of 0.7 m. 3 - More than one i n i t i a l f a i l u r e occurred, volume g i v e n a p p l i e s t o h i g h e s t headscarp. LEGEND FOR FIGURES 4 .3 , 4.4, 4.6 - 4.12 7 5 PS - P l a n a r s l o p e f o r m , u s u a l l y u n d e r l a i n b y b e d r o c k d i s c o n t i n u i t y . DD - D r a i n a g e d e p r e s s i o n f o r m w i t h i n t h e a v a l a n c h e s c a r p . IDD _ I n c o m i n g d r a i n a g e d e p r e s s i o n j o i n i n g t h e a v a l a n c h e s c a r p . ODD - O u t g o i n g d r a i n a g e d e p r e s s i o n d i v e r t i n g d e b r i s a w a y f r o m t h e a v a l a n c h e s c a r p . FOC - F i r s t o r d e r c h a n n e l . IFOC ~ I n c o m i n g f i r s t o r d e r c h a n n e l j o i n i n g t h e a v a l a n c h e s c a r p . LR - L o g g i n g r o a d c r o s s i n g s c a r p . SS - S i d e s c a r p . W i d e n i n g o f a p l a n a r a v a l a n c h e s c a r p , l i k e l y t r i g g e r e d b y c a s c a d i n g d e b r i s f r o m a b o v e . S S F - S i d e s l o p e f a i l u r e i n d r a i n a g e d e p r e s s i o n s , l i k e l y t r i g g e r e d b y d e b r i s u n d e r c u t t i n g i n d e p r e s s i o n a x i s . SHS - S e c o n d a r y h e a d s c a r p , d e v e l o p e d i n d e p e n d e n t l y o f m a i n h e a d s c a r p . LBL - L o c a l b a s e l e v e l . S m i t h C r e e k , i t s b r a n c h e s , o r L a k e W h a t c o m . D - D e b r i s d e p o s i t e d o n a v a l a n c h e s c a r p . P - H y d r a u l i c a l l y e r o d e d p i p e s o b s e r v e d . Thu ja - S p e c i e s o f t r e e t h a t r e s i s t e d f a i l u r e . So: N15 1E, 70SW- B e d d i n g a t t i t u d e i n r o c k e x p o s e d a t h e a d s c a r p ( s t r i k e , d i p i n d e g r e e s , d i p d i r e c t i o n ) . A A ' - c r o s s s e c t i o n l o c a t i o n s f o r F i g u r e 4 . 5 76 F i g u r e 4.1: Debris avalanche l o c a t i o n map S C A L E 1:24,000 ' kilometer 1 Contour interval : 200 ft 77 F i g u r e 4.2: Wedge headscarp geometries a.) Avalanche W-l b.) Avalanche W-2 W -1 HEADSCARP W - 2 H E A D S C A R P 446-, . . _ HORIZONTAL DISTANCE. METERS HORIZONTAL DISTANCE. METERS c.) Humphrey, 1982 PERSPECTIVE VIEW OF SOIL WEDGE THE TREE ROOTS MAY NOT REACH BEDROCK AT THE CENTER OF THE WEDGE F i g u r e 4 . 4 : Wedges: W-2 So : N140E.50SW F i g u r e 4.5; Drainage d e p r e s s i o n headscarp geometries 80 F i g u r e 4.6; Drainage d e p r e s s i o n s : DD-1 83 Figure 4.8: Drainage depressions: DD-3 So : N110E.61NE 84 Figure 4.9: Logging roads: LR-1 85 F i g u r e 4.10: Logging roads: LR-2 F i g u r e 4.11: D i s c o n t i n u i t y s u r f a c e s : DS-1 So: N33E.36NW Figure 4.12: Discontinuity surfaces: DS-2 87 F i g u r e 4.13: Debris scour zones a t W-2 F i g u r e 4.14: Debris scour zones a t DS-1 89 90 CHAPTER 5: ENGINEERING PROPERTIES OF SOILS AT DEBRIS AVALANCHE HEADSCARPS 5.1 I n t r o d u c t i o n H i l l s l o p e hydrology and s l o p e s t a b i l i t y are analyzed i n Chapters 6 and 7 t o e x p l a i n why d e b r i s avalanches were i n i t i a t e d r and what d i s t i n g u i s h e s the d i f f e r e n t avalanche types. E n g i n e e r i n g p r o p e r t i e s of s o i l s are needed t o perform the a n a l y s e s . For h i l l s l o p e hydrology, r e q u i r e d parameters i n c l u d e 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 i e s of s o i l , c o l l u v i u m / t i l l , and rock, p l u s unsaturated s o i l p r o p e r t i e s and f i e l d c a p a c i t i e s . The parameters are a p p l i e d to r e c o n s t r u c t e d s l o p e geometries and s o i l depths t o determine the water l e v e l s and r e s u l t i n g pore p r e s s u r e s under storm p r e c i p i t a t i o n . To e v a l u a t e s l o p e s t a b i l i t y , s o i l u n i t weights and shear s t r e n g t h parameters are used t o g e t h e r w i t h the pore p r e s s u r e s . In t h i s c hapter the methods used t o d e r i v e the parameters are d e s c r i b e d and r e s u l t s are presented. The t e x t begins with simple p r o p e r t i e s such as s o i l g r a i n s i z e d i s t r i b u t i o n s and d e n s i t y parameters. More complex h y d r o l o g i c and g e o t e c h n i c a l p r o p e r t i e s are then developed. S o i l p r o p e r t i e s are compared with p u b l i s h e d f o r e s t s o i l study r e s u l t s i n the west coa s t r e g i o n . These i n c l u d e t h r e e from the U.S. northwest (Megahan and C l a y t o n , 1983; Humphrey, 1982; Harr, 1977), two from B r i t i s h Columbia ( U t t i n g , 1978; 6'Loughlin, 1972), and t h r e e from southeast A l a s k a ( S i d l e , 1984; S i d l e and Swanston, 1982; Swanston, 1970). The study r e s u l t s are included i n the s o i l property tables. In a l l cases the studies involve h i l l s l o p e hydrology or slope s t a b i l i t y analyses, or both. 5.2 Grain s i z e d i s t r i b u t i o n s and c l a s s i f i c a t i o n s S o i l s at the avalanche headscarps were analyzed for grain s i z e d i s t r i b u t i o n and textural c l a s s i f i c a t i o n . An additional sample, labeled 'TEST1, was analyzed and compared with headscarp s o i l s . The sample was used i n c h a r a c t e r i s t i c curve determinations i n Section 5.6. Samples about .2 m3 i n volume were taken i n the B horizon from the .2 m depth to the gravelly C horizon. Grain sizes were analyzed according to ASTM D422-63 procedures (ASTM, 1978) except that sieves were used down to .063 mm rather than .075 mm. A hydrometer analysis was used for the s i l t and clay fractions. Grain size d i s t r i b u t i o n curves are plotted i n Figure 5.1. C l a s s i f i c a t i o n r e s u l t s are presented i n Table 5.1. A comparison with other studies i n Table 5.1 shows that study area grain sizes are s i m i l a r to those of west coast forest s o i l s . Most s o i l s are described as sands or loams with varying percentages of s i l t and gravel. 5.3 Density parameters S o i l void r a t i o , porosity, and unit weight a l l give an i n d i c a t i o n of the packing of s o l i d p a r t i c l e s i n a given volume. The void r a t i o , e, equals the volume of voids divided by the 92 volume of s o l i d s . The p o r o s i t y , n, i s the r a t i o o f the volume of v o i d s t o the t o t a l volume. The b u l k or mass d e n s i t y , /^b, i - s the t o t a l mass d i v i d e d by the t o t a l volume. The u n i t weight or weight d e n s i t y i s the f o r c e per u n i t volume d e r i v e d from the product of the mass d e n s i t y and the g r a v i t a t i o n a l a c c e l e r a t i o n . The equations used t o determine e, n, T , (dry u n i t weight), and T s ( s a t u r a t e d u n i t weight) are i n c l u d e d i n Appendix I I . To determine T or a t f i e l d c a p a c i t y the mass based moisture content, 0 m, i s r e q u i r e d . Q m i s d e f i n e d as the mass of water d i v i d e d by the mass of dry s o i l . Sampling procedures f o r G m and 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 5.7. To o b t a i n u n d i s t u r b e d samples from avalanche headscarps a S o i l Core Sampler ( S o i l M oisture Equipment C o r p o r a t i o n , Model No. 200A) was used. Samples were taken a c c o r d i n g t o ASTM D2937-71 procedures (ASTM, 1978), which are s i m i l a r t o procedures d e s c r i b e d i n M i l n e (1977 p.8). The s o l i d s s p e c i f i c g r a v i t y (G s) i s r e q u i r e d i n s o i l d e n s i t y parameter equations. T h i s parameter was measured u s i n g a pycnometer method with a kerosene f l u i d (ASTM D854-58; ASTM, 1978). G S v a r i e d between headscarps and w i t h i n a headscarp area. To assess t h i s v a r i a t i o n , 6 samples (5 - 7 g mass) from each s c a r p were t e s t e d . The t e s t s were performed i n the S o i l S c i e n c e Department a t the U n i v e r s i t y of B r i t i s h Columbia. Mean headscarp G S v a l u e s with t h e i r standard d e v i a t i o n s and c o e f f i c i e n t s of v a r i a t i o n are presented i n Table 5.2. The c o e f f i c i e n t of v a r i a t i o n (CV), equal t o the standard d e v i a t i o n 93 (s) divided by the mean, i s a s t a t i s t i c a l measure of r e l a t i v e v a r i a t i o n within a sample. G s values for Chuckanut sandstone minerals are included i n the Table. At each headscarp G s v a r i a t i o n i s small, with CV's ranging from .3% to 1.2%. The G s range of 2.48 - 2.66 between headscarps i s p o t e n t i a l l y s i g n i f i c a n t . Mean G s values at each headscarp are therefore used i n density parameter determinations, rather than the o v e r a l l mean. The v a r i a t i o n between headscarps i s attributed to differences i n s o i l mineral composition. S o i l density parameters varied throughout the study area. In t h i s study normal d i s t r i b u t i o n s for e, n, and T are assumed within each headscarp covering a maximum area of 400 m2. Nielsen, et a l . (1973), Rogowski (1972), and Warrick and Nielsen (1980) found bulk densities to be normally d i s t r i b u t e d i n space. A c h i squared te s t showed Milne's (1977) void r a t i o data to be normally d i s t r i b u t e d (see Appendix I I I ) . To calculate the mean and standard deviation i n the d i s t r i b u t i o n 15 samples were taken at each headscarp. The choice of 15 samples was based pa r t l y on a li m i t e d number of core cylinders and pa r t l y on the observation that the mean did not change s i g n i f i c a n t l y with N > 15. Freund (1979 p.288) states that a minimum of 3 0 observations are required to perform s t a t i s t i c a l t e sts for normality. However, i f the values plot as a st r a i g h t l i n e on p r o b a b i l i t y paper a normal d i s t r i b u t i o n i s q u a l i t a t i v e l y confirmed. Figure 5.8 shows 15 void r a t i o s from avalanche LR-2 plotted on normal p r o b a b i l i t y paper. The s t r a i g h t l i n e observed suggests a normal d i s t r i b u t i o n and validates use of a mean value. Void r a t i o s from other headscarps also plotted as straight l i n e s on p r o b a b i l i t y paper and are included i n Appendix I I I . Void r a t i o , porosity, and dry unit weight means, with s values and CV's from each headscarp, are presented i n Table 5.3. The r e s u l t s show lower e and n and higher values than many of the selected studies. This could be i n part because the cores were not large enough to represent the s o i l as a whole. However, core sampling methods were si m i l a r to those described i n Harr (1977) and O'Loughlin (1972). Also, study area void r a t i o s as high as e = 1.48 were recorded at avalanche DD-1; t h i s value i s i n the middle of Harr's range and greater than 0 1Loughlin's value. Megahan and Clayton's (1983) data show that s i m i l a r void r a t i o s and p o r o s i t i e s have been recorded i n forest s o i l studies. Density parameters are used to es t a b l i s h other s o i l properties i n t h i s study. In Section 5.4 the complete void r a t i o d i s t r i b u t i o n s are used to estimate f r i c t i o n angles at the avalanche headscarps. Mean po r o s i t i e s are used i n Sections 5.6 and 5.7 to determine volumetric water contents. In Chapter 7 mean T values are u t i l i z e d i n s t a b i l i t y analyses. CV's for the parameter populations are with one exception greater than the CV's at i n d i v i d u a l headscarps. For t h i s reason i n d i v i d u a l mean values at each headscarp were used. 5.4 Shear strength parameters The modified Mohr-Coulomb equation, as defined i n Eg. 4.1, expresses a s o i l ' s resistance to shear f a i l u r e . The s o i l shear strength parameters, cohesion, C 1, and int e r n a l f r i c t i o n , 0 ' , are required i n order to assess s o i l shear strength. The cohesion provided by vegetation, C r, must also be evaluated. 5.4.1 S o i l S o i l cohesion develops as a r e s u l t of overconsolidation and or bonding i n a clay and s i l t - r i c h s o i l (Sidle, et a l . , 1985 p.45). Study area s o i l s were cohesionless, with one exception. The f r i c t i o n angle i s the angle of a Mohr-Coulomb f a i l u r e envelope from the horizontal, and i t represents the degree of p a r t i c l e i n t e rlocking and f r i c t i o n a l resistance to shear. During shear, the peak f r i c t i o n angle, 0 ' , occurs when a peak deviator stress i s recorded on a s t r e s s - s t r a i n curve. As s t r a i n increases further the deviator stress becomes constant and the f r i c t i o n angle reaches a residual value, 0'j- 1. Void r a t i o variations most s i g n i f i c a n t l y affected the f r i c t i o n angle within each avalanche headscarp. A t y p i c a l 0 ' vs e r e l a t i o n i s presented i n Figure 5.2. On the steeply sloped segment of the curve the s o i l i s denser than i t s c r i t i c a l void r a t i o ( ^ c r i t i c a l ) ' a n d 0 ' increases l i n e a r l y with decreasing e. At ©critical ^ e s°ii w i l l neither expand nor contract upon - Also referred to as the constant volume f r i c t i o n angle, or 0 ' c v . shear (Craig, 1978 p.94). Above ©critical s°il contracts upon shear to reach a constant residual f r i c t i o n angle, and the slope of the curve approaches zero. In t h i s study, 0 ' vs e re l a t i o n s are produced for each avalanche. Linear segments approximate the e < S c j - i t i c a l and e > e c r ^ ^ i c a ^ (or residual f r i c t i o n angle) portions of the curves. Void r a t i o d i s t r i b u t i o n s were applied to the 0 ' vs e plots with a weighting scheme, and an integrated mean f r i c t i o n angle was derived. Direct shear tests were performed on disturbed samples from each headscarp. B horizon samples from the .2 m depth to bedrock or the gravelly C horizon were tested. The samples were a i r dried and the gravel size or greater f r a c t i o n (>2 mm) was removed by sieving. The 0 ' of the sieved sample i s assumed to be s i m i l a r to 0 ' for the unsieved sample. Direct shear te s t lab procedures conform to ASTM 3080-72 standards (ASTM, 1978). Between 6 and 8 data points were used to develop the 0 1 vs e pl o t s . In a l l cases the normal load applied was equal to the moist unit weight from each scarp (Table 5.6 and Appendix II) mult i p l i e d by the average depth of .74 m. The tests were performed i n the C i v i l Engineering Department at the University of B r i t i s h Columbia. A weighting scheme i n Sharma and Luxmoore (1979 p.1571) was modified to derive an integrated mean f r i c t i o n angle. The normal void r a t i o d i s t r i b u t i o n s were divided into f i v e equal area sections, each representing 20% of the d i s t r i b u t i o n . A Z - table (Mendenhall, 1983 p.A6) was used to determine - Z 5 values that bis e c t the areas. These values are used i n the Z equation to determine void r a t i o s e± - e 5. The void r a t i o s are then applied to the f r i c t i o n angle void r a t i o curve to determine f r i c t i o n angles (p* ^ - 0 ' s - The mean of the f i v e f r i c t i o n angles represents an integrated peak value. Figure 5.3 depicts t h i s method for s o i l at avalanche W-l. Table 5.4 summarizes weighted mean and residual f r i c t i o n angles for the nine avalanche s o i l s . These values are used i n the s t a b i l i t y analyses. Eight of the nine s o i l s had f r i c t i o n angles within the 27° -50° range expected by Craig (1978 p.95) for cohesionless sands and sandy gravel. One of the s o i l s , DD-3, had a low void r a t i o d i s t r i b u t i o n , causing an u n r e a l i s t i c a l l y high f r i c t i o n angle of 50.8°. The s o i l i s c l a s s i f i e d in Table 5.1 as a GM ( s i l t y sandy gravel, 17% s i l t and clay) and shows s l i g h t p l a s t i c i t y . A cohesion intercept of 9.6 kPa (200 psf) was determined using a maximum estimated f r i c t i o n angle of 34°. Calculations are included i n Appendix IV. Table 5.4 shows that some study area f r i c t i o n angles are s i g n i f i c a n t l y lower than those reported in other studies (W-l, DD-1, DD-2, and LR-1). This difference might have been caused by the removal of gravel-size grains i n d i r e c t shear t e s t s . However, a review of r e l a t i v e densities i n study area s o i l s indicates that the values are r e a l i s t i c . The r e l a t i v e density, RD, equals, (e m a x - e) / (e m a x - e m i n ) . The study area s o i l s showed cp' = 37..7° - 45.0° near the maximum RD of 1 i n the d i r e c t shear t e s t s . However, Table 5.4 shows that low 0' s o i l s were 98 near t h e i r loosest state (RD = 0) at the avalanche headscarps. Therefore, f r i c t i o n angles closer to the <p'r range of 27.8° -3 0.0° would be expected. The 0' r values i n Table 5.4 were confirmed by observing the angles of repose of poured dry sand p i l e s . The angle of repose i s roughly eguivalent to the residual f r i c t i o n angle (Brunsden and Prior, 1984). 5.4.2 Roots An approach s i m i l a r to Sidle and Swanston (1982) i s used to back calculate C r. In t h e i r study, pore pressures were estimated using piezometers adjacent to a headscarp of known geometry. At f a i l u r e the factor of safety equaled one and a peak C r of 2.02 kPa was back calculated i n Vaccinium alaskense (blueberry) and Oplopanax horridum (devils club) i n Alaska. In t h i s study, pore pressures are generated with an i n f i l t r a t i o n model and kinematic wave analysis. 5.5 Hydraulic conductivities Hydraulic conductivity, K, measures the a b i l i t y of a porous medium to transmit f l u i d . K i s used to evaluate r a i n f a l l i n f i l t r a t i o n into s o i l , and to evaluate the s o i l ' s a b i l i t y to discharge water downslope. K i s defined by Darcy's Law, K i n m/s = -Q / (<5H/<51) A (5.1) where, Q = discharge, mJ/s <SH/<S1 = hydraulic gradient 2 A = cross sectional area, m2 In t h i s study, separate K values were used to evaluate i n f i l t r a t i o n and then downslope movement of groundwater during the storm. The parameter K s a t (saturated hydraulic conductivity) was used to assess the i n f i l t r a t i o n of r a i n f a l l and discharge of water per unit area of s o i l . K s a^ i s controlled by s o i l matrix properties such as p a r t i c l e size, void r a t i o , composition, f a b r i c , and degree of saturation (Lambe and Whitman, 1979). Data were obtained using i n s i t u tests described i n Section 5.5.1. I n f i l t r a t i o n of r a i n f a l l took place i n unsaturated s o i l , so the unsaturated hydraulic conductivity must be assessed. Unsaturated conductivity data were obtained using lab tests, which are discussed in Section 5.6. A second K value, K ^ ] ^ , was used to assess saturated, downslope movement of water during the storm. K ^ ] ^ i s controlled by large voids i n the s o i l matrix, c a l l e d macropores, which increase the bulk hydraulic conductivity of the s o i l (Sidle, et a l . , 1985 p.43). In the study area, macropores r e s u l t mainly from decayed root channels and hydraulic pipes eroded by subsurface flow. K^y]^ estimation i s discussed i n Section 5.5.2. The rock and c o l l u v i u m / t i l l are assumed to be low K, 2 - The hydraulic head, H, i s defined as the sum of the pressure head, h, and the elevation head, z. 100 r e l a t i v e l y impermeable units through which n e g l i g i b l e seepage occurs. K assessments for these units are included i n Section 5.5.3 to v e r i f y t h i s assumption. 5.5.1 S o i l m a t r i x A well permeameter method developed by Talsma and Hallam (1980) was used to determine K s a^ for the s o i l matrix. The method i s based on the analysis of three dimensional, steady state water flow around a v e r t i c a l , c y l i n d r i c a l source. The tests are performed i n s i t u , with a minimum of s o i l disturbance. After a short, non-steady flow period, a steady i n f i l t r a t i o n rate, 0., occurs into an augered hole of radius, r, wetted depth, H, and depth to bedrock flow boundary, S, from the bottom of the augerhole. I f S > 2H the equation for saturated hydraulic conductivity i s , K s a t = Q / 2 Tr H 2 ( s i n n " 1 ( H / r ) - 1 ) (5.2) The f i e l d apparatus u t i l i z e d i s a constant head permeameter consisting of two concentric a c r y l i c tubes with a v e r t i c a l l y adjustable base. To determine Q , water le v e l s i n the tube are monitored as water i n f i l t r a t e s into a s l i g h t l y wider augerhole. Figure 5.4 depicts the permeameter and various parameters i n Eq. 5.2. The permeameter was constructed by Ray Rodway i n the Department of Geological Sciences at the University of B r i t i s h Columbia. The well permeameter i s r e l a t i v e l y new method of evaluating 101 Kgat i n forest s o i l s . One potential shortcoming i s that c a p i l l a r i t y i s not accounted for i n the ca l c u l a t i o n s . Stephens, et a l . (1987) used numerical solutions and borehole tests to show that c a p i l l a r i t y does not change Ksa-j- by a factor greater than two. To evaluate the method ten K s a t values were determined i n Milne's (1977) study area and compared to h i s established K s a^ population. A Student's - t te s t confirmed the hypothesis that the permeameter samples belonged to the same d i s t r i b u t i o n as Milne's K s a t population. Calculations are included i n Appendix V. In the study area, 7 to 10 hydraulic conductivity tests were performed at each headscarp. The number of measurements depended on the water haul distance and the consistency of r e s u l t s obtained. The t o t a l hole depth was t y p i c a l l y 20 - 30 cm, with the permeameter tube usually adjusted so that H = 10 cm. Plots of cumulative flow versus time showed that the sandy s o i l reached a steady state outflow rate within 10 minutes i f the s o i l was wetted p r i o r to the t e s t . A t y p i c a l i n f i l t r a t i o n rate versus time plot i s included i n Appendix V. K values vary s p a t i a l l y within the debris avalanche headscarps and throughout the basin. Freeze (1975 p.728), Nielsen, et a l . (1973 p.236), and Talsma and Hallam (1980 p.144) found K populations to be log normally d i s t r i b u t e d . In t h i s study the number of in s i t u readings from each headscarp i s too small to t e s t s t a t i s t i c a l l y for a log normal d i s t r i b u t i o n or to p l o t on normal p r o b a b i l i t y paper. A log normal d i s t r i b u t i o n i s 102 therefore assumed. K s a t i s influenced by the v i s c o s i t y of the f l u i d involved, hence a water v i s c o s i t y correction was applied. Water temperatures were recorded during permeameter tests and the v i s c o s i t y at those temperatures determined (Lambe, 1951 p.148). Calculated Ksa+- values were adjusted to water v i s c o s i t i e s at near freezing temperatures during winter storms. The 2°C (36°F) temperature i s considered representative, based on data i n Table 3.5. The conversion equation i s included i n Appendix V. Geometric mean K s a-£ values for each debris avalanche s o i l are summarized i n Table 5.5. Means, standard deviations, and CV's for the parameter Y are included to give an i n d i c a t i o n of K s a t v a r i a b i l i t y within each headscarp. CV magnitudes are dependent on the units chosen, therefore values are only useful for comparison between headscarps. Ksa-j- values are s i m i l a r to the selected studies l i s t e d . 5.5.2 Bulk s o i l ^bulk ^ s *-he saturated hydraulic conductivity of a s o i l mass i n which macropores contribute s i g n i f i c a n t l y to subsurface flow. Tracer and trench interception over short distances (commonly 1 m) are techniques used to evaluate K ^ ] ^ (Sidle et. a l , 1985). The r a t i o Kfc>ulk/ Ksat ranged from 10 - 2 0 in Megahan and Clayton's (1983) tracer study i n Idaho. Mosley (1979) determined maximum Kbu^/Kg^ values of about 20. Kbulk f ° r t h i s study w i l l be assessed in Chapter 6, when the e f f e c t s of 103 d i f f e r e n t estimates on the water table p r o f i l e s are plotted using a kinematic wave equation. 5.5.3 Rock and c o l l u v i u m / t i l l K values for the matrix and fractures of Chuckanut Formation rock were assessed. A f a l l i n g head hydraulic conductivity test was used to determine the matrix conductivity of a saturated, medium to coarse grained sandstone sample of porosity, n = .046. The te s t was carr i e d out i n the S o i l Science Department at the University of B r i t i s h Columbia. The K adjusted to 2°C was 3.4 X 10 - 9 m/s (calculations are included i n Appendix V). The rock matrix K i s about four orders of magnitude lower than s o i l matrix Ksa-j- values. F i e l d observations indicate that the fractures are also of low conductivity. Bedding and j o i n t d i s c o n t i n u i t i e s were t i g h t and did not produce flow i n discharge areas during rainstorms. Freeze and Cherry (1979 p.29) state that fractures increase the bulk K i n rock. The r e l a t i v e l y undeformed nature of the sandstone ( i . e . broad f o l d patterns, widely spaced d i s c o n t i n u i t i e s ) indicated low bulk K values. The above evidence suggests that seepage i n rock was n e g l i g i b l e r e l a t i v e to seepage through the higher K s o i l above i t . At some headscarps the bedrock was mantled by a thin, dense, c o l l u v i u m / t i l l without macropores. Well permeameter tests were used to assess K for t h i s material. The values determined were 2.9, 2.7, and 5.9 X 10~ 6 m/s at three d i f f e r e n t headscarps. 104 These values are s i g n i f i c a n t compared to the mean s o i l Ksa-j- of 4.5 X 1 0 - 5 m/s. However, the downslope seepage volume produced by the t h i n c o l l u v i u m / t i l l was n e g l i g i b l e r e l a t i v e to seepage through the s o i l . 5.6 S o i l matrix c h a r a c t e r i s t i c curves In unsaturated s o i l , negative pressure heads (or p o s i t i v e tension heads) occur and the hydraulic conductivity drops below Ksa-j-. As the pressure head, h, decreases the volumetric moisture content, 6 V, and hydraulic conductivity, K(h), decrease n o n l i n e a r l y 3 . 0 V equals the volume of water divided by the t o t a l volume i n a given sample. At saturation the moisture content (Q v-sat) equals the porosity. K(h) i s the unsaturated hydraulic conductivity, which i s pressure head dependent. The 6 V vs h and K(h) vs h rel a t i o n s are labeled by Freeze and Cherry (1979 p.41) as c h a r a c t e r i s t i c curves. The c h a r a c t e r i s t i c curves are hysteretic, meaning that the curve shapes depend on whether the s o i l i s wetting or drying. C h a r a c t e r i s t i c curve estimates are required for the r a i n f a l l i n f i l t r a t i o n analysis. Predictive equations i n Van Genuchten (1980) and Mualem (197 6) were used to determine the c h a r a c t e r i s t i c curves. The parameters m, n, and a were used to f i t curves to observed s o i l moisture retention data. The parameters were then used i n a 3 - Some s o i l s have 'tension-saturated' zones. At small negative pressure heads the s o i l i s s t i l l saturated and at i t s Ksat value. At the 'ai r entry pressure head' the s o i l becomes unsaturated. 105 p r e d i c t i v e equation for K(h) vs h. Nielsen, et a l . (1986 p.95S) note that the predictive equations work well i n medium to coarse textured s o i l s and are less accurate i n fine grained s o i l s . The p r e d i c t i v e technique i s therefore appropriate for study area s o i l s . S o i l moisture retention data were developed using 'TEST', a sample from the DS-1 headscarp. Table 5.1 and Figure 5.1 show that the sample's grain size d i s t r i b u t i o n i s s i m i l a r to d i s t r i b u t i o n s at other headscarps. Therefore, the sample was considered representative of study area s o i l s as a whole. A hanging water column device s i m i l a r to a suction table was used to determine moisture contents for pressure heads of 0 to -1.6 m. Klute (1986, p.657) found the method appropriate for heads of up to -2 m. A description of the apparatus and procedures used i s included i n Appendix V. Two retention curves were run under a tamped dense packing (average n = .456), and two were run under a poured loose packing (average n = .481). Porosities greater than n = .482 could not be produced with the reconstituted sample. Moisture contents at lower pressure heads were required to complete the retention curve. Both predictive equations require a parameter labeled the residual moisture content ( 6 v _ r e s ) . Van Genuchten (1980 p.894) defines © v-res a s t n e moisture content under a -150 m pressure head. The moisture content at h = -3 m was also evaluated as an estimate of f i e l d capacity, or the l i m i t of free drainage within a s o i l . Porous plate extractors 106 were used to determine the moisture contents at those two pressure heads. Test apparatus and procedures conformed to ASTM D2325-68 (ASTM, 1978) and descriptions i n Klute (1986, p.644). Five loose samples and f i v e tamped samples were tested at each pressure head. Consistent results were obtained for the ten samples, with CV's of 1.1% and 1.2% for h = -3 m and h = -150 m, respectively. The s o i l , therefore, compacts to a uniform minimum density under low pressure heads. Volumetric moisture contents were calculated under the assumption that the s o i l had compacted to the minimum n = .452 from Run 2 i n the water retention t e s t s . A l l retention tests were carr i e d out i n the S o i l Science Department at the University of B r i t i s h Columbia. Van Genuchten's (1980) equation was used to develop a s o i l moisture content versus pressure head curve to f i t the retention data, e v = ®v-res + ®v-sat ~ ®v-res (5.3) [ 1 + |ah|S ] m where, 0 v - r e s = residual moisture content ®v-sat = saturated moisture content h = pressure head a, n, m = equation parameters D i f f e r e n t i a t i o n of Eq. 5.3 leads to series of equations that are used to estimate m. The parameters n and a are related to m and can be derived once m i s known. In t h i s case the parameters derived from the equation gave a good f i r s t estimate of the 107 water r e t e n t i o n data. T r i a l and e r r o r adjustment of the parameters produced the f i t s seen i n F i g u r e 5.5. The curves are f i t t e d t o p o r o s i t y data f o r n = .481 and n = .456. Table 5.3 shows t h a t mean p o r o s i t i e s f o r avalanches LR-1 and LR-2 can be approximated by the n = .481 curve, and means f o r DS-1 and DS-2 are approximated by the n = .456 curve. Separate curves were c r e a t e d t o r e p r e s e n t the mean p o r o s i t i e s f o r oth e r d e b r i s avalanches. The m, n, and a v a l u e s used t o develop the new curves were estimated u s i n g the observed parameter d i f f e r e n c e s i n the n = .456 and n = .481 curves. The curves f o r W-l, W-2, and DD-2 are estimated u s i n g an average n = .513, and the curve f o r DD-1 i s estimated with n = .570. S o i l s a t the low d e n s i t y avalanche DD-3 were n e a r l y s a t u r a t e d at the onset o f r a i n f a l l , so the r e t e n t i o n curves f o r n = .456 were used as an approximation. The f o u r moisture r e t e n t i o n curves are presented i n F i g u r e 5.6. Curves from Humphrey (1982) and Hadas (1967) are i n c l u d e d t o d e p i c t r e t e n t i o n data i n h i g h e r and lower p o r o s i t y s o i l s , r e s p e c t i v e l y . The m, n, and a v a l u e s used t o produce the study area s o i l curves are i n c l u d e d i n Appendix V. Mualem's (1976) equation i s used t o develop a r e l a t i v e h y d r a u l i c c o n d u c t i v i t y vs h r e l a t i o n . The r e l a t i v e h y d r a u l i c c o n d u c t i v i t y i s the r a t i o of K(h) t o Ksa^-, K ( h ) / K s a t = s e ° ' 5 [ 1 - (1 - Se V ^ m ]2 ( 5 . 4 ) where, S e = e f f e c t i v e s a t u r a t i o n = ( 9 V - ©v-res) / (®v-sat ~ ®v-res) m = parameter from Eq. 5.3 108 At saturation K(h)/K s a-£ equals 1. Relative hydraulic conductivities were calculated for the four porosity groups. Figure 5.7 depicts the K(h)/Ksa-(- vs h curves for study area s o i l s plus data from Humphrey (1982) and Hadas (1967). The s o i l s show order-of-magnitude decreases i n hydraulic conductivity as negative pressure heads are applied. Relative c o n d u c t i v i t i e s were converted to absolute values by multiplying by the geometric mean K s a t values (Table 5.5). Both c h a r a c t e r i s t i c curves are assumed to be single valued. This means that the moisture retention curve approximates both the wetting and retention curves, and the drying conductivity curve represents both wetting and drying co n d u c t i v i t i e s . H i l l e l (1980a p.152, 201) states that t h i s approximation leads to s i g n i f i c a n t errors i n the G m vs h curve. The K(h) vs h r e l a t i o n i s affected by hysteresis, "to a much lesser degree". Single valued curves are often used i n modeling studies because wetting curves and scanning curves between the main branch curves are d i f f i c u l t to determine ( H i l l e l , 1980a). Humphrey (1982) used single valued c h a r a c t e r i s t i c curves i n a f i n i t e element model. In t h i s study the K(h)/Ksa-t- vs h curves are more important because hydraulic conductivity controls the discharge rate from the s o i l p r o f i l e . The error i n the 8 m values i s accepted due to the r e l a t i v e accuracy of the K(h) values. 5.7 Moisture contents The moisture content at which internal drainage becomes 109 n e g l i g i b l e has been described as the f i e l d capacity. H i l l e l (1980b p.67) found the term d i f f i c u l t to define, but stated that coarse textured s o i l s i n which internal drainage i s i n i t i a l l y rapid and then l e v e l s o f f are best suited to the description. In the study area, a 24-hour drainage period a f t e r s i g n i f i c a n t r a i n f a l l i s suggested as s u f f i c i e n t for drainage to the f i e l d capacity i n the coarse textured s o i l s . The f i e l d capacities were determined to esta b l i s h i n i t i a l conditions before the onset of the storms. The S o i l Core Sampler described in Section 5.3 was used to take samples for 0 m and 0 V at the avalanche headscarps. Samples were taken i n the B horizon from the .2 m depth to the gravelly C horizon. Time was not taken to trim the cores to a s p e c i f i c volume because numerous samples were required i n a short period. The cores were sealed i n t i n s with e l e c t r i c a l tape, weighed i n the lab, oven-dried at 105°C for 2 4 hours, and then reweighed. 6 m values were calculated and converted to 6 V using mean n values from Table 5.3. As with other s o i l properties the moisture content varied s p a t i a l l y at each headscarp. Nielsen, et a l . (1973 p.231) and Rogowski (1972, p.1017) found that moisture contents varied about a normal d i s t r i b u t i o n , s i m i l a r to bulk densities. To assess moisture content v a r i a b i l i t y a minimum of 15 samples were taken. Again N = 15 samples were chosen because of the limi t e d number of core cylinders and the fact that the mean did not vary with additional samples. If N was greater than 15 the 110 additional samples were incorporated into the mean and standard deviation. To tes t for normality the recorded moisture contents from LR-2 are plotted on normal p r o b a b i l i t y paper i n Figure 5.8. The s t r a i g h t l i n e q u a l i t a t i v e l y v e r i f i e s the normal d i s t r i b u t i o n . Normal p r o b a b i l i t y plots of moisture content d i s t r i b u t i o n s at other headscarps are included i n Appendix I I I . During sampling the moisture contents did not appear to change with depth i n the s o i l p r o f i l e s . This observation was confirmed by r e l a t i v e l y constant G m values at d i f f e r e n t points i n a given B horizon ( p r o f i l e 9 m values are included i n the calculated means). Study area s o i l s therefore drain a f t e r a r a i n f a l l to the f i e l d moisture contents i n Table 5.7. Because the s o i l s are coarse grained most v e r t i c a l drainage within the s o i l p r o f i l e probably occurs within a day. Downslope drainage depends on values. F i e l d moisture content r e s u l t s are presented i n Table 5.7. Comparison with other studies was not undertaken because of the climate dependency of moisture content values. 5.8 D i s c u s s i o n Numerous techniques have been used to determine the engineering properties of avalanche headscarp s o i l s . Most techniques allow numerous samples to be taken to account for s p a t i a l v a r i a b i l i t y i n parameters. Distributions have been established for density parameters, hydraulic conductivities, and f i e l d moisture contents. Qualitative tests show that density parameters and f i e l d moisture contents vary about a normal d i s t r i b u t i o n , as suggested i n the l i t e r a t u r e . Hydraulic conductivity i s assumed to vary over a log normal d i s t r i b u t i o n . Peak f r i c t i o n angles are determined using the normal void r a t i o d i s t r i b u t i o n . For some parameters i t i s not p r a c t i c a l to account for v a r i a b i l i t y , or techniques for parameter estimation were not ava i l a b l e . C h a r a c t e r i s t i c curves are estimated using a limited number of moisture retention runs on a single s o i l sample. Sophisticated techniques required to assess K ^ ] ^ were not available, therefore estimates are used in the h i l l s l o p e hydrology analyses. Most s o i l property data are s i m i l a r to west coast studies chosen for comparison. Void r a t i o s / p o r o s i t i e s and f r i c t i o n angles shower lower values, but the data are legitimized by the Use of sampling techniques si m i l a r to those found i n other studies. The f r i c t i o n angles are confirmed by comparison with r e s u l t s for s i m i l a r Unified S o i l C l a s s i f i c a t i o n s o i l s . 112 Table 5.1: Unified S o i l C l a s s i f i c a t i o n and Textural C l a s s i f i c a t i o n Avalanche number: W-1 W-2 DD-1 DD-2 DD-3 % gravel 45.2 37.0 33.7 37.8 46.0 % sand 49.2 49.2 57.9 55.7 37.0 % s i l t 5.4 13.3 8.2 6.3 15.6 % clay 0.2 0.5 0.2 0.2 1.4 U.S.C.2: SP - SM SW - SM SP - SM SP - SM GM % sand 89.8 78.1 87.3 89.6 68.5 % s i l t 9.8 21.2 12.4 10.1 28.9 % clay 0.4 0.7 0.3 0.3 2.6 T.C.3: Gravelly Gravelly Sand Gravelly Gravelly sand loamy sand sandy sand loam LR-1 LR-2 DS-1 DS-2 TEST % gravel 1 42.6 44.2 10.1 9.0 25.1 % sand 48.4 50.1 81.5 83.7 65.3 % s i l t 8.4 5.2 7.8 6.8 9.1 % clay 0.6 0.5 0.2 0.5 0.5 U.S.C.2: SP - SM SP - SM SW - SM SP - SM SW - SM % sand 84.2 89.8 90.7 92.0 87.2 % s i l t 14.7 9.3 8.7 7.5 12.2 % clay 1.1 0.9 0.6 0.5 0.7 T.C.3: Gravelly Gravelly Sand Sand Sand sand sand Sources: 1 - Defined according to MIT Classification System, Lambe and Whitman, 1979 2 - Craig, 1978 3 - Canada Soil Survey Coumittee, 1978 Selected studies - s o i l descriptions: Megahan and Clayton, 1983: Loamy coarse sand Humphrey, 1982: Loose gravelly loam Sidle and Swanston, 1982: U.S.C. - GM ( s i l t y sandy gravel) Utting, 1978: Podzol of sandy loam texture Harr, 1977: Gravelly clay loam - gravelly s i l t y clay loam O'Loughlin, 1972: Gravelly sandy loam (60% gravel, 30% sand, 10% s i l t / c l a y ) Swanston, 1970: Podzol of gravelly s i l t - l o a m texture 113 Table 5.2: S p e c i f i c g r a v i t i e s Mean S p e c i f i c Standard C o e f f i c i e n t Avalanche Gravity, deviation, of v a r i a t i o n , number G s s CV, i n % W-l 2.64 0. 01 0.3 W-2 2.55 0. 02 0.6 DD-1 2 . 63 0. 02 0.9 DD-2 2.64 0. 02 0.7 DD-3 2.48 0. 03 1.2 LR-1 2.64 0. 01 0.5 LR-2 2.57 0.03 1.1 DS-1 2.66 0. 02 0.7 DS-2 2.59 0. 03 0.9 Mean 2 . 60 0. 06 2 . 3 Minerals composing Chuckanut Formation. Bellingham Bay member: Quartz + Chert 3 9.9 Plagioclase feldspar 35.2 K feldspars 10.0 Mica 8.7 Source: Johnson, 1984 S p e c i f i c g r a v i t i e s of selected minerals: Quartz 2.65 Plagioclase 2.62 - 2.76 Microcline 2.54 - 2.57 Orthoclase 2.57 Muscovite 2.76 - 2.88 Source: Hurlbut and Klein, 1977 114 Table 5 . 3 : Void r a t i o s , p o r o s i t i e s , and unit weights Void ratio: Mean Standard Coefficient Avalanche Void Ratio, deviation, of variation. number e s CV, in % W-1 1.075 0.107 10.0 U-2 1.052 0.103 9.8 DD-1 1.331 0.108 8.1 DD-2 1.068 0.078 7.3 DD-3 0.627 0.148 23.4 LR-1 0.951 0.014 7.5 LR-2 0.951 0.027 14.8 DS-1 0.788 0.073 9.3 DS-2 0.842 0.086 10.2 Mean 0.965 0.217 22.5 Selected studies: e Megahan and Clayton, 1983 0.54 - 0.67 Humphrey, 1982 1.5 - 2.3 Harr, 1977 1.24 - 1.71 0'Lough I in, 1972 Porosity: Mean 1.21 Standard Coefficient Avalanche Porosity, deviation. of variation number n s CV, in % W-1 0.517 0.025 4.9 W-2 0.511 0.024 4.8 DD-1 0.570 0.020 3.6 DD-2 0.516 0.018 3.5 DD-3 0.380 0.053 14.0 LR-1 0.487 0.019 3.8 LR-2 0.485 0.038 7.9 DS-1 0.440 0.023 5.2 DS-2 0.456 0.025 5.5 Mean 0.481 0.062 12.8 Selected studies: n Megahan and Clayton, 1983 0.35 - 0.40 Humphrey, ' 1982 0.6 - 0.7 Harr, 1977 0.554 - 0.631 0'Lough I in , 1972 0.548 Unit weight: Avalanche Number W-1 W-2 DD-1 DD-2 DD-3 LR-1 LR-2 DS-1 DS-2 Mean Mean dry unit weight., T-I, in kN/m 12.47 12.61 11.10 12.50 15.99 13.24 13.30 14.46 14.04 13.30 Selected studies: Humphrey, 1982 Sidle and Swanston, Harr, 1977 O'Loughlin, 1972 1982 Standard deviation, s 0.66 0.63 0.52 0.46 1.38 0.48 0.98 0.59 0.65 1.53 7.8 - 9.8 11.52 7.92 - 10.59 10.89 Coefficient of variation CV, in % 5.3 5.0 4.7 3.7 8.6 3.6 7.4 4.1 4.6 11.5 115 Table 5.4: S o i l shear strength parameters Avalanche E f f e c t i v e peak E f f e c t i v e residual Relative Number f r i c t i o n angle, f r i c t i o n angle, Density 1, ep1, i n degrees 0 ' r , i n degrees RD W-l 31.8 29.8 .03 W-2 33.3 28.9 .32 DD-1 28.6 27 . 8 <0 DD-2 30.2 28.9 . 13 DD-3 34.0 2 30.0 >1 LR-1 30.6 28.9 <0 LR-2 34 . 2 28 . 6 .36 DS-1 40.4 29.2 .94 DS-2 35.1 29.2 .88 Mean 3 3.0 2 9.0 Selected studies: Peak <f>1 S i d l e , 1984 37° Humphrey, 1982 34° S i d l e and Swanston, 1982 43° O'Loughlin, 1972 34° - 41° Swanston, 1970 37° 1 - Based on mean void r a t i o s (Table 5.3) applied to void r a t i o range i n d i r e c t shear t e s t i n g . RD's < 0 indicate that high f i e l d void r a t i o s can not be duplicated i n lab. RD's > 1 indicate that low f i e l d void r a t i o s can not be duplicated i n lab. 2 - Weighting scheme value u n r e a l i s t i c a l l y high. Values adjusted to include s o i l cohesion, c'= 9.6 kPa. Explanation i n text. 116 Table 5.5; S o i l matrix hydraulic conductivities No. Geometric Parameter • Y = log K s a t : Avalanche of mean K s a t , Standard C o e f f i c i e n t number samples , m/s Mean, deviation, of va r i a t i o n N X 10~ 5 s CV i n % W-l 9 4.5 -4.35 .318 7.3 W-2 8 6.5 -4.19 . 149 3.6 DD-1 7 7.8 -4.11 .321 7.8 DD-2 10 11.0 -3.96 .480 12 .1 DD-3 8 2.9 -4.54 .292 6.4 LR-1 10 2.9 -4.54 . 282 6.2 LR-2 10 4.9 -4.31 .255 5.9 DS-1 7 2.1 -4 . 68 .293 6.3 DS-2 8 5.9 -4.43 . 282 6.4 Mean 9 4 . 5 1 -4 . 35 K c . a t selected studies: K s a t X 1 0 ~ 5 Technique: Megahan and Clayton, 1983 1.6 Cores Harr, 1977 4.4 - 114 Point flux 0'Loughlin, 1972 4.0 Cores K b u l i v selected studies: Ksat x 10-5 Technique: Megahan and Clayton, 1983 16. - 33. Trench/tracer Humphrey, 1982 40. Utting, 1978 8 . 0 Trench 1 - Inverse log of mean Y (=log K s a t ) value Table 5.6 : F i e l d capacities and degrees of saturation Volumetric moisture No. Mass based content, ©v 1 Avalanche of moisture Standard Degree of number samples, content, Mean deviation, saturation, N ©m s DS i n % W-l 15 . 197 . 251 . 038 48.4 W-2 17 . 286 .355 . 053 69.3 DD-1 27 .244 .276 . 048 48 . 3 DD-2 16 . 146 . 187 . 030 36.1 DD-3 15 . 244 . 373 . 054 96.8 LR-1 17 . 165 .223 . 038 45.7 LR-2 18 .220 .290 . 041 59.5 DS-1 15 . 185 .275 . 039 62 . 3 DS-2 15 . 184 .258 . 029 56.5 Mean 9 .208 .275 58.1 1 - Calculated using p o r o s i t i e s at each headscarp. Mean for nine scarps calculated with o v e r a l l mean porosity, n = .481. Figure 5.1; S o i l grain s i z e d i s t r i b u t i o n s CLAY 1 nn SILT SAND GRAVEL FINE MEDIUM COARSE FINE MEDIUM COARSE FINE MEDIUM COARSE 9 0 -8 0 -i— 1 7 0 -£5 6 0 -% 5 0 -^ 4 0 -L d O U J 3 0 -C L 2 0 -1 0 -o -DS D D V o -1, D S - 2 11 1/ if///// If//// 0 'J IIII * / E S T / f / D-1, D D - ; -2 , L R - V t i A i // / /// L R - 2 / A/ l l l \7n\ 1 1 l l l II | 1 1 l l l l | 1 l l 111| 1 l l l l l 10 " 3 10 " 2 10 " 1 1 10 10 GRAIN SIZE IN MM Figure 5.2: F r i c t i o n angle vs void r a t i o r e l a t i o n i n medium sand, from Lambe and Whitman, 1979. I T 3 OJ OO c c o 24 46 44 42 40 38 36 Porosity n before loading (%) 34 32 J 0.85 0.80 0.75 0.70 0.65 0.60 0.55 Void ratio eo before loading 0.50 0.45 Figure 5.3: Weighting scheme used to determine integrated mean f r i c t i o n angle for avalanche W-l /Lt - 1.075 a = 0.107 Z1 = -1.282, d Z2 = -0.524, e2 Z3 = 0.000, e3 = Z4 = 0.524, e4 = Z5 = 1.282. e5 = Z - { el-5 - /A) / a 23 e2 ,e3 T l I i | I I I I | i l I I | I I I I [ l l f~i p i I I I | I I I I | r 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 VOID RATIO, e 44 42-cr o 4 Q. Q 38-< 36-34-o i — o a: 32 L i _ 30-28 INTEGRATED MEAN FRICTION ANGLE: = 0'1 - 0'5 MEAN = 31.8 6 ^ - e RELATION: e<e critical: <fr = -4-7.3 e + 81.0 e>e critical: ^ r = 29.8 i i i i I i i i i I i i i i I i i i i I i i i i I i i i i I i i i i I i i 7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 VOID RATIO, e H H 00 F i g u r e 5.4: Well permeameter apparatus 119 B. A. v F. c. D. A. R u b b e r b u n g B. A i r - i n l e t t u b e , 1.6 c m OD C. W a t e r r e s e r v o i r , 7 c m O D , L e n g t h = 1 6 0 c m D. G r a d u a t e d s c a l e , c m E. A d j u s t a b l e b a s e F. W a t e r level in r e s e r v o i r G. A u g e r h o l e , r a d i u s , r , 2 r = 7 .6 c m I. Wa te r level in a u g e r h o l e J . B e d r o c k f low b o u n d a r y E. r ^ — A i r b u b b l e s NOT TO S C A L E G. H 2r S J . v. \ \ \ \ \ 120 F i g u r e 5.5; S o i l moisture r e t e n t i o n data and f i t t e d curves 0.50 0.15 ~] i i i i i i i i i | i i i i | i i— i—i 1 — i — i — i — -4.0 -3.0 -2.0 -1.0 0.0 1.0 P R E S S U R E H E A D , h , I N M F i g u r e 5.6: C h a r a c t e r i s t i c curves: Moisture content vs p r e s s u r e head 121 0.70 ^ 0.65 LU 0.60 ZD _ J O 0.55 > >- 0.50 OQ 0.45 > 0.40 h—" ~ZL LU | 0.35 1 o 0.30 o 1 1 1 0.25 cn 1— 0.20 CO o 0.15 0.10 0.05 0.00 P R E S S U R E H E A D , h , I N M 122 F i g u r e 5.7: C h a r a c t e r i s t i c curves: R e l a t i v e c o n d u c t i v i t y vs p r e s s u r e head P R E S S U R E H E A D , h , IN M 123 F i g u r e 5.8: Normal p r o b a b i l i t y p l o t s f o r v o i d r a t i o and moisture content a t avalanche LR-2 LR-2 AR XTHHETIC UALUES Pop . 1 VARIABLE s « UNIT 3 N 3 IS N C I a 12 POPULATIONS 3S33B3S3S33 H«an / s«d.D«u. 0 .951 0.1*C 100.0 USERS VISUAL PARANETER ESTIMATES ARITHHETXC UALUES UARIABLE 3 th«tl-N UNIT s N 3 15 N CI 3 12 POPULATIONS 333 3 3333333 Pop. Rean std.Dcu. */. 1 0.227 0.02* 100.0 USERS UISUAL PARANETER ESTINATES 124 CHAPTER 6: HILLSLOPE HYDROLOGY 6.1 Introduction A f i n i t e difference i n f i l t r a t i o n model i s combined with a modified kinematic wave equation to determine water tables at avalanche headscarps. The model provides v e r t i c a l s o i l discharge rates over the course of the comparison storms. Discharges are entered i n a modified kinematic wave equation, which i s used to estimate saturated zone thicknesses at avalanche headscarps. The main purpose of the analysis i s to dis t i n g u i s h s o i l discharge and saturated zone thicknesses during the January, 1983 storm from discharges and thicknesses during the other comparison storms. Input parameters c o n t r o l l i n g i n f i l t r a t i o n and water table build-up are also assessed. Implications for s o i l development and storm runoff are discussed. Water table estimates are entered i n slope s t a b i l i t y analyses i n Chapter 7. 6.2 I n f i l t r a t i o n program A one-dimensional, v e r t i c a l , transient, saturated-unsaturated, f i n i t e difference program i s used to estimate i n f i l t r a t i o n into a t y p i c a l s o i l p r o f i l e . Freeze (1969) f i r s t developed the program and outlined the equations of flow and the appropriate boundary conditions. Richard's equation governs flow i n the unsaturated zone, 125 S / S z [ K(h) (Sh/6z + 1) ] = C(h) S h / S t (6.1) where z = v e r t i c a l coordinate d i r e c t i o n h = pressure head K(h) = unsaturated hydraulic conductivity, pressure head dependent C(h) = s p e c i f i c moisture capacity, (68v/6~h) , pressure head dependent G v = volumetric moisture content t = time Under saturated conditions, C(h) = 0, K(h) i s constant, and Eq. 6.1 reduces to, <S2h/6z2 = 0 (6.2) In Freeze (1969), the r a i n f a l l rate, R, controlled flow at the ground surface. The boundary condition equation was, Sh/Sz = R / K(h) - 1 (6.3) A p o s i t i v e R represented r a i n f a l l , while a negative R represented evaporation. The basal boundary condition was considered as being a recharging or discharging groundwater flow system. The boundary condition equation was, Sh/Sz - Q / K(h) - 1 (6.4) A p o s i t i v e Q represented downward recharge away from the s o i l p r o f i l e , while a negative Q represented discharge upward into the p r o f i l e . 126 The f i n i t e difference equations for these flow and boundary conditions are reviewed i n Freeze's paper. The re s u l t s take the form of a two-dimensional matrix representing flow through a v e r t i c a l column of nodes as a function of time. 6.3 Assumptions The s o i l p r o f i l e i s assumed to be homogeneous and i s o t r o p i c . S o i l s i n the study area have consistent grain s i z e d i s t r i b u t i o n s and unlayered p r o f i l e s that meet these q u a l i f i c a t i o n s . The po t e n t i a l l y disrupting e f f e c t of macropores i s discussed i n Section 6.5. It i s also assumed that unsaturated flow occurs v e r t i c a l l y downward into the s o i l p r o f i l e . Weyman (1973) observed that flow between r a i n f a l l events was dominated by v e r t i c a l , unsaturated movement toward the p r o f i l e base. During storms a saturated, downslope flow component also emerged. Humphrey (1982) further confirmed t h i s assumption i n a two and three-dimensional f i n i t e element modeling study. Unsaturated flow i n two or three dimensions was always close to v e r t i c a l , and corresponded to one-dimensional approximations. Only when flow reached a saturated layer or impermeable boundary did i t s d i r e c t i o n rotate to flow downslope. Freeze included provisions for ponding at the ground surface i f R exceeded K s at-. A review of study area r a i n f a l l i n t e n s i t i e s and s o i l K s a t values showed that R was always less than Ksa^-. It i s therefore assumed that ponding w i l l not occur, and no 127 provisions for ponding were included. The program terminates when the top node becomes saturated from the base. 6.4 Input parameters To enter s o i l c h a r a c t e r i s t i c curves as program input, a number of points from the c h a r a c t e r i s t i c curve plots (Figures 5.6 and 5.7) are read into the program along with s o i l K s a t values. Relative conductivities multiplied by K s a t equal the actual conductivities at d i f f e r e n t pressure heads. The curves are approximated by a series of l i n e segments. I n i t i a l conditions before r a i n f a l l are also required. F i e l d capacities from Table 5.6 govern the i n i t i a l pressure head conditions i n the s o i l . In Section 5.7 i t was noted that the f i e l d capacity did not vary s i g n i f i c a n t l y i n a drained s o i l p r o f i l e . This observation suggests that moisture content, and therefore constant pressure head, are both constant with depth. The i n f i l t r a t i o n program generates i n i t i a l condition drainage p r o f i l e s when the r a i n f a l l rate i s equal set to zero. Two basal conditions are considered in t h i s study, and each produces a d i f f e r e n t i n i t i a l condition. The f i r s t condition i s an impermeable boundary, which generates hydrostatic p r o f i l e s . The second condition i s a discharging basal boundary, which generates curved p r o f i l e s s i m i l a r to those depicted by H i l l e l (1980b) for v e r t i c a l drainage i n a se m i - i n f i n i t e s o i l . Both i n i t i a l condition p r o f i l e s are depicted in Figures 6.2 and 6.3. The v a l i d i t y of the boundary conditions w i l l be discussed in 128 Section 6.5. Retention curve shapes from Figure 5.6 explain a pot e n t i a l c o n f l i c t between observed and computer generated i n i t i a l conditions. Below about -1 m, drops i n pressure head do not cause sharp decreases i n moisture content. What may appear to be a constant moisture content p r o f i l e could therefore be either of the computer generated i n i t i a l conditions. The li m i t e d accuracy of moisture content data makes either boundary condition possible. P r e c i p i t a t i o n i s applied in varying hourly i n t e n s i t i e s . The Smith Creek i n t e n s i t i e s for the comparison storms are applied as depicted i n Figure 3.3 c, d, and e. I t i s assumed that a l l the p r e c i p i t a t i o n occurs as r a i n f a l l , hence the term r a i n f a l l i n t e n s i t y i s used. It i s also assumed that evaporation i s n e g l i g i b l e due to low solar radiation intensity, low temperatures, and low forest f l o o r windspeeds. 6.5 Drainage condition t e s t s Test runs were carried out under both basal boundary conditions to determine which one best simulated subsurface flow i n forest s o i l s . Similar input parameters were used to standardize the r e s u l t s . A s o i l depth of .74 m i s assumed, based on f i e l d observation and Goldin*s (1984) s o i l p r o f i l e descriptions. The K s a^ of 4.5 X 1 0 - 5 m/s i s the mean value for the nine avalanches (Table 5.5). The c h a r a c t e r i s t i c curves for n = .481 i n Figures 5.6 and 5.7 were chosen to represent the 129 mean s o i l porosity (Table 5.3). A pressure head of -1 m occurs when the average © m = .27 5 (Table 5.6) i s used i n Figure 5.6. To develop i n i t i a l conditions, a s l i g h t l y higher pressure head of -.9 m i s assumed at the p r o f i l e base, with computer generated i n i t i a l conditions above. Continuous r a i n f a l l was entered at rate of 16.6 mm/hr (.65 in/ h r ) . The equivalent Nooksack Salmon Hatchery i n t e n s i t y applied over six hours has a 25-year recurrence i n t e r v a l . The same inte n s i t y applied over 12 hours has a recurrence i n t e r v a l of just over 100 years. 6.5.1 K s a t - c o n t r o l l e d drainage For t h i s case i n f i l t r a t i o n and downslope discharge are contro l l e d by K s a £ values and c h a r a c t e r i s t i c curves. I t i s assumed that permeameter Ksa-f- values account for the influence of macropores. The bedrock conductivity assessment in Section 5.5.3 showed that the rock may be considered an impermeable basal boundary, SH/Sz = 0 (6.5) Figure 6.1 depicts t h i s drainage condition. I n f i l t r a t i n g r a i n f a l l accumulates at the impermeable boundary, causing a saturated layer to develop. I t i s assumed that groundwater flowing downslope out of the v e r t i c a l p r o f i l e i s replaced by water flowing into the p r o f i l e from upslope. The water table therefore r i s e s p a r a l l e l to the ground surface. I n f i n i t e slope seepage theory i s based on a simi l a r assumption. 130 Use of permeameter KSat values i n downslope drainage calc u l a t i o n s supports t h i s assumption. Freeze and Cherry (1979) defined the average l i n e a r v e l o c i t y i n porous media as, c = -K s a t/n 6H/61 (6.6) For t h i n , saturated flow p a r a l l e l to the bedrock, 6H/rSl i s assumed to equal sin9, where 8 i s the bedrock slope angle. Given a t y p i c a l slope of 35° and K s a^ and n values from Chapter 5, the average downslope drainage rate i s , 6.5 X 1 0 - 5 m/s, or only 5.7 m/day. Over the course of a storm the v e r t i c a l water table r i s e i s more important than two and three dimensional flow patterns. A greater water table r i s e would d i s t i n g u i s h the January 9-10, 1983 storm from the other, less severe storms. Figure 6.2 depicts pressure head p r o f i l e s a f t e r r a i n f a l l i n i t i a t i o n . The s o i l p r o f i l e became saturated to the surface 9.67 hours a f t e r the onset of r a i n f a l l . The t o t a l r a i n f a l l volume to saturation was 160.5 mm. Figure 3.4 indicates that the December 13-14, 1979 storm would also have saturated the p r o f i l e , although over a longer time period. In addition, any storm with t o t a l r a i n f a l l greater than 160 mm would saturate the s o i l . I f t h i s were the case i t would be d i f f i c u l t to d i s t i n g u i s h pore pressures developed on January 9-10, 1983 from pressures developed during other storms. F i e l d evidence does not support the hypothesis that most of the basin regularly becomes saturated to the surface. Aft e r the January, 1983 storm, signs of overland flow included orientation 131 of twigs and vegetation p a r a l l e l to flow, deposition of mineral s o i l on the forest f l o o r , and small channels i n forest l i t t e r . The evidence was widespread, but lim i t e d to s p e c i f i c locations such as drainage depressions, C horizon seeps, and logging roads. In addition, the calculated drainage rates indicate that most of the slope would remain saturated a number of days a f t e r a storm. The low moisture contents i n Table 5.6 contradict t h i s hypothesis. The K s a^ controlled drainage assumptions do not appear r e a l i s t i c . 6.5.2 Kbuifc-controlled drainage In the second case the s o i l i s permeated with high conductivity macropores that drain the slope. A free draining basal boundary condition i s assumed at the base of the p r o f i l e , Sh/Sz = Q(t) / K(h) - 1 (6.6) where, Q(t) = v e r t i c a l discharge rate, m/s per unit area Figure 6.1 depicts t h i s boundary condition. The boundary continually changes as Q(t) adjusts to r a i n f a l l or drainage. Q(t) and K(h) increase as water content increases. Q(t) reaches a maximum at R, which means that the p r o f i l e remains unsaturated. I t i s assumed that groundwater i n tension w i l l not flow into macropores. Greater s o i l discharge rates would d i s t i n g u i s h the January 9-10, 1983 storm from less severe storms. Curves i n Figure 6.3 depict pressure head p r o f i l e s during i n f i l t r a t i o n to a discharging basal boundary. The p l o t i l l u s t r a t e s penetration of the wetting front u n t i l a near steady state condition i s reached, 13 hours a f t e r r a i n f a l l i n i t i a t i o n . Figure 6.4 depicts drainage p r o f i l e s a f t e r cessation of r a i n f a l l . The unsaturated flow discharges to a t h i n saturated zone. A modified kinematic wave equation was used to determine saturated zone thicknesses at d i f f e r e n t locations during the storm. The parameter K ^ ] ^ , discussed i n Section 5.5.2, i s entered i n the equation. It i s assumed that discharge from the p r o f i l e base approximately equals discharge at the water table l e v e l . Low discharge rates early in the storm cause small saturated zone thicknesses, therefore the assumption i s v a l i d under these conditions. The saturated zone thickness increases as discharge increases l a t e r i n the storm. Figure 6.3 shows that the discharge throughout the p r o f i l e becomes r e l a t i v e l y constant l a t e i n the storm, so the assumption again does not cause serious error. This basal boundary and drainage condition i s more r e a l i s t i c because, 1) the s o i l p r o f i l e remains unsaturated throughout the storm and drains to f i e l d moisture contents i n Table 5.6; 2) macropores become an important factor in s o i l drainage, as observed i n the f i e l d ; 3) the January 9-10, 1983 storm can be distinguished from less severe storms. 133 6 . 6 E f f e c t of v a r i a t i o n i n input parameters Different r a i n f a l l durations and i n t e n s i t i e s , i n i t i a l conditions, s o i l depths, and hydraulic conductivities were tested to observe the e f f e c t of t h e i r v a r i a t i o n on discharge rates at the base of the s o i l p r o f i l e . Figures 6.5 - 6.8 are plots of discharge at the base of the s o i l p r o f i l e versus time, as r a i n f a l l and drainage occur. 6 . 6.1 R a i n f a l l duration and in t e n s i t y Figures 6.5 and 6.6 show the e f f e c t of r a i n f a l l duration and inte n s i t y . The curves are i d e n t i f i e d by the duration of r a i n f a l l from zero time. After r a i n f a l l ceases the p r o f i l e i s allowed to drain. Figure 6.5 shows that a 2-hour r a i n f a l l at Rl = 16.6 mm/hr (.65 in/hr) has l i t t l e e f f e c t on s o i l discharge rates. A 6-hour storm at the same inte n s i t y produces a more s i g n i f i c a n t discharge. The 10-hour storm discharge peaked at 95% of the steady state discharge rate (equal to the r a i n f a l l i n t e n s i t y ) . However, steady state was s t i l l not achieved during a 13-hour storm (99% of steady state). Figure 6.6 includes a higher r a i n f a l l i n t e n s i t y , R2 = 22.3 mm/hr (.88 in/hr) applied over 3 and 13 hours. At Nooksack Salmon Hatchery, an equivalent 3-hour int e n s i t y has a greater than 100-year recurrence i n t e r v a l . The equivalent 13-hour i n t e n s i t y has a recurrence i n t e r v a l of greater than 10,000 years i n t h i s area. The results show that R2 applied over 3 hours s t i l l does not produce a s i g n i f i c a n t discharge. The th e o r e t i c a l 134 13-hour storm produces a steeper curve that reaches steady state. A r a i n f a l l duration of at least s i x hours, at i n t e n s i t i e s Rl or R2, was required to s i g n i f i c a n t l y r a i s e the discharge rate. R a i n f a l l durations of 1, 2, and 3 hours are therefore not as important i n increasing discharge. Pierson (1980 p.5) confirmed t h i s assessment with piezometers located i n the axes of drainage depressions on a h i l l s l o p e i n Oregon. Low in t e n s i t y , 1 to 3-hour r a i n f a l l had a n e g l i g i b l e e f f e c t on piezometric heads i n depressions. The above observations explain why high recurrence i n t e r v a l , 6, 12, and 24-hour r a i n f a l l s cause slope f a i l u r e i n study area s o i l s . Recurrence in t e r v a l s for 1, 2, and 3-hour storms are not important i f the r a i n f a l l i s iso l a t e d i n time or occurs near the beginning of a storm, while storage i s being f i l l e d . I f a high recurrence i n t e r v a l , 1 to 3-hour r a i n f a l l occurs l a t e i n a storm the discharge rates would increase above lower i n t e n s i t y l e v e l s . This did not occur in January, 1983. In general, higher intensity r a i n f a l l penetrates the s o i l p r o f i l e faster, reaches steady state more quickly, and produces higher discharge rates than lower in t e n s i t y r a i n f a l l . Pierson (1980) confirmed t h i s by reporting that higher i n t e n s i t y r a i n f a l l produces a shorter lag time between peak r a i n f a l l i n t e n s i t y and peak pressure head in piezometers. Lower in t e n s i t y rainstorms do not cause numerous slope f a i l u r e s , mainly because lower in t e n s i t y r a i n f a l l l i m i t s steady state 135 discharge rates (Figure 6.6). R a i n f a l l duration becomes less s i g n i f i c a n t during low in t e n s i t y storms. 6.6.2 I n i t i a l conditions Figure 6.6 also depicts the e f f e c t s of i n i t i a l conditions on discharge rates. I n i t i a l conditions of h = -1 m and h = -2 m are compared. S o i l discharge response i n d r i e r s o i l (h = -2 m) lags about 2.5 hours behind the wetter s o i l response. R a i n f a l l was sustained long enough for s i m i l a r peak discharges to be achieved, although wet i n i t i a l conditions caused a longer period of near steady state discharge. I f r a i n f a l l had not continued for 13 hours the i n i t i a l l y d r i e r s o i l would have shown a lower peak discharge. Drier s o i l responds more slowly because K(h) values are low under dry conditions. R a i n f a l l cannot penetrate the p r o f i l e u n t i l pore space storage has been p a r t i a l l y f i l l e d and K(h) increases. Pierson (1980) again confirmed t h i s by observing longer peak r a i n f a l l to peak discharge lags i n d r i e r s o i l s . 6.6.3 S o i l depth Figure 6.7 depicts the e f f e c t of s o i l depth on discharge rate. S o i l depths of .40, .74, and 1.00 m are included. The .40 m depth i s t y p i c a l of s o i l covering steep bedrock slabs at avalanches W-l and W-2. S o i l depths to 1.00 m commonly accumulate i n the axes of drainage depressions. R a i n f a l l penetrated the .40 m p r o f i l e and achieved steady state discharge 136 more quickly than r a i n f a l l penetrating deeper s o i l s . The shallow s o i l responded quickly because pore space storage was rapi d l y f i l l e d , and tr a v e l time to the base of the p r o f i l e was small. Shallow s o i l drained to lower discharge rates more quickly than deeper s o i l . 6.6.4 Hydraulic conductivity Figure 6.8 depicts the e f f e c t of study area K s a-r- values on discharge rate. High Ksa+- s o i l responded rapidly because r a i n f a l l was transmitted to the base of the p r o f i l e more quickly than i n lower Ksa+- s o i l . A difference of about 3 hours was observed between the high and low Ksa-f- responses. As the simulated storm progressed, the curves converged to reach steady state at s i m i l a r times. Drainage rates a f t e r r a i n f a l l cessation are independent of Ksa-(-. 6.7 Comparison storm discharge rates Discharge rates at the nine avalanches were modeled using r a i n f a l l and s o i l property input parameters. Storm hyetographs are included with discharge r e s u l t s . S o i l parameters include K s a t values from Table 5.5, i n i t i a l pressure heads from Table 5.7, and c h a r a c t e r i s t i c curves from Figures 5.6 and 5.7. Figures 6.9, 6.10, and 6.11 depict the hourly discharge rates during comparison storms. Comparison c l e a r l y shows that peak discharges were greater in January, 1983 than during the other storms. Higher r a i n f a l l i n t e n s i t i e s d i s t i n g u i s h January, 1983 and December, 1979 from the lower intensity, January, 1971 storm. The maintenance of high r a i n f a l l i n t e n s i t i e s over a longer time period distinguishes the January, 1983 storm from December, 1979. In January, 1983 the peak r a i n f a l l i n t e n s i t y (5.3 X 10~ 6 m/s) was maintained for 6 out of 12 hours (2100, January 9 to 0900, January 10). The same peak in t e n s i t y was maintained for only one hour i n December, 1979. As a r e s u l t the January, 1983 storm achieved a near steady state condition, with a peak r a i n f a l l intensity/peak discharge r a t i o of .9 for a l l avalanches. The December, 1979, R/Q was lower and varied among avalanches, ranging from .33 - .67. The record for that storm shows that the e f f e c t of that 1-hour peak was damped by pore space storage i n the s o i l . Of the input parameters, i n i t i a l conditions had the greatest influence on discharge response to a given r a i n f a l l . I n i t i a l conditions determine pore space storage i n the s o i l and influence lag between r a i n f a l l i n i t i a t i o n and discharge from the p r o f i l e base. Wetter i n i t i a l conditions cause shorter response lags at avalanches W-2, DD-3, LR-2, DS-1, and DS-2. As a r e s u l t , peak discharge magnitude and duration are also affected. In January, 1983, those i n i t i a l l y wetter s o i l s reached and sustained a peak discharge greater than 4.0 X 10"^ m/s per unit area for 8 hours. R a i n f a l l was sustained long enough for d r i e r s o i l s to reach t h i s peak discharge, but for durations of only 2-5 hours. December, 1979 peak discharges under wet i n i t i a l conditions are greater than peaks under dry i n i t i a l conditions. 138 In that case high i n t e n s i t y r a i n f a l l ceased just as pores i n i n i t i a l l y d r i e r s o i l became s u f f i c i e n t l y f i l l e d to produce s i g n i f i c a n t discharge. K s a t had a less s i g n i f i c a n t influence on discharge. This was demonstrated by s i m i l a r lag times and discharge response at LR-1 and DD-2, despite large differences in Ksa-t- (3.8 X greater at DD-2). K Sat was isolated as a factor when i n i t i a l conditions were s i m i l a r . LR-2 and DS-1 showed si m i l a r i n i t i a l conditions, but higher Ksa-j- s o i l at LR-2 responded more quickly to r a i n f a l l . As the rainstorms progress the avalanche discharges converge toward a single curve. I n i t i a l conditions, K s a t and s o i l depth therefore become less important late in a storm, and the s o i l s respond more d i r e c t l y to p r e c i p i t a t i o n input. Modeling observations are confirmed in other f i e l d studies. Ohta, et. a l (1985) found that pore space storage of r a i n f a l l was s i g n i f i c a n t on forest h i l l s l o p e s , and that i t delayed runoff from the slope. Plots i n Mosley (1979) and Pierson (1980) show lags between continuous r a i n f a l l i n i t i a t i o n and s o i l discharge or piezometric head r i s e . Pierson also found that drainage depressions showed d i f f e r e n t piezometric responses to the same rainstorm, and suggested d i f f e r e n t water retention capacities ( i . e . i n i t i a l conditions) as an explanation. 6.8 Kinematic wave equation A modified kinematic wave equation i s used to calculate headscarp saturated zone thicknesses during the comparison storms. The kinematic wave i s an approximation of the extended Dupuit-Forchheimer equation of subsurface flow over a sloping, impermeable bed. Beven (1981) introduces i t as a combination of Darcy's Law and the continuity equation. Darcy's Law i s rewritten, q = -K 6H/51 (6.7) where, q = s p e c i f i c discharge i n the downslope d i r e c t i o n In t h i s case the saturated zone i s t h i n r e l a t i v e to the length of the slope. The hydraulic gradient, 6H/51, i s therefore approximated by sine, where 6 i s the bedrock slope angle. Integrating Eq. 6.7 over the saturated zone and substituting into the continuity equation y i e l d s the kinematic wave equation, <Shw/rSt = -K sinG/e 5hw/5x + I/e (6.8) where, h w = saturated zone thickness, measured perpendicular to the impermeable bed t = time e = e f f e c t i v e porosity x = downslope distance I = input rate per unit area A s o l u t i o n to Eq. 6.8 i s described i n Tischer (1986) and Eagleson (1970) as the Method of Characteristics. A revised version of that solution i s used i n t h i s study. F i r s t , consider input to a horizontal surface in which no l a t e r a l outflow 140 occurs. In that case, 6hw/<5x = 0 i n Eq. 6.8, and <5hw/<5t = I/e. i n t h i s study the input rate i s separated into hourly input rates per unit area, II, 12, I j , from Figures 6.9 - 6.11. e equals the porosity minus the volumetric moisture content (n-8 V ) , and i t represents the available pore space i n the s o i l . e decreases as r a i n f a l l f i l l s the pore space during the storm, hence i t i s also incremented into hourly values, e l , e2, e j . The saturated zone thickness at time j and time increment t equals, h w at time j = I ( l ) t / e ( l ) + I(2)t/e(2) + (6.9) ... + I ( j ) t / e ( j ) On a sloping surface, downslope flow occurs during the course of the storm. A drainage divide i s a fixed head boundary of h w = 0 at x = 0 for a l l t. The downslope groundwater v e l o c i t y i s constant i n saturated s o i l , and i s governed by the l i n e a r v e l o c i t y equation (Eq. 6.6), dx/dt = c = Kfcuifc/n sine (6.10) After j hours, water input at the drainage divide w i l l have moved x = j c meters downslope (Kj-^ik entered i n m/h) . Downslope from that point h w i s constant and equal to h w at time j i n Eq. 6.9. The h w values between x = 0 and x = j c depend on the input rates, II through I j . For example, the water table at a point x = 3c meters downslope w i l l only have "seen" input from the l a s t three hours of the storm. In that case, h w equals, h w (x = 3c) at time j>3 = I(j-2)t/e(j-2) + (6.11) I ( j - l ) t / e ( j - l ) + K J ) t / e ( j ) Use of the kinematic wave equation i s based on the assumption that r e s u l t s approximate the extended Dupuit-Forchheimer solution. Beven (1981) developed the parameter, L, as an a c c e p t a b i l i t y c r i t e r i o n for using the kinematic wave, L = 4 I cose / K s i n 2 6 (6.12) For L > 0.75 the kinematic wave i s deemed inaccurate. In the study area, the maximum I = 4.6 X 10~ 6 m/s, 0 ranges from about 30° - 48°, and Kj-^ik i s shown i n Section 6.9 to be about 10~ 3 m/s. The maximum L = 0.03 under those conditions, hence the kinematic wave i s an appropriate solution. 6.9 Wave v e l o c i t y assessment The downslope groundwater v e l o c i t y depends mainly on K ^ ] ^ , which i s controlled by macropores i n shallow forest s o i l s . I f a K b u l k / K s a t r a t i o of 20 i s assumed from Section 5.5.2, the geometric mean K ^ ] ^ for the study area i s 9.0 X 10 - 4 m/s. For n = .481 (average of avalanche s o i l s ) the parameter Kj-^i^/n = 1.9 X 10" 3 m/s. An al t e r n a t i v e approach i s to estimate the v e l o c i t y by monitoring h i l l s l o p e s during rainstorms. Mosley (1979) and Pierson (1980) published results from which v e l o c i t y estimates are obtained. In Mosley, v e l o c i t y i s based on the time required for a pulse of heavy r a i n f a l l to t r a v e l from a drainage divide to a p i t i n which subsurface discharge i s recorded. The v e l o c i t y was 2.7 X 10 - 3 m/s i n a s i l t loam to s i l t y clay on a 20° slope. The parameter Kfc,ulk/n represents s o i l transmission c a p a b i l i t y without the influence of slope angle. In Mosley, K b u l k / n I s 7 , 9 x 10 - 3 m/s. In Pierson, the v e l o c i t y i s based on the time for piezometric head to drop to a base l e v e l following a heavy r a i n f a l l . The v e l o c i t y was 3.9 X 1 0 - 3 m/s i n a stony, sandy loam on a 32.4° slope. ^bulk/*1 ^ s 7 , 3 x 10~ 3 m/s. Calculations are included i n Appendix VI. Figure 6.12 depicts the e f f e c t of Kj 3 U^] c/n values on water table p r o f i l e s at avalanche W-l during the December, 1979 storm. In the 29-hour r a i n f a l l period groundwater flows 5 m down the 30° slope for K b u l k / n = 10 - 4, 50 m for 10 - 3, 250 m for 5 X 10 - 3, 500 m for 10 - 2, and 1000 m for 5 X 10 - 2. Downslope drainage rates for Kj^ij^/n less than 5 X 10 - 3 m/s are too slow to f i t the dry s o i l p r o f i l e s and lack of sheet overland flow evidence i n winter months, as discussed in Section 6.5. If Kbulk/ n were greater than 5 X 1 0 - 2 m/s s i g n i f i c a n t piezometric r i s e s would not be recorded i n f i e l d studies. Pierson (1980), O'Loughlin (1972) and others have shown that water l e v e l s r i s e to the surface on 200 m-long, planar slopes. Bedrock, s o i l s , vegetation, and slope angles i n Pierson (1980) are s i m i l a r to the Smith Creek study area. For that reason the K^^^/n = 7.3 X 10 - 3 m/s from that study i s used i n kinematic wave calculations. 143 6.10 Water t a b l e s Headscarp water tables are estimated during the January, 1983 and December, 1979 storms. The January, 1971 storm i s not included because i t produced lower discharge rates than the other two storms, and therefore did not t r i g g e r numerous debris avalanches. Kinematic wave results from the other storms are used to interpret why the January, 1971 storm i n i t i a t e d debris torrents. Uncertainty concerning snowmelt magnitudes and i n i t i a t i o n mechanics preclude a more detailed analysis. I t i s assumed that the i n i t i a l saturated zone thickness i s n e g l i g i b l e . This assumption i s r e a l i s t i c i f the s o i l had s u f f i c i e n t time to drain a f t e r the previous r a i n f a l l event. I f JXbulk/n = 7.3 X 1 0 - 3 m/s, a saturated wedge tr a v e l s 63 0 m down at a 30° slope i n 24 hours. This distance i s s u f f i c i e n t to drain study area slopes. Figure 3.3 c.) shows that four, low int e n s i t y , 1-hour r a i n f a l l s occurred in the 24-hour period before the January 9-10, 1983 storm. Figure 6.6 showed that short duration r a i n f a l l has a minor e f f e c t on discharge rates, therefore the saturated wedge was assumed to have been thi n at the beginning of the storm. No r a i n f a l l was recorded i n the 24 hours before the December, 1979 storm. The water table l e v e l at a given point on a slope changes in time. The maximum level s achieved at avalanche headscarps are assumed to have triggered the f a i l u r e s . A FORTRAN program was written to store hourly water table p r o f i l e s i n an array, and by comparison determine the maximum l e v e l at a known distance from 144 a drainage divide. Discharge p r o f i l e s were stored and maximum discharge rates were determined i n drainage depression analyses. The kinematic wave equation applies to planar slopes only; i t does not account for concentration of drainage i n depressions. Slopes are assumed to be planar at avalanches W-l, W-2, LR-2, DS-1, and DS-2. Water table p r o f i l e s at those avalanches are reviewed i n Section 6.10.1. In Section 6.10.2 the kinematic wave solution i s modified to account for flow from planar sideslopes into drainage depressions. The modified solution i s applied to avalanches DD-1, DD-2, DD-3, and LR-1. 6.10.1 Planar slope water table p r o f i l e s Figures 6.13 and 6.14 depict water table p r o f i l e s on planar slopes. The plots are p r o f i l e s of saturated zone thickness versus downslope distance from a drainage divide. Data symbols are spaced i n one hour in t e r v a l s , with an extra symbol included i f the water table reaches the ground surface. A l l the p r o f i l e s are rotated from the p r e v a i l i n g slope angle to horizontal. I n i t i a l conditions, porosity, Ksa-t-, slope angle, and t r a v e l time are noted above the plot, while the distance to drainage divide and headscarp saturated zone thickness are included with the p r o f i l e s . The t r a v e l time i s the time required for groundwater to t r a v e l from a drainage divide to the avalanche headscarp. Results c l e a r l y show that greater discharge rates i n January, 1983 generated higher water tables at avalanche headscarps. Greater discharge rates generally cause steeper 145 slopes i n the water table p r o f i l e s . Travel times show, with one exception, that groundwater takes about 1 and 3 hours to t r a v e l from a drainage divide to the avalanche headscarps. Maximum s o i l discharge rates over 1 to 3 hours therefore determined maximum saturated zone thicknesses. Under such circumstances v a r i a b l e i n i t i a l conditions become inconsequential, provided s i m i l a r peak discharges are attained at d i f f e r e n t avalanches. January, 1983 p r o f i l e s show that avalanche W-l produced a higher water table than LR-2, despite d r i e r i n i t i a l conditions (distances from drainage divide are about equal). December, 1979 p r o f i l e s show the opposite, because dry i n i t i a l conditions constrained the peak discharges. The water table quickly rose to the surface at DS-1 during both storms. Pore pressures are therefore indistinguishable and f a i l u r e should have occurred in both cases. K s a^- and n caused the rapid water table r i s e . The K s a-j- value was the lowest of the nine avalanches, while the n value was second lowest. C h a r a c t e r i s t i c curves show that K s a^ i s linked to 9 V. As Ksa^-decreases, the volume that a given s o i l discharge occupies increases, and 9 V increases. The 9 V increase during the comparison storms lowered e to near zero i n the low porosity s o i l . As a re s u l t , s o i l discharge rates comparable to other avalanches caused higher water tables. The slope of the p r o f i l e i s not as great i n a s o i l of sim i l a r porosity, DS-2, because the K s a t value i s higher. Higher slope angles promote s o i l drainage. Figure 6.13 146 shows that higher slope angles at W-2 (40°) and DS-2 (50°) lower the slope of the water table p r o f i l e . This e f f e c t i s c l e a r l y seen because other factors such as Ksa-(- and n are s i m i l a r at the d i f f e r e n t avalanches, and i n i t i a l conditions are not an influencing factor. December, 1979 storm r e s u l t s show that t h i s slope angle e f f e c t can be overshadowed under d i f f e r e n t discharge conditions. January, 1983 data indicate that the water table rose to the surface between 20 and 60 m from drainage divides. This r e s u l t apparently contradicts e a r l i e r observation that evidence for overland flow was limited during that storm. However, f i e l d inspection shows that study area h i l l s l o p e s are planar near drainage divides, then quickly develop into drainage depressions downslope. In the depressions the distance to a divide becomes the distance to the top of l o c a l sideslopes, which i s smaller than the distance to the ridge above the depression. 6.10.2 Drainage depression discharge p r o f i l e s Drainage depressions controlled saturated zone thicknesses at four of the debris avalanche headscarps, DD-1, DD-2, DD-3, and LR-1. The kinematic wave solution was again modified to account for concentrated flow i n depression axes. I t was assumed that the depressions take the form of a, "tipped, trian g u l a r trough" (Dietrich, et. a l , 1986), consisting of an axis fed by planar sideslopes. Groundwater input per unit length of a depression i s governed by discharge from the sideslopes. 147 Sideslope angles and distances to ridgelines are d i f f e r e n t than depression axis angles and distances to drainage d i v i d e s 1 . Sideslope angle was determined by rotating planar sideslopes i n a surveyed cross-section to the axis slope angle, using a Wulff stereonet. The maximum dip angle i n the rotated plane i s the sideslope angle, while the sideslope distance i s calculated using cross-section sideslope lengths. Ragan (1973 p.97) reviews t h i s rotation technigue. I t i s assumed that the complete sideslope lengths are contributing up to the ri d g e l i n e . Sideslope input rates are converted to discharges using Darcy's Law (Eq. 6.7). Figures 6.15 and 6.16 are plots of discharge versus downslope distance from drainage divide. I n i t i a l conditions, slope angle, sideslope length, sideslope angle, and t r a v e l time are noted above the plo t . Data point labels are plotted every three hours. January, 1983 discharges were, with one exception, about three times greater than December, 1979 discharges. DD-3 was i n i t i a l l y discharging under wet i n i t i a l conditions, therefore the difference i n storm discharges was not as great. Drainage depression avalanches generally occur further downslope from drainage divides, at longer t r a v e l times. Peak discharges therefore depend on the maximum discharge over a longer period. Under these circumstances i n i t i a l conditions become important, because they control the duration of peak discharge. Avalanches 1 - The term drainage divide applies to the highest elevation i n drainage depression axes, while the term r i d g e l i n e i s applied at divides between sideslopes. 148 with dry i n i t i a l conditions, such as DD-2, have lower discharge rates than avalanches with wet i n i t i a l conditions, such as DD-3. Discharge magnitude also depends on the sideslope angle and length. Sideslopes of greater length and steeper angle develop thic k e r saturated zones that discharge groundwater more rapidly into the axis of the depression. Avalanches DD-1 and LR-1 have s i m i l a r i n i t i a l conditions, but DD-1 produced greater discharges because of i t s longer, steeper sideslopes. 6.10.3 Drainage d e p r e s s i o n water t a b l e s Darcy's Law was used to convert discharge i n drainage depressions to cross-sectional areas of flow f i l l i n g the depressions, Q = Kbulk s i n 0 A (6.13) where, Q = maximum discharge at avalanche headscarp, from Section 6.10.2 sin8 = hydraulic gradient, a i s the depression axis slope angle A = cross-sectional area of saturated s o i l i n drainage depression The saturated cross-sectional areas were determined by t r i a l of d i f f e r e n t water table l e v e l s i n known depression geometry. During severe storms water tables r i s e to the surface of some depressions. Surface flow v e l o c i t i e s are greater than groundwater flow v e l o c i t i e s , which makes kinematic wave discharges erroneous. Manning's equation was used to develop rough estimates of surface flow v e l o c i t i e s and depths. In a l l 149 cases the distance traveled i n one hour i s equal to or greater than the distance from the drainage divide to the headscarp. The maximum surface water discharge was therefore less than the maximum 1-hour input a f t e r the water table reached the surface. The 1-hour input i s used as an approximate answer. Calculations are included i n Appendix VI. Drainage depression cross-sections and water tables are depicted i n Figure 6.17. January, 1983 and December, 1979 water tables at DD-1 and DD-2 are c l e a r l y distinguishable. Generally higher water tables at DD-1 are caused mainly by longer sideslopes and a greater distance to a drainage divide. Lower water tables at DD-2 r e f l e c t dry i n i t i a l conditions, short sideslopes, and a shorter distance to a drainage divide. At DD-3, a difference of 1 cm barely distinguishes the storm water tables. Wet i n i t i a l conditions indicate that water tables often r i s e to the surface. As a resu l t , pore pressures i n the depression axis do not distinguish the comparison storms. The data suggest that f a i l u r e occurred at DD-3 during a l l three comparison storms, and that other wet drainage depressions are susceptible to frequent f a i l u r e . A cross-section at LR-1 i s not included because flow from the drainage depression crosses a logging road, which formed the avalanche headscarp. To determine water tables i t i s assumed that groundwater discharging from the depression flowed i n a 6-meter wide swath over the r e l a t i v e l y impermeable road, and then discharged onto the f i l l slope. Downslope cross-sections of 150 comparison storm water tables are depicted i n Figure 7.7. A l l surveyed debris avalanches were i n i t i a t e d under January, 1983 water tables. In Chapter 7 those water tables are entered i n the s t a b i l i t y program and a factor of safety of one assumed. December, 1979 water tables are entered to assess factors of safety during that storm. 6.11 S o i l development and storm runoff implications Water table/discharge p r o f i l e s (Figures 6.13 - 6.16) have implications for s o i l development and slope s t a b i l i t y . The presence of drainage divides and ridgelines l i m i t s the saturated zone thicknesses at given distances downslope. The thicknesses are governed by the input rates, which depend ultimately on p r e c i p i t a t i o n . Physiographic l i m i t s on p r e c i p i t a t i o n i n t e n s i t y and duration (see Figure 3.1) control the maximum saturated zone i thickness at a given point on a slope. Tsukamoto, et. a l (1982) described three s o i l types progressing down a t y p i c a l planar slope: 1) Residual s o i l ; 2) Creeping s o i l ; 3) C o l l u v i a l s o i l . Water table and discharge p r o f i l e s provide an explanation for t h i s observation. Residual s o i l s develop i n s i t u near drainage divides because limited groundwater i s available to in s t i g a t e slope i n s t a b i l i t y or erosion. Extreme events such as the January 9-10, 1983 storm cause shear f a i l u r e s and removal of unstable s o i l accumulations. Downslope, the regular water table b u i l d up causes creep and slope i n s t a b i l i t y in c o l l u v i a l s o i l s . Water table analyses provided insight into runoff processes on forest h i l l s l o p e s . Freeze (1974) found that runoff models must account for the following, widely observed, f i e l d evidence: 1) overland flow occurring over small percentages of the basin; 2) sharp discharge peaks i n runoff hydrographs, r e s u l t i n g from rapid discharge of incoming r a i n water. Drainage depression configurations account for both these requirements. F i r s t , separation of long (200 m) h i l l s l o p e s into depressions separated by drainage divides causes a majority of the slope to remain unsaturated. Second, sideslope discharge converges i n depression axes to saturate the s o i l from below and cause surface flow. Dunne and Black (1970) found that r a i n f a l l on saturated s o i l contributed s i g n i f i c a n t l y to h i l l s l o p e hydrographs. In the study area, subsurface discharge and the Dunne and Black mechanism contribute water to the depressions which i s rapidly discharged to streams. 'Variable source area' i s a term used by Freeze (1974 p.634) and others to describe the expansion of the channel network. 6.12 Conclusion A f i n i t e difference i n f i l t r a t i o n model i s combined with a modified kinematic wave equation of flow to determine water tables at avalanche headscarps. V e r t i c a l i n f i l t r a t i o n i s governed by the s o i l matrix controlled hydraulic conductivity, i^sat' and i t s associated unsaturated flow rates. R a i n f a l l duration and intensity, i n i t i a l conditions, and s o i l depth are other factors that influence discharge from a s o i l p r o f i l e . 152 Modeling r e s u l t s show that variable r a i n f a l l i n t e n s i t y , r a i n f a l l duration, and i n i t i a l conditions most s i g n i f i c a n t l y influence the magnitude and duration of discharge response to a given storm. Downslope flow i s governed by the macropore-controlled conductivity, K^^k, which i s almost two orders of magnitude greater than the mean K s a f Kinematic wave tes t runs show that the parameter K^^^/n should range from 5 X 1 0 - 2 to 5 X 10 - 3 m/s i n forest s o i l s . Comparison storm te s t r e s u l t s show, with two exceptions, that water tables were higher i n January, 1983 than during the other rainstorms studied. I n i t i a l conditions, porosity, K s a t , slope angle, and slope length influence water table l e v e l s . Low porosity, low K s a t , headscarp s o i l at DS-1 became saturated to the surface during December, 1979 and January, 1983 storms, hence water tables are not e a s i l y distinguished. I n i t i a l l y wet s o i l s at DD-3 likewise became saturated during both storms. Variable source area stormflow appears to be the predominant runoff mechanism. Drainage depression configurations account for the presence of large areas of unsaturated s o i l along with sharp, high streamflow peaks. 153 F i g u r e 6.1: Flow equations, boundary c o n d i t i o n s , and assumptions f o r both drainage cases A.) K s a t - - c o n t r o l l e d drainage. Upper boundary c o n d i t i o n : Sh/Sz = R / K(h) - 1 _! I I I I I I ! _ SOIL Flow equations: Unsaturated: S/Sz [ K(h) (Sh/Sz + 1) ] = C(h) Sh/Sz S a t u r a t e d : S2h/Sz2 = 0 Q out < < Q i n ROCK Basal boundary c o n d i t i o n : SU/Sz - 0 Assumptions: 1) I n f i l t r a t i o n c o n t r o l l e d by K s a t , c h a r a c t e r i s t i c curves 2) Q i n = Q out f o r a l l t d u r i n g the storm. 3) Downslope drainage c o n t r o l l e d by K s a t -B.) K ^ ^ - c o n t r o l l e d drainage. Upper boundary c o n d i t i o n : Sh/Sz = R / K(h) - 1 T T Y T T T T T SOIL + MACROPORES Flow equations: Unsaturated: S/Sz [ K(h) (Sh/Sz + 1) ] = C(h) Sh/Sz Basal boundary c o n d i t i o n : Q out < Sh/Sz = Q(t) / K(h) - 1 < Q i n < x > ROCK Assumptions: 1) I n f i l t r a t i o n c o n t r o l l e d by Ksa-t-, c h a r a c t e r i s t i c curves. 2) V a r i a b l e b a s a l boundary c o n d i t i o n , dependent on Q(t) and K(h) at base o f p r o f i l e a t time, t . 3) Unsaturated flow occurs t o a t h i n s a t u r a t e d zone a t the base of t he p r o f i l e . Changes i n s a t u r a t e d zone t h i c k n e s s do not s i g n i f i c a n t l y e f f e c t d i s c h a r g e a t base. 4) Downslope drainage c o n t r o l l e d by K ^ ] ^ . 5) Q out = Q i n + Q(t) over X, a t steady s t a t e . 154 Figure 6.2: Pressure head p r o f i l e s during v e r t i c a l i n f i l t r a t i o n to an impermeable basal boundary P R E S S U R E H E A D , h , M O F W A T E R 155 F i g u r e 6.3: Pressure head p r o f i l e s d u r i n g v e r t i c a l i n f i l t r a t i o n t o a d i s c h a r g i n g b a s a l boundary THIN SATURATED ZONE •> "1 ' i i I i i i i 1 1 1 1 1 1 -1.4 -1.2 -1.0 -0 .8 -0 .6 -0 .4 -0.2 0.0 P R E S S U R E HEAD, h, M OF WATER Figure 6.4; Pressure head p r o f i l e s during v e r t i c a l drainage from a discharging basal boundary 156 LU O < U_ Crl ZD cn Q O Crl O O _ J UJ m x i — Q_ LU Q 0.00 - 0 . 0 8 - 0 . 1 6 - 0 . 2 4 - 0 . 3 2 - 0 . 4 0 - 0 . 4 8 - 0 . 5 6 - 0 . 6 4 - 0 . 7 2 - 0 . 8 0 GROUND SURFACE CURVE '0' EQUALS CURVE '13' IN FIGURE 6.3 CURVE LABELS ARE TIMES, IN HOURS, SINCE RAINFALL TERMINATION ' SANDY SOIL THIN SATURATED ZONE 1.4 - 1 . 2 - 1 . 0 - 0 . 8 - 0 . 6 - 0 . 4 P R E S S U R E HEAD, h, M OF WATER ~i i — 0.2 0.0 157 F i g u r e 6.5: E f f e c t o f r a i n f a l l d u r a t i o n on d i s c h a r g e r a t e R = 16.6 MM/HR (.65 IN/HR) 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 TIME, HOURS F i g u r e 6.6: E f f e c t o f r a i n f a l l i n t e n s i t y and i n i t i a l c o n d i t i o n s on d i s c h a r g e r a t e 158 0.040 0.035 jfj 0.030 cr < ±^ 0.025 -\ cr Ld Q_ _ 0.020 o l_J o cr < o CO Q 0.015 H 0.010 H 0.005 0.000 - f t R2 = 22.3 MM/HR (.88 IN/HR) • A O R1 = 16.6 MM/HR (0.65 IN/HR) - 13 HR. AT R1 13 HR. AT R1, IC = - 2 m 3 HR. AT R2 13 HR. AT R2 Ksat = 4.5 E - 5 IC = — 1 m, unless otherwise noted I i if I I i f I I C f I I I 0 1 2 3 4 H6.5E-006 > M-5E-006 m > 1.0E-006 J^5.0E-007 I M | I M | I M | I I I | M I | I M | I I I | I I I 6 7 8 9 1 0 1 1 12 13 14 15 TIME, HOURS 0.0E+000 F i g u r e 6.7: E f f e c t of s o i l depth of d i s c h a r g e r a t e 159 0.030 R = 16.6 MM/HR (.65 IN/HR) •5.0E-006 r-4.5E-006 GO -4.0E-006 O X > -3.5E-006 O m -3 .0E-006<^ CO I-2.5E-006 ^ -2.0E-006 % 1.5E-006 2> m > - 1.0E-006 •5.0E-007 0.000 1 1 ' I ' ' ' I ' i i | i I I | i I i | i i i | i I i | i i i | i i i | • 0.0E+000 0 2 4 6 8 10 12 14 16 18 TIME, HOURS Figure 6.8: E f f e c t of K s a t on discharge rate 160 0.030 R = 16.6 MM/HR (.65 IN/HR) CURVE LABELS ARE SATURATED HYDRAULIC CONDUCTIVITIES, IN M/S x 10 - 5 RAINFALL DURATION 13 HOURS AT RATE, R •5.0E-006 •4.5E-006 O GO -4.0E-006 O X > 70 3.5E-006 O •3.0E-006\_ GO •2.5E-006 ^ 70 -2.0E-006 0.000 1.5E-006 > 70 m > 1.0E-006 -5.0E-007 T | i i i | i i i | i i i | i i i | i i i | i i r 6 8 10 12 14 16 18 TIME, HOURS 0.0E+000 F i g u r e 6.9 0.035 -i < 0.030 -cr < ZD cr LU •_ 0.025 0.020 -CJ 0.015 -cr < JZ o 00 o 0.010 0.005 0.000 1 0 - 5 4.5 6.5 Avalanche Ksat in m/s Wedges: o - W - 1 , -1 .88 , * - W - 2 , - 0 .62 , Drainage depressions: A - DD -1 , - 2 .12 , 7.8 0 - DD-2 , -4 .80 , + - DD-3 , - 0 .35 , Logging roads: * - L R - 1 , - 2 .08 , x - L R - 2 , - 0 .95 , Discontinuities: * - D S - 1 , - 0 .96 , • - D S - 2 , - 1 .14 , Discharge r a t e s d u r i n g January 9-number, IC in m, 10, 1983 storm rainfall 0.0E+000 8 10 HOURS 12 14 16 18 20 SINCE 1000, JANUARY 9, 22 24 1983 26 28 0.035 - i < 0.030 -| Figure 6.10: Discharge rates during December 13-14, 1979 storm Avalanche number, IC Ksat in m/s X 1 0 - 5 : 0.000 O.OE+000 24 26 1979 to Figure 6.11: Discharge rates during January 29-30, 1971 storm 0.035 -i Avalanche number, Ksat in m/s X 10 0.000 O.OE+000 6 8 10 12 14 16 18 20 22 24 26 28 30 32 TIME, HOURS SINCE 0900, JANUARY 29, 1971 164 Figure 6.12: E f f e c t of K b u l k / n on water table p r o f i l e at W-l 0.7 0.6 H SLOPE: 34° STORM: DECEMBER 1 3 - 1 4 , 1979 zn 0.5 CL Ld Q W 0.4 H O N S 0.3 I— < cr ZD < 0 2 0.1 0.0 0 25 50 75 100 125 150 DOWNSLOPE DISTANCE FROM DRAINAGE DIVIDE, M F i g u r e 6.13: Water t a b l e p r o f i l e s on p l a n a r s l o p e s , January 9-10, 1983 AVALANCHE LABEL: W - 1 , INITIAL CONDITIONS IN M: - 1 . 88 , POROSITY: 0.52, KSAT IN M/S X 1 0 - 5 : 4.5, SLOPE ANGLE IN DEGREES: 34, TRAVEL TIME IN HOURS: 1.4 Wedge s : O- W-1. - 1 . 88 , 0.52, 4.5, 34, 1.4 * - W - 2 , - 0 . 62 , 0.51, 6.5, 40, 2.7 Logging roads: x - L R - 2 , - 0 . 95 , 0.49, 2.9, 34, 1.4 Discontinuities: * - D S - 1 , - 0 . 96 , 0.44, 2.1, 34, 5.4 • - D S - 2 , - 1 .14 , 0.46, 5.9, 50, 2.8 0.8 -i 0.7 - -£ 0 . 6 UJ z y 0.5 x Ld Z o N 0.4 a w 0 !< cr ZD *< 0.2 00 0.1 -0.0 DS-1: 0.74 (D = 80 m.) W-2: 0.70 GROUND SURFACE 0 25 50 DOWNSLOPE DISTANCE FROM DRAINAGE DIVIDE, M 166 F i g u r e 6.14: Water t a b l e p r o f i l e s on p l a n a r s l o p e s , December 13-14, 1979 AVALANCHE LABEL: W - 1 , INITIAL CONDITIONS IN M: - 1 . 88 , POROSITY: 0.52, KSAT IN M/S X 1 0 - 5 : 4.5, SLOPE ANGLE IN DEGREES: 34, TRAVEL TIME IN HOURS: 1.4 W e d g e s : O- VV-1. - 1 . 88 , 0.52. 4.5, 34. 1.4 * - W - 2 , - 0 . 62 , 0.51, 6.5, 40, 2.7 Logging roads: x - L R - 2 , - 0 . 9 5 , 0.49, 2.9, 34, 1.4 Discontinuities: * - D S - 1 , - 0 . 96 , 0.44, 2.1, 34, 5.4 • - D S - 2 , - 1 .14 , 0.46, 5.9, 50, 2.8 DS-V. Q.74 (D = 80 m.) GROUND SURFACE 0 25 50 DOWNSLOPE DISTANCE FROM DRAINAGE DIVIDE, M F i g u r e 6.15: Drainage d e p r e s s i o n d i s c h a r g e , January 9-10, 1983 AVALANCHE LABEL: DD-1, INITIAL CONDITIONS IN M: -2.12, SLOPE ANGLE IN DEGREES: 34, SIDESLOPE LENGTH IN M: 55, SIDESLOPE ANGLE IN DEGREES: 41 , TRAVEL TIME IN HOURS: 11.0 4 8 4 4 4 0 3 6 3 2 X :> 2 8 LU o 2 4 cn < X O 2 0 CO C J 1 6 1 2 8 Drainage depressions: A - DD-1, -2 .12, 34, 55, 41, 11.0 0 - DD-2, -4 .80, 38, 29, 18, 3.1 + - DD-3, -0 .35 , 34, 89, 39, 12.9 Logging roads: * - L R - 1 , -2 .08, 33, 43, 34, 18.2 DD-3: 41.3 4H 0 DD-1 : 25.3 L R - 1 : 15.81 DD-2: 2.5 " i — i — r 0 I I I I I I I 5 0 1 0 0 v- -0-1 I I I I I I I 1 I—I—I 2 0 0 2 5 0 3 0 0 D O W N S L O P E D I S T A N C E F R O M D R A I N A G E D I V I D E , M i | i i r 1 5 0 168 F i g u r e 6.16: Drainage d e p r e s s i o n d i s c h a r g e , December 13-14, 1979 AVALANCHE LABEL: DD-1 , INITIAL CONDITIONS IN M: -2 .12, SLOPE ANGLE IN DEGREES: 34, SIDESLOPE LENGTH IN M: 55, SIDESLOPE ANGLE IN DEGREES: 41, TRAVEL TIME IN HOURS: 11.0 Drainage depressions: A - DD-1, -2 .12 , 34, 55, 41, 11.0 0 - DD-2, -4 .80, 38, 29, 18, 3.1 + - DD-3, -0 .35 , 34, 89, 39, 12.9 Logging roads: * - L R - 1 , -2 .08, 33, 43. 34, 18.2 50 100 150 200 250 300 DOWNSLOPE DISTANCE FROM DRAINAGE DIVIDE, M 169 F i g u r e 6.17; Drainage d e p r e s s i o n c r o s s - s e c t i o n s and water t a b l e s 3H a.) D D - 1 ROCK I r 2 3 1 1 r 4 5 METERS n i i 1 1 6 7 8 b.) D D - 2 DEC, 1979 ROCK ~> r 0 "i r 2 4 METERS ~ l r 5 6 ~1 r 7 c.) D D - 3 ROCK DEC, 1979 0 ~ l 1 I 1 I 1 I i | i 1 1 1 1 1 1 2 3 4 5 6 7 8 METERS 170 i CHAPTER 7: SLOPE STABILITY ANALYSES 7.1 Introduction The f i r s t aim of the slope s t a b i l i t y analyses i s to back calcu l a t e the root cohesion, C r, assuming a factor of safety of one at f a i l u r e . The range of C r values i s compared to other published values, and patterns within the avalanches are discussed. F a i l u r e surface geometries are discussed and related to the c o n t r o l l i n g factors described i n Chapter 4. The e f f e c t s of s o i l depth, s o i l density, and slope angle are also quant i t a t i v e l y assessed. Back calculated C r values from January, 1983 are used to determine factors of safety (FS) under d r i e r , i n i t i a l conditions before the onset of the storm. A parameter c a l l e d the strength r a t i o i s developed to i l l u s t r a t e the loss of shear strength a f t e r f a i l u r e . FS values are determined under December, 1979 water tables. The minimum root strength required to maintain s t a b i l i t y i s also back calculated. The r e s u l t s explain the events of January, 1983 and December, 1979. Implications for the January, 1971 storm are also discussed. 7.2 Slope s t a b i l i t y program A computerized program, STABL2, was used to carry out the s t a b i l i t y analyses (Siegal, 1975). The program calculates the factor of safety against f a i l u r e using a two-dimensional l i m i t i n g equilibrium method. The Modified Bishop method of 171 s l i c e s i s the solution technique u t i l i z e d . Given input parameters the program searches for the f a i l u r e surface with the lowest FS. A microcomputer adaption has been developed by the Department of C i v i l and Environmental Engineering at the University of Wisconsin - Madison. 7.3 Assumptions ; Two-dimensional sections were analyzed. Craig (1983) states that 2D analyses give conservative results for a f a i l u r e on a three dimensional (dish shaped) surfaces. The root permeated s o i l showed uniform shear strength (as opposed to a high shear strength root mat at the surface), which minimized l a t e r a l boundary e f f e c t s on a 2D f a i l u r e surface. Three assumptions are inherent in u t i l i z i n g a C r value. F i r s t , roots are assumed to be oriented perpendicular to the shear plane. Gray and Ohashi (1983) found that randomly oriented roots i n forest s o i l s have about the same shear strength as roots oriented perpendicular to the f a i l u r e surface. Second, the t e n s i l e strength of a l l the roots i s f u l l y mobilized. Greenway (1987) states that roots w i l l break at d i f f e r e n t applied strains, and C r should be considered a "mean root t e n s i l e strength". F i n a l l y , roots are well anchored and do not p u l l out when tensioned. Gray and Leiser (1982) state that roots are generally long enough to f a i l in tension before p u l l i n g out of the s o i l . Under i n i t i a l conditions a constant, negative, pore pressure 172 i s entered i n the s t a b i l i t y program. Fredland, et. a l (1978) indicate that the strength provided by negative pore pressures i s c ontrolled by a second f r i c t i o n angle term, <p^, the f r i c t i o n angle with respect to s o i l matrix suction when t o t a l stress i s held constant. Tests showed 0 B to be si m i l a r , but s l i g h t l y lower, than 0 ' . I t i s assumed in using negative pore pressures i n the program that 0 ' = <}P. As a r e s u l t , shear strength and factors of safety are overestimated. However, approximate FS values demonstrate the e f f e c t s o i l matrix suction on slope s t a b i l i t y . F i n a l l y , under storm conditions i t i s assumed that the water table within discrete slope units i s p a r a l l e l to the bedrock substrate ( i . e . i n f i n i t e slope seepage). Kinematic wave equation r e s u l t s showed that the water table sloped away from the bedrock. However, the error i s not s i g n i f i c a n t within 5 -10 m slope segments. 7.4 Input parameters Input parameters include slope geometry, s o i l unit weights and shear strength parameters, and pore water pressures. Slope geometry and s o i l depths were derived from survey data i n Chapter 4. Unit weights and shear strength parameters were reviewed i n Chapter 5. I n i t i a l conditions and comparison storm water tables were analyzed i n Chapter 6. In the program, pore pressures can be expressed as a pore pressure r a t i o , pore pressure constant, or a s p e c i f i e d 173 piezometric surface. I f the s o i l was completely unsaturated, as under the i n i t i a l conditions, then a constant, negative, pore pressure constant was assigned. I f the s o i l was saturated to the surface then the pore pressure r a t i o , Ru, was used, Ru = u / TD (7.1) where, u = pore pressure T D = s o i l overburden Ru i s a convenient term for expressing pore pressures at any point i n saturated s o i l . I f the s o i l was p a r t i a l l y saturated, data on a piezometric surface were entered i n the program, and v e r t i c a l hydrostatic pressures were calculated beneath the surface. To account for seepage the piezometric surface was taken as the saturated zone thickness (perpendicular to the bedrock) from Figures 6.13, 6.14, and 6.17, times the cosine of the l o c a l slope angle (see Appendix VI). The piezometric surface i n the program i s therefore lower than the actual water table. Actual water table positions are depicted in Figures 7.1 - 7.9, 7.12, and 7.13. A uniform surcharge of .23 kPa was added to the slope to account for the weight of trees on the s o i l . The surcharge was calculated using a mean tree spacing of 9.3 m and a mean diameter of .3m. Calculations are included i n Appendix VII. Test runs showed that the surcharge had a n e g l i g i b l e e f f e c t on the FS values r e l a t i v e to the other variables involved. 174 7 .5 Back analyzed root cohesion C r values were entered i n STABL2 input f i l e s u n t i l the calculated FS f e l l between 0.995 and 1.000. Root cohesion was always required to make the FS = 1, which i l l u s t r a t e s i t s importance i n forest s o i l s t a b i l i t y . Results are presented i n Table 7.1 and compared with published studies i n Table 7.2. It was generally assumed that the roots completely permeated the s o i l p r o f i l e and anchored i n the sandstone bedrock. LR-2, DS-1, and DS-2 were also run with roots not anchored i n the rock. The water tables were approximate because h i l l s l o p e angles were averaged within slope segments, and water leve l s were averaged across headscarps. Discussion of C r i s based on the li m i t e d number of values obtained from back analyses. The number of observations i s too small to draw s t a t i s t i c a l l y s i g n i f i c a n t conclusions. 7 . 5 . 1 Results Table 7.1 shows that the C r of 2.7 3 kPa at W-2 i s near the maximum value back calculated, and i t represents a t y p i c a l C r for a mixed, regenerating forest logged between 1918 and 1943. The C r of 2.30 at W-l i s lower because scrub Tsuga was only 3 -4 m high and did not provide as much shear strength. DD-1 and DD-3 have notably lower C r values (1.75 and 1.65 kPa) because of weak understory vegetation t y p i c a l of deep, wet, drainage depressions. The C r at DD-2 (2.80 kPa) i s s i m i l a r to surrounding, mixed regenerating forest. The higher value occurs 175 because the depression was shallow and dry, therefore understory vegetation did not dominate i t s axis. At LR-1, discharge from the drainage depression crossed the logging road. The flow width across the road i s estimated as 4 -5m, based on f i e l d observation of overland flow. I t i s assumed that the road did not dive r t additional flow from adjacent slopes to the headscarp. The flow width range produces C r values from 2.63 - 2.99 kPa, which are again t y p i c a l of mixed forest. The lower C r of 2.63 kPa i s more probable because scrub Alnus and low shear strength Polystichum predominated on the logging road. The s o i l p r o f i l e at LR-2 i s d i f f e r e n t from other avalanches i n that thicknesses to 2.50 m were estimated. The back calculated C r of 2.01 kPa i s low for Alnus and Tsuqa forest with root penetration through the f u l l p r o f i l e . An al t e r n a t i v e i s to consider a li m i t e d rooting depth, underlain by cohesionless s o i l . A rooting depth of 1.60 m produces a C r of 2.87 kPa, which i s t y p i c a l of mixed f o r e s t 1 . Lower rooting depths cause higher C r values, which are u n l i k e l y considering the s i m i l a r i t y of regenerating study area forest cover. Roots therefore penetrate to at least the 1.60 m depth. At DS-1 the roots do not penetrate the downslope dipping bedding surface. To simulate t h i s , a thin, cohesionless layer i s included at the base of the s o i l p r o f i l e . The calculated C r - Other rooting depth r e s u l t s : 1.5 m: 3.3 5 kPa, 1.8 m: 2.06 kPa. 176 of 2.54 kPa i s s i g n i f i c a n t l y greater than the 2.32 kPa required i f f u l l root penetration i s assumed. Also, the t h i n layer can be assigned i t s residual f r i c t i o n angle, which Lambe and Whitman (1979) found appropriate for the f r i c t i o n a l resistance of sand s l i d i n g on a smooth surface. In that case the required C r increases to 3.16 kPa, which i s higher than the C r values at other avalanches. The f r i c t i o n angle l i k e l y f a l l s between 0 1 and 0 ' r , and the required C r i s in the 2.6 - 3.0 kPa range found at avalanches with s i m i l a r forest cover. The same techniques were applied to the dis c o n t i n u i t y surface underlying DS-2. For no root penetration, the back calculated C r of 2.44 kPa i s lower than the t y p i c a l range for mixed forests. The C r increases to 2.58 i f 0 ' r i s entered at the r o c k - s o i l contact. Sparse s t a b i l i z i n g trees and abundant understory vegetation on the steep, rocky slope cause the r e l a t i v e l y low value. 7.5.2 Conclusions and comparison with published values C r values are divided into four groups, based on re s u l t s i n Table 7.1 and published studies reviewed i n Table 7.2. The f i r s t three groups occur i n regenerating cover logged between 1918-1950. Group I includes understory vegetation such as grasses, sedges, and shrubs found i n drainage depressions at DD-1 and DD-3. The C r range of 1.6 - 2.0 kPa i s confirmed by Sidle and Swanston (1982), who back analyzed a C r value of 2.02 kPa i n understory vegetation. Group II includes the same understory 177 and numerous scrub trees stunted by exposure and poor s o i l cover. The C r range of 2.3 - 2.6 kPa includes W-l and DS-2. Group I I I , mixed forest, consists of understory and healthy forest to 15 m i n height. The C r range of 2.6 - 3.0 kPa includes W-2, DD-2, LR-1, LR-2, and DS-1. Scrub forest and mixed forest ranges are confirmed, but not distinguished, by b'Loughlin (1972). The f i n a l group consists of old growth vegetation, which was not back analyzed i n t h i s study. Burroughs and Thomas (1977) suggested that old growth Pseudotsuga has a C r range of 8.9 - 16.7 kPa, which would be expected i n the study area. Few debris avalanches originated in old growth forest because of higher C r values. 7 .6 F a i l u r e surface geometries Figures 7.1 - 7.9 depict s o i l p r o f i l e s and computer generated f a i l u r e surfaces producing the lowest FS values under January 9-10, 1983 water table conditions. Water tables from the December 13-14, 1979 storm produced s i m i l a r p o t e n t i a l f a i l u r e surfaces. C i r c u l a r f a i l u r e surfaces were generated at six of avalanches because the geometry was suitable and the s o i l had no inte r n a l planes of weakness. Wedge shaped surfaces occurred at W-l due to s o i l p r o f i l e geometry, and at DS-1 and DS-2 due weakness at the so i l - r o c k boundary. The depth and length of a l l f a i l u r e surfaces was limited by the high shear strength bedrock near the surface. Figures 7.1 and 7.2 depict W-l and W-2, respectively. Both 178 f a i l u r e surfaces started at the top of the s o i l wedge, passed through the thickest s o i l , then terminated midway through the wedge. The saturated, low angle section of the wedge was not involved i n i n i t i a l f a i l u r e , although i t encouraged downslope movement by o f f e r i n g l i t t l e resistance to f a i l e d debris. The thi c k pocket of s o i l i n the center of the wedge appears to be a c r i t i c a l c o n t r o l l i n g factor. The res u l t s do not agree with Humphrey's (1982) conclusion that pore pressure build-up at the downslope l i p of a wedge i s the p r i n c i p l e cause of f a i l u r e . Figures 7.3 - 7.5 depict DD-1, DD-2, and DD-3, respectively. The f a i l u r e surfaces at DD-1 and DD-2 both i n i t i a t e d at a break i n slope, which allowed f a i l u r e surfaces with greater d r i v i n g forces to develop. At DD-1 the f a i l u r e surface incorporated deep s o i l i n the low angle section of the wedge. At DD-2, the deep s o i l p r o f i l e on the steep headscarp slope provided dri v i n g forces for f a i l u r e . S o i l s at DD-3 were modeled as overconsolidated c o l l u v i u m / t i l l ( C = 9.6 kPa) overlain by loose, cohesionless s o i l . The shallow f a i l u r e surface was probably caused by erosion rather than Coulomb shear. Figures 7.6 and 7.7 depict LR-1 and LR-2, respectively. Computer generated surfaces corresponded well to observed headscarp geometries. In both cases the f a i l u r e surfaces were c i r c l e s , but the s o i l mass removed looked l i k e a wedge at LR-1 and a slab at LR-2. The cross sections both show that f a i l u r e s removed s o i l from the oversteepened f i l l slopes and l e f t the slopes closer to t h e i r governing bedrock angle. 179 ' Figure 7.8 and 7.9 depict DS-1 and DS-2, respectively. At DS-1, bedding d i s c o n t i n u i t i e s caused a wedge-shaped surface to occur i n a deep s o i l pocket. The steep crown scarp was confirmed by downdropped s o i l blocks at the headscarp and i n adjacent, p a r t i a l l y f a i l e d , slumps. Rotational movement at the toe of i n i t i a l f a i l u r e surfaces was confirmed by the back-t i l t i n g of trees. A s i m i l a r surface developed at DS-2, again because the discontinuity surface discouraged root penetration. However, the headscarp s o i l was thin, therefore the difference between wedge-shaped and c i r c u l a r surfaces was small. 7.7 E f f e c t s of factors c o n t r o l l i n g avalanche i n i t i a t i o n Slope angle, s o i l depth, s o i l density, vegetative cover, bedrock surface c h a r a c t e r i s t i c s , and snow were c i t e d i n Chapter 4 as factors c o n t r o l l i n g debris avalanche i n i t i a t i o n . The quantitative e f f e c t of these factors i s assessed to evaluate t h e i r r e l a t i v e importance and t h e i r implications for forest slope s t a b i l i t y . The e f f e c t of snow i s not considered because of uncertainty concerning snowmelt water volumes and the e f f e c t s of snow on surface water flow. 7.7.1 Slope angle Mean study area strength parameters were entered i n the ! program INSLOPE to assess the e f f e c t of slope angle. C r values i of 1.8 kPa and 2.7 kPa were chosen to represent Group I (understory) and Group III (mixed regenerating forest) ranges 180 discussed i n Section 7.5. Mean s o i l parameters from Chapter 5 were, 0' = 33.0°, T s = 17.9 kN/m3, and depth = .74 m. "Worst case" conditions of s o i l saturated to the surface were assumed. Results i n Figure 7.10 show that slopes covered by Group III vegetation f a i l at angles greater than 29.7°. Under Group I vegetation the slopes f a i l at angles greater than 24.6°. The l i m i t i n g angles are conservative because the i n f i n i t e slope analysis ignores end ef f e c t s , and s o i l s near drainage divides or rid g e l i n e s are seldom saturated. Slopes gentler than the l i m i t i n g angles generally remained stable on January 9-10, 1983. 7.7.2 S o i l density S o i l was described as loose i n wedges and drainage depressions, and Table 5.3 showed that void r a t i o s were high at those f a i l u r e geometries. Avalanches W-l and DD-1 were run under the f r i c t i o n angle range, and therefore density range, found i n d i r e c t shear t e s t s . Pore pressures from the January 9-10, 1983 storm were applied, and back calculated C r values from Table 7.1 were u t i l i z e d . Figure 7.11 shows that the s o i l would have been stable i n a denser condition. The r e s u l t s suggest that f r i c t i o n angles are lower and slopes are less stable i n wedges and drainage depressions than elsewhere on forest slopes. 7.7.3 S o i l depth F i e l d evidence from Chapter 4 and analyses from Section 7.6 showed that f a i l u r e surfaces developed i n deep s o i l p r o f i l e s . 181 To demonstrate the e f f e c t of s o i l depth, DD-2 and DS-1 were reanalyzed under January 9-10, 1983 pore pressures and back analyzed C r values, but with .3 m of s o i l removed from the p r o f i l e s . At DD-2, Figure 7.12 shows that the FS increased from 1.00 to 1.15. At DS-1, Figure 7.13 shows an FS increase to j 1.08. These r e s u l t s p a r t i a l l y explain why deep s o i l p r o f i l e s at a l l the headscarp geometries were susceptible to f a i l u r e , and why thinner s o i l p r o f i l e s on steeper slopes did not f a i l on January 9-10, 1983. At DS-1, f a i l u r e was i n i t i a t e d on a 26° -30° slope, while stable, 45° slopes with thinner p r o f i l e s are found adjacent to the middle of the scarp. The same p r i n c i p l e applies at W-l, W-2, and DS-1, where f a i l u r e s were i n i t i a t e d on shallow slopes with thicker s o i l , rather than on adjacent, steeper slopes with t h i n s o i l . 7 . 7 . 4 Root cohesion Root cohesion was shown in Section 7.5 to be an important i s t a b i l i z i n g influence i n forest s o i l s . The program INSLOPE demonstrated the e f f e c t of C r on factor of safety. Mean s o i l parameters were assumed on a saturated, 20° slope. Group I - IV vegetative covers produced the following factors of safety: Group I: FS = 1.03 (C r = 1.8 kPa); Group II: FS = 1.16 (C r = 2.45 kPa); Group I I I : FS = 1.23 (C r = 2.80 kPa); Group IV: FS = 2.45 (C r = 8.9 kPa, from Burroughs and Thomas, 1977). If Group IV forest can remain stable under saturated conditions, then the slope should not f a i l . Saturated 182 conditions produce near-maximum pore pressures, because high overland flow v e l o c i t i e s r e s t r i c t the overland flow thicknesses that can develop. To i l l u s t r a t e t h i s , W-l and W-2 were run under saturated conditions with C r = 8.9 kPa. FS values of 2.45, at W-l, and 3.40, at W-2, were produced, hence the slopes should not f a i l . However, the a l l u v i a l fan at the mouth of Smith Creek shows that numerous debris torrents occurred i n the p o s t - g l a c i a l period. F i r e , disease, or c l i m a t i c stress may have lowered the s t a b i l i z i n g C r values in parts of the study area, as Fraser (1986) suggested. In addition, f a i l u r e could s t i l l occur i n low C r drainage depressions or on discontinuity surfaces. Logging has the same e f f e c t as a natural disturbance i n that i t i reduces the C r from old growth values. 7 . 7 . 5 Discontinuity surfaces The e f f e c t of discontinuity surfaces was q u a n t i t a t i v e l y assessed i n Section 7.5. The r e s u l t s showed that a greater C r was required to maintain s t a b i l i t y , therefore the discontinuity surfaces lowered the o v e r a l l s t a b i l i t y of the slope. To demonstrate t h i s , factors of safety were calculated at DS-1 with the e f f e c t s of the discontinuity surface removed. I f no root penetration was assumed (Case B: C r = 2.54 kPa) the FS increased to 1.03 when root penetration was added. If no root penetration and residual f r i c t i o n angles were assumed (Case C: C r = 3.16 kPa) the FS increased to 1.13 when both were added. Thorsen (1987) noted a January, 1983 debris avalanche i n i t i a t e d on a 183 steep bedrock slab overlain by old growth forest. The example c l e a r l y showed that root cohesion and possibly f r i c t i o n angle were lower at the soil-bedrock interface than i n the p r o f i l e above. The reduced strength causes slopes underlain by d i s c o n t i n u i t i e s to f a i l on lower slope angles and i n higher C r forests than s t a b i l i t y analyses predict. 7.8 Slope s t a b i l i t y on January 9-10, 1983 Factors of safety before the January 9-10, 1983 storm were influenced by the negative pore pressures developed i n unsaturated s o i l . Constant, negative, i n i t i a l pore pressures and back calculated C r values from Section 7.4 were entered i n the s t a b i l i t y program. Results i n Table 7.4 showed that the slopes were quite stable under drained i n i t i a l conditions. The factors of safety were sharply reduced by increased pore pressures during the storm. At each avalanche, i n i t i a l f a i l u r e caused a loss of shear strength i n the s o i l mass. Root cohesion was eliminated as roots were sheared along the f a i l u r e surface. The f r i c t i o n angle was also reduced from a peak to a residual angle. A parameter c a l l e d the strength r a t i o , SR, i s used to evaluate t h i s strength loss. SR i s defined at each headscarp as the peak shear strength (at fa i l u r e ) divided by the shear strength of the f a i l e d s o i l mass, along the known f a i l u r e surface. I t i s assumed that shear stress remains constant during shear. Results i n Table 7.4 showed that SR ranged from 1.62 - 3.05. 184 This sharp loss of strength explains the high mobility of the avalanches a f t e r i n i t i a l f a i l u r e . 7.9 Slope s t a b i l i t y on December 13-14, 1979 Table 7.5 summarizes FS values for the December 13-14, 1979 i storm. Back calculated C r values from the January, 1983 storm were used i n the analysis. The r e s u l t s showed that lower water tables caused most of the avalanches to remain stable during i that storm. However, avalanches DD-2, DD-3, and DS-1 were only marginally stable, with FS values near 1. At DS-1, the water table reached the surface during both storms, and only small differences i n surface water depths distinguished the pore pressures. Partly revegetated slump scarps described i n Chapter 4 provided f i e l d evidence that f a i l u r e had occurred on the slope before 1983. The old slump scarps were e a s i l y discerned from fresh, January, 1983 scarps. Many of the slumps probably occurred i n 1979, but for an unknown reason did not develop into debris avalanches. At DD-3, a 1 cm difference i n surface water l e v e l s produced a n e g l i g i b l e difference i n FS values, and f a i l u r e probably occurred i n both cases. I t was shown in Section 7.5 that t h i s avalanche i s more l i k e a channelized washout than a Coulomb f a i l u r e . Localized washouts in deep drainage depressions therefore occurred during both storms. The January, 1983 storm was distinguished by the greater number of Coulomb f a i l u r e s on sideslopes, which contributed s o i l to the depression axis. 185 Unfortunately, f i e l d evidence for the 1979 washouts was destroyed i n 1983. At DD-2, extremely dry s o i l i n h i b i t e d water table b u i l d up, so the maximum water table only intersected a lim i t e d section of the p o t e n t i a l f a i l u r e surface. Despite the low FS i t i s un l i k e l y that the slope f a i l e d i n December, 1979. Smaller washouts may have occurred downslope i n the depression. Table 7.5 presents the C r value required to maintain an FS > ! 1 during the December, 1979 storm. Seven of the nine avalanches showed required C r values less than 2.0 kPa. The data indicate that s i m i l a r slope geometries with weak understory vegetation would have f a i l e d i n December, 1979. Such C r values could occur during a c r i t i c a l period a f t e r logging when the root strength from logged old growth had deteriorated. 7.10 Implications for the January 29-30, 1971 storm The January, 1983 and December, 1979 analyses have implications for the types of f a i l u r e s that occurred on January 29-30, 1971. In Chapter 3 the January, 1971 event was described ks a snowmelt/rainfall flood that mobilized stream channel debris into debris torrents. Of the avalanches studied, DD-3 i s the probably the best example of the type of f a i l u r e that occurred i n 1971. Hydrologic and slope s t a b i l i t y analyses showed that f a i l u r e at DD-3 was just as l i k e l y i n December, 1979 as i t was i n January, 1983. The combined snowmelt/rainfall event i n 1971 was probably equally capable of causing such 186 washout f a i l u r e s . Abundant sediment i n the drainage depressions and stream channels caused the washouts to develop into debris torrents i n the main channels, otherwise they would have been tloods. 7.11 D i s c u s s i o n A number of locations with the described f a i l u r e geometries did not f a i l on January 9-10, 1983. The c o n t r o l l i n g factors reviewed determined whether or not f a i l u r e occurred at a given location during the storm. Of these factors, slope angle, s o i l density, and discontinuity surfaces are r e l a t i v e l y constant. Root cohesion and s o i l depth are more important because they vary as a function of time. Changes i n root cohesion are caused by r e l a t i v e l y rapid processes such as f i r e , disease, or logging. As a r e s u l t , large areas of the forest become vulnerable to f a i l u r e . Burroughs and Thomas (1977), Zeimer and Swanston (1977) and Zeimer (1981) found that roots deteriorate rapidly a f t e r trees die, and root cohesion i s provided by regenerating vegetation. The number of avalanches i n i t i a t e d depends on the C r value and r a i n f a l l severity at a given time during the regeneration period. S o i l depth controls long term p r o f i l e s t a b i l i t y . In Zones 1 (planar slopes above drainage depressions) and 2 (drainage depressions) the accumulation of residual s o i l and colluvium gradually lowers the factor of safety over time. However, the increased s o i l depth acts as a storage buffer for i n f i l t r a t i n g 187 r a i n f a l l , and more severe rainstorms are required to t r i g g e r Coulomb f a i l u r e s . The avalanches scour s o i l and vegetation from the slope, and the cycle begins again. Logging roads are unigue because thick s o i l p r o f i l e s are produced instantaneously and do not redevelop a f t e r f a i l u r e . In Zone 2 and Zone 3 washouts of thi n p r o f i l e s of alluvium and colluvium occur. Hydrologic analyses have shown washouts to be higher frequency events. Sediment a v a i l a b i l i t y i n the depressions and channels i s the p r i n c i p l e factor c o n t r o l l i n g the severity of r e s u l t i n g debris torrents. Table 7.1: Root cohesion at f a i l u r e , Avalanche: C Wedges: Drainage depressions: Logging roads: Discontinuity surfaces: W-l W-2 DD-1 DD-2 DD-3 LR-1 LR-2 DS-1 DS-2 r (kPa) 2 .30 2.73 1.75 2.80 1.65 A. B. 2 . 63 2 . 99' A. 2.01, B. 2.87: A. B. 2 . 32 2.544 C. 3.16-A. B. C. 2 . 08 2 .44' 2 . 58! 188 January 9-10, 1983 : Vegetation: - Alnus, Tsuga, Pseudotsuga - Alnus, Tsuga, Thui a, Polystichum - Alnus, Tsuga. Polystichum - Tsuga, Thui a, Alnus - Alnus, Tsuga, Carex, Polystichum 1 _ Alnus, Tsuga, Thui a, Polystichum Alnus, Tsuga, Acer, Polystichum Tsuga, Alnus, Pseudotsuga, Polystichum. S a l a l Tsuga, Arbutus, Alnus, S a l a l , Acer 1 - Width of flow = 5 m. 2 - Width of flow = 4 m. 3 - Root depth = 1.5 m. 4 - Roots do not penetrate bedrock. 5 - Residual f r i c t i o n angle at so i l - r o c k interface, not used i n l a t e r c a l c u l a t i o n s . Table 7.2: Selected C r values Publication: S i d l e and Swanston, 1982 Burroughs and Thomas, 1977 8.9-16.7 p'Loughlin, 1972 Swanston, 1970 This study C r (kPa): Vegetation: 2.0 - Scrub Tsuga, Picea, Thui a, Phalanx  Pinus. Pseudotsuga, Picea 1.0 - 3.0 - Thui a, Tsuga, Pseudotsuga 3.4 - 4.4 - Thui a, Tsuga, Pseudotsuga 1.6-2.0 - Group I, Understory 2.3-2.6 - Group II, Scrub forest 2.6-3.0 - Group II I , Mixed for. > 8.0 - Group IV, Old growth 189 ! Table 7.3: Slope s t a b i l i t y on January, 9-10, 1983 F.S. - Strength Avalanche: i n i t i a l conditions: r a t i o : W-l 3 . 63 1.87 W-2 2 .35 3 . 05 DD-1 5. 04 2 .16 DD-2 7.50 2 .26 DD-3 2 . 09 2 .86 LR-1 A. 3 . 12 A. 1.98 B. 3 . 58 B. 2.19 LR-2 A. 1.72 1. 62 B. 1.73 DS-1 A. 3 . 02 2 .41 B. 3 . 02 2.19 DS-2 A. 3 . 37 2.54 B. 3 . 64 2.47 I n i t i a l conditions: Pore pressures throughout p r o f i l e governed by negative pressure heads i n unsaturated s o i l . Root cohesion equal to back calculated value at f a i l u r e . Strength r a t i o , SR: Shear strength at f a i l u r e (Integrated mean 0 ' , C = C + C r) divided by shear strength just a f t e r f a i l u r e ( 0 1 = 0 * r , C = 0). Shear stress assumed constant. Table 7.4: Slope s t a b i l i t y on December 13-14, 1979 Avalanche: Minimum Cj- 1, kPa: F. S. W-l 1.51 1. 23 W-2 1. 82 1. 17 DD-1 1. 44 1. 08 DD-2 2 . 68 1. 04 DD-3 1. 68 1. 01 LR-1 A. 1.34 1. 21 B. 1.39 1. 41 LR-2 A. 1. 68 1. 03 B. 2 .49 1. 20 DS-1 A. 2 . 32 ~1. 00 B. 2.54 «1. 00 DS-2 A. 1.15 1. 28 B. 1.29 1. 28 1 - Minimum root cohesion required to maintain an FS > 1. Figure 7.1; S t a b i l i t y analysis: W-l 446 - i 0 5 10 15 HORIZONTAL DISTANCE, METERS Figure 7.2: S t a b i l i t y analysis: W-2 H O R I Z O N T A L D I S T A N C E , M E T E R S Figure 7 . 3 ; S t a b i l i t y analysis: D D - 1 0 5 10 15 20 M HORIZONTAL DISTANCE, METERS S Figure 7.4: S t a b i l i t y analysis: DD-2 0 5 10 15 HORIZONTAL DISTANCE, METERS Figure 7.5: S t a b i l i t y analysis: DD-3 0 5 10 HORIZONTAL DISTANCE, METERS ELEVATION, METERS -F^ -f^  4> 4> O O O > cx> oo o S6T Figure 7.7: S t a b i l i t y analysis: LR-2 HORIZONTAL DISTANCE, METERS Figure 7.8: S t a b i l i t y analysis: DS-1 HORIZONTAL DISTANCE, METERS Figure 7.9: S t a b i l i t y analysis: DS-2 HORIZONTAL DISTANCE, METERS 199 F i g u r e 7.10: I n f i n i t e s l o p e a n a l y s i s : F a c t o r o f s a f e t y vs s l o p e angle under Group I and Group I I I v e g e t a t i o n 3.00 200 Figure 7.11: Factor of safety vs f r i c t i o n angle at DD-1 and W-l Figure 7.12; S t a b i l i t y analysis: DD-2 with shallow s o i l 0 5 10 15 HORIZONTAL DISTANCE, METERS Figure 7.13: S t a b i l i t y analysis: DS-1 with shallow s o i l ! 203 CHAPTER 8: CONCLUSIONS AND RECOMMENDATIONS 8.1 Conclusions Nine debris avalanches from the January 9-10, 1983 debris torrent event are back analyzed. Conclusions are drawn concerning, 1) c l i m a t i c controls on debris avalanche and debris torrent i n i t i a t i o n ; 2) debris avalanche c h a r a c t e r i s t i c s ; 3) h i l l s l o p e hydrology; and, 4) slope s t a b i l i t y . Frequency analyses are used in Chapter 3 to assess r a i n f a l l and snowmelt severity during the January, 1983 and January, 1971 debris torrent events, and during a December, 1979 storm that i s i included for comparison. Nooksack Salmon Hatchery r a i n f a l l data show the January, 1983 storm to be the worst on record i n the 12 and 24-hour durations. General equation data show a maximum recurrence i n t e r v a l of 71 years i n the 12-hour duration. Recurrence i n t e r v a l s of f i v e years or less are recorded i n the 1, 2, and 3-hour durations. Long duration, moderate in t e n s i t y storms therefore i n i t i a t e debris avalanches and debris torrents. In contrast, the January, 1971 storm has less than 2-year recurrence i n t e r v a l s i n a l l durations, and snowmelt i s an important contributing factor. The storm did not t r i g g e r numerous debris avalanches, but did mobilize s o i l and alluvium i n creeks into a debris torrent. Pore pressure increases triggered the f a i l u r e s , however a number of factors controlled avalanche i n i t i a t i o n on study area slopes. The modified Mohr-Coulomb equation shows that slope 204 angle, s o i l depth, s o i l density, vegetative cover, and snow are important factors. Avalanche descriptions i n Chapter 4 include reviews of c o n t r o l l i n g factors and t h e i r r e l a t i v e importance at each headscarp. The e f f e c t s of the factors are q u a n t i t a t i v e l y assessed i n Chapter 7. Four d i s t i n c t avalanche geometries are defined, based on published studies and f i e l d observations. The geometries are, 1) wedges; 2) drainage depressions; 3) logging roads; and, 4) d i s c o n t i n u i t y surfaces. The movement of debris to l o c a l headscarps i s also described. Three scour zones are defined. Zone 1 includes planar slopes above drainage depressions. Residual s o i l i s completely removed to expose rock at the i n i t i a l headscarp, but skims over s o i l and c o l l u v i u m / t i l l on slopes below. Zone 2 includes drainage depressions, where most or a l l of the residual s o i l and colluvium i s removed and debris becomes channeled. Zone 3 includes f i r s t order channels, where debris s t r i p s alluvium from bedrock. The parameters required to carry out hydrologic and slope s t a b i l i t y analyses are derived in Chapter 5 . Parameter d i s t r i b u t i o n s are established for void r a t i o s , f i e l d moisture contents, and s o i l matrix hydraulic conductivities. Void r a t i o s and f i e l d moisture contents f i t a normal d i s t r i b u t i o n , while hydraulic conductivity i s assumed to f i t a log normal d i s t r i b u t i o n . C h a r a c t e r i s t i c curves are estimated from a single, c h a r a c t e r i s t i c , sample. Results corresponded well with 205 west coast regional studies used for comparison. Chapter 6 included detailed study of pore pressure increases that triggered the debris avalanches. To model water table l e v e l s a one-dimensional, v e r t i c a l , transient, saturated-unsaturated f i n i t e difference i n f i l t r a t i o n program i s linked to a kinematic wave equation. The analysis i s undertaken to assess the factors c o n t r o l l i n g pore pressure increases, and to d i s t i n g u i s h pore pressures developed on January 9-10, 1983 from those developed during other storms. R a i n f a l l i n t e n s i t y and duration, i n i t i a l conditions, s o i l depth, and s o i l matrix hydraulic conductivity are the p r i n c i p a l factors c o n t r o l l i n g v e r t i c a l discharge from s o i l p r o f i l e s . R a i n f a l l i n t e n s i t y controls steady state discharge, and therefore l i m i t s the water table l e v e l s on a given slope. R a i n f a l l duration controls the achievement of steady state. Durations of 1 - 3 hours have l i t t l e impact on discharge rates, while durations greater than 10 hours produce near steady state discharges. Of the remaining factors, i n i t i a l conditions most s i g n i f i c a n t l y influence discharge response to r a i n f a l l . Dry s o i l responds to r a i n f a l l as many as f i v e hours l a t e r than wet s o i l , and reaches lower and/or less sustained peak discharges. Higher s o i l matrix hydraulic conductivity and shallower s o i l cause steady state to be achieved more quickly. High in t e n s i t y , long duration r a i n f a l l on January 9-10, 1983 caused a sustained, high discharge rate i n the s o i l . The January, 1983 discharge i s c l e a r l y distinguishable from 206 December, 1979 and January, 1971 discharges at a l l avalanches. Discharge rates are entered i n a kinematic wave equation to determine water table l e v e l s at the avalanche headscarps. The water l e v e l s are c l e a r l y distinguishable at seven of the nine avalanches. Levels are not c l e a r l y distinguished when s i g n i f i c a n t downslope discharge differences are expressed by small surface water l e v e l differences. The re s u l t s indicate that washouts i n drainage depressions occur more frequently than Coulomb f a i l u r e events. Back analyses are completed i n Chapter 7. The analyses begin with root cohesion back calculations. C r values for four vegetative covers are distinguished. Groups I - III were logged between 1918 and 1950. Group I includes r e l a t i v e l y weak understory vegetation (C r range: 1.6 - 2.0 kPa), which predominates i n drainage depression axes. Group II includes understory plus stunted trees (C r range: 2.3 - 2.6 kPa). Group III includes understory plus mixed, regenerating forest (C r range: 2.6 - 3.0 kPa). Group IV i s old growth forest, which i s hot back analyzed, but probably has a considerably higher C r range. Factors c o n t r o l l i n g i n i t i a t i o n are quantit a t i v e l y assessed. Conservative i n f i n i t e slope analyses show l i m i t i n g slope angles of 29.7° for Group I vegetation, and 24.6° for Group III vegetation. Factors of safety increase with decreasing void r a t i o . Wedge and drainage depressions had high void r a t i o s , therefore s o i l density i s an important c o n t r o l l i n g factor at 207 those geometries. Factors of safety increase with decreased s o i l depth. S o i l depth i s an important contributing factor at a l l avalanche geometries. Increased root cohesion caused increased factors of safety. Root cohesion i s c r i t i c a l to slope s t a b i l i t y at a l l the avalanches. Drainage depressions are p a r t i c u l a r l y vulnerable because of t h e i r low shear strength (Group I) vegetation. Logging roads are vulnerable because root cohesion does not apply to the complete p r o f i l e . Root cohesion and possibly f r i c t i o n angle are lower above smooth discontinuity surfaces, causing lower factors of safety than those above rough, fractured rock. Slope s t a b i l i t y on January 9-10, 1983 i s discussed. Under i n i t i a l pore pressures the headscarps had high factors of safety. On January 9-10, 1983, increased pore pressures reduced the factor of safety to one. The shear strength r a t i o , SR, i s introduced as the r a t i o of shear strength before f a i l u r e divided by shear strength a f t e r f a i l u r e . SR values between 1.62 and 3.05 explained the high mobility of avalanche debris. Back analyzed C r values are used to assess slope s t a b i l i t y on December 13-14, 1979. Six avalanches were c l e a r l y stable throughout the storm, while three were only marginally stable. A washout f a i l u r e was l i k e l y at one of the drainage depression avalanches. P a r t i a l l y revegetated slump scarps indicated that Coulomb f a i l u r e was l i k e l y at another. The t h i r d probably did hot f a i l . The December, 1979 analysis implies that washout f a i l u r e s 208 are a r e l a t i v e l y high frequency event i n wet drainage depressions. When washouts occur the i n i t i a t i o n of debris torrents depends on the a v a i l a b i l i t y of s o i l and alluvium i n i drainage depressions, f i r s t order channels, and creeks. The January, 1971 event appears to be a case of washout f a i l u r e s i n i t i a t i n g torrents. Root cohesion and s o i l depth are the two c o n t r o l l i n g factors that vary s i g n i f i c a n t l y as a function of time. Catastrophic changes i n root cohesion make h i l l s l o p e s vulnerable to tr i g g e r i n g rainstorms for r e l a t i v e l y short periods, before regenerating vegetation s t a b i l i z e s the slope. In t h i s case, a 12-hour, 71-year recurrence i n t e r v a l storm i s s u f f i c i e n t to i n i t i a t e numerous debris avalanches in regenerating forest. Increased s o i l depth gradually d e s t a b i l i z e s h i l l s l o p e s . In Zone 1 and Zone 2 low-frequency rainstorms t r i g g e r Coulomb f a i l u r e s i n residual s o i l and colluvium. Alluvium and colluvium are washed from Zone 2 and Zone 3 during high-frequency storms. 8.2 Recommendations Further research i n h i l l s l o p e hydrology and slope s t a b i l i t y should focus on K^^^ determination. K ^ ] ^ i s an important parameter that controls water table build-up during a storm. It i s evident from t h i s study that permeameter K s a t values cannot be applied to downslope drainage. Tracer studies have been demonstrated to be an appropriate method that requires further development. i 209 This study i s applicable to debris avalanche and debris torrent prediction. The task i s made d i f f i c u l t by the fact that avalanches and torrents are i n i t i a t e d by a 71-year recurrence i n t e r v a l r a i n f a l l with limited snowmelt, and torrents are i n i t i a t e d by an average annual storm with abundant snowmelt. Cannon and E l l e n (1985) have defined r a i n f a l l thresholds for, !*numerous slope f a i l u r e s " i n C a l i f o r n i a . They suggest monitoring r a i n f a l l as i t occurs and a n t i c i p a t i n g slope f a i l u r e s i f a threshold i s reached. A s i m i l a r system might be applicable i n the study area using the maximum 12-hour p r e c i p i t a t i o n i n t e n s i t y i n Table 3.4. Use of the lower Nooksack Salmon Hatchery i n t e n s i t y i n the study area would provide a safety factor for the warning system. Monitoring of the snowpack appears equally important considering the January, 1971 debris torrents. The presence of snow at elevations less than 160 meters ( i . e . near Lake Whatcom) in the basin indicates a thicker snowpack at higher elevations. I f the snowpack has a high snow water equivalent an average r a i n f a l l under high temperatures could t r i g g e r a debris torrent. The pack density and elevation should be monitored u n t i l the i snow recedes to at least the 370 m elevation. Results of t h i s study could also be used to locate areas of high debris avalanche r i s k . F i r s t , the "worst case" l i m i t i n g slope angles i n Section 7.7.1 could be used as conservative thresholds above which slope s t a b i l i t y problems occur. The thresholds would be adjusted to d i f f e r e n t root cohesion values. In addition, p o t e n t i a l l y unstable slope geometries could be i d e n t i f i e d i n land use hazard maps. Drainage depressions are i d e n t i f i e d by topographic contours. Slopes underlain by dis c o n t i n u i t y surfaces are i d e n t i f i e d with geologic maps and topographic maps. Exi s t i n g logging road locations could be mapped with a e r i a l photography. The building of new logging roads should be kept to a minimum when slopes are steep and unstable geometries are abundant. 211 REFERENCES American Society for Testing and Materials, 1978 Annual Book of ; ASTM Standards, Part 19, ASTM, Philadelphia, Pennsylvania, ! 1978. Anderson, M.G. and T.P. Burt, The role of topography i n | c o n t r o l l i n g throughflow generation, Earth Surface Processes, ! 3, 331-344, 1978. i Beven, K., Kinematic Subsurface Stormflow, Water Resources : Research, 17(5), 1419-1424, October, 1981. Beven, K., On Subsurface Stormflow: Predictions with Simple Kinematic Theory for Saturated and Unsaturated Flows, Water Resources Research, 18(6), 1627-1633, December, 1982. Brunsden, D. and D.B. Prior, Slope I n s t a b i l i t y , John Wiley and Sons, New York, 1984. Burroughs, E.R. and B.R. Thomas, Declining root strength i n Douglas-fir a f t e r f e l l i n g as a factor i n slope s t a b i l i t y , Research Paper INT-190, Forest Service, U.S. Department of Agriculture, Ogden, Utah, 1977. C a l i f o r n i a Department of Water Resources, R a i n f a l l Analysis for Drainage Design, Volume II, DWR B u l l e t i n No. 195, Sacramento, C a l i f o r n i a , October, 1976. Canada S o i l Survey Committee, Subcommittee on S o i l C l a s s i f i c a t i o n , The Canadian System of S o i l C l a s s i f i c a t i o n , | Can. Dept. of Agric. Publ. 164 6, Supply and Services Canada, Ottawa, Ontario, 1978. Cannon, S.H and S.E. E l l e n , R a i n f a l l Conditions for Abundant Debris Avalanches, San Francisco Bay Region, C a l i f o r n i a , C a l i f o r n i a Geology, 38(12), 267-272, December, 1985. Chow, Ven Te, Handbook of Applied Hydrology, McGraw-Hill Book Company, New York, 1964. Clague, J . J . , and J.L. Luternauer, Excursion 30A: Late Quaternary Sedimentary Environments, Southwestern B r i t i s h Columbia, Eleventh International Congress on Sedimentology, McMaster U., Hamilton, Ontario, August, 1982. Craig, R.F., S o i l Mechanics, 2nd Ed., Van Nostrand Reinhold Company, New York, 1978. Craig, R.F., S o i l Mechanics, 3rd Ed., Van Nostrand Reinhold Company, New York, 1983. 212 REFERENCES, Cont•d D i e t r i c h , W.E., C.J. Wilson, and S.L. Reneau, Hollows, colluvium, and landslides in soil-mantled landscapes, In, Abrahams, A.D. (Ed.), H i l l s l o p e Processes, A l l e n and Unwin, Boston, Massachusetts, 1986. Dingman, S.L., F l u v i a l Hydrology, W.H. Freeman and Co., New York, 1984. Dunne, T. and R.D. Black, An experimental investigation of runoff production i n permeable s o i l s , Water Resources Research, 6(2), 478-490, 1970. Eagleson, P.S., Dynamic Hydrology, McGraw-Hill, New York, 1970. Easterbrook, D.J., Late Pleistocene G l a c i a l Events and Relative Sea-Level Changes i n the Northern Puget Lowland, Washington, Geol. Soc. Amer. B u l l . , 74, 1465-1484, December, 1963. Easterbrook, D.J., and D.A. Rahm, Landforms of Washington, Union Pr i n t i n g Co., Bellingham, Washington, 1970. Eisbacher, G.H. and J . J . Clague, Urban landslides i n the v i c i n i t y of Vancouver, B r i t i s h Columbia, with sp e c i a l reference to the December, 1979 rainstorm, Canadian Geotech. J. , 18(2), 205-216, 1981. Fraser, D.L., Smith Creek, Unpub. report for Washington State Dept. of Natural Resources, May, 198 6. Fredland, D.G., N.R. Morgenstern, and R.A. Widger, The shear strength of unsaturated s o i l s , Canadian Geotech. J . , 15(3), 313-321, 1978. Freeze, R.A., The mechanism of natural groundwater recharge and discharge: 1. One-dimensional, v e r t i c a l , unsteady, ! unsaturated flow above a recharging or discharging groundwater flow system, Water Resources Research, 5, 153-171, 1969. Freeze, R.A., Streamflow Generation, Reviews of Geophysics and Space Physics, 12(4), November, 1974. Freeze, R.A., A Stochastic-Conceptual Analysis of One-Dimensional Groundwater Flow in Nonuniform Homogeneous Media, Water Resour. Res., 11(5), October, 1975. Freeze, R.A. and J.A. Cherry, Groundwater, Prentice-Hall, Inc., Englewood C l i f f s , New Jersey, 1979. 213 REFERENCES, Cont'd Freund J.E., Modern Elementary S t a t i s t i c s , Prentice-Hall, Inc., Englewood C l i f f s , New Jersey, 1979. Goldin, A., Interim S o i l Survey Report, Whatcom County Area, Washington, Four Volumes, USDA - S o i l Conservation Service, February, 1984. Gray, D.H. and Leiser, A.T., Biotechnical Slope Protection and Erosion Control, Van Nostrand Reinhold Co., New York, 1982. Gray, D.H. and H. Ohashi, Mechanics of f i b e r reinforcement i n sand, J . Geotech. Eng. Div., ASCE, 109(3), 335-53, 1983. Greenway, D.R., Vegetation and Slope S t a b i l i t y , i n Slope S t a b i l i t y , M.G. Anderson and K.S. Richards, Editors, John Wiley and Sons, New York, 1987. i Hadas, A., Evaporation and drying process i n layered s o i l s (in Hebrew), Ph.D. thesis, Hebrew Univ., Rehovot, I s r a e l , 1967. Haggard, W.H., Letter to Brett and Daugert, Bellingham, ! Washington, Re: Anderson Creek Rainfall/January 10, 1983, Climatological Consulting Corporation, Asheville, North Carolina, May 20, 1985. Harr, R.D., Water Flux i n s o i l and subsoil on a steep, forested slope, J . of Hydrology, 33, 37-58, 1977. Harter, H. Leon. A New Table of Percentage Points of the Pearson Type III Dis t r i b u t i o n , Technometrics, Vol. II, No.l, February, 1969. H i l l e l , Daniel, Fundamentals of S o i l Physics, Academic Press, New York, 1980a. H i l l e l , Daniel, Applications of S o i l Physics, Academic Press, New York, 1980b. Holtz, R.D. and W.D. Kovacs, An Introduction to Geotechnical ' Engineering, Prentice-Hall, Inc., Englewood C l i f f s , New Jersey, 1981. Humphrey, N.F., Pore Pressures i n Debris F a i l u r e I n i t i a t i o n , I Report 45, State of Washington Water Research Center, Pullman, Washington, September, 1982. Hunt, R.E., Geotechnical Engineering Investigation Manual, McGraw-Hill Book Co., New York, 1984. 214 REFERENCES, Cont'd Hurlbut, C.S. and C. Klein, Manual of Mineralogy, 19th Ed., John Wiley and Sons, New York, 1977. J o h n s o n , S.Y., Stratigraphy, age, and paleogeography of the Eocene Chuckanut Formation, northwest Washington, Can. J. ; Earth S c i . , 21, 92-106, 1984. Johnson, S.Y., Stratigraphy, sedimentology, and tectonic setting ! o f the Eocene Chuckanut Formation, Northwest Washington, Ph.D. thesis, University of Washington, Seattle, Washington, 1982. Klute, A., Water Retention: Laboratory Methods. In Klute, A., Ed. Methods of S o i l Analysis, Part I: Physical and Mineralogical Methods, No. 9 in Agronomy Series, American Society of Agronomy, Inc., Madison, Wisconsin, 1986. Lambe, T.W., S o i l Testing for Engineers, John Wiley and Sons, New York, 1951. Lambe, T.W. and R.V. Whitman, S o i l Mechanics, SI Version. John ; Wiley and Sons, Toronto, 1979. Megahan, W.F. and J.L. Clayton, Tracing Subsurface Flow on Roadcuts on Steep, Forested Slopes, S o i l S c i . Soc. Amer. J., 47(6), 1063-1067, November-December, 1983. Mendenhall, W., Introduction to Probabi l i t y and S t a t i s t i c s , 6th Ed., Duxbury Press, Boston, Massachusetts, 1983. Milne, D.M., The Variation i n Permeability Measured i n Quadra Sand as Determined From Sieve Analysis, Disturbed Sample Testing and Undisturbed Sample Testing, B.A.Sc. thesis, University of B r i t i s h Columbia, A p r i l , 1977. Morisawa, M., Streams, t h e i r dynamics and morphology, McGraw-H i l l Book Co., New York, 1968. Mosely, M.P., Streamflow Generation i n a Forested Watershed, New Zealand, Water Resour. Res., 15(4), 795-806, August, 1979. Mualem, Y., A new model for predicting the hydraulic conductivity of unsaturated media, Water Resour. Res., | 12(3), 513-522, December, 197 6. National Oceanic and Atmospheric Administration, National Environmental S a t e l l i t e , Data, and Information Service, Climatological Data, Volumes 75(1), 83(12), 87(1), National Climatic Center, Asheville, North Carolina, January 1971a; December, 1979a; January, 1983a. 215 REFERENCES, Cont'd National Oceanic and Atmospheric Administration, National Environmental S a t e l l i t e , Data, and Information Service, Hourly P r e c i p i t a t i o n Data, Volumes 16 - 35. National Climatic Center, Asheville, North Carolina, 1966 - 1985. National Oceanic and Atmospheric Administration, National Environmental S a t e l l i t e , Data, and Information Service, Hourly P r e c i p i t a t i o n Data, Volumes 21(1), 29(12), 33(1), l National Climatic Center, Asheville, North Carolina, ; January, 1971b; December, 1979b; January, 1983b. National Oceanic and Atmospheric Administration, National Environmental S a t e l l i t e , Data, and Information Service, I Hourly P r e c i p i t a t i o n Data, Volume 34(2,3,4). National Climatic Center, Asheville, North Carolina, February-A p r i l , 1984. National Oceanic and Atmospheric Administration, National Environmental S a t e l l i t e , Data, and Information Service, Storm Data, 25(1), National Climatic Center, Asheville, North Carolina, January, 198 3c. National Oceanic and Atmospheric Administration, National Weather Service, P r e c i p i t a t i o n - Frequency Atlas of the Western United States, Volume IX, Washington, NOAA Atlas 2, prepared by J.F. M i l l e r , R.H. Frederick, and R.J. Tracey, Washington D.C, 1973. Nielsen, D.R., J.W. Biggar, and K.T. Erh, Spatial V a r i a b i l i t y of Soil-Water Properties, Hilgardia, 42(7), 215-257, November, 1973. Nielsen, D.R., M. Th. Van Genuchten, J.W. Biggar, Water Flow and Solute Transport Processes i n the Unsaturated Zone, Water Resour. Res., 22(9), 89S-108S, August 1986. Ohta, T., Y. Tsukamoto, and T. Kido, The behaviour of rainwater on a forested h i l l s l o p e (II) Lateral flow on a slope, J . of Japanese Forestry Society 67(10), 383-390, 1985. 6'Loughlin, C.L., S t a b i l i t y of Steepland Forest S o i l s , Ph.D. thesis, University of B r i t i s h Columbia, October, 1972. Q'Loughlin, C.L., The e f f e c t of timber removal on the s t a b i l i t y of forest s o i l s , J. of Hydrology (New Zealand), 13(2), 1974. O'Loughlin, C L . and A.J. Pearce, Influence of Cenozoic Geology on Mass Movement and Sediment Response to Forest Removal, North Westland, New Zealand, B u l l . International Assoc. of Eng. Geol., 14, 41-46, 1976. 216 • REFERENCES, Cont'd O'Loughlin, C L . and R.R. Ziemer, The importance of root strength and deterioration rates upon edaphic s t a b i l i t y i n steepland forests, i n Carbon Uptake and A l l o c a t i o n i n Subalpine Ecosystems as a key to Management: Proceedings of the I.U.F.R.O Workshop, Aug. 2-3, 1982, 70-78, Oregon State Univ., C o r v a l l i s Oregon, 1982. Orme, Antony R., G.W. Thorsen and A.J. Orme, Recurrence Frequency of Major Debris Avalanches and Debris Torrents, Cascade F o o t h i l l s , Northwestern Washington, In, Geol. Soc. Amer., Cor d i l l e r a n Section, Abstracts with Programs, 18(2), 167, Boulder, Colorado, February, 1986. Orme, A.R., I n i t i a t i o n and mechanics of debris avalanches on steep forest slopes, In, Erosion and Sedimentation i n the P a c i f i c Rim, Proceedings of the C o r v a l l i s Symposium, IAHS Publ. No. 165, August, 1987. Pierson, T., S o i l pipes and slope s t a b i l i t y , Q. J . Eng. Geol., ' 16, 1-11, 1983. Pierson, T., Piezometric Response to Rainstorms i n Forested H i l l s l o p e Drainage Depressions, J . of Hydrology (New Zealand), 19(1), 1980. Riestenberg, M.M. and S. Sovonick-Dunford, The role of woody vegetation i n s t a b i l i z i n g slopes in the Cincinnati area, Ohio, Geol. Soc. Amer. B u l l . , 94, 506-518, A p r i l , 1983. Rogowski, A.S., Watershed Physics: S o i l V a r i a b i l i t y C r i t e r i a , Water Resour. Res., 8(4), 1015-1023, August, 1972. Selby, M.J., Earth's Changing Surface, Clarendon Press, Oxford, England, 1985. Sharma, M.L. and R.J. Luxmoore, S o i l Spatial V a r i a b i l i t y and i t s Consequences on Simulated Water Balance, Water Resour. Res., 15(6), 1567-1572, December, 1979. Si d l e , R.C., Relative importance of factors influencing ; l a n d s l i d i n g i n coastal Alaska, Proc. of 21st Annual Eng. Geology and S o i l s Symp., Univ. of Idaho, Moscow, 311-325, 1984. Sid l e , R.C., A.J. Pearce, and C L . O'Loughlin, H i l l s l o p e S t a b i l i t y and Land Use, Water Resources Monograph No. 11, American Geophysical Union, Washington, D.C, 1985. S i d l e , R.C, and D.N. Swanston, Analysis of a small debris s l i d e i n coastal Alaska, Can. Geotech. J., 19, 167-174, 1982. 217 REFERENCES, Cont'd Siegal, R.A., STABL - User Manual, Joint Highway Research Project JHRP-75-9, Engineering Experiment Station, Purdue University, Indiana, June, 1975. S i n c l a i r , A.J., Application of Probability Graphs i n Mineral | Exploration, Association of Exploration Geochemists, Special Volume 4, 95 p, 1976. Stanley, CR., PROBPLOT - An i n t e r a c t i v e program to F i t Mixtures of Normal (or Log-Normal) Distributions using Maximum Likelihood Optimization Procedures, Association of Exploration Geochemists, Special Volume 14, 40 p, 1987. Stephens, D.B., K. Lambert, and D. Watson, Regression Models for Hydraulic Conductivity and F i e l d Test of the Borehole Permeameter, Water Resources Research, 23(12), December, 1987. Swanston, D.N., Mechanics of debris avalanching i n shallow t i l l s o i l s of Southeast Alaska, Research Paper PNW-103, Forest : Service, U.S. Dept. of Agriculture, Portland, Oregon, 197 0. j Syverson, T.L., History and Origin of Debris Torrents i n the Smith Creek Drainage, Whatcom County, Washington, M.Sc. thesis, Western Washington University, Bellingham, Washington, December, 1984. Takahashi, T., Debris Flow, Annual Review of F l u i d Mechanics, 13, 57-77, 1981. Talsma, T. and P.M. Hallam, Hydraulic Conductivity Measurement of Forest Catchments, Australian J. S o i l Res., 18, 139-148, 1980. Thorsen, G.W., Dept. of Natural Resources, Olympia, Washington, personal communication, 1987. i Tischer, E., Hydrologic behavior of a forested mountain slope i n coastal B r i t i s h Columbia, M.Sc. thesis, University of B r i t i s h Columbia, September, 198 6. Tsukamoto, Y. and 0. Kusakabe, Vegetative influences on debris s l i d e occurrences on steep slopes in Japan, Proceedings of i the symposium on the e f f e c t of forest land use on erosion I and slope s t a b i l i t y , Environment and Policy I n s t i t u t e , Honolulu, Hawaii, May, 1984. 218 REFERENCES, Cont•d Tsukamoto, Y., T.Ohta, and H. Noguchi, Hydrological and geomorphological studies of debris s l i d e s on forested h i l l s l o p e s i n Japan, In, Recent Developments i n the Explanation and Prediction of Erosion and Sediment Yiel d , Proceedings of the Exeter Symposium, IAHS Publ. No. 137, July, 1982. U.S. S o i l Conservation Service, unpub. data tabulations, 1960 - 1986, Portland, Oregon, 1987. Utting, M.G. The Generation of Stormflow on a Glaciated i H i l l s l o p e i n Coastal B r i t i s h Columbia, M.Sc. thesis, ! University of B r i t i s h Columbia, December, 1978. VanDine, D.F., Debris flows and debris torrents i n the Southern Canadian C o r d i l l e r a , Can. Geotech. J., 22, 44-68, 1985. Van Genuchten, M. Th., A closed-form equation for predicting the hydraulic conductivity of unsaturated s o i l s , S o i l S c i . Soc. Amer. J., 44(5), 892-898, 1980. Varnes, D.J., Slope Movement Types and Processes, In Landslides: Analysis and Control, Transportation Research Board, National Academy of Sciences, Washington, D.C, Special Report 176, Chapter 2, 1978. Weichert, D.H. and G.C Rogers, Seismic Risk i n Western Canada, In, Earthquake Geotechnique, Canadian Geotechnical Society, ! Vancouver, B.C., May, 1987. Weyman, D.R., Measurements of the downslope flow of water i n a s o i l , J . of Hydrology, 20, 267-288, 1973. Wilson, C.J. and W.E. Die t r i c h , The contribution of bedrock i groundwater flow to storm runoff and high pore pressure | development i n hollows, In, Erosion and Sedimentation i n the P a c i f i c Rim, Proceedings of the C o r v a l l i s Symposium, IAHS Publ. No. 165, August, 1987. Wooldridge, D. Personal communication, 1984. Zeimer, R.R., Roots and the s t a b i l i t y of forest slopes, Erosion and Sediment Transport in P a c i f i c Rim Steeplands, IAHS Publication 132, London, 343-361, 1981. Zeimer, R.R. and D.N. Swanston, Root Strength Changes a f t e r Logging i n Southeast Alaska, Research Note PNW-306, P a c i f i c Northwest Forest and Range Experiment Station, US Forest Service, Portland, Oregon, 9 p, 1977. 2 1 9 ! A P P E N D I X I : T A B L E 3.4 I N I M P E R I A L U N I T S i T a b l e 3.4: R e g i o n a l c l i m a t o l o g i c a l d a t a b e f o r e a n d d u r i n g i c o m p a r i s o n s t o r m s . C o d e s : D = D a y o f m o n t h P = P r e c i p i t a t i o n , i n i n c h e s i S / S G = S n o w f a l l / Snow o n g r o u n d , i n i n c h e s T m a x / T m i n = M a x i m u m t e m p e r a t u r e / M i n i m u m t e m p e r a t u r e , i n d e g r e e s F a h r e n h e i t Station: Bellingham Newhalem Diablo Dam (elev. 42.7 m) (elev. 160 m) (elev. 271 1.6 m) January. 1983: S/ Tmax/ S/ Tmax/ S/ Tmax/ D: P: SG: Tmin: P: SG: Tmin: P: SG: Tmin: 2 48/40 41/34 .1 T/ 38/30 3 .3 47/39 .5 i/1 38/32 1.2 2/2 37/31 4 .3 50/39 .3 .5/1 37/32 .6 T/2 37/32 5 1.6 48/39 1.4 T/T 38/32 2.5 1/3 37/31 6 .2 45/34 .1 40/31 .3 /2 42/31 7 .2 55/42 .3 40/32 .8 1/3 43/32 8 .9 53/42 1.8 51/35 3.1 T/T 52/33 9 .2 48/38 .4 41/32 .6 41/33 10 1.8 53/45 3.2 37/32 4.7 39/35 11 58/40 1.0 40/33 .8 46/36 December, , 1979: S/ Tmax/ S/ Tmax/ S/ Tmax/ D: P: SG: Tmin: P: SG: Tmin: P: SG: Tmin: 5 .1 48/40 .4 47/39 .2 47/38 6 .1 50/45 .6 42/38 .4 44/38 7 .1 48/44 .2 44/40 .1 46/39 8 .3 50/42 .1 45/40 45/39 9 .4 46/42 1.1 49/34 1.3 44/40 10 .1 45/29 2.0 39/30 2.1 T/T 51/32 11 42/30 T 43/31 T/T 38/31 12 47/30 1.2 40/31 1.1 T/T 42/32 13 .5 48/32 1.0 5/- 36/- .5 1/1 38/33 14 2.0 52/40 4.7 2/- 41/29 3.7 40/33 January. 1971: S/ Tmax/ S/ Tmax/ S/ Tmax/ D: P: SG: : Tmin: P: SG: Tmin: P: SG: Tmin: 21 .3 .5/ 41/32 .9 /4 39/32 .5 3/7 38/31 22 .4 41/32 .4 6.5/11 38/30 .5 7.5/15 34/29 23 .2 46/39 1.7 5.5/16 36/30 1.1 /13 38/29 24 1.1 46/39 2.2 /14 37/30 2.3 /8 38/34 25 .1 40/30 1.1 /8 39/30 1.0 /7 40/32 26 2.0 50/35 2.4 .5/8 36/30 2.2 4/11 36/32 27 .1 52/46 .6 /8 36/30 .5 /10 40/33 28 51/35 /8 41/31 /9 43/33 29 .5 52/41 n 40/31 /8 43/34 30 1.9 51/43 1.9 /5 39/31 2.3 /6 45/37 31 .5 51/47 .7 /3 40/31 .9 /T 47/37 1 - * = Data not recorded ! 220 lAPPENDIX II - DENSITY PARAMETER AND MOISTURE CONTENT EQUATIONS Void r a t i o , e = (V t - V s) / V s (A2.1) where, V-j- = t o t a l volume i V s = volume of s o l i d s = Ms / (G s /? w) (A2.2) where, M s = dry mass of s o l i d s G s = Solids s p e c i f i c gravity >^ w = mass density of water Porosity, n = e / (1 + e) (A2.3) i i i i I f the terms i n Eq. A2.1 are divided by V s, then V v equals e units and the following equations apply, Dry unit weight, = G s / ( 1 + e ) r w (A2.4) Saturated unit weight, T s = ( G s + e ) / ( 1 + e ) r w (A2.5) where, T w = unit weight of water Mass based moisture content, 0 m = Mw / M s (A2.6) where, M w = mass of water Using the same assumptions as for Eq.'s A2.4 and A2.5, the moist unit weight i s , Unit weight at f i e l d capacity, r = G s ( 1 + 0 m ) / ( 1 + e ) r w (A2.7) | ; Volumetric moisture content, e v = V w / V-f- (A2.8) ! where V w = volume of water ( = Mw //O w ) I V-,- = t o t a l volume 221 APPENDIX I I I : PARAMETER DISTRIBUTIONS 1. Evidence for normal d i s t r i b u t i o n i n density parameters 2. P r o b a b i l i t y plots for e and 0m I I I . l : Evidence f o r normal d i s t r i b u t i o n i n density parameters i S o i l density parameters also varied within a headscarp and between headscarps. Nielsen et. a l (1973 p.227) found that the frequency histogram for 72 0 bulk density samples taken from a 150 hectare s i t e compared well with a normal density function curve based on the population mean and standard deviation. Rogowski (1972 p.1019) performed chi-squared tests on 100 bulk density samples taken randomly from a s o i l study i n New England. For n = 100 and d.f. = 7, % 2 = 9.05 and }^o.05 = 14.07, so the bulk densities were normally d i s t r i b u t e d at the 90% confidence l e v e l . A chi-squared t e s t was applied by the author to Milne's '(1977) void r a t i o data. For a 10 m transect with n = 100 and d.f. =6 , 2 = 7.40 and ">^o.052 = 12.59, hence the void r a t i o s were also considered normally d i s t r i b u t e d . The analyses suggest that s o i l density parameters are normally d i s t r i b u t e d i n space. I I I . 2 : Normal p r o b a b i l i t y plots for e and 8 M The program PROBPLOT, written by C l i f f o r d R. Stanley, was iised to produce the normal p r o b a b i l i t y p l o t s . References: S i n c l a i r (1976) and Stanley (1987). 1 1.) Avalanche W-l 222 ARITHMETIC U A L U E 5 s s a a s a a a a a a a s a a s VARIABLE a « UNIT a N a IS N C I a 12 POPULATIONS a a a a a a s a a a a Pop. H t a n S«d.o«u. z 1 1.07S 0.111 100.0 USERS UISUAL PARANETER ESTINATES -~i r— 99 18 ARITHMETIC UALUES UARIABLE a t l t e t a - H UNIT a N a IS N CI a 13 POPULATIONS Mean S t d .Deu. v. 0 . 197 0.029 100 . 0 USERS UISUAL PARANETER ESTIMATES 2.) Avalanche W-2 223 U -2 L U i H I M M i W l ARITHMETIC UALUES s s a s a s s s s i assess UAR IABLE s a UNIT s N a 15 N C I s 12 POPULATIONS a s s s s a s i B s g Pop. Mean S«4.D«U. * 1 1.052 0.107 100.0 USERS UISUAL PARAMETER ESTIMATES U-2 ARITHMETIC UALUES UAR TABLE s th«ts-M UNIT s N s 15 N C I s 12 POPULATIONS Pop. Mean s t d . D e u . y. 1 0.285 O.Otf 100.0 USERS UISUAL PARAMETER ESTIMATES 3.) Avalanche DD-1 224 225 4.) Avalanche DD-2 ARITHMETIC UALUES UARIA8LE a e UNIT « N a 15 N C I = 12 POPULATIONS a a a s s s s a a a a Pop. Mean SttJ.Oeu. •/. 1 1.0S« 0.080 100.0 USERS UISUAL PARAMETER ESTIMATES ARITHMETIC UALUES UARIABLE a t n e t a - n UNIT a N a IS N C I a 13 POPULA TIONS Pop. Mean std.Deu. v. 1 0 . l f 6 0 . 02* 100.0 USERS UISUAL PARAMETER ESTIMATES 5.) Avalanche DD-3 226 6.) Avalanche LR-1 227 2 2 8 7.) Avalanche DS-1 ARITHMETIC UALUES 3 S B 8 S S 3 S S S 3 3 3 3 3 3 UARIABLE 3 e UNIT 3 N a 15 N C I 3 12 POPULATIONS 3 3 3 3 3 3 3 3 3 3 3 Pop. Moan S t d . D e u . 0 .799 0.07C 100.0 USERS UISUAL PARAMETER ESTIMATES ARITHMETIC UALUES UARIABLE 3 t h e t a - n UNIT 3 N 3 IS N C I 3 12 POPULATIONS Pop. Mean S t d . D e u . Y. 1 0.18* 0.025 100.0 USERS UISUAL PARAMETER ESTIHATES 8.) Avalanche DS-2 229 DS-2 ARITHMETIC UALUES - i 1 1 1—i—i—i i 1 i 1 r — i — i — i — i 1 1 1— 99 »« 95 *5 70 50 30 15 5 2 1 USERS UISUAL PERCENT PARAMETER ESTIMATES APPENDIX IV: SHEAR STRENGTH PARAMETERS 1. Void r a t i o - f r i c t i o n angle r e l a t i o n s 2. S o i l cohesion determination at DD-3 IV. 1 : V o i d r a t i o - f r i c t i o n angle r e l a t i o n s From d i r e c t shear te s t r e s u l t s : W-l: 0 1 = -46.65 e + 80.36 ; r 2 = 0.96, n = 7, e range = 0.85 - 1.08 j W-2: 0 ' = -57.62 e + 93.11 r 2 = 0.99, n = 7, e range = 0.81 - 1.17 DD-1: 0 ' = -41.58 e + 80.94 r 2 = 0.93, n = 7, e range = 0.92 - 1.22 DD-2: 0 ' = -57.65 e + 86.03 r 2 =0.87, n = 7, e range = 0.77 - 0.98 DD-3: 0 ' = -38.10 e + 74.06 r 2 = 0.91, n = 7, e range = 0.78 - 1.16 LR-1: 0 ' = -56.69 e + 83.69 r 2 =0.97, n = 7, e range = 0.75 - 0.94 LR-2: 0 ' = -37.75 e + 69.49 r 2 = 0.87, n = 7, e range = 0.81 - 1.05 DS-1: 0 1 = -39.71 e + 71.66 r 2 =0.92, n = 8, e range = 0.77 - 1.11 DS-2: 0 1 = -25.67 e 4- 56.69 r 2 = 0.93, n = 8, e range = 0.74 - 1.06 231 IV.2: S o i l c o h e s i o n d e t e r m i n a t i o n a t DD-3 Hunt (1984, p.202) has indicated that 34° i s the maximum f r i c t i o n angle for compacted s i l t y sands with cohesion. C at DD-3 i s determined by passing a 34° l i n e through the i n t e r s e c t i o n of a = 18.2 kPa ( s o i l overburden) and the hypothetical 0 ' = 50.8° for e = 0.63, extrapolated from the e vs 0 1 data. Figure A4.1: DD-3 cohesion value determination NORMAL LOAD, c r , kPa 232 APPENDIX V: HYDRAULIC CONDUCTIVITY PARAMETERS j 1. Well permeameter evaluation i 2. Cumulative outflow vs time p l o t i 3. K s a-r- temperature adjustment 4. Hydraulic conductivity of rock matrix 5. Hanging water column device 6. Char a c t e r i s t i c curves: m, n, and a i i i ! V . l : Well permeameter evaluation To evaluate the v a l i d i t y of the well permeameter method, ten hydraulic conductivity tests were compared to a population surveyed by Milne (1977) on the Quadra Sands unit. The tests were conducted on the Point Grey Bl u f f s just west of the University of B r i t i s h Columbia campus. Milne determined K with a f a l l i n g head apparatus applied to intact cores. A Student - t procedure was used to t e s t the hypothesis that the populations came from the same d i s t r i b u t i o n . For 100 random v e r t i c a l l i n e samples Milne found a mean K s a-r- = 4.45 X 10~ 4 m/s with a standard deviation, a = 1.80 X 10~ 4 m/s. The 10 well permeameter tests at the same location showed a mean K s a - j - = 4.60 X 10~ 4 m/s and a standard deviation, s = 1.52 X 10~ 4 m/s. I f t lf *-0.05 then the hypothesis i s accepted at the 0.05 l e v e l of sig n i f i c a n c e . In t h i s case t = 0.227, t 0 > U 5 = 1 « 6 6 (d.f. = 108), and the hypothesis i s accepted. The well permeameter method i s therefore v a l i d for testing K s a t -233 V.2: Cumulative o u t f l o w vs time p l o t The steady state discharge rate, Q, i s entered i n Eq. 5 . 2 . Steady state i s indicated by a constant water l e v e l drop with time, which plots as a straight l i n e . Figure A 5 . 1 : Cumulative outflow vs time plo t 234 V.3: Ksa-t- temperature adjustment K s a t (2C) = K s a t (T) /i (T) / M (2C) (A3 .1) where, K s a^ (2C) = K s a^ at 2°C, m/s K s a t ( T) = K s a t a t t e s t temperature, m/s /x (T) = v i s c o s i t y at tes t temperature, cP /i (2C) = v i s c o s i t y at 2°C, cP Source: Lambe, 1951 V.4: Hydraulic conductivity of the rock matrix Darcy's Law with known discharge (Q), gradient (6H/51), and cross sectional area (A) i s used to determine K s a-j-. To set up the experiment a rectangular block of known length j(0.08 m) and cross sectional area (0.00368 m2) was sealed with wax into a ple x i g l a s s cylinder. The base of the sample was submerged i n water and a negative pressure head of 5.44 m was applied to the sealed top of the cylinder. The gradient equaled the head difference over the sample length (5.44/0.08 = 68). Discharge was calculated by measuring the rate of water l e v e l irise i n the top of the cylinder (1.03 X 10~ 9 m3/s) . K = 5.20 X 10 - 9 m/s. The value adjusted to 2°C was 3.4 X 1 0 - 9 m/s. y.5: Hanging water column device j The apparatus consisted of a plexiglass cylinder, 6.9 cm i n diameter, with a perforated base and an outlet attached to tubing adjustable to d i f f e r e n t elevations. About 55 - 70 cm3 of s o i l was placed i n the cylinder on a combination powdered graphite-blotter paper f i l t e r . The s o i l was then wetted to i t s saturated moisture content, © v-sat (~ n)• T n e outlet was lowered i n steps to decrease the pressure head. Cumulative outflow from s o i l pores was monitored at each step and moisture contents were calculated. The porosity was calculated with the oven dry s o i l weight, the measured s o i l volume in the cylinder, and s p e c i f i c gravity data from Table 5.2 in Eq. A2.1, A2.2, and A2.3. V.6: C h a r a c t e r i s t i c curves: m, n, cr values S o i l density m n a i V.V. Loose 0.3028 1.4343 0.0382 V. Loose Loose Dense 0.3351 0.3567 0.3754 1.5039 1.5544 1.6009 0.0329 0.0304 0.0279 235 APPENDIX VI: HYDROLOGY ANALYSIS VI. 1: Kj-)U|]C calculations VI.2: Maximum surface water depths VI. 1 : Kjjjjifc c a l c u l a t i o n s Pierson (1980): The piezometer chosen for analysis i s 3D, which i s 168 m from a drainage divide on a 32° slope. Figure 3 depicts hourly r a i n f a l l and simultaneously recorded piezometric head i n a study hollow. A severe rainstorm on January 7, 1976 caused a sharp r i s e i n piezometric head at 3D. After r a i n f a l l ceased the piezometric l e v e l dropped sharply for 12 hours, then leveled o f f to background l e v e l s . This i s interpreted as the downslope t r a v e l time for the saturated wedge caused by the rainstorm. Seepage v e l o c i t y = 168 m / 12 hr = 14 m/hr = .00389 m/s = K b u l k / n s i n 32.4° K b u l k / n = '00727 m/s t Mosley (1979): The seepage p i t chosen for analysis i s Site A, which i s 222 m from a drainage divide on a 20° slope. Figure 8 depicts hourly r a i n f a l l and simultaneously recorded discharge i n the p i t , which was i n the axis of a hollow. A rainstorm on July 4, 1978 caused an increase i n discharge. After the r a i n ceased the discharge dropped for 2 hours before reaching background l e v e l s . Seepage v e l o c i t y = 19.4 m / 2 hr = 9.7 m/hr = .002 69 m/s = Kj-^i^/n s i n 20° K b u l k / n = -00787 m/s 236 VI.2: Maximum surface water depths j Manning's empirical equation of surface flow i s used to estimate surface flow v e l o c i t i e s : V, m/s = (1/n * R 2 / 3 * S V 2 ) where, n = a dimensionless roughness c o e f f i c i e n t R = hydraulic radius, cross sectional area of flow divided by the wetted perimeter, i n m S = water surface slope, taken to equal the slope of the underlying bedrock The parameters n and R change as flow depth increases. At DD-1 the flow depth i s assumed as 3 cm, as a f i r s t approximation. The channel geometry, outlined i n Figure 6.17, shows that R = .0304 at that flow depth. Dingman (1984) estimated n = 0.4 i n dense shrubbery and l i t t e r . Therefore, V, m/s = (1/0.4) * (.0304) 2/ 3 * (.49)1/2 = 0.17 m/s = 614 m/hr This t r a v e l distance i s much greater than the 150 m length of the scarp. The calculations show that high overland flow •velocities do not allow appreciable surface water thicknesses to b u i l d up. As a rough estimate, the maximum 1-hour discharge yalues (Q = V * A) a f t e r saturation are used to determine surface water l e v e l s at DD-1 and DD-3. 237 APPENDIX V I I : SLOPE STABILITY ANALYSIS 1. Pore p r e s s u r e s under i n f i n i t e s l o p e seepage 2. Surcharge c a l c u l a t i o n s V I I . 1 : Pore p r e s s u r e s under i n f i n i t e s l o p e seepage Under i n f i n i t e s l o p e seepage groundwater flows downslope p a r a l l e l t o the bedrock boundary. E q u i p o t e n t i a l l i n e s are p e r p e n d i c u l a r t o t h a t boundary. Given a v e r t i c a l s o i l t h i c k n e s s , t , the t h i c k n e s s p e r p e n d i c u l a r t o the s l o p e i s , t l = t cose, where 9 i s the s l o p e angle. The v e r t i c a l component of t h i s t h i c k n e s s i s the h y d r o s t a t i c head, h = t2 = t l cosG = t c o s 2 G . The head i s m u l t i p l i e d by the u n i t weight of water, T W , t o determine the pore press u r e , u = h T W . VII.2: Surcharge c a l c u l a t i o n s Surcharge = V e g e t a t i o n weight, Wv i n kN = 0.2 3 kPa F o r e s t f l o o r area, A i n m2 Wv = Tree Volume X Weight = 15.6 kN Volume = 7r (0.15 m) 2 15 m = 1.06 m3 D e n s i t y = 1500 kg/m3 (O'Loughlin, 1974) A = 7T (4 . 65 m) 2 = 67 . 9 m2 

Cite

Citation Scheme:

        

Citations by CSL (citeproc-js)

Usage Statistics

Share

Embed

Customize your widget with the following options, then copy and paste the code below into the HTML of your page to embed this item in your website.
                        
                            <div id="ubcOpenCollectionsWidgetDisplay">
                            <script id="ubcOpenCollectionsWidget"
                            src="{[{embed.src}]}"
                            data-item="{[{embed.item}]}"
                            data-collection="{[{embed.collection}]}"
                            data-metadata="{[{embed.showMetadata}]}"
                            data-width="{[{embed.width}]}"
                            async >
                            </script>
                            </div>
                        
                    
IIIF logo Our image viewer uses the IIIF 2.0 standard. To load this item in other compatible viewers, use this url:
http://iiif.library.ubc.ca/presentation/dsp.831.1-0302653/manifest

Comment

Related Items