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UBC Theses and Dissertations

Groundwater hydrology and slope movement at Pavilion, B.C. Nadler, Denise Eileen 1984

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GROUNDWATER HYDROLOGY AND SLOPE MOVEMENT AT PAVILION, B.C. by DENISE EILEEN NADLER B . S c , The U n i v e r s i t y of Washington, 1980 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE THE FACULTY OF GRADUATE STUDIES (The Department of Geography) We accept t h i s t h e s i s as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA August 1984 (c) Denise E i l e e n Nadler, 1984 3 In presenting t h i s thesis i n p a r t i a l f u l f i l m e n t of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available for reference and study. I further agree that permission for extensive copying of t h i s thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. I t i s understood that copying or publication of t h i s thesis for f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of C\ejQQ^ f-?a^ >/"vy  The University of B r i t i s h Columbia 1956 Main Mall Vancouver, Canada V6T 1Y3 Date DE-6 (3/81) ABSTRACT OF THESIS "GROUNDWATER HYDROLOGY AND SLOPE MOVEMENT AT PAVILION, B.C." The. r a t e of earthflow motion at P a v i l i o n , B r i t i s h Columbia v a r i e s s e a s o n a l l y w i t h maximum displacements o c c u r r i n g from e a r l y winter (December) to mid- J u l y , and minimum movement occurs during the period September to December. Earthflow motion i s a moisture-dependent process; t h e r e f o r e increased movement r a t e s i n s p r i n g and summer may be a t t r i b u t e d to high pore water pressures i n the f a i l u r e zone as a r e s u l t of seasonally high groundwater recharge from snowmelt and r a i n f a l l . The o b j e c t i v e s of t h i s t h e s i s are to examine the causal r e l a t i o n s h i p between the climate, moisture regime and subsurface hydrology, and to c o r r e l a t e cumulative movement w i t h piezometric observations from s e v e r a l s i t e s on the earthflow. The h y d r o l o g i c budget f o r the drainage basin from August 1981 to June 1982 was estimated from p r e c i p i t a t i o n records and p o t e n t i a l evapo-t r a n s p i r a t i o n values. The clim a t e record i n d i c a t e d that more than 95% of the annual groundwater recharge occurred from snowmelt and r a i n f a l l i n A p r i l and May 1982. Two notable increases of groundwater l e v e l s were observed i n 1981, and one r i s e of groundwater e l e v a t i o n was seen i n 1982. The f i r s t r i s e i n 1981 and the r i s e noted i n 1982 occurred between May and J u l y and were c o r r e l a t e d w i t h l o w - e l e v a t i o n snowmelt from March 1 to A p r i l 1. An estimated one-month phase l a g between recharge and piezometer response suggests that recharge from e a r l y snowmelt occurs r e l a t i v e l y q u i c k l y . Tension cracks and f r a c t u r e s at shallow depths (10-20m) of the earthflow and i t s surrounding areas probably provide conduits f o r e f f i c i e n t groundwater flow and r a p i d piezometric response. The second groundwater l e v e l r i s e i n 1981 d i d not occur u n t i l the period from October to November and was s u b s t a n t i a l l y greater than the e a r l i e r r i s e . This was a t t r i b u t e d to snowmelt higher i n the basin during May and June, and r a i n f a l l i n May and J u l y 1981. Thus a f o u r - to five-month time l a g between r e g i o n a l groundwater recharge and piezometer response i s i n d i c a t e d . This i s accounted f o r , i n p a r t , by t r a v e l time from Mt. Cole, south of the e a r t h f l o w , to the earthflow i t s e l f and by slow upward movement of groundwater from the basal f a i l u r e zone i n t o the standpipes i n the earthflow. Earthflow a c c e l e r a t i o n commenced i n December 1981 as a r e s u l t of high pore water pressures from the impulse of recharge from Mt. Cole observed i n November 1981, and the r a t e of movement remained r e l a t i v e l y high at l e a s t u n t i l J u l y 1982. The r a t e of earthflow motion was a l s o high from March to J u l y 1981, wnich c o r r e l a t e s very w e l l w i t h high groundwater l e v e l s observed i n s p r i n g 1981. These observations i n d i c a t e that the earthflow i s s e n s i t i v e to changes of the e f f e c t i v e normal s t r e s s according to the seasonal h y d r a u l i c head f l u c t u a t i o n s on the basal s l i p surface of the earthflow. i i i TABLE OF CONTENTS TITLE PAGE ABSTRACT OF THESIS i i TABLE OF CONTENTS i v LIST OF TABLES v i i LIST OF FIGURES i x ACKNOWLEDGEMENTS x v i CHAPTER 1: INTRODUCTION 1 1.1 Problem statement 1 1.2 Approach 2 1.3 Previous work 3 1.4 Thesis organization 6 CHAPTER 2: SETTING 7 2.1 Location 7 2.2 Bedrock geology 10 2.3 Pleistocene h i s t o r y ' 13 2.4 Climate 15 2.5 Vegetation 16 2.6 Hydrology 19 i v TITLE PAGE CHAPTER 3: EARTHFLOW CHARACTERISTICS 25 3.1 Morphology 25 3.2 Character of movement 26 3.3 Debris character 32 3.4 Earthflow movement i n the Holocene period 35 CHAPTER 4: CLIMATE AND SUBSURFACE HYDROLOGY AT PAVILION 42 4.1 Introduction 42 4.2 Approach and data sources 44 4.3 Climate record 47 4.4 Basic operation of a standpipe piezometer 57 4.5 Groundwater flow and hydraulic conductivity 60 4.6 Piezometric observations 61 4.7 Relationship between climate and groundwater l e v e l s at P a v i l i o n 71 4.8 Summary 75 CHAPTER 5: SLOPE MOVEMENT AT PAVILION 77 5.1 Introduction 77 5.2 Method of observation 79 5.3 Movement i n the source area 82 5.4 Movement of the main flow 88 5.5 Movement of the west lobe 90 5.6 Movement of the east lobe 94 v TITLE PAGE 5.7 Cor r e l a t i o n of movement with groundwater hydrology 95 5.8 Fluctuations of earthflow motion i n the Holocene period 104 5.9 Summary 110 CHAPTER 6: CONCLUSION 112 6.1 Summary 112 6.2 Earthflow motion i n the I n t e r i o r Plateau 114 6.3 Future work at P a v i l i o n 116 BIBLIOGRAPHY 118 APPENDIX I: Atterberg l i m i t s : Test procedure and r e s u l t s 123 APPENDIX I I : Reduction of slope movement data 128 APPENDIX I I I : Periodic and t o t a l displacements calculated f o r P a v i l i o n 133 v i LIST OF TABLES TITLE PAGE TABLE 2.1. A summary of the l i t h o l o g i e s and formations i n the P a v i l i o n region (a f t e r T r e t t i n , 1961). 12 TABLE 3.1. Average consistency l i m i t s and natural water contents of near-surface samples of the earthflow debris and reworked t i l l . 34 TABLE 4.1. Summary of the depth of piezometer t i p s below the l o c a l ground surface. 45 TABLE 4.2, Det a i l s of the shallow i n s t a l l a t i o n s at P a v i l i o n . piezometer nest 48 TABLE 4.3. Mean monthly temperatures and po t e n t i a l evapo-transpiration estimates computed from the P a v i l i o n Mountain temperature data. 54 TABLE 4.4, Estimated values of groundwater recharge f o r the period August 1, 1981 to June 30, 1982. A l l values are i n mm. 56 TABLE 4.5, H y d r a u l i c c o n d u c t i v i t y v a l u e s of the earthflow debris obtained from i n s i t u measurements. 62 TABLE 4.6, Summary of the measured values of pressure head from the earthflow. 63 TABLE 4.7, The basic hydrostatic time lags computed from the slug test data used to estimate the hydraulic conductivity of the earthflow debris at P a v i l i o n . 74 v i i TITLE TABLE 5.1. TABLE A I . l . TABLE A I I I . l . TABLE AIII.2. TABLE AIII.3. TABLE AIII.4. PAGE Movement measured i n the source area. Stake 85 array locations are given i n figu r e 5.2. The r e s u l t s of A t t e r b e r g l i m i t t e s t s 126 performed on s e v e r a l samples of the earthflow debris. Movement i n the x - d i r e c t i o n calculated f o r 134 the stake arrays along the l a t e r a l shear zones. A l l values are i n centimeters. Movement i n the y - d i r e c t i o n calculated from 135 the state array measurements. A l l values are i n centimeters. Movement between the 1,5 and 4,5 stake pair 136 at the five-stake arrays. A l l values are i n centimeters. Measured displacement ( i n c e n t i m e t e r s ) 137 between the 2,3 stake pair situated on the unstable earthflow debris. v i i i LIST OF FIGURES PAGE Map of the r e g i o n i n which P a v i l i o n 8 earthflow i s located. The earthflow i s indicated by the s t r i p p l e d region east of the v i l l a g e of P a v i l i o n . Base map of the earthflow which includes delin e a t i o n of the earthflow d i v i s i o n s , c r o s s s e c t i o n t r a n s e c t s , l o c a t i o n s of seismic measurements, s u r f i c i a l geology, and _ * stake array locations. ( i n p^e4&e*--aife--baek ^ HP^'I^P*-• Longitudinal p r o f i l e of the earthflow along 9 the section A-B-C shown i n fig u r e 2.2. Vegetation map of the region surrounding 17 P a v i l i o n . The map shows the d i v i s i o n s between the biogeo-climatic zones discussed by Mathewes (1978). The earthflow i s the hatched region near the center of the map. (aft e r Mathewes, 1978) Base map of the e a r t h f l o w showing the vegetative cover and s u r f i c i a l hydrology of the earthflow. Included on the map are the locations of the piezometers and r a i n gauges at the s i t e . ( i n pocket at back of thesis) Photograph of the west lobe looking south 20 toward the central tension zone. Note that the large Douglas f i r stand i s t i l t e d from r e l a t i v e l y rapid motion of the west lobe. i x FIGURE 2.7. Photograph of a large Douglas f i r on the upslope side of a large tension crack i n the source area. Expansion of the tension crack i n the downslope d i r e c t i o n has caused removal of earthflow debris from the root zone of the tree. The d i r e c t i o n of movement i s to the l e f t i n the photograph. 20 FIGURE 2.8. Photograph of a s p l i t Douglas f i r on the east l a t e r a l shear zone of the east lobe. 21 FIGURE 2.9. Photograph of the l a t e r a l deposit west of the east lobe showing the extension of Douglas f i r roots downslope. The d i r e c t i o n of motion i s to the r i g h t . 21 FIGURE 2.10. FIGURE 2.11. FIGURE 3.1. Photograph of Pond 3 taken i n July 1981. Photograph of Pond 3 taken i n October 1981. A e r i a l view of P a v i l i o n earthflow. The earthflow boundaries are shown by the pen l i n e s . (airphoto number BC 7788-242, taken i n 1977) 23 23 27 FIGURE 3.2. Cross s e c t i o n a l view of three of the 29 earthflow d i v i s i o n s : (a) the source area; (b) the main flow; and (c) the east lobe, flow b i f u r c a t i o n , and west lobe. Transect ^ ^ l o c a t i o n s are g i v e n i n f i g u r e 2.2 ( i n C*MUJU*H«y< frorrket). FIGURE 3.3. Johnson's analysis of the development of 30 l a t e r a l ridges using the concept of "dead" zones within the flow channel. View (a) shows the concept of a r i g i d "plug" of debris flowing on a r a f t of viscous debris i n the flow channel. View (c) shows the evolution of multiple l a t e r a l deposits which develop when successively smaller waves of debris move through the channel. ( a f t e r Johnson, 1970) x FIGURE 3.4. Photograph of the west l a t e r a l shear zone of the west lobe near the apex of the debris fan. Note the large conglomerate boulder at the toe of the advancing west lobe, near the l a t e r a l deposit. FIGURE 3.5. Photograph of a large conglomerate boulder on the west side of the west lobe. The exposed surface appears to have been sheared while i n transport within the earthflow debris p r i o r to extrusion from the flow. FIGURE 3.6. Grading curves for the reworked g l a c i a l t i l l and the red-brown debris. FIGURE 3.7. G e n e r a l i z e d contour map of the south sideslope of P a v i l i o n Creek approximating the v a l l e y t o p o g r a p h y f o l l o w i n g d e g l a c i a t i o n . FIGURE 3.8. Schematic p r o f i l e of the main v a l l e y constructed from the generalized contour map d e p i c t i n g the c o n f i g u r a t i o n f o l l o w i n g d e g l a c i a t i o n , and the longi t u d i n a l p r o f i l e of the present earthflow surface. Transects are g i v e n i n f i g u r e s 3.7 and 2.2, respectively. FIGURE 4.1. (a) Schematic diagram of the f l o a t and counterweight ins i d e the piezometer. (b) Photograph of the c o n t i n u o u s , f l o a t -activated recorder mounted on BH-5. FIGURE 4.2. P r e c i p i t a t i o n r e c o r d s from P a v i l i o n Mountain, Hat Creek Va l l e y , and P a v i l i o n earthflow. FIGURE 4.3. Plot of B r i t i s h Columbia snow survey data from P a v i l i o n Mountain (1,250 m), Harry Lake (1,350 m), and Cornwall H i l l s (2,000 m). The snowpack depth over Mr. Cole (1,725 m) was estimated from the plots for each water year. The snow depths are a l l expressed as water equivalent values. x i FIGURE 4.4. Estimated change of snowpack depth (water equivalent) for Mt. Cole from January 1 to June 30, 1981 and 1982. FIGURE 4.5. Schematic diagram demonstrating the use of a standpipe piezometer for measuring hydraulic head h and i t s two components. The pressure head i s the height of the water column i n the standpipe and the elevation head z i s the elevation of the piezometer t i p above the datum (z = 0). FIGURE 4.6. Diagram showing the use of a piezometer to measure hydraulic head i n a h i l l s l o p e where the groundwater table i s below the ground s u r f a c e . The piezometer of l e n g t h d measures the pore water pressure at P. Since P i s located on the equipotential where h=2.8, the height of the water i n the piezometer i s equal to 2.8 on the v e r t i c a l scale to the l e f t of the diagram. FIGURE 4.7. Plot of the piezometer l e v e l f l u c t u a t i o n s against time. The data from BH-2, BH-3, BH-5, and BH-6 are shown for the duration of the study. The data plotted for BH-4 are the only points considered r e l i a b l e from the piezometer, while the data from the period p r e c e d i n g October 1981 are c o n s i d e r e d anomalous possibly due to a crack i n the standpipe. The data for BH-1 are the only two points that were measured at t h i s s i t e . FIGURE 4.8. The continuous records from the f l o a t -activated continuous water l e v e l recorders at BH-2, BH-4, and BH-5. No continuous record i s a v a i l a b l e from the i n t e r v a l s when the clocks stopped i n the winter, but the magnitude of the change that occurred i n these i n t e r v a l s could be determined from the charts because an ink trace was made when a change occurred, although the chart remained stationary. x i i FIGURE 4.9. Water l e v e l f l u c t u a t i o n s measured at the shallow piezometer nest s i t e s from June 6, 1981 to July 7, 1982. The s i t e s are (a) Nl, (b) N2, (c) N3, (d) N4, (e) N5, and ( f ) N6. FIGURE 5.1, Schematic diagram of the s t r a i n nets used at Pa v i l i o n for monitoring earthflow movement. FIGURE 5.2. Enlargement of the source area showing the arrangement of the s t r a i n nets i n the upper tension zone. The lower case l e t t e r s on the segments of WUT-3, WR9, and WUT-4 ind i c a t e the segment referred to i n Table 5.1. The in d i v i d u a l stakes at WUT-3 and WUT-4 are shown by upper case l e t t e r s . The "x" north of ESR-1 i s the peg attached to the recorder. FIGURE 5.3. Schematic diagram showing the i n s t a l l a t i o n of the continuous earth s l i d e recorders. FIGURE 5.4. Continuous movement record from ESR-1 i n the source area for the period May 15 to July 9, 1982. FIGURE 5.5. Photograph looking southeast at WUT-4 and ESR-1. Note the greenish-grey P a v i l i o n Group rocks that crop out from the red-brown earthflow debris (contact i s shown by the dashed l i n e ) . FIGURE 5.6. Cumulative movement of the main flow for the period March 21, 1981 to July 9, 1982. FIGURE 5.7, The continuous movement record from ESR-2 (adjacent to WR8) for the period November 16, 1981 to July 9, 1982. FIGURE 5.8. Cumulative movement of the west lobe for the period March 21, 1981 to July 9, 1982. x i i i FIGURE 5.9. The continuous movement record from ESR-3 (between WR2 and WR3 on the west lobe) f o r the period November 16, 1981 to July 9, 1982. There are two diagrams because the recorder was not i n the f i e l d from May 15 to June 9, 1982, hence movement i n t h i s period could not be accounted f o r i n the p l o t . 93 FIGURE 5.10. Cumulative movement of the east lobe for the period March 21, 1981 to July 9, 1982. 95 FIGURE 5.11. FIGURE 5.12. A segment of an unstable h i l l s l o p e with a groundwater table at height h above the f a i l u r e plane, and the depth to the f a i l u r e surface i s z. The pore water pressure u above the f a i l u r e plane i s found by computing: u = h cos^9. Sketch of the l i n e a r r e l a t i o n s h i p between shear strength and e f f e c t i v e normal s t r e s s . The shear strength parameters c' and are d e f i n e d by the diagram. The f a i l u r e envelope i s the common tangent to the Mohr c i r c l e s which represent various states of s t r e s s at f a i l u r e and are found by laboratory t e s t i n g of s o i l samples for determination of the shear strength of a s o i l ( s ) . 96 98 FIGURE 5.13 Plot of movement of the main flow measured at WR-8 and ESR-2, piezometric data from BH-2, BH-3, and BH-4, and the estimated groundwater recharge for the second water year. 100 FIGURE 5.14. Plot of movement of the west lobe measured at WR-2 and ESR-3, piezometric data from BH-5 and BH-6, and the estimated groundwater recharge f o r the second water year. 101 xiv FIGURE 5.15. Cumulative movement earthflow from June (from Bovis, 1980). measured 1978 to at P a v i l i o n August 1980 FIGURE 5.16. (a) Photograph of the road cut through the west l a t e r a l shear zone of the east lobe showing Mazama ash i n t e r s t r a t i f i e d with earthflow debris. (b) Scale drawing of the same photograph giving dimensions of the tephra layer and l a t e r a l deposits. FIGURE 5.17. Photograph of Mazama ash i n t e r s t r a t i f i e d with earthflow debris deriving from g l a c i a l t i l l i n the central tension scarp. FIGURE A I I . l . Sketch of a four-stake array at P a v i l i o n showing the s i x possible distances that were measured at each stake array. FIGURE A l l . 2 . Diagram showing the variables used f o r c a l c u l a t i n g earthflow movement from the stake array measurements. FIGURE A I I I . l . Diagram of stakes 1, 4, and 5 i n the f i v e -stake arrays which shows the p o s i t i v e and negative components of motion. FIGURE AIII.2. Diagram showing the p o s i t i v e and negative components of movement between the 2,3 stake pair within the stake arrays. xv ACKNOWLEDGEMENTS Completion of t h i s thesis would not have been possible without the assistance of several persons i n the f i e l d . In p a r t i c u l a r , I am indebted to Anne Hanssen, James Gallager, Sandy Brown, Neil Wanless, Gary Barrett, Chris Nadler, and Penny and Don Jones. Dr. M.A. Church offered several thoughtful comments on preceding d r a f t s of the t h e s i s , and Dr. M.J. Bovis not only guided me through the various phases of research, but also contributed many hours i n the f i e l d . To Dr. Bovis and Dr. Church I am grat e f u l for t h e i r time and patience. F i n a l l y , I am g r a t e f u l to my family and to my colleagues at UBC for t h e i r continued support and encouragement which have led to the completion of my endeavors. xvi " . . . the displaced surface seems to move p a i n f u l l y and grindingly over the bedrock, and the surface i s broken into a thousand i r r e g u -l a r i t i e s . . . . the l a n d s l i p s i n t h i s area look not unlike an earthern g l a c i e r . " M. B. Begbie, 1871, p. 141 (reference to P a v i l i o n earthflow) x v i i CHAPTER 1 INTRODUCTION 1.1 PROBLEM STATEMENT Common among large-scale mass movements i n the t e r r a i n of B r i t i s h Columbia's I n t e r i o r Plateau are slow earthflow-type features such as that present at P a v i l i o n , B r i t i s h Columbia. Passing reference has been made to several of these features i n regional geologic reports and t e r r a i n analyses (Bovis, 1980). The factors i n f l u e n c i n g earthflow movement within the I n t e r i o r Plateau, though, remain to be systematically investigated. In t h i s study, the r e l a t i o n s h i p between p r e c i p i t a t i o n , groundwater l e v e l s , and slope movement i s examined. The objectives of the project are: ( i ) to develop a d e s c r i p t i o n of the causal linkages between climate and subsurface hydrology, and ( i i ) to co r r e l a t e groundwater l e v e l f l u c t u a t i o n s with the seasonal v a r i a t i o n of movement rate. The r e s u l t s of several mass movement studies i n North America and Europe (VanDine, 1974, 1980; G i l and Kotarba, 1977; Swanson and Swanston, 1977; and Kelsey, 1978) have indicated that moisture a v a i l a b i l i t y i s important i n determining the rate of earthflow motion. For example, from these studies i t was found that maximum movement i s 1 observed sh o r t l y a f t e r commencement of the wet season or concurrent with peak discharge from snowmelt depending on the l o c a t i o n of the research. Bovis (1980), i n the preliminary i n v e s t i g a t i o n of earthflow motion at P a v i l i o n , found that slope displacement was highest during spring and early summer, and suggested that t h i s could be at t r i b u t e d to high groundwater l e v e l s from spring snowmelt and p r e c i p i t a t i o n i n the spring and summer. This i s not unreasonable as the ro l e of high pore water pressures i n reducing the shearing resistance of s l i d e debris has long been established. However, s u f f i c i e n t groundwater data were not ava i l a b l e from Bovis' research to allow c o r r e l a t i o n with movement. The goal of the present study at P a v i l i o n i s to conduct a more thorough i n v e s t i g a t i o n of the r e l a t i o n s h i p between climatologic and hydrologic f a ctors and t h e i r e f f e c t on earthflow motion. Therefore, i t i s desirable to have a good understanding of the influence of p r e c i p i t a t i o n and snowmelt on the piezometric conditions of the earthflow. The groundwater hydrology can be investigated to the depth of piezometer penetration i n the earthflow debris. This can l a t e r be applied i n the i n v e s t i g a t i o n of the causal linkages between subsurface hydrology and earthflow movement. ,2 APPROACH The data required to achieve the primary objectives of t h i s thesis include slope movement and piezometric measurements c o l l e c t e d at regular i n t e r v a l s during the study, and a climate record of s l i g h t l y longer duration. Slope movement was monitored at 33 stake arrays d i s t r i b u t e d around the earthflow margins, with continuous records of movement at three s i t e s . Groundwater l e v e l s were obtained from s i x standpipe piezometers i n s t a l l e d to depths ranging from 7.62m to 16.55m, with continuous records of piezometric l e v e l s at three boreholes. The data were c o l l e c t e d i n the period from mid-March 1981 to early July 1982, with continuous records from November 1981. Piezometer nests were placed at s i x s i t e s adjacent to ponds and seeps on the earthflow and the deeper groundwater l e v e l s obtained from the boreholes. P r e c i p i t a t i o n was measured for s i x months i n 1981 with f i v e storage gauges placed on the earthflow, and a d d i t i o n a l climate data were obtained from two climate stations located near P a v i l i o n operated by B r i t i s h Columbia Hydro and Power Authority. Snowpack information for the winters 1980-1981 and 1981-1982 was obtained from the B r i t i s h Columbia Snow Survey B u l l e t i n s , (the P a v i l i o n Snow Course, Harry Lake Snow Course, and Cornwall H i l l s Snow Course). F i e l d i n v e s t i g a t i o n s to determine the l o c a l geology and surface hydrology were also c a r r i e d out. These factors are important for determination of the regional and l o c a l groundwater flow regimes. 1.3 PREVIOUS WORK Earthflow motion has been studied i n the western United States by Kelsey (1978) and Swanson and Swanston (1977), and i n Poland by G i l and Kotarba (1977). In B r i t i s h Columbia, VanDine (1974, 1980) did a 3 geotechnical survey on the Drynoch earthflow, and Bovis (1980) has described the character of earthflow motion at P a v i l i o n from i t s topographic features and v e l o c i t y p r o f i l e s . G i l and Kotarba (1977) developed a model of slope evolution for the P o l i s h Carpathians, based on an active Holocene earthflow. They correlated climate and hydrologic data with the dynamic features of the earthflow over a period of 3 years. Their r e s u l t s showed that the t o t a l rate of displacement was dependent on the p r e c i p i t a t i o n regime. In southwestern Oregon, Swanson and Swanston (1977) also related earthflow motion to p r e c i p i t a t i o n and found that the antecedent moisture content, rather than r a i n f a l l rate i t s e l f , was an important factor a f f e c t i n g the rate of movement. Early i n the wet season, for example, the l e v e l of p r e c i p i t a t i o n was high while the flow v e l o c i t y remained low. Swanson and Swanston explained that t h i s resulted from low antecedent moisture present i n the s o i l at the beginning of the wet season. The low moisture content i n the earthflow early i n the wet season was a t t r i b u t e d to a delay i n the groundwater recharge. During spring snowmelt, occurring l a t e i n the wet season, the earthflow had become s u f f i c i e n t l y saturated to cause an increase of movement which became high r e l a t i v e to the p r e c i p i t a t i o n volume received (Swanson and Swanston, 1977). A s i m i l a r observation was made by Kelsey (1978). Kelsey (1978) was interested i n determining the sediment contribution from earthflows undercut by r i v e r s i n the Van Duzen River 4 Basin, C a l i f o r n i a . He found that the sediment load was highest when the rate of earthflow motion was maximum, shortly a f t e r the moisture content of the s l i d e debris had increased to a value great enough to promote more rapid flow—probably as a r e s u l t of increased pore water pressures. Kelsey further surmised that while heavy p r e c i p i t a t i o n i n i t i a t e d a c c e l eration of the flow, the removal of material from the base of the s l i d e by stream erosion enhanced earthflow response. In B r i t i s h Columbia, VanDine (1974, 1980) studied the geotechnical properties of Drynoch earthflow located on the Thompson River south of Spence's Bridge, and found that peak runoff from snowmelt correlated well with increased pore water pressure along the basal f a i l u r e surface and with maximum movement rate. Flow acceleration occurred both immediately following a groundwater l e v e l r i s e and up to one month aft e r snowmelt, depending on the point of observation. Like Kelsey (1978), VanDine (1974, 1980) concluded that while pore water pressure values remain important i n determining flow rate, active undercutting of the Drynoch s l i d e by the Thompson River also contributes to earthflow ac c e l e r a t i o n . Although P a v i l i o n earthflow has been described by several workers (Begbie, 1871; Dawson, 1896; D u f f e l l and McTaggart, 1952; T r e t t i n , 1961; and Ryder, 1976), Bovis (1980) was the f i r s t to examine the dynamic features of the earthflow. Bovis explained the character of earthflow-type movement by discussing the s p a t i a l and temporal v a r i a t i o n s of 5 movement rate, as well as d e t a i l s of the l o c a l topography. Many s i m i l a r i t i e s between the p r i n c i p l e s of debris flow mechanics described by Johnson (1970) and the apparent nature of earthflow movement were noted. From t h i s , Bovis suggested that c e r t a i n aspects of Johnson's analysis might be appropriate f o r the analysis of earthflow motion. 1.4 THESIS ORGANIZATION The thesis i s organized into s i x chapters. Chapter Two i s a s i t e d e s c r i p t i o n and includes discussions of the l o c a l geology, the Quaternary h i s t o r y , climate and vegetation, and surface hydrology. The morphology of the earthflow i s described i n Chapter Three. This includes an introduction to the character of earthflow motion at P a v i l i o n . The climate record and piezometric data are presented i n Chapter Four. These data are compared temporally and the r e l a t i o n s h i p between these time series i s analyzed. The r e s u l t s of slope movement measurements are presented i n Chapter Five. This includes analyses of the s p a t i a l and temporal v a r i a t i o n s of earthflow motion, and a de s c r i p t i o n of the r e l a t i o n s h i p between slope movement and piezometric l e v e l f l u c t u a t i o n s i n the earthflow. Chapter Six i s a summary of the r e s u l t s of the r e s u l t s and includes a b r i e f discussion on the character of earthflow motion i n the I n t e r i o r Plateau region of B r i t i s h Columbia. 6 CHAPTER 2 SETTING 2.1 LOCATION P a v i l i o n earthflow i s located 370 km northeast of Vancouver, B r i t i s h Columbia and i s 30km northeast of L i l l o o e t just south of Highway 12, opposite the Indian v i l l a g e of P a v i l i o n ( f i g u r e 2.1). P a v i l i o n i s situated on the southeastern portion of the Fraser Plateau, part of the physiographic region which Holland (1964) referred to as the I n t e r i o r Plateau. The earthflow issues from the watershed flanking the north side of Mt. Cole (figure 2.1). An unimproved track runs the length of the flow and allows easy access to the study area. The earthflow ranges from an average of 720 m at the toe to 1080 m at the upper headscarp (figure 2.2). It is. 2.2 km i n length, and varies i n width from 80 m to 500 m. The average width i s about 300 m. The slope of the earthflow surface ranges from 4° to 20 °; the average slope i s 10.5° (fi g u r e 2.3). Based on morphologic c h a r a c t e r i s t i c s , the earthflow i s d i v i s i b l e i n t o seven units (figure 2.2): ( i ) the source area ref e r s to the region of extending flow within 200 m north of the headwall; 7 FIGURE 2 . 1 . Map of the r e g i o n i n which P a v i l i o n e a r t h f l o w i s l o c a t e d . The e a r t h f l o w i s i n d i c a t e d by the s t i p p l e d r e g i o n e a s t of the v i l l a g e of P a v i l i o n . 8 FIGURE 2.3. Longitudinal p r o f i l e of the earthflow along the section A-B-C shown i n f i g u r e 2.2. ( i i ) the main flow i s the region bounded to the south by the source area and by the c e n t r a l t e n s i o n zone to the nort h ; ( i i i ) the c e n t r a l tension zone r e f e r s to the region of extending flow w i t h i n 100m north of the mainflow; ( i v ) the west lobe i s the lobe which flows around the west s i d e of the flow b i f u r c a t i o n and i s by f a r the l a r g e r of the two lobes; (v) the east lobe i s the lobe which flows to the east of the flow b i f u r c a t i o n ; ( v i ) the flow b i f u r c a t i o n i s the lens-shaped region which d i v i d e s the earthflow i n t o the two lobes; and ( v i i ) the deb r i s fan i s the l a r g e , hummocky feature contiguous w i t h the east and west lobes. 2.2 BEDROCK GEOLOGY The ensuing d i s c u s s i o n on the geology of P a v i l i o n i s derived l a r g e l y from a r e g i o n a l geologic study by T r e t t i n (1961), and to a l e s s e r extent from D u f f e l l and McTaggart (1952), Preto et a l . (1979), Monger (1981), and Shannon (1981). A d d i t i o n a l f i e l d observations were made during the present study. A geologic map ( f i g u r e 2.2, i n pocket) was compiled i n the f i e l d w i t h reference to T r e t t i n ' s r e g i o n a l map, and the formations and l i t h o l o g i e s are summarized i n Table 2.1. 10 The oldest rocks found i n the P a v i l i o n area were mapped by T r e t t i n as the P a v i l i o n Group (Permo-Triassic). These rocks occur between 1010 m and 1060 m elevation i n the southern portion of the study area. The l i t h o l o g y consists primarily of metavolcanic rocks interbedded with c l a s t i c marine sediments. The P a v i l i o n Group i s at least 100 m i n thickness, and s t r i k e s northwest, dipping steeply to the east. The topography overlooking the most constricted portion of the earthflow i s underlain by g r a n o d i o r i t i c i n t r u s i v e s , which have been correlated with the Mt. Martley Stock (figure 2.1), south of the earthflow (Preto et a l . , 1979). B i o t i t e from samples c o l l e c t e d near P a v i l i o n gave a potassium-argon date of 141 m i l l i o n years Before Present (BP) (Preto et a l . 1979), i n d i c a t i n g Jurassic age. The i n t r u s i o n of these rocks caused considerable contact metamorphism and a l t e r a t i o n of the P a v i l i o n Group ( T r e t t i n , 1961). Continental sedimentary deposits unconformably o v e r l i e the P a v i l i o n Group and i n t r u s i v e rocks, and include carbonaceous shale (the basal member), conformably ov e r l a i n by i n t e r s t r a t i f i e d l i t h i c sandstone, conglomerate and additional carbonaceous shale s t r a t a ( f i g u r e 2.2). The conglomerate and sandstone contain angular fragments of P a v i l i o n Group rocks, feldspar and quartz, and rounded c l a s t s of chert and a r g i l l i t e set i n a fine-grained matrix that varies from brick red to pale yellow i n c o l o r . The coarse, well-sorted sandstone and conglomerate are probably f l o o d p l a i n deposits—their color i n d i c a t i n g deposition i n a 11 TABLE 2.1. A summary of the l i t h o l o g i e s and formations i n the P a v i l i o n region ( a f t e r T r e t t i n , 1961 and Monger; and McMillan 71984). ERA PERIOD EPOCH LITHOLOGY AND CONTACTS Cenozoic Quaternary Recent and Pleistocene alluvium, volcanic ash, mudflows, g l a c i a l t i l l , g l a c i a l outwash, earthflow debris, colluvium —unconformity-Lower Upper or Upper Middle Cretaceous l i t h i c arenite, conglomerate, carbonaceous shale —unconformity— Mesozoic Lower Cretaceous or older granodiorite, q u a r t z - d i o r i t e stocks — i n t r u s i v e contact— T r i a s s i c GROUP:Pavilion FORMATION: D i v i s i o n II ASSEMBLAGE: P a v i l i o n t u f f , volcanic arenite, greywacke, volcanic flows, a r g i l l i t e , chert, limestone 12 well-drained, o x i d i z i n g environment ( T r e t t i n , 1961). The presence of carbonaceous shale shows that backswamp conditions existed l o c a l l y . S tructural and s t r a t i g r a p h i c r e l a t i o n s h i p s suggest that these rocks are Pre-Miocene i n age ( D u f f e l l and McTaggart, 1952; T r e t t i n , 1961). T r e t t i n (1961), therefore i n f e r r e d that the deposits were T e r t i a r y i n age. Recently, Monger (1981), Shannon (1981), and Monger and McMillan (1984) have correlated the rocks with s i m i l a r units that crop out near Lytton and i n the Upper Hat Creek Valley, which contain Albian (Middle Cretaceous) f l o r a . Pollen remains from the carbonaceous shale near the base of the unit at P a v i l i o n i n d i c a t e Cretaceous age for the sediments (Bovis, personal communication, 1984). The upper conglomerate and sandstone sequence i s younger. The s t r a t a , then, are considered to be Upper Middle to Lower Upper Cretaceous i n age (Table 2.1). The depth to bedrock i s v a r i a b l e , as revealed by a seismic survey c a r r i e d out at several s i t e s on the earthflow. In the source area, c r y s t a l l i n e rock was detected near the surface, and rocks from the P a v i l i o n Group crop out i n several places (fi g u r e 2.2). In the center of the earthflow, on the other hand, bedrock was not detected at 12 m, the maximum depth attainable with the S o i l test MD-9 seismograph. 2.3 PLEISTOCENE HISTORY Fulton and Smith (1978) provide a late-Quaternary chronology for south-central B r i t i s h Columbia. Although t h e i r study i s centered i n the 13 Okanagan area, east of P a v i l i o n , t h e i r chronology can be extended to the P a v i l i o n region because the enti r e I n t e r i o r Plateau i s considered to have had a s i m i l a r g l a c i a l h i s t o r y . The following discussion i s derived from topographic and s t r a t i g r a p h i c observations made at P a v i l i o n i n addition to the work of Fulton and Smith (1978) and Ryder (1976). Most of south-central B r i t i s h Columbia was not completely covered by i c e u n t i l 19,000 years BP, and deglaciation was probably complete by 10,000 years BP (Fulton and Smith, 1978). This period corresponds with the Fraser G l a c i a t i o n i n the I n t e r i o r Plateau region (Fulton and Smith, 1978). There i s no evidence at present to indicate that stades and interstades of the Fraser G l a c i a t i o n can be recognized i n the In t e r i o r Plateau. The i c e cap of the Kamloops Lake G l a c i a t i o n attained a maximum height of 2,400 m along the eastern edge of the Coast Mountains ( D u f f e l l and McTaggart, 1952; Ryder, 1976). The major v a l l e y s were buried by i c e over 2,100 m thick and i c e depths over the plateau were generally less than 900 m (Ryder, 1976). The i c e sheet moved from west to east over the Clear Range from the Fraser Valley (Ryder, 1976) and deposited a thick basal t i l l blanket over the earthflow s i t e and the surrounding uplands. During de g l a c i a t i o n , upland areas became i c e - f r e e while the major valle y s remained occupied by g l a c i e r lobes (Ryder, 1976). The h i l l s l o p e s surrounding P a v i l i o n earthflow are mantled by a 14 blanket of g l a c i a l t i l l . The flow b i f u r c a t i o n , and the higher l a t e r a l ridges appear to consist of t i l l (figure 2.2), and patches of translocated t i l l are found on the main flow between 940 m and 1,000 m elevation and on most of the east lobe. It appears that continued movement of the earthflow has removed most of the t i l l from the source area, depositing the reworked t i l l on the debris fan where i t has l a t e r been overridden by flows con s i s t i n g primarily of weathered Cretaceous sediments (Bovis, 1980). 2.4 CLIMATE The present climate i n the v a l l e y s of the southwestern I n t e r i o r Plateau region i s semi-arid to sub-humid, with p o t e n t i a l evaporation often exceeding p r e c i p i t a t i o n . The annual p r e c i p i t a t i o n i n the region ranges from 300 mm to 400 mm, and maximum p r e c i p i t a t i o n occurs from November to February, and usually during June. Snowfall i s most frequent from November to March. The mean d a i l y temperature remains below freezing i n winter, therefore winter p r e c i p i t a t i o n i s stored i n the snowpack. During spring snowmelt, then, the moisture surplus from winter accumulation i s av a i l a b l e for spring runoff and groundwater recharge. In the period from A p r i l to October the average temperature remains above freezing; therefore some evaporation occurs with maximum evaporation l i k e l y from May to September when the temperatures a t t a i n the greatest annual values. 15 The mean d a i l y temperature i n January averages -7.7 C i n the region and the mean d a i l y temperature i n July averages 14°C. As the elevation of the I n t e r i o r Plateau region increases above 600 m a . s . l . the trend i s toward increased moisture and cooler temperatures (Mathewes, 1978). Total p r e c i p i t a t i o n at P a v i l i o n Mountain (figure 2.1) from September 1, 1980 to August 31, 1981 was 329 mm. The B r i t i s h Columbia snow survey data from P a v i l i o n Mountain indicate that there was no snowpack af t e r January 1, 1981. In the second water year, September 1, 1981 to July 10, 1982, 210 mm of p r e c i p i t a t i o n f e l l and the snow survey data show that at least 94 mm (water equivalent) of that quantity was snow. 2.5 VEGETATION A regional map showing the biogeoclimatic zones i s provided i n figu r e 2.4. Precise delineation of the zones i s not possible because many plant species occur i n more than one zone and there i s usually a gradual t r a n s i t i o n from one zone to another (Mathewes, 1978). A map of the vegetation on the earthflow i s provided i n fig u r e 2.5 ( i n pocket) which shows that sage brush (Artemisia tridentata) and bunchgrass (Agropyron spicatum) dominate the debris fan. I n t e r i o r Douglas f i r (Pseudotsuga menziesii) and Ponderosa pine (Pinus ponderosa) are present over most of the lower part of the earthflow fan, with Douglas f i r dominant above 875 m. Sage brush, bunchgrass, and Ponderosa pine generally occur i n the Ponderosa Pine-Bunchgrass Zone (figure 2.4), 16 FIGURE 2.4. Vegetation map of the region surrounding P a v i l i o n . The map shows the d i v i s i o n s between the biogeoclimatic zones discussed by Mathewes (1978). The earthflow i s the hatched region near the center of the map. (af t e r Mathewes, 1978) 17 which i s the d r i e s t , and i n summer the warmest, zone i n B r i t i s h Columbia (Mathewes, 1978). I n t e r i o r Douglas f i r favors areas where the moisture a v a i l a b i l i t y i s s l i g h t l y higher than that required by Ponderosa pine and bunchgrass. Hence Douglas f i r often grows along stream edges within the Ponderosa Pine-Bunchgrass Zone (Mathewes, 1978), and occurs at high elevations where the influence of strong p r e c i p i t a t i o n gradients increases the moisture supply. At P a v i l i o n , the stand density of Douglas f i r i s high on the lower part of the east lobe and along the toe of the debris fan adjacent to P a v i l i o n Creek (figure 2.5). This i s probably associated with the elevation of the groundwater table, which commonly r i s e s along v a l l e y bottoms. Aspen (Populus tremuloides) and willow ( S a l i x lasiandra) are found where surface runoff and groundwater discharge are abundant, for example along the margins of ephemeral streams and around poorly-drained depressions. Also found around ponds on the earthflow are aquatic sedge (Carex a q u a t i l i s ) and other wetland vegetation. Wild rose (Rosa nutkana, R. gymnocarpa) i s common throughout the map area as i t can to l e r a t e a v a r i e t y of moisture regimes ranging from moderately wet to very wet. Some logging a c t i v i t y was conducted at P a v i l i o n i n the lat e 1950s and early 1960s. As w e l l , forest f i r e s have swept through the area within the l a s t f i f t y years. Many of the trees alongside the l a t e r a l shear zones and on the debris lobes are t i l t e d from large downslope displacements ( f i g u r e 2.6). Many of the trees i n the source area have 18 been uprooted (fig u r e 2.7) as a r e s u l t of slumping, a few trees that straddle the shear zones are s p l i t ( f i g u r e 2.8), and the roots of several large Douglas f i r have been extended downslope (fig u r e 2.9). In contrast, coniferous trees growing on the abandoned l a t e r a l deposits appear to be undisturbed. This implies that movement along the flanks of these landforms has not occurred since the trees became established. 2.6 HYDROLOGY Ephemeral streams are e a s i l y located by the abundance of phreatophytes that grow along t h e i r flow paths (fig u r e 2.5). These drainage routes are most active during spring and early summer when discharge from snowmelt i s highest. By l a t e summer and autumn most of them are dry. P a v i l i o n Creek runs along the base of the earthflow fan (fig u r e 2.5), but does not cause s i g n i f i c a n t undercutting of the debris fan. Ponding of water on the earthflow appears to r e s u l t from runoff accumulation and groundwater discharge i n poorly-drained topographic depressions. These basins are commonly located on low-lying f l a t portions of the earthflow underlain by reworked t i l l . The l e v e l of water i n the major ponds (numbered i n figure 2.5) was much higher i n 1981 than i t was i n 1982. For example, a dramatic change of water l e v e l elevation was observed i n Pond 3 during the study. The photograph i n 19 FIGURE 2.6. Photograph of the west lobe looking south toward the cent r a l tension zone. Note that the large Douglas f i r stand i s t i l t e d from r e l a t i v e l y rapid motion of the west lobe. FIGURE 2.7. Photograph of a large Douglas f i r on the upslope side of a large tension crack i n the source area. Expansion of the tension crack i n the downslope d i r e c t i o n has caused removal of earthflow debris from the root zone of the tree. The d i r e c t i o n of movement i s to the l e f t i n the photograph. 2U FIGURE 2.9. Photograph of the l a t e r a l deposit west of the east lobe showing the extension of Douglas f i r roots downslope. The d i r e c t i o n of motion i s to the r i g h t . 21 figure 2.10 was taken i n mid-July 1981, and figure 2.11 i s a photograph of Pond 3 early i n October 1981. In May and early July 1981 i t was noted that the l e v e l of Pond 3 had not recovered from the huge drawdown depicted i n fi g u r e 2.11, despite intense r a i n f a l l and a high volume of snowmelt from May to July 1982. Seasonal surface storage basins much smaller than the numbered ponds are usually situated i n grassy, bowl-shaped depressions, often less than 1.0 m i n diameter. The l o c a t i o n of these saturated basins varied from spring 1981 to spring 1982, and they were completely dry i n 1981 by mid-July. In May 1982 small snowpatches persisted i n the source area and once melt began i n t h i s area i n May, i t continued r a p i d l y . There was add i t i o n a l evidence of recent snowmelt on the earthflow at several locations on the main flow during May 1982 where water from snowmelt tended to pool within the small surface storage basins. In winter 1981, however, no snowpack had accumulated on the earthflow nor was any snowpack measured at the P a v i l i o n Mountain snow survey s t a t i o n . As a r e s u l t , the saturated depressions observed on the earthflow i n May and June 1981 must have been associated with groundwater discharge and heavy r a i n f a l l i n May 1981. The subsurface hydrology of the earthflow i s not well-understood at present, but i t i s probably very complex. Nonetheless, a b r i e f overview can be given. It i s l i k e l y that much of the groundwater recharge at P a v i l i o n occurs by i n f i l t r a t i o n of surface runoff into the 22 FIGURE 2.10. Photograph of Pond 3 taken i n July 1981. s o i l and rock debris capping the north drainage basin of Mt. Cole. This includes slopes immediately below Mt. Cole and most of the upper and middle portion of the earthflow. Piezometric data measured adjacent to several of the major ponds of the earthflow ind i c a t e flow of water toward the groundwater table, or groundwater recharge. Groundwater discharge i s evident from the base of the headwall throughout most of the source area. Apparently, slumping of the earthflow and tension crack development i n the source area are associated with extension of the b r i t t l e debris over a steep gradient. If the water table i s s u f f i c i e n t l y close to the ground surface i n t h i s case, discharge w i l l occur through the f i s s u r e d debris. Discharge across the width of the middle tension zone i s evident from the abundance of phreatophytes (aspen, willow) extending to about 850 m elevation on the west lobe and to about 830m elevation on the east lobe (fi g u r e 2.5). Below these elevations, the dominant vegetation changes to sage and bunchgrass which remain dominant to about 760 m, where coniferous stands become abundant. The l a t t e r i n dicates an abrupt r i s e of the groundwater table at the v a l l e y bottom where the groundwater flow system discharges into P a v i l i o n Creek. 24 CHAPTER 3 EARTHFLOW CHARACTERISTICS 3.1 MORPHOLOGY According to Varnes' (1978) c l a s s i f i c a t i o n of mass movement types, P a v i l i o n i s a slow earthflow. Varnes (pp. 19-20) describes these features as "flows occurring i n r e l a t i v e l y dry, p l a s t i c earth where there i s the combination of clay or weathered clay-bearing rocks, moderate slope, and adequate moisture." Varnes further describes (p. 20) "a common elongation of the flow with channelization and depression i n the slope, and spreading of the debris occurring at the toe." Indeed, the topography of P a v i l i o n earthflow (figures 2.2 and 3.1) suggests that slow, viscous flow has taken place. The earthflow discharges from a bowlr-shaped headwall i n the south end of the study area (fi g u r e 2.2). Its surface i s hummocky with several poorly-drained topographic depressions. Open cracks up to 1.0 m i n width and back-rotated slump blocks are present where the debris has f a i l e d i n tension. The earthflow divides into two lobes around the flow b i f u r c a t i o n about 1,200 m upslope from Highway 12. An abrupt change i n lon g i t u d i n a l gradient at t h i s l o c a t i o n (figure 2.3) suggests that a bedrock "step" i s close to the surface. La t e r a l deposits run p a r a l l e l 25 to the earthflow margins and consist of mixed unconsolidated earthflow debris and reworked g l a c i a l t i l l . These are analogous to l a t e r a l ridges which often define debris flow channels. The debris fan which consists of material deposited by the earthflow i s very large and hummocky i n character (figures 2.2 and 3.1). Topography suggests that the areal extent of the earthflow was once much greater than i t i s at present. For example, a large landform about 600 m south of the present source area appears to be an ancient bowl-shaped headwall (fi g u r e 2.2). Further suggestion of a once-larger earthflow volume i s derived from the size of the abandoned l a t e r a l deposits, or l a t e r a l ridges (fi g u r e 2.2) located well outside the present earthflow margins. .2 CHARACTER OF MOVEMENT Earthflow movement at P a v i l i o n i s i n many ways s i m i l a r to g l a c i e r flow. Tension cracks and back-rotated slumps i n the area of the active headwall and middle tension zone appear to be related to flow extension—perhaps a r e s u l t of a break i n slope of the underlying bedrock surface. Compressional stresses probably occur within the main flow and at the apex of the debris fan where the surface gradient and movement rate are quite small and the width of the flow i s large compared to that of the source area and the east and west lobes (fi g u r e 2.2). 26 27 The well-developed l a t e r a l deposits which define the earthflow boundaries (fi g u r e 3.2) are analogous to l a t e r a l moraines (Johnson, 1970). L a t e r a l deposits develop from d i f f e r e n t i a l movement between the edges of the flow and the center of the channel. According to Johnson (1970), the material tends to accumulate on the margins of the channel where there are "dead" zones, or regions of l i t t l e or no movement i n the channel (fi g u r e 3.3). The debris tends to be pushed upward along the margins of the flow so that accumulation of the s l i d e debris may occur with some " s p i l l i n g " of debris out of the channel. At P a v i l i o n , sandstone and conglomerate boulders up to 1.0 m i n diameter are common along the lower portion of the west l a t e r a l shear zone of the west lobe (figure 3.4). Many of these blocks exhibit one or more sheared surfaces (figure 3.5), i n d i c a t i n g that they had been transported along the basal shear zone p r i o r to extrusion from the earthflow. Johnson (1970) further stated that multiple l a t e r a l deposits develop when successive waves of material moving i n a channel become smaller (fi g u r e 3.3), each wave leaving l a t e r a l deposits on i t s flow margins. Linear ridges higher than the present earthflow surface, a feature also noted by VanDine (1974), are undoubtedly l a t e r a l deposits produced when the earthflow was much larger than i t i s at present (Bovis, 1980). In cross section (fi g u r e 3.2), the earthflow has a concave p r o f i l e . The depressed center indicates that movement occurs at the 28 FIGURE 3.2. Cross s e c t i o n a l view of three of the earthflow d i v i s i o n s : (a) the source area; (b) the main flow; and (c) the east lobe, flow b i f u r c a t i o n , and west lobe. Transect locations are given i n fig u r e 2.2 ( i n pocket). 29 (c) FIGURE 3.3. Johnson's analysis of the development of l a t e r a l ridges using the concept of "dead" zones within the flow channel. View (a) snows the concept of a r i g i d "plug" of debris flowing on a r a f t of viscous debris i n the flow channel. View (c) shows the evolution of multiple l a t e r a l deposits which develop when successively smaller waves of debris move through the channel. ( a f t e r Johnson, 1970) 30 FIGURE 3.4. Photograph of the west l a t e r a l shear zone of the west lobe near the apex of the debris fan. Note the large conglomerate boulder at the toe of the advancing west lobe, near the l a t e r a l deposit. FIGURE 3.5. Photograph of a large conglomerate boulder on the west side of the west lobe. The exposed surface appears to have been sheared while i n transport within the earthflow debris p r i o r to extrusion from the flow. 31 highest rate i n the center of the channel where the influence of boundary resistance i s minimum. As the flow boundaries are approached, the rate of movement decreases r a p i d l y . This causes the debris to bui l d up along the flow margins according to Johnson's (1970) theory of "dead" zones. The rapid movement i n the center of the channel, then, causes the debris to exhibit a concave p r o f i l e , with the e f f e c t being more pronounced where the flow i s most cons t r i c t e d (e.g., the east lobe and tne west lobe, fi g u r e 3.2). Bovis (1980) has shown that the horizontal v e l o c i t y p r o f i l e i s s i m i l a r to that of a Bingham v i s c o - p l a s t i c substance flowing i n a semi-c i r c u l a r channel. From t h i s , he speculated that a r i g i d "plug" flows on a r a f t of viscous debris i n the flow channel (fi g u r e 3.3), with l i t t l e or no deformation of the "plug" occurring above a c r i t i c a l depth. This, however, requires much further i n v e s t i g a t i o n , and i s beyond the scope of t h i s t h e s i s . .3 DEBRIS CHARACTER The earthflow debris i s a mixture of poorly sorted, rounded and angular l i t h i c c l a s t s of v a r i a b l e s i z e within a red-brown s i l t y - c l a y matrix ( f i g u r e 3.6). Apparently, the debris i s derived from weathered, sheared Cretaceous deposits. X-ray analysis of the shale indicated the presence of i l l i t e with some expanding laye r s , k a o l i n i t e , and vermiculite (P. Jones, personal communication, 1982), and quartz and feldspar were recognized i n t h i n section. It was found that the shale 32 too-SAND SILT CLAY COARSE MEDIUM F/AIE COARSE MEDIUM FINE COARSE MEDIUM FIHE i 0 X SIEVE HYDROMETER 'RED-BROWN DEBRIS REWORKED TILL-^ RED-BROWN DEBRIS REWORKED TILL 17% GRAVEL 30% GRAVEL 2>r/* SAND 45% SAND zr/a SILT IV/. SILT ZO'U CLAi (<O.OOZn,m) 6 7, CLAY "1 I — T 0.1 0.001 DIAMETER (mm) OOOOI FIGURE 3 . 6 . Grading curves f or the reworked g l a c i a l t i l l and the red-brown d e b r i s . weathers to montmorillonite (about 60%) with some k a o l i n i t e (35%). Evidently the smectite i s derived from mica i n the shale, while k a o l i n i t e comes from the weathering of abundant feldspar fragments (P. Jones, personal communication, 1982). Entrainment of blocks of younger sandstone and conglomerate s t r a t a within the s l i d e debris i s evident i n the f i e l d (figures 3.4 and 3.5), and probably occurred due to f a i l u r e of the underlying shale (Bovis, 1980). The reworked t i l l c onsists of poorly sorted angular fragments of granodiorite, P a v i l i o n Group c l a s t s , and minor limestone set i n a sa n d y - s i l t matrix ( f i g u r e 3.6). Laboratory t e s t i n g of near-surface samples of the earthflow debris and reworked g l a c i a l t i l l was conducted to determine the consistency l i m i t s . The d e t a i l s of the tests are given i n Appendix I. Both the l i q u i d l i m i t and the p l a s t i c i t y index of the s l i d e material were found to be moderately low (Table 3.1). This can be accounted for by the r e l a t i v e abundance of k a o l i n i t e i n the debris which reduces the consistency l i m i t s of montmorillonitic deposits. TABLE 3.1. Average consistency l i m i t s and natural water contents of near-surface samples of the earthflow debris and g l a c i a l t i l l . M aterial type L.L.(%) P.L.(%) P.I.(%) w(%) Earthflow debris Translocated t i l l 41.5 30.2 26.2 20.3 15.3 9.9 30.6 18.6 34 Near-surface samples of the earthflow debris and reworked t i l l taken from saturated zones suggested that the natural water contents of the red-brown debris are s l i g h t l y over the p l a s t i c l i m i t (Table 3.1). Bovis (1980), however, reported f i e l d water contents s u b s t a n t i a l l y lower than the p l a s t i c l i m i t to depths of 8-10 m, below which values s l i g h t l y exceeded the p l a s t i c l i m i t . Low natural water contents, as discussed by Bovis (1980), are consistent with the b r i t t l e behavior of the earthflow debris manifest by the abundant tension cracks present where flow extension i s occurring—contrary to the i l l u s i o n of slow, viscous flow derived from the a e r i a l view of the earthflow (fi g u r e 3.1). 3.4 EARTHFLOW MOVEMENT IN THE HOLOCENE PERIOD A sequence of events which may have led to the development of the earthflow as i t appears at present has been constructed on the basis of topographic and s t r a t i g r a p h i c r e l a t i o n s h i p s noted on the earthflow and i t s surrounding slopes. The chronology i s tentative and u n t i l i t can be supported by deep borehole data from the debris fan, i t remains conj e c t u r a l . From the topographic map of the earthflow ( f i g u r e 2.2) and the a e r i a l photograph (fig u r e 3.1), i t appears that the slopes south of P a v i l i o n Creek situated on the east and west boundaries of the earthflow were once contiguous with a steep v a l l e y wall at the s i t e of the earthflow. In addition, i t i s evident that the north side of the flow b i f u r c a t i o n has a s i m i l a r topographic configuration to the steep slopes 35 bounding the earthflow. Based on these observations, the contour map i n f i g u r e 3.7 depicts the topography of the main v a l l e y as i t may have appeared following d e g l a c i a t i o n . The map i s schematic and was constructed by l i n k i n g the contours of the steep slopes with the contours on the north side of the flow b i f u r c a t i o n . The l a t t e r appears to be displaced downslope from i t s o r i g i n a l p o s i t i o n , therefore i t s contours i n fi g u r e 3.7 were elevated accordingly. The east and west lobes appear to occupy former water courses around the flow b i f u r c a t i o n , therefore runoff channels were included i n these positions (f i g u r e 3.7). If f i g u r e 3.7 approximates the topography of the v a l l e y about 10,000 years BP, then i t seems that at least two processes may have caused i n i t i a t i o n of mass movement at P a v i l i o n . F i r s t , oversteepening of the v a l l e y sidewall by g l a c i a l scour increased the horizontal stress on the weakly indurated sediments and caused them to f a i l . The second process i s that f a i l u r e of the Cretaceous deposits was i n i t i a t e d by high runoff through the steep channels at the s i t e of the earthflow. Continued p o s t - g l a c i a l slumping and debris flow of the shale and clay-bearing c l a s t i c deposits resulted i n extension of the s l i d e into the main v a l l e y ( f i g u r e 3.8), with retrogressive f a i l u r e proceeding upslope from the v a l l e y sidewall. Thus, movement of the earthflow through the channels of the east and west lobes, accompanied by accumulation of the debris fan, has concealed the topography of the main v a l l e y that prevailed p r i o r to earthflow movement. In contrast, the 36 FIGURE 3 7 G e n e r a l i z e d contour map of the south sideslope of P a v i l i o n Creek approximating the v a l l e y t o p o g r a p h y f o l l o w i n g deglaciation. VERTICAL EXAGGERATION-4X 4 — ~ \ 1 1 1 1 1 1 1 1 1 1 I I 1 I Y " A  A FIGURE 3.8. Schematic p r o f i l e of the main v a l l e y constructed from the generalized contour map d e p i c t i n g the c o n f i g u r a t i o n f o l l o w i n g d e g l a c i a t i o n , and the l o n g i t u d i n a l p r o f i l e of the present earthflow surface. Transects are g i v e n i n f i g u r e s 3.7 and 2.2, respectively. v a l l e y p r o f i l e east and west of the earthflow remains r e l a t i v e l y undisturbed apparently because of the r i g i d plutonic and metamorphic rocks which dominate these slopes. The flow b i f u r c a t i o n consists primarily of g l a c i a l t i l l and probably sheared to i t s present p o s i t i o n ( f i g u r e 2.2) shortly a f t e r removal of the i c e buttress. S t a b i l i z a t i o n of the b i f u r c a t i o n while earthflow motion has continued might be explained by a differ e n c e i n shearing resistance between the t i l l and the red-brown debris. The shear strength of each material was not determined, but the coarser t i l l matrix ( f i g u r e 3.6) suggests that the angle of shearing resistance would be greater than that of the red-brown earthflow debris. The average slope of the earthflow at present i s about 10°—sufficient f or movement of saturated clay debris to p e r s i s t . Some of the t i l l i n the P a v i l i o n region, however, may have s i m i l a r mechanical properties to the red-brown debris as Ryder (personal communication, 1982) noted that t i l l deposits across the main v a l l e y from P a v i l i o n -earthflow contain the same l i t h o l o g i e s as the earthflow debris. The large topographic depression surrounding the source area and the depressed configuration of the main flow and the source area ind i c a t e that a large quantity of debris has been removed from the upper and c e n t r a l portions of the basin by earthflow movement. The volume of debris removed was estimated f o r l a t e r comparison with a volume estimate from the debris fan. Since very l i t t l e erosion of the debris fan i s 39 evident, the d i f f e r e n c e between the two volume estimates was expected to be very small. The volume of debris removed from the south end of the study area was estimated by f i r s t approximating the topography p r i o r to debris removal. The maximum elevation of the l a t e r a l ridges east and west of the present shear zones (fig u r e 2.2) was considered to represent the slope elevation p r i o r to displacement. The southernmost boundary used was the headwall-like landform south of the earthflow, the top of which was considered, again, to approximate the elevation of the slope p r i o r to displacement. The contours pre-dating earthflow movement were then extrapolated across the source area from the tops of the l a t e r a l ridges so that a plateau-like feature was constructed. The depth of the debris removed was found by computing the d i f f e r e n c e between the extrapolated contour and the present earthflow topography at several randomly selected l o c a t i o n s . The area of a column at each l o c a t i o n considered was m u l t i p l i e d by the depth of that column. The sum of these values i s 7 3 approximately 5.4 x 10 m . The volume of the debris fan and the east and west lobes was estimated by using the base map of the earthflow ( f i g u r e 2.2) and the generalized contour map (fi g u r e 3.7). The method was s i m i l a r to that used to f i n d the volume estimate i n the source area: the area and the depth of the fan and the two lobes were estimated at several locations and l a t e r m u l t i p l i e d together to obtain the volume of a column of 40 debris. The volume of the debris fan and the contiguous lobes, then, found by summation of the volume estimates, i s approximately 6.1 x 10^ m3. Although the two values are the same order of magnitude, the ft 3 di f f e r e n c e of 1.0 x 10 m may be accounted f o r several ways. F i r s t , measurement error could cause a discrepancy between the r e s u l t i n g q u a n t i t i e s . This source of error could e a s i l y a r i s e from bias introduced i n the extrapolation of contours over the study area i n the construction of the pre-earthflow topography. Second, a discrepancy may also a r i s e from addition of material to the volume estimate of the debris fan that should a c t u a l l y be included i n the estimate of the source area volume. For example, the central tension zone was chosen at the boundary between the two regions. In r e a l i t y , however, the source area-main flow unit may extend further downslope. Third, the bulk density of the translocated earthflow material i s probably less than the bulk density of the undisturbed Cretaceous sedimentary deposits. This would cause the volume estimate for the debris fan to be larger than the volume estimate of the source area. 41 CHAPTER 4 CLIMATE AND SUBSURFACE HYDROLOGY AT PAVILION 4.1 INTRODUCTION The r e s u l t s from Bovis' (1980) study of earthflow movement at P a v i l i o n suggested that the high rate of motion observed from March mid-July was a t t r i b u t a b l e to high piezometric l e v e l s present i n the earthflow during that period. However, the piezometric l e v e l s were not related to basin hydrology. G i l and Kotarba (1977), Swanson and Swanston (1977), and Kelsey (1978) found that the regional p r e c i p i t a t i o n regime was c l o s e l y linked to earthflow movement. They noted that the v e l o c i t y of earthflow movement increased shortly a f t e r commencement of the winter rainy season, and Swanson and Swanston (1977), and Kelsey (1978) found that the regional p r e c i p i t a t i o n regime was c l o s e l y linked to earthflow movement. They noted that the v e l o c i t y of earthflow movement increased sh o r t l y a f t e r commencement of the winter rainy season, and Swanson and Swanston (1977) noted an increase of the flow rate commensurate with snowmelt. S i m i l a r l y , VanDine (1974, 1980) found that maximum displacement of the Drynoch landslide occurred within one month of peak discharge from snowmelt. VanDine surmised that there was no co n t r i b u t i o n to the groundwater flow system from r a i n f a l l because the estimated evapotranspiration f i g u r e was greater than p r e c i p i t a t i o n . 42 Hence, only snowmelt was considered by VanDine to contribute to groundwater recharge. In addition, peizometric data from Drynoch pre-dating VanDine's research indicated that increased groundwater l e v e l s correlated well with peak discharge form snowmelt. These r e s u l t s suggest an unmistakable moisture-dependence of earthflow-type motion, which accounts for seasonal v a r i a t i o n i n the rate of movement. Groundwater observations are important i n any slope movement study because of the ro l e of pore water pressure i n determining the e f f e c t i v e stress i n the s l i d e debris, and therefore the shearing resistance. If the pore water pressure within the landslide debris i s below the maximum possible value but i s s u f f i c i e n t to allow slope displacement, an increase i n pressure w i l l cause a c c e l e r a t i o n of the flow. I n f i l t r a t i o n of a s u f f i c i e n t volume of snowmelt, and runoff due to excess p r e c i p i t a t i o n during p a r t i c u l a r seasons w i l l cause an increase i n groundwater l e v e l . Piezometric data can then be related to slope movement. Thus, the seasonal v a r i a t i o n of earthflow movement may be explicable from the groundwater flow regime and, i n d i r e c t l y , from the seasonal climate v a r i a t i o n s . The primary objective of t h i s chapter i s to examine the r e l a t i o n s h i p between subsurface hydrology and climate at P a v i l i o n . To do t h i s , the po t e n t i a l groundwater recharge (or moisture surplus) must be estimated from p r e c i p i t a t i o n , temperature, and snowpack records for the P a v i l i o n area relevant to the present study. 43 4.2 APPROACH AND DATA SOURCES P r e c i p i t a t i o n and temperature records from September 1, 1980 to July 10, 1982 were obtained from two climate stations near P a v i l i o n , operated by B r i t i s h Columbia Hydro and Power Authority. The f i r s t i s located on P a v i l i o n Mountain (elev. 2115 m), 14.5 km northeast of the earthflow (fi g u r e 2.1); the second at the mouth of Upper Hat Creek Valley (elev. 864 m), 22 km southeast of P a v i l i o n on Highway 12. Topography no doubt influences the flow paths of various weather systems passing through the region; nevertheless, the B r i t i s h Columbia Hydro p r e c i p i t a t i o n data are assumed to represent p r e c i p i t a t i o n over the earthflow. The contribution from snowmelt on Mt. Cole was estimated by i n t e r p o l a t i o n of snow survey data from three s i t e s near the earthflow: P a v i l i o n Mountain (elev. 1,250 m), Harry Lake (1,340 m), and Cornwall H i l l s (2,000 m). R a i n f a l l on the earthflow was also measured for a short time (May 15, 1981 to November 14, 1981) i n f i v e storage gauges (locations i n f i g u r e 2.5). Groundwater observations were made i n six open standpipe piezometers i n s t a l l e d on the earthflow (locations i n f i g u r e 2.5). Three of the piezometers were 1 1/12-inch diameter PVC tubing and the remaining three were 1 1/2-inch ABS inclinometer tubing. The piezometer depths ranged from 10.47 m to 16.55 m. The d e t a i l s of each standpipe are summarized i n Table 4.1. 44 TABLE 4.1. Summary of the depth of piezometer t i p s below the l o c a l ground l e v e l . Borehole Depth (m) BH-1 10.47 BH-2 15.75 BH-3 16.55 BH-4 13.95 BH-5 13.72 BH-6 9.05 Water l e v e l s were measured r e g u l a r l y between March 22, 1981 and July 9, 1982 witii an e l e c t r i c water l e v e l sensor lowered into each standpipe. Borehole 1 (BH-1), located i n the source area, was sealed at the bottom to prevent i n t r u s i o n of muddy debris u n t i l mid-November 1981 when the cap was pierced, allowing water to enter the standpipe. Therefore, the record from BH-1 spans only the l a t t e r portion of the study. Continuous-recording f l o a t - a c t i v a t e d water l e v e l recorders were i n s t a l l e d i n BH-2, BH-4, and BH-5 i n November 1981. Each instrument was mounted on a covered platform and centered over the top of each pipe (f i g u r e 4.1). A f l o a t and counterweight were attached to opposite ends of a f i n e wire cable draped over a pulley wheel. The f l o a t s were very s e n s i t i v e to water l e v e l changes. 1.0mm wire Cable {P/ywtod hose for recorder housing F/Oar (b) FIGURE 4.1. (a) Schematic diagram of the f l o a t and counterweight inside the piezometer. (b) Photograph of the continuous, f l o a t - a c t i v a t e d recorder mounted on BH-5. 46 The record i s discontinuous from the date of i n s t a l l a t i o n u n t i l mid-February 1982. Apparently, the battery-operated clocks stopped when the winter temperatures f e l l below -20°C, and since the clocks needed to be restarted by hand, only the portion of the record obtained p r i o r to clock f a i l u r e i s a v a i l a b l e . To examine the r e l a t i o n s h i p between ponds and seeps on the earthflow and the subsurface flow regime, t h i r t e e n shallow peizometers were placed at s i x locations on the earthflow (fi g u r e 2.5). The d e t a i l s of these i n s t a l l a t i o n s are summarized i n Table 4.2. Two pairs (N4 and N5) were placed i n areas that were saturated during summer 1981; Nl, N2, and N6 were placed adjacent to Pond 2, Pond 3, and Pond 4, r e s p e c t i v e l y ; and N3 consisted of three piezometers traversing the c e n t r a l tension zone immediately north of Pond 3 ( f i g u r e 2.5). 4.3'CLIMATE RECORD Monthly p r e c i p i t a t i o n data from P a v i l i o n Mountain and Hat Creek are summarized i n fi g u r e 4.2. R a i n f a l l measured on the earthflow i s included. The data show that the seasonal d i s t r i b u t i o n of p r e c i p i t a t i o n near P a v i l i o n was v a r i a b l e over the 22 months of record. For example, the t o t a l p r e c i p i t a t i o n from September 1 to November 30, 1980 was 166 mm; during the same period i n 1981 only 43 mm f e l l . S i m i l a r l y , 62 mm of r a i n f e l l i n May 1981, while only 3 mm were recorded i n May 1982. 47 TABLE 4.2. at P a v i l i o n . D e t a i l s of the shallow piezometer nest i n s t a l l a t i o n s Piezometer nest Nl-A Nl-B N2-A N2-B N3-A N3-B N3-C N4-A N4-B N5-A N5-B N6-A N6-B Depth (m) 2.18 1.63 1.93 1.13 2.02 2.23 1.57 1.73 1.42 1.53 1.13 1.60 1.62 The data show that the t o t a l p r e c i p i t a t i o n at Hat Creek was s l i g h t l y lower than at P a v i l i o n Mountain. For example, the p r e c i p i t a t i o n at Hat Creek totaled 487 mm from September 1, 1980 to July 10, 1982; i n the same period 554 mm f e l l at P a v i l i o n Mountain. The average r a i n f a l l on the earthflow (figure 4.2) exceeded r a i n f a l l at Hat Creek twice and i t exceeded P a v i l i o n Mountain r a i n f a l l twice. Since runoff and groundwater recharge over Mt. Cole and i t s flanks undoubtedly contribute to groundwater flow i n the earthflow, the climate record from 48 I 80-70-60-SO-i 44-30-ZO-10-MOU/VTAlM ll HAT WEEK VALLE1 I PA V/L/ON £APTHFL0IV ll 0 N D J F M A M J J- A S 0 N D ? F M A M T T A 'WO mi I98Z 4 * FIGURE 4.2. P r e c i p i t a t i o n records from P a v i l i o n Mountain, Hat Creek Valley, and P a v i l i o n earthflow. P a v i l i o n Mountain i s considered most appropriate f o r c o r r e l a t i o n with piezometric f l u c t u a t i o n s i n the earthflow. The B r i t i s h Columbia Hydro climate data do not, however, separate snow from r a i n . Therefore, the amount of groundwater recharge contributed by snowmelt over Mt. Cole i n spring 1981 and spring 1982 was estimated from snowpack data from P a v i l i o n Mountain, Hat Creek, and Cornwall H i l l s . The snow survey data were used to construct a plot of snowpack depth ( i n mm of water) with elevation ( f i g u r e 4.3). Assuming that snow depth increases l i n e a r l y with el e v a t i o n , the snowpack 49 thickness at the summit of Mt. Cole (1,725 m) was estimated from figure 4.3. In 1981, snowpack depth increased l i n e a r l y with elevation. In 1982, though, s p a t i a l v a r i a b i l i t y of snowfall apparently resulted i n less accumulation of snow at Harry Lake than at P a v i l i o n Mountain, making i t necessary to i n t e r p o l a t e between these data points to a t t a i n the gradient up to the Cornwall H i l l s snow survey s t a t i o n . The estimated depths of snowpack for Mt. Cole a f t e r A p r i l 1, 1981 and May 15, 1982 (fig u r e 4.3) are regarded as maximum values because the snow at Harry Lake and P a v i l i o n Mountain had completely ablated by those dates. The depth of snowmelt from Mt. Cole was approximated by computing the amount by which the depth of snowpack had decreased during each month i n the spring. Figure 4.4 shows that about 26 mm (water equivalent) of net melt occurred between March 1 and May 1, 1981. By May 15, 1981, at least 26 mm of net accumulation took place, followed by ablation of 120 mm of the snowpack a f t e r May 15 and during June. Apparently, net ablation i n 1982 did not begin u n t i l a f t e r A p r i l 1, then 168 mm of melt occurred by the end of June. Since the mean d a i l y temperature remains below 0°C i n the period from November u n t i l March, the majority of the p r e c i p i t a t i o n i n t h i s period f a l l s as snow. Although the Mt. Cole water equivalent of snowpack i s not equal to p r e c i p i t a t i o n at P a v i l i o n Mountain from November to March, the snowpack depths estimated over Mt. Cole are assumed to account for p r e c i p i t a t i o n from November u n t i l the 50 /?dl ZOOO 1 /// -i 1 1 ? i 1 1 i i i i r 90 100 IZO 14-0 l(>0 180 200 ZZO 2-W 2(,0 ZSO 300 W.E SNOW (mm) (a) ZOOO -i ZO 40 60 80 ~WO ~lJo ~HC ~160 m> ZOO Z20 240 Z60 ZSO ~30O hi- £ SNOW (mm) (b) FIGURE 4.3. P l o t o f B r i t i s h C o l umbia snow survey d a t a from P a v i l i o n M o u n t a i n (1,250 m) , H a r r y Lake (1,350 m», and C o r n w a l l H i l l s (2,000 m) The snowpack depth over Mt. C o l e (1.725 m ) was e s t i m a t e d from the p l o t s f o r each w a t e r y e a r . The snow dep t h s a r e a l l e x p r e s s e d as wat e r e q u i v a l e n t v a l u e s . 51 (b) FIGURE 4.4. Estimated change of snowpack depth (water equivalent) for Mt. Cole from January 1 to June 30, i n 1981 and 1982. commencement of snowmelt—after March 1, 1981 and a f t e r A p r i l 1, 1982. Since the P a v i l i o n Mountain p r e c i p i t a t i o n record does not separate snow from r a i n , p r e c i p i t a t i o n i n the periods September 1 to October 31, 1981, and A p r i l 1 to July 10, 1982 i s considered to be r a i n f a l l . The groundwater recharge from snowmelt, then, w i l l be estimated from the snowpack figures found f o r Mt. Cole and p r e c i p i t a t i o n measured at P a v i l i o n Mountain w i l l be used to estimate recharge from r a i n f a l l . It i s important to r e a l i z e that s p a t i a l v a r i a b i l i t y of p r e c i p i t a t i o n may influence the transposition of P a v i l i o n Mountain observations and the B r i t i s h Columbia snow survey data to Mt. Cole. It 52 was noted, f or example, that many summer storm c e l l s which pass over P a v i l i o n Mountain miss Mt. Cole and the earthflow e n t i r e l y . Nevertheless, the B r i t i s h Columbia Hydro climate record and the snow survey data are the most r e l i a b l e data sources at present. Daily temperature data from P a v i l i o n Mountain were used to ca l c u l a t e the mean monthly temperatures for the duration of the present study (Table 4.3). The mean monthly temperature values were then used to estimate the pot e n t i a l evapotranspiration by a p p l i c a t i o n of the Thornthwaite method (described by Dunne and Leopold, 1978, pp. 136-137): Er> — 1.6 !0 ^ (4.1a) where E j i s the pot e n t i a l evapotranspiration i n cm/month, T a i s the mean monthly a i r temperature i n degrees C e l s i u s , I i s the annual heat index and i s a function of the mean annual temperature, and a = 0.49 + 0.01791 + 0.0000771I2+ 0.0000006I 3. (4.1b) Evapotranspiration estimates were not computed for the periods November 1980 to A p r i l 1981 and November 1981 to A p r i l 1982 when the mean monthly temperature was less than 0°C. In addition, temperature data were not a v a i l a b l e for s i x months of the study, therefore only eight values of potential evapotranspiration were computed (Table 4.3). 53 TABLE 4.3. Mean monthly temperatures and po t e n t i a l evapotranspiration estimates computed from P a v i l i o n Mountain temperature data. MEAN MONTHLY POTENTIAL MONTH TEMPERATURE (°C) EVAPOTRANSPIRATION (mm) 9/80 10.2 64.0 10/80 4.7 20.7 11/80 -0.2 X * * 12/80 -1.1 X 1/81 2.1 0.9 2/81 -3.0 X 3/81 * — 4/81 -0.1 X 5/81 — — 6/81 — — 7/81 — — 8/81 9.7 68.5 9/81 7.9 46.5 10/81 2.2 8.0 11/81 -3.1 X 12/61 -6.1 X 1/82 -7.7 X 2/82 -7.7 X 3/82 -2.7 X 4/82 2.7 X 5/82 3.9 23.3 6/82 10.5 84.5 7/82 — — *No data **Erj, was n e g l i g i b l e because the mean monthly a i r temperature was below 0°C. 54 The p o t e n t i a l evapotranspiration values are used together with the r a i n f a l l measured at P a v i l i o n Mountain and with estimated snowmelt depths from Mt. Cole to approximate the monthly groundwater recharge to the earthflow (Table 4.4). No temperature data were a v a i l a b l e f o r the period May to July 1981, hence no r e l i a b l e estimates of groundwater recharge could be found for the f i r s t water year. A continuous record of temperature, however, was av a i l a b l e for the period August 1, 1981 to June 30, 1982. The estimated groundwater recharge values for th i s period are summarized i n Table 4.4. The t o t a l recharge from snowmelt over Mt. Cole was about 97.7 mm and occurred i n A p r i l and May 1982. Snowmelt i n A p r i l and May then, contributed 82% of the t o t a l recharge i n the period August 1981 to June 1982. The recharge from r a i n f a l l was about 21 mm, with 15 mm of that occurring i n A p r i l 1982. Mt. Cole snow p e r s i s t s through June, therefore r a i n f a l l from A p r i l to June f a l l s on snow i n the upper part of the basin. As a r e s u l t , the evapotranspiration and recharge estimates for t h i s period may not be those calculated from the P a v i l i o n mountain record; rather, the actual spring recharge would be s u b s t a n t i a l l y greater at Mt. Cole. Since about 76% of the t o t a l recharge i n the second water year occurred between A p r i l and May 1982 the piezometric l e v e l s are expected to be maximum i n thi s period or shortly a f t e r . Furthermore, the rate of earthflow motion should also be high late i n spring. 55 TABLE 4.4. Estimated values of groundwater recharge f or the period August 1, 1981 to June 30, 1982. A l l values are i n mm. Aug Sept Oct Nov Dec Jan Feb Mar Apr May Jun Total Recharge from r a i n f a l l * 0 0 6.0 - - - - - 15.0 - 0 21.0 Recharge from snowmelt** — — — 0 0 u 0 22.0 27.0 48.7 0 97.7 Total recharge 0 0 6.0 0 0 0 0 22.0 42.0 48.7 0 118.7 Percentage of annual t o t a l (water year) 0 0 5 0 0 0 0 19 35 41 0 100 *Estimated from P a v i l i o n Mountain p r e c i p i t a t i o n f o r the period August to October 1981 and for the period A p r i l 1 to June 30, 1982. **Estimated from snowpack values for Mt. Cole f or the period January to June 1982. 4.4 BASIC OPERATION OF A STANDPIPE PIEZOMETER A standpipe piezometer i s a simple device used for groundwater observation. The apparatus i s a tube placed i n the ground with i t s t i p below the groundwater table (figure 4.5). The tube must be sealed along i t s entire length because the point of measurement, P, i s at the base. The standpipe should be open to water flow only at the bottom, and open to the atmosphere at the top (Freeze and Cherry, 1979, p. 23). The parameter measured i s the f l u i d p o t e n t i a l <J> (energy per unit mass) which i s the hydraulic head h (energy per unit weight) m u l t i p l i e d by the acceleration due to gravity (Freeze and Cherry, 1979, p. 21): Flow always occurs from regions where h has higher values to regions components (fig u r e 4.5): ( i ) the elevation head z, which i s the elevation of the base of the piezometer above a s p e c i f i e d datum (e.g. sea l e v e l ) , and ( i i ) the pressure head f , which i s the height of the water column i n the standpipe above point P. Hydraulic head can therefore be written as: (J = gh (4.2) where i t i s lower (figure 4.6). Hydraulic head i s the sum of two h = z + 4» (4.3) 57 Point of measurement T .0477///, FIGURE 4.5. Schematic diagram demonstrating the use of a standpipe piezometer for measuring hydraulic head h and i t s two components. The pressure head H* i s the height of the water column i n the standpipe and the elevation head z i s the elevation of the piezometer t i p above the datum (z = 0). 5-1 FIGURE 4.6. Diagram showing the use of a piezometer to measure hydraulic head i n a h i l l s l o p e where the groundwater table i s below the ground surface. The piezometer of length d measures the pore water pressure at P. Since P i s located on the equipotential where h = 2.8, the height of the water i n the piezometer i s equal to 2.8 on the v e r t i c a l scale to the l e f t of the diagram. 58 The dimension of head i s length [ L ] , and for the present study i t i s expressed as "centimeters of water" or "meters of water." Hydraulic head i s determined by measuring the elevation of the water l e v e l i n a piezometer and c a l c u l a t i n g h from equation 4.3. By d e f i n i t i o n , then, the hydraulic head i s the f l u i d pressure at the piezemeter t i p P and i s not the elevation of the groundwater table. The groundwater table, or phreatic surface, however, i s a surface of v a r i a b l e head (fig u r e 4.6) over which the pressure head f i s equal to atmospheric pressure. The operation of a standpipe piezometer for observing groundwater behavior i n a simple h i l l s l o p e i s i l l u s t r a t e d i n fi g u r e 4.6. The flow net i s drawn so that the equipotential l i n e s ( l i n e s of constant head) are dotted and the flow l i n e s , which indi c a t e the d i r e c t i o n of groundwater flow, are orthogonal to the equipotential l i n e s . The diagram shows that flow occurs from the high elevations of the h i l l s l o p e (regions of high hydraulic head) toward the lower elevations (regions of low head). If a piezometer i s placed i n the h i l l s i d e to a depth d (figure 4.6), the hydraulic p o t e n t i a l at point P w i l l cause the column of water to r i s e to the elevation equal to the value of hydraulic head at P. 59 4.5 GROUNDWATER FLOW AND HYDRAULIC CONDUCTIVITY The rate of groundwater flow can be found from Darcy's law: v d h v = K — dl (4.4) where v i s the flow rate, K i s the hydraulic conductivity, and dh/dl i s defined as the hydraulic gradient, the differe n c e i n head (dh) between two points of measurement divided by distance, 1, measured normal to conductivity K i s a v e l o c i t y term that expresses the rate at which a f l u i d can t r a v e l through a porous medium; therefore i t has dimensions of v e l o c i t y ( [ L / T ] ) . The conductivity of the earthflow debris was estimated i n s i t u by i n j e c t i n g a slug of water i n t o each piezometer. Two head measurements were then used to estimate the time required f o r the water l e v e l to completely recover the head measured p r i o r to slug i n j e c t i o n , assuming that the o r i g i n a l rate of inflow was maintained. The hydraulic conductivity was then estimated from Hvorslev's (1951) equation: where L i s the length of the piezometer intake, R i s the radius of the intake, r i s the radius of the piezometer, and T q i s the time required for f u l l water l e v e l recovery. In t h i s case, the piezometer intake was equipotential l i n e s passing through the points. The hydraulic K = r 2 l n (L/R) 2LT o (4.5) 60 simply the base of the tube, hence L was considered to be 1.0 mm, and r was equal to R whose value was 1.9 cm. The r e s u l t i n g hydraulic conductivity values from the earthflow are summarized i n Table 4.6. The boreholes not shown were s i t e s where no change of head occurred i n the observation period following slug i n j e c t i o n . At these s i t e s i t i s p l a u s i b l e that the conductivity i s s l i g h t l y less than the values l i s t e d , perhaps from a greater concentration of f i n e p a r t i c l e s within the earthflow debris. The range of hydraulic conductivity values found for the earthflow i s s i m i l a r to the range of values for s i l t and s i l t y - s a n d deposits. Thus the measured values from P a v i l i o n earthflow seem reasonable according to the grain s i z e d i s t r i b u t i o n of the material (fi g u r e 3.6). The presence of f i s s u r e s within the earthflow w i l l also a f f e c t the value of the bulk hydraulic conductivity. Fissures provide conduits for e f f i c i e n t groundwater flow to occur thus increasing the bulk conductivity of the material. In the source area and central tension zone, then, i t i s possible that the hydraulic conductivity of the earthflow debris i s greater than the measured values summarized i n Table 4.5. 4.6 PIEZOMETRIC OBSERVATIONS The periodic measurements of hydraulic head obtained from the piezometers on the earthflow are used for determination of the seasonal pattern of groundwater l e v e l change at P a v i l i o n . The absolute water l e v e l measurements for the duration of the study are summarized i n Table 61 TABLE 4.5. Hydraulic conductivity values of the earthflow debris obtained from i n s i t u measurements. Borehole K (m/sec) BH-3 1.5 x IO" 8 BH-4 1.6 x 10" 8 BH-5 1.6 x 10~ 7 Nl-A 1.5 x 10" 7 N2-A 2.9 x 10"° N2-B 2.4 x 10~ 8 N3-B 5.8 x 10" 8 N3-C 2.1 x I O - 8 N4-A 1.1 x 10" 7 N4-B 4.1 x 10" 8 -8 N5-B 4.6 x 10 62 TABLE 4 . 6 . Summary of the measured values of pressure head from the earthflow. P r e s s u r e h e a d (m) D a t e BH-1 B H - 2 B H - 3 BH-4 B H - 5 B H - 6 2 1 / 3 / 8 1 - 8 . 3 2 1 3 . 7 8 7 . 0 6 7 . 2 5 1 .51 2 2 / 5 / 8 1 - 8 . 10 1 3 . 6 9 8 . 0 0 7 . 0 9 1 . 4 3 11/6/81 - - 1 3 . 8 8 8 . 3 1 7 . 0 7 1 . 4 2 16/6/81 - 8 . 14 - - - -3 / 7 / 8 1 - 8 . 10 1 3 . 7 7 8 . 8 1 7 . 1 3 1 . 5 0 2 0 / 7 / 8 1 - 8 . 10 1 3 . 7 7 9 . 13 7 . 1 2 1 .51 3 / 1 0 / 8 1 - 7 . 8 7 1 3 . 7 5 9 . 6 3 6 . 9 2 1 .47 16/1 1/81 - 8 . 3 4 1 4 . 6 2 1 0 . 5 0 7 . 0 9 2 . 3 0 1 3 / 2 / 8 2 - 8 . 1 2 1 4 . 6 0 1 0 . 6 1 6 . 9 1 2 . 3 7 1 5 / 5 / 8 2 0 . 6 4 8 . 0 6 1 4 . 6 1 1 0 . 5 3 7 . 1 8 2 . 3 2 1 / 6 / 8 2 - - - - 7 . 2 4 -9 / 7 / 8 2 0 . 9 4 8 . 0 8 1 4 . 7 4 1 0 . 7 1 7 . 2 2 2 . 6 0 63 4.6. The piezometric f l u c t u a t i o n s measured at BH-4 from mid-March to early October 1981 are anomalous compared with the hydraulic head changes observed at BH-2, BH-3, BH-5, and BH-6. For example, while a net decrease of the piezometric l e v e l s occurred i n the l a t t e r four boreholes between March 21 and October 3, 1981, the head i n BH-4 increased s t e a d i l y by 3.34 m. The continuous r i s e of hydraulic head at BH-4 during the f i r s t seven months of the study i s probably a t t r i b u t a b l e to a fracture along a portion of the length of the piezometer. Assuming that t h i s was the case, the point of measurement was not at the piezometer t i p . Instead, water would seep i n t o the tube through the crack u n t i l the water l e v e l i n the standpipe became equal to the head at the top of the flaw. Thus, the top of the fracture would be the point of measurement. This i s the most l i k e l y explanation for the anomalous water l e v e l r i s e observed i n BH-4 p r i o r to October 1981. In October 1981, the behavior of the water l e v e l s i n BH-4 became more uniform with the l e v e l s observed at the other s i t e s . The absolute changes of hydraulic head measured at BH-2, BH-3, BH-5, and BH-6 for the duration of the study are plotted against time i n f i g u r e 4.7. The data from BH-4 are plotted for the period October 3, 1981 to July 9, 1982 when these measurements were more s i m i l a r to the measurements from the other boreholes on the earthflow. The measurements from BH-1 are plotted f or the period May 15 to July 9, 1982—the only two measurements av a i l a b l e from t h i s s i t e . 64 M A M J J - A S O N b J F M A M J T A 1181 MB2. FIGURE 4.7. Plot of the piezometric l e v e l f l u c t u a t i o n s against time. The data from BH-2, BH-3, BH-5, and BH-6 are shown for the duration of the study. The data plotted f o r BH-4 are the only points considered r e l i a b l e from the piezometer, while the data from the period preceding October 1981 are considered anomalous possibly due to a crack i n the standpipe. The data f o r BH-1 are the only two points measured at th i s s i t e . 65 In the period from mid-March to mid-November 1981, the trend of groundwater f l u c t u a t i o n s was s i m i l a r at BH-2, BH-3, BH-5, and BH-6, although they were out of phase from mid-May to mid-July. Apparently the small peak observed at BH-2 and BH-3 (both on the main flow, figure 2.5) i n the middle of June did not occur at BH-5 and BH-6 (on the west lobe) u n t i l early July ( f i g u r e 4.7). It can be i n f e r r e d from t h i s that groundwater flow occurs through the earthflow i n a wave-like form. An increase i n groundwater l e v e l at an upslope s i t e w i l l not be registered downslope u n t i l the "wave" has propagated through the earthflow. Contrary to t h i s pattern was the marked increase of water l e v e l s i n a l l the piezometers except BH-5 between October 3 and Nobemver 16, 1981, which appears to be i n phase everywhere (fig u r e 4.7). There may have been a s i m i l a r wave-like propagation of groundwater through the earthflow i n the f a l l but the time r e s o l u t i o n of the data c o l l e c t i o n at t h i s time did not permit observation of the character of the water l e v e l r i s e . A f t e r November 16, 1981 the pattern of groundwater f l u c t u a t i o n s was v a r i a b l e over the earthflow. For example, at BH-2 the water l e v e l f e l l 22 cm between mid-November and February 1982; a f t e r February 13, 1982 a net decrease of only 4 cm was noted. At BH-3 the head decreased 2 cm from November 16, 1981 to February 13, 1982. At BH-5 the water l e v e l f e l l 9 cm from November 16 to the end of December 1981 and rose 5 cm by mid-February 1982. Between mid-February and July 10, 1982, then, 66 the water l e v e l s i n BH-3 and BH-5 rose 14 cm and 41 cm, r e s p e c t i v e l y . The continuous record from BH-5 ( f i g u r e 4.8) shows that the water l e v e l increased 12 cm between May 15 and May 22, 1982, then the piezometric l e v e l s rose 4 cm from May 23 to July 10. A net increase of hydraulic head was also observed i n BH-4 and BH-6 i n the period November 16, 1981 to July 10, 1982. At BH-4 the head increased 38 cm and the r i s e appears to have been steady but no continuous record i s a v a i l a b l e f o r the e n t i r e period to v e r i f y t h i s assertion ( f i g u r e 4.8). The water l e v e l i n BH-6 rose 13 cm between November 1981 and February 1982, followed by a 5 cm drop noted on May 15, 1982 (figure 4.7). The head i n BH-6 then increased 28 cm between May 15 and July 10, 1982. In the l a t t e r i n t e r v a l the head i n BH-1 also increased 30 cm. The data from the shallow piezometers are summarized i n f i g u r e 4.9. The piezometric l e v e l at N2-B was lower than the piezometer t i p by October 3, 1981. The cross section ( f i g u r e 4.9b) shows that the t i p of N2-A was located i n the red-brown debris, while the t i p of N2-B was more shallow and i t s t i p was i n the reworked t i l l . Thus, the lower water l e v e l i n N2-B appears to c o r r e l a t e with the time of drawdown of the surface of Pond 3 l a t e i n summer 1981. The depth of N2-A was s u f f i c i e n t to continue monitoring subsurface water l e v e l s i n the earthflow debris, and i n October 1981 the water l e v e l i n N2-A was high r e l a t i v e to previous measurements made i n the summer (figure 4.9b). By November 16, 1981, however, the t i p of N2-A was also r e s t i n g i n dry debris, 67 1 ^ 30.0-\ ^ zo.o 5 m o -% „] vj o.0A I -30.0, I5fl B H - S ^ No conHnuous record Mailable //fl/ ///2 //l/8Z //2- 1/4 —r -//5 i r FIGURE 4.8. The continuous records from the f l o a t - a c t i v a t e d continuous water l e v e l recorders at BH-2, BH-4, and BH-5. No continuous record i s available from the intervals when the clocks stopped i n the winter, but the magnitude of the change that occurred i n these i n t e r v a l s could be determined from the charts because an ink trace was made when a change occurred, although the chart remained stationary. STRATIGRAPHY NS-4 115-6 1(0 HI N6-A (f) A/6-3 N6-A N6-S FIGURE 4.9- Water l e v e l f l u c t u a t i o n s measured at the shallow piezometer s i t e s from June 6. 1981 to J u l y 7, 1982. The s i t e s are (a) N l , (b) N2, (c) N3, (d ) , N4, (e) N5, and ( f ) N6. i n d i c a t i n g a profound decrease of the near-surface groundwater l e v e l elevation between early October and mid-November 1981. During i n s t a l l a t i o n of N3 i n l a t e June 1981, groundwater discharge was occurring through the central scarp face and there was standing water at N3-B, N3-C, and downslope at N4 ( f i g u r e 2.5). The surface at these s i t e s was dry i n May and July 1981, although the water l e v e l s i n N3-A, N3-B, and N3-C were greater than the 1981 values ( f i g u r e 4.9c). The increased water l e v e l s at N3 i n 1982 suggest a r i s e of the groundwater table elevation around Pond 3 despite the lower elevation of the pond surface. It was noted while augering the hole for N3-A that the grey reworked t i l l was saturated (about 1.0 m i n thickness) while the underlying red debris was unsaturated. This suggests that Pond 3 i s l o c a l l y perched i n a poorly drained depression. The lowered elevation of the surface of Pond 3 could be explained, then, by drainage through the base of the translocated t i l l i n t o the underlying red debris. This, i n turn, would cause the water table i n the red debris to r i s e . An elevated groundwater table due to recharge from the base of Pond 3 may p a r t i a l l y account for the marked increase of head i n BH-3 and BH-4 between October and November 1981. The two piezometers at N5 were also dry on October 3, 1981. These were located at the north end of the major discharge zone on the west lobe ( f i g u r e 2.5). In June 1981, surface runoff from groundwater discharge was abundant at t h i s s i t e , but i n May and July 1982 the s i t e 70 was dry. The water l e v e l s measured i n June, October, and November 1981 indi c a t e groundwater recharge. Thus, discharge and runoff from upslope on the west lobe were i n f i l t r a t i n g the earthflow surface to recharge the subsurface flow regime. This, too, may p a r t i a l l y account for the r i s i n g groundwater l e v e l s observed i n BH-5 and BH-6 lat e i n 1981. The increase i n BH-5, however, did not become as marked as the water l e v e l r i s e noted at the other boreholes u n t i l February 1982 (figu r e 4.7). 4.7 RELATIONSHIP BETWEEN CLIMATE AND GROUNDWATER LEVELS AT PAVILION The f i r s t event of groundwater l e v e l r i s e during the study occurred between mid-May and mid-June 1981 at BH-2 and BH-3 (figure 4.7). This was followed by a peak of the hydraulic head at BH-5 and BH-6 between mid-June and early July. These peaks i n the piezometric record are believed to be associated with groundwater recharge from e a r l y , or low-elevation, snowmelt and spring r a i n f a l l . Snowmelt up to at least 1350 m was complete by A p r i l 1, 1981 (figu r e 4.3) and r a i n f a l l i n May was very high (fi g u r e 4.2). Since the l e v e l of the e a r l i e s t snowmelt was not f a r above the elevation of the earthflow i t s e l f , recharge probably occurred r a p i d l y through tension cracks and f i s s u r e s i n the debris surrounding the source area. In May 1981, heavy r a i n f a l l over the earthflow and i t s surrounding slopes would also tend to penetrate the flow r a p i d l y v i a f i s s u r e s and enhance the e f f e c t of the pulse of groundwater recharge from early snowmelt with l i t t l e delay i n piezometric response. 71 The marked r i s e of groundwater l e v e l s observed i n November 1981 at BH-2, BH-3, BH-4, and BH-6 (fig u r e 4.7) probably represents groundwater recharge caused by snowmelt and p r e c i p i t a t i o n over Mt. Cole for the period May to July 1981. If i t i s assumed that groundwater flows p a r a l l e l to the ground surface from Mt. Cole to the earthflow, the hydraulic gradient can be taken to be equal to the average surface gradient over the distance, which i s 0.136. Groundwater recharge at Mt. Cole must pass i n t o the P a v i l i o n Group rocks from the overlying g l a c i a l t i l l and then be transported through the P a v i l i o n Group to the elevation range of the earthflow. Knowing the hydraulic gradient and the approximate time when each of the piezometers i n the earthflow respond to recharge from Mt. Cole, the hydraulic conductivity of the P a v i l i o n Group rocks can be estimated by backcalculation: K = v ( d h / d l ) " 1 = 4 , 1 3 0 m (0.136)" 1 = 2.3 x 10 _ 3m/sec (4.6) 5 mo. The distance used to estimate the v e l o c i t y v of groundwater transport to the standpipes i s the average distance from Mt. Cole to the boreholes i n the earthflow. The r e s u l t i n g hydraulic conductivity, 2.3 x 10 m/sec, i s reasonable for densely fractured bedrock. An element of piezometric response not accounted for i n equation 4.6 i s that the t i p s of the piezometers do not penetrate the f a i l u r e plane. Hence no increase of the hydraulic head can be observed i n the standpipes u n t i l s u f f i c i e n t time has elapsed to allow the impulse from 72 spring snowmelt and p r e c i p i t a t i o n i n early summer to flow into the earthflow debris from below and e q u i l i b r a t e i n the piezometers. The time lag for each standpipe has been estimated from the slug test data that were used to estimate hydraulic conductivity. The hydrostatic time lag estimates are given i n Table 4.7, these were computed by the following equation provided by Hvorslev (1951): T = — - (4.7) ln(H D/H) where T i s equal to T Q i n equation 4.5 and i t i s the time required for equali z a t i o n of a pressure d i f f e r e n c e between the standpipe and the surrounding debris when the rate of intake remains constant. The v a r i a b l e t i s the length of the observation period for the slug i n j e c t i o n at time t=0, and H i s the water l e v e l at time t a f t e r slug i n j e c t i o n . The time lags over the earthflow vary according to the v a r i a b i l i t y of the hydraulic conductivity values. The shorter hydrostatic time lags probably represent s i t e s where the piezometer rests within densely f i s s u r e d debris. This would permit more rapid piezometric response to a change of the external pore water pressure. The data for the period mid-May to early July 1982 show another groundwater l e v e l r i s e everywhere, except BH-2 where the l e v e l remained r e l a t i v e l y uniform. As surmised for the elevated piezometric l e v e l s observed i n t h i s period i n 1981, the r i s i n g water l e v e l s e arly i n summer 1982 are a t t r i b u t e d to low-elevation snowmelt during A p r i l and May 1982. 73 TABLE 4.7 The basic hydrostatic time lags computed from the slug test data used to estimate the hydraulic conductivity of the earthflow debris at P a v i l i o n . Borehole BH-3 BH-4 BH-5 Nl-A Nl-B N2-A N2-B N3-B N3-C N4-A N4-B N5-B Time lag (days) 92 82 8 9 22 46 57 23 65 13 33 29 74 4.8 SUMMARY Analysis of the seasonal occurrence of groundwater recharge for the period August 1981 to June 1982 indicated that snowmelt i n A p r i l and May 1982 accounted for 64% of the t o t a l groundwater recharge estimated for the second water year, while 13% of the net recharge was derived from r a i n f a l l i n A p r i l 1982. In the second water year, then, about 77% of the t o t a l annual recharge estimates was contributed i n A p r i l and May 1982. C o r r e l a t i o n of the moisture budget with the piezometric f l u c t u a t i o n s showed that the two time seri e s are out of phase. There were two events of groundwater l e v e l r i s e i n 1981, each believed a t t r i b u t a b l e to climate input. The f i r s t event, observed from May to July 1981, was a t t r i b u t e d to low-elevation snowmelt and r a i n f a l l d i r e c t l y over the earthflow i t s e l f and over i t s surrounding slopes. As a r e s u l t of tension cracks and f i s s u r e s near the earthflow surface, the groundwater l e v e l s i n the earthflow appear to respond to the early snowmelt and spring p r e c i p i t a t i o n with l i t t l e delay. However, the standpipes i n the earthflow apparently do not show increased water l e v e l s from recharge transported from Mt. Cole u n t i l about f i v e months have elapsed. The delay a r i s e s from the combination of transport time from Mt. Cole v i a fractured bedrock to the earthflow, slow upward flow of groundwater from the basal f a i l u r e zone in t o the piezometers, and the hydrostatic time lag for pressure equalization within the standpipes. 75 l e v e l s from recharge transported from Mt. Cole u n t i l about f i v e months have elapsed. The delay arises from the combination of transport time from Mt. Cole v i a fractured bedrock to the earthflow, slow upward flow of groundwater from the basal f a i l u r e zone into the piezometers, and the hydrostatic time lag for pressure e q u a l i z a t i o n within the standpipes. Since earthflow motion i s a moisture-dependent process i t i s l i k e l y that the earthflow accelerates synchronous with increased pore water pressures along the f a i l u r e plane. Therefore, i t may be possible to corroborate the assertion that the delay between groundwater input over Mt. Cole and piezometer response does r e f l e c t v e r t i c a l t r a v e l time through the earthflow from i t s basal surface and the hydrostatic time lag. This w i l l be examined i n the following chapter a f t e r the character of earthflow movement at P a v i l i o n has been discussed. 76 CHAPTER 5 SLOPE MOVEMENT AT PAVILION 5.1 INTRODUCTION Bovis (1980) suggested that high rates of earthflow movement observed during spring and early summer may be a t t r i b u t a b l e to high piezometric l e v e l s r e s u l t i n g from snowmelt and spring r a i n f a l l . This i s not unreasonable because several studies of earthflow movement (VanDine, 1974, 1980; G i l and Kotarba, 1977; Swanson and Swanston, 1977; and Kelsey, 1978) have shown that the rate of movement i s dependent on the amount of moisture a v a i l a b l e to the groundwater flow system. The hydrologic data required to q u a l i f y Bovis' (1980) assertion, however, were i n s u f f i c i e n t at that time to investigate the r e l a t i o n s h i p between slope movement and groundwater l e v e l f l u c t u a t i o n s . The piezometric data obtained during the current research, on the other hand, are considered to be adequate f o r c o r r e l a t i o n with movement. In t h i s chapter, slope movement data are examined so that the s p a t i a l and seasonal pattern of earthflow motion can be ascertained and the seasonal character of movement i s correlated with the piezometric data. The l a t t e r w i l l suggest connections between the rate of slope displacement and changes i n the e f f e c t i v e normal s t r e s s . i 77 VanDine (1980, pp. 32-33) presented groundwater l e v e l and movement data from the Drynoch earthflow for the period June 1957 to June 1960. These records show that earthflow acceleration at Drynoch commenced concurrent with the r i s e of groundwater l e v e l s usually between December and February, depending on the l o c a t i o n of measurement. Accelerated motion then continued at a r e l a t i v e l y constant rate u n t i l June at three s i t e s and u n t i l September at one s i t e , although the piezometric l e v e l s had begun to f a l l at least one month p r i o r to earthflow deceleration. The data show that the Drynoch earthflow accelerates i n d i r e c t response to increased pore water pressure along the basal f a i l u r e plane. The maintainence of rapid flow while the groundwater l e v e l s were f a l l i n g suggests that the moisture contents of the c l a y - r i c h earthflow debris remained s u f f i c i e n t to maintain an increased unit weight of the debris and, therefore, r e t a i n the shear stress that was acquired with r i s i n g groundwater l e v e l s . Peak discharge i n the Thompson River, apparently due to snowmelt (VanDine, 1980) occurred from May to July, and discharge remained r e l a t i v e l y high u n t i l August as .a r e s u l t of summer r a i n f a l l . The timing of the groundwater l e v e l increase and earthflow acceleration i n winter r e l a t i v e to peak discharge i n summer suggests at least a four-to five-month delay between groundwater recharge to the regional flow regime and earthflow response. This may be accounted for by recharge to a regional system underlying the Drynoch earthflow whose recharge area i s located upslope a considerable distance from Drynoch. Swanson and Swanston (1977) and Kelsey (1978) also found that 78 seasonally large displacements of earthflows i n western Oregon and northern C a l i f o r n i a , r e s p e c t i v e l y , were caused by large amounts of moisture contributed seasonally to the earthflow. In both cases, increased flow rate did not occur concurrent with commencement of the wet season. Rather, a few days following the onset of the wet season groundwater recharge had occurred through the flow and raised the s o i l moisture contents s u f f i c i e n t l y to cause a reduction of the shearing resistance of the earthflow debris and subsequent earthflow acc e l e r a t i o n . Crandell and Varnes (1961) studied the Slumgullion earthflow near Lake C i t y , Colorado f o r several years and found that the rate of movement was invariant throughout the year, regardless of the v a r i a b i l i t y of the moisture input. They suggested that the f l u i d pressure along the f a i l u r e plane had attained i t s maximum possible value and neither dissipated nor increased i n response to seasonally v a r i a b l e moisture regimes. Hence, Crandell and Varnes show that seasonal f l u c t a t i o n s i n the rate of earthflow motion are not u n i v e r s a l . But they do v e r i f y that the nature of the groundwater flow system ( i n conduction with the debris character) determines earthflow behavior. 5.2 METHOD OF OBSERVATION The pattern of earthflow movement at P a v i l i o n was determined through periodic measurement of t h i r t y - t h r e e stake arrays d i s t r i b u t e d around the earthflow perimeter (locations i n figu r e 2.2). This allows examination of the s p a t i a l and temporal v a r i a b i l i t y of movement. Twenty-seven stake arrays along the l a t e r a l shear zones were used to monitor l a t e r a l displacements. At each l o c a t i o n four wooden stakes were arranged as a s t r a i n net, with one pair serving as a baseline for computing displacements (fi g u r e 5.1). Each of s i x dimensions was measured repeatedly by extending a s t e e l surveying tape between the six possible pairs of pegs i n the s t r a i n net. . These data were used to c a l c u l a t e the gross displacement during successive measurement periods. Two values of movement i n the x - d i r e c t i o n ( f i g u r e 5.1) and two values i n the y - d i r e c t i o n could be found by applying the law of cosines and the law of sines, r e s p e c t i v e l y . The d e t a i l s of the data reduction method are provided i n Appendix I I . Usually the two values of displacements computed for the x - d i r e c t i o n were not i d e n t i c a l which suggests that the earthflow does not move as a r i g i d body ( i . e . without deformation) on the scale of an i n d i v i d u a l stake array. As a r e s u l t there could be movement between any of the two stake pairs within the s t r a i n net which i s not accounted for i n the f i n a l movement f i g u r e s . In most cases the average of the two values of displacement i n the x - d i r e c t i o n was taken to represent the net displacement for a given time i n t e r v a l . There were, however, a few s i t u a t i o n s where t h i s could not be done. At WL3 ( f i g u r e 2.2), for example, the calculated values of movement i n the x-d i r e c t i o n departed s u b s t a n t i a l l y from one another so that the quantity judged to be the most reasonable with respect to r e s u l t s from the 80 £ n _ > T 3 ! \ . . / ' j EARTHFLOW ! . / X I SHEAR ZONE /«& 3ASELINE ^ _ \ / ' • 0EP05IT (STABLE) BS FIGURE 5.1. Schematic diagram of the s t r a i n nets used at P a v i l i o n for monitoring earthflow movement. adjacent stake arrays was believed to represent the net displacement. At some of the other s i t e s (WR1, ELO, and ERO, fi g u r e 2.2) one of the baseline stakes became unusable for r e l i a b l e measurement and the amount of displacement could not be computed. Six of the stake arrays monitoring downslope movement had a f i f t h stake located on the abandoned l a t e r a l deposits f l a n k i n g the presently active shear zone (fig u r e 5.1), the purpose being to check for movement of the baseline r e l a t i v e to the ancient l a t e r a l s . In the source area, four sets of stakes were extended between slump blocks and across tension cracks (fi g u r e 5.2) to monitor the rate of separation of the flow from the source area. In addition, 81 d i f f e r e n t i a l movement between blocks of debris was determined. Three continuous e a r t h s l i d e recorders were added to the stake network i n November 1981. The i n s t a l l a t i o n of the recorders i s shown i n fig u r e 5.3. The recorders were located i n the source area of WUT-4 (figures 2.2 and 5.2), adjacent to WR8 on the east l a t e r a l shear zone of the main flow, and between WR2 and WR3 on the east l a t e r a l shear zone of the west lobe. The recorders were l a b e l l e d ESR-1, ESR-2, and ESR-3, res p e c t i v e l y . The clocks on the recorders were battery-operated, and, unfortunately, they stopped due to exposure to the very cold winter temperatures (le s s than -20°C). Therefore the continuous record from these s i t e s i s l i m i t e d . 5.3 MOVEMENT IN THE SOURCE AREA From Table 5.1, i t can be seen that d i f f e r e n t i a l movement between blocks of debris at WUT-3 (figur e 5.2) was su b s t a n t i a l . Total displacement, for example, ranged from -0.8 cm to 5.8 cm for the period of study. Displacement i n the negative d i r e c t i o n observed between stakes B and C (Table 5.1) represents either backward r o t a t i o n of the slump block or closure of the tension crack over which they were extended. Negative displacements was also observed at the north side of WUT-4 (fi g u r e 5.2, Table 5.1), where -4.4 cm of movement was measured. This suggests that the lower block i s r e l a t i v e l y stable while the upslope block i s progressing slowly downslope ( i . e . retrogression). 82 /'1,250 Contour Interval- 5m FIGURE 5.2. Enlargement of the source area showing the arrangement of the s t r a i n nets i n the upper tension zone. The lower case l e t t e r s on the segments of WUT-3, WR9, and WUT-4 i n d i c a t e the segment r e f e r r e d to i n Table 5.1. The i n d i v i d u a l stakes at WUT-3 and WUT-4 are shown by upper case l e t t e r s . The "x" north of ESR-1 i s the peg attached to the recorder. FIGURE 5.3. Schematic diagram showing the i n s t a l l a t i o n of the continuous e a r t h s l i d e recorders. 84 TABLE 5.1. Movement measured i n the source area. Stake array locations are given i n figure 5.2. Displacement (cm) Array 15/5/81 3/7/81 3/10/81 16/5/82 9/7/82 Total WR9a * — — 12.3 12.3 WR9b — — — 2. 1 2. 1 WR9c — — — 0.6 0.6 WR9d — — — 8.2 8.2 WUT-1 -0.4 0.7 2.8 1.4 4.5 WUT-2 -0.3 0.3 8.4 0.9 9.3 WUT-3a — — 5.0 -0.2 4.8 WUT-3b — -1.0 1.6 -2.0 .0.6 -0.8 WUT-3c — — 6.0 -0.2 5.8 WUT-4a — — — ' -0.3 2.9 2.6 WUT-4b — — -0.8 5.2 4.6 WUT-4c — — -1.9 -2.5 -4.4 Not a v a i l a b l e . 85 The continuous e a r t h s l i d e recorder (ESR-1) adjacent to WUT-4 was i n s t a l l e d to observe the r e l a t i v e displacement between the upslope blocks at WUT-4. Total movement for the period May 15 to July 9, 1982 was 3.5 cm. From figure 5.4 i t can be seen that no movement occurred from May 17 to June 4. After t h i s , the flow accelerated with the rate of movement ranging from 0.1 cm/day to 0.5 cm/day. During the same i n t e r v a l , 5.2 cm of movement was recorded along the stake l i n e WUT-4A,B. This discrepancy may be a t t r i b u t a b l e to the locations of peg WUT-4B and the peg attached to the continuous recorder on the downslope block (fig u r e 5.2). For example, the recorder peg was located on the upslope side of the block, while stake WUT-4B was on the downslope side. It may be, then, that slumping of the downslope side of the debris s l i c e i s causing a greater rate of change of distance between stakes A and B than the rate of change on the upslope side of the slump block. A "stream" of fragmented P a v i l i o n Group rocks crops out at WUT-4 (figure 5.5), i n d i c a t i n g that bedrock l i e s near the surface. Apparently the earthflow debris i s s l i p p i n g over a steep bedrock surface, a s i t u a t i o n believed to favor rapid displacement. Seepage of groundwater from the exposed P a v i l i o n Group indicates that the groundwater table springs to the surface at t h i s l o c a t i o n . I f t h i s were not the s i t u a t i o n , the debris probably would not be moving as r a p i d l y . 86 FIGURE 5.5. Photograph looking southeast at WUT-4 and ESR-1. Note the greenish-grey P a v i l i o n Group rocks that crop out from the red-brown earthflow debris (contact shown by the dashed l i n e ) . 87 5.4 MOVEMENT OF THE MAIN FLOW Lat e r a l movement of the main flow (as defined on fi g u r e 2.2) was monitored at six stake arrays (WR8, WR6, WL6, WL5, WL4, and ER5, figure 2.2). D i f f e r e n t i a l rates of flow within t h i s segment are c l e a r l y demonstrated by the r e s u l t s shown i n figure 5.6. For example, at WR6 (fi g u r e 2.2), where the slope angle i s approximately 12° and the flow width i s about 225 m, the t o t a l measured displacement was 23.8 cm during the study period. On the other hand, the t o t a l movement measured at WL6 was only 3.5 cm, and the corresponding slope angle i s 4° and the flow width i s about 480 m. Assuming, then, that flow depth remains constant, i t appears that the rate of motion i s a function of surface gradient and flow width i n accordance with the continuity p r i n c i p l e . Following t h i s , at WL4, which i s adjacent to the middle tension zone (fig u r e 2.2), where the slope angle increases to 12° and the flow width i s 310 m, the net displacement recorded was 14.6 cm. The continuous e a r t h s l i d e recorder ESR-2 was adjacent to WR8 on the east l a t e r a l shear zone of the main flow (figure 2.2). The record from ESR-2 (figure 5.7) suggests that the earthflow moved downslope i n a surge-like fashion a f t e r periods of very l i t t l e or no flow. Except for periods of stagnation, the rate of motion was f a i r l y uniform throughout the record. This i s consistent with the cumulative record from WR8 (figure 5.6) where the v e l o c i t y of flow did not increase s i g n i f i c a n t l y l a t e i n spring, although net displacement was greater a f t e r A p r i l 1, 1982. 88 M A M J A S 0 N D J F M A M J J A /90I I1BZ FIGURE 5.6. Cumulative movement of the main flow f o r the period March 21, 1981 to July 9, 1982. 3.0 -i FIGURE 5.7. The continuous movement record from ESR-2 (adjacent to WR8) for the period November 16, 1981 to July 9, 1982. 89 The movement calculated from the stake array data from WR8 was 4.5 cm for the period October 3, 1981 to May 17, 1982. The trace from the continuous record accounts f o r only 2.1 cm of movement. If the stake array data are correct, 2.0 cm of movement were not recorded by ESR-2 and probably occurred i n the i n t e r v a l from October 3 to November 16, 1981 ( p r i o r to i n s t a l l a t i o n of the continuous recorder). 5.5 MOVEMENT OF THE WEST LOBE The highest rate of earthflow movement at P a v i l i o n was recorded from the west lobe. The high flow v e l o c i t y i s evident from the r e s u l t s shown i n figu r e 5.8. The net displacement for the duration of the study was highest i n the most constricted portion of the channel where 63.4 cm and 62.6 cm of movement were observed at WL2 and WL3 (figures 2.2 and 5.8), r e s p e c t i v e l y . The rate of flow on the east l a t e r a l shear zone was s l i g h t l y lower—58.7 cm and 52.6 cm recorded from WR3 and WR2—apparently due to "dragging" of the flow around the b i f u r c a t i o n point. The consistency of the data i n figu r e 5.8 demonstrates that acceleration and deceleration of the flow occurs everywhere at the same time which suggests that the earthflow moves as a r i g i d body. Deceleration of the west lobe where the earthflow spreads l a t e r a l l y onto the debris fan i s shown by the low movement at WLO (figures 2.2 and 5.8). Again, t h i s i s believed to be a t t r i b u t a b l e to the combined e f f e c t of the decreased slope angle and widening of the flow channel i f the depth i s considered to be constant. For example, 90 ?0-i FIGURE 5.8. Cummulative movement of the west lobe f or the period March 21, 1981 to July 9, 1982. 91 t o t a l movement recorded at WRO-B (figure 2.2) was 27.7 cm, while displacement at WRO-A (upslope of WRO-B) was 47.0 cm for the entire observation period. On the west l a t e r a l shear zone, the e f f e c t of widening of the flow channel below WL1 (figur e 2.2) i s much more pronounced and probably accounts f o r only 4.6 cm of movement measured at WLO from May 22, 1981 to July 9, 1982. The stake array data from WR2 and WR3 indicated that 16.5 cm of movement occurred between October 3, 1981 and May 17, 1982 and that 5.0 cm and 7.5 cm of movement occurred at WR2 and WR3 between May 17 and July 9, 1981 (figur e 5.8). However, the continuous record from ESR-3 accounts f o r 15.6 cm of movement from November 16, 1981 to May 17, 1982 and 2.3 cm of movement were recorded between June 9 and July 9, 1982 (figure 5.9). Movement that occurred while the recorder clocks were not functioning was noted on the chart, although not as a continuous trace over time. Therefore the t o t a l displacement was recovered from the chart and has been included i n figu r e 5.9 to complete the record. The continuous record, of course, does not include data for the period from October 3 to November 16, 1981 because the continuous recorder had not yet been i n s t a l l e d . Data are also absent from May 17 to June 9, 1982. The pattern of movement shown by the continuous record from ESR-3 i s consistent with that found from the stake array data. For example, the rate of movement was low during November and early i n December, and increased around December 9, 1981 to a rate (about 0.05 cm/day) which 92 /6.0-£0 -MO 13.0 IZ.O no H ~ 10.0 % ^ 9.0 I § 8.0 40 -3.0-2.0 -1.0 0.0 Ass—/V« continuous record I available II, l/l 11/5 IZ/I li/lS I'/I 1/15 z}\ Z/IS3/I 3/iS 4/l 4/ff S/l $(s ' 3/K ill ill* 7/1 7/15 mi FIGURE 5.9. The continuous movement record from ESR-3 (between WR2 and WR3 on the west lobe) f o r the period November 16, 1981 to July 9, 1982. There are two diagrams because the recorder was not i n the f i e l d from May 15 to June 9, 1982, hence movement i n t h i s period could not be accounted for i n the p l o t . 93 was maintained throughout the spring. There was no movement recorded for the 15-day period i n March 1982. The next period of record from ESR-3 indicates that the flow accelerated to 0.07 cm/day for one month. Arrays on the west lobe having f i v e stakes were WR2, WR1, WRO-A, and WRO-B. The calculated displacement figures along the 1.5 and 4,5 li n e s are provided i n Appendix I I I . In each case, no s i g n i f i c a n t change of distance was recorded from the baseline to the f i f t h peg which indicates that the baseline stakes are stable r e l a t i v e to the l a t e r a l deposits from e a r l i e r flows (fi g u r e 5.1). 5.6 MOVEMENT OF THE EAST LOBE Earthflow movement on the west side of the east lobe was f a i r l y uniform along the ent i r e length of the l a t e r a l shear zone. From figure 5.10 i t can be seen that the t o t a l displacement ranged from 33.2 cm at ELI to 27.8 cm at EL2 (figur e 2.2). The shear zone on the east side of the east lobe branches into two segments at about 882 m elevation ( f i g u r e 2.2) and movement was monitored along both of these segments. Maximum displacement along the i n t e r i o r shear zone (proximal to the east lobe) was 24.1 cm, while along the ext e r i o r branch movement was generally less than 4.8 cm. The only f i v e stake array on the east lobe was EL4. Unfortunately, the f i f t h stake became loose during the study so the measurements from t h i s l o c a t i o n are probably u n r e l i a b l e . It i s believed, however, that very l i t t l e movement occurred between these pegs. 94 M A M J J A S 0 N D 7 F M A M 7 7 A I18Z FIGURE 5.10. Cumulative movement of the east lobe f o r the period March 21, 1981 to July 9, 1982. .7 CORRELATION OF MOVEMENT WITH GROUNDWATER HYDROLOGY The development of an explanation of earthflow motion as a function of pore water pressure f l u c t u a t i o n s f i r s t requires that the re l a t i o n s h i p between the groundwater table elevation and pore water pressure be understood, and that the p r i n c i p l e of e f f e c t i v e stress i s known. These items w i l l therefore be reviewed b r i e f l y before the groundwater data from the earthflow are correlated with seasonal movement. Figure 5.11 i s a lon g i t u d i n a l section through a hypothetical h i l l s l o p e with a f a i l u r e plane at depth z below the ground surface and the groundwater table i s at height h above the f a i l u r e plane. Hence, i f the l o c a t i o n of the f a i l u r e surface i s known, the pore water pressure u can be found from the equation: 95 FIGURE 5.11. A segment of an unstable h i l l s l o p e with a groundwater table at height h above the f a i l u r e plane, and the depth to the f a i l u r e surface i s z. The pore water pressure u above the f a i l u r e plane i s found by computing: u = h cos 8. u = T w h c o s 2 6 (5.1) where Tw i s the unit weight of water and 9 i s the angle of the f a i l u r e surface from the horizontal d i r e c t i o n (which i s assumed to be p a r a l l e l to the ground surface). Now, consider a saturated s o i l column. Because i t i s saturated, the column consists of two phases: ( i ) the s o l i d p a r t i c l e s which make up the s o i l skeleton, and ( i i ) the water f i l l i n g the i n t e r p a r t i c l e voids. At the base of the column, the t o t a l normal stress 0" on the s o i l has two components: ( i ) the e f f e c t i v e normal stress cT i s the component 96 derived from the i n t e r p a r t i c l e contacts of the s o i l skeleton and i s the only component that contributes to the shear strength of the s o i l , and ( i i ) the pore water pressure u i s the component due to water within the i n t e r p a r t i c l e voids. Total normal s t r e s s , then, can be expressed as: a = c7 / + u (5.2a) o r o'= 6 - u (5.2b) If the t o t a l normal stress of the s o i l i s increased, for example by r a i s i n g the elevation of the groundwater table, with no deformation occurring i n the horizontal d i r e c t i o n ( i . e . the s o i l i s confined l a t e r a l l y ) , the increase i n pore water pressure w i l l be equal to the increase i n t o t a l normal s t r e s s . In other words, the increase i n normal stress i s c a r r i e d e n t i r e l y by the pore water. From equation 5.2b, then, i t i s c l e a r that an increase of the pore water pressure causes the e f f e c t i v e stress to be reduced, thus reducing the shearing resistance of the s o i l . The shear strength of a s o i l at a point on a p a r t i c u l a r plane was o r i g i n a l l y expressed by Coulomb as a l i n e a r function of the normal stress tT at the same point: s = c + cT tan <j> (5.3) where s i s the shear strength (or shearing resistance) of the s o i l , and c and d> are the apparent cohesion and the angle of shearing resistance of the s o i l (Craig, 1978). Since shear stress i s only r e s i s t e d by the s o i l skeleton, the shear strength i s usually expressed i n terms of e f f e c t i v e s t r e s s : s = c'+ a' tan 4' (5.4a) s = c' + (cT-u)tan (/>' (5.4b) where c' and <(>' are the shear strength parameters i n terms of e f f e c t i v e stress ( f i g u r e 5.12). Equations 5.4a and 5.4b show how the shearing resistance of a s o i l i s c o n t r o l l e d by the pore water pressure conditions and, therefore, by the e f f e c t i v e normal s t r e s s . O ' (kN/ma) FIGURE 5.12. Sketch of the l i n e a r r e l a t i o n s h i p between shear strength and e f f e c t i v e normal s t r e s s . The shear strength parameters c' and are defined by the diagram. The f a i l u r e envelope i s the common tangent to the Mohr c i r c l e s which represent various states of stress at f a i l u r e and are found by laboratory t e s t i n g of s o i l samples for determination of the shear strength of a s o i l ( s ) . 98 The r e l a t i o n s h i p between earthflow motion and groundwater f l u c t u a t i o n s at P a v i l i o n i s most e a s i l y examined by p l o t t i n g the piezometric l e v e l f l u c t u a t i o n s with movement data. The r e s o l u t i o n of the piezometric data from October 1981 to May 1982 i s greater than the res o l u t i o n of the stake array data i n t h i s period. Thus the stake array data alone are i n s u f f i c i e n t to show the pattern of earthflow displacement i n winter. The continuous movement records from ESR-2 and ESR-3 can be used to f i l l the void i n the period October 1981 to May 1982. To demonstrate movement of the main flow for the duration of the study, the data from WR8 and ESR-2 have been merged to form one movement curve (fi g u r e 5.13). This i s v a l i d since ESR-2 i s located at the same s i t e as WR8. S i m i l a r l y , movement of the west lobe i s represented i n figure 5.14 by the merged data sets from WR2 and ESR-3. Figure 5.13 shows that the rate of movement at WR8 was high between March 21, 1981 and July 3, 1981. In the period from July to early October 1981, deceleration of the earthflow i s indicated. Between mid-November 1981 and lat e January 1982, s l i g h t a c c e l e r a t i o n of the main flow occurred, followed by a lesser rate of displacement u n t i l the end of March 1982. In the period from A p r i l to July 1982 the net displacement was r e l a t i v e l y high, however, phases of rapid motion occurred a f t e r periods of stagnation. At WR2, displacement i n spring and summer 1981 was maximum between May 15 and July 3 (fig u r e 5.14). The west lobe also shows deceleration from July 3 to October 3, 1981. At ESR-3 (figu r e 5.14) the rate of 99 FIGURE 5.13. Plots of movement of the main flow measured at WR8 and ESR-2, piezometric data from BH-2, BH-3, and BH-4, and the estimated groundwater recharge for the second water year. 100 100 m\ iiez FIGURE 5.14. Plot of movement of the west lobe measured at WR-2 and ESR-3, piezometric data from BH-5 and BH-6, and the estimated moisture surplus f o r the second water year. 101 movement was s u b s t a n t i a l l y greater from early December 1981 to l a t e January 1982, followed by deceleration u n t i l l a t e March 1982. The west lobe accelerated again at the end of A p r i l 1982 to a rate maintained u n t i l July 9, 1982. The phases of accelerated earthflow motion c o r r e l a t e well with the r i s e of piezometric l e v e l s observed i n May and June 1981 (figures 5.13 and 5.14), i n November 1981, and i n May and June 1982. The continuous records of earthflow movement and groundwater l e v e l f l u c t u a t i o n s show that earthflow acceleration i n December 1981, most prominent at WR2/ESR-3 (figu r e 5.14), followed the r i s e of groundwater elevation by about one-half a month. In spring 1982 the earthflow accelerated about one month p r i o r to the r i s e of groundwater l e v e l s . From these observations, i t i s believed that a c c e l e r a t i o n of the earthflow i n spring 1981 also preceded the observed r i s e of the piezometric l e v e l s , but the r e s o l u t i o n of the data i n t h i s period does not permit observation of the phase lag. The r e l a t i o n s h i p between earthflow motion and piezometric l e v e l change suggests that earthflow acceleration occurs when the pore water pressure i s maximum i n the f a i l u r e zone as a r e s u l t of recharge from snowmelt and summer p r e c i p i t a t i o n . Since none of the piezometers measured water pressure d i r e c t l y on the basal s l i p surface, the phase lag between earthflow acceleration and piezometric l e v e l r i s e i n the earthflow can be explained by flow of the groundwater from the f a i l u r e plane into the standpipes and by the hydrostatic time lag discussed i n Chapter 4 (pp. 72-74). 102 Figures 5.13 and 5.14 also show that the earthflow accelerated i n spring 1982 a short time following groundwater recharge from r a i n f a l l and snowmelt. This indicates e f f i c i e n t transport of groundwater recharge from areas surrounding the earthflow i t s e l f . Acceleration i n winter i n d i c a t e s transport from Mt. Cole through the underlying P a v i l i o n Group rocks and into the basal f a i l u r e zone of the earthflow. Although the earthflow accelerated s l i g h t l y i n winter 1981 to 1982 (figure 5.13), the movement records i n d i c a t e that earthflow response to the large groundwater l e v e l r i s e between October and December 1981 was not as great as earthflow response to spring recharge. This may be explained by the difference between the mechanism of groundwater recharge i n the spring from snowmelt and r a i n f a l l , and delayed recharge from Mt. Cole i n the f a l l . For example, percolation of spring snowmelt and r a i n f a l l wets the earthflow from the top down and the r i s e of groundwater l e v e l s occurs uniformly throughout the earthflow, causing an o v e r a l l increase of the e f f e c t i v e normal stress and s i g n i f i c a n t a c c e leration of the earthflow. Delayed discharge from Mt. Cole i n winter causes wetting of the earthflow from the bottom up. As a r e s u l t , only the highest conductivity zones within the earthflow w i l l undergo an increase of e f f e c t i v e normal s t r e s s , while the e f f e c t i v e stress i n the low-conductivity portions of the earthflow w i l l remain r e l a t i v e l y unchanged. Since the increase of e f f e c t i v e normal stress i s not uniform i n the earthflow, the rate of movement does not increase as much as i t does i n response to spring recharge. 103 The movement data presented i n t h i s chapter agree with data presented by Bovis (1980) ( f i g u r e 5.15). The smallest displacements can be recognized from August to December 1979 and from March to June 1980. Displacement was maximum between March and August 1979, June and August 1980, and i n the period December 1979 to March 1980. Bovis' data, therefore, also show earthflow acceleration i n winter and earthflow deceleration from la t e summer to the end of autumn. 5.8 FLUCTUATIONS OF EARTHFLOW MOTION IN THE HOLOCENE PERIOD There i s s t r a t i g r a p h i c and morphologic evidence on the earthflow that suggest that the volume of moving debris has changed at least three times since movement began at P a v i l i o n . For example, a layer of volcanic ash up to 3.0cm i n thickness i s exposed i n a road cut on the west side of the east lobe (fi g u r e 5.16). Smith and Westgate (1969) i d e n t i f i e d the ash as Mazama, which f e l l 6,600 years BP. In the road cut the tephra separates l a t e r a l deposits c o n s i s t i n g of unconsolidated earthflow debris (fi g u r e 5.16) that are associated with d i f f e r e n t flow events. The debris overlying the ash ranges from 3.5 to 4.0 m i n thickness ( f i g u r e 5.16). From t h i s i t can be i n f e r r e d that the earthflow volume has not remained constant through time. This i m p l i c a t i o n i s further supported by the superposition of a 1.0m thick layer of debris deriving from g l a c i a l t i l l over the ash i n the central tension scarp ( f i g u r e 5.17). 104 o im mo FIGURE 5.15. Cumulative movement measured at P a v i l i o n earthflow from June 1978 to August 1980 (from Bovis, 1980). (a) FIGURE 5.16. (a) Photograph of the road cut through the west l a t e r a l shear zone of the east lobe showing Mazama ash i n t e r s t r a t i f ied with earthf1 ow debris. (b) Scale drawing of the same photograph giving dimensions of the tephra layer and l a t e r a l deposits. 106 FIGURE 5.17. Photograph of Mazama ash i n t e r s t r a t i f i e d with earthflow debris deriving from g l a c i a l t i l l i n the cen t r a l tension scarp. Judging from the size of the l a t e r a l deposit beneath the tephra (fi g u r e 5.16), the volume of the earthflow was r e l a t i v e l y small p r i o r to 6,600 years BP. Following deposition of the ash, the volume of the debris i n transport increased s u b s t a n t i a l l y , permitting accumulation of the larger l a t e r a l deposit over the ash. Subsequent decrease of the volume of moving debris occurred u n t i l the present l e v e l was attained. It i s believed that f l u c t u a t i o n s of the earthflow volume during the Holocene period may be explained by several climate changes since retreat of the l a s t major i c e sheet 10,000 years BP. Ryder (1978), for 107 example, suggested that slow movement of saturated, weathered rock material continued at a rate commensurate with the rate of supply of debris and the p r e v a i l i n g moisture regime. The influence of climate f l u c t u a t i o n s upon landform development, however, has not been studied i n great d e t a i l i n B r i t i s h Columbia. If the v a r i a t i o n of earthflow rate during the Holocene period can be at t r i b u t e d to climate change, the wetter periods would be associated with growth of the earthflow while dry phases would correspond with low flow. The reasoning here i s that wet phases would correspond with periods of high piezometric l e v e l s which favor renewed slumping and earthflow ac c e l e r a t i o n . Dry phases would show f a l l i n g piezometric l e v e l s and, by inference, decreased earthflow a c t i v i t y . The Holocene environment of southern B r i t i s h Columbia has been summarized by Clague (1981), and l o c a l studies of the Holocene environment were conducted by A l l e y (1976) and King (1980). According to Clague (1981), the climate during early p o s t - g l a c i a l time i n most areas of B r i t i s h Columbia was as warm as or warmer than present. This period i s often referred to as the 'hypsithermal' (Clague 1981). Cores of lake sediments from the L i l l o o e t area were analyzed by King (1980) and showed that warm, dry conditions prevailed u n t i l about 6,100 to 6,200 years BP. After 6,200 years BP, the moisture l e v e l s increased u n t i l about 4,000 years BP. The l a t t e r corresponds to the co l o n i z a t i o n of modern vegetation. Since 6,000 years BP, several f l u c t u a t i o n s of 108 temperature and p r e c i p i t a t i o n have occurred, some s u f f i c i e n t to cause expansion of alpine g l a c i e r s , although the climate gradually became cooler and wetter u n t i l present. In the l a t t e r period, three main Neoglacial i n t e r v a l s have been i d e n t i f i e d (Ryder, 1978, p. 67): ( i ) 5,800-4,900 years BP; ( i i ) 3,300-2,300 years BP; and ( i i i ) 1,000 years BP to present. Apparently a very moist phase prevailed i n the Kelowna area from 3,200 to 2,000 years BP ( A l l e y , 1976). The low earthflow volume i n f e r r e d from the r e l a t i v e l y small l a t e r a l deposit underlying Mazama ash, then, probably ref e r s to the hypsithermal i n t e r v a l . At th i s time, the climate was warm and dry so that the net input of meteoric water to the earthflow and the volume of debris i n transport would have been small. It can be argued that accumulation of the l a t e r a l deposit overlying the ash commenced shortly a f t e r the end of the hypsithermal, which King (1980) estimated to be about 6,100 years BP for the L i l l o o e t area, and continued to accumulate u n t i l about 4,000 years BP. A large portion of the debris may have been deposited during the f i r s t Neoglacial i n t e r v a l (5,800-4,900 years BP). From t h i s i t can be i n f e r r e d that the large, abandoned l a t e r a l deposits date back at least as far as 6,000 years BP. Subsequent reduction of the elevation of the earthflow surface ( i . e . a reduced flow rate) i s probably a t t r i b u t a b l e to a decreased moisture supply a f t e r 4,000 years BP, as well as a decrease of the debris supply from upslope sources i n the basin ( i . e . a reduced rate of retrogressive slumping). 109 10 SUMMARY The r e s u l t s of the slope movement observations i n d i c a t e that the amount of earthflow displacement depends on the l o c a t i o n of measurement. From the s p a t i a l v a r i a b i l i t y of movement i t was suggested that the width of the flow channel and the surface gradient are important factors determining the rate of motion. That i s , i f the depth i s considered constant, the v e l o c i t y was lowest where the flow width was large and the slope angle was small. Higher rates of movement were observed i n areas of the flow that were most constricted and where the gradient was r e l a t i v e l y high. The highest v e l o c i t y was observed i n the narrowest length of the west lobe where the slope was about 10°. Comparison of the continuous movement record from ESR-1 (figure 5.4) with the records from ESR-2 and ESR-3 (figures 5.7 and 5.8) reveals that movement i n the source area i s more sporadic than i t i s downslope. The lack of c o r r e l a t i o n between movement of the debris i n the source area and movement at the downslope s i t e s suggests that loading i n the headwall region i s unimportant i n determining movement of the earthflow lobes. For example, several slope movement studies have shown that loading of the headwall, perhaps from large-scale slumping of debris, causes accelerated downslope movement commensurate with the rate of loading. At P a v i l i o n , however, the volume of debris input i n the source area i s very small compared to the amount of debris already i n motion downslope. 110 Correlation of the piezometric data with movement has shown that the earthflow i s s e n s i t i v e to the p r e v a i l i n g hydrostatic conditions i n the f a i l u r e zone. The r e s u l t s , then, show that the earthflow responds to changes of the e f f e c t i v e stress i n the f a i l u r e zone as determined by the amount of surplus moisture that i s recharged to the groundwater system. I l l CHAPTER 6 CONCLUSION 6.1 SUMMARY Analysis of the climate record indicated that most of the c l i m a t i c input to the groundwater system at P a v i l i o n i s derived from snowmelt and r a i n f a l l i n the spring and early summer. In the period from August 1981 to June 1982 64% of the groundwater recharge was contributed by snowmelt i n A p r i l and May 1982, and 13% of the recharge was due to r a i n f a l l i n A p r i l . The piezometric record from the earthflow showed two phases of groundwater l e v e l r i s e i n 1981, both of which were out of phase with the apparent climate input. The f i r s t of these was a small pulse observed between mid-May and early July. It i s believed that t h i s increase i n head was caused by e a r l y , or low-elevation, snowmelt from slopes surrounding the source area. The e f f e c t was probably enhanced i n 1981 by abundant r a i n f a l l i n May d i r e c t l y on the earthflow i t s e l f and on the surrounding slopes. Recharge from these sources can occur quickly because of the disturbed, fractured debris i n the source area and on the surrounding slopes which are believed to have undergone retrogressive 112 f a i l u r e . The presence of fractures i n the earthflow debris can greatly increase the hydraulic conductivity of the material and, therefore, permit rapid groundwater recharge through the debris. The second phase of piezometric l e v e l r i s e occurred between October 3 and November 15, 1981. This event was a t t r i b u t e d , i n part, to groundwater recharge derived from snowmelt l a t e i n spring, which i s expected to be greater i n volume than the e a r l i e r snowmelt because of a p r e c i p i t a t i o n gradient. This pulse, too, was probably enhanced by p r e c i p i t a t i o n during May and July 1981. The f i v e month delay between piezometer response and actual groundwater recharge i s att r i b u t e d to the tr a v e l time required f o r the groundwater input at Mt. Cole to flow to the elevation range of the earthflow and to upward passage of groundwater i n t o the standpipes from the basal s l i p plane. The movement data show earthflow acceleration i n winter which undoubtedly was caused by the high pore water pressure i n the f a i l u r e zone from the delayed recharge from Mt. Cole. The slope movement record for the period October 3, 1981 to July 7, 1982 showed that the rate of displacement decreased from the values found i n 1981. This observation may i n d i c a t e that the volume of groundwater recharge i n the second water year was lower than i t was i n the f i r s t water year, which would cause the pore water pressures i n the f a i l u r e zone to be less than i n the previous year. In addition, c o r r e l a t i o n of the movement data with piezometric l e v e l change indicated 113 that the earthflow i s s e n s i t i v e to changes of the e f f e c t i v e normal str e s s on the basal s l i p plane. From these r e s u l t s , i t can be surmised that temporal v a r i a b i l i t y of earthflow movement rate i s determined by the p r e v a i l i n g hydraulic conditions within the basal f a i l u r e zone, which are affected by the climate regime and r e s u l t i n g groundwater recharge. The observed s p a t i a l v a r i a b i l i t y of displacement depends on the l o c a t i o n of measurement on the earthflow. The greatest movement values were obtained from the most con s t r i c t e d and steepest portions of the east and west lobes. If the depth of the earthflow i s considered constant, an increase of the s u r f i c i a l gradient while the width i s unchanged w i l l r e s u l t i n earthflow a c c e l e r a t i o n . S i m i l a r l y , an increase of the flow width causes the earthflow to decelerate, i f the gradient and depth are constant. Movement data from the source area indicated that the input of debris i n the source area does not a f f e c t the pattern of motion observed downslope along the l a t e r a l shear zones. 6.2 EARTHFLOW MOTION IN THE INTERIOR PLATEAU • Observations from P a v i l i o n earthflow and from Drynoch earthflow on the Thompson River may allow some generalizations about the seasonal character of earthflow-type motion i n the I n t e r i o r Plateau region of B r i t i s h Columbia. A four-year movement record from Drynoch (VanDine, 1980) shows that maximum displacement occurred at a f a i r l y uniform rate from December to August, and movement was minimum from September to December. This trend i s nearly i d e n t i c a l to the pattern of movement 114 observed at P a v i l i o n i n 1981 and 1982. Since i t has been shown that earthflow acceleration occurs from increased pore water pressures i n the basal f a i l u r e zone, the earthflows at P a v i l i o n and Drynoch receive adequate moisture i n December to cause earthflow a c c e l e r a t i o n i n winter, and the moisture surplus remains adequate at least u n t i l July. The groundwater data from Drynoch show that the piezometric levels usually rose from December to February to a peak where the water l e v e l remained unchanged u n t i l about A p r i l . This was followed by a second r i s e of groundwater l e v e l s between May and June, apparently i n response to discharge from snowmelt as indicated by hydrographs for the Thompson River over the same time period as the groundwater and movement records. The period from December to June, then, was when the groundwater elevation was maximum and t h i s c o r r e l a t e s very well with the timing of acceleration of the Drynoch earthflow. This i s s i m i l a r to observations made at P a v i l i o n during the present study. Transposing the conclusions drawn from the observations made at P a v i l i o n to Drynoch, i t appears that the r i s i n g groundwater l e v e l s that occur i n winter at Drynoch may be the r e s u l t of maximum annual recharge during the preceding spring and summer. The increased groundwater l e v e l s i n winter, therefore, probably represent the passage of the impulse from spring and summer recharge through a regional groundwater system underlying the Drynoch s l i d e , whose recharge area i s located well beyond the maximum extent of the earthflow i t s e l f . The secondary peak 115 of groundwater elevation recorded i n May and June at Drynoch i s the i n i t i a l response to recharge from snowmelt over the slopes surrounding the earthflow, which would pass quickly to the basal f a i l u r e zone and show up as a piezometric l e v e l r i s e shortly following input. From the trends of the movement data and groundwater l e v e l f l u c t u a t i o n s , from Drynoch, and from the hydrographs for the Thompson River, i t appears that the behavior of Drynoch earthflow i s much the same as the behavior observed at P a v i l i o n . If t h i s i s the case, then i t i s expected that other earthflows i n the I n t e r i o r Plateau region would also behave s i m i l a r l y to P a v i l i o n earthflow. . 3 FUTURE WORK AT PAVILION As a r e s u l t of annual climate v a r i a b i l i t y i n the region, i t would be useful to monitor both the groundwater l e v e l s i n the earthflow and the regional climate f o r a longer duration than was possible f o r the present study. A record of groundwater f l u c t u a t i o n s and estimated groundwater recharge of f i v e or ten years.duration would allow one to inves t i g a t e annual changes i n the behavior of these parameters. In addition, i t might be possible to determine the v e r t i c a l range of groundwater l e v e l f l u c t u a t i o n s i n the earthflow. An i n v e s t i g a t i o n of the c h a r a c t e r i s t i c s of the basal region of the earthflow at several locations on the earthflow would c e r t a i n l y provide answers to many questions which, within the scope of the present research, remain unsolved. F i r s t , the depth to the basal s l i p surface 116 could be found, together with the degree of v a r i a b i l i t y of earthflow thickness. It would also be desirable to place several piezometers i n the debris with t h e i r t i p s located along the f a i l u r e plane. This would allow the pore water pressure to be monitored and l a t e r correlated with earthflow movement; perhaps one would f i n d less delay between the groundwater changes and earthflow response. The conclusions of t h i s thesis suggest that the pore water pressure f l u c t u a t i o n s i n the f a i l u r e zone c o r r e l a t e reasonably well with movement. 117 BIBLIOGRAPHY A l l e y , N.F., 1976, The palynology and paleoclimatic s i g n i f i c a n c e of a dated core of Holocene peat, Okanagan Valley, southern B r i t i s h Columbia: Can. Journal Earth S c i . , 13(8): 1131-1144. Armstrong, J.E., Crandell, D.R., Easterbrook, D.J., and Noble, J.B., 1965, Late Pleistocene s t r a i t i g r a p h y and chronology i n southwestern B r i t i s h Columbia and northwestern Washington: Geol. Soc. Am. B u l l . , 76: 321-330. Begbie, M.B., 1871, On the benches or v a l l e y terraces of B r i t i s h Columbia: Royal Geog. Soc. P r o c , Session 1870-1871, 15: 133-145. Bjerrum, L. , 1967, Progressive f a i l u r e i n slopes of overconsolidated p l a s t i c clay and clay shales: Jour. S o i l Mech. and Found. Div., Am. Soc. C i v i l Engrs., v. 93, SM5, Paper 5456, Sept., pp. 1-49. Bovis, M.J., 1980, The character and extent of earthflow-type mass movement, Fraser V a l l e y , B r i t i s h Columbia, Canada: Int. Geog, Union Symp. F i e l d Experiements i n Geomorph., Kyoto Univ., Japan, Aug. 25-30. Clague, J . J . , 1981, Late Quaternary geology and geochronology of B r i t i s h Columbia. Pt. 2: Summary and discussion of radiocarbon-dated Quarternary h i s t o r y : Geol. Survey of Canada, Paper 80-35, 41 p. Craig, R.F., 1978, S o i l Mechanics: Van Nostrand Reinhold Company, Ltd., pp. 83-84. Crandell, D.R. and Varnes, D.J., 1961, Movement of the Slumgullion earthflow near Lake C i t y , Colorado: U.S. Geol. Survey, Prof. Paper 424B: 136-139. Dawson, G.M., 1879, Report on exploration i n the sourthern portion of B r i t i s h Columbia: Geol. Survey of Canada, Report of Progress 1877-1878, part B, pp. 1-173. 118 Dawson, G.M., 1896, Report on the area of the Kamloops map-sheet, B r i t i s h Columbia: Geol. Survey of Canada, Annual Report for 1894, v. 7, part B, pp. 1-427. D u f f e l l , S. and McTaggart, K.C., 1952, Ashcroft map-area, B r i t i s h Columbia: Geol. Survey of Canada, Memoir, pp. 64-66. Dunne, T. and Leopold, L.B. , 1978, Water i n Environmental Planning: W.H. Freeman and Co., pp. 136-137. Freeze, R.A. and Cherry, J.A., 1979, Groundwater: P r e n t i c e - H a l l , 604 p. Freeze, R.A., and Witherspoon, P.A., 1967, Theoretical analysis of regional groundwater flow: 2. E f f e c t of water table configuration and subsurface permeability v a r i a t i o n : Water Resources Res., 3: 623-634. Fulton, R.J., 1971, Radiocarbon geochronology of southern B r i t i s h Columbia: Geol. Survey of Canada, Paper 71-37, 28 p. Fulton, R.J. and Smith, G.W., 1978, 1978, Late Pleistocene stratigraphy of south-central B r i t i s h Columbia: Can. Jour. Earth S c i . , 15: 971-980. G i l , E. and Kotarba, A., 1977, Model of slope evolution i n Flysch Mountains: an example drawn from the P o l i s h Carpathians: Catena, 4: 233-248. Holland, S.S., 1964, Landforms i n B r i t i s h Columbia— a physiographic o u t l i n e : B r i t i s h Columbia Dept. of Mines and Petroleum Resources, B u l l . 48, 138 p. Hvorslev, M.J., 1951, Time lag and s o i l permeability i n groundwater observations: . U.S. Army Corps of Engineers, Waterways Experiment Sta t i o n , Vicksburg, M i s s i s s i p p i , B u l l . 36, 50 pp. Johnson, A.M., 1970, Physical Processes i n Geology: Freeman, pp. 433-571. 119 Kelsey, H.M., 1978, Earthflows i n Franciscan melange, Van Duzen River basin, C a l i f o r n i a : Geology, 6: 361-364. Kenney, T.C., 1977, Residual strengths of mineral mixtures: Proceedings of the 9th International Conference on S o i l Mech. and Found. Engineering, 9(1): 155-160. King, M., 1980, Palynological and macrofossil analyses of lake sediments from the L i l l o o e t area, B r i t i s h Columbia: [Unpublished M.Sc. t h e s i s ] , Simon Fraser U n i v e r s i t y , Burnaby, B r i t i s h Columbia, I25p. Lambe, W.T., 1951, S o i l Testing for Engineers: John Wiley and Sons, pp. 22- 28. Matheson, D.S. and Thompson, S., 1973, Geological implications of v a l l e y rebound: Can.Jour. Earth S c i . , 10: 961-978. Mathewes, R.W., 1978, The environment and b i o t i c resources of the L i l l o o e t area. Reports of the L i l l o o e t Archaeological Project No. 1, Introduction and Setting: i n Stryd, A.H. and Lawhead, S. (eds.), Nat. Mus. of Man Mercury Ser., Archaeol. Survey of Canada, Paper 73: 68-99. Mathewes, R.W. and Heusser, L.E., 1981, A 12,000 year palynological record of temperature and p r e c i p i t a t i o n trends i n southwestern B r i t i s h Columbia: Can. Jour. Botany, 59: 707-710. Meinzer, O.E., 1927, Plants as in d i c a t o r s of groundwater: U.S. Geol. Survey Water Supply Paper 577, 95 p. Monger, J.W.H., 1981, Geology of parts of western Ashcroft map-area, southwestern B r i t i s h Columbia: Current Research, Part A, Geol. Survey of Canada, Paper 81-1A: 185-189. Monger, J.W.H. and McMillan, W.J., 1984, Bedrock geology of Ashcroft (921) map area: Geological Survey of Canada, Open F i l e 980. 120 Noble, H.L., 1973, Residual strengths and landslides i n clay and shale: Jour. S o i l Mech. and Found. Div. , Am. Soc. C i v i l Engrs., 99: 705-719. Preto, V.A., Osatenko, M.J., McMillan, W.J., and Armstrong, R.L., 1979, Isotopic dates and strontium i s o t o p i c r a t i o s for plutonic and volcanic rocks i n the Quesnel Trough and Nicola B e l t , southcentral B r i t i s h Columbia: Can. Jour. Earth S c i . , 16: 1658-1672. Ryder, J.M., 1976, Terrain inventory and Quaternary geology, Ashcroft, B r i t i s h Columbia: Geol. Survey of Canada, Paper 74-49, 17 p. Ryder, J.M., 1978, Geomorphology and the Quaternary h i s t o r y of the L i l l o o e t area. Reports of the L i l l o o e t Archaelogical Project. No. 1, Introduction and Setting: i n Stryd, A.H. and Lawhead, S. (eds.), Nat. Mus. of Man Mercury Ser. Archaeol. Surv. Canada, Paper 73: 56-67. Shannon, K.R., 1981, The Cache Creek Group and contiguous geology, Ashcroft, B r i t i s h Columbia: Current Research, Part A, Geol. Survey of Canada, Paper 81-lA: 217-221. Smith, D.G.W. and Westgate, J.A., 1969, Electron probe technique for cha r a c t e r i z i n g p y r o c l a s t i c deposits: Earth and Planetary S c i . L e t t e r s , 5: 313-319. Swanson, F.J. and Swanston, D.N., 1977, Complex mass-movement t e r r a i n s i n the western Cascade Range, Oregon: Geol. Soc. Am., Boulder, Colorado, Reviews i n Engineering Geology, 3: 113-124. T r e t t i n , H.P., 1961, Geology of the Fraser River Valley between L i l l o o e t and Big Bar Creek: B r i t i s h Columbia Dept. of Mines and Petroleum Resources, B u l l . 44, 109 p. Varnes, D.J., 1978, Slope movement types and processes: Schuster, R.L. and Krizek, R.J. (eds.), Landslides, analysis and c o n t r o l , National Academy of Sciences, Transportation Research Board, Special Report 176: 11-33. 121 VanDine, D.F., 1974, Geotechnical and geological engineering study of Drynoch Landslide, B.C.: [Unpublished M.Sc. t h e s i s ] , Queens Univ e r s i t y , Kingston, Ontario, 123 p. VanDine, D.F., 1980, Engineering geology and geotechnical study of Drynoch Landslide, B r i t i s h Columbia: Geol. Survey of Canada, Paper 79-31, 34 p. Williams, R.E. and Farvolden, R.N., 1967, The influence of j o i n t s on the movement of groundwater through g l a c i a l t i l l : Jour, of Hydrol., 5: 163-170. Yatsu, E., 1967, Some problems on mass movements: Geog. Annaler, Ser. A, 49: 396-401. 122 APPENDIX I ATTERBERG LIMITS: TEST PROCEDURE AND RESULTS 123 TEST PROCEDURE The Atterberg consistency l i m i t s are used, i n general, i n conjunction with the mechanical analysis or gr a i n - s i z e d i s t r i b u t i o n curves for i d e n t i f i c a t i o n and c l a s s i f i c a t i o n of a s o i l . The Atterberg l i m i t t e s t s (described i n d e t a i l by Lambe, 1951, pp. 22-28) are performed on the f r a c t i o n of the s o i l passing through a no. 40 sieve (le s s than 0.42 mm). The test r e s u l t s allow one to estimate the engineering behavior of a s o i l at varying water contents. The consistency l i m i t s that are most frequently used are the l i q u i d l i m i t , the p l a s t i c l i m i t , and the p l a s t i c i t y index. A l l of these s o i l c h a r a c t e r i s t i c s are affected by the amount of clay present i n the s o i l , as well as the type of clay. The l i q u i d l i m i t i s the moisture content (%) above which the s o i l becomes l i q u i d when agitated, thus, the wetter the mixture, the less a g i t a t i o n required to cause i t to flow as a viscous material. The p l a s t i c  l i m i t , on the other hand, i s the minimum water content (%) at which the soil-water mixture behaves as a p l a s t i c s o l i d . The p l a s t i c i t y index i s the numerical diff e r e n c e between the l i q u i d l i m i t and the p l a s t i c l i m i t of the s o i l . I t , therefore, represents the range of moisture contents within which the s o i l behaves as a p l a s t i c s o l i d . The p l a s t i c i t y index increases as the clay content of a s o i l increased, and, conversely, i t decreases as the clay content decreases. 124 The r e s u l t s of the Atterberg l i m i t tests performed on several samples of the earthflow debris are summarized i n Table A I . l . The values are consistent and are therefore considered r e l i a b l e within the l i m i t s of the laboratory procedure. 125 TABLE A I . l . The r e s u l t s of Atterberg l i m i t tests performed on several samples of the earthflow debris. Sample No. Depth (cm) Description LL(%) PL(%) PI(%) DEM-81-5a 70 fine-grained, red-brown 34.2 24.5 9.7 -5b 80 fine-grained, red-brown 43.4 25.7 17.7 -5c 120 fine-grained, red-brown 38.2 25.4 12.8 -5d 140 fine-grained, red-brown 39. 1 22.6 16.5 -5f 65 fine-grained, red-brown 55. 1 — -5g 110 fine-grained, red-brown 50.6 28.3 26.2 -6a 70 reworked t i l l , grey, coarse-grained 23.9 15.4 8.5 -6b 120 reworked t i l l , grey coarse-grained 27.7 19.6 8. 1 -6c 160 reworked t i l l , grey, coarse-grained 25.3 15.9 9.4 -6d 8 reworked t i l l , grey, coarse-grained 28.5 19. 1 9.4 -6e 140 reworked t i l l , grey, coarse-grained 21.2 16.2 5.0 -7b 100 reworked t i l l , grey, coarse-grained 23.7 19.3 4.4 -7c 145 reworked t i l l mixed with red-brown, fine-grained debris, near contact 24.6 19.9 4.7 -7d 160 fi n e , red-brown mixed with reworked t i l l 31.6 2 1.6 10.0 -8a 65 organic-rich reworked t i l l 34.9 26.3 8.6 -8b 85 reworked t i l l , grey, sandy matrix 33.8 25.4 8.4 -8c 130 reworked t i l l , grey, somewhat clayey 36.3 24.3 12.0 * No data a v a i l a b l e . 126 TABLE A I . l . (continued) Sample no. Depth (cm) Description L i . m PL(%) PI(%) DEM-8l-8d 165 reworked t i l l , grey, gravelly 34.6 30.4 4.2 -8e 90 very s t i f f f i n e -grained, red-brown 45.0 24.6 10.4 -8f 145 very s t i f f f i n e -grained, red-brown 43.8 24.6 19.2 -8g 240 very s t i f f f i n e -grained, red-brown 46.0 25.9 20. 1 -10a 65 reworked t i l l with organics mixed in 29.4 20.4 9.0 -10b 130 reworked t i l l mixed with' red-brown debris 25.9 18. 1 7.8 -10c 65 reworked t i l l , grey, stoney 28.9 19.4 9.5 -lOd 155 sandy red-brown debris 32.2 18.8 13.4 -1 la 65 stony grey, c l a y - r i c h 30.6 26.3 13.4 - l i b 65 coarse, grey, has c l a y - r i c h matrix 44.5 28.2 16.3 -13b 100 coarse, grey mixed with f i n e , red-brown debris 34.2 20.0 14.2 -13c 140 coarse, grey mixed with f i n e , red-brown debris 20.6 16.5 4. 1 -14a 65 sandy, organic-rich reworked t i l l 23.2 17.0 6.2 -14b 150 sandy, organic-rich reworked t i l l 20.2 17.3 8.9 -16a 65 red-brown, f i n e -grained mixed with organic debris 54.3 32.2 22. 1 -16b 105 red-brown, f i n e -grained 45.4 27.4 18.0 -16c 130 red-brown f i n e -grained 47.6 28.7 18.9 127 APPENDIX II REDUCTION OF SLOPE MOVEMENT DATA 128 REDUCTION OF SLOPE MOVEMENT DATA Each four-stake array was set up i n the manner shown i n figure A I I . l . At each s i t e , the distance between s i x possible stake pairs ( f i g u r e A I I . l ) was measured using a s t e e l surveying tape. Two oblique t r i a n g l e s r e s u l t whose length on a l l sides i s known. With the ap p l i c a t i o n of the cosine law, each t r i a n g l e was used to c a l c u l a t e the net downslope movement i n the x - d i r e c t i o n and the sine law was used to f i n d displacement i n the y - d i r e c t i o n for a given time i n t e r v a l . As an example, the procedure for c a l c u l a t i n g the change i n distance, Ax w i l l be shown for the t r i a n g l e 1-2-4 shown i n fi g u r e A l l . 2 . F i r s t , the distance x, i s calculated i n the following manner: (A l l . 1 ) 2 a l c l and x^= a^ cos . ( A l l . 2 ) S i m i l a r l y , x~ i s calculated by: (All.3) and X 2 = a 2 cos /2>z (All.4) 129 \ I EARTHFLOW v i 'WFWI 20A/£ 4 FIGURE A I I . l . Sketch of a four-stake array at P a v i l i o n showing the six possible distances that were measured at each stake array. FIGURE A l l . 2 . Diagram showing the variables used to c a l c u l a t e earthflow displacement from the stake array measurements. 130 From equations A l l . 2 and A l l . 4 the change i n distance, A x can be computed: A : x^ - X2 ( A l l . 5 ) A s i m i l a r process i s used to f i n d A x ' for the 1-3-4 t r i a n g l e shown i n figure A l l . 2 with lengths on a l l sides being l a b e l l e d a', b', and c'. Idea l l y , A x and ZAX' for a given stake array would be i d e n t i c a l for a c e r t a i n time duration, however t h i s was r a r e l y the case. Although manual f i e l d measurements were assumed to be within 1.0 mm of accuracy i t i s l i k e l y that the error was greater, perhaps as a r e s u l t of thermal expansion and contraction of the s t e e l tape due to temperature changes from season to season. Since the value of Ax and Ax' were not always equal, the arithmetic mean was considered to represent the net displacement for a given time i n t e r v a l . Once ^ and were known, computation of the y-componenf of movement was done by using: y^ = a^ s i n (A l l . 6 ) y2 = a2 s i n (All.7) Hence, A y = I y x - y 2 (All.8) 131 In general, the value of /\y was very small. For most of the s i t e s i t was l e s s than 5.0 mm, i n d i c a t i n g that the majority of movement occurred i n the x - d i r e c t i o n . 132 APPENDIX III PERIODIC AND TOTAL DISPLACEMENTS CALCULATED FOR PAVILION 133 TABLE A I I I . l . Movement i n the x - d i r e c t i o n calculated f o r the stake arrays along the l a t e r a l shear zones. A l l values are i n centimeters. ARRAY 15/5/81 3 / 7 / 8 1 3 / 1 0 / 8 1 1 6 / 5 / 8 2 9 / 7 / 8 2 T o t a l WR$ 4 . 4 4 . 8 5 . 1 8 . 2 1 . 3 2 3 . 8 WR8 5 . 2 4 . 5 3 . 5 4 . 5 1 . 0 1 8 . 7 WL6 0 . 5 0 . 2 0 . 4 0 . 6 1 . 8 3 . 5 WL5 1. 1 1 . 3 1 . 0 2 . 5 0 . 8 6 . 7 WL4 3 . 1 1 . 2 1 . 9 7 . 3 1. 1 1 4 . 6 ER5 0 . 9 1.7 4 . 2 2 . 3 0 . 8 1 5 . 5 WL3 1 1 . 9 1 3 . 9 1 3 . 2 8 . 7 1 4 . 9 6 2 . 6 WL2 1 1 . 2 1 2 . 3 15. 1 1 8 . 8 6 . 0 6 3 . 4 WL1 * — 9 . 5 1 1 . 3 2 . 2 2 3 . 0 WLO — — — — 4 . 6 4 . 6 WRO-B 3 . 4 3 . 1 4 . 7 9 . 0 2 . 5 2 2 . 7 WRO-A 7 . 1 9 . 6 1 1 . 0 1 4 . 7 4 . 6 4 7 . 0 WR1 8 . 0 11.7 — — — 1 9 . 7 WR2 9 . 3 1 2 . 0 1 1 . 3 1 8 . 4 5 . 2 5 6 . 2 WR3 1 2 . 3 1 0 . 5 1 2 . 6 1 6 . 0 7 . 3 5 8 . 7 EL4 8 . 8 1 4 . 6 — 4 . 8 2 . 3 3 0 . 5 E L 3 9 . 8 8 . 2 4 . 2 6 . 3 2 . 4 3 0 . 9 E L 2 8 . 8 7 . 8 3 . 5 5 . 6 2 . 1 2 7 . 8 E L I 8 . 6 9 . 0 6 . 4 5 . 8 3 . 4 3 3 . 2 ELO 9 . 5 8 . 4 5 . 0 — — 2 2 . 9 ER4 6 . 0 3 . 6 0 . 2 4 . 9 0 . 8 1 5 . 5 ER3 7 . 5 5 . 5 3 . 3 5 . 2 1 . 5 2 3 . 6 E R 2 a 1 . 8 1 . 4 0 . 4 1. 1 0 . 1 4 . 8 ER2b 5 . 0 4 . 7 3 . 1 3 . 5 1 . 4 17.7 ER1 7 . 3 5 . 5 4 . 1 5 . 2 2 . 0 2 4 . 1 ER20 0 . 4 0 . 4 0 . 3 — 2 . 0 3 . 1 ER21 0 . 7 1 . 0 0 . 5 1.7 0 . 2 4 . 1 ER22 — 0 . 6 1 .4 1 .4 0 . 6 4 . 0 No d a t a a v a i l a b l e . 134 TABLE A I I I . 2 . Movement i n the y - d i r e c t i o n calculated from the stake array measurements. A l l values are i n centimeters. ARRAY 15/5/81 3 / 7 / 8 1 3 / 1 0 / 8 1 1 6 / 5 / 8 2 9 / 7 / 8 2 T o t a l WR9a 1.7 1.7 WR9b — — — — 1 . 0 1 . 0 WR9c — — — — 0 . 5 0 . 5 WR9d — — — — 2 . 5 2 . 5 WR8 1 . 3 1 . 2 0 . 3 1 . 3 0 . 2 4 . 3 WR6 1 . 0 1 . 8 2 . 3 1 . 8 0 . 8 7 . 7 WL6 0 . 1 0 . 3 1 . 6 0 . 5 0 . 3 2 . 8 WL5 1 . 5 0 . 4 0 . 1 0 . 8 1 . 2 4 . 0 WL4 0 . 2 0 . 3 0 . 8 0 . 4 0 . 1 1 . 8 ER5 1.1 0 . 4 2 . 3 1 . 2 3 . 3 8 . 3 WL3 0 . 4 1 . 0 0 . 3 0 . 2 1 . 5 3 . 4 WL2 0 . 8 1 . 2 2 . 5 1 . 6 0 . 5 6 . 1 WL1 — — 0 . 8 0 . 4 0 . 8 2 . 0 WLO — — — — 3 . 6 3 . 6 WRO-B 0 . 2 0 . 2 1.1 0 . 3 0 . 1 1 . 9 WRO-A 0 . 4 0 . 8 0 . 8 0 . 1 0 . 0 2 . 1 WR1 0 . 9 0 . 7 — — — 1 . 6 WR2 0 . 7 0 . 7 0 . 8 1.7 0 . 2 3 . 1 WR3 2 . 0 0 . 8 1. 1 3 . 2 1. 1 8 . 2 EL4 0 . 8 0 . 7 — 0 . 5 0 . 3 2 . 3 E L 3 0 . 4 0 . 1 0 . 9 0 . 4 0 . 3 2 . 1 E L 2 0 . 3 0 . 3 0 . 5 0 . 7 0 . 3 2 . 1 EL 1 3 . 4 3 . 4 1. 1 0 . 4 0 . 4 8 . 7 ELO 0 . 6 0 . 4 1 . 9 — — 2 . 9 ER4 0 . 9 0 . 9 0 . 3 0 . 9 0 . 1 2 . 8 ER3 1 . 3 0 . 3 1 . 0 1 . 0 0 . 1 3 . 7 E R 2 a 0 . 7 0 . 9 1 . 0 0 . 3 0 . 1 3 . 0 ER2b 0 . 1 0 . 3 0 . 1 0 . 4 0 . 3 1 . 2 ER1 0 . 4 0 . 7 0 . 9 0 . 6 0 . 1 2 . 7 E R 2 0 0 . 3 0 . 2 0 . 4 — 0 . 9 1 . 8 ER21 0 . 2 0 . 2 0 . 4 0 . 1 0 . 2 1. 1 ER22 — 0 . 1 0 . 6 0 . 4 0 . 2 1 . 3 135 TABLE AIII.3. Movement between the 1,5 and 4,5 stake pair at the f i v e -stake arrays. Values are i n centimeters. ARRAY 15/5/81 3 / 7 / 8 1 3 / 1 0 / 8 1 1 6 / 5 / 8 2 9 / 7 / 8 2 T o t a l WRO-B 1 , 5 4 , 5 - 0 . 1 0 . 1 0 . 0 - 0 . 4 0 . 4 0 . 5 - 0 . 7 - 0 . 5 - 0 . 2 0 . 2 - 0 . 6 - 0 . 1 WRO-A 1 , 5 4 , 5 - 0 . 9 - 0 . 5 - 0 . 3 - 0 . 9 0 . 7 1.1 - 0 . 1 - 0 . 6 0 . 7 0 . 2 0 . 0 - 0 . 7 WR1 1 , 5 4 , 5 0 . 0 0 . 1 0 . 0 0 . 0 - 0 . 5 — — - 0 . 5 0 . 1 WR2 1 , 5 4 , 5 - 0 . 1 0 . 2 - 0 . 4 - 0 . 6 0 . 9 0 . 2 - 0 . 1 - 0 . 4 — - 0 . 6 - 0 . 6 WL6 1 , 5 4 , 5 1 . 3 0 . 5 0 . 1 - 0 . 5 0 . 9 - 0 . 1 2 . 0 - 0 . 4 0 . 2 0 . 0 4 . 5 - 0 . 5 EL4 1 , 5 4 , 5 - 0 . 4 - 0 . 8 — 0 . 1 0 . 3 - 0 . 4 1 . 5 1 . 0 1 . 6 0 . 3 2 . 6 OLD LATERAL 0EPOS/T STABLE UNSTABLE FIGURE A I I I . l . Diagram of stakes 1, 4, and 5 i n the five-stake arrays. The diagram shows the negative components of motion. 136 TABLE AIII.4. Measured displacement ( i n centimeters) between the 2,3 stake pair situated on the unstable earthflow debris. ARRAY 15/5/81 3/7/81 3/10/81 16/5/82 9/7/82 Total WR8 0.2 -0.5 0.6 0.0 0.0 0.3 WR6 -0.3 0. 1 -0.7 -4.8 -0.2 -4.1 WL6 0. 1 -0.3 0.7 -0.6 -0. 1 -0.2 WL5 0.0 -0. 1 0. 1 -0.5 0. 1 -0.4 WL4 0. 1 -0. 1 -0.5 0. 1 0.3 -0. 1 ER5 -0.5 0.2 -2.8 -0.9 0.6 -3.4 WL3 0.0 0.2 0.6 -1.8 0.2 -0.8 WL2 0.0 -0.7 1.6 -1.2 0. 1 -0.2 WL1 -1.5 0.3 -0.5 -2.5 -0. 1 -4.3 WLO — — — — -0.8 -0.8 WRO-B 0.2 -0.3 0.4 -0.6 0. 1 -0.2 WRO-A -0.5 0. 1 -0.2 -0.4 0.1 -1.0 WR1 -0. 1 0.0 0.4 -0.5 — -0.2 WR2 0.3 -0.6 -0.4 -0.1 0.0 -0.8 WR3 -0.6 0.3 0.5 -0.9 -0.3 -1.0 EL4 -0.9 — 0. 1 -0.7 0. 1 -1.4 EL3 0.4 -0.2 0.5 0.0 -0. 1 0.6 EL2 -0.2 -0. 1 0. 1 -0.4 -0. 1 -0.7 ELI 0.5 0.4 1. 1 0.1 0.0 2.1 ELO 0.3 0. 1 0.9 — -0. 1 1.2 ER4 -0.5 0.0 0.3 -0.1 0.4 0.1 ER3 — — — — — — ER2a -0.2 0.2 0.2 -0.3 -0.1 -0.2 ER2b 0.0 0.0 0.3 -1.2 0.0 -0.9 ER1 -0.3 0.0 0.6 -0.5 -0.3 -0.2 ER20 -0. 1 0.0 0.4 — -0.6 -0.3 ER21 0. 1 -0.2 -0. 1 0.0 -0.1 -0.3 ER22 0.0 -0.1 2.4 -1.0 0.2 2.4 137 . UNSTABLE 4 STABLE FIGURE AIII.2. Diagram showing the p o s i t i v e and negative components movement between the 2,3 stake pair within the stake arrays. 138 

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