Ministry of Province of Environment British Columbia WATER and Parks MANAGEMENT MEMORANDUM

BRANCH

W To: A.P. K o h u t

Senior Geological Engineer Groundwater Section

Date: April 9 , 1988

Our File: 82E/12 #31

Re: Trout Creek Landslide

INTRODUCTION

As requested by D . Dobson, Engineering Section Head, Penticton Regional Office, a preliminary assessment of the groundwater conditions i n the area of the above has been completed. The sl ide i s situated a1 ong the north bank of Trout Creek i n the Trout Creek Canyon, 3 miles south of Summerland and has been moving continuously for about the last 60 years ( F i g . 1). Besides threatening residential and agricultural development i n the terrace area back of the slide (Paradise Flats), the slide is continually sloughing and moving material into Trout Creek and i s contributing t o the creek's si1 t 1 oad.

The Regional Office's main concern i s primarily w i t h the heavy s i l t load i n the creek because i t kills aquatic l i f e (fish food) , "chokes" the fish and s i l t s u p wave1 Sr>awnihg beds for the kokanee (pers comm. C.J. B u l l , 1987) The creek must be si1 t-free from f a l l t o spring i f i t is t o benefit kokanee (assuming t h a t water quantity and qual i ty conditions outside of the slide d u r i n g t h a t period are a1 so adequate t o sustain the kokanee). The true worth of Trout Creek from kokanee i s estimated a t $3,000,000 ( B u l l , 1987). These considerations affect the p l a n n i n g of any remedi a1 measures t o stabil ize the slide for Fisheries' benefit.

Numerous prel imi nary studies have been done on the sl ide. Much of this memorandum is based on a review of these previous studies, especially work by Rig1 i n (1977) as we1 1 as from interpretation o f a i r photos covering different periods and field visits by Groundwater Section staff i n June, 1987. This memorandum out1 i nes the physical and hydrogeological conditions a t the slide and recommends further investigative work t o determine stratigraphic and hydrogeologic factors of the slide i n greater detail so t h a t appropriate and effective remedi a1 measures can be planned.

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*E DESCRIPTION

The s l ide has devel oped a1 ong the edge of a sandy terrace above Trout Creek. From the ground (and from a i r photos) the sl ide is obvious and we1 1 defined (Photo 1 and 2 ) . The bounding failure surface is evident a1 ong the back and side scarps and i s exposed a1 ong the toe, 150 f ee t above the creek. W i t h i n the slide the land surface is stepped i n appearance and has an overall sl ope of about 22%.

Three main sl ide blocks - the upper, middl e , and 1 ower blocks - occupy most of the slide (Figs. 2 and 3 ) . Failure surfaces separate these main blocks as a resul t of differential movement. Numerous other failure surfaces exist w i t h i n the main blocks themselves especi a1 l y i n the middl e and lower blocks as a resul t of differential movements w i t h i n the blocks. The appearance and geometry of the sl ide indicate t h a t differential slumping and backward rotation has occured and t h a t the blocks have a1 so moved la te ra l ly downhi 11 . Overall, the slide measures a maximum 1,400 f ee t i n length from head to toe and maximum 1,400 f e e t from side to side ( F i g . 2 ) . However, the slide volume i s unknown because the bounding failure surface beneath the sl ide i s unknown.

LOCAL GEOLOGY

Terraced gl aciofl uvial sand and gravel deposits line the lower reaches of Trout Creek, including the sl i de area. Presence of the gl aciofl u v i a1 deposits as far up the creek as the slide area suggests that the water level of Okanagan Lake was probably about 200 f e e t higher during the ice age than today. These deposits were probably formed during the las t i ce age where Trout Creek emptied into Okanagan Val ley (Nasmith, 1962).

Geologically, the sl ide area i s situated a1 ong the southern boundary of the Summerland Basin, an assemblage of Tertiary volcanic rocks (Church, 1980). Pre-Tertiary granitic basement rocks bound the Summerland Basin to the west and south (Figure 1 ) . The southern pa r t of the Summerland Basin is truncated by the Summerland F a u l t which is believed to run subparallel t o the 1 ower reaches of Trout Creek and near the sl ide area (Figure 1).

Much of the slide involves the u n i t of glaciofl uvia l sand and gravel which i s saturated a t i t s bottom section (F igs . 2 and 3 ) . The overall thickness and saturated thickness of this u n i t beneath the sl ide i s unknown. Beneath this sand and gravel u n i t is a thinner assemblage of t i l l , clay, and s i l t and weak bedrock comprised of light-grey volcanic sandstone, claystone, and coal. This weak bedrock may be part of the Tertiary volcanics mapped by Church ( 1980). T h i s assemblage is exposed only i n the 1 ower block a1 ong the toe and i s broken up, sheared, and repeatedly upthrusted (Photo 3 ) . A1 ong the toe, the failure surface occurs w i t h i n this weak assemblage and above competent orange and green granitic bedrock (probably the Pre-Tertiary basement rocks) which forms the Canyon Wall (Photo 4 ) . The slide doesn't

appear to involve the granitic rock, b u t only the materials overlying i t .

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V F i e l d i n s p e c t i o n and l i m i t e d d r i l l l o g i n f o r m a t i o n from Golder Associates L td . (1979) ind ica te tha t the whole sect ion of unconsol idated sediments and weak T e r t i a r y bedrock i s t h i c k e r a t t h e s l i d e t h a n i n t h e a d j a c i e n t areas; the competent Pre-Tertiary bedrock appears to be loca ted h igher on e i t h e r s i d e o f t h e s l i d e t h a n a t t h e s l i d e . I n fac t , t he weak T e r t i a r y bedrock appears to e x i s t o n l y l o c a l l y a t t he s l i de . Th i s sugges ts t ha t t he s l i de occurs wi th in a Te r t i a ry bu r ied va l l ey f ea tu re wh ich i n te rsec ts T rou t Creek Canyon.

SPRING DISCHARGE AND GROUNDWATER FLOW

Much of the lower block appears saturated. Upper spr ings occur a1 ong the head o f the lower b lock and t h e s p r i n g w a t e r e i t h e r c o l l e c t s i n h o l l ows f u r t h e r down the lower b lock and then resur faces a t the toe as lower spr ings o r i n c i s e s a steep channel i n the s l i de ma te r ia l (F igs 2 and 3 ) . On reach ing t he t oe o f t he s l i de t he wa te r f l ows ove r t he g ran i t i c bed rock l i p and then down to Trou t Creek. Spr ing water cont inual ly erodes and moves the s l i d e m a t e r i a l to the toe where i t sloughs b i t by b i t onto the ta l us s lopes be l ow. Spr ing f low over the lower par t o f the s l ide and down the loose, s teep ta l us becomes s i 1 t laden as it reaches Trout Creek (Photos 4.W)Total d ischarge i n June, 1987 i s e s t i m a t e d a t a few hundred gpm. Nowhere e lse a long the Canyon Wall i s groundwater discharge as obvious as a t t h e s l i d e .

The groundwater feed ing the spr ings l i ke ly o r ig ina tes f rom the te r race a rea beh ind the s l ide and f lows through the sand and g r a v e l a q u i f e r u n i t and maybe a1 so through the under ly ing weak T e r t i a r y assemblage as we1 1 . T h i s i s i n f e r r e d from:

1. inspect ion o f topography - t h e s l i d e l i e s d i r e c t l y d o w n h i l l f r o m t h e ter race area

2. prescence o f n i t ra tes in the spr ing water (Rig l in , 1977) suggests that t he wa te r l i ke l y o r i g ina tes f rom ag r i cu l tu ra l a reas ( the most immediate i s t h e t e r r a c e a r e a )

3. peak spr ing d ischarge cor re la tes with i r r i g a t i o n p a t t e r n s i n t h e terrace area (Rig1 in, 1977).

No springs are apparent i n t h e P r e - T e r t i a r y g r a n i t i c r o c k .

SLIDE MOVEMENTS

Removal o f s l i d e m a t e r i a l a t t h e t o e by spr ing d ischarge removes the b u t t r e s s s u p p o r t f o r t h e s l i d e and a1 1 ows the 1 ower b lock t o move cont inuously as a waterlogged mass downhi l l probably a1 ong the sur face o f the Pre-Ter t ia ry g ran i t i c rock . Downh i l l movement o f the lower b lock reduces support for the middl e block causing it t o move and so on as f a i l u r e g radua l l y and continuously reaches back up-the s l ide . Prev ious ground

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w measurements by R i g l i n (1977) and Madsen (1974) and f rom i nspec t i on o f a i r photos cover ing d i f fe ren t years show t h a t , i n general , recent movements i ncrease from the upper block ( about 1 foot /year) downhi l l to the lower b lock (up to 35 fee t /year ) and from the sides to the cen t re o f t he s l i de (F igs. 2 and 3 ) . W i th in a yea r t he ra te o f movement seems to co inc ide w i th t h e r a t e o f s p r i n g d i s c h a r g e , o c c u r r i n g g r e a t e s t i n l a t e fall ( R i g l i n , 1977 1.

Rate of movement over the years i s uncer ta in because no long-term data are ava i lab le . However, t h e s l i d e i s bel ieved to have i n i t i a t e d i n t h e p e r i o d 1914 t o 1917, abou t 10 yea rs a f te r i r r i ga t i on wa te r was first introduced onto the terrace area. Mathews (1975) in fer red that over ha1 f of t h e t o t a l s l i d e movement occu r red du r ing t he i n i t i a l f a i l u re . Apparen t l y t he s l i de has been moving cont inuously a t a steady rate eversince.

I n recen t yea rs , much of the overburden cover on the upper block was s t r i p p e d away and pa r t o f t he b lock was mined fo r g rave l (photo 1 ) . Th is would tend to enhance i n f i l t r a t i o n o f any p r e c i p i t a t i o n and i r r i g a t i o n r u n o f f i n t o t h e upper block which would promote greater sl ide movement.

TALUS SLOPES

Steep t a l us cones have formed bel ow the l i p o f t h e s l i d e down to the bank o f Trou t Creek as a resul t of accumulat ion of s loughed mater ia l f rom the lower block (Photo 2 ) . The slope ravels c o n s t a n t l y t r y i n g t o a d j u s t i t s slope angle. Materi a1 i s c o n t i n u a l l y b e i n g added t o the ta l us s lopes f rom the top and removed from the bottom by Trout Creek. This eros ion a t the bot tom apparent ly a1 so cont r ibu tes to the c reek s i1 t load especial l y dur ing f r e s h e t . I n June 1987, however, it was apparent that spr ing f low was b r i n g i n g more s i1 t in to the c reek than tha t caused by eros ion o f the ta l us slopes (Photo 5). Movement o f t h e t a l us slope has no t been we1 1 studied.

DISCUSSION

Evident ly local geology and groundwater condi t ions great ly in f luence the s l i d e s t a b i l i t y w h i c h i n t u r n a f f e c t s t h e s i 1 t a t i o n of Trout Creek. The t h i c k o v e r b u r d e n s e c t i o n a t t h e s l i d e a c t s as a bas in o r channel f o r groundwater fl ow and the occurrence of the til 1 , clay, and weak T e r t i a r y volcanic sandstone, claystone, and coal render the s lope suscept ib le to f a i l u r e . A1 though groundwater di scharge was l i k e l y p r e s e n t even before the i n i t i a l s l i d e f o r m a t i o n (some spr ings had always been present along the Canyon Wall 1, h i s t o r y suggests t h a t i n c r e a s e d i r r i g a t i o n on the te r race probab ly t r iggered the sl i d e by increasing recharge into the groundwater system and ra is ing the groundwater leve l thereby decreas ing the e f fect ive shear s t rength o f the ground thus leading to fa i lure. Factors such as topography, spring water chemi s t r y , i r r i g a t i o n , and s p r i ng d i scharge p a t t e r n s s u g g e s t t h a t r e c h a r g e f o r t h e s p r i n g s i s p e r p e t u a t i n g i n s t a b i l i t y and comes from the terrace area, where i r r i g a t i o n i s o c c u r r i n g . S p r i n g

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V runoff is eroding the 1 ower portion of the sl ide and the tal us sl opes b r i n g i n g s ignif icant amounts of s i l t to Trout Creek. Control 1 i ng groundwater flow and protecting the talus slopes against erosion by Trout Creek on a year-round basis are potential measures which would l ikely s tab i l ize s l ide movement and minimize the si1 tation problem for Fisheries' benefit. Remedi a1 measures for sl ide stabil ization may involve for example (refer a lso t o F i g . 4 ) :

A. dewatering we1 1 s on the terrace back of the sl ide to 1 ower the groundwater level by intercepting the flow before i t reaches the s l i de

B. horizontal drains a1 ong the lower block t o dewater the toe and 1 ower section of the sl i de

C. an interceptor ditch a1 ong the toe t o col lect spring runoff a f t e r i t comes o u t of the slope and t o protect the toe from erosion as

recommended by Smythe (1975).

The water collected woul d have to be piped away from the sl ide. The rate of movement a t the toe suggests t h a t an interceptor ditch however woul d need frequent maintenance and may even be destroyed w i t h i n a year. Furthermore, an interceptor ditch i n t h i s case woul d simply col lect spring discharge and would n o t dewater the sl ide.

Horizontal drai ns may work provided that a d r i l l i ng rig can be b rough t into the lower section of the sl ide and the holes can penetrate far enough into the slope to effectively intersect the sand and gravel u n i t beneath the sl ide to lower the groundwater level . Maintenance and possible rep1 acement for the horizontal dra ins may a1 so be required.

On the other hand dewatering we1 1 s i n the up1 and terrace above the sl ide could remain as permanent i n s t a l l a t i o n s , largely unaffected by s l ide movements.

A t the p resent , there i s insuf f ic ien t unders tanding of the local stratigraphy , structure ( i ncl udi ng the f a i l ure surfaces) , hydro1 ogy (water 1 eve1 s , hydraulic conductivit ies, storativit ies, f low), and geotechnical aspects (soil strength, resistance a1 ong the failure surfaces, rates of movement) of the slide t o recommend the most cost-effective measures:

1. woul d dewatering we1 1 s be effective for example i n intercepting groundwater flow before i t reaches the sl ide? How many we1 1 s would be required, where, what size, how deep, the pumping rates, costs of construction and pump instal la t ions, e tc .

2. would horizontal drains a t the toe be effective? how far into the slide can horizontal drains be constructed (will depend on local geology and access), will they be relatively maintenance-free etc .

The significance of the talus slopes on si1 ta t ion of Trout Creek has not been we1 1 documented. More information such as the nature and rates of

movement, t iming , and degree of tal us slope erosion i s needed to decide what

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V approp r ia te p ro tec t i ve measures should be taken such as r i p rapping, bu i ld ing p ro tec t ive wa l ls , p lan t ing vegeta t ion , e tc .

Under present condi t ions, not on ly will s i 1 t a t i o n o f T r o u t Creek from spr ing discharge and e ros ion a t t he s l i de con t inue bu t t he s l i de wou ld con t i nue moving, f a i l i n g p r o g r e s s i v e l y f u r t h e r back onto the terrace area.

RECOMMENDATIONS

The f o l l owing i n i ti a1 steps are recommended i n gather ing necessary s i te s p e c i f i c i n f o r m a t i o n on t h e s l i d e t o h e l p p l a n a p p r o p r i a t e s t a b i l i z a t i o n measures:

1. d r i l l up t o f i v e t e s t w e l l s t o d e t e r m i n e w a t e r l e v e l s , h y d r a u l i c gradients , groundwater f low, hydraul ic conduct iv i t ies and s t o r a t i v i t i e s o f t h e s l i de mater i a1 s. The t e s t w e l l s shoul d be 1 ocated a1 ong the s l i de cross-sec t ion where access ib le and along the area back o f t h e s l i d e . The tes twe l ls shou ld be d r i l l ed t h rough the sand and g r a v e l u n i t and under ly ing til 1-clay-weak Tert iary bedrock assemblage to the sur face o f the g ran i t i c bedrock . The t e s t w e l l s should be d r i l l e d by the cab le too l method with cont inuous tube sampl ing and completed with screens and pump tes ted to de termine aqu i fe r charac ter is t i cs . Cos t o f f i ve I ) - inch , ZOO-foot deep tes twe l ls wou ld cos t up to roughly $110,000 exc lud ing engi neer i ng supervisory costs (Appendix I ) . Af te r we l l comp le t i on and tes t ing , the we l ls shou ld be f i t t e d w i t h automat ic water leve l recorders to mon i to r f luc tua t ion o f the water 1 eve1 i n t h e s l i d e area.

2. document i r r i g a t i o n p r a c t i c e s i n t h e t e r r a c e a r e a and, i f poss ib le , check f o r any leakages i n t h e i r r i g a t i o n p i p e l i n e system.

3 . i n c o n j u n c t i o n w i t h t h e above steps, a comprehensive geotechnical appra isa l o f the s l ide shou ld be undertaken so t h a t any hydroqeologic work undertaken i s i n t e g r a t e d w i t h i n an overa l l s tudy o f t h e s l i d e complex.

Once subsurface information, groundwater condit ions, and s l i d e movements are b e t t e r understood, the most appropriate remedial measure, i f any, can then be designed. Monitor ing of s l ide movements, groundwater levels, and water qual i ty a t T r o u t Creek a f t e r imp1 ementation of remedi a1 measures would be imperat ive to assess the actua l e f fect iveness of any remedial program.

REFERENCES

B u l l , C.J . 1987. Trou t Creek. Unpublished Memorandum. M i n i s t r y of Environment and Parks. F i l e 40.3502 Trout Creek.

Church, B.N. 1980. Anomalous Uranium i n t h e Summerland Cal dera. Paper 1980-1, Geological F i e l d Work, 1979. M i n i s t r y o f Energy, Mines and Resources.

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L, Golders Associates. 1979. Review of Monitor ing for Paradise Flats Go1 f Course, Summerl and, B.C. Unpublished Repor t to The Corp. of the District of Summerl and.

Madsen, K . 1974. Letter t o K. Smyth, Hydraulic Engineer, Water I n v e s t i g a t i o n s Branch.

Mathews, W .H. 1975. Perpetual S1 ide Maps - Summerl and, B .C . Letter t o K . Smyth, Water I n v e t i g a t i o n s Branch.

Nasmith, H.W. 1962. Late Glacial His tory and S u r f i c i a l D e p o s i t s of the Okanagan Val l e y , British Columbia. Bulletin No. 46. Department of Mines and Petroleum Resources.

Rig1 i n , L . 1977. The Perpetual Lands1 ide, Summerland, British Columbia. M.Sc. Thesis, U.B.C.

Smyth, W.W.K. 1975. Trout Creek Ecological Overview ( Including Perpetual S1 ide) and Ten ta t ive Recommendations. Draft Report . Department of Lands, Forests and Water Resources.

Mike Wei Geological Engineer Groundwater Section 387-9463

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APPENDIX 1

- -e l i rninary Cost Es t ima te , Tes twe l l Dr i l l i ng P rogram, T rou t Creek S l i d e , h n m e r l a n d . B.C. *

ITEM - Mobi l iza t ion and d e m o b i l i z a t i o n Move between s i t e s and se t up

I n s t a l l l O " P , s u r f a c e c a s i n g t o 1 5 f e e t 8"B Drive shoe 8"P) over1 ap casing 8"8 c a s e d d r i l l i ng and sampl i ng

Hourly work Screens and f i t t i n g s Standby

COST

1 4

7 5 ' 5

7 5 ' 925 I

200 hrs.

40 hrs. 4

Mobil izat ion and demobil izat ion of t e s t pump 1 Ins t a l 1 and remove t e s t pump 5 Pumping t e s t 120 hrs.

+15% cont ingencies

$ 2,000 $ 600

$ 3,375 $ 1,200 $ 1,500 $ 37,000

E 20,000 $ 15,000 $ 2,000

$ 2,000 $ 2,500 $ 9,000

$ 96,175 $ 14,425

*Excluding engineering s u p e r v i s o r y a n d a d m i n i s t r a t i v e costs