LES COULEES BOUEUSES DE CATHEDRAL MOUNTAIN, CANADA. L.E. JACKSON ... `5 sc produire en 1925 et lear frEquence a augmentd jusqu'en 1985.
35
BULLETIN i i
of the International Association of ENGINEERING GEOLOGY de I'Associationlnternationalede GEOLOGIE DE L'INGENIEUR
CATHEDRAL MOUNTAIN DEBRIS FLOWS, LES COULEES
BOUEUSES
DE CATHEDRAL
L.E. JACKSON,
Jr.*, O. H U N G R * * ,
N~
40
PARIS
1989
CANADA MOUNTAIN,
J.S. G A R D N E R * * * ,
CANADA
C. M A C K A Y * * * *
Abstract Historic debris flow activity along the north side of Cathedral Mountain in the southern Rocky Mountains of British Columbia. began in 1925 and has increased in frequency up to 1985. A typical debris flow event involves approximately 100,000 m 3 of material. Debris flow velocities and discharges above the head of the fan crossed by the Trans-Canada Highway and the C.P.R. mainline are 5.5 m/sec and 210 m3/sec. Most of the large debris flow events are associated with jEkulhlaups from Cathedral Glacier. JEkulhlaup discharges of at least 10,000 and perhaps as much as 24.000 m 3 of water mobilize these debris ['lows. Part of the water may have come from a small ephemeral lake on the south side of the glacier. The balance must have been stored within the glacier. The onset and acceleration of debris flow activity was apparently induced by the recession of Cathedral Glacier. Source areas of debris flow sediments have retreated upslope since initiation of debris flow activity. C.P.R. began pumping meltwater from the glacier in 1985 and no j/Jkulhlaups or significant debris flows have occurred since. This preventive measure should either eliminate j'Okulhlaups or reduce their magnitudes should they occur. Without jEkulhlaups, debris flow hazard in the area should be reduced both in frequency and in magnitude.
REsumE Les coulees boueuscs sur le c6td Nord de Cathedral Mountain. partie sad des Montagnes Rocheuses de Colombie-Britannique ont commence `5 sc produire en 1925 et lear frEquence a augmentd jusqu'en 1985. Une coulee typique se caractErise par un d,Splacement d'environ 100 0 0 0 m 3 de matEriaux. La vitesse et le debit des coulees au-dessus du cEne d'Eboulis traverse par I'autoroute trans-canadienne et par la voie ferrde sont respectivement de 5,5 m/sec et 210 m3/sec. La plupart des coulees se produisent lots de venues d'eau brutales provenant du glacier de Cathedral Mountain. Ces venues d'eau d'au moins 10 000 m 3 et peut-~.tre jusqu'`5 24 000 m 3 sont .5 I'origine des coulees. Une partie de l'eau provient probablement d'un petit lac temporaire situE sur le c6td sad du glacier. Le reste est stock6 dans le glacier. La frdquence plus dlevEe des coulees est apparemment liEe au fecal du glacier. Les zones de mobilisatioo des debris entra~nEs par les venues d'eau sont situEes plus haut sur les pentes qu'au debut de I'activitE des coulees. La sociEtE gestionnaire de la vole ferrde a commence `5 pomper I'eau de fonte darts le glacier en 1985 et depuis aucune venue d'eau brutale ni coulee boueuse importante ne se sont produites. Cette mesure preventive devrait ou dliminer les venues d'eau brutales ou rEduire tear importance si elles se produisent. Sans ces venues d'eau, le risque de coulees boueuses devrait ~tre rdduit, aussi bien en frEquence qu'en volume.
Introduction Debris flow is a c o m m o n phenomenon in t h e Canadian Cordillera (Van Dine, 1985). Passes t h r o u g h m o u n t a i n r a n g e s w i t h i n this r e g i o n are f e w and routes must of necessity cross snow and rock avalanche courses and debris flow fans. C o n s e q u e n t l y , rail a n d h i g h w a y r o u t e s h a v e b o r n e the b r u n t o f d e b r i s f l o w d a m a g e in t h i s r e g i o n . A d e b r i s flow fan below the north slope of Cathedral Mountain in K i c k i n g H o r s e P a s s h a s b e e n t h e s c e n e o f h i s t o r i c d e b r i s f l o w a c t i v i t y ( F i g . 1). D e b r i s f l o w s h a v e blocked the Canadian Pacific Railroad (C.ER.) main line and the Trans-Canada Highway on a number of
occasions since 1925. The investigations of the c a u s e s o f d e b r i s f l o w s at t h i s s i t e b e g a n w i t h J a c k s o n ( 1 9 7 9 a a n d b; 1 9 8 0 ) f o l l o w i n g d e b r i s f l o w e v e n t s in 1 9 7 8 . S i n c e t h i s w o r k , o t h e r a u t h o r s ( G a r d n e r , 1 9 8 2 ; S a u c h y n et al., 1 9 8 3 ; D e s l o g e s a n d G a r d n e r , 1 9 8 4 ) a n d u n p u b l i s h e d w o r k b y O. H u n g r a n d G . C . Morgan of Thurber Consultants Ltd., staff engineers w i t h C.P., Rail, R . M . H a r d y a n d A s s o c i a t e s ( 1 9 7 8 ) Ltd., and glaciologists with Environment Canada, have further elucidated debris flow triggering processes and have apparently achieved some control over them. This paper presents a case history of the e v o l u t i o n o f d e b r i s f l o w a c t i v i t y at this site b a s e d upon this collective work.
* Geological Survey of Canada Terrain Sciences Division 100 West Pender St. Vancouver. British Columbia V6B IR8 Canada. Geological Survey of Canada contribution 14088. ** Thurber Consultants Ltd. Suite 200. 1445 West Georgia Street Vancouver. British Columbia V6G 2T3 Canada. *** Department of Geography University of Waterloo Waterloo. Ontatrio N2L 3GI Canada. **** C.P. Rail Ltd. Engineering-System Windsor Station P.O. Box 6042. Station A Montreal, P.Q. Canada H3C 3E4.
36 L o c a t i o n a n d site history
The pass was first explored by the Palliser Expedition in 1858, The C.P.R. route was constructed in I884. The original 4.5 % rail grade down the west side of Kicking Horse Pass was replaced by C.P.R. in 1909. The revised 2.2 % grade crosses the debris flow fan of Cathedral Gulch at three levels, switching back in two spiral tunnels. The three track levels are named, from top to b o t t o m , Partridge, Yoho and Cathedral. The debris flow fan of Cathedral Gulch is located midway b e t w e e n the tunnels. All three levels of track cross the fan.
The northern slopes of Cathedral Mountain are located in Kicking Horse Pass, the lowest point in the central Continental Divide of the Rocky Mountains, which separates drainage to the Pacific Ocean and Hudson Bay (Fig. 1). The pass and the adjacent upper Kicking Horse River valley feature some of the most spectacular scenery in the Rocky Mountains. Glacier-mantled peaks including Cathedral Mountain rise up to 1 000 m above the valley floor which is less than 1 km wide.
-"(
l
L J\ British
~
The original rail grade was used by the Kicking Horse Highway, c o n s t r u c t e d in 1924, and sub-
520 \
Alberta
PAGETp AK
Kicking Horse Pass
C.~
Columbia ~
_
5000 "
~
Wapta
.
~, 5
~
/(
c
,
"~CATHEDRAL MOUNT"
"-
STEPHEN,
/
Lake
La
!
'
1 "CATIEIRA L
~VANGUARD
~
F-"
LEGEND Debris Flow Path Drainage Area Bdry.
0 I
1 t
2 t km
Fig. 1 : Location of Kicking Horse Pass and Cathedral Gulch and geographical features refered to in the text.
3 I
37 s e q u e n t l y by the T r a n s - C a n a d a H i g h w a y c o n s t r u c t e d in 1958.
T a k i n g into a c c o u n t the e f f e c t s o f a l t i t u d e (Janz and Storr, 1977), the a v e r a g e annual p r e c i p i t a t i o n at C a t h e d r a l G u l c h is e s t i m a t e d to be a p p r o x i m a t e l y 750 mm at the track l e v e l , rising to about 1 250 mm at the s u m m i t of C a t h e d r a l C r a g s . M o r e than 70 % o f this falls in the f o r m o f snow a b o v e the 2 000 m elevation.
Climate The c l i m a t e o f the K i c k i n g H o r s e Pass area is continental m o d i f i e d by m o u n t a i n o u s terrain. F i g u r e 2 s u m m a r i z e s the m e a n m o n t h l y p r e c i p i t a t i o n and t e m p e r a t u r e for the two c l i m a t e stations nearest to the site, B o u l d e r C r e e k 12 km to the west at El. 1 295 m and W a p t a L a k e 4 km east at El. 1 591 m. Both have r e l a t i v e l y short p e r i o d s o f record. The d a t a shown in the f i g u r e are " a d j u s t e d n o r m a l s " , that is the b r i e f r e c o r d p e r i o d has been c o m p e n s a t e d by m e a n s o f a r e g i o n a l s t a t i s t i c a l c o m p a r i s o n c a r r i e d out by E n v i r o n m e n t C a n a d a .
The m a x i m u m 24 h o u r p r e c i p i t a t i o n at L a k e L o u i s e 15 km east o f the site w a s 52 m m on June 26, 1915. The m o n t h l y m e a n d a i l y t e m p e r a t u r e at W a p t a L a k e is a p p r o x i m a t e l y 0.2 ~ C. A s s u m i n g a t e m p e r a t u r e g r a d i e n t of 0.5 ~ C p e r 100 m, the m o n t h l y m e a n d a i l y t e m p e r a t u r e at C a t h e d r a l G l a c i e r is about 6.8~ A p p l y i n g the s a m e c o r r e c t i o n to the m o n t h l y m e a n v a l u e s shown in F i g u r e 2, the m o n t h l y m e a n d a i l y t e m p e r a t u r e s at the g l a c i e r are a b o v e the f r e e z i n g
140 120
BOULDER CREEK El. 1 2 1 9 m
100 O
80
Annual T o t a l s :
,.J ~ ""
60
Rain 3 0 8 . 6 m m Total 557.3mm
n
40
(1970-1980)
,-, ~ E •:g E
20 0 J
F M A M J
J
A SO
N
D
15
lO ~
5
o
E
-5
9
-10
~-
-15
l +0"2~
1
,120 ,o ._oc
I
WAPTA LAKE El. 1 6 5 0 m
100
Annual T o t a l s : c ~ ,,-,, = ..='_ E
80
-
Rain 3 2 2 . 6 m m Total 946.6mm
60 O
(1974-1980)
ft.
4o
2o 0
I J
*
I I 1 I FMAMJ
I
I J
I I ASON
I
I D
Adjusted Normals
Fig. 2 : Summary of mean monthly precipitation for Boulder Creek (El. I 295 m) and mean monthly precitation and temperature data Wapta Lake (El. I 591 m) climate stations (data courtesy of Environment Canada).
38 point for only 3 m o n t h s of the year, with a m a x i m u m of 4.9 ~ C in Julv.
/,
//
Historical debris flow events
/ // 0
The f o l l o w i n g are b r i e f a c c o u n t s of 7 debris flow events b a s e d on e y e w i t n e s s t e s t i m o n i e s and newspaper a c c o u n t s . The extent of the main deposits of each of the seven d e b r i s flow events of this century has been m a p p e d (Fig. 3a-f) by an a n a l y s i s of airphotos (1946 and 1962 events), ground and helicopter p h o t o g r a p h s (1925, 1978, 1982 and 1984) and dated deposits m a p p e d in the field during field work in 1985. W h e r e v e r possible, the thickness of the debris was based on e y e w i t n e s s reports or g r o u n d photographs. The a v e r a g e t h i c k n e s s of the deposits and v o l u m e of the d e b r i s flows were estimated from these n u m b e r s (Table 1).
0.1
Date
Aug. 5. 1925 II Aug. I6. 1946 i July 1. 1962 Sept. 6, 1978 Aug. 18. 1982 Aug. 27, 1984 Aug. 29, 1984 * No watererosion,
Area Covered (m22) 40.000 50.000 12,000 77,000 5,000 5,000 52,000
' :~.c- _
q
Estimated Magnitude (m3) 80.000* 90;000 24.000* 136.000 7.000 5.000 87,000
d e b r i s t h i c k l y p i l e d up.
At m i d n i g h t , A u g u s t 5, 1925, debris coverd the Partridge and Yoho tracks (Fig. 3a; Fig. 4). The track was c l e a r e d after one day. The next m a j o r flow started at 5.30 P.M. on August 16, 1946, and c o n t i n u e d u n t i l 8.00 PM of the same day, b u r y i n g each of the 3 track levels with over 3 m of debris (Fig. 3b). Some a m o u n t of e r o s i o n also occurred on the Partridge Track by a water flood which preceded the debris flow. Traffic r e s u m e d after 4 days. A s m a l l e r flow o c c u r r e d on July 1, 1962, i m m e d i a t e l y f o l l o w i n g h e a v y rains (Fig. 3c). This caused no erosion and b l o c k e d o n l y the Partridge Track. The slide occurred at n i g h t and the d e p o s i t r e m a i n e d very fluid until m i d - m o r n i n g , g r a d u a l l y spreading out on the track. The track was clearcd at n o o n of the same day. Parts of the deposit had already b e c o m e c o n s o l i d a t e d and were d i f f i c u l t to e x c a v a t e by that time. Two surges o c c u r r e d w i t h i n several m i n u t e s at 9.00 PM on S e p t e m b e r 6, 1978 (Jackson, 1979a and b). Both surges were partly d e f l e c t e d by the superele-
Fig. 3 : Reconstructions of debris flow limits (stipple pattern) on the fan of Cathedral Gulch during a. 1925, b, 1946. c. 1962, d. 1978, e. 1982. and f. 1984 events. The Trans-Canada Highway was constructed in 1958 and consequently did not play a role in deflecting prc-1962 debris flows.
/
, "'\
0.2kin
Table I : Documenteddebris flow events
1
....
I
,,f'
39
d
~
;
cAuG.2 9 ~ ~ f
40
Fig. 4 : Deposits of the August 5, 1925 debris flow (a) Partridge Track (b) Yoho Track (bath looking west; photos courtesy C.P. Rail). vated surface of the Trans-Canada Highway and advanced along it behind steep fronts over 1 m high. The first surge had a reported velocity of about 0.3 m/sec and the second 6 m/sec. Most of the deposits remained on the highway, but a smaller lobe breached the safety barrier and reached as far as the Cathedral Track. Large volumes of water flowed along the debris flow path between the surges, and following them, until the next morning. This water eroded the fill embankment at the Partridge Track up to 6 m deep (Fig. 3d). A minor event took place at 1.00 A.M. on August i8, 1982. The Partridge Track was covered, then eroded to a depth of 3 m by an extremely heavy flow
of water. A small amount o f debris was deposited on the Yoho track (Fig. 3e). In early morning of Augut 27, 1984, following a day of record rainfall, a debris flow covered the Partridge Track. The flow front was observed to move "quite slowly" onto the track. H e a v y water flow followed during the rest of the day. A much larger event occurred two days later on the morning of August 29 (Fig. 3f and 5). The event began with a major surge, which was described as resembling a rapidly m o v i n g dust cloud flowing down the slopes of the Upper Bench by an eyewitness 2 km away on the Trans-Canada Highway (Lang-
41 shaw, p e r s o n a l c o m m u n i c a t i o n , 1985). This impression p r o b a b l y refers to the a b u n d a n t spray of silty water s u r r o u n d i n g the front of the flow. Later surges were p r o b a b l y smaller, but moved down c h a n n e l above the tracks at several metres per second, without s l o w i n g d o w n or halting. This covered both the Partridge and Yoho tracks as well as the TransC a n a d a Highway. U n l i k e the 1978 event, however, this flow did not o v e r f l o w the highway barrier and only silty water reached the Cathedral Track. The main part of the debris was deposited b e t w e e n 8.30 and 10.00 AM. The greatest debris discharge probably o c c u r r e d in the initial surges, which were observed m o v i n g d o w n the upper slopes of the Cathedral M o u n t a i n by people on the highway. A large n u m b e r of s m a l l e r surges c o n t i n u e d for about one hour. After c o v e r i n g the Yoho and Partridge tracks, debris oozed onto the T r a n s - C a n a d a H i g h w a y and flowed a l o n g it at w a l k i n g speed. A b u l l d o z e r was able to work the deposits on the highway d u r i n g the event. Water c o n t i n u e d to flow down the debris flow path d u r i n g the rest of the day, but at a reduced rate. Traffic was restored in 28 hours. No other debris flows are k n o w n to have reached the Partridge Track. H o w e v e r , some debris flow activity above the Partridge c r o s s i n g may have gone u n n o ticed. For e x a m p l e , on July 10, 1980, one of the authors (J.S.G.) o b s e r v e d a surge of liquefied debris m o b i l i z e d in the talus slopes below the S u m m i t G u l l y d u r i n g a r a i n s t o r m and d e p o s i t i n g some distance downslope.
The
debris
flow p a t h
K i c k i n g Horse-Yoho R i v e r c o n f l u e n c e at 1 340 m, 7 km west of the C o n t i n e n t a l Divide (Fig. I. 6. 7 and 8). Cathedral G u l c h can be d i v i d e d into 5 distinct segm e n t s (Figs. 1 and 8), the m a i n features of which are s u m m a r i z e d in Table 2. The flow path begins in the S u m m i t G u l l y at the toe o f the north m a r g i n of the Cathedral Glacier, at El. 2 750 m. The S u m m i t G u l l y is a steep c h a n n e l cut in talus and ice-cored m o r a i n e ; The u n d e r l y i n g b e d r o c k are the c l i f f - f o r m i n g limestones and d o l o m i t e s of the Middle C a m b r i a n Cathedral F o r m a t i o n which are e x p o s e d at the head of the S u m m i t Gully. Debris flow is well c o n f i n e d and very erosive in this s e g m e n t . The U p p e r B e n c h is a r e l a t i v e l y p l a n a r area i n c l i n e d u n i f o r m l y at a b o u t 20 ~ . It is the p h y s i o g r a p h i c expression of the recessive silty shale and m a s s i v e grey l i m e s t o n e of the M i d d l e C a m b r i a n M o u n t W h y t e Form a t i o n . Its u p p e r part is covered by ice-cored m o r a i n e , rock glacier, and talus. The i c e - c o r e d m o r a i n e was d e p o s i t e d d u r i n g the Little Ice Age advance of the C a t h e d r a l G l a c i e r over the past several centuries. This a d v a n c e was the most e x t e n s i v e since the end of the P l e i s t o c e n e more than 10,000 years ago (Gardner, 1982). Till o f late P l e i s t o c e n e age lies b e y o n d the i c e - c o r e d m o r a i n e . Boulders of rockslide origin up to 30 m in d i a m e t e r are e x p o s e d along the lower m a r g i n o f the U p p e r B e n c h (Fig. 6, 8, and 9). Below the U p p e r B e n c h , the path a b r u p t l y d e s c e n d s a scarp u n d e r l a i n by the L o w e r C a m b r i a n Gog G r o u p quartzites. At least 3 c h a n n e l s d e s c e n d the 30 ~ Scarp s e g m e n t , where they are i n c i s e d into the shallow c o l l u v i a l v e n e e r as well as s l i g h t l y into the bedrock. The s o m e w h a t less steep A p r o n s e g m e n t beneath the Scarp is c o v e r e d by r o c k s l i d e blocks. The debris flow c h a n n e l is i n c i s e d a m o n g the largest of these and occupies a w e l l - d e f i n e d c o r r i d o r a p p r o x i m a t e l y 40 m wide.
Physiography and geology The d e b r i s flow path. i n f o r m a l l y k n o w n as Cathedral G u l c h (Jackson, 1979a), d e s c e n d s the north side of Cathedral M o u n t a i n from Cathedral G l a c i e r located b e t w e e n Cathedral M o u n t a i n summit (3 170 m) and Cathedral Crags (3 050 m) to a debris flow fan at the base of the slope. The fan t e r m i n a t e s near the
The 6-10 ~ Debris F a n c u l m i n a t e s 100 m upstream of the Partridge S i d i n g a n d has a r e l a t i v e l y regular c o n i cal shape.
Table 2 : Main featuresof the debris flow path segments
Segment 1. SummitGully 2. UpperBench 3. Scarp 4. Apron 5. DebrisFan
Approx . Length (m)
Altitude (rn)
Typical Slope Angle
820
2 150-2750
34~
l 080
l 850-2 150
20-22~
740
l 620-1 850
30~
290
1 540-I 620
27-31o
1 100
1 340-1 540
6-19~
Relief
Deeply incisedgully Planarwith shallow channelsand moundsof large boulders Steep, featurelessslope, main channelslightly incised Sloping, main channel slightly incised Fan-like
Underlying Material
Description
Talus, till, bedrock
The gully is incisedinto a steep apron of talus and moraine. Ice-cored colluviumand till The surface of the loose deposits is beingreworked by debris flows. Shallow colluvialveneer over bedrock
Colluvium,till (rockslide) Debris - coarse above highway, fine below
Several debris flow channelsare cut throughthe forested soil veneer to bedrock. Channelscut amonglarge boulders. Channels.levees and boulder accumulations.Older rockslide deposits showingthrough in several places.
42
Fig. 5 : August 29, 1984 debris flow deposits (a) overview of Partridge and Yoho tracks and Trans-Canada Highway (b) close-up of debris flow lobe advancing down the Trans-Canada Highway. Debris flow initiation, debris sources and transport along the debris flow path are detailed in the following sections. Cathedral Glacier The glacier (Fig. 1 and I0) has a surface area of approximately 0.59 km 2. A s s u m i n g a mean ice thickness o f 30 m, its total volume is in the order o f 30,000,000 m 3. A p r o m i n e n t feature o f the glacier is a snow ridge which extends east-west along its length and was p r o b a b l y built by westerly winds a c c u m u lating snow in the lee of the Cathedral Crags summit. There is a 25 m deep depression behind the snow ridge south o f Cathedral Gulch (Fig, 10). An ephemeral lake f o r m s partly on the surface o f the ice and partly on the adjacent bedrock at the center o f this depression. T h e depression in which the lake is situated has a v o l u m e o f about 190,000 m 3. However, observations o f the lake beginning in the late 1940's suggest that the lake has never contained more than 4 000 m3of water. The lake is n o r m a l l y no more than a knee-deep puddle which drains freely down an adjacent fissure in Cathedral Glacier. This fissure has been a permanent feature since at least 1979 (Fig. 11; Jackson, 1980). The drainage area o f the lake depression is a p p r o x i m a t e l y O. 113 km 2. A s s u m i n g a mean annual precipitation on the g l a c i e r of 1 250 mm and losses
due to evaporation and infiltration o f 35 %, the mean v o l u m e of yearly s n o w m e l t into the ephemeral lake and d o w n the drainage fissure is estimated to be 106,000 m 3. H o w e v e r , d r a i n a g e d o w n this fissure is periodically i m p e d e d c a u s i n g the lake surface to rise m a r k e d l y (Fig. I1). S u b s e q u e n t work by C.P.R. has d e m o n s t r a t e d a pressure c o n n e c t i o n between this drainage fissure and the m e l t w a t e r outlet at the toe of Cathedral Glacier a b o v e the S u m m i t Gully (See J6kulhlaup abatement measures). The north tongue o f the g l a c i e r at the head o f the Cathedral Gulch has been the source o f several catastrophic outbursts o f water or j6kulhlaups (Jackson, 1979a and b). The e p h e m e r a l lake behind the snow ridge (Fig. 6, 10 and 11) has been suspected by various investigators as b e i n g either the sole source (Brawner, 1978, written c o m m u n i c a t i o n ) , or a seco n d a r y source (Jackson, 1980; Holdsworth, 1984, written c o m m u n i c a t i o n ) o f the ourburst waters. Debris flows w h i c h have o c c u r r e d f o l l o w i n g days o f little or no precipitation (Table 3), namely on September 6, 1978, A u g u s t 18, 1982, A u g u s t 29, 1984 and A u g u s t 5, 1925, were u n q u e s t i o n a b l y caused by j6kulhlaups. Only the 1962 and the A u g u s t 27, 1984 events were p r e c e d e d by substantial rainfall. High discharges o f water out o f outlets near the ice terminus in the S u m m i t G u l l y were observed shortly after the events o f 1978 (Jackson, 1979 : Fig. 12a), 1982, and both A u g u s t 27 and 29, 1984.
43 Table 3 : Rainfall records relative to the historic debris flow events Toial Rainfall (ram) Date
, Aug. 5. 1925 ! Aug. 16, 1946 July 1, 1962 ! Sepi. 6, 1978 , Aug. 18. 1982 I Aug. 27. 1984 LAug. 29, 1984
Day of event 0 0 17.5 3 0 8.2 0
Ww o
days* --V-
* includingthe day of the event ** .gaugeoverflowedon Aug. 26 I LakeLouise.Albeaa Yoko Park (Wapta), B.C.
11.9 17.5 6 0 4 ~ ~**
1;o
Seven days*
Fourteen days*
0 18.8 25.8 29.5 27.8 49.2** 50.2**
6.6 24.1 25.8 42.7 44.6 58.9** 58.7**
i
Station
T h e a m o u n t o f a d d i t i o n a l w a t e r c o n t e n t r e q u i r e d to m o b i l i z e d e b r i s a l o n g C a t h e d r a l G u l c h i n t o the d e b r i s f l o w s m a y b e r e l a t i v e l y s m a l l . J a c k s o n ( 1 9 7 9 a ) rep o r t e d a water c o n t e n t of the l i q u e f i e d debris ranging f r o m 13 to 27 %. In c o m p a r i s o n , t y p i c a l in s i t , w a t e r c o n t e n t s o f s a t u r a t e d t i l l s o f s i m i l a r d e s c r i p t i o n in t h e R o c k y M o u n t a i n s r a n g e f r o m 10 % to 25 % ( J a c k s o n , 1987a). T h e q u a n t i t y o f w a t e r , V. r e q u i r e d to i n c r e a s e t h e water content of a sediment from a natural value w to a h i g h e r v a l u e w I in a d e b r i s f l o w is g i v e n b y t h e equations : V w =M
(w 1 - w ) / ( 0 . 3 7
+ w 1)
w h e r e M is t h e t o t a l v o l u m e o f the m o b i l i z e d d e b r i s . A s s u m i n g an i n i t i a l w a t e r c o n t e n t in u n d i s t u r b e d s e d i m e n t s o f 10 % , a f i n a l w a t e r c o n t e n t o f l i q u i f i e d
Fig. 6 : Generalized bedrock and surficial geology of the Cathedral Gulch area : 1) Cambrian carbonates and quartzites. 2) Pleistocene glacial deposits. 3) Early tlolocene debris avalanche deposits. 4) Talus. 5) Rock glacier. 6) Debris flow deposits. 7) Alluvial deposits,
44
Fig. 7 : Overall view Cathedral Gulch (debris flow path) looking south. The fan is divisible into the active eastern one-third (A to B) and the inactive western two-thirds (B to C). The dashed line separates the active from the inactive areas. Hyphenated letters indicate locations of cross sections shown in Fig. 14. Numbers indicate superelevated sections measured to compute debris flow velocity and discharge in Table 4.
45 [
FAN
APRON! -S-C-~,-R~'-- UPPER BENCH
--GULLY
',
I
GLACIER
t
Lake
g 3 ~ j -
~
- ~
3000
F 2800
-
~ c ~
$ ~,
F
Cathedral Forrn,ation (Carbonates)
~2600
I 2400
o
t,
~
~
-~
9
.~
/
~
,~
,~ ~
~-
~ ~ O
I
[ 300
I~.~
I
' - >
9~
23o -
~
_
~
~ "
"
~
~
s
l
i
d
L
e
Deb . . . . .
~ e o t r GogGroup
L
~
-
-
~ ~
I
I
F2 o0
LEGEND
~
r~
I
~
.
~
I
"
Moraine
F
Coarse Till
and T a l u s
(ice-cored)
" " Debris FIow Deposits
0 21
o 4,
o ~,
0~
1 o,
,2
~ 1800 ~1600
IIl!!l Ice and Firn
i
=. !..., .~:~'.""'!.,-'~"'~," : .'.b"~'~~ 0l
2000
i
1400 1 4'
1 ,'
1 ~'
2 0i
~2
Distance
~ 4'
~'
2 ~'
~ 0I
~2
~:4
~ '0
~'
40
(km) 0
LONGITUDINAL
200
400
600
800 ~m
PROFILE
Fig. 8 : Profile along tile centre of Cathedral Gulch showing underlying geology.
d e b r i s o f 15 % to 25 %, and a total d e b r i s flow v o l u m e o f 100 000 m 3, a m a x i m u m w a t e r quantity range o f 10,000 to 24,000 m 3 is r e q u i r e d to m o b i l i z e s e d i m e n t along C a t h e d r a l G u l c h into a d e b r i s flow. A v a i l a b l e e v i d e n c e i n d i c a t e s that the lake basin n e v e r held m o r e than 4 000 m 3 of water. This s u g g e s t s that s i g n i f i c a n t i n t r a g l a c i a l s t o r a g e of water, p o s s i b l y in c r e v a s s e s or m e l t w a t e r tunnels, c o n t r i b u t e s a d d i t i o n a l water to j 6 k u l h l a u p s .
Summit Gully The S u m m i t G u l l y (Fig. 9 and 12) is the p r e s e n t m a i n source o f d e b r i s for d e b r i s flow. A single w e l l - i n c i s e d c h a n n e l e x t e n d s for n e a r l y 800 m b e l o w b e d r o c k exposure. Its w a l l s a n d b a s e c o n s i s t o f g l a c i a l till, d e p o s i t e d by C a t h e d r a l G l a c i e r during f o r m e r adv a n c e s , and talus w h i c h a c c u m u l a t e s by m e a n s r o c k f a l l and s n o w a v a l a n c h e a c t i v i t y from the a d j a cent c l i f f s (Fig. 13). The c h a n n e l at El. 2 150 m is a p p r o x i m a t e l y 8 m deep a n d 12-15 m w i d e . The sides are s t e e p and unstable and the b a s e is c o v e r e d with m a t e r i a l h e a v i l y r e w o r k e d and s o r t e d by water. The soil b e n e a t h these r e w o r k e d d e p o s i t s is p r e s u m a b l y frozen, as i n d i c a t e d by 1984 p h o t o s . R e c o n s t r u c t i o n o f t o p o g r a p h i c c o n t o u r s across the c h a n n e l i n d i c a t e s that a p p r o x i m a t e l y 600,000 m 3 o f m a t e r i a l has b e e n e r o d e d and p r e s u m a b l y c o n t r i b u t e d to d e b r i s f l o w e v e n t s . This is e x c l u s i v e o f m a t e r i a l such as talus, b r o u g h t into the channel by o t h e r processes.
Upper Bench The U p p e r B e n c h ( F i g . 9) has b e e n both a source o f d e b r i s f l o w s e d i m e n t s and an area o f d e b r i s flow storage. S i g n s o f d e b r i s flow a c t i v i t y are c o n f i n e d to
a n a r r o w f a n - l i k e f e a t u r e s t a r t i n g at the toe o f the S u m m i t G u l l y and g r a d u a l l y w i d e n i n g to a m a x i m u m width o f 230 m n e a r El. 2 000 m. The g r o u n d rises g e n t l y to each side o f the n a r r o w fan. It c o n s i s t s o f m a t e r i a l w h i c h is b e i n g p e r i o d i c a l l y p i c k e d up, rem o v e d and r e d e p o s i t e d , as s h o w n by the l a y e r i n g o f the d e p o s i t s e x p o s e d in the s i d e s o f the m a i n c h a n nels. T h i r t e e n b r a n c h i n g p o i n t s (Fig. 9) c o n n e c t a n e t w o r k o f c h a n n e l s w i t h i n the n a r r o w fan. T y p i c a l c r o s s - s e c tions are s h o w n in F i g u r e 14. The m a i n c h a n n e l , a l i g n e d d i r e c t l y d o w n the fall line, is by far the l a r g e s t , b e i n g at l e a s t 4 m d e e p , with a m i n i m u m b a s e width o f 5 m. T h e m a i n c h a n n e l in S e c t i o n B-B is a t y p i c a l c r o s s - s e c t i o n . T h e l a r g e s t o f the s e c o n d ary c h a n n e l s b r a n c h e s from the m a i n one n e a r El. 2 200 m. The m a i n c h a n n e l has cut 8 m d e e p b e l o w the base l e v e l o f the side c h a n n e l at the j u c t i o n . The s e c o n d a r y c h a n n e l f l o w s n e a r the l e f t - h a n d m a r g i n o f the n a r r o w fan, with a t y p i c a l c r o s s - s e c t i o n less than 2 m d e e p and 3 m w i d e at the b a s e . This is the o n l y c h a n n e l , a s i d e f r o m the m a i n one, which has the c a p a c i t y to c a r r y the full d i s c h a r g e o f a m a j o r d e b r i s f l o w event. It last d i d so in 1925. T h e h e a d s o f the o t h e r c h a n n e l s lie a l o n g the i n f e r r e d L i t t l e Ice A g e t e r m i n u s o f the C a t h e d r a l G l a c i e r . C o n s e q u e n t l y , these l i k e l y c a r r i e d m e l t w a t e r and d e b r i s f l o w s from the m a r g i n o f the C a t h e d r a l G l a c i e r d u r i n g the c l i m a x o f this a d v a n c e . The q u a n t i t y o f m a t e r i a l r e m o v e d in e r o d i n g the c o n t e m p o r a r y m a i n c h a n n e l on the U p p e r B e n c h is app r o x i m a t e l y 150,000 m 3.
Bedrock Scarp and Apron The d e b r i s flow a c t i v i t y on the b e d r o c k scarp is c o n f i n e d to the m a i n c o n t e m p o r a r y c h a n n e l and two sec-
46
Fig. 9 : Summit Gully and Upper Bench. Note the network of channels adjacent to the main channel.
Fig. 10 : View looking west. ephemeral lake and snow ridge (July 19. 1985).
47 o n d a r y d i s t r i b u t a r y c h a n n e l s (Fig. 7). A l l the c h a n nels are cut d o w n to the b e d r o c k in t h e i r t r a v e l o v e r the scarp. On the A p r o n u p s t r e a m o f the tracks, the m a i n c h a n nel is i n c i s e d in a s i n u o u s c o u r s e a m o n g l a r g e b o u l d e r s w i t h i n r o c k s l i d e and till d e p o s i t s . T h e total v o l u m e e r o d e d f r o m the c h a n n e l s y s t e m on the S c a r p and A p r o n is e s t i m a t e d as 1 4 0 , 0 0 0 m 3.
Debris Flow Fan T h e D e b r i s F l o w F a n ( F i g . 6, unit 6) has an a r e a o f 9 "9 . approximately 0.36 kin-. Assummg a mean thickness of 5 m o v e r the e n t i r e a r e a o f the fan, the total v o l u m e o f d e b r i s d e p o s i t e d is 1 , 7 8 0 , 0 0 0 m 3. T h i s is t w i c e the e s t i m a t e d v o l u m e e r o d e d f r o m the c h a n n e l s u p s t r e a m o f the fan, w h i c h a m o u n t s to 8 9 0 , 0 0 0 m 3. T h e r e are at l e a s t t w o p o s s i b l e e x p l a n a t i o n for this d i s c r e p a n c y : a) the e r o d e d v o l u m e m a y be u n d e r e s t i m a t e d , as it is d i f f i c u l t to r e c o n s t r u c t p r e - d e b r i s f l o w t o p o g r a p h y ; b) the d e b r i s v o l u m e o f the fan m a y be o v e r e s t i m a t e d . P o s s i b l y , b o t h e x p l a n a t i o n s are v a l i d to s o m e e x t e n t . N e v e r t h e l e s s , the total a m o u n t o f m a t e r i a l t r a n s p o r t e d by d e b r i s f l o w f r o m the u p p e r s l o p e s to the v a l l e y f l o o r is p r o b a b l y b e t w e e n 1 • 10~' and 2 x 106 m ~.
Fig. 11 : A slush-filled ephemeral lake and drainage fissure. September 13, 1979 (looking northeast). The dashed line marks the limit of the lake on the day of the photograph (dimensions 20 by 33 m). The dottd line marks the high water mark of the lake from earlier that summer (dimensions 35 by 73 m). The volume within the upper water mark is estimated at between _'~ ~'_60 and 3 53-9 m3 . The lake , was freely draining into the fissure at the time of inspection.
T h e f a n has a s u b d u e d s u r f a c e that s l o p e s a p p r o x i m a t e l y 14 ~ n e a r t h e a p e x and 11 ~ n e a r the m a r g i n s . T w o k i n d s o f s u r f a c e f e a t u r e s are w i d e s p r e a d and h a v e b e e n m a p p e d . 1) E l o n g a t e l o b e s or t r a i n s o f debris, t y p i c a l l y less t h a n 1 m h i g h and 10 to 20 m wide, containing relatively well sorted bouldery material. The modal boulder sizes range from several c e n t i m e t r e s to a p p r o x i m a t e l y I m. 2) C h a n n e l s issuing f r o m the d i s t a l e n d s o f the l o b e s . B y far the l a r g e s t c h a n n e l s are the f o u r that w e r e m o s t r e c e n t l y a c t i v e , e x t e n d i n g b e t w e e n the P a r t r i d g e and Y o h o t r a c k s n e a r the e a s t e r n m a r g i n o f the fan. T h e l a r g e s t o f t h e s e is up to 6 m d e e p and 3-5 m w i d e at the b a s e . In c o n t r a s t , c h a n n e l s f o u n d on the o l d e r fan surface to the w e s t a r e t y p i c a l l y s h a l l o w e r than 2 m.
O n all parts o f the f a n , t h e r e are i s o l a t e d l a r g e b o u l d e r s , up to a b o u t 5 m in d i a m e t e r . S e v e r a l o f t h e s e w e r e d e p o s i t e d on t h e h i g h w a y p a v e m e n t d u r i n g the 1978 e v e n t .
T h e d e b r i s m a t e r i a l r a n g e s w i d e l y in t e x t u r e . D e b r i s d e p o s i t e d n e a r the T r a n s - C a n a d a H i g h w a y d u r i n g the 1984 e v e n t is v i s u a l l y e s t i m a t e d to be 3 0 - 4 0 % c o b b l e s and b o u l d e r s up to 1 m d i a m e t e r , in a m a t r i x consisting of gravel with minor a proportion of sand
and silt and a t r a c e o f c l a y . T e x t u r a l a n a l y s i s o f the f i n e r than 30 m m m a t r i x i n d i c a t e s silt and c l a y to m a k e up less t h a n 15 % by w e i g h t . T h i s a g r e e s w i t h the f i n d i n g s o f J a c k s o n ( 1 9 7 9 a , 1 9 8 7 b ) w h o e s t i m a t e d the l a r g e r t h a n 2 m m f r a c t i o n in 1978 d e b r i s f l o w s to be 60 to 70 % a n d m e a s u r e d t o t a l c l a y c o n t e n t o f the f i n e r than 64 m m f r a c t i o n to be less than 5 % t h r o u g h o u t the R o c k y M o u n t a i n s .
Calculation of debris flow velocity and discharge above the debris fan Eyewitness information concerning estimates of v e l o c i t i e s a n d d i s c h a r g e s o f d e b r i s f l o w s at C a t h e d r a l G u l c h e x i s t s o n l y f o r the m o s t r e c e n t e v e n t s and for v a n t a g e p o i n t s c l o s e to t h e C.P.R. t r a c k s and the Trans Canada Highway (See Historical Debris Flow Events).
Table 4 : Summary.of velocities and discharges estimated from superelevated flow cross-sections August 29, 1984 event
Cross
Section
Location Upstream Yoho (left branch) Upstream Yoho (right branch) Upstream Partridge right bend Upstream partridge left bend Upper Bench Air. 1990m
Area A
(m/sec)
(m 2)
Depth
m'/sec
12~
3.4
8.1
2.3
28
12~
4.8
36
4.9
170
15.5~
5.5
37
5.0
206
17.5~
4.7
45
4.4
211
Slight overflow of bank
22~
3.1
34
3.4
104
Cross-section may have been modified by erosion
Superelevation Angle
Velocity v
Mean
?
Comment Total 198 mS/sec in both branches
48
Fig. 12 : Cathedral Gulch at the head of Summit Gully. (a) The morning of September 7, 1978. The torrent exiting the glacier is residual flow from tile j6kulhlaup of the previous evening. (b) July 19. 1985. The arrow indicates the upper limit of stagnant ice and rockfall debris in (a).
Fig. 13 : Cathedral Mountain viewed from Yoho Valley. Talus accumulated from rockfalls forms a nearly continuous apron below the prominent cliffs of the Cathedral Formation.
49 A-A
El. 2200 -
-
'
~
a
v
e
;
~
Boulders 7
B-B
C-C
D-D
El. 1 8 ~ ~ . .
E-E
El. 1840
El. 2020
El. 1800
o i
10 . . . . . .
20
30m
:-~-~2: -~Y-. . . .
1
Fig. 14 : Typical channel cross-sections at and below Upper Bench.
The maximum discharge of the August 29, 1984 debris flow upstream of the fan was estimated using a method based on superelevation in bends. This method is based on the forced vortex equation of hydrodynamics (e.g. Johnson and Rodine. 1984). A correction factor to account for the lateral rigidity of
NO. 1
R= 34m
NO. 4
NO, 3
The maximum discharge calculated for the cross section on the Upper Bench (104 m3/sec) is approximately one-half of this range. Although spillout along the left bank of the channel during the event (Fig. 17) may have led to underestimation of peak discharge, the greatest source of discrepancy is probably nonsteady flow along the debris flow path. Debris flows move as slug-like surges (Costa and Williams, 1984). The height of the maximum surge probably increased between cross sections 5 and 4.
R:22m
NO. 2
the debris tongue and for supercritical flow effects has been used, as suggested by Hungr et al. (1984). Five cross-sections were analyzed (Fig. 15 and 16). These are located on the 2 channels between Partridge and Yoho tracks, just upstream of the fan apex and on the upper bench (Fig. 7). The input data and results are summarized in Table 2. The results from cross-sections 1 to 4. all in the vicinity of the debris fan apex, are quite consistent and indicate a peak debris discharge in the range of 198-211 m3/sec.
The high velocities obtained from these channel cross-sections upstream of the fan contrast with the slow oozing movements described by eyewitnesses from parts of the deposition area, especially the Trans Canada Highway. This illustrates a change of the dynamic character of debris flow consequent on the loss of confinement and momentum at the head of the deposition area.
R=24m
R =24rn
Historical development of the debris flow path NO.
5
R=46m
0
5m
Fig. 15 : Cross-sections of superelevated debris flow deposits on channel bends. R numbers indicate the radii of channel bend curvature. The other numbers identify the sections and correspond to Table 2 (no vertical exaggeration).
Changes in the Cathedral Glacier Historic photographs (Fig. 18) and former ice marginal features and stagnant ice indicate that Cathedral Glacier may have terminated near the base of the Summit Gully (as low as perhaps 2 200 m), at the
50
Fig. 16 : Channel bends 3 and 4 immediately upstreana of Partridge Track. Arrows mark upper limits of fresh debris used to measure superelevation.
Fig, t7 : Upper Bench, August 29, 1984 a.m. A water surge is traversing the channel following a debris flow earlier in the day. The arrow indicates an area covered by fresh debris flow deposits which overflowed the channel earlier that morning.
51 climax of the Little Ice Age in the mid-l.800"s. Cathedral Glacier terminated at El. 2 500 na in 1910. Between 1910, the date of the first available photograph, and 1925. the glacier thinned and the terminus retreated approximately 300 m (Gardner, 1982). The terminus retreated by approximately 400 m to near El. 2 700 m between 1925 and 1952, retaining a bulbous front. The present ice front is tapered and indistinct, at approximately El. 2 750 m. Continued retreat of ice from the Summit Gully is also suggested by the disappearance of buried stagnant ice from the base of the Summit Gully, as documented by the comparison of 1978 and 1985 photos (Fig. 12).
Evolution of the Flow Path Flow path development was reconstructed by dating channels using standard dendrochronological techniques (e.g; Sauchyn et al., 1983; Jackson, 1975, 1987), and from historic photographs and airphotos. Upper Bench. A 1923 photo by pioneer photographer Byron Harmon, taken from Yoho Valley, shows no large channel on the Upper Bench; the debris flow path appears as a smooth-surfaced apron (Fig. 18a). A 1925 photo shows two smaller secondary channels starting near the lower end of the Upper Bench; these apparently were created by the 1925 debris flow (Fig. 18b). There was no evidence of the contemporary large main channel which prominently bisects Upper Bench deposits. This channel was first documented in the 1949 airphotos. It probably was carved by the 1946 debris flow. All subsequent debris flows followed the main channel on the Upper Bench although both 1962 and 1978 flows spilled over into a secondary channel. Scarp and Apron Prior to 1925, only the course of the contemporary main channel was visible on the Scarp and Apron (Fig. 18a), The 1925 debris flow travelled down the path followed by the contemporary channel and two other channels to the west (right on Fig. 18b). Of these, the main discharge of that event followed the middle channel. A minor overflow discharge took the westerly branch. Dendrochronological evidence indicates that these apparently new channels were in existence prior to 1925 and cleared of obscuring vegetation by the 1925 debris flow. The bulk of the 1946 and all subsequent debris flows used the contemporary main channel over the scarp. It is estimated that the channel cross-section was enlarged by at least 500 percent during this period. Apron and Debris fan A presently inactive channel carried most of the debris to the Apron and Debris Fan during the 1925 event and the contemporary main channel course has carried debris flows to them since 1946 (Fig. 7). However, it is not certain which of the channels transported the substantial volumes of debris found
in the undated part of the Debris Fan (see Development of the Deposition Area). Physical barriers limit possible past courses to between the alignments of the contemporary course and the main channel of the 1925 event.
Development of Debris Sources Since the prominent main channel did not exist on the Upper Bench during and prior to the 1920's, it must have been cut by the 1946 and later debris flow events. The total volume of sediments removed from the Upper Bench as the result of channel cutting is estimated to be 150,000 m 3. In comparison, the total volume of the latest 6 events (Table I) is estimated to be 349,000 m 3. The remaining approximately 200,000 m 3 could have been derived only from the Summit Gully and the Scarp and Apron. The volume of sediment eroded from the Scarp and Apron is estimated to be approximatel~v 70,000 m 3. This leaves approximately 139,000 m ~ which could have been derived only from the Summit Gully. These estimates are cumulative for the 6 latest events. There are indications that erosion of the main channel on the Upper Bench and on the Scarp and Apron has been relatively minor since 1980. Photos were taken of the main channel at elevations of approximately 1 820, 1930 and 2 120 m in 1980 and 1985, All show surprisingly little change in channel depth, width and shape, despite the fact that the 1982 and 1984 debris flows, with a cumulative volume of 99,000 m 3, occurred in the interval. On the other hand, there was substantial deepening in the Summit Gully area, involving both removal of debris and melting of ice (Fig. 12). This indicates that the Upper Bench channel recently has become less prone to erosion, possibly due to the removal of loose easily erodible material by the debris flows prior to and including 1978. Conversely, the Summit Gully has been a major source of the debris material largely due to the melting of stagnant and buried glacial ice, The present quantity of moraine and talus in the gully in estimated as several millions of m 3, even allowing for a certain percentage of clean ice.
Development of the Deposition Area The debris fan can be divided into two parts based upon ages of deposits. The smaller, eastern part contains recently active channels and dated deposits of historic events (Fig. 9). Older debris underlies these deposits, as indicated by exposures in the channel walls and the presence of fragments of old debris surfaces. One clearly defined debris lobe at the extreme eastern margin below the partridge Track is mantled by trees dating to 1780 A.D. The westerly two-thirds of the fan is mantled by the remains of old mature forest which was at least 150 to 200 years old when destroyed by a forest fire in 1890.
52
Fig. 18 : The north side of Cathedral Mountain showing a single channel in 1923 (a) and multiple paths following the 1925 debris flow (b). The present channel follows (a). The main flow of the 1925 debris flow followed the middle channel in (b). The channel to the west (right) probably carried minor overflow discharge (post card photographs by Byron Harmon).
53 Deposition has therefore shifted from west to east with all the events of this century being confined to the extreme eastern margin.
J6kulhlaup abatement measures Noted previously, the ephemeral lake at the south margin of Cathedral Glacier has been identified as a significant source of water involved in past j6kulhlaups. While the lake has never been observed to contain more than 4 000 m 3, the estimated 106,000 m 3 of meltwater which could pass through the lake and adjacent drainage fissure represent a significant percentage of the meltwaters which are shed by the northern sector of Cathedral Glacier. Pumping of meltwater out of the lake as a means of controlling the storage of meltwater within the glacier and the lake was proposed by one of the authors (L.E.J.) in 1979 and undertaken by C.P.R. beginning in August, 1985 when rapid rising of the lake suggested that a j6kulhlaup might be imminent (Olsen, 1988 written communication). Between August 12 and Aueust~ 2 9 , 1985, approximately 7 400 m 3 of water were pumped from the lake and harmlessly discharged along the west margin of the glacier. A pressure connection between the lake/fissure system and the meltwater outlet at the head of Cathedral Gulch was demonstrated because the meltwater stream on the north side of Cathedral Glacier dried up following pumping. A similar pumping program was carried out in 1986. However, the lake did not form in 1987 and consequently no pumping was required (Arnold, 1988 personal communication). No j6kulhlaups have occurred since the initiation of pumping.
pre-1925 record useless as a guide for debris flow hazard between 1925 and 1985. During the Little Ice Age, Cathedral Glacier was significantly more extensive than today and provided lateral support for marginal glacial deposits and talus in Summit Gully. A cooler climate probably kept nearby unconsolidatd deposits frozen for longer periods which decreased their susceptibility to mass wasting. Retreat of the glacier since the late 19th century has left unstable accumulations of sediment supported by melting and collapsing stagnant ice. Such sediment/ice configurations are themselves conducive to the generation of debris flow (Eisbacher and Clague, 1984). What sets apart debris flow activity in Cathedral Gulch from debris flow activity noted in similar small steep watersheds elsewhere in the Rocky Mountains (Desloges and Gardner, 1984) are the large volumes and relatively high frequency of Cathedral Gulch events. Both are a result of j6kulhlaups from the Cathedral Glacier. The j6kulhlaups may also be related to glacier recession. Glacial thinning attendant with recession has likely made the glacier less plastic at its base. Lower plasticity allows meltwater tunnels to form and remain open providing potential internal water storage. Given the right conditions, these systems could become confined, fill and catastrophically drain. Regardless of the mechanisms which cause j6kulhlaups, the frequencies and volumes of debris flows in Cathedral Gulch would be greatly reduced if j6kulhlaups did not occur. Consequently, the practice of pumping large quantities of meltwater away from the glacier is a positive measure to reduce the risk of j6kulhlaups and to limit the volume released should a j6kulhlaup OCCUr.
Discussion Conclusions Debris flow fan stratigraphy and chronology has been used to establish debris flow recurrence frequencies (e.g. Jackson, 1975 and 1987; Kochel, 1987). However, in order for recurrence frequencies to have value, basin conditions such as sediment supply, and the hydrometeorological regime must remain constant. This is not the case in glacierized basins (Jackson et al., 1987). Glaciers introduce many variables to debris flow occurrence that are largely unrelated or indirectly related to basin morphometry and hydrometeorology, for example, debris flows may result from the failure of glacier and moraine-dammed lake (Eisbacher and Clague, 1984; Clague et al., 1985). The Little Ice Age advances of the past several centuries were unprecedented during the past 10,000 years (Luckman and Osborn, 1979). Evaluation of debris flow frequency on the debris flow fan of Cathedral Gulch in the year 1900 A.D. would have indicated no flows for more than 180 years, based upon estimates of the ages of the oldest trees growing on fan deposits at that time. Progressive glacial recession during this century altered slope conditions in upper Cathedral Gulch and consequently made the
Historic debris flow activity at Cathedral Gulch has been caused directly by the recession of the Cathedral Glacier. This has resulted in an unstable juxtaposition of stagnant ice with glacial and colluvial sediments and changes in the morphology and internal hydrology of the glacier. The latter has probably resulted in the periodic storage and catastrophic release of large volumes of meltwaters from the glacier. J6kulhlaups mobilize unstable glacial and colluvial sediments into debris flows. The largest historic debris flow, in 1978, had a volume of approximately 136,000 m 3. Three other events have been in the range of 80,000 to 100,000 m 3. The maximum calculated debris velocity and discharge upstream of the Partridge Tracks are 5.5 m/sec and 210 m3/sec respectively. There is no evidence of large scale events between the time of railroad construction and 1925 and it is possible that the first historic event marked the end of a long quiescent period. Debris flows cut a prominent main channel between 1946 and 1984.
54 Over most of the period between 1946 and 1984, debris flows originated in approximately equal proportions from the Upper Bench (El. 1850-2150) and the Summit Gully (above 2150). It appears, however, that toward the end of that period, the balance changed in favour of the Gully. This was the result of frequent debris flow activity which kept the lower channel clean and ice melting which accelerated instability of the upper slopes. Pumping of the ephemeral lake, which began in 1985, has apparently reduced storage of meltwater within the glacier. This positive measure should reduce significantly the likelihood of jrkulhlaups or at least limit the volume released should one occur. This, in turn will reduce the debris flow hazard to the C.P.R. tracks and the Trans-Canada Highway.
Acknowledgements Part of the data for this paper was produced under contract to C.P. Rail, Special Projects Division by Thurber Consultants Ltd. Permission to publish this material is gratefully acknowledged by the authors. D.E VanDine of VanDine Geological Engineering Services Ltd. prompted the authors to write this paper. John Clague of the Geological Survey of Canada gave a rigorous review to an earlier draft. This paper was improved greatly thanks to his diligent efforts. Our thanks to Bey Vanlier [-or word processing and Christine Davis for drafting.
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