Urban landslides in the vicinity of Vancouver, British ...

Urban landslides in the vicinity of Vancouver, British Columbia, with special reference to the December 1979 rainstorm

Can. Geotech. J. Downloaded from www.nrcresearchpress.com by Hunan Normal University on 06/05/13 For personal use only.

Geological Sicrvey of Canada, 100 West Pender Sweet, Varrcouver, B.C., Carrada V6B IR8

Received May 2, 1980 Accepted December 30, 1980 Historical landslides in the urbanized Vancouver region, southwestern British Columbia, have almost commonly occurred along escarpments within and at the margins of gently rolling upland surfaces underlain by Pleistocene unconsolidated sediments. The most common and most destructive landslides are debris avalanches and debris flows. They are triggered by intense autumn and winter rainstorms, when water infiltrates and saturates the surficial layer of weathered colluvium. After failure the veneer of debris gains momentum and picks up additional soil and uprooted vegetation. Debris avalanches may temporarily block gullies swollen with runoff water, thus changing into rapidly moving debris flows. A severe rainstorm in December 1979 was accompanied by destructive debris avalanches and debris flows in urban areas in the vicinity of Vancouver. A search of local newspapers and meteorological records back to 1900 indicates that this event was not unique, for at least 26 other comparable storms have trigzered landslides in the Vancouver region during this century. Thus it is likely that landslides similar to those of December 1979 will occur repeatedly in the future. The danger of such landslides to life and property will grow if potentially hazardous sites are urbanized without appropriate protective measures. Historiquement, les glissements de terrain dans la rCgion urbanisCe de Vancouver, dans le sud de la Colombie Britannique, se sont gCnCralement produits le long de talus bordant des hautes terres vallonies, formks de sidiments pleistoc&nesnon consolidCs. Les glissements les plus courants et les plus destructeurs sont des avalanches et des coulCes de debris. 11s sont dCclanchCs par des pluies torrentielles d'automne et d'hiver, lorsque l'eau s'infiltre et sature la couche superficielle de colluvium altCrC. Aprks rupture, le couvert de debris acquiert de la vitesse et entraine du sol additionnel et de la vCgCtation dCracinCe. Les avalanches de debris peuvent bloquer temporairement des ruisseaux gonflCs par les eaux de ruissellement, et se transformer en coulCes de debris ti mouvement rapide. Une pluie torrentielle en dkembre 1979 a CtC accompagnCe d'avalanches et coulCes de debris destructrices dans la rigion urbaine de Vancouver. Une recherche dans les journaux locaux et les relevis mCtCorologiques depuis 1900 a montrC que cet Cvknement n'Ctait pas unique, puisqu'au moins 26 autres temp@tesde ce type ont dCclenchC des glissements dans la rigion de Vancouver depuis le deblit du sikcle. I1 est donc probable que des glissements semblables 51 ceux de dCcembre 1979 se rCpktent ti l'avenir. Les dommages produits par ces glissements aux propriCtCs et aux personnes vont augmenter ti mesure que des sites dangereux sont urbanisCs sans mesure protectrice approprik. [Traduit par la revue] Can. Geotech. J., 18, 205-216 (1981)

Introduction Between December 12 and 18,1979, two periods of intense rainfall triggered widespread flooding and slope failure throughout coastal southwestern British Columbia and the adjacent United States. Damage caused by landslides was particularly significant in recently urbanized sections of the Vancouver area. A systematic search of climatological records and Vancouver newspapers revealed that storms of the variety experienced in December 1979 are recurrent events and that the type of slope failures which occurred during this storm have their historical precedents. In this paper we shall summarize and discuss the special geologic and climatic conditions

which render some parts of Vancouver more prone to slope failure than others. Although the geologic factors which contributed to landslides during the 1979 rainstorm are broadly similar to those prevailing in previous storms, slope stability in this region has been affected during the last three decades by changes in land use, slope management, and intensified urbanization. The Geologic-Climatic Setting The geology of the Vancouver region has been documented in a number of specialized government reports and scientific papers. The bedrock geology has been described by Roddick (1965); surficial

0008-3674/8 1 /020205-12$01.00/0 @ 1981 National Research Council of Canada/Conseil national de recherches du Canada

CAN. GEOTECH. J. VOL. 18, 1981

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. . . . * - *

* ? + - * - + -

MOUNTAIN

SLOPES- THIN

UNCONSOLIDATED

[--? LOWLANDS-

SEDIMENTS

SEDIMENTS

19351

HOLOCENE

PEAT A N D ALLUVIUM

YEAR OF L A N D S L I D E ( S )

FIG. 1. Geologic terranes i n the Vancouver region and locations of landslides that occurred during severe storms between 1900 and 1979 (see Table 1).

deposits have been mapped and interpreted by Armstrong (1956, 1957, 1960, 1980a,b, in press), Armstrong c.t 01. (1965), Clague (1976), Armstrong and Clague (1977), and Armstrong and Hicock (1980a,b); and the history of late Quaternary crustal movements has been documented by Mathews ei 01. (1970). Much of this information has been summarized by Eisbacher (1973). The urbanized Vancouver region can be subdivided broadly into three geologic terranes (Fig. 1). The first comprises mountain slopes underlain mainly by Mesozoic granitic igneous rocks and gently south-dipping Cretaceous-Tertiary sandstone and conglomerate. In most places this terrane is the northernmost of the three and forms the southern flank of the Coast Mountains. However, spurs of Tertiary bedrock protrude southward beyond the mountain front at several places, for example, at Stanley Park, Simon Fraser University, and east of Abbotsford (localities are shown in Figs. 1 and 2). The bedrock slopes are covered in most places by thin till and colluvium. Locally, however, there are steep bare rock faces prone to sporadic rock falls. A second terrane comprises gently rolling and

terraced uplands, up to 200 m in elevation, composed of unconsolidated Pleistocene sediments. Although the stratigraphy, facies, and thickness variations of these sediments are complex, some generalizations are possible. On uplands in the west and southwest (Vancouver, Surrey, and White Rock areas), a thin layer of till and glaciomarine stony clay generally overlies thick glaciofluvial sand and gravel. Farther east, the upland surface is underlain by thick glaciomarine sediments complexly interstratified with till and deltaic and ice-contact sand and gravel. The northern upland area at the foot of the Coast Mountains is underlain in places by deltaic and marine silt, sand, and gravel, and elsewhere by thin till and glaciomarine sediments above glaciofluvial and ice-contact sand and gravel. Escarpments up to 125 m high occur within and at the edges of the uplands (Fig. 2). Some of these are the product of late Pleistocene and early Holocene downcutting accompanying isostatic uplift of the region. Others probably originated by a combination of glacial erosion and ice-marginal deposition during one or more Pleistocene glaciations. All the escarpments are breached by ravines and gullies carrying


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TABLE1. Severe storms between 1900 and 1979 that triggered landslides in the Vancouver region


Precipitation (mm)

I

I

,

1

Date

Daily max.

Two-day max.

Storm total

Sept. 1905 Nov. 1908 Nov. 1909 Dec. 26,1917- Jan. 1, 1918 NOV.13-18, 1919 Oct. 24-29, 1921 Feb. 11-12, 1924 Jan. 7-11, 1932 Feb. 24-27, 1932 Jan. 20-25, 1935 Oct. 17-20, 1940 Oct. 8-15, 1941 Feb. 6-7, 1945 Nov. 22 - Dec. 1, 1949 Jan. 20-25, 1951 NOV.17-21, 1954 Nov. 2-3, 1955 Oct. 16-19, 1956 Dec. 8-9, 1956 Oct. 20-23, 1960 Feb. 18-20, 1961 Dec. 22-23, 1963 Dec. 9-18, 1966 Jan. 17-20, 1968 July 11-12, 1972 Dec. 15-26, 1972 Dec. 12-18, 1979

69 64 111 70 68 59 65 40 28 95 80 59 72 65 41 60 85 101 84 40 31 69 54 124 77 107 77

Unknown Unknown Unknown 87 101 83 82 66 57 167 109 105 109 81 64 94 125 112 154 70 56 115 81 175 103 149 141

Unknown Unknown Unknown 206 160 144 82 100 97 329 138 200 109 213 128 160 125 151 154 115 84 115 23 1 , 227 103 390 268

NOTES:Only major storms during which there were documented landslides in the Vancouver region are tabulated. Other storins undoubtedly triggered landslides, but documentation is inadequate or completely lacking. Storm dates provided by E. Keranka (British Columbia Ministry of the Environment). Precipitation data are for the downtown Vancouver area and were provided by Atmospheric Environment Service. Landslide areas for these storms are shown in Fig. 1 and were derived from contemporary newspaper accounts.

ephemeral streams fed by groundwater and sporadic surface runoff from the flanking uplands. The third terrane breaches the other two and is a nearly flat surface, about 3-10m above sea level, underlain mainly by Holocene peat and fluvial deposits of the Fraser River and tributary streams. This surface is protected from flooding by a network of dykes. Most historical landslides in the Vancouver region have occurred during periods of intense precipitation in autumn and winter. Storms during these seasons are frontal and generally are accompanied by strong flows of moisture-laden Pacific air from the southwest. Approaching the Coast Mountains, the air masses rise and release large amounts of precipitation. Torrential autumn and winter rains not only trigger landslides in the Vancouver region, but also cause rock falls along transportation routes in the

adjacent Coast Mountains (Peckover and Kerr 1977, pp. 490-491). Table 1 lists those storms between 1900 and 1979 that were accompanied by landslides in the Vancouver region. The data in Table 1 indicate that storms capable of triggering landslides are relatively common in this area. A few landslides in unconsolidated sediments on moderate and steep slopes may occur during rainstorms with maximum 24 h precipitation values as low as 50 mm.' However, widespread landsliding probably is not initiated until 24 h rainfall exceeds 100-150 mm. O'Loughlin (1972, p. 109) likewise found that debris avalanches and debris flows on sediment-veneered rock slopes in the Coast Mountains north of Vancouver commonly occur when 24 h rainfall exceeds 150mm. These values are attained on the average about once every 2 years in the southern Coast Mountains and once every 3-5 years immediately south of the mountain front, but are much less frequent farther south (Bruce 1961). The dates and approximate locations of landslides triggered by intense storms in the Vancouver region are shown in Fig. 1. Most of these landslides occurred on escarpments of uplands underlain by Pleistocene sediments. Commonly the failure sites are steep walls of ravines or gullies. The sediments underlying such steep slopes often have high background moisture content and are prone to creep and small slumps. Rainstorm runoff from urbanized areas bordering the gully heads intensifies saturation of surface materials and promotes incipient instability. Mode of Slope Failure The most common and destructive slope failures in the urbanized Vancouver region are debris avalanches, debris flows, and earth s l ~ m p sRock . ~ falls are locally significant (e.g., at Stanley Park). Because during rainstorms debris avalanches and earth slumps are generally immediate precursors to debris flows, the three will be discussed together (see also Stini 1910; Clar 1959; Williams and Guy 1973; Varnes 1978). In Vancouver these rapid mass movements are set off by sudden increases in runoff from uplands into gullies and ravines. The considerable flow of water accompanying slope failures accounts for the common use of the terms "washout" and "mudslide" by local newspapers. The geology of the steep ravines and gullies pre>Thevalues in Table 1 are for fixed calendar days, generally 8:00 A.M. to 8:00 A.M. local time, and are lower than extreme

24 h values. *Landslide terminology after Varnes (1978).


EISBACHER AND CLAGUE

destines debris avalanches and shallow slumps as the most common types of failure. In their undisturbed state gully walls are inclined between 10 and 40'. A blanket of weathered colluviun~,in general less than 1 m thick, covers the slopes and supports the characteristic Pacific coast forest, dominated by tall, shallow-rooted conifers. Along steep escarpments and at the heads of gullies, slips and slumps involve both the surficial colluvial layer and underlying stratified sediments. This is particularly the case where landfill has been emplaced on an escarpment for the purpose of extending the upland for development. During storms, discrete slumps near the top of the escarpments merge downslope into a zone of accelerated creep involving only the saturated soil layer. Thus, when failure occurs, a relatively long but thin layer of saturated sediment and vegetation breaks away from the slope and avalanches downward. Typical debris avalanches occurred in the urbanized Seymour-Riverside area in December 1979 (see section i n Landslides of December 1979 below). A second, potentially even more dangerous situation is created when an earth slump or debris avalanche blocks a rain-swollen stream. A mass of sediment and trees may cascade into a high-gradient, narrow ravine and temporarily stem the flow of water down the ravine. Eventually, however, this plug fails, and a mobile debris flow consisting of water, mud, stones, and uprooted trees is set in motion. Transported boulders and tree trunks up to about I m in diameter are formidable tools of destruction when they encounter natural or artificial obstacles. Such a debris flow caused major damage in Port Moody in December 1979 (see below). Finally, a third type of storm-induced slope failure occurs in the Vancouver region, which, although rarer than those described above, is potentially dangerous. Short, steep gullies along uplands underlain by loose sand and gravel may be catastrophically eroded by uncontrolled runoff during periods of heavy rain. Torrents cascading down these normally dry gullies rapidly cut downward and headward to form spectacular canyons bounded by steep walls. The canyon walls collapse and retrogress as sediment is eroded from their base. The eroded sediment generally is deposited as a fan at the gully mouth. The most spectacular example of such an event occurred on the campus of the University of British Columbia in January 1935 following 2 days of torrential rain after a week of heavy snowfall (Williams 1966). Over a period of 2 days, a raging torrent in a formerly minor gully removed about 100 000 m3 of sand, creating a badland canyon of impressive proportions (Fig. 3). During this period of erosion, the steep walls

209

of the canyon repeatedly collapsed, sending surges of sand and water down to the sea where a large fan formed. Similar catastrophic gully erosion, bank collapse, and mass transport occurred in Coquitlam Valley during the winter of 1951-1952, when approximately 300 000 m 3 of sand and minor gravel, silt, and clay were dumped on the valley floor, blocking Coquitlam River for several days (Armstrong 1957, p. 13). Vegetation to some extent lessens the likelihood of slope failures such as those described above. Plant cover reduces or modulates runoff and increases the shear resistance of the surface sediments. Thus deforestation due to urbanization and clear-cut logging may increase the incidence of debris avalanches and debris flows on moderate and steep slopes (e.g., O'Loughlin 1972). In addition, logging debris along stream courses may impede surface drainage and cause additional slope failures.

The Rainstorm of December 1979 During December 1979 much of British Columbia was under the influence of a southwesterly flow of mild, moist Pacific air. Throughout the early part of the month, near normal precipitation fell over coastal southwestern British Columbia. On December 12 a Pacific frontal system became nearly stationary over southern Vancouver Island, and a series of minor depressions inhibited the eastward movement of this front. This generated alnlost continuous heavy rainfall in the Vancouver region from the evening of December 12 to the evening of December 14. These rains decreased as the front moved southward in response to the strengthening of an arctic high pressure system over the British Columbia interior, and on December 15 no precipitation was recorded in the vicinity of Vancouver. However, there was renewed flow of moist air from the southwest on the morning of December 16, and heavy rains fell for the next 2 days. The total amount of rainfall during this storm in the Vancouver region increased in a north-northeasterly direction, reflecting uplift of air masses along the mountain front north of the city (Fig. 4). This precipitation pattern is typical of nearly all frontal storm systems in the Vancouver area and is due to longer duration and greater intensity of rainfall near the mountains (Schaefer 1973). The result is a fourfold increase in annual precipitation as one approaches the Coast Mountains from the southwest (Wright 1966). The rainfall pattern at Port Moody, where landslide damage was greatest, is similar to that at



FIG.3. "Campus Canyon" shortly after the January 1935 storm during which it was created. In a 2-day period the canyon was carved by torrential runoff from the campus of the University of British Columbia. Photo by M. Y. Williams, provided by University of British Columbia Special Collections.

Burnaby Mountain, 6 km to the southwest, and to that at Pitt Polder, 16 km to the east (Fig. 4). At Burnaby Mountain 124 mm of rain were recorded during the first phase of the storm (December 12-14) and 148 mm during the second phase (December 16-18), a total of 272 mm. Corresponding values at Pitt Polder were 144 mm and 169 mm, a total of 313 mrn. Comparable amounts of rain fell in the Seymour-Riverside area, the other serious landslide zone during the 1979 storm. A climatological station less than 2 km from Seymour-Riverside recorded 131 mm and 171 mrn, a total of 302 mm. The two most destructive mass movements occurred towards the end of the two storm phases, respectively (Fig. 4). A comparison of the 1979 rainstorm with previous storms ii the Vancouver region indicates that such events are not rare (Table 1). The most recent storm of comparable size, in December 1972, caused some landsliding and flooding in West Vancouver. Prior to 1972, at least 25 storms in this century triggered landslides. All of these except one struck in autumn or winter. The December 1979 rainstorm, although somewhat more severe than most of those listed in

Table 1, was exceeded by storms in November 1909, January 1935, January 1968, and December 1972. Storms in October 1940, November 1955, October and December 1956, and July 1972 were as intense as that in 1979, although shorter in duration. Landslides of December 1979 It is not possible to deal comprehensively with all the types of mass movements that resulted from the storm of December 1979. However, the main types, debris avalanches and debris flows, are well illustrated by the two most destructive landslides which hit in the Seymour-Riverside and Coquitlam - Port Moody areas, respectively.

Seymour-Riverside (Locaied at R in Fig. 2; Tol~ographicProfile in Fig. 5; Details in Fig. 6) An 80 m high escarpment separates two urbanized terraces east of Seymour River. Near the end of the second phase of the December storm (Fig. 4), a thin layer of weathered colluvium failed near the top of this escarpment, cut a swath through a thick stand of tall conifers, and avalanched onto the terrace below


EISBACHER A N D CLAGUE

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Seymour-Riverside d e b r ~ savalanche

.OO

Coquitlam-Port Moody debris flow

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12 Dec. 12

2h

I

15 Dec. 13

24

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15 Dec. 14

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1'2 Dec.15

2k

I

ii Dec. I6

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Vancouver t+orimur/

22

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