CGRG Bibliography of Canadian Geomorphology
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Author : Evans, S.G.
Date : 2000.
Title : Catastrophic Landslides in Canada.
Publication : GeoCanada 2000. Calgary, Alberta. May 29-June 2, 2000.
Issue : Abstract
Page(s) :
Abstract
Canada has a wide range of landslide types reflecting the diverse geomorphic and geologic environments in the nation’s landscape. In addition to being an important geomorphic process in many regions of the country, landslides are a significant geological hazard that have resulted in considerable property damage and significant loss of life in the historical period, taken to be 1840 to 1999. Catastrophic landslides are defined as those that have mean velocities of 5m/s (18 km/hr) or greater Rapid landslides in rock slopes ; Rock avalanches involve the initial failure and subsequent disintegration of a large rock mass on a mountain slope and the rapid downslope movement of this debris into a valley or onto a glacier surface. Rock avalanches are typically greater than 1 M m3 in volume and can be very mobile, i.e., debris may travel long distances (up to 10 km in some Cordilleran cases) from their source. Rock avalanches are the fastest type of landslide and may reach very high speeds. According to eye witnesses, for example, the 1903 Frank rock avalanche had a duration of about 100 seconds in its travel over 3.12 km, indicating an average velocity of 31.2 m/s (112 km/hr). In the 1959 Pandemonium Creek rock avalanche in the Coast Mountains of British Columbia, the debris may have reached speeds of up to 100 m/s (360 km/hr). Rock avalanches are very destructive when they impact on human activity as exemplified by the 1903 Frank Slide which struck the outskirts of the town of Frank, Alberta killing around 75 people. Rock avalanches are most common in the Cordillera where hundreds of prehistoric rock avalanche deposits exist, the largest being the Valley of the Rocks deposit (est. vol. 1B m3) in the Rocky Mountains of southeastern British Columbia. 30 high-velocity rock avalanches are known to have occurred in the Cordillera since 1855, the largest being the 1965 Hope Slide (est. vol. 48 M m3 ) Rock avalanche deposits have also been noted in mountainous areas of Canada’s Arctic Archipelago. The flanks of volcanoes are particularly prone to catastrophic failure; rock avalanches that originate on the slopes of volcanoes are termed volcanic rock avalanches. Several have taken place in the twentieth century on the Quaternary volcanoes of the Garibaldi Volcanic Belt, southwestern British Columbia, the largest being the 1975 Devastation Glacier rock avalanche (est. vol. 12 M m3). Like many other volcanoes in the world, volcanoes of the Garibaldi Volcanic Belt have also been subject to massive large-scale flank collapse in the Quaternary Period. The Late Pleistocene collapse of the western flank of Mount Garibaldi involved a volume of 2.5 – 3.0 B m3, comparable in volume to the 1980 rockslide-avalanche at Mt. St Helens, Washington, U.S.A. Other massive flank collapses occurred in the Holocene in the Garibaldi Volcanic belt at Mount Cayley and at the Mount Meager Volcanic Complex, the latter event being associated with the explosive eruption of Plinth Peak at about 411 B.C. Both events temporarily blocked major rivers in the southern Coast Mountains. Rapid Rockslides are transitional to rock avalanches. They occur when a rock mass slides on a detachment surface, such as a bedding plane or fault surface, and the debris only partially leaves the source area. Most of the debris is deposited only a limited distance downslope. Rockfalls involve a much smaller rockmass than rock avalanches or rapid rockslides, which disintegrates into numerous blocks that fall, bounce, and roll on steep slopes after detachment from a rock slope. Rockfalls are typically less than 50,000 m3 but may attain high velocities in the range 35-40 m/s (126 –144 km/hr). Rockfalls are a constant problem along transportation routes through rocky terrain across Canada and have also impacted on buildings located below bedrock slopes. Debris avalanches and channelised debris flows form a group of landslides that are characterised by the rapid flow of saturated materials in steepland terrain. Typically, debris avalanches and debris flows consist of boulder and gravel size clasts that are transported in a finer grained slurry. A debris avalanche occurs when a saturated mass fails on an open slope and does not become channelised. It is differentiated from a channelised debris flow which occurs when a saturated mass of debris moves downslope confined in a steep stream channel. Debris avalanches are common in forested steeplands. Landslide volumes generally range from 20,000 to 60,000 m3. Debris avalanche paths may exceed 500 m in length but involve only a very thin veneer (less than 2 m) of surficial materials. Based on eye witness accounts, the velocity of debris avalanches commonly exceed 10 m/s (36 km/hr). An analysis of the 1990 Belgo Creek debris avalanche (estimated volume 23,000 m3) indicated local velocities as high as 20 m/s (72 km/hr;). The behavior and occurrence of debris avalanches is in contrast to channelised debris flows which generally recur in a channel course and the velocity of which rarely exceeds 7 - 8 m/s (25 – 28 km/hr). Debris flows are frequently generated by the channelisation of an initial debris avalanche triggered in the steep upper slopes of a watershed. In other cases, stream bed material may be mobilised when stream discharge exceeds a critical threshold such as during a severe rainstorm or as a result of a sudden draining of an ice-dammed or moraine-dammed lake. In Klattasine Creek, British Columbia, for example, a massive debris flow (estimated volume ~2 M m3) was triggered in the early 1970s when a moraine-dammed lake burst in the headwaters of the Creek. They may also be triggered by the impact loading of stream bed materials by rockfalls or falls of glacier ice. Debris flows and debris avalanches occur widely in the Cordillera of western Canada. They also occur in the Appalachian and Laurentian Mountains of eastern Canada where their occurrence is invariably linked to heavy rains. Two debris avalanches triggered by heavy rains were responsible for a total of 6 deaths in 1973 at Harbour Breton, Newfoundland and in 1996 at La Baie, in the Saguenay region of Quebec, when they impacted on homes located at the base of the failed slope. Since 1976, a number of debris flow defensive structures have been constructed in the Cordillera of western Canada. They include debris retention structures that stop and dewater debris in containment basins upstream of debris fans, channelisation works that confine the debris in its passage over a fan surface, and deflection berms that either divert the flow away from potential impacts on infrastructure facilities into a predetermined deposition area, or create open containment areas. Rapid slides and earthflows (flowslides) in Quaternary sediments; Glaciomarine sediments; Rapid landslides are especially common in the sensitive glaciomarine Pleistocene sediments of both eastern and western Canada. A number of significant landslides have occurred in the glaciomarine sediments (commonly called Leda Clay) of the St. Lawrence Lowlands of eastern Canada during the historical period, with the earliest description dating from 1771. In addition, numerous scars of old landslides are present along river courses and terrace edges in the region. The behaviour of Leda Clay landslides reflect the complexity of the glaciomarine source materials, which were initially deposited in salt or brackish water, and the extent to which their chemistry has been modified by post-depositional processes. Initial failure may be followed by a process of retrogression which is dependent both on the initial strength of the material and its ability to disintegrate and flow after failure which is in turn a function of its strength loss, i.e., sensitivity. Development of rapid retrogressive earthflows has been found to be associated with the presence of sensitive clay zones at the base of the initial slopes. High sensitivity in turn is associated with the reduction of the original salt content by groundwater leaching. In spite of the fact that Leda Clay landslides involve original limited slope heights (usually less than 30 m), both the retrogression and the flowage phase of the landslides can be extremely rapid and thus very destructive. During retrogression, survivors have sometimes had to run to avoid being swallowed up by the debris, indicating retrogression speeds of approximately 5 m/s or greatrr. For example, at the 1894 St-Alban landslide, Quebec, survivors report that in some places the failing earth was moving faster than they could run in the opposite direction. A similar experience occurred at the 1971 St-Jean-Vianney landslide, Quebec, where a man running to escape the rapidly retrogressing failure, reported having to run on what seemed like moving stairs, indicating that the final stages of the retrogression matched his running speed. It is noted that all 31 deaths in the St-Jean-Vianney landslide, occurred when houses were engulfed in the retrogression phase of the landslide. The movement of the disintegrating landslide debris may also be extremely rapid and a number of casualties have resulted from being overwhelmed by fast flowing Leda Clay debris. The velocity of clay blocks being carried along on the surface of the flowing clay in the 1898 St-Thuribe landslide, Quebec, was estimated by eyewitnesses to be moving as "fast as a horse can gallop" i.e., approximately 10-13 m/s. Where the distal flow is channelised, velocities may be estimated from the superelevation of the debris surface as it passed through bends. In the St-Jean-Vianney landslide, the distal flow travelled through the narrow Rivière des Vases down to the Saguenay River at a velocity of approximately 7 m/s (25 km/hr) destroying a bridge in its path and carrying the concrete piers into the Saguenay River. Landslides in Leda Clay may involve substantial volumes of material. Approximately 17 historical landslides involving volumes in excess of 1 M m3 have occurred in the period 1840-1999. The largest is the remarkable 1894 St-Alban landslide, Quebec, with a volume that is estimated to be 185 M m3; the landslide scar covers an area of 462 ha and has an average depth of 40 m. Although the movement took place in multiple phases over a 3 hour period, the 1894 St-Alban landslide is arguably Canada's largest known landslide in the period 1840-1999. More recently, a large landslide took place in Leda Clay at Lemieux, Ontario in 1993 and temporarily blocked the South Nation River. In April 1996, the largest Leda Clay landslide of the twentieth century occurred at St-Boniface-de-Shawinigan, Quebec, (est. volume 7-8 M m3) Landslides in Leda Clay occur most commonly in the spring during the melting of the winter snow pack. Rapid slides and earthflows have also occurred in the less well known sensitive glaciomarine sediments of coastal British Columbia. In January 1880, a major landslide (est. volume ~ 1 M m3) occurred at Haney, on the north bank of the Fraser River, in the Lower Mainland of southwest British Columbia. The slope involved in the failure is about 30 m high and the retrogression of the slide was in the order of 250 m. Debris partially blocked the Fraser River which at this point is about 475 m wide; approximately 8 ha of land were displaced very rapidly downward and outward, and the debris generated a deadly displacement wave in the Fraser River. More recently, around Christmas 1993, a large earthflow, covering an area of 23 ha and involving a volume of 1.4 M m3 occurred in sensitive glaciomarine sediments near Terrace, northwestern British Columbia. It is one of numerous flowslides to have occurred in the glaciomarine sediments of the Terrace-Kitimat area in the Holocene. Glaciolacustrine sediments; rapid slides and, less commonly, rapid earthflows may also occur in glaciolacustrine sediments. The mechanisms for the development of such landslides in these sediments is not fully understood at present. A striking example of a rapid flowslide in glaciolacustrine sediments is the 1973 Attachie landslide in the Peace River Valley, 50 km west of Fort St. John, British Columbia. The total volume of material involved in the movement was about 12.4 M m3. Sliding was seated in silty-clay glaciolacustrine materials underlying a cap of glacial till and took place on a valley side that had undergone considerable pre-failure displacement. About 6.0 M m3 remained on the failure surface as slide blocks but the remainder (6.4 M m3) formed a high speed flow that traveled almost a kilometre across the flat bottomed Peace valley. An emplacement velocity of 20 m/s (72 km/hr) was estimated for the distal flow. Similar high-velocity flowslides occurred in silty glaciolacustrine sediments in the Thompson Riverv alley of southwestern British Columbia in 1880 at Ashcroft (est. vol 15 M m3) and in 1908 at Spences Bridge (described below). In eastern Canada, similar landslides have occurred in glaciolacustrine sediments which have been shown to be sensitive (e.g. 1990 Nipigon River landslide, Ontario). In the case of the 1946 Beattie Mine disaster at Duparquet, western Quebec, a flow generated in sensitive glaciolacustrine materials around the rim of the open-pit, or “Glory Hole” became fluid enough to penetrate underground workings of the mine where 4 miners were killed by the flowing debris. The secondary effects of landslides; The secondary effects of landslides (landslide generated waves and landslide dams) extend the potential for damage beyond the limits of the debris and can be very destructive. Where rapid landslides enter rivers, lakes, artificial reservoirs, or the sea, they may generate destructive water waves (also known as landslide tsunamis). They have caused substantial damage in Canada in the historical period as in the cases of the 1905 landslide at Spences Bridge, British Columbia and the 1908 Notre Dame de la Salette landslide disaster in Quebec. Underwater landslides may also cause landslide-generated waves particularly in the confined fiords and bays of Canada’s Pacific Coast. Landslide dams cause damage upstream due to back-flooding and downstream due to an outburst flood which may develop as a result of a sudden dam break. Large landslides in glaciolacustrine sediments along the Thompson River, near Ashcroft British Columbia, for example, have blocked the river on several occasions since the middle of the nineteenth century; in 1880 a large landslide blocked the river for 44 hours forming a lake that extended 14 km upstream. Landslides in Leda Clay have also formed significant landslide dams which have caused both upstream and downstream flooding in the historical period. Landslide Disasters in Canada; recent work defined a landslide disaster in the Canadian context as a landslide event, or related geotechnical failure, that directly or indirectly results in at least 3 deaths. A total of 43 events, that meet this nominal national landslide disaster criterion, occurred in Canada in a period of 160 years between 1840 and 1999. These disasters resulted in a minimum of 570 deaths. Landslide disasters are heavily concentrated in the Provinces of British Columbia (21 events; 49% of the total) and Quebec (17 events ; 39.5% of the total) which together account for 88.5% of Canada’s landslide disasters. 239 (42%) of the deaths have occurred in British Columbia, followed closely by Quebec with 213 deaths (37%). In the Province of Quebec, 44% (93) of the deaths were as a result of the four rockslides on Champlain Street, Quebec City. The 1903 Frank Slide is Alberta’s only landslide disaster, but is Canada’s worst. 55% of the deaths occurred in the Cordillera of western Canada and 32.6 % in the St-Lawrence Lowlands of eastern Canada. The most destructive landslide type are rock avalanches involving volumes equal to or in excess of 100,000 m3. Rock avalanches caused 23.5% (134) of the total deaths in only 4 disaster events. Second, in terms of destructiveness, were small scale rockslides and rockfalls involving volumes of less than 100,000 m3. These caused 17.4% (99) of the deaths in six events. Third, were landslides in Leda Clay which caused 17.2% (98) of the deaths. Many of the landslide deaths were caused by the secondary effects of landslides and related geotechnical failure, i.e., landslide-generated displacement waves and outburst floods. These caused 147 (26%) of the total number of deaths in only 5 events.
Bibliography of Canadian Geomorphology