ABSTRACT: The August 1999 debris flow in the vicinity of Banff, Banff National Park, Rocky Mountains, Alberta.

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Abstract

In the Rocky Mountains of western Canada, debris flow is an important geomorphic process that transfers a large amount of sediment from low order mountain watersheds to high order valley bottoms (Winder 1965; Desloges & Gardner 1984; Jackson, 1987; Jackson et al. 1987; VanDine, 1985). Debris flow is also an important hazard and has had a major impact on transportation routes in the Rocky Mountains in thehistorical period (Hungr et al., 1987; Eisbacher, 1980; Evans and Clague, 1989; Podor, 1992). This paper documents a major debris flow and its consequences that occurred near Banff in August 1999 and contains detailed field observations of the depositional and source areas of the debris flow.At about 6:45 p.m. on Wednesday, August 4th, 1999, a debris flow, originating in the Five Mile Creek watershed, buried all four lanes of the Trans-Canada Highway which runs along the north side of the Bow Valley, west of the town of Banff in Banff National Park, Alberta. Thousands of motorists were affectedby the closure of the highway at the height of the tourist season and had to take alternative routes, or wait 24 hours for the partial reopening of the highway. The debris flow also cut buried fiber optic cables depriving the community of Lake Louise of telephone communication. Outdoor activities in this area of the Banff National Park were also affected ; hiking trails were cut and access to warden cabin was thus impossible .. This part of the Bow valley is also utilized by the Canadian Pacific Railway which runs along the Bow River below the highway; the railway was not affected by the debris flow.Debris originated in colluvial material on steep mountain slopes in the upper part of the Five Mile Creek basin and traveled more than 3 km from the source area down to the Bow River valley bottom. At the highway, debris, including mud, boulders and tree limbs, rapidly filled a large culvert beneath the road and then overwhelmed the highway, covering a 200 m wide stretch. The uppermost layer of the debris was essentially fine material. Thickness of debris varies between 2 m on the westbound lanes to 50 cm on the eastbound lanes. 150 m-long portions of Five-Mile Creek upstream and downstream of the highway were rapidly and completely filled by debris. At its distal limit, the debris spread out in a forested area, and mud and water reached the Canadian Pacific Railway, but without any damage being caused to the railway embankment. Our calculations indicate that a total of 45,000 m3 of debris was removed from the highway and from upstream and downstream portions of Five-Mile Creek. According to Banff National Park personnel, a localized, high intensity thunderstorm provided intense rainfall over the mountainous terrain of the Five-Mile Creek watershed, several hours before the debris flow. Although severe rainfall seems to have been triggered the debris flow, weather stations located at Lake Louise (about 45 km north west of Five Mile Creek) and at Banff (10 km east of Five-Mile Creek) did not record the high rainfall, due to the isolated nature of the thunderstorm. Daily rainfall data recorded at the Banff weather station indicates that the total precipitation for the month of July 1999 exceeded twice the average monthly precipitation recorded for the period 1887-1990. It is thus probable that colluvial and other surficial deposits on the steep mountain slopes of the upper part of Five Mile Creek watershed would have been saturated when the thunderstorm event triggered the debris flow on August 4thThe Five-Mile Creek event is characteristic of a Canadian Rocky Mountains debris flow in that the flow had multiple source areas of debris. Over ten small gullies contributed debris from the eastern slopes of the Five-Mile Creek valley. However, the largest proportion of debris came from four major gullieslocated on the western slopes of the Five-Mile Creek valley. In addition debris was mobilised from the main channel of Five Mile Creek. Much of the surface of the source area is covered by bare rock, leading to low infiltration into the ground and a high level of runoff. The high localised rainfall provided an extraordinary amount of runoff, which had the capacity to erode a significant amount of colluvial material from the steep mountain slopes. It is noted that no single rock mass failure, or slide in colluvial debris initiated the debris flow. According to the records and recollection of Banff National Park and Highway Maintenance Service personnel, the 1999 debris flow is the first to have occurred in Five-Mile Creek since the construction of the Canadian Pacific Railway in the 1880s. Examination of aerial photographs taken since 1947 does notshow evidence of debris flow activity in the Five-Mile Creek basin or on its fan. Thus although Five Mile Creek is not recognized as a problematic watershed , it was the location of a large debris flow suggesting that debris flows similar to the 1999 event have long return intervals. Reconnaissance observations in the vicinity of the Town of Banff indicated potential locations of future debris flows. Some creeks were noted to be charged with colluvial debris, which could generate a debris flow if high rainfall or an important snow melting event would occur. Other major debris flows have occurred in the vicinity of Banff in recent years. On July 31, 1998, two large debris flows occurred in the Spray River valley, south of Banff. One of them partially blocked the Spray River impounding a small lake.

Bibliography of Canadian Geomorphology