Formation and failure of natural dams in the Canadian Cordillera

Abstract

In western Canada, existing and former lakes dammed by landslides, moraines, and glaciers have drained suddenly to produce floods orders of magnitude larger than normal streamflows. Landslide dams consisting of failed bedrock generally are stable, whereas those comprising Quaternary sediments or volcanic debris fail soon after they form, typically by overtopping and incision. Moraine dams are susceptible to failure because they are steep-sided and consist of loose, poorly sorted sediment. Irreversible rapid incision of a moraine dam may result from a large overflow associated with a severe rainstorm, avalanche, or rockfall. Some glacier-dammed lakes drain suddenly through englacial and subglacial tunnels to produce large floods. Most outburst floods are characterized by an exponential increase in discharge, followed by an abrupt drop to background levels when the water supply is exhausted. Peak discharges are controlled by lake volume, dam characteristics, failure mechanisms, and downstream topography and sediment availability. An appraisal of the likelihood that a natural dam will fail can be made by studying the dam, the reservoir, and the surrounding terrain. A moraine dam may be hazardous if it contains ice or has a low width-to-height ratio, or if the reservoir is bordered by steep, rockfall- or avalanche-prone slopes. All glacier-dammed lakes with a recent history of outburst floods should be considered hazardous, but even stable lakes may drain suddenly after a long period of glacier retreat. Many landslides dams fail soon after they form, consequently hazard appraisal may be concerned more with the location and size of potential dams than with the stability of existing dams.