CGRG Bibliography of Canadian Geomorphology
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Author : Geertsema, M.
Date : 2011.
Title : Quick clay landslides, landscape evolution, and climate change - a perspective from British Columbia.
Publication : Putting Science into Practice. The Second World Landslide Forum. October 3-9, 2011. FAO Headquaters, Rome, Italy.
Issue : Abstract Book.
Page(s) : 12.
Abstract
Stream erosion is believed to be the most common trigger of retrogressive landslides in marine sediments. Both Bjerrum in Norway, and Lefebvre in Quebec, Canada used a landscape evolution approach to map zones of earth flow potential in sensitive clay areas. They were interested in stream incision. Bjerrum and colleagues defined three zones of erosion for streams and related them to earth flow hazard. 1. A lower zone, no longer actively downcutting, characterized by old slide scars, and localized cutbank failures in stream bends. This is a mature zone and the danger of quick clay landslides is small. 2. An intermediate zone, where streams flow almost exclusively on displaced landslide deposits, and in general, the streams are eroding back to their old levels. As long as streams are cutting through old landslide deposits the risk of quick clay landslides triggered by erosion is low. 3. An upper zone, at the original horizontal plateau where streams are cutting into undisturbed clay. It is at the boundary between these zones 2 and 3 that the risk of retrogression was found to be the greatest. They called this boundary the front of aggression. Each time an earth flow occurred, the intermediate zone would be extended, moving the front of aggression upstream. Lefebvre described three phases of valley formation with associated groundwater flow regimes. An early phase involves relatively shallow stream incision into deep mud deposits where groundwater flow is not influenced by underlying (more pervious) till. An intermediate phase is characterized by strong artesian pressures and thus favours large, retrogressive landslides. A late phase occurs when streams have incised through the lower pervious till (if it exists) resulting in downward flows. Only small landslides are expected during the early and late phases of valley formation. While Bjerrum's zones do not relate directly to Lefebvre’s phases of valley formation, in both approaches the intermediate zones present the greatest hazards for retrogressive flowsliding. It appears that Lefebvre’s valley formation approach is more applicable to British Columbia than the front of aggression approach of Bjerrum because prehistoric landslides are already well distributed throughout the area and large landslides are occurring in areas with previous retrogressive failures. I examine prehistoric and historic quick clay landslide scars in two landslide rich valleys in northwestern British Columbia. Streams in the Terrace-Kitimat valley are still incising deep glaciomarine sediments and appear to be in the early and intermediate stages of valley formation, thus large landslides are still occurring and more are expected. In constast, streams in the Nass Valley are incised into bedrock - early evidence suggests most of the landslides are old, and more large landslides are not expected to be triggered by bank erosion. Rapid incision in the Nass valley may have overwhelmed climatic influences, however in the Terrace-Kitimat valley identified wetter climate regimes seem to correspond to higher earth flow activity. Here, a future warmer and wetter climate, as predicted by most global circulation models, will likely lead to increased landsliding.
Bibliography of Canadian Geomorphology