A slope failure caused by drainage cutoff through the advancement of seasonal frost, Hudson Bay Lowland
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. P. van Zeyl I L. A. Penner I R. A. Halim
A slope failure caused by drainage cutoff through the advancement of seasonal frost, Hudson Bay Lowland
Abstract A failure occurred between December 14 and 17, 2008 in the upper part of a 45-m high, northwest facing bank of the Nelson River in northern Manitoba (56.687°N, 93.777°W). The slope failure occurred at a spring site in a bay associated with a buried valley. The sediment input to the river from this event is roughly 20,000 to 25,000 m3. The source zone is made up of 25 m of water-bearing sand and gravel confined between ice-rich silty clay at the top of the bank and laminated to rhythmically bedded silt and clay at the base of the section. The collapse was confined to the material above the basal silts and clays and was associated with a perched groundwater flow system. A strong argument for drainage cutoff by the advancement of seasonal frost has been demonstrated through the correlation of the bank collapse with the timing of a significant cold snap recorded at two nearby weather stations. The failure illustrates the importance of stratigraphy in controlling bank erosion in this area. Previously, fluvial erosion was seen as an important control on mass wasting in Horseshoe Bay. However, surface information suggests that no toe erosion except to remove the slide deposit has occurred at this site since 2004. Keywords Frost penetration . Layered slope . Perched aquifer . Bank erosion . Hudson Bay Lowland . Subarctic climate Introduction A significant volume of sediment was delivered into the Nelson River by a bank failure that occurred in December 2008, about 90 km inland of Hudson Bay (Figs. 1 and 2). The failure appears to have been caused by the advance of seasonal frost during a cold snap, a mechanism not often described in the literature but that should be expected in certain geological settings in northern regions. The landslide was studied during an erosion assessment for the potential Conawapa hydropower project being planned by Manitoba Hydro. The failure occurred 1 km downstream of the potential generating station and represents the single largest erosional event observed in the area during erosion studies to date. By providing a record of the failure mechanism and failure zone stratigraphy, this case record illustrates the importance of stratigraphy in controlling bank erosion in the area. Bank erosion in this area can result in ecological impacts associated with loss of riparian habitat, disturbance to fisheries through increased turbidity, and the destruction of heritage resources. In addition to generating potentially destructive waves, sediment delivered by landslides into hydroelectric reservoirs can reduce reservoir capacity. Previous work During reconnaissance work from Split Lake to Hudson Bay, Piteau (1972) found that actively failing slopes were a common feature along the banks of the Nelson River. Dredge and Nielsen (1985) described bank failures along the Nelson estuary. Upstream of Horseshoe Bay,
Donnelly et al. (1990) described minor mudsl
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