HR: 1340h
AN: OS23B-1314    [Abstracts]
TI: Multibeam Mapping of Active Slope Instability Features: Examples from the Fraser River and Squamish River Deltas, British Columbia, Canada
AU: * Hill, P R
EM: phill@nrcan.gc.ca
AF: Natural Resources Canada, Geological Survey of Canada, Box 6000, Sidney, BC V8L 4B2 Canada
AB: Multibeam mapping of the coastal waters of British Columbia has immensly improved our ability to identify and assess submarine landslide and tsunami hazard. This paper will present analysis of high-resolution images of slope instability features from two delta slopes where recent slope failure can be documented through repetitive multibeam mapping and/or comparison with previous single-beam hydrographic soundings. Numerous mass movement features characterize the slope of the Fraser River delta, all the recent features being located at the mouths of distributary channels. Engineering works have maintained the main channel in a fixed position since the 1930's, contributing to over-steepening of the slope and development of a network of submarine channels. Repetitive multibeam mapping shows that recent slope failures have occurred in numerous locations around the main channel lobe, some at the head of a large submarine channel system and others as isolated small failures that form the headwalls of small submarine channels. The scalloped morphology and association with channels, together with volume estimates derived from repetitive multibeam mapping, indicate that these features result from shallow, small volume liquefaction failures. Smaller scale, shallow slides are present on the very shallow water slope area adjacent to the channels, raising the possibility of groundwater seepage as an influence on slope stability. The slide masses from these failures are rapidly transformed into gravity flows that carve the submarine channels. Slides and channels of a similar scale are found at the mouth of a secondary distributary channel and an abandoned distributary channel. The multibeam imagery allows discrimination between recent slide features and relict features, the latter showing infilling or reworking by bottom currents. An area of undulatory seafloor, located on the flank of the main distributary channel lobe, has been cited as a possible creep displacement feature and a potentially high risk of subsequent slope failure and tsunami generation. Repetitive mapping of this feature suggests relative movement of ridges and trenches as well as general subsidence of the lobe flank. This feature has been selected as a site for pore fluid pressure monitoring within the VENUS cabled observatory program. Recent multibeam mapping of the Squamish River delta followed a major flood in October 2003. The survey of the delta was completed in less than 12 hours on an opportunity basis. Preliminary results and comparison with maps created from more labor-intensive traditional techniques will be presented.
DE: 3022 Marine sediments--processes and transport
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting