Storm-triggered mass failure and sediment liquefaction in the Swatch of No Ground canyon, offshore Bangladesh
Abstract
In November 2007 the eye of a major tropical storm, Cyclone Sidr, tracked over the offshore canyon associated with the Ganges-Brahmaputra river delta. Known as the Swatch of No Ground (SoNG), this large canyon incises within 30 km of the deltaic coast and is accreting rapidly (10 to >50 cm/yr) with fluvially derived sediments. Historically numerous tropical storms have affected the northern Bay of Bengal each year, but more recently cyclogenesis has been weak and Sidr is the first major cyclone to impact the Bengal shelf since 1991. As part of an ongoing study, our research team had just completed a high-resolution sub- bottom sonar (chirp) survey of the SoNG canyon head six months prior to Cyclone Sidr. Following the storm, we organized a second sub-bottom survey to assess the storm's impact on canyon morphology and sedimentation patterns. A comparison of our pre- and post-storm surveys reveal dramatic mass failures around the canyon wall, but also vast areas that were largely unaffected. Among the failures, many had dimensions 10s of meters thick and >1 km wide. These large-scale failures appear to have been preferentially located where there is pre-storm evidence for fluid escape, perhaps via submarine groundwater discharge through onshore sandy delta complex. Fluid chimneys, apparently gas charged, are also associated with many of the large failures. However, many locations with prominent shallow gas show no disruption after the storm, suggesting that zones prone to major failure are associated fluid flow. Other areas of the canyon show storm-induced liquefaction of surface sediments (<5 m below seabed) and resulting mudflows. Runout distances are only a few hundred meters, though, suggesting rapid consolidation of the flows even on sloping surfaces (1-3°). As interesting as these storm-related failures are the actively accreting, steep-walled (>5°) gullies that show no apparent effect of the storm. Overall it appears that areas of the canyon most susceptible to failure are not necessarily the steepest or most rapidly accreting, but rather those associated with subsurface fluid flow and planes of weakness along escape structures.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMOS54A..02G
- Keywords:
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- 1817 Extreme events;
- 3025 Marine seismics (0935;
- 7294);
- 3070 Submarine landslides;
- 4219 Continental shelf and slope processes (3002)