From UNH's 2013-2014 CCOM/JHC Seminar Series: John Hughes Clarke of the Ocean Mapping Group at the University of New Brunswick, presents, "Imaging Active Turbidity Currents on a Fjord Delta." This talk was recorded on May 2, 2014 at UNH's Jere A. Chase Ocean Engineering Laboratory.

Abstract

Using repetitive multibeam surveys at time scales ranging from annual to daily and even hourly, the temporal evolution of active channels on the front of a fjord delta in British Columbia has been monitored. During the peak summer discharge period, rapidly migrating bedforms on the delta top channel document the flux of sediment into the fjord. The lip of the delta is seen to progressive advance over a period of several days interspersed with abrupt regression during discrete mass wasting events. Those events are clearly strongly correlated with the low water spring tides when the off-delta currents are highest and the hydrostatic pressure at the seabed is lowest. During those events, bedforms in the channel abruptly migrate upslope.

Most recently, imaging multibeam sonars have been suspended from a moored vessel directly above the active channels. Using this geometry, the passage of the head, body and wake of multiple turbidity current flows can be observed with a half second update rate. Massive release of gas as a result of the head passage is clearly documented. Together with current velocity and suspended sediment measurements from co-located ADCP’s and optical backscatter probes, insights into the flow dynamics of the turbidity currents are inferred.

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