TIMESCALES OF BED RESPONSE IN A LOW ENERGY SURF ZONE

Philip D. Osborne, Christopher E. Vincent

Abstract


Direct and nearly continuous measurements of bed position changes, near-bed velocities and suspended sand concentrations have been made at a single location on a macro tidal beach in the south west of England. The study was conducted in 0.5- 2.25 m water depth under low-energy swell conditions with weak currents present. Bed positions and suspended sand concentrations were measured using the 3 transducers of a multi-frequency acoustic backscatter sensor with 5 mm vertical resolution. Large (upto 10 cm) and rapid (upto 1.0 cm min-1) changes in bed elevation were observed in association with local hydrodynamic forcing, bedform development and bedform migration. Large-scale bedforms observed under breaking waves were highly mobile with measured migration rates of upto 2.2 cm min"1. Suspended sediment concentrations and transport rates are particularly sensitive to changes in bed position changes and subject to potentially large errors (upto 30 %) without compensation for bed elevation changes relative to sensor position.

Keywords


surf zone; low energy surf zone; bed response

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