A PRE-DREDGING SAND MOBILITY STUDY USING A RADIOISOTOPE TRACER

Abstract

The wave-driven movement of sand across the alignment of a proposed navigation channel was investigated using radioactive chromium-51 labelled tracer sand. The mean particle velocity and thickness of the mobile layer were determined over a two-month period, and an annual infill rate estimated. Wave height and period were measured concurrently. Despite two storms, during which near-bed oscillating velocities of 1.5 m s-1 were calculated the sand transport at 10 m (BMWL) appears to occur within the wave boundary layer. Onshore transport in the direction of wave propagation, due to mass transport velocity and wave asymmetry effects, was easily identified. Tidal currents up to 1.2 m s-1 (at 3 m above bed) had less than the expected effect on the tracer dispersion pattern.