Sediment transport under waves and tidal currents: a case study from the northern Bristol Channel, UK

Bed-load transport under tidal currents and waves, and waves alone, is predicted from near-bed self-recording current-meter data and observations from a wave-rider buoy. Transport rates and paths are derived using seven different unidirectional transport formulae and those of Bagnold (1963), Bijker (1967) and Madsen and Grant (1976) for combined flows. Predicted rates are compared with the results of a fluorescent sand tracer study.

The data are available from Swansea Bay, a high tidal- and wave-energy embayment on the British continental shelf. Sediment transport paths in this region had been determined previously using a variety of techniques, including bedform orientation and sea-bed drifter recovery patterns.

Transport is enhanced under the superimposed effect of waves and, in some circumstances, the direction of the resultant vector is altered. Such storm-induced catastrophic transport is different to that predicted under tidal currents alone; this could provide a mechanism for long-term sediment supply. In some areas, there is correspondence between bedform orientation and storm-induced transport paths; in others, there is opposition.