Estimates of sand transport in the Oosterschelde tidal basin using current-velocity measurements

Abstract

Estimates of sand transport have been made from mid-depth velocity measurements in the Oosterschelde tidal basin, southwestern Netherlands. This involves: (1) The use of a two-region composite boundary-layer model in deriving the local spatially averaged bed shear stress from velocity distributions over large bedforms (megaripples and sandwaves). The two regions are divided at 100 cm above the bed, with an outer-region roughness length of 1.5 cm and an inner-region roughness length of 0.15 cm. (2) The application of a modified Bagnold's transport equation with the transport coefficient K depending on dimensionless excess shear stress in the form of a power law. The calculations indicate very active sand movement in the present-day Oosterschelde tidal basin. Local sediment circulation may develop around shoals. Channel-floor erosion may occur in parting areas of sediment transport. The net sediment transport is mainly in the ebb direction. Estimates of sediment budget for the last 20 years show that large amounts of sand have been eroded from the Oosterschelde tidal basin and transported to the ebb-delta area. These results are comparable with other estimates from echo-sounding data, reflecting the rapid adjustment of the basin to the increased tidal prism. Such active sediment redistribution and major morphological evolution processes would have important influence on the structural organization and sequential development of estuary-ebb delta deposits. Information about the sedimentary processes in present-day estuary tidal basins is valuable for a better understanding of ancient examples.