Dune-associated sand fluxes at the nearshore termination of a banner sand bank (Helwick Sands, Bristol Channel)

Abstract

Sand dunes typically migrate in opposing directions along the two sides of sand banks, reflecting a circulation driven by tidal current asymmetry, but it has been less clear how this pattern is distorted where banks intersect the coastline. The nearshore end of Helwick Sands, a banner bank in the Bristol Channel, was surveyed three times over three years, twice with a high-resolution multibeam echo-sounder. In both multibeam surveys, an unusual geometry was found over the crest of the bank, whereby dunes connect continuously with the dunes on the flanks, despite the flank dunes migrating in opposite directions. The crestal dunes thus appear to realign rapidly. We suggest that this morphological behaviour arises here because of vigorous wave-driven transport and because surface waves propagate almost exactly parallel to the crestal dunes. Sand transported parallel to the crestal dunes ensures that efficient reconnection occurs with dunes migrating along the flanks, particularly at low tide when wave currents are more strongly felt at the bed.Dunes were tracked between the surveys to assess the bedload transport fluxes associated with dune migration. Fluxes on the two flanks of the bank were found to be similar. The coarse-scale pattern of deposition and erosion was calculated from divergence of the fluxes using the continuity relation. That pattern differs strongly compared with the deposition/erosion pattern derived more directly from the bathymetry change between successive surveys. This implies that the dune-associated fluxes fail to represent all the sediment movements in this shallow area; significant contributions from storm wave-assisted transport, suspended load and/or sand transport during the reversed tide also probably affect transport flux besides dune-associated bedload.