Morphology and dynamics of headland connected sandbanks from high resolution bathymetric surveys : Helwick and Nash Sands, Bristol Channel, U.K

摘要

Morphology and movements of sand dunes are studied using repeated high-resolution bathymetric data in areas where banner banks approach the shore. Two sites in the Bristol Channel were selected for their contrasting environments. The Helwick Sands is characterised by deeper water-depths, stronger wave climates and weaker tidal currents than the Nash Sands. At the Helwick, migrations of the dunes were measured ranging between 21 and 109 hlv"1. Dunes crossing its crest and connecting despite opposite direction of migration on either flank were observed. This geometry is interpreted to be the result of the strong wave climate coupled with a nearly rectilinear tidal flow, which are leading to dune crests extension. A morphometric study of the sand dunes has revealed the tendency for the dunes to flatten in shallow water, which can also be attributed to the effect of the waves. At the Nash, strong currents and breaking waves have created a strong crestal escarpment. Dune migration rates along the flanks were measured to range from 34 to 180 m.y"1 {Text not available}, based on a comparison of surveys 263 days apart. However, in the Nash Passage (between East Nash and the coast) short term (19 days) sand dunes migration measurements revealed very fast moving (up to 715 m.y_1) small short-living sand dunes. Celerity and morphology of the dunes were used to compute sand transport specific fluxes. Such fluxes are broadly compatible with fluxes computed from tidal current data using bedload transport formulae. For both banks, the geometry of the flux vector field suggests a clockwise sand transport pattern around the banks. Although different intensities of the fluxes were expected at the two sites, the fluxes of corresponding morphological areas are similar. Differences in the tidal current asymmetry and reduced effective threshold of sand transport due to the wave energy are invoked to explain theses similarities. Pattern of erosion and deposition were evaluated from the divergence of sand fluxes. This partem has revealed the occurrence of transients in the sand transport, which are the result of complex interactions between the flow, the headland and the bed morphology.