What is it about?
Sand banks are shaped by tidal currents, typically with sand moving on average with different directions on the two sides of the bank. This is reflected in the shapes of sand dunes, which have steeper lee slopes in the direction of sand movement. Helwick Sands, a sand bank in the Bristol Channel, unusually reveals dunes over its crest that connect up with the dunes on the flanks, despite them moving in different directions. We attributed this to an effect of strong oscillating currents due to surface waves, which create a strong mobility of the sand across the bank crest. We have also attempted to quantify the rates of sand movement ("fluxes") by seeing how far the dunes migrate between sonar surveys, showing that the fluxes are roughly equal on the two sides of the bank.
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Why is it important?
The unusual crestal morphology of flank-connecting dunes has only been reported in one or two instances, as far as we are aware. This may suggest that such instances require rather unique configurations of water depth, tidal currents and waves. In this case, the wave field is parallel with the dune crests and tidal oscillations are perpendicular to dune crests.
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This page is a summary of: Dune-associated sand fluxes at the nearshore termination of a banner sand bank (Helwick Sands, Bristol Channel), Continental Shelf Research, March 2014, Elsevier,
DOI: 10.1016/j.csr.2014.01.003.
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