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 from the comparison of surveys 263 days apart. However, in the Nash Passage (between East Nash and the coast) a 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 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 pattern 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.
Two banner sandbanks, Helwick and Nash Sands, were investigated. The banks were chosen because of their contrasting environments. The Helwick Sands is characterised by deeper water-depths, stronger wave climates and weaker tidal currents than the Nash Sands. The comparison of both sites is based on the interpretation of bathymetric data acquired repeatedly around the connection of these banks with the shore. Multibeam swathe (Reson Seabat 8101) and single-beam sonar surveys were undertaken, providing the first multibeam survey of this type of environment. A method for estimating the relative vertical uncertainties between co-located bathymetric data was developed and provides constraints on the morphological comparison and kinematics analysis of bank-associated sand dunes. The sense of the asymmetry of sand dunes is indicative of the residual tidal currents around the banks. At the crest, dunes are more symmetrical, indicating a zone of convergence. Migration rates were derived by dune tracking between surveys, giving rates between 21 m.y-1 and 109 m.y-1 at the Helwick site and between 34 and 180 m.y-1 at the Nash sites (on the basis quasi-annual surveys comparison). In the Nash Passage (area between East Nash and the coast) a short term (19 days) repeated survey revealed very fast moving (up to 715 m.y-1) small short-living sand dunes. The celerity, heights and spacing of the dunes were used to compute sand transport specific flux (typically of the order of 0.02 kg.m-1.s-1). The fluxes compare well (within an order of magnitude) with bedload fluxes computed from current meter and sediment texture data using the widely Gadd transport formula. Comparison of sand dune associated sand transport with numerical predictions are supporting the idea that sand transport occurs in large part by sand dune migration associated with bedload movements.
For both banks, the geometry of dune migration indicates a clockwise sand transport pattern around the banks, which are compatible with previous observations elsewhere. Moreover, the sand budget parallel to both banks flanks is in rough equilibrium, within the limits of the estimation method. Across-bank sand transport is more difficult to estimate but is predominantly induced by wave-induced currents. On the Helwick Sands this is particularly evident. There sand dune crests extend across the crest of the bank, despite the opposite direction of migration observed along each of the flanks. This atypical morphology is explained to be the result of the strong wave climate coupled with an elliptic tidal flow, which are leading to dune crests extension.
The pattern of erosion and deposition was derived from divergence of the specific sand flux data using the Continuity Equation. For the Nash area and with less confidence for the Helwick area, the magnitudes of the calculated erosion and deposition generally agree with bathymetric changes measured by the direct differening of co-located bathymetric data. The result revealed transient patterns of erosion and deposition along the banks near their respective headlands. Although no data of the structure of the flow was collected during the period of the study, it could be hypothesised that this transients patterns of sand deposition and erosion could be related to transient eddies observed near other banner sandbanks.
Reference:
Schmitt, T., Sediment dynamics in the Bristol Channel and Severn Estuary, PhD thesis, Cardiff University, 2006. abstract.
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