The effect of velocity veering on sand transport in a shallow sea

Abstract

This study aims at comparing and contrasting two different models for sand transport by currents in a shallow sea to illustrate the effect of velocity veering. The first model uses the Bailard-type formulation, which allows calculation of erosion/deposition rates at a fixed location on the sea floor via the divergence of horizontal sediment fluxes. The second model is a semi-analytical 2.5-dimensional model, which takes into account the time lag between erosion and deposition events and the velocity veering within the sediment-laden (nepheloid) layer caused by the Coriolis force. The velocity veering implies that the direction of the sediment flux is generally different from the direction of the surface flow. The latter model was designed for rapid, semi-analytical computations of sediment transport, using flow fields from 2-DH numerical models. The two models use a matching set of parameters to provide identical values for the bottom stress and suspended sediment load for a uniform steady current at any given surface velocity. The two models were compared in a range of sand grain sizes 50–500 μm and current speeds up to 1 m s⁻¹ for an idealised square region (100 × 100 km) of a shelf sea of constant depth. The erosion/deposition patterns and suspension load were examined in three settings: (1) uniform steady flow, (2) straight jet, (3) meandering jet. It was found that both the rates and, in particular, the spatial distribution of the areas of erosion/deposition differ significantly between the models in cases (2) and (3). This difference can be attributed to additional flux divergence due to velocity veering. A comparison of model results with field data, collected at Long Island Shelf, supports the relevance of Coriolis-induced veering of currents on the direction of the sediment flux.