THE INFLUENCE OF RESIDUAL EFFECTS, COASTAL SLOPE AND BATHYMETRIC PERTURBATIONS ON HEADLAND ASSOCIATED SEDIMENT DYNAMICS

摘要

The sediment transport and sandbank building mechanisms of the Bristol Channel have been investigated using an idealised numerical model. The model consists of a theoretical semi-Gaussian shaped headland comparable to Nash Point in the Central Bristol Channel U.K. Under low energy conditions we see two principal zones of sediment accretion develop, a larger one extending into the constricted upstream channel and a smaller deposit in the downstream wake of the headland. Large gradients in transient tidal velocities mark these principal depositional areas. The relationship between the residual velocity field and areas of sediment accretion is less clear. However, confined regions of upwelling to the north east and south west of the headland, correspond well with positions of cubic velocity residual gyre centres and mark limited regions of sediment accretion. This suggests the possible influence of secondary flows on post-depositional sandbank building processes. Adjustment of the near-headland geometry, to include a gently sloping coastal ‘apron’ surrounding the headland, has the effect of increasing the amount of sediment transport. This is in response to both continuity and bed slope effects, which augment offshore acceleration and induce enhanced sediment scour. Under higher energy conditions, we see the disappearance of the upstream accretionary zone and a large increase in deposition downstream of the headland. These results show good agreement with conditions observed in the real environment, where strong tidal flows upstream of Nash Point restrict accretion east of the headland. The final test, conducted using an initial bed perturbation to represent a bedrock outcrop or sandbank ‘core’, demonstrate that under low energy conditions such a feature promotes secondary flow mechanisms and local accretion.