Numerical modelling of storm and surge events on offshore sandbanks

Abstract

This contribution uses a 3 dimensional coastal area numerical model, DHI's MIKE3, to simulate the impact of storm and surge events on offshore sandbanks. Three offshore sandbanks in the Bristol Channel are considered due to the region's sensitivity to anthropogenic pressures and the gradients in wave and tidal forcing in the area. Two extreme storm and surge events are simulated: one co-incident with spring tide and the other with neap tide. Reference simulations of astronomical tidal forcing only are also presented. It is shown that for the two sandbanks with greater wave exposure, storm conditions can provide a mechanism for the maintenance of crest position. For these cases, bed level changes over the crest are in the opposite direction compared to astronomically forced change. For the least wave exposed bank, both wave and tide only cases exhibit similar patterns of bed level change. Volumetric changes under astronomical forcing are shown to vary with changing maximum tidal current. Accretion occurs over a neap tidal cycle for all three sandbanks and as maximum tidal current increases the amount of accretion increases; however, over a spring tidal cycle accretion is observed for the less tidally dominated site but increasing maximum tidal current leads to reduced accretion and then erosion for the most tidally dominated bank. Volumetric changes under storm conditions are related to sandbank morphology and setting rather than relative wave exposure. The two single banks closely tied to headlands show similar magnitude of percentage volumetric change despite being at the two extremes of wave exposure and greater erosion occurs over the neap tide event. The sandbank that has associated secondary banks shows lesser percentage change and greater erosion over the spring tide event.