Broad-Scale Hydrodynamic Simulation, Wave Transformation and Sediment Pathways

Abstract

The stretch of coastline off East Anglia (UK) is characterised by the presence of large-scale seabed features, tidal sandbanks and shore-connected sand ridges, which determine tidal flow movement and wave propagation. This chapter describes the method used for transforming extreme offshore events into shallow waters which influences local flooding and erosion. The climate projections presented in Chap. 2 are at a regional scale that allows the assessment of the influence of climate change on offshore wave and storm-surge conditions; in this case the effect is small. In order to propagate offshore wave conditions into shallow water and ultimately the coast, fine-resolution numerical wave and surge models are necessary to capture the spatial variability of the bathymetry and the physical processes associated with wave propagation in shallow water. Here, this is achieved by the TELEMAC hydro-informatics system. More precisely it is used to generate a look-up table to compute inshore waves from 1960 to 2099 taking account of the cumulative effect of climate change on atmospheric forcing and sea-level rise. Simulations take into account four scenarios of linear sea-level rise and a baseline climatic scenario for storm surges and offshore waves. Extreme event analysis is performed to estimate the effect of sea-level rise on nearshore wave height and the occurrence of extremes. For nearshore areas, it is shown that an increase of mean sea level leads to a reduction in the return period of extreme waves. The wave simulations above are made assuming a fixed bed. Changes in wave climate and sea level potentially modify the sediment dynamics and the supply of sediment for the maintenance of offshore sandbanks. Here, the evolution of the sandbanks is analysed by simulating sediment transport and seabed morphodynamics using a process-based approach. The complexity of long-term residual flows and sediment pathways is shown by nearshore and sandbank interaction. Wave transformation methods were used to provide the inputs to the erosion and flood analyses in Chaps. 7 and 8. The sandbank dynamics are important, but the understanding was too limited to include. This is an important topic for further research.