Sand transport under full-scale progressive surface waves

Abstract

The morphology of coastal areas is constantly changing under the influence of sediments being transported to, from and along the coast. Under storm conditions with high waves and flow velocities, bed forms are being washed out and large quantities of sand are transported in a thin, mm to cm thick layer close to the bed called the sheet-flow layer. Since sand transport under storm conditions is primarily controlled by small scale near-bed processes, development of well-founded methods for predicting near-bed sand transport are critical for estimating sand budget in coastal areas. Various transport models have been developed to predict both the quantities and directions of sediment transport under storm conditions. The majority of the existing models are based on data obtained from oscillatory flow tunnel experiments. Even though oscillatory flow tunnels provide a good approximation of the flow experienced at the sea bed, theory and former experiments indicate that flow differences between full scale progressive surface waves and oscillatory flow tunnels may have a substantial effect on the net sand transport. The research presented in this thesis focuses on the influence of surface wave effects on sand transport under sheet-flow conditions. For the first time, detailed measurements of wave boundary layer flow and sheet-flow layer transport processes under full scale surface waves are presented and analysed. These results give new insights and provide quantitative data of wave boundary flow, sheet-flow layer concentrations, sediment fluxes and net transport rates under velocity skewed surface waves for different wave conditions and types of sediment.