Author(s): Kate Steenhauer; Dubravka Pokrajac; Gustaaf Kikkert; Tom O'Donoghue
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Abstract: This study contributes to the ongoing investigation of fundamental aspects of bore-driven swash, in particular examining the effects of permeability on the surface and subsurface hydrodynamics using large-scale laboratory experiments and numerical modelling. Detailed surface and subsurface flow measurements were conducted for two steep immobile beaches with different roughness and permeability (1. 5mm and 10mm sediment) using Particle Image Velocimetry, Laser Induced Fluorescence, pressure transducers and wetting front digital image recordings. A numerical model for bore-driven swash on coarse-grained beaches has been developed, consisting of a surface and subsurface flow model. The subsurface model includes filtration, air pressure and groundwater modules, capable of simulating the vertical filtration processes, and the horizontal movement of air and groundwater within the beach. The numerical and experimental results are in good agreement. They reveal the distinctly different behaviour of filtration and pressure within the beach for the two types of sediment. The considerable build-up of air pressure, induced by entrapped air below the wetting front in the unsaturated region of the 1. 5mm sediment, minimises the loss of surface water to the beach by reducing infiltration rates. Moreover it causes the flow within the bed to reverse and induce exfiltration. In contrast, the build-up of air pressure in the10mm beach is considerably lower, resulting in higher infiltration rates and a greater exchange of water from surface flow to the subsurface. The combined insights from the experimental and numerical work significantly contribute to the better understanding and modelling of key physical processes for swash and similar flows.
Year: 2010