Sand transport processes in the surf and swash zones

Long-term predictions of beach morphology using numerical models contribute to cost-effective coastal protection strategies. The physics of sand transport in the wave breaking region and the swash zone are not fully understood, leading to poor predictive capability of existing sand transport models for these regions. The aim of this thesis is therefore to examine the sand transport physics in the breaking and swash zones, through controlled experiments in a large-scale wave flume. The experiments involved high-resolution measurements of fluid and sand transport dynamics under energetic breaking waves over a medium-sand bed, using novel acoustic and conductivity-based measurement instruments. Hydrodynamic measurements in the breaking region show that wave breaking-generated turbulence invades the wave bottom boundary layer. This enhances suspended sediment pick-up rates (by an order of magnitude) in the breaking region relative to the wave shoaling region. Sand transport is composed by generally onshore-directed bedload and offshore-directed suspended transport. Suspended transport is dominated by current-related advection at outer-flow elevations; the wave-related transport is generally confined to the wave bottom boundary layer. The cross-shore distribution of the transport components is related to the breaker bar morphologic evolution and to cross-shore grain size sorting. Measurements of sheet flow processes in the swash zone using a novel conductivity-based measurement instrument reveal substantial intra-swash bed level changes (of up to 1 cm) due to the strong cross-shore non-uniformity of the flow and sediment transport. In addition, bore turbulence and cross-shore sediment advection lead to an increase in the swash zone sheet flow layer thickness.