Narrow Results

Detailed measurements of the hydrodynamics of bore-generated swash on steep beaches with different roughness have been obtained. They were used for investigating a number of approaches to determine the bed shear stress in order to assess their theoretical and practical viability. The relationship between the bed shear stress in the swash zone and the main flow parameters is further discussed.

The shoreline along sandy beaches is located at a unique position on the earth's surface where marine and terrestrial processes converge. The swash zone distinguishes the landward-most reach of wave action. Field observations from this shallow and highly energetic region reveal that individual waves regularly deposit or remove hundreds of kilograms of sand per meter width of beach. Such high rates of sand movement represent several centimeters of bed-level change and far exceed the underlying pace of beach evolution. Relatively large morphological changes caused by single swashes might suggest that very rapid beach erosion or accretion is a common occurrence. The contrasting reality shown by these new observations is that beaches generally exhibit a state of dynamic equilibrium.

A method to determine swash zone boundary conditions is developed using remote sensing by video and ARGUS. Guard and Baldock (2007) proposed new numerical solutions for swash hydrodynamics, where the flow field varies according to a free parameter, k. k=0 in the Shen and Meyer swash solution but k=1 appears more realistic for real swash. This study has developed a semi-automated method to obtain values for k from multiple waves in field conditions using ARGUS video data and image analysis techniques. The results indicate that k is greater than 0 and close to 1 for natural swash. No trends were observed between k and run-up length, offshore significant wave height or tidal phase, consistent with the self-similarity of the swash hydrodynamics. In conjunction with modelling, the method offers the possibility of remote sensing of the asymmetry in the swash zone hydrodynamics.