Flow evolution and solute dispersion in the swash of regular waves

Beaches are sources and sinks for solutes and debris, and thus wave-driven flow on beaches is important in the dispersion and accumulation of contaminants at coastlines. The swash zone, the most shoreward extent of wave-driven flow, is highly complex and the flow evolution and transport processes there are some of the least understood. In this study, we show via high resolution surface velocity measurements of periodic waves that the swash zone flow evolution has strong agreement with models of swash dynamics generated from dam-break scenarios with constant forward moving characteristics. The results reveal continuous stretching of the flow, acting as the dominant mechanism for solute dispersion in the swash zone. We derive a new dispersion regime for this flow and show that the prediction agrees with the measured growth of dye clouds. We further confirm the existence and measure the flow convergence at the swash tip, the likely mechanism by which buoyant debris accumulates at the maximum run-up.