Flow evolution and vertical accelerations in wave-swash interactions

We report on a laboratory study of wave-swash interactions, which occur in the very nearshore environment of a beach when the shallow swash flow of a breaking wave interacts with a subsequent wave. Wave-swash interactions have been observed in the field, hypothesized to be important for nearshore transport processes, and categorized into different qualitative types, but quantitative descriptions of their dynamics have remained elusive. Using consecutive solitary waves with different wave heights and separations, we are able to reliably and repeatably generate a wide variety of wave-swash interactions with large flow velocities and vertical accelerations. We find that wave-swash interactions can be quantitatively characterized in terms of two dimensionless parameters: H/H and T/T, where the first is the wave height ratio for consecutive waves and the second is a dimensionless measure of the time separation between consecutive wave crests. We find that wave-swash interactions commonly involve three main stages and that the cross-shore location of these stages is dependent on the interaction type. Using measurements of bed pressure and free-surface displacement, we estimate the total vertical accelerations and focus on the peak upward-directed acceleration. We find that wave-swash interactions can generate vertical accelerations that can easily exceed gravity, despite occurring in very shallow water depths. The vertical velocities during large vertical accelerations are upward-directed and are quickly followed by onshore-directed horizontal velocities. Together, our findings suggest that wave-swash interactions are capable of inducing large material suspension events of sediment or solutes in sediment pores, and transporting them onshore.