Swash zone boundary conditions derived from optical remote sensing of swash zone flow patterns

Optical remote sensing is used to measure flow patterns in the swash zone. Timestack images are analyzed to measure the asymmetry and the relative duration of the inflow into the swash zone. This varies significantly between individual swashes, contrary to the classical analytical swash model for runup induced by bores, which predicts a similar flow pattern for all events. For swash forced by breaking bores, the gradient of the x‐t locus of flow reversal varies over a wide range and flow reversal can occur simultaneously across the whole swash zone. This variation of the gradient of the locus of flow reversal in x‐t space can be parameterized in terms of a single free variable in recent solutions to the nonlinear shallow water equations, which fully defines the swash boundary inflow condition. Consistent with the theory, the horizontal runup, the swash period, and the swash similarity parameter were observed to be independent of the swash inflow conditions but the flow asymmetry is not. Only a weak correlation was observed between the swash boundary condition and the Iribarren number and beach slope. Conversely, the analysis suggests that the degree of swash‐swash interaction does influence the swash boundary condition and the resulting internal flow kinematics. The variation in inflow conditions is expected to influence the magnitudes of the velocity moments within the swash zone and therefore sediment transport rates.