Theoretical and experimental analysis of swash flow due to obliquely incident breaking waves.

We present a quasi-analytical solution to the two-dimensional nonlinear shallow water equations using the method of characteristics, specifically to model swash zone flows generated by obliquely incident breaking waves. We use a weakly two-dimensional modelling framework, where the wave approach angle is small. This framework leads to a one-way coupling from the cross-shore flow to the alongshore flow. Motivated by the similarity of swash flow to dam-break flow, we solve the cross-shore dynamics based on the invariance of the forward-moving characteristic variable and subsequently solve for the alongshore flow. The new solution's ability to predict real flows is tested using fifteen large-scale wave basin experiments, including normally incident and obliquely incident waves of different heights and periods. Measurements of flow velocity and water depth are obtained via in situ sensors and the shoreline motion is tracked using quantitative imaging. The data confirm that the key theoretical assumptions are satisfied: the flow exhibits a constant value on forward-moving characteristics and the shoreline motion obeys Snell's law. The results demonstrate that the proposed solution accurately captures swash zone flow behavior across a range of wave conditions.