Model of bores interaction in the swash

This paper examines the impact of wave interactions in the inner-surf zone, including the swash, on shoreline excursion dynamics. Specifically, the study focuses on how the time lag between consecutive waves and the slope of the beachface affect shoreline dynamics. To investigate this, a laboratory experimental setup is developed to study the behaviour of two consecutive bore-type waves travelling over a fluid layer with constant depth, representing an idealized inner-surf zone, before impacting an inclined solid plane with slope $\beta$, representing the beachface. Bore waves are generated using two dam-break flow devices, with a controlled time lag $\Delta t$ between them. This simplified physical model allows for the assessment of semi-theoretical predictive models based on shallow water approximation, resulting in ballistic-type models for shoreline motion. By combining these approaches, the study characterizes different flow regimes in the $(\Delta t,\beta )$ parameter space. It is observed that varying $\Delta t$ at a fixed $\beta$ distinguishes four interaction regimes, either wave–wave interaction in the inner-surf zone or wave-swash interaction in the swash, with possibilities of merging or collision depending on wave orientation. In particular, increasing $\Delta t$ leads to either bore–bore merging in the inner-surf, bore-runup merging in the swash, bore-backwash collision in the swash or bore–bore collision in the inner-surf. Bore–bore merging and bore-runup merging enhance runup, peaking when $\Delta t$ is such that merging occurs near the transition from the inner-surf to the swash. On the contrary, bore-backwash collision results in additional dissipation of the second bore’s dynamics, leading to a reduced shoreline extension compared to that induced by the first bore. All processes within the swash, including the transition between bore-runup and bore-backwash regimes, as well as the enhancement and extra dissipation of the second bore’s swash excursion, exhibit some level of dependence on $\beta$. Overall, the results from this physical model help characterize and explain local mechanisms triggered by wave interaction near the shoreline. Validating this model’s relevance warrants specific attention through comparisons with field data. As an initial validation attempt, field data extracted from the literature are compared to the physical model, showing promising agreement.