Investigations on Bore Capture Modulation on a Macrotidal Beach

The ability to empirically and accurately predict runup on natural beaches is made difficult by the random nature of waves and extreme runup events. In some cases, extreme runup events are the result of bore capture where one broken wave passes over the front of and merges with another broken wave or shoreline capture where capture occurs at the instantaneous shoreline. Here we use high resolution Lidar data to investigate potential drivers of bore and shoreline capture on a macro-tidal dissipative beach. The proportion of runup events that are derived from capture(s) was identified within normalized runup elevation percentiles which increased from 15% in the lowest tenth percentile of runup elevations to 55% of runup events in the highest tenth percentile. Bore capture was found to occur primarily on the rising infragravity wave in both space and time, whereas shoreline capture occurred predominately during the rising and peak phases of the infragravity wave. The occurrence of bore capture was not, however, fully restricted to particular infragravity phases, suggesting multiple drivers of capture. Bore trajectories of pairs of captured bores and non-captured bores were tracked showing that the probability of capture is also a function of normalized interwave proximity, the ratio of depths beneath consecutive wave crests, and normalized proximity to the mean shoreline. More dissipative beaches therefore not only have more infragravity energy within the surf and swash zones (thus infragravity modulation of bore capture), the wider surf zones provides greater time for bores to capture preceding bores.