Quantifying Bore-bore Capture on Natural Beaches

Abstract

Bore-bore capture occurs when a faster moving bore captures a slower moving bore whilst both are propagating shoreward in the surf or swash zones. This phenomenon occurs frequently on natural beaches, but has not yet been quantified in the literature. Novel application of wave tracking methods allowed for investigation of this phenomenon at seven sandy, micro-tidal, wave-dominated Australian beaches. The results showed that, for the locations where beach slope and environmental conditions allowed for bore-bore capture to occur, there was a high probability 40% of one bore capturing another bore in the surf or swash zones. The landward-most 10% of the nearshore region (i.e., the time-varying surf-swash extent) was found to be the most likely location for a bore-bore capture event. Amplitude and frequency dispersion and the interaction between bores and infragravity waves are (indirectly) shown to to be equally important for driving bore-bore capture. Capture events infrequently led to extreme horizontal shoreline maxima (approximately 20% of the cases), however, most extreme shoreline maxima were directly driven by bore-bore capture events >97% of the cases). For the analysed data, there was a direct relation between the probability of bore-bore capture driving extreme shoreline maxima and the Iribarren Number and beach morphodynamic state: the steeper or more reflective the beach, the higher the probability of a bore-bore capture event causing an extreme shoreline maxima event. Such correlation has direct importance for the future development of predictive runup models, which currently do not account for this phenomenon.