Quantifying Bore‐Bore Capture on Natural Beaches

Abstract

AbstractBore‐bore capture occurs when a faster moving bore captures a slower moving bore or the shoreline while both are propagating shoreward in the surf or swash zones and is quantified here for the first time using wave tracking methods applied to data collected at seven sandy Australian beaches. The results showed that, for the locations where bore‐bore capture occurred, there was a high probability (≈40%) of bore‐bore capture occurring in the surf or swash zones. Of these ≈45% consisted of a bore capturing the instantaneous shoreline (i.e., they occurred in the swash zone). 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 capture event. Infragravity wave energy (Eig) at the shoreline, number of bore‐bore capture events, and beach morphodynamic state are shown to be correlated: more dissipative beaches had higher levels of Eig at the shoreline which directly correlated with higher rates of bore‐bore capture. Approximately 20% of the capture events led to extreme horizontal shoreline maxima; however, >97% of extreme shoreline maxima were directly driven by bore‐bore capture events. There was a clear relation between the probability of bore‐bore capture driving an extreme shoreline maxima and beach morphodynamic state: The more dissipative the beach, the lower 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 do not currently account for this phenomenon.