  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.
