Bore-bore capture and large runup events

Understanding the mechanisms that lead to the largest possible runup events for a given sea state has implications for beachgoer safety, dune erosion and coastal flooding. Much progress has been made in recent decades to quantify and predict bulk runup parameters such as the 2% runup exceedance level based on bulk environmental parameters. Much less emphasis has been given to the conditions that lead to individual extreme runup events. Optical observations (Argus) at Agate Beach, OR, suggest that the largest runup events are correlated to bore-bore capture (BBC). To further investigate, multiple numerical simulations for different sea states were performed using a phase-resolving model. To isolate the effect of BBC, multiple simulations with regular wave trains interrupted by a wave with a different wave height are performed. When BBC occurs, two or more bores combine and result in one single runup event. The runup associated with such a BBC event is much larger than the one expected from the individual waves involved. This runup increase can be explained in terms of onshore directed momentum flux, which increases non-linearly as the bores interact through BBC. In the presence of more realistic sea states, maximum runup occurs when BBC events coincide with the crest of the underlying infragravity waves. Under the evaluated conditions (mildly dissipative), the momentum flux due to the incident wave and infragravity wave interactions, including BBC, is larger than the one resulting from the infragravity waves. In addition, the physical mechanisms that control BBC are identified in order to achieve a predictive understanding of the largest runup events. When predicting the bore trajectories using an enhanced dispersion relation, amplitude dispersion is found to dominate from the breaker location to the mid-surfzone while bore interaction with infragravity waves dominates in the inner-surfzone.