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Observations on a mildly sloping beach suggest that the largest runup events are related to bore‐bore capture (BBC). A numerical model based on the Reynolds‐averaged Navier‐Stokes equations is implemented to evaluate the effects that BBC have on runup. From simulations with realistic sea states, BBC is found to be a necessary but not sufficient condition for large runup generation. The dominant dynamics leading to BBC are amplitude dispersion and interactions with infragravity waves in the outer and inner surf zone, respectively. When the effects of BBC are isolated, it is found that the runup associated with the merging of two bores is at least 50% larger than that associated with the larger of the two waves in a monochromatic wave train. The phase difference of the incident waves with the infragravity wave can generate up to 30% variability of the runup maxima. The majority of the shoreward directed momentum flux, prior to runup initiation, is related to the interaction between the bores and the infragravity wave followed by that of the incident infragravity waves alone.

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