Beach Cut in Relation to Surf Zone Morphodynamics

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Abstract
Field experiments on the coast of Southeastern Australia, supplemented by systematic observations in a variety of environments in other parts of the world, indicate at least three quasi-discrete modes of subaerial beach cut, each of which is related to a distinct set of mechanisms. For any given set of incident wave conditions, the operation or non-operation of a particular mode of cutting depends on the morphodynamic state of the surf-zone and beach. Steep, reflective, beaches are susceptible to cut under moderate swell conditions by accentuated runup and berm overtopping associated with subharmonic resonance. Appreciably more energy is required to cut flat dissipative beaches. Cut of dissipative beaches involves high setup which oscillates at infragravity frequency and allows the bores of broken waves to penetrate to the backshore. Beach states intermediate between the reflective and dissipative extremes are subject to cut by both the modes just described as well as by scour in the embayments of topographically arrested rips which can cause significant localized erosion even when the coast regionally is accreting. Beaches which most commonly exhibit intermediate topographies are the least stable and most mobile.

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