The morphodynamics of rip channels on embayed beaches

Abstract

We use a nonlinear morphodynamic model to examine the formation and nonlinear evolution of surfzone rip channels on embayed beaches. Starting from a range of embayed beach bathymetries characterized by different length and curvature, and under different time-invariant and time-varying wave conditions, the numerical model can reproduce the flow circulation and morphological characteristics observed on natural embayed beaches: (1) normal beach circulation, characterized by rips similar to non-embayed beaches and the presence of headland rips, (2) cellular circulation, with either headland rips only occurring at one or both ends of the embayment or a single rip at the centre of the beach and (3) transitional circulation, where both topography and currents influence rip location and behaviour. Time-invariant simulations show that, under oblique-wave forcing, rip spacing is systematically larger updrift than downdrift. Headland rips are preferably observed for straight beaches, with no clear dependence on wave angle. Wave shadowing and resulting alongshore gradients in wave height against the headland are the primary driving mechanism for headland rips. The formation of a single central rip is observed for short, curved embayed beaches, with no clear dependence on the wave angle as well. We use a novel non-dimensional embayment scaling parameter to quantify the degree of headland impact on beach circulation. Our simulations with shore-normal waves and initially alongshore-uniform embayed beaches show the parameter is consistent with observations. Our simulations also suggest that for high wave obliquity or time-varying wave angle to the shore, the influence of the headlands can progressively propagate into the whole domain. A time-varying wave angle results in persistent migration of rips towards the downcurrent headland rip, the splitting of shoals, an increase in merging of rip channels and more alongshore-variable rip spacing. The longshore variability of rip channel wavelength along embayed beaches is consistent with the hypothesis that rips are self-organized patterns and is consistent with recent field observations.