Rip channel development on nonbarred beaches: The importance of a lag in suspended‐sediment transport

Abstract

On approximately planar beaches, rip channel development is often preceded by a period in which jet‐like rip currents develop in apparently random locations, and dissipate after minutes to tens of minutes. The subsequent development of sharp‐edged, trough‐like channels extending across the surf zone suggests that rip currents on planar beaches cause local erosion. Conversely, channels are known to cause localized offshore‐directed flow, and once channels have formed on approximately planar beaches, rip currents no longer occur in apparently random locations, but are restricted to the locations of the channels. Apparently, the excavation of channels by rip currents on planar beaches triggers a positive feedback between the morphological development and the flow. However, theoretical analysis indicates that, when depth increases with distance from shore, and sediment transport is treated as a function only of local flow conditions, channel development in the vicinity of a rip current may not occur. In numerical simulations (using a “cellular” model of nonbathymetrically driven rip currents) in which sediment flux on a planar beach is driven by approximately realistic rip current velocity patterns, deposition occurs under parts of the rip currents (especially in the seaward half of the surf zone), and these “rip ridges” cause a negative feedback. In these simulations, as in most models treating surf zone sediment transport, sediment flux is assumed to be strictly a function of local hydrodynamic conditions. However, Observations of sediment‐laden rip‐current plumes extending well beyond the surf zone suggest that suspended sediment transport is not always in equilibrium with local conditions. Other simulations employ a treatment of suspended‐sediment transport that allows for a lag in the adjustment of fluxes to the changes in local hydrodynamic conditions that the sediment is advected through. With this nonlocal sediment‐transport, the flux of sediment out of the surf zone in a rip current is increased, ridges do not develop under any part of the rip currents, and the morphodynamic feedback becomes positive.