Modeling formation and subsequent nonlinear evolution of rip channels: Time-varying versus time-invariant wave forcing

We use a nonlinear morphodynamic model to demonstrate that time-varying forcing, in particular the time-varying angle of wave incidence, is crucial to the development of rip channels in terms of rip channel morphology, nonlinear behavior, longshore migration, and mean rip spacing. The time-varying angle of incidence leads to different mean rip spacings than the time-integrated time-invariant forcing and to systematically less developed bar and rip morphologies at more alongshore variable scales. This supports the common field observation of irregular and random alongshore rip spacings, and contrasts with the regular spacing predicted by existing time-invariant template, and instability models. Time-varying wave incidence also generally results in the onset of splitting of shoals and an increase in merging of rip channels. In addition, a time-varying angle of incidence with zero mean can drive a significant net alongshore migration of the rip channels. Abrupt changes in wave conditions are responsible for this net longshore migration through cumulative effects of the mismatch between wave conditions and bar and rip morphology orientation.