Formation of shoreface-connected sand ridges: effects of rigid-lid approach, quasi-steady approach and wave-topography feedbacks

Abstract

The initial- and long-term evolution of shoreface-connected sand ridges (sfcr) is investigated with a new nonlinear model (MORFO56) that employs finite difference methods, rather than spectral methods. MORFO56 uses depth-averaged shallow water equations, including sediment transport and bed updating. Moreover, it includes full wave-topography feedbacks, wave shoaling and refraction, and wave radiation stresses. First, effects of relaxing the rigid-lid assumption and quasi-steadiness on the initial growth and migration of sfcr are quantified, by conducting a series of short-term runs. It turns out that assuming a free lid and unsteady flow results in larger wavelengths and larger migration speeds of sfcr. Furthermore, the new model is able to simulate the finite amplitude behaviour of sfcr for more realistic bottom slopes than earlier spectral models. Finally, the role of wave-topography feedbacks in the initial formation of sfcr is examined. Model simulations show that sfcr in the presence of these feedbacks are more trapped to the shoreface, with an offshore extent of approximately 1 km. Moreover, growth of sfcr is enhanced considerably by wave-topography feedbacks. The specific inclusion of radiation stresses does not affect this result.