The ability to model bar migration in the nearshore zone is critically important for predicting coastal morphology. We present a cost-effective wave-averaged 3D model (CROCO) with parameterization of wave-related bedload transport (adapted SANTOSS), and evaluate/calibrate its performance in comparison with data measured in a two-dimensional wave channel. In this model, suspended-load transport (both in and above the wave bottom boundary layer) is resolved by the flow model. We assume that if the tuning parameters are not robust to varying forcing conditions, the parameterization can be considered a failure. The present work is therefore a first step towards a numerical model capable of predicting the onshore and offshore migration of a sandbar under storm and post-storm conditions using the same set of parameters. One way to achieve this goal is to treat the effects of waves and currents in separate formulations to avoid conflicts and redundancy. We show that the implementation and adaptation of SANTOSS in CROCO can achieve this goal, provided that the hydrodynamics are accurately reproduced and the separation between the wave-related bedload and current-related suspended load formulations is respected.

The problem of sandbar migration on the storm timescale is revisited with a 3D wave-resolving hydro-sedimentary model. The latter presents an intermediate approach between expensive wave-resolving two-phase flow models and highly parametrized wave-averaged models. Innovative features include the use of weakly compressible assumptions in the hydrodynamics and morphological acceleration of bed changes to speed up numerical simulations. The model accurately simulates the successive offshore and onshore bar migration observed in a large-scale flume experiment in response to wave forcing representing storm and post-storm (recovery) conditions. The diagnosis of sand transport and the analysis of an ensemble-averaged asymmetric wave cycle reveal the migration mechanisms in each phase. In all cases, sediment resuspension is impacted by breaker-induced turbulence, while sediment transport and bed evolution are primarily the result of the undertow distribution – the breaker-induced seaward undercurrent – across the sandbar. There is also a significant contribution from asymmetric wave-related onshore fluxes, due to greater mobilization and currents during the wave crest period.