Predicting sandbar migration using a phase resolving 3D numerical model

Abstract:

Understanding the response of the beach profile and its shallow features (sandbars) in varying wave climates has important implications on a range of fields that span from coastal protection, tourism, military activities, to ecology. Sandbars provide coastal protection by causing the waves to break over the bar and thus dissipating the wave energy before reaching the shoreline. This reduction in wave energy results in a calmer wave climate near the coastline and helps to reduce coastal erosion. The ability to accurately model sandbar variation is essential in predicting shoreline erosion during storms, which has significant consequences for coastal communities. Successful modeling of sandbar behavior is challenging due to the complex three-dimensional hydrodynamics, which drives sediment transport and morphological changes.

It is understood that offshore sandbar migration is predominantly due to offshore-directed currents (undertow), while wave skewness and asymmetry favor shoreward bar migration. Most wave-averaged models perform unsatisfactorily in modeling onshore sandbar migrations due to the inability to reproduce the intra-wave phenomena correctly. A 3D wave resolving model can explicitly solve for the complex non-linear wave effects, such as velocity and acceleration skewness, and the vertical profile of undertow.

We successfully implemented sediment transport and morphology into the 3D wave resolving model SWASH. Sediment transport fluxes are calculated, taking acceleration and velocity skewness into account. The sediment transport of the model was validated using flume data, and it was able to reproduce the bedload and suspended load correctly. Using this new implementation, we can assess the ability to reproduce onshore and offshore sandbar migration observed in measurements. This work primarily focuses on data collected at the Field Research Facility in Duck, NC. The newly implemented sediment transport and morphology feature in SWASH provides a valuable tool that will be used to improve the parameterization of the velocity profile and wave-shape-related sediment transport in wave averaged models such as XBeach and COAWST.