Unraveling the Mechanisms That Cause Cyclic Channel-Shoal Dynamics of Ebb-Tidal Deltas: A Numerical Modeling Study

The morphodynamics of many ebb-tidal deltas include cyclic patterns of channel deflection and breaching associated with sandy shoals migrating and attaching to the downdrift coast. These channel-shoal dynamics result from the complex combination of tides and waves; however, their precise role and the physical mechanisms forcing the cyclic patterns are unknown. Here, we use Delft3D/SWAN to simulate their periodic behavior and to study the underlying mechanisms during different phases of the cyclic behavior. The model forcing was such that different ratios between tidal prism and littoral drift were obtained. Two different types of cyclic behavior were modeled, namely, ebb tidal delta breaching and outer delta breaching. The typical time scale of modeled cyclic behavior generally increases with increasing tidal prism and decreasing littoral drift. No cyclic channel-shoal dynamics were found for high-ratio tidal prims/littoral drift. Classically, channel deflection and shoal growth have been attributed to waves and the breaching solely to tidal currents. We confirm that shoal formation and channel deflection are forced by wave-induced currents and sediment concentrations. However, the subsequent shoal growth can only be explained by tides and waves operating simultaneously. Moreover, the breaching of ebb tidal deltas is not a tide-only effect because waves entrain the sediment that is transported by the ebb flow. In fact, there exists an optimum wave height; that is, larger waves cause further channel rotation rather than channel breaching. Furthermore, temporal variations in the sediment exchange between the back-barrier basin and the sea are found to be an inherent feature of the modeled ebb tidal delta breaching.