Wave-driven tidal inlet migration: mechanics and effects on barrier morphology

Littoral sediment transport on barrier island coasts can cause tidal inlets to migrate alongshore up to hundreds of meters per year and pose significant hazards and challenges to coastal communities, infrastructure, and ecosystems. Surprisingly little is known about either the mechanisms or the expected rates of tidal inlet migration. Here we propose and test a simple framework of inlet migration that allows us to investigate the movement of sediment around tidal inlets and predict the corresponding migration rates. We test this framework using a combination of observed migration rates and idealized inlet simulations from the coupled hydrodynamic and morphodynamic model Delft3D-SWAN. In the Delft3D experiments, the tidal inlets quickly reach a dynamic state where the inlet cross-sectional area, the tidal prism and the migration rate, all of which are emergent characteristics of the experiment, remain constant through time. Tracking the sources of sediments deposited around the tidal inlet, we find that the eroded downdrift barrier is a significant source of sediment to both the flood tidal delta and the newly constructed barrier updrift of the inlet. The alongshore sediment bypassing volumes and pathways affecting inlet migration depend strongly on wave and tidal conditions. Furthermore, we find that migrating flood tidal deltas can act as a net sink of up to 80% of the littoral sediment flux. This sink reduces alongshore sediment bypassing of tidal inlets and thins the barrier downdrift. These modeled tidal inlets can therefore act as a migrating "buzzsaw" across barrier coasts that leave a zone of flood tidal delta deposits in their wake, an efficacious sediment mover that contributes significantly to the long-term landward migration of barrier islands with or without sea level rise.