QUANTIFYING TIDAL INLETS AS A SINK OF LITTORAL SEDIMENT

Coastal barrier islands, such as those found along the Gulf Coast and Mississippi Delta, are transgressive landforms that migrate landwards through two main mechanisms: punctuated storm-driven overwash fluxes and gradual tidal-inlet-driven flood delta deposition. If these landward-directed sediment fluxes are insufficient, sea-level rise can cause barrier islands to drown, with potentially disastrous consequences for ecology, fisheries, and coastal defense. Here, we propose and test a simple framework of tidal inlet migration that allows us to investigate the potential for inlets to act as long-term sinks of littoral sediment. We quantify this framework using idealized inlet simulations in the coupled hydrodynamic and morphodynamic model Delft3D-SWAN.

Investigating the pathways of sediment transport around migrating tidal inlets, we find that both the eroded downdrift barrier and the wave-driven littoral sediment flux can be a significant source of sediment for the flood tidal delta. Additionally, inlet migration opens up new accommodation space for flood tidal delta deposits. Inlet migration therefore is an efficient mechanism for the landward transport of barrier island sediments. With not only the littoral zone but also the eroded downdrift inlet bank as a sediment source, these migrating inlets can act as a net sink of up to 160% of the littoral sediment flux magnitude. As a sediment sink, inlets can cause significant erosion of the downdrift barrier, making this coast more susceptible to storm-driven overwash fluxes. The potentially interesting feedbacks between tidal inlets and storm-driven overwash highlight the need for coupled models to investigate the long-term morphologic evolution of barrier islands in the face of accelerating rates of sea-level rise.