Long-term sea level rise modeling of a basin-tidal inlet system reveals sediment sinks

Abstract

Much of the world’s population lives close to coastlines and this proximity is becoming increasingly impactful because of sea-level rise (SLR). Barrier islands and backbarrier saltmarshes, which comprise >10% of these coasts, are particularly susceptible. To better understand this risk, we model backbarrier morphologic and hydrodynamic evolution over a 200-year period of SLR, incorporating an erodible bed and a range of grain sizes. Here, we show that reduction in intertidal area creates negative feedback, shifting transport of coarse sediment (silt and sand) through the inlet from net export to net import. Imposing a modest marsh vertical accretion rate decreases the period of silt and sand import to 40 years (years 90 to 130) before being exported again. Clay is continuously exported thereby decreasing inorganic deposition on marshes and threatening their sustainability. Simulated marsh loss increases tidal prism and the volume of sand contained in ebb deltas, depleting coastal sand resources.

Storm and tidal interactions control sediment exchange in mixed-energy coastal systems

“…Storm impacts on basin hydrodynamics, sediment transport, and exchange between morphologic elements are investigated with an idealized basin model so that relationships and storm responses can be more easily quantified and compared. The model hydrodynamic and morphology grids were adopted from a previous study ( 22 ) and consist of an elongated basin of ∼15 km by 5 km (with 50 m resolution) and a 30-km alongshore by 10-km cross-shore section of the nearshore, nested within a 60-km by 15-km coarse-resolution (500 m) wave grid (Fig. 1 ).…”

“…Although tidal hydrodynamics and asymmetries are heavily influenced by the long-term evolution of tidal inlet morphology and basin hypsometry ( 9 , 21 , 22 ), in a regime of accelerating SLR, infrequent impacts from intense storms can contribute residual fluxes into or out of the basin, thereby influencing the morphologic trajectory of the basin ( 20 , 21 , 23–26 ). For example, sand import and deposition on flood deltas during large storms are important processes that help maintain tidal flats and build backbarrier marshes ( 27 ).…”

“…c) High-resolution hydrodynamic and morphologic model grid, nested within coarse-resolution wave model grid with offshore wave boundaries. Initial bathymetry represents an approximately equilibrium condition reached after a 1.5-m amplitude semi-diurnal, sinusoidal tide and 0.25 m significant wave height, 6-s peak period wave imposed for 100 years of simulation time with a morphologic acceleration factor of 50 (model adopted from Hanegan et al ( 22 )).…”