Abstract
Inundation of barrier islands can cause severe morphological changes, from the break-up of islands to sediment accretion. The response will depend on island geometry and hydrodynamic forcing. To explore this dependence, the non-hydrostatic wave model SWASH was used to investigate the relative importance of bedload transport processes, such as transport by mean flow, short- (0.05–1 Hz) and infragravity (0.005–0.05 Hz) waves during barrier island inundation for different island configurations and hydrodynamic conditions. The boundary conditions for the model are based on field observations on a Dutch barrier island. Model results indicate that waves dominate the sediment transport processes from outer surfzone until landwards of the island crest, either by transporting sediment directly or by providing sediment stirring for the mean flow transport. Transport by short waves was continuously landwards directed, while infragravity wave and mean flow transport was seaward or landward directed. Landward of the crest, sediment transport was mostly dominated by the mean flow. It was forced by the water level gradient, which determined the mean flow transport direction and magnitude in the inner surfzone and on the island top. Simulations suggest that short wave and mean flow transport are generally larger on steeper slopes, since wave energy dissipation is less and mean flow velocities are higher. The slope of the island top and the width of the island foremost affect the mean flow transport, while variations in inundation depth will additionally affect transport by short-wave acceleration skewness.
Highlights
The response of barrier islands to overwash and inundation differs strongly between islands
The boundary conditions are primarily based on field observations collected in 2017 [7] to give insight into sediment transport processes during the inundation of barrier islands in the Wadden Sea region, and how these processes change with a change in forcing and island geometry
For an easier comparison of variations in slope steepness, all experiments will be presented on a “normalized” grid, xx, for which output locations are chosen to be at the same depth with respect to mean sea level (MSL)
Summary
The response of barrier islands to overwash (water overtops the beach- or dune crest without directly returning to the sea [1]) and inundation (the area is continuously submerged [2]) differs strongly between islands. It can include erosion such as shoreline retreat and breaching [1,3], the landward migration of islands, termed “roll-over” [1], and sediment accretion such as gains in subaerial areas [4]. The choice of a 1:120 seaward slope was made as an average lower bound for slope steepness on barrier islands in the Netherlands as it was measured at the field site [7,8], while the landward sloping island top was typical at the field location (Figure 3)
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