Tidal inlet sediment bypassing at mixed-energy barrier islands

The large-scale littoral sediment drift along barrier island coasts involves alongshore sand transport at the islands' surf zone and across tidal inlets. Mechanisms of sediment bypassing at non-migrating, mixed-energy tidal inlets are discussed based on process-based numerical modeling of sediment dynamics at an exemplary barrier island system in the southern North Sea. Residual transport pathways of distinct sediment grain-size fractions and morphological changes reveal the complexity of processes at tidal inlets driven by the non-linear interaction of competing tidal and wave forces. Sediment bypassing from the updrift to the downdrift shoreline of a tidal inlet is evaluated and the ratio of bypassed to recirculated sand volumes is estimated. Common inlet bypassing schemes are discussed and verified. It is shown that the different bypassing mechanisms can happen simultaneously and are related to the transport behavior of particular sediment grain-size fractions: Very fine to fine sand bypasses the inlet along the entire ebb-tidal delta periphery by wave-induced transport processes. Fine to medium sand is transported by the ebb-directed tidal currents in the inlet throat to the ebb-tidal delta where wave-driven transport processes and bar welding bypasses the sediments to the downdrift shore. For fine to medium and medium sands, a sediment recirculation cell at the downdrift ebb-tidal delta and reversal to the main inlet throat is identified.