Nearshore sediment pathways and potential sediment budgets in embayed settings over a multi-annual timescale

Abstract

Embayed beaches constitute a large proportion of the world's rocky coastlines, but there is a paucity of studies focusing on the longshore sediment exchange between embayed beaches separated by rocky stretches of coast. Here, we investigate the nearshore sediment transport dynamics along a 15-km stretch of the embayed coastline of north Cornwall, SW England, using Delft3D. Numerical simulations (coupled wave and tide) are conducted to compute major circulation modes and sediment fluxes, including order of magnitude for sediment bypassing between bays, for a wide range of modal and extreme wave conditions. Results indicate that extreme events cause (i) multi-embayment circulation and mega-rip formation where an alongshore current is deflected offshore (0.7 m s−1 at >20 m depth) in the down-wave sectors, (ii) large bypassing rates (103–104 m3 day−1 bypassing) and (iii) exchanges extending to depths that exceed the base of the headlands. Accretionary phases over moderate-high swell periods were associated with clockwise intra-embayment circulation with simulated currents inducing redistribution towards the south in the long embayments (>103 m3 day−1 longshore). This circulation mode is combined with significant bypassing rates around the shallower and wider headlands (102–103 m3 day−1). Predictions of sediment fluxes along the lower shoreface are based on the correlation between the modelled sediment fluxes and offshore wave-conditions (r > 0.92), providing insights into the potential sediment budget over multi-annual timescales. Hindcasted yearly bypassing rates around the headlands range between 103 and 105 m3 y−1 under an assumption of unlimited sediment supply, with the bulk of the transport occurring mainly during high energy events. Hence, the magnitude of the computed potential sediment fluxes presented here has implications for coastal evolution at longer timescales (>10 years), especially along sediment-starved coastlines.