Modelling nearshore sediment fluxes in embayed settings over a multi-annual timescale

Predicting coastal system response and evolution requires an accurate delineation and understanding of coastal cell boundaries and sediment transport pathways. Recent studies along highly embayed sandy coastlines show that important sediment transport into and out of the embayments may occur under particular conditions; however, key processes (e.g., mega-rips, headland bypassing), driving forces, flux rates and local factors (e.g., headland/embayment morphometric parameters) influencing these sediment fluxes are still poorly resolved. Here, we investigate the nearshore sediment transport dynamics along a 15-km stretch of the embayed coastline of SW England using the process-based numerical model Delf3D. 

Numerical simulations (coupled wave and tide) are conducted to compute major circulation modes and sediment fluxes for a wide range of modal and extreme conditions. Based on the hindcast wave data, predictions of sediment fluxes over multi-annual timescales are then produced allowing for resolution of potential sediment budgets. 

Results indicate that extreme events (H_s  > 7 m) involve multi-embayment circulation and mega-rip formation (0.7 m s^-1 at > 20 m depth) in the down-wave sectors of the embayments with subsequent significant sediment flushed beyond the base of the headlands (c. 10⁴ m³ day^-1 cross-shore and 10³ m³ day^-1 bypassing). Accretionary phases over moderate-high swell periods (up to H_s = 4 m) are characterized by the presence of clockwise intra-embayment circulation with predicted currents (0.4 – 0.5 m s^-1 flow below 10 m depth) inducing a slow transport of sand from the updrift to the downdrift part of all the embayments (c. -10² – -10³ m³ day^-1). This circulation mode is combined with weaker bypassing rates around the shallower and wider headlands (10² – 10³m³ day^-1) that is partially conditioned by the direction of the waves.

 Our study suggests that major mechanisms for redistributing material to and along the lower shoreface (up to 25 m depth) for embayed coastlines are the longshore residual flow around headlands, the presence of mega-rips and the embayment-scale circulation, with the latter being a function of embayment length and headland configuration. Hindcasted yearly bypassing rates around the headlands are episodic, occur mainly during high-energy events and range between 10³ and 10⁵ m³ y^-1. Hence, the magnitude of this bypass suggests that lower shoreface sediment fluctuations should be considered a critical mechanism that will inevitably affect coastal evolution over longer temporal scales (> 10 years), specifically along high energy and sediment starved coastlines.