Nearshore sediment pathways and potential sediment budgets in embayed settings over a multi-annual timescale
Introduction
Embayed beaches constitute a large proportion of the world's rocky coastlines. Highly embayed beaches are often considered closed cells with the prominent headlands acting as barriers to littoral drift, such that sediment transport into and/or out of adjacent cells is insignificant. Consequently, there is a paucity of studies focusing on the longshore sediment exchange between embayed beaches separated by neighbouring rocky stretches of coast. Nevertheless, recent studies on sandy beaches show that important sediment transport pathways offshore and/or beyond the headlands may occur under particular conditions (Short, 2010; Aagaard, 2011; Goodwin et al., 2013; George et al., 2018; McCarroll et al., 2018; Vieira da Silva et al., 2018; King et al., 2019; Valiente et al., 2019a).
The physical coupling between the beach and the inner shelf is of major interest to coastal researchers, but key processes are still poorly resolved as this coupling is generally considered relatively limited. Recent observational studies of beach storm response and evolution emphasize that substantial transport occurs to large depths (George et al., 2018; Valiente et al., 2019b; McCarroll et al., 2019). Niedonora and Swift, (1981) showed that sediment may be permanently lost to the inner shelf during storms. Later, Wright, (1995) studied the surf zone processes connected to the inner shelf from a morphodynamic point of view, finding that part of the infragravity oscillations contributing to cross-shore transport and reaching the inner shelf area were originated in the surf zone. More recent studies of mega-rips and beach response to extreme storm events along embayed coastlines also point in this direction (Gallop et al., 2011; Loureiro et al., 2012; Castelle and Coco, 2013), revealing that a significant amount of sediment can be ejected outside the surf zone mainly due to the presence of mega-rips (Short, 2010; Castelle and Coco, 2013; McCarroll et al., 2016), and between adjacent beaches through headland bypassing (Duarte et al., 2014; McCarroll et al., 2018; Vieira da Silva et al., 2018). Short, (1985) defined mega-rips as large-scale erosional rips which can reach up to 1 km offshore and occur when the headland\embayment topography forces wave refraction and surf zone longshore gradients. Moreover, largest alongshore sediment fluxes occur during large oblique waves (McCarroll et al., 2019). Along rocky coastlines, sediment transport may be disrupted and/or altered by the regional topography (e.g., embayment and headland configuration) introducing complexity in the general transport patterns that needs to be better understood.
For embayed coastlines, bypassing and embayment-scale cellular circulation involving mega-rips (Castelle et al., 2016) are the main mechanisms responsible for sediment exchange between embayments and neighbouring areas. Based on observations, Gallop et al. (2011) and Loureiro et al. (2012a) linked morphological change with embayment-scale circulation (rip and mega-rip formation) on several embayments of varying size and orientation. Additionally, Castelle and Coco (2013) studied the role of embayment morphometry in governing ejection outside the surf zone using simulations of passive tracers. They showed that the surf zone of embayed beaches systematically flushes out more floating material than on open beaches, with most exits occurring through the headland rips, and provided retention rates (in percentage) for varying beach length and constant headland length. More recent studies were more focused on the driving forces for sediment bypass around natural headlands. Vieira da Silva et al. (2018) investigated the influence of wind and waves on headland bypass whereas McCarroll et al. (2018) studied the role of embayment-scale circulation modes inducing bypass.
Sand bypassing rates are often predicted using simple analytical solutions such as one-line models on straight shorelines (Ab Razak et al., 2013; Brown et al., 2016). More sophisticated approaches include 2D and 3D process-based numerical modelling that is able to simulate horizontal and vertical currents. McCarroll et al. (2018) first introduced a site-specific bypass parameter for a multi-year period based on modelled sand bypassing rates on a single headland. A recent study by George et al. (2019) examined the impact of idealised headlands of varying size and shape on rates of headland bypass and determined that longshore sediment fluxes around headlands are mainly determined by the degree of blockage. However, despite these later efforts, prediction of sediment bypass in embayed beaches of different geometry and complex circulation remains poorly resolved.
A quantitative understanding of sediment pathways in littoral cells is fundamental when investigating beach response and evolution along embayed coastlines (Komar, 1998; Rosati, 2006; Thom et al., 2018). For closed cells, coastal changes need only be attributed to a redistribution of the sediment within the embayment, but for open or leaky cells, sediment exchanges within a larger area, including neighbouring embayments, need to be considered. Balances and imbalances between incoming and outgoing sediment fluxes encompassing several embayments, even when open, can ultimately provide essential information on the major sediment transport pathways, as well as help to derive sediment budgets within the inter-connected cells. Valiente et al. (2019b) followed a total sediment budget approach based on morphological observations in an embayed beach in SW England concluding that, despite the deeply embayed nature of the beach, the system was open. The approach allowed quantification of sediment gains and losses; however, the understanding of the system was incomplete as it lacked information on the directional sediment fluxes.
The aim of this study is to determine sediment transport pathways between embayments on a high-energy coastline, providing order of magnitude estimates for potential bypassing rates between sediment compartments, across event to multi-annual timescales. Following the observational study conducted by Valiente et al. (2019b), variable local factors (wave exposure and sheltering, headland bypassing and embayment scale circulation) influencing the inter-annual sediment transport dynamics for 15 km of the macrotidal, exposed and embayed coastline of north Cornwall, SW England, are investigated using numerical simulations. A description of the study area is presented in Section 2. Section 3 describes the observational dataset used for calibration and validation purposes, model set up and selected modelled scenarios. Model results including major sediment pathways and fluxes are outlined in Section 4. Section 5.1 introduces the main mechanisms for redistributing material on the lower shoreface. Sediment exchange between the different embayments is reconstructed over multi-annual timescales and potential sediment budgets are provided in Section 5.2. Finally, conclusions are presented in Section 6.
Section snippets
Study area
The study encompasses a 15 km-long section of the macrotidal, exposed and embayed coastline of north Cornwall from Chapel Porth (Chapel) to Holywell (Holy) (Fig. 1). This stretch of coast includes five sandy beaches delineated by sharp headlands of diverse morphometric characteristic (Fig. 1e) that alternate with rocky sediment-free areas backed by cliffs 50–90 m high. The beaches are characterized by a wide low-gradient (mean bed slope ranging 0.018–0.021) sandy platform facing W with a slight
Waves, water levels and current observations
Waves, currents and water levels were measured using three 600 kHz RDI WorkHorse Monitor Acoustic Doppler Current Profilers (ADCPs) deployed for 2–3 months in summer 2016 (AN17 and AN25) and winter 2016/17 (AS20, AN20 and AN27) off the two headlands delineating Perranporth beach in 15–30 m depth relative to ODN (Fig. 1d). Two ADCPs located shore-normal to the apex of the northern headland (same transect, 475 m apart) were moored during summer and winter periods with an extra ADCP located
Calibration and validation
WAVE output over the validation period is compared with observations in Fig. 4. Visual inspection indicates that the model correctly reproduced the experienced wave conditions. The model satisfactorily replicated wave height (averaged model skill values for all locations were RMSE = 0.40 m, MAE = 0.29 m, R2 = 0.79 and bias = 0.05; Table 2) for both summer and winter validation periods. The peak period prediction was good (average model validation coefficients for all locations were RMSE
Sediment transport mechanisms
A conceptual model of headland bypassing and major sediment transport pathways for increasing wave forcing conditions (Hs = 0–2 m, 2–5 m, >5 m) along an idealised embayed coastline based on the study area in SW England with two types of embayment lengths is shown in Fig. 13. Major mechanisms for redistributing material to and along the lower shoreface for embayed coastlines are the longshore flow around headlands (northward for SW England), the presence of mega-rips, and the embayment-scale
Conclusions
This study presents a numerical modelling investigation of the processes redistributing material along the lower shoreface of a complex high-energy embayed coastline. Numerical simulations of wave- and tide-induced currents were used to model the main sediment transport pathways and potential sediment fluxes due to headland bypassing at event to multi-annual timescales. This study provides an order of magnitude analysis of sediment bypassing rates both around headlands and intra-embayment using
Acknowledgments
This work was supported by UK Natural Environment Research Council grant (NE/M004996/1; BLUE-coast project), and by EPSRC through an Overseas Travel Grant awarded to GM (Coastal modelling of extreme storms and sea-level rise; EP/T004304/1). Thank you to all those who assisted with the ADCP deployment, in particular Peter Ganderton and Aaron Barrett. Wave and bathymetric data were provided by the Plymouth Coastal Observatory (PCO) and United Kingdom Hydrographic Office (UKHO), respectively.
Declaration of Competing Interests
None.
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