Multi-annual embayment sediment dynamics involving headland bypassing and sediment exchange across the depth of closure
Graphical abstract
Introduction
Sandy beaches and coastal dunes have significant natural capital through representing efficient and natural coastal defenses that can protect the hinterland from coastal flooding. In a context of increasing winter-wave conditions (Castelle et al., 2018) and rate of sea-level rise (Church and White, 2011), it is important to understand how coasts respond and evolve as a result of changing boundary conditions, as this significantly affects continued human occupation of the coastal zone. Predicting coastal system behavior requires an accurate delineation and understanding of coastal cell boundaries, sediment sources and sinks, and transport pathways. The difficulties with identifying these key sediment-related factors, attributed to large uncertainties associated with sediment transport modelling and a paucity of high-quality field datasets extending from the top of the dunes to depths beyond the depth of closure (Aagaard, 2011; Coco et al., 2014), inhibit accurate quantification of sediment fluxes in a particular littoral cell. Moreover, long-term beach response is controlled by the sediment exchanges between the different beach sub-components (e.g., dunes, supratidal beach, inter-tidal zone, and sub-tidal zone), and these sub-components tend to operate over different time scales (Castelle et al., 2017).
A quantitative understanding of littoral cells and sediment budgets is a fundamental element of coastal sediment studies (Bowen and Inman, 1966; Caldwell, 1966; Komar, 1998; Rosati, 2005). Littoral cell (self-contained or semi-contained; refer to Fig. 1) and sediment budget concepts were introduced in the 1960s through several regional studies based upon coastal geology (rocky headlands) and estimates of longshore sand transport along specified sources and sinks (Bowen and Inman, 1966; Caldwell, 1966; Inman and Frautschy, 1966). Littoral cells are essentially defined as self-contained coastal units over a period of time, usually separated by prominent features (often headlands or jetties) that impede transfer of sediment (Kinsela et al., 2017). These cell boundaries delineate the spatial area within which the budget of sediment is known, providing the framework for the quantification of coastal erosion and accretion (CIRIA, 1996). Whether a littoral cell can be considered contained (Fig. 1-left panel) or semi-contained (Fig. 1-middle and right panels) depends on the timescale of consideration. Often, a compartment or coastal cell may appear closed, but over longer timescales during which long return period events inducing severe sediment transport are included, it may actually be open or semi-contained. Therefore, primary sediment compartments (self-contained/closed) are those that capture the limit in the sediment pathway within a large sediment-sharing area for long timescales (101–102 years); while sub-cells are usually finer in scale, identify semi-contained/open systems at timescales > 101 years and can appear closed in the short-term (<101 years) (Rosati, 2005; Kinsela et al., 2017; Thom et al., 2018).
Highly embayed beaches are often considered closed cells (Fig. 1-left panel) with the prominent headlands acting as barriers to littoral drift, such that sediment transport into and/or out of adjacent cells is insignificant. Nevertheless, recent studies show that significant sediment transport offshore and/or beyond these barriers exists under particular conditions, inducing headland bypassing (Short, 1985; Short and Masselink, 1999; Short, 2010; Cudaback et al., 2005; Loureiro et al., 2012; George et al., 2015; Vieira da Silva et al., 2017; McCarroll et al., 2018). Short (1985) suggested that major storm wave events are one of the key drivers of headland-attached bar bypassing, allowing sand to be transported to the morphological depth of closure (DoC) and beyond the headland position. Additionally, recent studies of mega-rips and beach response to extreme storm events also reveal important cross-embayment exchanges across the shoreface to deeper water (Short, 2010; Loureiro et al., 2012; McCarroll et al., 2018) and between adjacent beaches (Cudaback et al., 2005; Vieira da Silva et al., 2017). Furthermore, new research also emphasizes the influence of the strong tidal currents registered around headlands in facilitating bypassing at macrotidal environments (McCarroll et al., 2018; King et al., 2019; Valiente et al., 2019). All these studies demonstrate that under certain conditions of wave-tidal current interaction, important sediment transport paths occur at depths that well exceed the depth of the base of headlands, challenging the notion of embayments as closed coastal cells and highlighting limitations to the littoral cell and the depth of closure, critical concepts for long-term coastal evolution studies (e.g., application of the Bruun rule) and shoreline modelling (e.g., one-line models).
A total sediment budget approach to a coastal cell enables derivation of incoming and outgoing sediment fluxes from the rate of sediment volume change within the cell. A significant research gap exists in quantification of sediment budgets, in that many studies examine parts of the budget (e.g., the inter-tidal), while extremely few studies capture the entire system. This information helps with confirming the status of a closed cell and estimating the long-term coastal evolution (Wiggins et al., 2018). For a given coastal cell, the sediment budget (dQnet) is expressed by the balance of volumes between sediment supply (ΣQsource) and sediment losses (ΣQsink) in the compartment (Rosati, 2005; Aagaard, 2011). In both closed (Fig. 1-left panel) and balanced systems (Fig. 1-middle panel) dQnet = 0; however, for unbalanced systems (Fig. 1-right panel), dQnet ≠ 0 and in this case the volume of incoming sediment is not the same as the volume that exits the system. For prograding shorefaces and retrograding shorefaces, dQnet > 0 and dQnet < 0, respectively. Sediment sources include longshore transport of sediment into the area, cross-shore supply of sediment from offshore (beyond the cell seaward limit), anthropogenic interference (beach nourishment), in-situ production of sediment (Kinsela et al., 2017) and supply from autochthonous sources, such as rivers and dune and cliff erosion (Aagaard, 2011). Sediment losses from the upper shoreface can be accomplished through longshore and cross-shore processes. Sediment can leave embayments through headland bypassing, onshore aeolian transport beyond the coastal dune region (e.g., into a back-barrier lagoon) and offshore exchange from the upper shoreface to larger depths, i.e., beyond the depth of closure from where sediment may not be transported back onshore.
Most of current coastal research based on observations lack rigorous uncertainty calculation, potentially identifying measurement artefacts as real morphological changes and consequently, misrepresenting sediment fluxes. For a robust quantification of cross-shore and longshore sediment fluxes within coastal cells, is important to distinguish real changes from noise (Lane et al., 1994; Milne and Sear, 1997; Lane, 1998; Brasington et al., 2000; Lane et al., 2003; Wheaton et al., 2010; Wiggins et al., 2018; Guisado-Pintado and Jackson, 2018). Sandy coastlines commonly exhibit vertical morphological fluctuations of similar magnitude to the uncertainty associated with the measurement. In order to account for this uncertainty, but retain information on real morphological change, effective spatially-variable uncertainty computation techniques are required (Brasington et al., 2003; Lane et al., 2003; Wheaton et al., 2010).
In this study, we apply a total sediment budget approach based on field observations and spatially-variable uncertainty analysis. We evaluate the inter-annual dynamics of Perranporth beach, a sandy, exposed and embayed coastal system located on the north coast of SW England, over multi-annual time scales. Recent model-based studies investigated the potential for headland bypassing and offshore shoreface limits for significant sediment transport across Perranporth (McCarroll et al., 2018; Valiente et al., 2019). These indicated that the sub-tidal zone is potentially as dynamic as the rest of the beach system, and that, despite the cross-shore dominated nature of this type of embayment, alongshore processes and sediment fluxes may play an important role in the sediment balance of the system. Hence, we examine: (1) inter-annual morphological evolution of the inner embayment, including cross-shore and longshore sediment exchanges between sub-systems; (2) multi-annual full embayment morphological response to the 2013/14 winter, which represents the most energetic period along most of the European Atlantic coast since at least 1948 (Masselink et al., 2016b), using a total sediment budget approach; (3) relationship between wave forcing and embayment response; and (4) the nature of Perranporth's coastal cell (closed or open).
A description of the study area together with the methodology applied to estimate the total sediment budget is presented in Section 2. A comprehensive analysis of quasi-full embayment beach morphology (inter-annual records of dune, inter-tidal and sub-tidal regions) is presented in Section 3. This analysis is extended spatially (for multi-annual epochs) to the full embayment (coastal cell) by including observations offshore (up to −40 m Ordnance Data Newlyn, ODN) and beyond the bounding headlands for the years 2011, 2016, 2017 and 2018 in Section 4. Links between wave forcing and embayment morphological change are presented in Section 5. Section 6 presents discussion with a conceptual sediment budget model. Finally, conclusions are presented in Section 7.
Section snippets
Perranporth and Penhale Sands beach
Perran and Penhale Sands beach (hereafter noted as Perranporth beach) is a sandy, exposed, dissipative and macrotidal embayment located on the north coast of Cornwall, SW England (Fig. 2a). The configuration of the beach is typical of this coastline (Burvingt et al., 2018), which is characterized by sandy beaches embayed by sharp headlands (Fig. 2b). The site represents a 3.5-km long wide sandy beach facing 290° at the south and 280° at the north, backed by an extensive and high dune system
Quasi full embayment beach response and evolution (volume time series)
Fig. 5 shows beach volumetric time series for each of the sub-systems considered (dunes, inter-tidal and sub-tidal) for the north and south sectors of the beach (red and blue boxes, Fig. 2a). Sediment volumes are plotted relative to the reference state, January 2011, as a topographic and bathymetric survey is available for that time for both north and south sectors of the beach. The beach/dune morphology is significantly different for the two sectors: the inter-tidal beach in the north is
Full embayment total sediment budget
In this section we will present the full embayment analysis for two epochs, representing extreme storm response (Fig. 6) and post-storm recovery (Fig. 7). The results for both epochs are then summarized in Fig. 8 and Table 2. It is noted that in the figures the sediment volume changes are presented in units of m3 per unit meter beach width, whereas in the table the total volume changes in m3 are listed.
Relating wave forcing and morphological change
To determine the sediment budget for any coastal domain, it is necessary to understand the forcing controls on sediment fluxes within, and in and out of the system, with waves being the primary forcing control in this instance. In the study area, the wave climate is strongly seasonal (Fig. 3f), such that the larger waves over winter periods are also slightly more northward in direction. Therefore, winters are associated with greater absolute wave power (forcing offshore transport), but also
Sediment budget conceptual model
This study has demonstrated that, with reference to Fig. 1, Perranporth is an open system, that does not have a balanced sediment budget at the short to medium temporal scale (up to 10 years), and displays multi-annual accretional or erosional trends (Fig. 5e). Computed DoDs based on full embayment observations show significant morphological change in front of the headland bases and beyond the DoC in some sectors (Fig. 8, Fig. 9). The alongshore continuity of the DoC contour line off the
Conclusions
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A total sediment budget approach was implemented across the macrotidal, high energy Perranporth embayment for the period 2011–2018, using a multi-method surveying approach and accounting for measurement uncertainties.
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Inter-tidal volumetric changes indicate a longshore coherent, cross-shore dominant behavior, following a seasonal cycle superimposed by a multi-annual oscillation induced by extremely energetic winter seasons, with full recovery taking at least 5 years.
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Total embayment (combined
Acknowledgments
This work was supported by UK Natural Environment Research Council grant (NE/M004996/1; BLUE-coast project). This study is only possible thanks to the efforts of the members of the Coastal Process Research Group who have been, and still are, collecting observations at Perranporth. Special thanks to Aaron Barret, Peter Ganderton, Richard Kenyon, Oliver Bilson, Olivier Burvingt and Erin King for supporting the many field efforts during the last years. The Channel Coastal Observatory is project
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