Multi-annual embayment sediment dynamics involving headland bypassing and sediment exchange across the depth of closure

Highlights

• Full sediment budget was produced along an exposed macrotidal embayment and linked to wave forcing.

• Inter- and sub-tidal volumes are not inversely correlated, indicating decoupling.

• Sediment budget suggests significant exchange around headlands and/or across depth of closure.

• Beaches on this coast type can be considered part of an extended coastal cell.

Abstract

Predicting the future behavior of beach and nearshore systems requires an accurate delineation and understanding of coastal cell boundaries, sediment transport pathways, and sediment sources and sinks. Unfortunately, there is a paucity of field datasets on beach and nearshore morphological change that extend fully from the top of the dunes to beyond the depth of closure to enable quantification of the sediment budget. Here, for the first time, we employ a total sediment budget approach, examining a sandy and embayed beach located in the north coast of SW England, to investigate inter- and multi-annual embayment scale sediment dynamics over a 10-year period that includes extreme storm erosion and post-storm recovery. We demonstrate that, despite the deeply embayed nature of the beach, the shoreline orientation roughly parallel to the dominant wave direction and the overwhelmingly cross-shore forcing of the inter-tidal beach volume, the system is neither closed, nor balanced. The very significant net changes in the recorded sediment volume from dune top to depth of closure (−14.5 m ODN), representing a loss of c. 100 m³ m⁻¹ during the extreme storm period and a gain of c. 200 m³ m⁻¹ during the recovery period, indicate that significant sediment transport occurs seaward of the base of the terminating headlands and beyond the morphological depth of closure. The results further indicate that the inter-tidal region is partly uncoupled from the sub-tidal region, with the former region dominated by cross-shore sediment fluxes, whereas the subtidal region is also significantly affected by longshore sediment fluxes. A conceptual model is presented that balances the observed volume changes with inferred fluxes, forced by variations in total and alongshore wave power. This study contradicts the general assumption that when sediment exits the inter-tidal, it rests undisturbed in the sub-tidal, waiting for a period of low-moderate energy to bring it onshore. The large sediment volumetric variations across the lower shoreface (depth of 5–20 m), which are of the same order of magnitude as, but uncorrelated with, those occurring in the inter-tidal region, are suggestive of an energetic longshore transport system across this deeper region. It is possible that this transport system extends along the whole north coast of SW England and this finding may lead to a shift in understanding of sediment budgets along exposed and macrotidal embayments globally.

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 (dQ_net) is expressed by the balance of volumes between sediment supply (ΣQ_source) and sediment losses (ΣQ_sink) in the compartment (Rosati, 2005; Aagaard, 2011). In both closed (Fig. 1-left panel) and balanced systems (Fig. 1-middle panel) dQ_net = 0; however, for unbalanced systems (Fig. 1-right panel), dQ_net ≠ 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, dQ_net > 0 and dQ_net < 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.