Review
Geologically controlled sandy beaches: Their geomorphology, morphodynamics and classification

https://doi.org/10.1016/j.scitotenv.2020.139123Get rights and content

Highlights

  • Beaches geologically controlled by rock and coral formations are common globally.

  • We review the state of knowledge of geological control of sandy beaches.

  • There was no encompassing classification system for these beaches.

  • We present longshore and cross-shore models from low to high geological control.

  • There is poor applicability of models for management of this common beach type.

Abstract

Beaches that are geologically controlled by rock and coral formations are the rule, not the exception. This paper reviews the current understanding of geologically controlled beaches, bringing together a range of terminologies (including embayed beaches, shore platform beaches, relict beaches, and perched beaches, among others) and processes, with the aim of exploring the multiple ways in which geology influences beach morphology and morphodynamics. We show how in addition to sediment supply, the basement geology influences where beaches will form by providing accommodation, and in the cross-shore, aspects of rock platform morphology such as elevation and slope are also important. Geologically controlled beaches can have significant variations in sediment coverage with seasons and storms, and geological controls have fundamental influences on their contemporary morphodynamics. This includes wave shadowing by headlands and rock/coral formations inducing strong alongshore gradients in wave energy, resulting in corresponding variations in morphodynamic beach state and storm response. Geologically-induced rip currents including shadow rips, deflection rips and mega-rips that can develop on embayed beaches during storms, are an integral feature of the nearshore circulation and morphodynamics of geologically controlled beaches. We bring these processes together by presenting a conceptual model of alongshore and cross-shore levels of geological control. In the longshore dimension, this ranges from beaches that are slightly embayed, through to highly embayed beaches where headlands dominate the entire beach morphodynamic response. In the cross-shore dimension, this ranges from beaches without discernible geological controls, through to relict beaches above the influence of the contemporary littoral zone. Given the prevalence of geologically controlled beaches along the world's coasts, it is paramount for coastal management to consider how these beaches differ from unconstrained beaches and avoid applying inappropriate models and tools, especially with our uncertain future climate.

Introduction

Strong feedback loops exist within sandy beach systems, where a change in a single driver such as wave period and height, or sediment size, may result in an adjustment to beach form, whose interaction was termed morphodynamics by Wright and Thom (1977) and synthetized by Wright and Short (1984) for sandy beach environments. Most research on beach morphodynamics focuses on cross-shore and alongshore sediment exchange that is assumed to be unconstrained by geology or other hard substrates (Cowell and Thom, 1994; Short and Jackson, 2013; Feal-Pérez et al., 2014; Trenhaile, 2018). Classic examples include the beach change frameworks developed for single, double (Wright and Short, 1984; Wright et al., 1985) and multi-barred (Short and Aagaard, 1993) wave-dominated beaches, and the model of Masselink and Short (1993) that accounts for tidal range using the Relative Tidal Range (RTR) parameter. In these models, the surf zone and beach morphology is essentially a function of grain size, wave and tide hydrodynamics, conveniently described through the surf scaling parameter, Dean's parameter and RTR (Jackson et al., 2005; Jackson and Cooper, 2009). However, many beaches have significant geological controls due to headlands, reefs, platforms, rock outcrops and islets (Short, 2006), which determine beach boundaries, beach morphology, morphodynamics and long-term evolution (Jackson et al., 2005; Gómez-Pujol et al., 2007; Short, 2010). An increasing number of studies show that beaches with geological controls have distinctly different behaviour compared to unconstrained beaches (González et al., 1999; Muñoz-Pérez et al., 1999; Jackson et al., 2005; Jackson and Cooper, 2009; Loureiro et al., 2013; Gallop et al., 2011b, Gallop et al., 2012, Gallop et al., 2013, Gallop et al., 2015a; Trenhaile, 2016), which causes significant complications for coastal managers as traditional erosional models are generally not directly applicable in such settings. However, geologically controlled beaches are still largely not classified as a distinct type, there is still a fundamental lack of data of their behaviour, and there is no commonly-accepted terminology and classification system of their morphology.

Thus, the aims of this critical review are to first understand our current state of knowledge on how geological control affects sandy beach morphology and morphodynamics, and second to identify key research needs and management implications of these understudied, globally distributed coastal systems. In Section 2 we review the terminology used for geologically controlled beach systems. Section 3 focuses on the morphodynamics of sandy geologically controlled beaches, starting with conditions necessary for beach accumulation in terms of the underlying geological surface morphology (Section 3.1), followed by a discussion of the sometimes stark temporal variations in sediment coverage that can occur in these systems (Section 3.2). This is followed by a synthesis of how geological controls can reduce beach wave exposure and filter wave energy, increasing the dominance of infragravity waves (Section 3.3). We then discuss the range of geologically controlled rip currents in Section 3.4, followed by a summary of beach rotation in Section 3.5. Section 4 presents conceptual models of geological control in the longshore (based on existing models) and cross-shore directions (a new model developed in this review). We discuss the management of these beaches in Section 5 followed by conclusions in Section 6.

Section snippets

Defining geologically controlled beaches

Various terms have been applied in the geomorphological and engineering domains to describe geologically controlled beaches and their morphology (Table 1, Fig. 1). The terms geologically controlled and geologically constrained have been used interchangeably, both to describe beaches with alongshore geological controls (Short, 2006, Short, 2010) and/or where there is a geologically-influenced cross-shore beach profile (Jackson and Cooper, 2009; Muñoz-Pérez and Medina, 2010). In particular,

Beach accumulation on shore platforms

Beaches that develop by sand accumulation on shore platforms are probably the most well-studied form of cross-shore geologically controlled beach (Trenhaile, 2016). On shore platform beaches, a rocky surface occupies at least part of the intertidal zone. The degree to which sediment can accumulate, and therefore the level of beach profile development, is a product of the elevation of the platform and its slope (Trenhaile, 2004; Kennedy and Milkins, 2015). Trenhaile (2004) modelled the

Models of geological control

Beach-state classifications and conceptual models provide a framework for understanding the beach environment through distinguishing beaches by the morphology of the depositional landforms and coupled morphodynamic processes (Wright and Short, 1984; Wright et al., 1985). In the sections below, we consider existing models and classifications for beaches with longshore and cross-shore geological control of beach morphodynamics, and build on these to systematise new conceptual models for

Management of geologically controlled beaches

In addition to the many services provided by beaches themselves, these coastal systems also provide an important form of natural protection from the impacts of waves and sea level rise to coastal communities, infrastructure and habitats which lie behind. As shown by the review above, our knowledge of geologically controlled beach morphodynamics and therefore how to manage them, is limited. There are few case studies on the management of beach sediment erosion on geologically controlled beaches.

Conclusions

Geologically controlled beaches are a distinct beach type, and have their own unique morphodynamic processes that make them behave differently to unconstrained beaches. This review brings together the various naming conventions and studies of what geologically controlled beaches are, and focused on the morphology and morphodynamics of those composed of sand. In addition to sediment supply, key factors that determine where geologically controlled beaches form are determined by basement geology,

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

LAN appreciates the support of Prof. Viles for her doctoral research on ephemeral beaches in Wales that is presented in this paper. SLG's contribution to this project received funding from the Australian Research Council (ARC) Discovery Project DP160102561. CL's contribution is developed in the framework of H2020 MSCA NEARControl project, which received funding from the European Commission under grant agreement no. 661342.

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