Geologically controlled sandy beaches: their geomorphology, morphodynamics and classification

Beaches that are geologically controlled by rock and coral formations are the rule, not the exception. This paper reviews 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 rocky/coral formations inducing strong alongshore gradients in wave energy, resulting in corresponding variations in morphodynamic beach state and storm response. Geologically-induced rip currents such as shadow rips and deflection rips, and even 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