A quantitative approach to classify the shape of beach profiles

Sandy beaches in estuaries and bays (BEBs) are common globally and their existence relies on the complex interactions of regional and local waves, limited sediment input, tidal dynamics, and human interventions. BEB morphodynamics are poorly defined with only a few studies attempting to qualitatively relate different BEB profile shapes with relevant physical drivers. Here, we propose a novel Profile Morphotype Index (Γ) to quantitatively classify the shape of beach profiles in BEBs into distinct morphotypes to allow an objective analysis of changes in beach morphodynamic states. For this study, beach profiles in three swash-aligned and one drift-aligned BEBs in Gamay–Botany Bay (NSW, Australia) were measured periodically between 2016 and 2023, resulting in a total of 774 beach profile surveys. Based on this dataset, we defined equations that compute the profile shape assigning it to a specific morphotype. Ten unique morphotypes form the new morphometric index (Γ) varying gradually between Concave (Γ = -1.000) and Convex (Γ = 1.000) profile shapes, which indicate more erosive and accretive beach states, respectively. Our results show that those BEBs closest to the estuary entrance favoured the Convex morphotype, while BEBs farthest from the entrance and under erosive conditions were Mostly Concave (Γ = -0.570 to -0.950) morphotypes. The morphometric index also captured changes in profile shape due to high-energy storm waves propagating into the estuary. This new morphotype index (Γ) provides an objective classification of BEB profile shape allowing us to better predict beach state response to changes in coastal processes and hydrodynamics. • A novel quantitative Profile Morphotype Index (Γ) was proposed to objectively classify BEB profiles into distinct morphotypes. • Ten distinct profile morphotypes, from concave to convex, were identified, representing net changes in BEBs upper beach. • Morphotype changes indicated shifts in the beach state, transitioning between erosive and accretive conditions and vice-versa, driven by natural events and human interventions.