Beach morphodynamic characteristics and classifications around Hainan Island, China

Climate change and anthropogenic activities have profoundly affected coastal systems, making geomorphological research a critical focus for coastal protection and sustainable development. In this study, a comprehensive classification of beach states around Hainan Island is conducted for the first time by utilizing the Ω-RTR model and geological control modes. Six distinct classic beach states ranging from dissipative to reflective are identified: barred dissipative beaches or no-barred dissipative beaches (BD or NBD), barred beaches (B), low-tide terrace or low-tide bar with rip (LTTR or LTBR), and reflective state (R). Among these, the BD and B types are predominant on Hainan Island. Notably, the beach states are subject to multiple factors, such as hydrodynamic forcings, geomorphic features and underlying substrates, and exhibit remarkable spatiotemporal variability. During extreme events, hydrodynamic forcings impact beach states more substantially than geological and geomorphic features do, leading to a more homogeneous distribution of beach states. Under normal circumstance, beach states are predominantly controlled by geological and geomorphic features. Coastal geological and geomorphic features have a pronounced influence on beach morphology and stability. For example, hard substrates underpin wide and stable dissipative beaches, whereas softer substrates lead to narrower, erosion-prone beaches. Three geological control modes are identified, namely, gently sloping hard substrates with dissipative beaches, moderately sloping hard substrates with seasonally variable reflective beaches, and steeply sloping soft substrates with dynamic sandbar-dominated beaches. These findings highlight the necessity of integrating geological settings in tandem with hydrodynamic forcings into coastal management practices. A dual- mode strategy is proposed: maintaining geomorphic self-organization on hard-substrate coasts under normal conditions and implementing hybrid engineering–ecological measures (e.g., artificial sand replenishment and vegetation restoration) on erosion-prone soft substrates.