Multi-site assessment of microtidal wave-dominated active beach state and morpho-sedimentary parameters using optical satellite imagery

Abstract

Abstract Traditionally, beach states are defined from visual observations, in-situ measurements and/or video imagery, which limits their application to a handful of well-instrumented sites. In this work, we propose a different approach by focusing on a remotely observable quantity: the cross-shore distance between the offshore wave-breaking and the shoreline position, denoted $$X_b$$ . This metric defines the active beach state, capturing where waves dissipate energy relative to the underlying morphology. Using 10 years of Sentinel-2 imagery, $$X_b$$ is evaluated across 30 wave-dominated microtidal sandy beaches spanning reflective to fully dissipative conditions. The metric reproduces the structure of classical beach state frameworks and enables classification into five active states (R, LTT, TBR/RBB, LBT, and D) using transferable thresholds. $$X_b$$ is continuous, thus it also reveals how beach state evolve through time, allowing quantification of state occurrence, residence time, and transitions, with seasonal variability consistent with independent classifications at well-studied sites. Furthermore, using empirical relationships, we demonstrate that $$X_b$$ carries first-order information about beach-face slope ( $$\tan \beta$$ ) and sediment grain size ( $$D_{50}$$ ), opening a pathway toward systematic satellite-based monitoring of coastal morphodynamics at regional to global scales.