Structural impacts under shared regional forcing in sandy beach systems

Sandy beaches are often assessed under the assumption that morphodynamic impact scales with the intensity of external forcing. This perspective underpins many geomorphological frameworks that prioritise wave and meteorological descriptors to infer erosion and recovery. However, beaches exposed to comparable regional forcing can exhibit contrasting morphodynamic responses. Here, it is shown that intrinsic system structure and connectivity, rather than forcing intensity alone, govern how morphodynamic impact is expressed at the interval scale. Repeated cross-shore topographic surveys from four sandy beaches along the Galician coast (NW Spain) are analysed. Morphodynamic response is characterised using three complementary metrics: net volumetric change (ΔV total ), the cross-shore footprint of adjustment (L90 change ), and alongshore coherence as an indicator of system-scale organisation. Results indicate that ΔV total and L90 change do not scale systematically, with intervals of similar sediment exchange associated with contrasting spatial footprints of adjustment. Alongshore coherence introduces a third, partially independent dimension of response, distinguishing between integrated and fragmented behaviour that is weakly related to forcing magnitude and strongly site-dependent. Forcing-response relationships are inconsistent for sediment budgets, whereas the spatial footprint of change exhibits a clearer regional signal that is constrained by intrinsic system characteristics. Rainfall acts as a regime-modulating factor that alters system state and spatial organisation without directly mobilising sediment. These findings demonstrate that coastal impacts are filtered by system structure and connectivity, allowing moderate forcing to generate persistent structural reorganisation in constrained settings. Incorporating spatial organisation and system state is therefore essential to move beyond forcing-centric interpretations of sandy beach morphodynamics.