Wave–current interactions within microtidal systems

This study presents the first structured and comprehensive analysis of wave–current interactions at microtidal river mouths, a topic often overlooked in estuarine flood studies. Using long-term observations and modeling from the Misa River (Italy), we reveal how opposing currents significantly alter incoming sea waves, through processes such as wave blocking, steepening, and infragravity wave upriver propagation, leading to increased flood risk and morphological changes. A key contribution of this work lies in the detailed physical analysis of wave–current interaction mechanisms, developed through a combined theoretical and observational approach. The superposition of opposing flows and wave fields modifies wave propagation, bottom boundary layer dynamics, sediment transport, and energy dissipation at river mouths. Observations at the Misa River from monitoring system showed a shift from frequent moderate floods to fewer but more intense events. This change triggered cyclical sediment dynamics and mouth bar reshaping, driven by alternating low-flow accumulation and flood-induced erosion. The upriver propagation of IGWs, typically linked to tidal forcing, was detected despite negligible tides, confirming the dominant role of wave–current interactions. These dynamics, also documented at other microtidal rivers (e.g., Rhône, Mississippi), govern key processes such as sediment transport, nearshore wave patterns, and compound flooding. When river floods coincide with high sea levels, due to storm surge or sea level rise, the extent of flooding can increase substantially, particularly in low-lying urban areas. We give evidence of such process through the results of a novel numerical analysis performed at the Misa River estuary. The paper is an elaboration of the keynote lecture given by the first Author on the same topic at the 38th International Conference on Coastal Engineering. • Wave–current interactions at microtidal river mouths significantly influence flooding and sediment dynamics. • Long-term observations reveal upriver propagation of infragravity waves even for small tidal forcing. • Opposing river currents lead to wave blocking, steepening, and enhanced bed shear, affecting mouth bar evolution. • A regime shift towards less frequent but more intense floods is observed in the Misa River, altering estuarine morphology. • When flood events coincide with high sea levels (e.g., storm surge or sea level rise), flood extent increases significantly, especially in low-lying urban areas.