Estuarine flooding is driven by extreme sea-levels and river discharge, either occurring independently or at the same time, or in close succession to exacerbate the hazard, known as compound events. Understanding compound flooding in the face of climate change is crucial for anticipating and mitigating heightened risks. Rising sea levels, increased storm intensity, and changing precipitation patterns can amplify the simultaneous occurrence of extreme storm surges and river flows. It is necessary to assess changing patterns of timing and intensity in extreme storm-driven compound events to inform future incident and hazard management strategies. Understanding whether these events will intensify or diminish is crucial for adapting and developing effective mitigation measures.

This research represents the first time that projections of future sea-level, storm surge, and river discharge to assess changes in the magnitude and timing of storm-driven compound events in an estuary particularly vulnerable to compound flooding (Dyfi, west Wales). Sub-daily projections of river discharge from a hydrological model and sea level and residual surge from a shelf sea model are assessed independently to identify changes in their magnitude and return periods. Projections are then assessed in combination to identify future extreme dependence and timing of compound events. The analysis provides forcing conditions representative of a 1 in 20-year and 1 in 50-year event to simulate the impacts of future return periods in the Dyfi Estuary.

The research shows that more extreme river discharge and storm surges will occur up to 2100, and the severity of a 1 in 1-year to a 1 in 5-year event will become more severe into the future. There is a stronger likelihood of an extreme river discharge occurring at the same time as an extreme skew surge in the future, more often per storm season, and with greater dependence. Further to this, as storm-driven compound events become more prevalent in the future, the associated flood impacts are anticipated extend over larger areas and occur with increased severity.

This research presents a scalable methodology for comprehensive assessment and analysis of the future likelihood and impacts of storm-driven compound events, that can be applied worldwide where sub-daily river and sea level projection forced by the same global climate model are available.