Climate Change Likely to Intensify Storm-Driven Compound Flooding in an Exemplar UK Estuary

Flood protection authorities are not prepared for compound flood risk in estuaries—now and in the face of climate change. Climate projections are rarely downscaled appropriately to assess future changes in storm surge and concurrent river discharge extremes, and their interactions to exacerbate flooding. This is the first time that hourly and fine spatial resolution (7/2.2 km sea level/precipitation), physically consistent, climate projections are used to assess changes in storm surge and river discharge-driven compound events. The analysis, applied to the Dyfi estuary, western UK, uses 12 downscaled perturbed parameter ensembles for the high-emissions “RCP8.5” scenario from a global climate model (HadGEM3-GC3.0). Residual surge and river discharge projections are assessed independently to identify changes in magnitudes and return periods—then combined to identify changing patterns of dependence and timing of compound events. Under RCP8.5 scenario to 2080, river discharge is expected to increase by 28%–29% for 1/20 and 1/50-year events. Extreme (95th percentile) discharge events are more likely to occur concurrently with extreme surges, and compound events will occur more often, and with a shorter time lag between peak surge and peak discharge—potentially compounding flooding further. The analysis provided forcing conditions representative of future 1 in 20-year and 1 in 50-year events used to simulate a potential increased flood footprint in the estuary. The research raises the question of the wider pattern of future compound events throughout the UK, and worldwide, highlighting the critical need for downscaled, coastal and fluvial projections to futureproof flood management strategies.