Flood hazard amplification by intra-event sediment transport

Rivers are dynamic, with channel size and shape adapting to fluctuations in water and sediment supplied from their upstream catchments. These changes directly affect flood conveyance capacity, yet sediment transport processes are often overlooked in flood hazard prediction and management, where channels are treated essentially as static pipes through landscapes. Recent global floods show this assumption can be flawed, as extreme rainfall events can liberate and transport vast volumes of sediment, and in doing so potentially amplify flood hazard. Here we show, using a prototype catchment in the UK and rainfall data, including that derived from an extreme event associated with Storm Desmond in 2015, the critical role of intra-event sediment transport on flood inundation levels. Our analysis reveals a > 400% increase in flood inundation volumes compared to projections that exclude sediment transport processes. Extending these simulations to a range of storm scenarios, we find that both event duration and intensity can significantly influence sediment-driven flood amplification processes, with longer-duration floods of the same magnitude increasing inundation by up to 560%. These findings underscore the need to consider incorporating intra-event sediment fluxes into flood hazard assessments and that failing to address and integrate these processes could underestimate future risks under climate change.