The Sensitivity of Urban Pluvial Flooding to the Temporal Distribution of Rainfall Within Design Storms

The risk posed globally by pluvial flooding to people and properties is growing due to urbanisation, infrastructure development and intensification of rainfall due to climate change. Whilst tools to model pluvial flood hazard have also advanced, there remains a knowledge gap around whether design storms used in modelling adequately represent the temporal distribution of rainfall within the extreme convective storms which drive flooding. In the UK, the industry standard design storm considers rainfall events to always have a singular, central intensity peak. Study of UK extreme rainfall observations suggests that loading of rainfall towards the start or end of events is in fact more common. This study highlights the sensitivity of pluvial flood extent, hazard and timing to the shape of the design rainfall profile for two urban catchments in northern England. We demonstrate that for events with the same accumulated rainfall depth, there is up to a 25% increase in total flood-affected area with a back-loaded compared to a front-loaded profile. Failing to account for the variability in event profile shapes observed in real events may result in substantial inaccuracies in the design of flood risk management solutions, leading to both underestimation and overestimation of the required measures.