Towards new design rainfall profiles for the United Kingdom

The Flood Studies Report (FSR) summer and winter design profiles are a key component of rainfall design guidance in the United Kingdom (UK). We have examined the rainfall profiles of over 70,000 extreme rainfall events with the original FSR profile methodology. This analysis reveals that rainfall profiles change with rainstorm duration but not season, contradicting one of the key assumptions in current UK rainfall design guidance. By using a method that does not artificially generate symmetrical and centred profiles we show that profile shapes are highly variable and strongly related to event duration and magnitude. Short events tend towards front-loaded profiles, while heavy long-duration events tend towards centred profile shapes. Finally, manual, automatic and mixed methods of deriving new design profiles for use in the UK were trialled, with consistent results. These could be used to derive new design profiles to supersede the FSR profiles. Notably, peak intensities in observed profiles and trialled summary profiles often exceeded those found in both FSR profiles. We conclude that current design profile guidance for the UK fails to account for the observed variability in event profile shapes and peak intensities and may lead to significant under- or over-design of flood risk management solutions.