The sensitivity of urban surface water flood modelling to the temporal structure of rainfall

The risk posed globally by surface water flooding to people and properties is growing due to rapid urbanisation and the intensification of rainfall due to climate change. Whilst tools to model urban flood risk have also been rapidly developing, there remains a knowledge gap around the sensitivity of urban hydraulic modelling methods to the temporal structure of rainfall. In the UK, the industry standard process considers rainfall events to always be symmetrical, and with a singular peak in intensity. Previous studies of observed UK extreme rainfall events suggests that loading of rainfall towards the start or end of events is in fact more common. In this study, the sensitivity of an urban catchment in the north of England is tested using fifteen realistic rainfall profiles derived from these observed extremes. Additionally, idealized systematic variations are made to the industry standard profile to shift the single peak towards the start or end of the event, and to split the rainfall volume over multiple peaks. We demonstrate that the positioning of the peak, as well as its magnitude, influences the severity, timing and nature of the associated flooding. The profile with the peak nearest the end of the event is associated with an 18% larger flooded area than the early peaking profile which is associated with the smallest flooded area.