September 2008 Morpeth Flood addition - Performance of ensemble rainfall forecasts in relation to pluvial flooding impacts

Following the major floods in 2007, the UK Government commissioned a report (the Pitt Review) which formulated the actions needed to respond to this issue. A key recommendation of the report was the need for 'improved modelling of all forms of flooding'. Most attention so far has been paid to developing computer models of flooding from rivers (fluvial flooding), but a significant percentage of flooding is from rainfall events which lead to localised runoff (pluvial flooding). These flood events are difficult to predict as they are often have local high intensity rainfall (typically associated with convective storms) and runoff in urban areas through subsurface drains as well as overland. A new version of the Met. Office's operational weather forecast model has recently been developed with a high (1.5km) resolution that is capable of representing these local rainfall events more accurately, with the potential to be used to support better predictions of pluvial flooding. This proposal aims to use the outputs from the new model for the case study event of the 6th Sept. 2008 Morpeth flood that affected around 1,000 properties. This case study is particularly valuable, as it is one of the very few events for which the dynamics of the flooding have been measured in detail in a previous project from publicly-sourced information, which has allowed detailed reconstruction of the different sources of flooding for comparison against flood models. The quality of the detailed rainfall data from the 1.5km weather forecast model for the Sept. 2008 Morpeth flood event will be assessed through comparison with raingauge observations and other standard approaches currently used in the UK, including assessment of the uncertainty in the rainfall data. These data will then be fed into hydrological models of runoff from the localised catchment areas, and used to simulate the volumes of water flowing into the town, and how the floodwater reaches the different areas of the floodplain, allowing comparison against the observed flood levels for this event. This case study is the first to use the new weather forecast model rainfall predictions including uncertainty for flood modelling, and will help to support development of flood prediction modelling from pluvial sources across the UK.