Future Change in Urban Flooding Using New Convection-Permitting Climate Projections

Rainfall intensity in the United Kingdom is projected to increase under climate change with significant implications for rainfall-driven (combined pluvial and fluvial) flooding. In the UK, the current recommended best practice for estimating changes in pluvial flood hazard under climate change involves applying a simple percentage uplift to spatially uniform catchment rainfall, despite the known importance of the spatial and temporal characteristics of rainfall in the generation of pluvial floods. The UKCP Local Convective Permitting Model (CPM) has for the first time provided the capacity to assess changes in flood hazard using hourly, 2.2 km CPM precipitation data that varies in space and time. Here, we use an event set of similar to 13,500 precipitation events across the three UKCP Local epochs (1981-2000, 2021-2040, and 2061-2080) to simulate rainfall-driven flooding using the LISFLOOD-FP hydrodynamic model at 20 m resolution over a 750 km2 area of Bristol and Bath, UK. We find that both the event set and uplift approaches indicate an increase in flood hazard under near-term (2021-2040) and future (2061-2080) climate change. However, the event set produces markedly higher estimates of flood hazard when compared to the uplift approach, ranging from 19% to 49% higher depending on the return period. This suggests including the full spatiotemporal rainfall variability and its future change in rainfall-driven flood modeling is critical for future flood risk assessment. Climate change is making rainfall more extreme. However, the detailed change in the space and time characteristics of rainfall are not well known and not represented in flood risk assessment. Instead, standard practice uses spatially uniform rainfall derived from historical records and increases them by a fixed percentage uplift to account for the impact of climate change. These uplifts are generated from climate model simulations that are analyzed to determine average rainfall changes. Failure to account for the changing space-time characteristics of rainfall means that this standard approach likely mis-estimates future changes in flooding. To test this hypothesis, we use high space and time resolution predictions of future rainfall patterns considering climate change to drive a detailed flood inundation model over the Bristol and Bath urban region in south-west UK. Flood hazard is projected to increase in future, with increases greater when using full space-time rainfall data rather than the standard uplift approach. This has important implications for how we estimate changes in flooding from changing rainfall with global warming. We produce flood hazard estimates using spatiotemporal rainfall data from new convection permitting climate projections (UKCP Local)We find the UKCP Local rainfall data gives higher estimates of flood hazard compared to standard climate change uplift approachesRepresenting full spatiotemporal rainfall and its future change is crucial for future flood risk assessment