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 ∼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.