Exploring the extent to which climate change and urban growth both influence future urban flood events

Flooding is a known and increasing risk under a changing climate, especially in urban areas where greater proportions of populations now reside, with predictions only showing this to continue to increase. However, climate change is driving an increase in the frequency and intensity of periods of extreme rainfall and thus the likelihood of ‘flash flood’ events, as seen by a number of such events throughout Europe and the globe, in recent year. It is in urban areas where the greatest levels of exposure to such events occur, where population is the greatest and most dense, and it also these areas which change the most, particularly with urban expansion to accommodate the growing demands for residential units. However, most current modelling work fails to account for these different drivers; (a) changes in urban form through urban expansion and (b) model climate induce uplifts to storm intensities and durations. Therefore, these results may mis-represent or mis-capture the true levels of exposure and risk to the population in these areas.

In our work we address these issues through employing a 2D high-resolution hydrodynamic flood model, CityCAT, coupled with an urban development model, UDM, which can generate plausible building level scenarios of urban growth. This approach allows our modelling to not only capture both the changes in extreme rainfall but also changes in the urban landscape at building level and explore the relationships between these as drivers for urban flooding and it’s potential impacts in the future. Additionally, we are able also look at the impact of adaptation, such as green infrastructure, on the outcomes of extreme rainfall and the subsequent flood events in the urban landscape as a method of reducing exposure and risk.

Applying to this to a number of cities in Great Britian, we use a downscaled UK specific version of the Global SSPs (Socio-Economic Pathways) to model plausible urban change outcomes at building level scale, using this to then also update land-use scenarios in the hydrodynamic model. Together when coupled with rainfall storm profiles using uplift values, we are able to investigate the outcome of both these drivers, climate and urban change, on flood outcomes for future scenarios, including changes in economic damages and exposure levels, in urban areas.

Our results therefore explore the interplay between climate change and urban development on the impacts of exposure to flooding events, and the extent to which adaptation measures can play a role in reducing these. While the results show changes in flood extents, potential economic damages and exposure, they also show the influence of the analysed drivers and how these can vary and therefore highlight the need for city-specific analysis.