Impact of soil moisture and rainfall on future urban flood

Soil moisture dynamics and rainfall patterns can play a major role in flood generation, yet there has been little research into their impact and how it might change in a future climate. New research, led by Youtong Rong and whose co-authors include Fathomers Prof Paul Bates, Prof Jeffrey Neal and Dr Simbi Hatchard, helps address this gap.

Extreme rainfall events are expected to become more frequent and intense due to climate change. It is often assumed that this will lead to more severe floods in urban areas, where the prevalence of impervious surfaces exacerbates the flood risk.

However, it can be misleading to look only at rainfall patterns when assessing urban flood risk. The links between precipitation and flooding are extremely complicated and there are many other factors, such as the physical features of catchments and antecedent soil moisture, that influence the extent and depth of flooding. There is evidence that even when rainfall extremes increase, changes in soil moisture can lead to a decrease in flood volume. 

There have been many studies of changes in rainfall and streamflow and their impact on flood risk. But changes in rainfall distribution patterns and soil moisture due to climate change have been neglected.

To address this, the research team developed a new method of assessing future flood hazard that incorporates soil moisture and rainfall patterns, using the city of Bristol in the UK as an example. 

The research: A new flood assessment framework for the UK

Bristol is one of the 10 cities in the UK most susceptible to flooding. But assessing flood hazard under climate change in urban areas is a major challenge, not least because most models cannot handle the complex interactions between multiple flood-generating mechanisms at a resolution of ∼30 m or less over such large areas. 

To deal with this, the researchers developed a new framework that incorporates four flood-generating processes (water balance, precipitation, drainage loss and infiltration) into a physics-based flood model. 

Using the latest UKCP Local dataset, a very high resolution (2.2 km) regional climate model over the UK, the team extracted over 30,000 rainfall events from the past, present and future and used these in a high-resolved hydrodynamic model to evaluate pluvial flooding hazard in Bristol, accounting for changes in the precipitation distribution patterns and soil moisture dynamics.

Results: More flooding annually, less flooding in summer 

Key findings of the research include:

• Soil moisture levels at the start of rainfall events significantly modulate the resulting runoff, and accounting for this effect is important to correctly forecast climate change impacts over urban areas.
• Whilst future rainfalls are higher over Bristol, higher future temperatures can also mean that, at times, these events occur when soils are drier.  This reduces the resulting flood volume.  How this affects changes in the future is seasonably dependent, with a marked reduction in summer flooding.
• Overall, in a future climate (2060–2080), the total annual rainfall is projected to increase, leading to a significant rise in the frequency of extreme floods. The annual urban flood volume is also expected to increase up to 52.6% when compared with 1980–2000.  

The results confirmed that it is not possible to look solely at changes in rainfall alone to accurately predict current and future  flooding and that soil moisture dynamics are an important factor in urban flood modeling. They also underline the importance of adaptation to flooding in cities as the climate changes and the risk increases.