Throughout November, we have been exploring the climate theme of cities – looking at the ways that cities could be impacted by climate change and learning about some of the research that is being undertaken to better understand and quantify these impacts.
As global warming continues, we are seeing increasing occurrences of weather extremes as a result of human-induced climate change. In a previous blog post, we explored the increasing frequency of extreme heat events and the way this heat is compounded in cities due to the nature of a typical urban landscape. Higher temperatures are not the only extreme that can be linked to climate change, however. As the planet warms, we can expect to see an increase in intense rainfall events and consequent flooding.
Some areas are more vulnerable to flooding than others, and cities can be particularly at risk of flash flooding. In this blog post we’ll look into why this is and explore some of the research being carried out to better predict future changes in flooding in the city of Bristol.
Is the UK getting more rain?
The Met Office State of the UK Climate report for 2021 was published by the Royal Meteorological Society in July 2022. The report highlighted the evidence that the UK’s climate is already changing, with recent decades warmer, wetter and sunnier than the 20th century. For rainfall, the most recent decade was 2% wetter than 1991-2020 and 10% wetter than the 1961-1990 period. Recent trends in local rainfall extremes have also been detected, with some evidence that these are already being influenced by climate change (Cotterill et al 2021).
In another piece of research from the UK Climate Resilience (UKCR) Programme, researchers examined how different levels of global warming could affect the number of ‘high-impact’ weather days that we see in the UK. They found a positive correlation between increased global warming and instances of high-impact days, with projections indicating an increase of three days per year for high-impact heavy rainfall days under a high emissions (+4℃) scenario.
Cities and heavy rainfall
Cities are particularly vulnerable to surface water flooding due to the nature of the terrain in urban areas. Pavements and tarmacked road surfaces are impermeable, meaning that rainwater cannot pass through and instead accumulates on the surface and eventually runs off. By contrast in rural areas rainfall is able to penetrate into the ground, reducing the risk of surface water flooding. Cities therefore require adaptive capacity (such as sustainable urban drainage systems), informed by the latest climate research and observational data, in order to minimise these risks.
Modelling flooding events to predict their severity
In a recent study led by Bristol University in collaboration with the Met Office as part of the UKCR Programme, researchers input UKCP local (2.2km) rainfall data directly into a flood impacts model to predict future flood risk. UKCP local simulations operate at a higher resolution of 2.2km, meaning they can more explicitly represent convective storms and provide improved estimates of hourly rainfall extremes. They also better capture local surface features such as cities, mountains and coastlines. The study found that using UKCP local data as opposed to the standard uplift approach* to drive
the LISFLOOD-FP flood inundation model, led to very different predictions of flood areas and depths for the Bristol metropolitan area. In particular, future changes in flood extent are higher when using the full UKCP local data.
The study highlighted the fact that the 1 in 30-year flood event does not simply correspond to the 1 in 30-year rainfall event (assumed in standard uplift approaches), since the degree of flooding is controlled by the complex interactions between the rainfall and the landscape (in this case the city environment). Also, the detailed variation of rainfall in space and time, and how this changes in the future, is important for future changes in flooding. On including the temporal and spatial interaction of the rainfall with the landscape, researchers achieved significantly different flood hazard estimates, when compared to using the standard approach.
Professor in Faculty of Science at Bristol University, Scientific Manager and Met Office Science Fellow, Professor Lizzie Kendon, who led on the research, said: “The climate is already changing – and we are starting to see this in observations here in the UK.
“Moving into the future the character of rainfall is expected to change with more intermittent but heavier rainfall. This could have devastating consequences for our cities that are particularly vulnerable to flash flooding. Existing drainage systems have been designed based on historically lower rainfall intensities and do not have the capacity for the projected increases in rainfall. Therefore, it is vitally important that we understand potential increases in flooding, so that we can make informed adaptation decisions.”
With climate projections indicating a future increase in the occurrence of heavy rainfall events, research like this demonstrates the value of high-resolution local data to ascertain the level of flood risk for a particular area. This enhanced understanding of flood risk enables informed decision-making when considering climate adaptation in UK cities.
If you would like to learn more about the flood risk where you live and how you can prepare for high-impact weather, visit our seasonal advice pages.
* The standard uplift approach refers to the more traditional approach of inputting observational rainfall data, to which a standard uplift (percentage increase in rainfall for a given return level, based on climate model output) has been applied, into a hydrological model to generate flood projections.