There can be few people in the UK unaware of the risk of flooding rising year on year. The effects of climate change, the fact that more homes are being built for a growing population, and the pressure on green spaces are all causal factors that have been highlighted in the news for years. But the picture has become increasingly grave.
Last year, a University of Bristol* study estimated that annual damage caused by flooding could rise by more than a fifth over the next century. However, although this is quite a terrifying prospect from a disruption and safety point of view, and a significant worsening of the risk, many don’t realise that significant sustainable drainage mitigations can be made, and that these measures stack up quite well in terms of cost compared to the high costs of reparation.
Inevitable – yet unpredictable – expenditure
Flooding doesn’t just damage properties and risk lives (if that weren’t reason enough to invest in mitigation strategies), it disrupts businesses, communities and economies, which means that the financial implications of repairs and rebuilding is only one aspect of the costs involved in the clean-up operation. And nobody can ever quite be sure how bad it will be, or give an accurate long-term warning about when disaster might strike.
The Office for Budget Responsibility recently calculated that the direct fiscal cost of the 2007 floods was £1.3bn (at 2024-25 prices), while the equivalent calculation for the 2015-16 floods – which were less severe – resulted in a figure of £1.4bn. At a community level, the impact is perhaps more easily quantified in context, and the dreadful economic effect on business owners, workers and their families understood. Business in the Community / Aviva figures show that during the 2013-14 floods, the average cost of flood damage to small businesses was £82,000, an average of 50 working days were lost, and 40% failed to reopen after a flood.
Just considering flood damage costs doesn’t provide a full picture of the economic impact either. There are several categories of direct and indirect costs involved after a flood event:
Economic costs of flooding: Most obviously, there is the cost involved in damage to homes, businesses and infrastructure, but repairs and rebuilding are only part of the equation. Insurance premiums skyrocket, so many flood victims struggle to find affordable coverage afterwards, while significant downtime for businesses results in supply chain disruptions and lost revenue, perhaps also closures and job losses, which in turn affects local economies.
Social costs of flooding: Flooding uproots lives. Families are displaced, communities are fractured, and the mental health toll can linger for years. The World Health Organization has highlighted the psychological strain of repeated flooding and other climate-change effects, which can lead to anxiety, depression and post-traumatic stress disorder. These struggles put additional pressure on the NHS and can also affect individuals’ earning power and therefore the economic wellbeing of their households.
Environmental costs of flooding: Flooding wreaks havoc on ecosystems. Eroded landscapes and polluted water systems are just the beginning. Flooding can compact the soil through sheer weight of water, while waterlogged conditions can cause the breakdown of soil aggregates and organic matter, and displace air, so soil particles can pack more tightly together when the floodwaters recede, leading to further compaction. In addition, heavy equipment, vehicles or even foot traffic during clean-up or repairs can significantly compact wet soils, which are more vulnerable to compaction than dry soils.
Over time, this degradation leads to a vicious cycle of reduced infiltration, poor plant growth, reduced biodiversity, poor water quality, and heightened vulnerability to future floods.
The role of SuDS
Investing in preventative sustainable drainage systems (SuDS) can help prevent flooding by managing rainfall, slowing and reducing surface water flow, and incorporating natural processes such as infiltration and evaporation. Instead of relying solely on traditional drainage networks, SuDS use a combination of permeable materials, green roofs, tree pits, raingardens and swales to slow down water flow, allowing it to infiltrate the ground naturally.
This holistic approach not only minimises flood risks but also delivers broader environmental and social benefits, helping to ensure more resilient and liveable cities that can thrive in the face of climate change. SuDS also filter out pollutants, improving water quality, further supporting healthy green environments.
Why trees are an important element of SuDS
Trees are nature’s flood defence. Their extensive root systems act as natural sponges, absorbing excess water, reducing runoff and boosting soil infiltration. By improving water infiltration, trees help prevent surface water accumulation – a key cause of urban flooding. Unlike impermeable surfaces like concrete, the healthy soil around tree roots can store significant water, making trees essential to regulating the urban water cycle, provided they are thriving.
Tree canopies also play a critical role in flood prevention. By intercepting rainfall before it reaches the ground, they slow runoff and reduce its volume, giving stormwater systems more time to manage heavy rain. This added buffer can be the difference between a system that functions and one that floods.
The benefits of SuDS with trees incorporated also extend far beyond flood prevention. These solutions also help address pressing environmental and social challenges:
- Boosting biodiversity SuDS designed to include urban trees create habitats for wildlife, encouraging insects and birds to thrive (increasing pollination), as well as other species. In cities where biodiversity has been declining due to urbanization, green infrastructure offers a lifeline for ecosystems.
- Building climate resilience As climate change intensifies, urban areas face more frequent and severe weather events. SuDS don’t just reduce the impact of heavy rainfall, they mitigate the urban heat island effect. They also contribute to carbon sequestration, helping local authorities and other organisations to meet their sustainability goals.
- Enhancing amenity value
Well-designed green spaces improve quality of life. Parks with raingardens, tree-lined streets, and community-focused green infrastructure become places where people want to live, work and spend time. These spaces foster social cohesion and provide opportunities for recreation, which are essential for both mental and physical health.
Cost-benefit analysis: prevention vs. cure
One of the biggest hurdles to implementing SuDS and urban tree planting is the perception of high upfront costs.
Installing SuDS is a significant project. Initial outlays for materials and labour are required, in some cases there are costs involved in traffic management, too – not to mention disruption during installation. More broadly speaking, there can be a financial impact on local businesses, which affects the local economy. For this reason, replacing impermeable surfaces with permeable paving, installing tree pits, or constructing swales may seem expensive compared to traditional drainage solutions.
However, these costs pale in comparison to the long-term benefits. Local authorities and property owners who invest in this sort of green infrastructure see reduced expenditures on flood repairs, lower maintenance costs for conventional drainage systems and fewer insurance claims. Research shows that every pound spent on SuDS can yield multiple pounds in avoided flood damages and ecosystem services – for example, this academic study (published in the International Journal of Environmental Impacts) demonstrates that a leisure centre spent £39,000 on a SuDS solution, but provided a net value of over £100,000 over a ten-year period, achieving a return on investment in just three years.
There are plenty of other examples around the world. Take Copenhagen as one example. By integrating green infrastructure into its urban planning, to have an estimated 30% of its rainfall managed in this way instead of through conventional sewers, the city has saved millions in flood damage costs while creating vibrant public spaces. Similarly, Singapore’s rain gardens and green roofs have proven highly effective in reducing runoff and improving water quality.
Savvy investments to safeguard future communities
The cost of retrofitting green infrastructure into existing urban areas may evoke a sharp intake of breath for budget-balancers, while developers might need to weigh up the cost/benefit analysis of SuDS for new-build sites (though it can be a win-win – density increase, along with compliance and planning benefits, can be achievable). Ultimately, though, the costs of inaction are too high; something must be done. As urbanization continues, impermeable surfaces are increasing, putting more areas at risk of flooding. Meanwhile, climate change is amplifying the severity of extreme weather events. Waiting for disaster to strike before taking action is simply not logical or ethical. Investing in SuDS, and incorporating tree planting, is a proactive approach that safeguards our urban areas and their residents for many years to come; they are not just costs, but investments in economic stability, environmental health, and community wellbeing.
