Stricter controls on new property developments alongside up to £12bn of investment in drainage infrastructure over the next thirty years will be necessary to stop thousands more homes and businesses from flooding due to inadequate drainage, according to a new report by the National Infrastructure Commission.
Localised floods across England last month alongside a series of major flooding incidents in London in 2021 – which affected 1,500 properties and disrupted healthcare and transport networks – have highlighted the risks posed by surface water flooding.
At present, 325,000 English properties are currently in areas at high risk from this type of flooding, which means they have a sixty per cent chance of being flooded in the next thirty years.
But a combination of the impacts of extreme weather caused by climate change and increasing pressure on drainage systems caused by new developments could push an additional 230,000 properties into the high risk category by 2055. The spread of impermeable surfaces – such as the paving over of gardens – could move another 65,000 properties into a high risk area.
The report says the first step is to stop adding to the problem by limiting the knock-on effects of urban spread on existing drainage systems. Further action to expand existing drainage capacity above and below ground, including the use of nature-based solutions such as green gulleys and flood storage ponds, and addressing the fragmented way in which the problem is managed and funded, will improve the resilience of drainage systems to more frequent and heavier downpours.
The report calls for an expanded role for the Environment Agency and Ofwat in overseeing the delivery of joint local plans for high risk areas, developed in partnership by local authorities and water companies.
The Commission’s modelling suggests its recommended levels of investment in new infrastructure could move 250,000 properties out of the high risk category, and boost protection levels for thousands more properties. Action on new developments could prevent a further 95,000 properties from facing a high risk of surface water flooding in their area.
Professor Jim Hall, National Infrastructure Commissioner, said:
“It’s clear that faced with more intense rainfall and increased urbanisation, we need to start taking this type of flooding far more seriously.
“The solution is clear – reducing the amount of water flowing into drains, whilst also improving the capacity of those drains. That means stopping urban creep from increasing the amount of storm water that drainage systems have to cope with and giving nature more opportunities to hold on to excess water, as well as targeted investment to ensure sewers can cope with growing pressures.
“While sustained investment is needed, the estimated additional costs are relatively modest. At least as important is a more joined-up approach to owning and acting on the problem.”
Reducing the risk of surface water flooding makes a series of recommendations for action by government, water companies, local authorities, the Environment Agency and Ofwat in three areas:
- Reduce the amount of run-off water entering drainage systems – government should strengthen legislation and standards to discourage new developments from connecting to existing drainage infrastructure in favour of wider uptake of sustainable systems, and review options for managing the unplanned growth of impermeable surfaces;
- Expand the capacity of drainage systems – better maintenance of existing drainage networks and expanding the use of lower cost above ground measures (such as channels and drains) should be considered before new pipes and sewers, and priority should be given to nature-based solutions such as roof gardens, drainage ponds and rain gardens. Ofwat should ensure that water and sewerage companies play their part, by enabling efficient investment in both above and below ground drainage infrastructure;
- Create more joined-up, targeted governance and funding – the Environment Agency should be actively involved in assessing surface water flood risk and government should set national risk reduction targets. Local authorities and water companies should then work together to develop fully costed joint plans which deliver locally agreed targets, with public funding devolved to local areas.
The Commission estimates the new infrastructure elements will cost £12 billion spread over 30 years, split between both public and private funding. However, this total amount could be reduced as a result of action undertaken as a result of the separate £56 billion already earmarked to reduce combined sewer overflow spills.
The report also identifies that improving the breadth and coverage of local flood risk data would make it easier for the Environment Agency to show risk mapping at street or property level, giving national and local officials and members of the public a better understanding of risks.
