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 in 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.

Notes

The Commission’s full set of recommendations are:

• Recommendation 1: By the end of 2023, government should implement Schedule 3 of the Flood and Water Management Act 2010 and update its technical standards for sustainable drainage systems.
• Recommendation 2: Government should undertake a comprehensive review of the effectiveness of all available options to manage unplanned increases in impermeable (or hard) surfaces, including those described above, and their costs and benefits. By the end of 2024, government should decide whether policy changes are required to reduce the impacts on surface water flooding or adjust investment levels for flood risk reduction accordingly.
• Recommendation 3: Government should:
  • require the Environment Agency to use the results of the second National Flood Risk Assessment in 2024 to identify new flood risk areas
  • from 2025, require upper tier local authorities, water and sewerage companies, and other relevant authorities in the new flood risk areas to, where necessary, develop detailed local risk maps that can be integrated into the Environment Agency’s national map, and models that can be used to plan future management of surface water flooding.
• Recommendation 4: By early 2025, government should set a long term target for a percentage reduction in the number of households at high and medium risk of surface water flooding.
• Recommendation 5: The government should require risk management authorities in the new flood risk areas to agree appropriate local targets by mid 2025.
• Recommendation 6: Government should clarify in its strategic priorities for Ofwat that it should enable water and sewerage companies to invest in solutions to manage surface water flooding including sustainable drainage.
• Recommendation 7: Government should require:
  • upper tier local authorities, water and sewerage companies, and, where relevant, internal drainage boards in the new flood risk areas to produce and deliver costed, joint investment plans for managing surface water that achieve the agreed local objectives and follow the ‘solutions hierarchy’
  • the Environment Agency to review and assure the final plans with input from Ofwat and support from Regional Flood and Coastal Committees, and publish data on progress against local and national targets
  • joint plans to be completed by 2026 and revised every five years following the review of flood risk areas the year before, and to inform the following Ofwat Price Review.
• Recommendation 8: By the end of 2025, government should devolve public funding to upper tier local authorities in or containing new flood risk areas, based on the Environment Agency’s assessment of the levels of risk in each new flood risk area. The funding allocation should be reviewed every five years, in line with single joint plan cycles.
• Recommendation 9: By the end of 2024, government should explore options for funding property level measures for those properties that remain at high risk of surface water flooding because improving drainage infrastructure is not cost effective.

The government is expected to respond formally to the Commission’s recommendations within 12 months at the latest, as set out in the Framework Document between HM Treasury and the Commission.