More than three million homes are at risk from surface water flooding and climate change is making the problem worse. Thomas Lane looks at the measures being taken to mitigate the risk

July 2021 was a month that many Londoners might prefer to forget. On 12 July, a month’s worth of rain fell in the space of an hour, flooding buildings, Tube stations and major roads. Less than two weeks later, on 25 July, history repeated itself with torrential rain dumping a further month’s worth of rain in two hours. In total over 1,000 buildings were flooded.

Flooding is a particularly devastating type of natural disaster as it destroys people’s homes and their possessions. To add insult to injury for those being flooded, floodwater is often mixed with raw sewage. 

In areas with so-called combined drainage systems, where foul and surface water is mixed, heavy rain overwhelms these and causes this vile mix to bubble up out of street manholes and people’s toilets inside their homes. In a worst-case scenario people die, particularly in basement flats. 

A couple of months after the London floods, an extreme storm in the wake of Hurricane Ida killed 13 people in New York, with 11 of those known to have drowned in basement flats.

What marks these floods out is that they were a consequence of torrential rain overwhelming drainage systems, so-called surface water rather than river or coastal flooding. While river and coastal flooding has the potential for far greater damage given the volumes of water involved, it has the merit of being relatively predictable as it only affects those living near rivers or the sea.

Surface water flooding is much less predictable as torrential rain can strike anywhere. Some areas are at greater risk, particularly densely occupied urban areas as there is less permeable land available to soak up rainwater. And a prolonged dry spell, such as that experienced this summer, can extend the risks more generally as soil becomes much less permeable when it is hard and dry. 

River and coastal flooding has tended to dominate the flooding debate because water levels are much higher than surface water floods and last much longer. Surface water floods are typically less than 20cm deep and drain away relatively quickly. But 3.2 million homes are at risk from surface water flooding, more than the 2.7 million at risk from river and coastal flooding and the problem is getting worse. So why is surface water flooding as bad as it is and what is being done to tackle the problem?

Urban creep

Climate change is a key driver of increased surface water flooding as warmer air can hold more water, which increases overall rainfall levels. That rain is also more likely to be intense. The Met Office predicts that by 2070 winters will be 30% and summers 60% wetter. 

So-called urban creep is also a significant contributor to the risk as it reduces the amount of available permeable ground to soak up rainfall,  which means more water is directed towards the drains. Urban creep includes new extensions, with a significant contribution from paved-over front gardens to create off-street car parking. A 2005 London Assembly report called Crazy Paving estimated that 32km2 of front gardens – equivalent to an area of 22 Hyde Parks – had been paved over in London alone. 

“The incremental loss of front gardens and side returns is having a huge impact on our systems,” explains Alex Nickson, water resources and growth lead for Thames Water. “We estimate that in a thunderstorm one patio delivers as much water to the network as 100 homes.” 

Tackling this issue is a major challenge as property owners can build small extensions and pave over their front gardens under permitted development rights with no requirement to register the work. This makes it difficult for water companies to establish where increased drainage capacity is needed. 

“We can plan for 100 homes, but we don’t know where the patios are,” says Nickson, adding that Thames Water is turning to remote sensing to keep on top of the issue.

New development can also increase pressure on drainage systems. In theory, planners can insist on measures to ensure that water run-off is no greater than before the site was developed. Steven Wade, the technical director for climate resilience at Atkins, says that this is achievable if flood mitigation measures are considered carefully at an early stage. 

But problems can arise if a developer tries to pack as many homes onto a site as possible. “You don’t want to find yourself in a position where a scheme is designed in a certain way, and then you realise more space is needed to manage the flood issue,” Wade explains. 

“It’s always harder to find solutions later rather than at the beginning.” He adds that another problem is a failure to maintain flood mitigation measures which don’t work as designed when needed.

In theory well-designed brownfield redevelopments could reduce water run-off as undeveloped sites often consist entirely of impermeable surfaces and buildings. Nickson says Thames Water’s experience of the quality of redevelopment is variable. “Some boroughs in London are wonderful and we are confident that the new development has a net reduction in surface water run-off. We have other boroughs that are perhaps yet to fully attain those standards,” he says. He adds that the London Plan helps as this has strong rainwater management policies adopted by most London boroughs.


So what measures help to mitigate surface water flooding risk? All water companies were required to produce a draft drainage and wastewater management plan by this summer. A DWMP is a 25-year strategic plan with the idea that all those with an interest in drainage and wastewater management work collaboratively to ensure that drainage systems are robust and resilient in the face of growing populations and climate change, and to improve environmental water quality.

Thames Water’s DWMP aims to address surface water flooding primarily by increasing the area of sustainable urban drainage systems (SUDS). SUDS are a basket of measures that mimic natural rainwater drainage through soil. These include swales, vegetated channels that retain rainwater, soakaways, wetlands and retention basins, areas of land that retain water and allow it to drain away slowly. The DWMP estimates that the Thames area needs 7,200ha of SUDS by 2050, an area equivalent to 50 Hyde Parks.

London’s density means that some degree of SUDS retrofitting will be necessary to provide enough permeable land. This could include planting more street trees and installing water retention tanks under pavements and car parks.

Another solution could be “rain gardens”; a car-sized, planted space situated at the sides of roads designed to retain and slowly release water. New York has installed 11,000 rain gardens in a bid to reduce the risk of surface water flooding.

To cope with extreme events, “cloudburst boulevards” may be necessary. These were installed in Copenhagen after the Danish capital  was hit by a devastating storm in 2011.

Conventional roads are raised in the centre to direct water to drains at the sides. A cloudburst boulevard is the opposite; higher at the edges so that the road becomes a channel which is given added capacity with raised kerbs. The channel retains water during an extreme event which seeps away slowly into the ground – the low, absorbent central part of the road can be greened to add amenity value and provide a cooling effect during hot weather.

The main stumbling block to implementing these measures is that responsibility for surface water flooding is split between the water companies which look after sewers and drains and lead local flood authorities (LLFAs) which are county councils and unitary authorities and are responsible for surface water flooding. The Environment Agency has strategic oversight of the management of all types of flooding as well as responsibility for managing the risks of flooding from coastal and river flooding. 

A flood is often a mixture of rainwater and water or sewage that has overflowed from sewers with this all ultimately ending up in a river. Untangling responsibilities is all but impossible.

“We are legally responsible for sewer flooding, but the integration of the different drainage systems makes it difficult to say this bit of flooding here is sewer flooding, and that bit of flooding is surface water flooding,” says Nickson, adding that improving conventional drainage could make river flooding worse.

The London Flood Review, an independent report commissioned by Thames Water into the July 2021 London floods, recommended that a body with strategic oversight of the issues should be established to tackle the problem. This should have representation from all the organisations responsible for different aspects of surface water flooding, a conclusion Nickson agrees with and is working on implementing. 

You don’t want to find yourself in a position where a scheme is designed in a certain way, and then you realise more space is needed to manage the flood issue

Alex Nickson, Thames Water

“What we are doing now is working with all the key partners in London to say we need a shared and collaborative approach to managing this risk because you can’t define that this is sewage and that is surface water flooding. What we need is a high-level group to steer it and that is something that will be more formally announced towards the end of this year.”

Thames Water has worked with a wide range of stakeholders to formulate its DWMP and is moving towards establishing a group to steer its implementation. But this is not being repeated evenly across the country. London is ahead of the game because the July 2021 flood prompted the mayor to set up a roundtable group to look into the issues and Thames Water commissioned its London Flood Review to do the same.

DWMPs are supposed to encourage all water companies and interested stakeholders to work collaboratively, but John Chappell, senior policy adviser at the National Infrastructure Commission which has been commissioned by the government to investigate how those responsible for surface flooding can better manage and mitigate the impacts, has reservations about them. As a water regulator requirement, they are being driven by the water companies and have varying levels of engagement with other, critical stakeholders. Some don’t even mention the management of surface water flooding. “These are important but are only part of the solution,” Chappell says.

Like Nickson, Chappell thinks surface water flooding needs to be managed by one body. “You need a long-term strategic assessment programme for all forms of flooding and people who are responsible for administering that and planning it and prioritising it,” he says. 

Unlike Nickson, who thinks the strategic oversight body can be drawn from the stakeholders in different regions, Chappell thinks a national body would be better as this would mean the approach would be consistent across the country. He says the Environment Agency, given its current strategic role, could be a suitable candidate.


A major obstacle to better surface water management is the expense. As an example, the strategy set out in Thames Water’s DWMP will cost £24bn over 25 years. Quite how this will be paid for is not clear. And the DWMP says the £24bn is only a starting position that indicates the scale of the task.

Until recently surface water flooding has been the poor relation of coastal and river flooding investment. LLFAs are responsible for surface water flooding but funding to tackle the issue is not ring-fenced and, given the pressures on local authority budgets, is unlikely to be at the top of the priority list. 

Much of the funding to tackle surface water flooding, other than that which comes from water companies, comes in the form of Environment Agency grants. Allocation is based on competitive bids with the money going to those with the best cost-benefit ratio. 

Because the cost-benefit ratio of surface water flooding mitigation is less than that for coastal and river flooding, it barely gets a look in. But things are improving somewhat: until recently it received just 6% of the money, now it gets 25%. 

Chappell questions whether there should be ring-fenced funding for surface water flooding alleviation. “If you speak to people who have just been flooded, they won’t care if it is a surface water or a river flood, but they will care if it isn’t dealt with.”

One of the main reasons why the government commissioned the NIC report is to find the most cost-effective ways of addressing the issue. An example is tackling two different objectives at the same time – for instance, the government’s storm overflows discharge reduction plan which aims to tackle the issue of raw sewage discharges into rivers and the sea. 

Water companies use storm overflows when heavy rain risks overloading combined sewage systems and treatment plants which would back up into buildings. Addressing this will cost £56bn over 25 years, adding an average £42 a year to water bills to 2050. This will not tackle surface water flooding on its own as the measures are designed to cope with a rain event that typically happens four times a year. 

Surface water flooding measures need to cope with much more extreme, one-in-30-year rain events. “We can get efficiency savings from that to also address surface water flooding because some of the solutions are the same, they are just bigger for flooding because you are dealing with bigger rainfall events,” explains Chappell. 

Greater collaboration will help save money – for example, if a council is building a new car park it will be cheaper if a water retention tank is installed underneath at the same time rather than different agencies doing this separately.

Ultimately, it is not possible to protect all properties all the time so it is down to individual building owners to take action. An easy first step is to check the Environment Agency’s surface water flooding risk maps to see if a property is likely to be affected. 

Wade says Atkins does carry out surface water flood risk assessments, typically for government departments or private sector companies with large property portfolios. “You tend to find that most of the risk centres on 10 to 20 buildings out of a portfolio of 1,000 buildings,” he says.

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Wade adds that client response varies. A building owner might decide to get involved in a local flood resilience scheme whereas an organisation leasing an at-risk building might decide not to renew the lease when the time comes for renewal. 

He adds that some clients are only assessing the risk because of corporate social responsibility or regulatory requirements and take the view that people can work from home if the building floods or that insurance will cover the cost. Companies relying on critical infrastructure tend to take the problem much more seriously.

The National Infrastructure Commission report is due to be published at the end of November and will include recommendations for action. Although Chappell cannot say what these will be, all the signs suggest they will include recommending a national body to ensure there are robust plans in place to tackle the issues and that there is a strategic investment programme to make this happen. 

The recommendations will form part of the second National Infrastructure Assessment, a five-yearly report to government that makes recommendations on how to address major, long-term infrastructure challenges. This is due out in the second half of 2023, which means it is unlikely much will happen on the ground before then. 

Indeed, it will take years to mitigate the risks which are increasing all the time with climate change. For now, all people can do is look at the sky and pray that those angry-looking clouds dump their contents somewhere else.