SuDS: Natural solutions to sponge up cities

Government has given the green light to making sustainable drainage systems a mandatory requirement for all developers. This is the huge step the water sector has long been calling for but there is much more work to be done. Utility Week talks to Thames Water and United Utilities about their strategies to avoid surface water flooding, while the Greater London Authority discusses the hurdles to adding sustainable drainage in a dense, complex urban space.

It is July 2021. A mother passes her baby through a basement flat window as flood waters rise towards the ceiling. Hospitals are forced to transfer patients as they close their doors and extreme weather brings transport systems to a halt. Such scenes are familiar in news reports from less developed parts of south Asia but this is the experience of Londoners as the very real impacts of climate change begin to hit home.

“We were really lucky that we didn’t have any deaths,” Victoria Boorman, policy and programme water lead in the climate change adaptation team at the Greater London Authority (GLA) tells Utility Week. “There were stories of water reaching ceilings in basement flats. It shows how vulnerable major cities like London with high concentrations of people are to climate shifts.”

Urban creep has come at the expense of permeable green spaces, meaning that rainfall has nowhere to go but overwhelmed drains and sewers, which can lead to flooding as well as triggering combined sewer overflows (CSOs). The water industry has long been calling on government to change key policy that will ease the strain on sewers. Nearly three years after its first report on implementing sustainable drainage systems (SuDS), the Department for Environment, Food and Rural Affairs (Defra) has responded by mandating the approach for all home-building schemes and removing developers’ automatic right to connect to drainage systems.

But the green light from Westminster is only the first hurdle to implementing SuDS as the complexity of stakeholders and funding pools means collaboration on an epic scale is required.

Facing into that challenge, what are the hurdles, and what progress is being made?

Problems floating on the surface

Surface water poses both the greatest flooding risk and the least understood. That is according to the National Infrastructure Commission (NIC), which delivered a landmark report to government at the end of last year in which its top recommendation was to mandate sustainable drainage systems within all new developments in England.

Government has taken this onboard and started 2023 off by granting this addition, which should be implemented by 2024 after consultation. Enacting schedule 3 of the Flood and Water Management Act will also see developers lose the automatic right to connect to sewer systems without consent of the operator. This will mirror the approach adopted in Wales from 2015.

This is a huge step in the right direction to mitigate the risk of surface water flooding and something the water sector has calling for in the battle against changing climates and population growth that has in places overstretched infrastructure.

Towns and cities that have lost permeable land to urban creep face the risk of flooding from surface water that must be redirected away from sewer networks. This presents an opportunity to use a nature-based solution as opposed to traditional engineering that can be carbon intense. SuDS mimic natural water retention and can include wetlands, raingardens, retention ponds and tree planting to slow the flow of excess surface water.

What is the concern?

The amount of water from surface runoff and increased rainfall is hitting the combined sewer networks and reaching wastewater treatment plants, which can be 30 times the normal flow rate in medium to heavily urbanised areas that in turn can trigger sewer overflows (CSOs). Rising public awareness of CSOs and the risk of harm they pose to bathing waters, has prompted government action.

But this means an urgent need to address the causes of so much water hitting those systems and rethinking how surface runoff is managed. In many built-up areas, permeable spaces have been lost to roads, buildings and the trappings of urban life. Redirecting this away from sewer networks to prevent them being burdened at times of high rainfall needs sustainable drainage systems. In London alone, 7,200 hectares – an area equivalent to 50 Hyde Parks – needs to be “spongified” with drainage by 2050 according to Thames Water’s drainage and wastewater management plan.

Spongifying London – a challenge across 32 local authorities

Victoria Boorman, policy and programme lead on water, the climate change adaptation team at the Greater London Authority (GLA) explains how the intense flooding in London in July 2021 was a catalyst for action.

Mayor of London Sadiq Khan, who has no formal responsibility on flooding, decided to bring together all the parties with a responsibility or roles related to surface water flooding to create a London-wide strategy.

“There are international examples of city-wide strategies, but nothing on the scale or complexity as London, so it’s a really big job to not only write that plan but make sure it can be delivered,” Boorman says.

“What became obvious from these meetings was the need to work across the boroughs, because rain can fall in one place but its impact is felt as it flowed into another. We need to encourage cross-boundary working through the London strategy to identify and facilitate work done by one borough to benefit another. That isn’t easy with the current legislation, which is why collaboration is so important.”

Funding pots

Boorman and her team felt the NIC’s report could have gone further on funding, which she said remains a significant issue. “Local flood authorities in different boroughs talk about the difficulties they have in securing funding for SuDS schemes. There is a big issue with scaling up and delivering the amount of sustainable drainage needed, which the NIC report didn’t really draw out. The government really needs to focus on that to enable us to deliver the schemes needed to manage the risk going forward and the increasing risks climate change will bring.”

Funding comes from multiple pots, which again requires cooperation and coordination to apply for multiple funds – including from water companies – and some administered through the Environment Agency.

Compared to flood defense schemes at rivers and coastal locations, surface water flooding schemes tend to be smaller scale and spread across a wider catchment area, Boorman explains, which makes it difficult in terms of the EA allocating and distributing money.

Collaboration is essential here and via a pilot by the Mayor SuDS are being added where possible after utilities have dug up a road.

“We want to use these as opportunities to build that back better and green that up with permeable surfaces,” Boorman says. The approach has the potential to cut delivery costs by 25%, as a project in Enfield where SuDS were added concurrently to streetworks shows. “We’re aligning infrastructure work with potential SuDS schemes by local authorities to reduce the overall cost of delivering SuDS, which would be fantastic to help funding go further.”

Essentially it will take countless small pockets of activity across the city to reach Thames’ estimation of needing 7,200 hectares by 2050.

Boorman says of this ambitious target : “It’s important that we aim high so we can manage these risks and there is a lot already being deliver.”

More than £20 million in green infrastructure programmes have been funded by the Mayor since 2016 to support green space projects.

“The more green the better so we have that water draining away to the ground as opposed to it running off causing issues.”

Spongifiying the city

Thames Water’s strategy supports borough councils to include SuDS in routine work especially in high risk areas.

“We are rethinking our approach to rainfall,” Alex Nickson, wastewater systems strategy manager at Thames Water say. “Green infrastructure solutions, such as street trees and raingardens, are now the central element of our long-term plan to manage surface water flood risk in the capital.”

The company “strongly supports” the NIC’s call to tackle the loss of green spaces and manage the impacts of new developments using existing legislation.

Thames’ ambition to drain 50 Hyde Parks worth of impermeable areas to sustainable drainage by 2050 is a tall ask compared to the amount delivered in recent years. Nickson says it will take a total shift in thinking. “This will be delivered through changing our approach to implementing SuDS, moving from a reactive, opportunistic approach to a target-driven outcome focussed mentality, achieved through improving new developments, retrofitting existing urban areas and encouraging and incentivising Londoners to unwind the urban creep that has been enabled over decades.”

He says based on examples in other major cities like Copenhagen, Rotterdam and New York, SuDS can take the strain off of drains and protect the most vulnerable properties and people from flood risk.

Smart control of existing sewer networks to optimise capacity and address “pinch points” in the network should be done before increasing capacity in high flood risk areas. Finally, he says learning from other examples stresses the importance of having a plan to manage a bigger storm than the capacity of the systems by temporarily holding stormwater on the surface.

Funding challenge

Thames has funded 20 nature-based flood protection schemes this AMP and is developing strategic partnerships through a surface water management programme with the aim of giving councils a ring-fenced budget for SuDS.

So far in one borough, the authority has used Thames’ money to leverage further investment. Lambeth’s Victorian sewer network is predominately combined and has one of the lowest capacities in the capital. Dense urbanisation means upgrading buried infrastructure is not suitable and space is restricted so conventional flood mitigation options are not possible, With the funding, it has identified 30 places to build SuDS across highways, streets, open spaces, housing estates and schools between by 2025.

Gains made with developers

Getting ahead of the curve, prior to Schedule 3 being approved, United Utilities has been working with developers across the northwest on incentive schemes for including SuDS such as offering a 90% reduction of infrastructure charges for not connecting into the sewer network.

Samuel Fox, network strategy and planning manager at UU explains: “It’s not about inhibiting growth, but how can we support that growth without flood risks.”

He says the company is looking at incentivisation and adoption of SuDS as well best approaches to maintenance once they are installed.

“We’re excited for the opportunity to start investing outside of our traditional below-ground asset base but we’re very mindful of affordability challenge and making sure we deliver things that actually address the core service we provide for customers whilst also delivering things that have wider multi-capital benefits.”

Another challenge the company is looking at is managing unpermitted developments, such as the expansion of impermeable surfaces like driveways and covering domestic garden spaces. The NIC called for a clarity and focus from government on planning requirements for people undertaking projects to tighten up local authority rules.

To address the challenge at domestic level, UU is exploring property level separation on streets where all the surface water is connected to a combined sewer to disconnect and reroute flows into SuDS.

“One challenge we have is on a street that is completely connected to the combined system, you need 100% of the properties to buy into that disconnection. Otherwise you leave a path for all the rainwater to still enter the system. We’re running trials to understand customer engagement and willingness to participate .”

This could be a constructed wetland or trees planted with a retention pond or space to collect rainwater to channel it away from the sewer.

“There are different options in terms of disconnection at property level, or disconnection within the public sewer itself. We’re exploring the cost benefit and the disruption.”

On affordability he says UU is looking at alternative routes of investment that would deliver multiple benefits on top of alleviating flood risk. “SuDS has a core part to play with regards to enhancing the aesthetic appeal of an area. They provide wellbeing benefits, cooling urbanised areas, carbon sequestration, all of those really positive things from sustainable drainage.”

“Surface water management and SuDS has not necessarily been lost in the conversation around spill reduction, but there’s obviously been a heavy focus on that. We want to make sure we don’t disconnect those two things.”

Ultimately, the company is looking at spill reduction and surface water management through SuDS in tandem through its rainwater management strategy, which recognises the value of every drop that falls.

Water companies have been working to overcome the hurdles long before government gave the green light and now blue-green infrastructure can develop at pace with developers and local authorities working with the industry and stakeholders to bring natural solutions to mitigate climate change.

It remains to be seen how government chooses to apply Schedule 3, but mandating SuDS will force a complete rethink of managing surface water, which cannot come soon enough.