Is retrofitting sustainable drainage (SuDS) the key to building flood resilience in the UK

min read time
2022-12-22 11:42:10

It is hard to ignore the current predictions of flooding in urban areas. Extreme weather caused by climate change and the ever-growing population of the UK combined with urban creep are all playing a part in increasing the risk of floods. In a speech Sir James Bevan, Chief Executive of The Environment Agency, has described surface water as “the UK’s biggest flood risk, representing a growing threat to life, property and the economy”. Now, more than ever, the UK needs to look for ways to ensure the resilience of its cities and built environments.

Why sustainable drainage (SuDS)?

Sustainable Drainage Systems (SuDS) offer an approach to managing rainwater in development that mimics natural drainage in nature. In urban areas, natural infiltration is limited by hard impermeable surfaces which increases runoff diverting water into local water courses and causing problems with both the quantity and quality of water. SuDS aim to manage surface water as close as possible to where it falls.

SuDS are widely recognised as being beneficial in reducing flood risk, reducing the volumes of water in sewers and making urban development more sustainable and resilient to the pressures of climate change and population growth. SuDS also offer many other benefits beyond just surface water management including improvement of air and water quality, increased biodiversity, temperature regulation, improvements to the urban landscape and health benefits for inhabitants.

A combination of both engineered and soft SuDS features is generally considered to be the most effective approach to SuDS design combining the filtering and cleansing ability of natural features such as trees, grass and soil with the more space-efficient storage of large volumes of water provided by engineered SuDS components. The four pillars of SuDS design are set out by CIRIA in The SuDS manual reinforcing the importance of combining hard and soft SuDS features to not only manage water quantity but also water quality, biodiversity and amenities for local communities.

Geocellular structures such as Wavin’s Q-Bic Plus compliment soft SuDS features and could prove particularly beneficial, over conventional in-situ tanks or large diameter pipes, in SuDS projects due to their spatial versatility, ease of installation and maintenance and the option for infiltration through a geotextile liner in suitable soils.
Martin Lambley , Product Manager for Stormwater Management at Wavin

The need for retrofit

Ofwat codes for adoptable sewers came into force in England in April 2020 replacing all versions of Sewer for Adoption. This guidance is for use by developers when planning, designing and constructing foul and surface water drainage systems intended for adoption by English water and sewage companies under a section 104 agreement. It differs from the outgoing Sewers for Adoption guidance as compliance is mandatory. There is however some debate on whether this will be enough to ensure flood resilience in the UK.

New builds only make up a small part of our current urban areas so whilst the implementation of SuDS on new developments is important, the impact of retrofitting of SuDS in existing urban areas is likely to offer much greater opportunities for increasing flood resilience of cities. According to Susdrain, (the independent community on sustainable drainage created by CIRIA) the increase in capacity of traditional methods of surface water management such as sewers and underground storage is unsustainable and not adaptable for the future. They believe that the retrofitting of SuDS will give a more joined-up approach to managing surface water across wider areas and support the water cycle as a whole.

The Local Government Association (LGA) advises that all local authorities should seriously consider the opportunity that retrofit SuDS offers to local stakeholders with an innovative approach to exploiting existing urban spaces. It also recognises that each urban community is different and so a range of options should be considered with urban design being key to each solution. CIRIA’s document on retrofitting SuDS (C713) combines the work of UK experts with industry-wide collaboration to create a vision of a transformational approach to surface water management. The 272 pages of guidance have clear guidelines on how to retrofit SuDS should be designed to not only manage surface water but also to create habitats for wildlife and more pleasant places to live.

In practice retrofitting SuDS is far from straightforward. In UK legislation, the Flood and Water Management Act 2010 and Flood Risk Management (Scotland) Act 2009 require responsible bodies to cooperate in order to manage flood risk. However, with such a wide range of organisations, which often work in isolation, being involved in developing and managing urban areas, a joined-up and strategic approach to urban design and retrofitting of SuDS could prove challenging.

Greener grangetown SuDSimage source: Susdrain

Retrofitting schemes in the UK

Retrofitting of SuDS is beginning to happen in the UK and there are a growing number of case studies emerging which demonstrate how successful SuDS can have positive impacts on communities, ecosystems and finances.

Welsh Water in partnership with Cardiff Council and Natural Resources Wales have been instrumental in promoting the retrofit of SuDS through its award-winning Greener Grangetown scheme which involved SuDS retrofit in central Cardiff and is widely considered to be an excellent example of the benefits that can be gained from a well designed and implemented retrofit of SuDS. Water and Waste Water Treatment reported on how the scheme takes inspiration from schemes in Malmo, Sweden and Portland, Oregon where a more eco-friendly area and attractive area has been delivered with substantially reduced costs. The project involved trees, planters, grass channels, drainage curbs and rain gardens being installed across 12 existing residential streets with a result of water being cleaned and diverted into the River Taff and removing over 40,000 cubic metres of surface water being removed from the sewer network each year.

Llanelli, a coastal town in South Wales is also a great example of large-scale retrofitting of SuDS to combat frequent flooding or properties and pollution of local seawater and shellfish populations. The SuDS-based approach proves to be just a quarter of the £377m cost of the traditional solution of building additional storage. Institution of Civil Engineers (ICE) reported on how the roadside swales and planters performed better than expected whilst creating a ‘greener’ town and that all the benefits are likely to increase as the plants grow and mature. Post-construction monitoring shows a 25% reduction in peak flow during some storms.

Thames Water has implemented an ambitious sustainable drainage system aimed at reducing the pressure on the existing drainage system. The £14.5m scheme represents the UK’s largest ever SuDS project and aims to separate surface water falling in the area between Vauxhall Bridge and Battersea Power Station from wastewater and sewage, channelling it through large underground pipes and discharging it into the Thames via a large pumping station. The large-scale engineering project will be completed by soft SuDS features in new housing developments planned as part of the regeneration of the area including green roofs, ditches containing filtering vegetation and streets with rainwater gardens.

Anglian Water has also recently rolled out a SuDS scheme aimed at reducing flood risk by reducing hard surfaces in urban areas and aiding natural drainage. A school in Newmarket was one of the first to implement a scheme with school playgrounds and carparks being identified as large areas of hard surface. The scheme included funding for a tree pit, a water droplet-shaped rain garden, a series of planters, a watering can and a cloud rainfall system to flow rainwater into the planters. The scheme also maximised community benefits by including an educational aspect which featured educational materials, a gazebo-style outdoor classroom for the children to learn about the water cycle and a weather station for the children to use as part of their learning.

Strategic v opportunistic retrofitting of SuDS

With an issue as complex as stormwater management, the solution is likely to be just as complex. Water companies are responsible for the management of a large proportion of the assets involved in current stormwater management and their role in the planning and design of strategic retrofitting will be important in ensuring a catchment-wide and systematic approach but there are also many other agencies and organisations that will play a part.

There are many opportunities for opportunistic retrofit of SuDS on a smaller scale including small local incremental improvements which can often form part of green network strategies. Whilst these schemes utilise smaller measures, they can still significantly reduce overall runoff. A coordinated approach to identifying and managing these opportunities is needed to ensure that they are not missed and that relevant parties fully understand how SuDS can help them to achieve their own objectives for example by sharing the cost of the scheme to cut costs.

Water and Waste Water Treatment reported on how Anglian Water has worked with engineering and design consultancy Atkins using their unique SuDS studio toolkit, to identify the most cost-beneficial solutions and create a strategic plan for retrofitting SuDS. The work aimed to not only manage surface water but also to maximise other benefits of SuDS including water and air quality, reduced noise or heat island effects and improved biodiversity and amenity. The plans allow Anglian water to share key opportunities with flood risk partners including leading local flood authorities and district councils to enable the delivery of partnership-funded opportunities.


image source: Susdrain

The enablers - funding and co-operation

There are questions facing the retrofitting of existing urban areas over who should be responsible for the funding of such schemes and how the different parties involved can effectively cooperate to find innovative solutions.

Whilst the government and other key organisations are likely to lead on the retrofitting of SuDS homeowners will also have a role to play. A study by DEFRA in 2010 looked at water retrofit policies including permeable paving, green roofs and rainwater harvesting with the recognition that its objectives could not be met by the government alone and that initiatives would be required to influence homeowners' behaviours. It found that the attitudes of homeowners to retrofitting SuDS was mixed with many factors likely to influence potential uptake including awareness of SuDS, perceived risk of flooding, cost, aesthetics, windows of opportunity during the maintenance cycle, maintenance, proven effectiveness, confidence in installers and safety. The study found that general schemes that provided incentives were generally favoured over those providing compulsion.

Innovative thinking around the financial case for funding SuDS is already providing a persuasive case with incentives for businesses and organisations and it is possible that this could also extend to individual homeowners. In 2018, The Princes Responsible Business Network created an interesting business case for investing in flood resilience in Greater Manchester with SuDS. With 250,000 properties currently at risk of surface water flooding in Greater Manchester there was a clear case for retrofitting SuDS but the proposal also highlighted both cost savings and the creation of blue and green spaces. The business case was based on the savings that could be made by businesses moving down a charging band with United Utilities by using SuDS to disconnect from the wastewater system. A key driver for this financial approach to retrofitting is the changing of the charging structure by United Utilities as a result of OFWAT guidelines. These savings would not only offset the initial construction costs for SuDS but also provide longer-term savings. The overall calculations at the programme level found that 249 schools and NHS sites could make combined savings of over £300,000 per year, seeing a return on investment in SuDS within 5 years. By extending the return period to 15 years, 598 sites could invest in SuDS, saving over £800,000 per year, creating over 300,000m2 of green and blue space and delivering over £83million worth of social and environmental benefits. A demonstration site at Moorlands Junior school involved disconnecting 497m2 of hard standing from wastewater sewers by installing rain gardens with a pathway and an area of permeable paving in the carpark. The result was an annual saving of £1475 which includes the 50% discount applied to schools. Other non-domestic customers do not receive this discount so would potentially benefit from a saving of £2950 each year on a similar scheme.

Water companies are likely to play an important part in retrofitting SuDS and their recognition of the benefits and commitment in terms of investment will be important in driving overall implementation. WWT online recently reported on how SuDS and wastewater management are becoming more important for water companies. Thames Water recognises the expense of maintaining grey infrastructure, particularly in built-up areas and also the benefits of managing surface water at source including the value of amenities and biodiversity. They talk of their current commitment in their 2020 programme to investing £20m to disconnect 20 hectares of surface water from their combined sewer network and plans. They also outline how they are planning for even greater investment in the future with partnerships with local authorities and other key organisations being important to them.

Innovative projects across Europe can also help to find new ways for collaboration. Plymouth City Council is a lead partner in a Water Resilient Cities project to increase urban resilience to climate change through improved stormwater management. The project aims at improving the adaptive capacity of cities to heavy rainfall by demonstrating how SuDS can be retrofitted in public areas that are normally constrained by existing uses and infrastructure both above and below ground or ‘historic environment protection. Lack of drainage for stormwater has proved to be a constraint for new developments in Plymouth, especially in low-lying areas when tides are high and prevent drainage into the sea. The project is demonstrating a new approach to retrofitting SuDS with the use of public realm land combined with additional resources provided by developers to improve quality and bring new innovative approaches. Plymouth’s first SuDS pilot site was unveiled in August 2018, a retrofit of existing urban areas with tree pits to help manage surface water and prevent city centre flooding. The tree pits were part of a bigger project by Plymouth City Council partnered with Aspire Student Living 3 to redevelop the area with new paving, street lighting and planting of trees and shrubs to create an attractive safe and high-quality area in the city centre. The scheme was described by Cllr Mark Lowry as a “good example of what can be achieved when private and public sector works together. “

To summarise

Retrofitting of SuDS to existing urban areas offers significant opportunities to reduce flood risk and manage surface water whilst also providing huge benefits to communities and ecosystems. However, there are also many challenges in identifying opportunities, finding workable financial models and enabling cooperation between relevant organisations, bodies and individuals.

Legislation, prioritisation by the government, clear demonstration of the benefits in terms of finance, environment and health, innovation and cooperation could all play a part in driving the large-scale retrofitting of existing urban areas with SuDS.