Sustainable drainage systems

Sustainable drainage systems (SuDS) are drainage solutions that mimic natural drainage systems, rather than relying upon a network of pipes and sewers to direct excess surface water to nearby watercourses. What potential benefits do they have for towns and cities and how can local authorities and businesses harness them to their advantage?

In urban areas, most surfaces have been paved over or are made of impenetrable materials, which means that rainwater can no longer naturally soak away. Coupled with climate change, which is making rainfall both much more intense and more frequent, the result of this is lots of surface runoff, excess water with nowhere to go, which heads towards drain and sewer networks. The lack of natural drainage means that, during a period of intense rainfall, sewers run the risk of overflowing, polluting nearby water sources, and causing severe floods.     

Surface run-off from roads and buildings contains elements of those surfaces; water flowing into drains from roads will contain pollution like oils and heavy metals from cars, which then in turn can end up polluting water in rivers. Risks here include harming wildlife, and eutrophication, excess algae growth in water bodies due to increased levels of phosphorous often due to fertilisers or chemical pollution reaching water sources.     

Traditional drainage systems were built many years ago for a smaller population with different needs who lived in a different climate to ours. Now, with the UK population set to exceed 70 million and the weather shifting to wetter winters and hotter summers, the current drainage system is outdated and unable to sustainably continue to support a healthy society. In recent decades, there has been a growing need to address several interconnected environmental problems – water quality, flood management, lack of green spaces in urban environments, falling biodiversity, societal wellbeing – simultaneously in the form of SuDS.     

A different approach to traditional pipe-based drainage systems is rather than treat surface run-off as a nuisance, to treat it as a valuable resource that can be harnessed. This approach tends to not only reduce the risk of flooding in urban areas, but also have numerous knock-on effects, like encouraging biodiversity, promoting better water quality, and improving the amenity of urban areas by paying greater attention to green spaces.       

SuDS can either work by slowing water down before it enters a watercourse (attenuation), used to allow water to soak (infiltrate) into the ground preventing it from entering a watercourse, or can be evaporated from surface water and/or transpired from vegetation (evapotranspiration). Communities using several components often incorporate a combination of all of these to ensure maximum removal of surface runoff.     

SuDs can be small property level mitigation measures, such as a water butt, or large chains of components within a community that tackle excess surface runoff from multiple angles, like the London Sustainable Drainage Action Plan, published in 2015.

Types of SuDS

SuDS can vary in their aim and scale, but are often unobtrusive, natural-looking, and do more than blend into the environment. Rather, they often improve the aesthetic or environmental quality of an area.     

Rain gardens are designed to catch water before it enters the drainage system, providing places rain can soak into, rather than run down streets and off buildings. Not only are they a great habitat for wildlife, but they can also improve the appearance and quality of an area. Green roofs operate in a similar vein by soaking up rainwater from buildings, which are an easy way of incorporating green spaces into otherwise concrete environments. They can retain between 70-80 per cent of runoff in summer months, and can be used on small or large scales. Detention basins control the flow of water and the attenuation of runoff by not only catching water, but also some sediment and particulate pollutants that would otherwise end up in other watercourses. They are easy to design, construct and maintain, and improve the landscape and ecological value to an area. Retention ponds operate similarly, except they are often wet rather than dry like basins. They catch rainfall but also slow release the runoff collected and stored over time to reduce the risk of flooding. Wetlands are very similar to retention ponds.     

Permeable paving is easy to maintain and unobtrusive. It consists of paving stones that are not composed of porous materials, but rather paving with gaps between the flagstones which allow for water infiltration. When used in tandem with greener components, permeable paving is a great way to incorporated SuDS into built-up areas, as they can typically drain double its area, preventing surface runoff that could overflow drainage sites.     

Swales, rather than absorb runoff, intercept it, as they are shallow vegetated channels that move water away and provide temporary storage, which slows down the flow rate. They also remove pollutants by acting as a filter, and are easy to maintain.     

Harvesting rainwater is an easy, scalable operation for individual buildings and local authorities alike as it can be collected in a water butt or larger underground tanks. As well as reducing flooding risks, initiatives to collect rainwater can help reduce waste or expenses through reusing the water to flush toilets or to water gardens.

Benefits of SuDS

SuDS take into account not only the short-term need to prevent excess surface water on roads and buildings, but also the long-term environmental and social factors in being strategic in managing excess rainwater. Their benefits exceed improving human quality of life, such as encouraging biodiversity or having a softer impact on the climate.     

SuDS can reduce the frequency and/or severity of floods through removing or redirecting water from roads and buildings, where it collects, or from traditional pipes and sewers, where it can overwhelm them and cause a surge. The quality of water can also be improved using SuDS, by ensuring less water enters drainage systems. Human development has led to several pollutants such as oil, pesticides, animal waste, litter, and fertilisers that, when it rains, can be washed into sewers and eventually watercourses in surface water runoff. Reducing the number of pollutants, contaminants, and untreated sewage in water bodies can improve the quality of the water, which has health and recreational benefits for humans, as well as opportunities for wildlife and biodiversity. Biodiversity and ecology value underpins other functions within the environment, and so this is an especially important benefit.     

In urban settings, there is an increasing pressure for planners to incorporate green spaces for the benefit of both wildlife and those living in the area. SuDS present a great opportunity to achieve these targets, as well as reducing other risks like flooding. In a twofold approach, SuDS components like ponds and wetlands provide a plethora of amenity, recreational and biodiversity benefits, which improve the wellbeing of those that live and visit the area. Components like ponds also present educational opportunities for children to explore and learn from.     

Ecological or aesthetic SuDS components like trees and green roofs also have the added benefit of improving local air quality, especially in areas where air pollution is a known issue, like in London. These green components are able to remove certain pollutants like nitrogen dioxide or carbon dioxide, and have a positive effect on local health and better quality of life. Green roofs have the added bonus of regulating the temperature of buildings by keeping them shaded in the summer and providing insulation in the winter, cutting bills and reducing energy consumption.     

An additional bonus of SuDS is that there is some evidence that they can stimulate local economic growth, perhaps due to the increased aesthetic value of an area due to green SuDS components, pulling in more customer spending, business investment, or the creation of green jobs. In the UK, green infrastructure has displayed positive employment effects, with developers willing to pay, on average, three per cent more for land close to proximity to open space.  SuDS systems can also provide savings on the overall construction and maintenance costs of drainage schemes, with Lamb Drove SuDS in Cambridge saving ten per cent on capital costs compared with traditional drainage systems.

SuDS in action

SuDS are a recognised initiative in reducing flooding in cities and building climate resilience as our weather continues to change and put more pressure on our mitigation strategies. New planning policy released in 2015 has meant that SuDS should be provided on all developments, particularly major development, and the government have produced guidelines for all SuDS to ensure they uphold standards on water quality and reduce the risk of surface water flooding.

In 2023, not-for-profit Thames21 published their five-year plan on cleaning up the Thames and its surrounding sources, with an emphasis on the role of SuDS to maintain water quality. By 2028, Thames21 has pledged to create two integrated construction wetlands, plant 30,000 trees, and support ten SuDS projects, among many other missions. Collaborating with Enfield Council, The Environment Agency and Thames Water, Thames21 are working on a Salmons Brook wetland system to treat polluted water before it can enter the watercourse.     

In 2023, Thames Water published their drainage and wastewater management plan (DWMP) for 2025-2050 which aims to invest £31.9 billion over the next 25 years, protect 187,000 properties from the risk of flooding, and manage rainwater falling on more than 7,500 hectares of land across the London region. Initiatives like these spotlight SuDS as the forefront of flooding management strategies and work towards implementing them on a variety of scales, and provide inspiration for other towns and cities to take action to manage their stormwater more effectively.