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What are SuDS?

Environment news round-up Detention basin

An increasing number of development projects, both residential and commercial, refer to something called SuDS. The UK Government is also in the process of implementing legislation that requires all new developments in England to include them. But what are SuDS and why are they so important?

Sustainable Drainage Systems

In non-built-up areas, rain soaks into the ground fairly easily due to its permeable nature in a process called infiltration. In an urban environment, however, buildings and paving seal off the underlying soil. This means that less surface water is able to soak into the ground. There is also less vegetation to absorb water. Traditional methods of dealing with this water include culverts and pipe and sewer networks which transfer the water elsewhere. However, a combination of aging networks and increasingly wet winters means that these are often overwhelmed. As well as leading to an increased risk of flooding, there is also a pollution risk. Inundated sewer systems overflow, and contaminated water ends up in rivers and streams.

Avon flooding
SuDS can help prevent urban flooding

SuDS are sustainable drainage systems. They are alternatives to pipe and sewer systems and use elements of natural water drainage mechanisms to cope with excess surface water. Crucially, they deal with this excess water at or near the place where it falls. Some SuDS, such as water butts and tanks, collect rain for use in the household or garden. Others, including ponds and wetlands, allow water to slowly drain naturally into the ground whilst also filtering it of pollutants, thus improving the water quality of nearby watercourses. Many not only help alleviate the risk of flooding but increase a development’s biodiversity by attracting wildlife. They can also enhance the appearance of a development and improve residents’ or workers’ wellbeing by bringing nature closer. If not part of a development when it is first built, many SuDS can also be retrofitted.

Types of SuDS

The types of SuDS appropriate for a particular development depend on a number of factors. These include available space, local soil type, type of development and whether the system is part of the original work or being retrofitted. Most are low-cost and easily installed.

Detention Basins and Retention Ponds

Basins and ponds are similar but differ in that detention basins only store excess water temporarily. During dry spells, they are usually flat, grassy areas that can be used for various recreational activities. After heavy rainfall, however, they are designed to hold excess water until it has drained away. Ponds, meanwhile, have some water in them permanently but have capacity for any excess during wet periods. Both attract wildlife and are attractive amenities. They are an increasingly important part of many sustainable developments, such as the Brooks Dye Works site in Bristol. They can also be added to urban settings at a later date, such as Cumbernauld’s SuDS ponds.

Detention basin
Detention basins are designed to hold excess water temporarily


Wetlands usually include a range of different zones and often cover a larger area than basins or ponds. Deeper ponds combine with shallower marshy areas to create a mosaic of habitats. They are designed to hold excess water for prolonged periods, drastically reducing the rate of surface water runoff. This in turn allows pollutants to filter out and sediments to settle. Any excess that does then reach nearby watercourses has higher water quality. Wetlands are well vegetated which also helps retain water and filter contaminants. They are also an important biodiversity boost for a development, as well as being attractive amenities.

Swales and Bioretention Areas

Swales are broad, shallow, linear channels, lined with vegetation. As well as temporarily storing excess water, they transport it elsewhere, thus reducing flood risks. In addition, the vegetation and soil help filter out pollutants and sediment. Bioretention areas are similar but aren’t necessarily linear and use gravel and/or sand along with vegetation to filter water and remove pollutants. Both can be incorporated into sites in a range of ways that don’t use additional land, unlike ponds and wetlands. These include on verges, traffic islands and parking areas, thus offsetting the high levels of impermeable paving these features usually have. Swales and bioretention areas are easier to retrofit than basins and wetlands due to their smaller size yet provide the same benefits.

Swales are vegetation-lined channels

Permeable Paving

Standard hard paving allows little or no water to permeate to the soil underneath. This increases the amount of surface water and the risk of flooding. Permeable paving uses porous stone and/or gaps between slabs to allow water to infiltrate better and reduce runoff. They can also help filter out pollutants, depending on the type of subsoil. It can be used in residential and commercial settings, and it is reasonably easy to implement. They can be incorporated in individual drives, on whole streets or entire districts.

Permeable paving
Permeable paving may be made of porous material and/or have gaps between slabs

Grass Roofs

Green, or living, roofs are entirely or partly covered in vegetation. These plants, and the material they grow on, help hold water and reduce runoff. Although there is a limit to the amount of water they can soak up, they can still have an impact. Roof-top vegetation also helps to reduce energy use by adding insulation to a building in winter and cooling it in summer. Additional benefits include increased noise insulation and wildlife attraction, as well as counteracting the urban heat island affect caused by lack of green spaces in cities. Green roofs can be included at the build stage or retrofitted, provided the existing roof’s structure is suitable. They can also be installed on single properties or on whole developments. Bristol City Council is even planning to install them on bus shelters and has already put them on some toilet blocks in the city centre.

Green Roofs
Green roofs can be installed on big projects, like these apartments in Bergen
Green roof Bristol
They can also be placed on smaller buildings such as these old toilets in Bristol

Rain Butts, Tanks and Rain Gardens

Another beneficial aspect of SuDS is their potential for harvesting rain for use in the home and garden. This means they can help to reduce water consumption at the same time as preventing flooding. Downpipes can be easily disconnected from the sewer system and diverted to water butts or tanks. At peak rainfall times, this takes water away from overstretched pipes and enables water to be stored for use in toilets or the garden. Alternatively, downpipes can be directed via channels to rain gardens. Rain gardens are shallow dips that collect excess water and allow it to slowly drain. Water tolerant plants are planted in the areas likely to be inundated. Rain gardens are easy to install at the individual householder level and are beneficial to wildlife.

The Future of SuDS

As our climate warms, the UK is predicted to see warmer and wetter winters. Our existing aging water treatment systems are already finding it difficult to cope with periods of heavy rainfall, leading to flooding and pollution events. SuDS provide a cost-effective and natural means of alleviating these risks. They not only help to prevent flooding by reducing surface water runoff, but also help filter out contaminants and improve the water quality of nearby streams and rivers.

In light of this, the UK Government has confirmed that it will at last be implementing Schedule 3 of the Flood and Water Management Act 2010. This makes it compulsory for all new developments to include SuDS that have met local authority approval. Implementation is likely to take place sometime in 2024. As a result, development projects will soon all have to incorporate SuDS into their designs. However, individual homeowners can also use smaller scale SuDS to reduce the risk of local flooding and increase biodiversity. SuDS are therefore going to be an increasingly important tool for alleviating the effects of climate change.

Further Reading

Remember to check local planning laws before beginning any SuDS installation, even at the individual house level. Permeable paving, for example, may be subject to planning laws if the area you plan to cover on your property is over a certain size.

For developers and homeowners alike, the susdrain website is a brilliant resource. This community contains a wealth of information for anyone involved in installing SuDS of all kinds. As well as lots of detail about the benefits and drawbacks of individual types of SuDS, there are lots of case studies of real projects.