What are SuDS?

Sustainable Drainage Systems (SuDS) are engineered solutions that are designed to reduce the quantity of surface water runoff that drains into local sewer networks, while also improving its quality. SuDS can manage rainfall close to where it falls, which is where it differs from traditional drainage, all the while mimicking natural systems. Here we explain more about SuDS, what they are, the different types and why they are so important.  

SuDS are designed to reduce the impact of rainfall on new developments in urban areas. Buildings and paving seal a large number of surfaces, which limits the amount of natural infiltration that can occur.

Drainage networks play an important role, helping to divert surface water to local watercourses to lower the risk of flooding. However, there are still instances where downstream flooding occurs, which happens when sewers receive too much surface water, leading to the release of dirty water into nearby rivers.

SuDS are intended to minimise these problems by decreasing flow rates to watercourses, storing or re-using surface water at source and generally improving water quality.

The introduction of a SuDS scheme offers numerous benefits, such as:

• Flood risk management by reducing the risk of flooding
• Water quality management by preventing pollutants from urban runoffs washing into groundwater or local rivers
• Biodiversity and amenity are improved by creating recreational, habitat and biodiversity areas
• Additional urban growth occurs through the building of new developments in areas impacted by established drainage networks and SuDS is part of planning permission requirements
• Groundwater supplies are recharged, and rainwater can be captured and harvested for reuse
• Peak flows to treatment works are reduced using SuDS

There are a number of different types of SuDS and they can be put to use both above and below ground level. Some involve the use of landscaping, while others rely on manufactured products to manage the flow of water.

A variety of factors must be taken into consideration before being chosen for a particular project and traditionally it is divided into two different types: engineered (grey) and soft (green).

Engineered SuDS are best used for high density, commercial and industrial developments and include infiltration tanks, geocellular attenuation and permeable paving solutions.

These types of SuDS are often used to manage surface water runoff and are typically modular in design, which allows them to adapt to their surroundings, while also being easy to install. They enable the storage of large volumes of water in a relatively small site, while capacity and flow rates can be accurately measured which makes it easier to model and predict future events.

Examples of landscaped SuDS include ponds, swales and rills, all of which can be built into the existing landscape to collect water during periods of heavy rainfall. Once collected, the water can then be slowly dispersed into the soil. In some instances, these features may also include drought resistant vegetation that is used to slow down the flow of water and clean it.

Once water starts to flow through the framework of a SuDS, the flow velocity is managed and controlled while pollutants are also removed. The process often includes the following stages:

Source control

This is used to reduce the volume of water passing onto the drainage or river network. Runoff water is intercepted from roofs for re-use (irrigation, for example) or to be stored.

Pre-treatment

Filter trenches or vegetated swales offer pre-treatment solutions that remove pollutants from the water before it is discharged to aquifers or watercourses.

Retention systems

The discharging of surface water to watercourses is delayed through use of these systems. Retention basins, ponds or wetlands are some examples of this.

Infiltration systems

This can be things such as soakaways or infiltration trenches that can mimic natural recharge, enabling water to soak into the soil slowly and safely.

There are a variety of stormwater solutions that can be used for different applications, depending on the location, area size and amount of storage needed:

Stormwater crates

Stormwater crates can be installed underground and are modular in design, allowing them to be constructed in a variety of ways if needed. They are a cost-effective solution that simply connect together and store water before it is discharged.

Stormwater attenuation tanks

Another good way to minimise the risk of flooding, attenuation tanks are used to store water before discharging it to a local water course. They aren’t limited in size or shape and offer a bespoke and cost-effective solution for a variety of applications.

Stormwater infiltration tunnels

These are ideal for private and rural locations and help to save on space while remaining a cost-efficient option. The equipment is lightweight, and installation is easy, offering a way to store stormwater safely and effectively before disbursement.

As set out in the manual produced by the Construction Industry Research and Information Association (CIRIA), SuDS design should always take the following into account:

Water quantity

This refers to the controlling of runoff to support the flood risk management, while maintaining and protecting the natural water cycle.

Water quality

To prevent pollution, the quality of the water runoff must be managed, ensuring that sediment and contaminants are removed before being discharged.

Amenity

The creation and sustainability of better places for people is another important factor that must be considered, adding value through the construction of play areas, walkways etc.

Biodiversity

This refers to the creation and sustainability of better places for wildlife and ensuring that habitats are not negatively affected by new developments. Use of swales or balancing ponds are particularly affective at doing this.