What are SuDS?

“The SuDS approach involves slowing down and reducing the quantity of surface water runoff from a developed area to manage downstream flood risk, and reducing the risk of that runoff causing pollution. This is achieved by capturing, infiltrating, slowing, storing, conveying and treating runoff on site and, where possible, on the surface rather than underground. Water then becomes a much more visible and tangible part of the built environment, which can be enjoyed by everyone.”

The SuDS Manual (CIRIA, 2015)

Sustainable Drainage Systems, or SuDS, are drainage components designed to mimic the way rainwater drains in a natural landscape. Traditionally, rain falling on roads, roofs and pavements has been collected in underground pipes and tanks, which emptied quickly into the nearest sewer or river.

SuDS design, Water Quantity, Water Quality, Amenity, Biodiversity

The Four Pillars of SuDS (CIRIA, 2015)

However, this has led to:

Flooding
Pollution entering rivers
Low river flows in summer months, where rainfall cannot drain into groundwater reserves.

SuDS aim to slow the flow of water, by:

allowing rain which falls on roads, roofs and pavements to soak naturally into the ground
storing water on the surface in ponds and basins

This helps to manage flooding during storms and also naturally filters pollution (such as silt and petrol), preventing it from entering rivers. At the same time SuDS create spaces for wildlife and places for people to enjoy, and make our developments more resilient to climate change.

There are a wide range of SuDS components which either reuse water, allow water to soak into the ground, move water or store water. A SuDS system has several interconnected components which form part of a management train. The management train should begin with managing rainwater as close to where it falls as possible. This is referred to as 'source control'.

The Susdrain website has more information on the principles of sustainable drainage.

Reusing water

SuDS features can be used to collect rainwater and allow it to be re-used in homes and buildings, for example for toilet flushing, or for watering the garden. This also helps to reduce the demand for drinking water. For more information and design criteria, see CIRIA SuDS Manual Chapter 11.

Rainwater harvesting tanks

Rainwater is collected from roofs and other surfaces into a water tank for re-use within a property or a series of properties. More complex systems may include treatment and 'smart rainwater harvesting systems'. This can be particularly beneficial for commercial developments with high water demand.

Water butts

Rainwater is collected from roofs into a barrel. Generally used on individual hoes, it allows small sale water re-use, but no treatment is provided.

Allowing water to soak

In nature, rain which falls on the surface soaks slowly into the ground and enters a store of water beneath the ground. This process is known as infiltration. However when rainfall on a paved surface, such as a roof, driveway or road, it is unable to soak away. Instead it ponds on, or runs off, the ground surface. Infiltration SuDS components mimic nature by allowing rain to soak away where it falls. This is either through using grassy, gravely or specially paved surfaces.

For more information and design criteria, see CIRIA SuDS Manual Chapter 13.

Infiltration basins

Collect and store runoff allowing it to infiltrate into the ground. Vegetation and unsaturated soils can protect groundwater from pollution risks.

Photo credit: Susdrain

Filter drains

Runoff is temporarily stored below the surface in a shallow trench filled with stone/gravel, providing attenuation, conveyance and treatment by filtration.

Photo credit: Susdrain

Soakaways

A below ground structure into which surface water is conveyed, designed to promote infiltration. May be concrete structures with holes or modular crate units. Source control with minimal land take, but provides no nature-based benefits.

Photo credit: DIY Doctor

Permeable pavements

Runoff is allowed to soak through roads and parking areas, either through gaps between the paving blocks or through porous blocks. Water can be stored in the gravel sub-base and infiltrated where ground conditions allow. For more information, see CIRIA SuDS Manual Chapter 20.

Trees

Trees can be stand-alone features, or within tree pits or planters. They improve the performance of infiltration systems as their roots increase infiltration capacity. They also provide biodiversity benefits, shade and cooling, helping developments to be climate resilient. For more information, see CIRIA SuDS Manual Chapter 19.

Green roofs

Planted soil layer on the roof of a building. Water is stored in soil and absorbed by plants. Ideal for storing the 'interception' or 'everyday event' (first 5mm rainfall).

Moving water

When rain falls on a slope, it naturally runs off the ground surface as sheets or forms small streams. In traditional drainage systems, pipes are used to carry (or convey) water from the surface underground, and into the nearest sewer or watercourse. SuDS move water across the ground surface, using channels and wide strips of grass, which also allow some of the water to soak into the ground.

Swale

A wide vegetated channel with shallow side slopes is used to convey and treat runoff by filtration. For more information, and design criteria see CIRIA SuDS Manual Chapter 17.

Channels and rills

Small channels on the surface. Can be built into the hard landscape and/or planted to increase biodiversity benefits.

Photo credit: Cambridge City Council

Filter strips

Runoff from a paved area is allowed to flow across a grassed/vegetated area which binds and collects soluble nutrients, metals, and hydrocarbons. For more information, and design criteria see CIRIA SuDS Manual Chapter 15.

Photo credit: Susdrain

Storing water

When heavy rainfall collects in a low point in the landscape, it builds up and forms a puddle or pond. SuDS act in the same way to store water.

There are two types of storage SuDS:

those which permanently store water (e.g. ponds and wetlands)
those which store water during times of flood and then drain down afterwards (e.g. rain gardens and detention basins).

Water can be stored at a range of scales, from planters on individual properties, to large ponds and lakes.

Planters

Rainwater is collected from roofs and filtered through a layer of plants and soil, slowing the flow and improving the quality of water. Source control method which is ideal for homes, businesses, schools and community buildings.

Photo credit: SuDSPlanter Ltd.

Bioretention systems / rain gardens

A shallow landscaped depression allows runoff to pond temporarily on the surface, filtering it through vegetation and underlying soils. It can then be discharged to a sewer, river or the sea, or infiltrated into the ground. Source control method. For more information, and design criteria see CIRIA SuDS Manual Chapter 18.

Ponds and wetlands

Permanent water bodies used to provide attenuation and treatment of runoff. Wetland vegetation helps to enhance treatment and increase biodiversity. Should be used in combination with source control methods to allow sufficient water treatment. For more information, and design criteria see CIRIA SuDS Manual Chapter 22 and 23.

Permeable pavements

Runoff can soak through paving either through gaps between the paving blocks or through porous blocks. Water can be stored in the gravel sub-base and infiltrated where ground conditions allow. Source control method. For more information, and design criteria see CIRIA SuDS Manual Chapter 20.

Detention basins

Landscaped depression with an outlet that restricts flows, designed to fill during a rainfall event. Usually dry between events. Should be used in combination with source control methods to allow sufficient water treatment. For more information, and design criteria see CIRIA SuDS Manual Chapter 22.

Blue roofs

Blue roofs store water at roof level without vegetation.

Photo credit: Livingroofs.org

Go to the CIRIA SuDS Manual to find out more.....

Developers Designers Community Planners and approvers