Sustainable Drainage Systems (SuDs)
Learn about how SuDs can reduce flooding…
Sustainable drainage systems (SuDS) offer a more natural approach to managing drainage systems in and around properties and developments than traditional drainage systems. They are designed to temporarily store water during storm events, reduce peak flows and reduce surface water runoff, by mimicking the natural cycle of water management by retaining water where it lands.
In urban areas, where many surfaces are sealed by buildings and paving, natural infiltration is limited. Instead, drainage networks consisting of pipes and culverts divert surface water to local watercourses. In some cases, this has resulted in downstream flooding and deterioration in river water quality. This is caused when foul sewers are overwhelmed by surface water, leading to a release of dirty water into rivers.
In natural environments, rain falls on permeable surfaces and soaks into the ground; this process is called infiltration.
Types of SuDS
Source Control – These deal with water at the place where it falls (the source), reducing the volume of runoff from a particular site by intercepting water and increasing storage whilst slowing infiltration. By storing water close to the source, the water does not flow to a different area and cause problems elsewhere.
There are three types of drainage methods involved in source control, they allow for the interception, infiltration and storage of water and are outlined below:
- Infiltration – Source control methods involve the infiltration of the collected runoff into the subsoil layer. Eventually, this water is returned through the subsoil layer to the water table below.
- Retention – Retention methods involve permanently or temporarily storing surface water runoff in storage areas at the surface and can be in the form of ponds, underground tanks etc.
- Conveyance – These methods involve the transfer of surface water runoff to a point of discharge. There are a number of methods which can be used, for example, underground pipes with no water treatment or through vegetated channels in the surface which allow for some treatment and attenuation.
SuDs examples:
Greening
Natural surfaces can temporarily hold onto water and slow the time between precipitation falling and water entering sewage systems.
Green Roofs
Also known as ‘living roofs’, green roofs have plants growing on them that help to soak up rainfall by increasing infiltration and reducing the amount of runoff. This allows the incorporation of green spaces within properties – even with limited space.
Green roofs are a source control method as they store and hold back water, providing interception storage for, on average, the first 5mm of rain. They remain effective during long and intense rainfall events as they still provide a significant reduction in peak runoff rates, retaining between 70-80% of runoff in summer months. They can be retrofitted to a property; however, this does depend on the structural capacity of the existing roof. They not only help to manage flood risk but also provide noise and sound insulation and provide areas to grow vegetation.
Rain Gardens
They help gardens deal with rainfall more effectively and can be combined with rainwater harvesting measures; they are shallow depressions with absorbent, free-draining soil and vegetation that can withstand occasional inundation.
Rain gardens are an infiltration method which increases the amount of water entering the soil and in turn, reduces rates of runoff and volumes of surface water.
Downpipes are often disconnected from sewers and redirected into the garden; they can also be established in planters which are supplied with water by downpipes in properties with limited space.
They do not require changes to the existing drainage system and can be installed virtually anywhere, having significant advantages for surface water quality and habitat creation as well.
Swales
Swales are shallow, broad vegetated channels used to collect and move water to reduce peak flows in rivers. Their sides are slightly sloping, and they are flat at the bottom. Ideally, they are better installed in areas which do not have steep slopes.
Swales provide temporary storage and infiltration by storing and conveying stormwater runoff. They remove pollutants by encouraging infiltration.
Swales can be ‘wet’ and store water above ground, or, they can be ‘dry’. When they are dry, water collects in a pipe or gravel layer beneath. In wet weather, the rainwater flows down the sloped sides of the swale, along its length and infiltrates through the grass, which acts as a filter, trapping sediment. Some of the runoff is also lost at the surface through evaporation and transpiration.
Further benefits include the reduction in runoff and discharge into local water bodies which helps to reduce flood risk. Swales are also useful for removing any pollutants from runoff and therefore improve the water quality.
Permeable Surfaces
Put simply, this is paving that allows water to soak through, whether in the form of paving blocks with gaps or porous paving which filters water through the blocks themselves. These can be installed at properties and reduce the peak flows to watercourses. Most people opt to have permeable paving on their property; however, some opt for pervious or porous paving which is less common.
Permeable Paving
Permeable paving is not composed of material which is permeable or porous but relies on gaps and voids in the surfacing to allow infiltration. Porous and permeable surfaces allow water to infiltrate through the surface itself, which reduces and delays runoff and peak flows in watercourses.
There are also alternatives to paving such as wood chippings or recycled aggregates which also have the same effect.
Most of the water that falls on hard surfaces within a property will run off into drains rather than being absorbed into the ground, which is not the natural cycle of drainage and increases the risk of flooding elsewhere.
Permeable surfaces can be applied to both front and back gardens in residential areas, as well as in larger, non-residential areas. An advantage of using permeable surfacing is that it can typically drain double its area, allowing for more water to be stored and infiltrated than directly entering the watercourse. If the surface has a good aggregate sub-base, then it can also provide benefits for water quality treatment as well.
Further benefits of installing permeable surfacing are:
- They can be easily applied to properties in both high- and low-density residential areas.
- SuDS allow for the dual use of space, for example on driveways, and do not require any extra land take.
- They are easy to maintain but can lead to weed growth and clogging if they are poorly maintained.
Water Storage
Water storage methods can be either at or below the surface. Water can be stored in tanks on a property for treatment and can often be reused again. Rainwater can be collected from the roof of a building or paved surface and be released slowly into the ground once the storm is over, allowing for natural drainage to take place. This reduces the risk of flooding as water is retained and slowly drains away rather than quickly overwhelming drainage systems.
Rainwater Harvesting – Water butts
Rainwater is collected from the roof of a building or paved surface and stored in over or underground tanks for treatment and reuse in toilet flushing, irrigation and gardening. Water butts are the most common measure used for property level SuDS, providing a source control of rainwater, whilst also reducing a property’s water usage.
Water butts can be used to collect rainwater from roofs by connecting them to the property’s downpipe. An overflow is required to prevent the water butt from flooding and to create capacity for the next storm event. This is also redirected away from the combined sewer and instead connects a perforated hose to allow slow release of stormwater into the ground once a storm event has passed. Tanks, geo-cellular structures and plastic crates beneath the ground can also be used for storing rainwater. They can be used to convey and infiltrate the water back into the ground to reduce peak flows by allowing natural drainage.
As well as reducing flood risk, rainwater harvesting has many other benefits. Firstly, the equipment is easy to maintain and reduces water wastage as the harvested water can be reused again within a property, for example for toilet flushing, irrigation, or watering gardens, as rainwater is often free of chemicals so will not harm plants. In turn, this has a positive effect by reducing water bills as families reuse the harvested water instead of using it more.
