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Sustainable Urban Drainage Systems (SUDS)
Traditional drainage practice is designed to move rainwater as rapidly as possible from the point at which it has fallen to a discharge point, either a watercourse or soakaway. This approach has a number of adverse impacts:
- run-off from hard paving and roofing can increase the risk of flooding downstream, as well as causing sudden rises in water levels and flow rates in watercourses
- surface water run-off can contain contaminants such as oil, organic matter and toxic metals. Although often at low levels, cumulatively they can result in poor water quality in rivers and groundwater, affecting biodiversity, amenity value and potential water abstraction. After heavy rain, the first flush is often highly polluting
- by diverting rainfall to piped systems, the amount of water infiltrating the ground is reduced, depleting ground water and reducing flows in watercourses in dry weather
As a result, many urban watercourses are lifeless and unattractive, and are often hidden in culverts under the ground.
The SUDS approach to drainage incorporates a wide variety of techniques. As a result, there is no one correct drainage solution for a site. In most cases, a combination of techniques are required.
Prevention
Prevention plays an essential part in the process. This includes both the prevention of run-off and the prevention of pollution.
The prevention of run-off requires careful consideration of the need to pave a surface – a gravel surface for a car park may be an acceptable alternative – and the disposal of roofwater. For example, it may be possible to discharge roofwater onto a lawn, where it can soak into the ground.
The prevention of pollution includes a wide range of measures designed to tackle urban diffuse pollution. These include:
- Safe oil and chemical storage at industrial sites;
- Safe disposal of waste oil and chemicals from domestic and industrial premises;
- Minimal use of herbicides or the use of alternative techniques for weed control in public areas;
- Proper disposal of wash water from cleaning activities to the foul sewer;
- Steps to prevent the illegal connection of foul sewage flows to surface water sewers;
- A dog litter policy.
Green roofs and rainwater re-use
Green roofs can improve water quality and reduce the peak flow and the total volume discharged from a roof.
In addition, they can enhance insulation and increase the lifespan of the roof. Rainwater reuse (or harvesting) involves the collection and storage of rainwater on site and its use as a substitute for mains water, for example in watering gardens or for flushing toilets.
Permeable pavements
The need for surface water drains and off-site sewers can be reduced or eliminated where run-off is encouraged to permeate through a porous pavement, such as permeable concrete blocks, crushed stone or porous asphalt.
Depending on the ground conditions, the water may infiltrate directly into the subsoil or be stored in an underground reservoir (for example, a crushed stone layer) before slowly soaking into the ground. If infiltration is not possible or appropriate (for example, because of ground contamination), an impermeable membrane can be used with an overflow to keep the pavement free from water in all conditions. Pollutant removal occurs either within the surfacing or sub-base material itself, or by the filtering action of the reservoir or subsoil.
Infiltration trenches and filter drains
Infiltration trenches
An infiltration trench is a shallow, excavated trench that has been filled with stone to create an underground reservoir.
Stormwater entering the trench is gradually infiltrated into the ground. Their longevity can be enhanced by providing pre-treatment of the stormwater using a filter strip, gully or sump pit to remove excessive solids.
Filter Drains
Filter drains are widely used by highway authorities for draining roads.
They are similar structures through which a perforated pipe runs. This facilitates the storage, filtering and some infiltration of water passing from the source to the discharge point. Pollutants are removed by absorption, filtering and microbial decomposition in the surrounding soil. Systems can be designed to successfully incorporate both infiltration and filter systems.
Swales and basins
These can be created as features within the landscaped areas of the site, or they can be incorporated into ornamental, amenity and screen-planted areas where they would be looked after as part of the normal maintenance contract. Swales and basins are often installed as part of a drainage network connecting to a pond or wetland, prior to discharge to a natural watercourse.
Swales
Swales are grassed depressions which lead surface water overland from the drained surface to a storage or discharge system, typically using the green space of a roadside margin.
They may be used to replace conventional roadside kerbs, saving construction and maintenance costs. Compared to a conventional ditch, a swale is shallow and relatively wide, providing temporary storage, conveyance, treatment and the possibility of infiltration under suitable conditions.
Basins
A basin is designed to hold back storm runoff for a few hours and to allow the settlement of solids.
They are dry outside of storm periods. They provide temporary storage for storm water, reduce peak flows to receiving waters, facilitate the filtration of pollutants (deposited and incorporated into the substrate) and encourage microbial decomposition, as well as allowing water infiltration directly into the ground.
By kind permission of The Environment Agency
Wolseley has a number of products when combined can provide effective and practical sustainable urban drainage systems.
SUDS System
The SUDS approach to drainage incorporates a wide variety of techniques. As a result, there is no one correct drainage solution for a site. In most cases, a combination of techniques are required.
