Sustainable Drainage systems (SuDS) alleviate flooding and associated pollution during heavy or prolonged rainfall. Traditionally, surface water drainage has been combined with foul drainage in many areas. Even though they are separated in new developments, they often discharge into the same combined sewer. During heavy rainfall, the treatment systems cannot cope with this combined effluent, and untreated water is discharged via overflows into watercourses, an obviously unsatisfactory situation. Even where surface water is separated and conveyed directly to watercourses, there are issues with pollution from the surfaces drained and overloading of watercourses, leading to scouring and flooding during severe rainstorms.
The key concepts behind SuDS are:
- Rainwater should be encouraged to infiltrate into the ground within the local area if possible
- Surface water run-off from a development should be as free from pollution as if the area were still in an original natural state
- Rate of flow of surface water run-off should not be greater than that for an equivalent undeveloped area.
SuDS systems use a variety of techniques to control surface water run-off locally:
Filter strips and swales
Filter strips are areas of ground, covered in vegetation, that absorb runoff from adjacent hard surfaces. Swales resemble long shallow ditches, and can have the additional function of carrying the collected water elsewhere. These features slow down the flow of surface water, and remove pollutants through the filtering effect of the planting, which may be slow-growing grasses or wild flower mixtures. Gradients must be gentle to avoid erosion, and deep hollows where planting may become waterlogged should be avoided.
In NBS Building and Landscape, ground shaping, e.g. excavating, filling, and embedded erosion control, is covered in section D20. Grass and wild flower seeding is covered in section Q30. Erosion control using landscape materials is specified in section Q31.
In NBS Create, dry and wet swales are covered in section 50-50-75. Excavating and filling is covered in section 10-35-35. Planting is covered in sections 40-35-00, 40-35-30 and 40-35-35.
Permeable surfaces or paving
Paving is traditionally impervious and laid to falls to gullies or slot drainage, with below-ground pipework to sewers. In a SuDS scheme, this run-off may have local treatment and disposal, e.g. via soakaways. Alternatively, permeable paving may allow below-ground drainage to be eliminated or much reduced.
In NBS Building and Landscape, specify Gravel surfacings in section Q23, Paving blocks with open or coarse aggregate filled joints in section Q24 (which also has guidance on permeable paving), Grass (with reinforcement of various kinds for trafficked locations) in section Q30, and Grass-filled or gravel-filled concrete paving, regarded as ‘semipermeable’, in sections Q21 for the in situ version and Q25 for precast units.
In NBS Create, specify unbound paving systems in section 20-15-85 and permeable unit paving systems in section 20-15-90.
Attenuation and storage below ground
Soakaways, infiltration trenches and filter drains allow retention and gradual release of water to the sewerage system, or into the ground. Water may infiltrate generally, or be piped from a remote location such as an area of impervious paving. Plastics honeycomb units wrapped in geotextile are commonly used to create underground voids where water can be held. In traditional soakaways, an excavated pit or trench is filled with coarse aggregate or rubble. There may also be a geotextile lining and a precast concrete or brick structure to retain the fill. Vertical pipes capped by inspection covers may be used to monitor water levels and assist maintenance. An infiltration trench is, essentially, a linear soakaway, and a filter drain is an infiltration trench with a horizontal pipe to assist water flow.
In NBS Building and Landscape, soakaways, infiltration trenches and filter drains can be specified in section R13. Underground tanks, e.g. constructed of a geotextile lining and plastics honeycomb units, should be specified in section R12.
In NBS Create, appropriate content can be found in sections 50-10-45 (Land drainage), 50-50-05 (Below ground drainage) and 50-50-75 (Sustainable drainage systems).
Attenuation and storage above ground
Flood plains and detention basins are normally dry but flood harmlessly during heavy rainfall. Basins and ponds store water above ground, temporarily or permanently. Balancing and attenuation ponds are permanent water features whose level fluctuates to control flow and reduce flooding. Retention ponds retain the water for longer periods to improve quality. With the addition of specific planting to form constructed wetlands, the quality improvement is even greater, allowing secondary or tertiary treatment of effluent from septic tanks or sewage treatment units.
In NBS Building and Landscape, specify compacted clay or bentonite pond linings in NBS section D20, or sheet material linings, including geosynthetic clay, in section J44. Septic tanks and sewage treatment units are covered in section R12. For planting, including aquatic and marginal plants, see section Q31.
In NBS Create, specify bentonite and other linings in section 30-70-90, ponds in section 50-50-75 and wetland planting in section 40-30-00.
The permissions and approvals necessary for SuDS as the project evolves may be unfamiliar and more wide-ranging compared to those needed for traditional piped drainage. For example:
- Planning permission may be needed for some features
- Building regulation approval is needed where a SUDS system is used for run-off from building roofs and paving associated with buildings
- The sewerage undertaker approves connections to adopted sewers, and sustainable drainage systems that are to be adopted
- The highway authority gives approval of highway construction and drainage
- The local authority should be involved where public land is to be maintained
- The environmental regulator (e.g. Environment Agency, Scottish Environmental Protection Agency) gives consent to discharge, e.g. to a watercourse.