Sustainable Urban Drainage Systems (SUDS)

Technical author John Glen dips his toes into some unwanted urban water, and shows how many NBS work sections can contribute to resolving this issue.

Many specifiers are aware of the basic concepts of Sustainable Urban Drainage Systems (SUDS), but may be unfamiliar with the specific techniques used, and where they can be specified in NBS. This article gives a brief description of some of the options, together with some notes on the approvals and consents needed, as these can be more wide-ranging than for a conventional drainage system.

Traditionally, surface water drainage has been combined with foul drainage in many areas. Even though they must usually be 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.

SUDS systems use a variety of techniques to control surface water run-off locally, avoiding or considerably reducing these problems.

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, 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.

Permeable surfaces or paving

Paving is traditionally laid to falls to gullies or slot drainage, with below-ground pipework to sewers. In a SUDS scheme, the run-off may have local treatment and disposal, e.g. via soakaways. A more radical solution is to use permeable paving, where the use of below-ground drainage can be eliminated or much reduced.

The alternatives and their locations in NBS are as follows: Gravel surfacings are specified in section Q23. Paving blocks with open or coarse aggregate filled joints are specified in section Q24, which also has guidance on permeable paving. Grass, with reinforcement of various kinds for trafficked locations, is specified in section Q30. Grass- or gravel-filled concrete paving, regarded as ‘semipermeable’, is specified in sections Q21 for the in situ version and Q25 for precast units.

Soakaways, infiltration trenches and filter drains

These features all include some degree of below-ground water storage. The release of water to the sewerage system, or into the ground, can thereby be delayed. Water may infiltrate generally, or be piped from a remote location such as an area of impervious paving. 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. A filter drain is essentially an infiltration trench with a horizontal pipe to assist water flow. Alternatively, plastics honeycomb units, again wrapped in geotextile, can be used to create underground voids where water can be held.

Soakaways, infiltration trenches and filter drains can be specified in NBS section R13. Underground tanks, e.g. constructed of a geotextile lining and plastics honeycomb units, should be specified in section R12.

Basins and ponds

These store water above ground, temporarily or permanently. Flood plains and detention basins are normally dry but flood harmlessly during heavy rainfall. 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.

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.

Regulatory framework

Because SUDS is a new and developing phenomenon and departs radically from traditional ‘pipe away and treat remotely’ systems, the permissions and approvals necessary as the project evolves may be unfamiliar and conflicting. 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 sewerage 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.