8.6 Choice of SuDS for Streets

The choice of SuDS features will be influenced by the site location and must also factor in SCC’s sustainability hierarchy (Section 8.2) which sets out a preference of methods based on sustainability.

Green Roofs

Lightweight green roofs can be installed on structures, such as bus stops, bin and bike stores, etc. These incorporate a thin layer of soil and vegetation which helps to intercept rainfall, as well as enhancing biodiversity.

Roofs should be designed to be low or zero maintenance.
Planting should be locally appropriate with a variety of flowering species. Sedum roofs should not be used.
An adequate soil depth and a reservoir board layer must be included to maintain moisture.

Permeable Surfaces

The use of permeable surfaces throughout a development can be space efficient, avoiding the need for overly engineered drainage solutions and increasing land for housing or open space. Permeable surfacing can be used in most contexts, depending on the surfacing system used.

Must be designed in accordance with materials guidance in Section 5.10.
Must not be laid over any existing or proposed services wherever possible.
Materials should complement the palette of neighbouring non-permeable materials to create a coherent streetscape.
Must be designed in accordance with the local ground conditions.
The porous sub base should be fully lined and include a connection to the drainage system. Unlined systems, those that allow infiltration into the sub-grade below, should be avoided unless no other solution is feasible. In all cases a geotechnical expert must be consulted.

Rain Gardens

These are landscaped areas that intercept rainfall and allow it to soak into the ground.

Must be relatively shallow with a gentle slope below or be slowly released into the drainage system. They are an ideal feature to use on new and existing streets, particularly in verges and any left-over spaces.
Must provide above ground storage and be set at least 200 millimetres below the adjacent paving level.
Must be protected from vehicles through use of kerbs of other methods.
Planting should be locally appropriate with a variety of flowering species.

Swales and Ditches

Swales are shallow, vegetation lined channels that collect and convey runoff, typically running alongside streets. They slow the flow of water, cleaning it in the process. Swales and ditches can be dry or contain a permanent water level. They are typically grassed but can contain larger planting where appropriate.

Must be accessible for maintenance, such as mowing.
Should ensure a gentle slope profile, typically not steeper than 1 : 3.
Planting should be locally appropriate with a variety of species.
Should be protected from vehicle overrun.

Filter Strips and Verges

Filter strips and verges are gently sloped areas that can slow surface run-off and are very effective at filtering pollutants. Runoff from the filter strip will normally be collected by a French drain, swale, or other linear feature.

Must be accessible for maintenance, such as mowing.
Should be protected from vehicle overrun.
Must be at least 1 metre wide, ideally more than 2.5 metres. However, special requirements must not impact on urban design and layout considerations.

Channels and rills

Channels, often constructed from cobbles or setts and running down the middle of a street, are an effective method of conveying surface water on narrow urban streets. Rills are deeper channels, typically with a permanent water level and planting, also effective for streets where space is constrained.

Should be constructed from durable, attractive high-quality materials such as natural stone.
Rills should be planted to enhance visual amenity and improve water quality, using locally appropriate species.

Tree Pits

Tree pits can be designed to receive additional runoff from adjacent paved areas, reducing runoff and removing pollutants.

Must be designed in accordance with the requirements of Chapter 6.
Must not be allowed to become waterlogged. The infiltration capacity of the soil must be assessed, and positive drainage provided if possible.

Attenuation Ponds and Basins

Ponds and basins are a very effective way of providing attenuation volume and removing pollutants as well as amenity and biodiversity. Multiple ponds can be linked together to spread the benefits around a site. Smaller ponds can be integrated into streets and squares, such as by creating a ‘village pond’, and they can be soft or hard landscaped. Large, single features should be avoided. Basins are normally dry and only flood in extreme rainfall events, and can be used for other functions, such as play. Where ground conditions permit, basins can drain via infiltration into the soils below.

Must be downstream of other SuDS features, as part of the management train, and not used as an ‘end of pipe’ feature.
Should not be allowed to conflict with good density and layout principles. Through careful design it should be feasible to accommodate ponds in denser developments.
Should be multifunctional and provide additional amenity benefit and be integrated into the landscape.
Have due regard to safety and be protected from vehicle overrun.

Soakaways

Soakaways allow infiltration into the ground, they are constructed as either gravel filled pits or trenches, concrete rings surrounded by gravels, deep boreholes or by using proprietary crate systems.

Must be downstream of other SuDS features to provide pollution control, as part of the management train.
Must be situated 5m from any building foundation, increasing to 15 metres in chalk areas. Specialist advice must be sought where soakaways are required in chalk.
Must be designed in accordance with CIRIA34 or BRE35 guidance as well as SCC guidance.

Rainwater Planters

These are a low cost and low maintenance form of SuDS which can be used to collect roof drainage from new or existing buildings, providing a dual function of greenery and interception of rainfall. They are particularly appropriate on constrained sites and high streets.