Sustainable Drainage Systems (SuDS): guidance for developers

Infiltration rate testing must be carried out in accordance with Building Research Establishment Digest 365 Soakaway Design (2016) standards, in the location and to the effective depth of the proposed component, with test pits draining beyond the 25% effective depth.

A minimum infiltration rate of 1x10-6 m/s must be achieved.

If groundwater level monitoring is required, groundwater monitoring must be undertaken during the winter months (from November until April).

The base of the soakaway must be at least 1m above the groundwater level to protect the functionality of the infiltration process.

Shallow soakaways must be located a minimum of 5m away from any building foundation, retaining structure, highway or site boundary.

Half-drain time of the soakaway must be within 24 hours for the 1 in 30 year rainfall event.

Half-drain time for the 1 in 100 year storm event.

A separation distance of 10m between individual soakaways, if separation distance cannot be achieved calculations must account for reduction is capacity.

Maximum impermeable area of 1000m2 served by individual soakaway.

Soakaways must not be constructed in contaminated ground.

Maximum side slopes of 1 in 3, however, 1 in 5 slopes are preferred for maintenance access

Base width of between 0.5 and 2m to prevent channelling and erosion

Maximum depth of swale must be no deeper than 600mm

Underdrains should use a perforated pipe with a minimum diameter of 100mm, with 150mm of clean gravel above the pipe, enclosed in a geotextile fabric which must be shown on construction drawings

For the 1 in 1 year event the maximum flow velocity must be no greater than 1.2l/s

If velocity exceeds the recommendation, then erosion reduction measures are required

For extreme events the maximum flow velocity must be no greater than 1l/s ( 1in 100 year plus climate change event)

For the requirements for infiltration swales see ‘Shallow Infiltration Systems’ above

For slopes 3% (1:20) or greater, internal terracing or check dams should be incorporated

To accommodate services, an impermeable central carriageway might be employed to contain services, visually differentiated from pervious parking bays.

For the requirements for Type A permeable paving see ‘Shallow Infiltration Systems’.

A minimum infiltration rate of 1x10-8 m/s must be achieved.

Discharge receptor following the drainage hierarchy.

Lined permeable paving can be used when the water table is within 1m of the sub-base, and/or where the site is contaminated.

Discharge receptor following the drainage hierarchy.

Floatation calculations where high groundwater levels are encountered.

Liner to be shown on construction drawings.

Any pond, wetland, infiltration basins or detention basins must be designed/carried out to the following standards:

• maximum side slopes of 1 in 3, however 1 in 5 side slopes are preferred
• maximum depth for permanent open water areas of 1.2m
• maximum depth of water should not normally exceed 2m in the most extreme design event
• minimum 300mm freeboard between bank-full and the 1 in 100 year plus climate change
• demonstration of overspill for extreme events
• they must be natural in shape, with topography and vegetation reflecting the landscape and enhancing the character of the area

In locations with high groundwater levels, the ponds, wetlands and detention basins must be lined and floatation calculations provided.

Ponds, wetlands, infiltration basins and detention basins should have appropriate access to allow maintenance to be carried out. Access is needed to all inlets, outlets and control structures. Access should be 3.5m wide.

You may require reinforced grass where vehicles will need access. Fencing is not desirable, however toddler proof fencing, 600mm to 750mm is acceptable where required.

The requirements for infiltration basins is set out above in ‘Shallow Infiltration Systems’.

Detention basins must:

• have a low flow channels should be incorporated
• be designed as multifunctional spaces within a development

Above-ground SuDS components must be utilised unless the applicant can robustly demonstrate that they are not feasible; even where significant constraints exist, above and below ground components can be used in combination.

Effective upstream treatment is an important consideration to control the risk of the system performance being compromised by sediment accumulation.

Complicated shapes with lots of angles should be avoided.

In locations with high groundwater levels floatation calculations must be provided.