Sewers for Adoption: SuDS at the Heart of New Development
Water UK has just released the 8th edition of Sewers for Adoption, providing clearer guidance for developers on what Sustainable Drainage Systems (SuDS) can be adopted.
Its publication marks a significant moment in the history of SuDS and could lead to a far wider acceptance of their role in surface flood risk management. In the first of a two part blog, we examine the guidance in detail and what this means for developers.
The latest version of Sewers for Adoption (SfA8) sets out standards for adoptable SuDS and which can be adopted by water companies.
SuDS – the preferred drainage method
For a component to be considered for adoption by a sewerage undertaker, surface water drainage components must fall within the legal definition of a ‘sewer’ or ‘lateral drain’ in accordance with the Water Industry Act 1991.
In the Building Regulations and Government guidance to local Planning Authorities (LPAs), there is a clear surface water hierarchy for developers to follow:
a) Surface water runoff is collected for use;
b) Discharged into the ground via infiltration;
c) Discharged to a watercourse or other surface water body;
d) Discharged to a surface water sewer, highway drain or other drainage system, discharging to a watercourse or other surface water body;
e) Discharged to a combined sewer.
Clearly, discharging surface water to an existing sewer is the least desirable. Developers will have to submit compelling evidence, reviewed by the Lead Local Flood Authority (LLFA) and accepted by the LPA that demonstrates that it is not reasonably practicable to discharge surface water to the ground via infiltration or to a watercourse for the whole or part of the site.
Given this and now that the revised code means that water companies are able to adopt effective and well constructed SuDS, developers should look to incorporate them as part of increasingly more viable sites with less onerous drainage maintenance.
What SuDS can be adopted
An adoptable SuDS must have all of the following elements in place:
a) constructed for the drainage of buildings and yards relating to buildings;
b) has a channel (a depression between banks or ridges with a definite boundary);
c) conveys and returns flows to a sewer or to a surface water body or to groundwater; and,
d) has an effective point of lawful discharge into a watercourse or other water body or onto or into land. This right to discharge must be secured by the developer and transferred to the water company on adoption.
There are a number of SuDS components which are not considered to be adoptable by water companies because they do not meet these conditions. These include permeable pavements, green roofs, and filter strips.
They can be included as part of the overall drainage design to minimise surface flood risk, as long as they are upstream of the main SuDS features which are to be adopted.
Understanding groundwater impact and infiltration
It is also important to stress that a water company will adopt surface water run-off from a managed system, such as an approved SuDS. It has no duty to accept land drainage runoff, flows from watercourses or groundwater.
It is therefore essential to understand groundwater emergence risk. High water levels in a groundwater body, watercourse or other surface water body can restrict the discharge of surface water from the adopted system. The design should take into account the likely frequency and duration of these conditions.
The highest groundwater level should be at least 1m below the base of the proposed infiltration component design. If the site is on a sloping profile, its impact on neighbouring sites should also be taken into account.
Understanding the infiltration potential of the soil and subsoil through the hydraulic design of infiltration components is an essential part of the planning submission and should be confirmed by tests in accordance with the CIRIA Manual.
This should be completed and verified via the infiltration potential checklist (page 847 in the guide) and submitted with the S104 Application.
Other Code Conditions
Proximity of SuDS features to buildings: There are no constraints for permeable pavements. If they are concentrated in a small area and there are other proposed design features within 5m of building foundations, sign off is needed from a geotechnical specialist and submitted to the water company with the S104 application.
Retention basins: These need to be at least 500mm below the lowest floor level of any adjacent properties.
Infiltration trench or filter drains: These should be filled with permeable granular material, which promotes rapid of surface water to the ground. They should be designed with a perforated pipe along the whole length and adequate access chambers to provide access for inspection and maintenance.
Soakaways: A point feature without a channel, these temporarily store water before it soaks in to the ground. They must be fed by an upstream channel that is legally a sewer or lateral drain that is also proposed for adoption. It should include the whole structure up to the external face, including any external rubble fill or membrane.
Access to SuDS: Access should be provided so that removal and replacement of any fill material can be effected.
Differences to the Local Authority guidance
SfA8 requires the 1 in 30 year to be stored below ground. However, the LLFA may impose restrictions on the method of discharge of surface water from a development, on the rate and volume of discharge. They can also specify the frequency and duration(s) of the design rainfall used to calculate the discharge, including any allowance for climate change.
Currently an assessment of the impact of the 1 in 100 year plus climate change event is required by the LLFA. A solution will be required to accommodate the additional water that exceeds the 1 in 30 year run-off, by mapping exceedance flow routes and potential temporary storage on the surface in non-essential areas.
It will be necessary to ensure that the SuDS design is able to meet the requirements of both sets of guidance if the scheme is to gain planning permission and be adopted.
Code-Compliant Site Reports
In order that water companies are more likely to adopt the chosen SuDS schemes, developers must ensure that the planning submission has a professional site appraisal.
Our SuDSmart reports are compliant to the new Water UK SFA code, identifying the surface water infiltration and groundwater risk. You can add an outline scheme design to the desktop report, to meet the 1 in 30 chance rainfall event including runoff calculations and exceedance flow requirements.
There is a clear net benefit to spending time implementing the right SuDS: It reduces flood risk for residents, adds amenity and quality and can raise the value of property with attractive environments, especially in urban areas.
Now, the right design can also support adoption by the water company, so they can be seen less as a hindrance and more of an enabler.
The second part of this blog will look at Drainage Strategy and design requirements in the new code.
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