New Sewerage Sector Guidance documentation published
New Sewerage Sector Guidance documentation has been approved by Ofwat under its Code for Adoption Agreements, and has been implemented from 1 April 2020. The rules will allow English water and sewerage companies to adopt a wider range of sewer types, including those with sustainable drainage (SuDs) elements.
Work to prepare the 8th edition of Sewers for Adoption, SFa8 , was superseded by Ofwat’s decision to require all water and sewerage companies to commit to a single, mandatory national approach to sewer adoption. The rules governing the transfer can be found in the Design and Construction Guidance (DCG) documentation.
The change is very significant for SuDS and could lead to far wider implementation as part of SuDS’ role in surface flood risk management. We assess the guidance in detail and what this means for developers in this post.
What SuDS can be adopted?
The Sewerage Sector Guidance states that sewer elements in public open spaces will be potentially adoptable by the water and sewerage company, if they serve more than one property and meet the criteria set out in the DCG. Detention Basins, Swales, Rills, Under-drained Swales, Ponds, Wetlands, Infiltration Basins and Soakaways are all suitable candidates for adoption. Ultimately, it is for the water and sewerage company to assess the assets that are being offered for transfer which must have all of the following elements in place:
a) constructed for the drainage of buildings and yards relating (appurtenant) to buildings;
b) has a channel (a depression between banks or ridges with a definite boundary, inlet and outlet);
c) conveys and returns flows to a sewer or to a surface water body or to groundwater;
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.
e) Some infiltration into the system may be allowed provided that is not the designed purpose of the system.
The following are excluded from adoption or represent negative criteria:
a) Watercourses as defined in law
b) Systems built primarily for the drainage of surface water from streets or for the drainage of land
c) Systems built to manage groundwater
d) Suds that form part of the structure of a building or yard or an integral part of the structure of a street
e) Systems that forms part of a private curtilage
The water and sewerage company will need to be satisfied that the proposed minimum standards of operation and maintenance are appropriate, for both adoptable elements and any connecting non-adoptable elements of the system. The water company will be responsible for the conveyance and storage functions of the SuDS features including the surface and contents of the channel, such as vegetation, inlets, outlets and flow control devices, up to the extent of the asset needed to deal with 1:100-year rainfall events plus climate change.
There are a number of SuDS components which are not considered to be adoptable by water companies because they do not meet the DCG criteria. These include private drainage features, such as water-butts, rainsave planters, permeable paving, and green roofs, which are classed as building drainage, even where flows from more than one property are conveyed. These will remain the responsibility of the homeowners. 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. Any system with the main purpose of highway drainage is not adoptable.
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. Sewers are especially vulnerable to groundwater ingress forced through cracks in the access chambers or the pipes themselves. 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 Application.
SuDS –hierarchy of drainage options
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.
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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