Who can certify a drainage design for adoption

There is no statutory certificate for a drainage design and no register of people entitled to sign one. Who certifies drainage design for adoption in practice is the water and sewerage undertaker, through the design check it carries out before entering an adoption agreement under section 104 of the Water Industry Act 1991. The designer's job is to produce a submission that survives that check. The undertaker's job is to satisfy itself that it can own and maintain the asset for the next century.

That distinction trips up developers who expect a chartered signature to be the deciding factor. It helps, but the undertaker checks the design regardless of who signed it.

What the undertaker checks before entering an agreement

The adoption process in England runs under the Code for Adoption Agreements, with the technical requirements in the sector's design and construction guidance and the procedural requirements in the sewerage sector guidance. The check covers:

• Compliance of the layout, materials, gradients, cover depths and manhole arrangements with the design and construction guidance.
• Hydraulic performance, demonstrated by modelling for the design storms the guidance requires, including the surcharge and flooding checks.
• Capacity in the receiving network, and whether the point of connection is acceptable.
• Land rights, so that every length of adoptable sewer is within a dedicated highway, a public open space or an easement in a form the undertaker accepts.
• Build-over, proximity and tree planting arrangements where the layout brings structures near the sewer.
• The bond or other security, sized against the value of the works.

Items that fail most often are land rights and the connection point, not the hydraulics. A pumping station in a rear garden with no vehicular access, or a sewer running under a plot curtilage with no easement, will stop the agreement regardless of how good the modelling is.

Designer competence, indemnity and the design check

What the undertaker and the developer both look for is a designer who can carry the technical risk. In practice that means a civil engineer or drainage specialist with demonstrable experience of adoptable schemes, professional indemnity insurance at a level proportionate to the scheme, and familiarity with the current edition of the design and construction guidance rather than the version in force when they last did one.

Several undertakers operate accredited or approved designer arrangements, under which a pre-qualified designer can self-certify parts of the submission and the check becomes an audit rather than a full review. Where such a scheme exists it shortens the programme considerably, so ask early whether the undertaker in your area runs one and whether your designer is on it.

Professional membership is a reasonable proxy for competence, and chartered status with one of the engineering institutions is what most developers specify. The better test is the same one that applies to other specialist authors: ask for a redacted example of a previous adoption submission with the hydraulic calculations included. The approach to checking a specialist's credentials is set out more fully in the guide to who can prepare a transport assessment, and the logic transfers directly.

Hydraulic modelling against layout drafting

These are two different skills and they are frequently held by two different people.

Layout drafting produces the drawing: pipe runs, manhole positions, levels, cover depths, connection details. It is drawing office work and it can be done well by a technician who knows the guidance.

Hydraulic modelling produces the evidence that the system works: rainfall inputs from the current national rainfall model, the climate change allowance applied to peak rainfall intensity, pipe-full and surcharged conditions, attenuation volumes, flow control settings, exceedance routing for events beyond the design standard, and the half-drain-down time for infiltration and attenuation features. That work needs someone who understands why the model is producing the answer it is.

A submission with beautiful drawings and a thin modelling appendix is the commonest form of weak design, and it is the one that produces a round of comments that costs six weeks.

Where the lead local flood authority reviews instead

Adoption and planning approval are separate processes with different reviewers, and conflating them causes real programme problems.

The lead local flood authority is interested in the surface water hierarchy, the discharge rate relative to greenfield run-off, water quality treatment and long-term maintenance. It does not adopt anything. Schedule 3 to the Flood and Water Management Act 2010, which would create SuDS approving bodies with their own approval and adoption duties, has not been brought into force in England, and government has signalled an intention to implement it, so check the current position before assuming either way.

Where surface water features are not adoptable by the undertaker, somebody has to own and maintain them, and the authority will want that arrangement secured through a management company, a planning obligation or a condition. Set out the whole picture in the Drainage Strategy rather than leaving the maintenance question for the discharge of conditions stage.

Site supervision and as-built records

An adoption agreement is not complete at design stage. During construction the undertaker inspects at defined hold points, and the works have to be tested before vesting: air or water tightness tests, CCTV survey of every length, infiltration testing where relevant, and commissioning records for any pumping station.

The record set the undertaker needs at the end is specific. As-built drawings showing what was actually built rather than what was designed. A manhole schedule with cover and invert levels surveyed. The CCTV survey with the coding report. Test certificates. Pumping station operation and maintenance manuals and electrical certification. Confirmation of the easements and transfers.

Keeping that record as the works proceed costs far less than reconstructing it at the end, and the maintenance period and bond release both depend on it.

Briefing a designer for an adoptable scheme

Give the designer the things that control the design before they start, because retrofitting them is expensive. The topographical survey with levels tied to Ordnance Datum. The point of connection confirmed by the undertaker in writing. Infiltration testing to the recognised method, carried out at the right depth and in the right season. The flood risk position and any watercourse consent requirements. The layout, including plot curtilages, adopted highway extents and open space, so that easements can be designed in rather than discovered.

And tell them whether adoption is the intention from the outset. A scheme designed as a private system and then put forward for adoption usually needs redesigning, because the private layout puts pipes where no easement is available. Where foul capacity is the constraint, the capacity enquiry evidence belongs in a Foul Sewage Assessment and the wider service connections in a Utilities Assessment. For schemes where an independent check is wanted before submission, the professional directory is the route to a reviewer.

This is general guidance, not legal advice. Adoption codes, design guidance editions and the status of Schedule 3 to the 2010 Act change, so confirm the current requirements with your water undertaker and lead local flood authority before designing.

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