Surface water and highway drainage: who takes the flow

There is no right to connect site surface water into a highway drain. Section 106 of the Water Industry Act 1991 gives a right to communicate with a public sewer, and nothing equivalent exists for highway drainage, which belongs to the highway authority and is there to drain the road. A highway connection is a permission the authority may grant, usually on terms, often with a commuted sum, and sometimes not at all. That single point changes how a drainage strategy should be written.

It also changes the order in which you ask. Many schemes arrive at a highway connection because the soakaway failed and the sewer is on the wrong side of the site, which means the application is a request for a favour rather than the next step in a hierarchy. Highway authorities notice.

Two systems that meet at the kerb

A development's surface water system and a highway drainage system have different owners, different design standards and different purposes. The road gullies, carrier drain and outfall exist to keep the carriageway safe in a storm, sized for the road's catchment and no more. Highway drains are generally vested in the highway authority under the Highways Act 1980, and they are not public sewers unless they form part of a combined system vested in the sewerage undertaker, which happens often enough in older towns to be worth checking.

The first job on any site is therefore to establish what the pipe in the road actually is. Ask the highway authority for its drainage records, ask the sewerage undertaker for an asset plan, and compare them. On a surprising number of roads the answer is that nobody is certain, and a short CCTV survey costs less than a wrong assumption carried into a planning condition.

The hierarchy that decides whether you should be asking

Approved Document H and the national guidance on sustainable drainage both set the same order of preference: dispose of surface water to the ground by infiltration, then to a surface water body, then to a surface water sewer, combined sewer or highway drain. A highway drain sits at the bottom with the sewers.

So the Drainage Strategy has to show the hierarchy was worked through rather than asserted. In practice that means:

1. Infiltration tested, not assumed. Soakage tests to the recognised method, at the depth and position of the proposed feature, with the groundwater level recorded and the season noted. A single trial pit in August is not evidence.
2. Watercourses identified within a reasonable distance, including ditches that only appear on historic maps, with the riparian ownership and the route of any easement set out.
3. Sewer records obtained and the levels checked, because a surface water sewer 200 metres away at the wrong invert is as useless as no sewer at all.
4. Only then the highway drain, with an explanation of why the options above are unavailable.

Lead local flood authorities are statutory consultees on surface water drainage for major development, and their response is where an unevidenced hierarchy gets caught.

Capacity evidence a highway authority will accept

Highway authorities refuse connections for two reasons: there is no spare capacity, and they do not want someone else's water and someone else's liability. Evidence addresses the first and the agreement addresses the second.

What tends to be asked for:

• A CCTV survey of the relevant length, with a pipe schedule giving diameters, materials, gradients, invert levels and condition grades.
• A hydraulic assessment of the existing highway system for the design event, demonstrating the spare capacity available at the proposed connection point and the effect of the additional flow upstream and downstream.
• The proposed discharge rate and volume, with the flow control device specified.
• Confirmation that the connection point is a manhole or catchpit rather than a gully pot, since connecting to a gully is almost always refused.
• Evidence that the receiving outfall can take the flow, which is the step most often skipped. A highway drain with spare pipe capacity and a surcharged outfall has no capacity at all.

Where the road is part of the strategic network, National Highways applies its own process and its own standards, and the timescales are longer than a local highway authority's.

Attenuation and treatment ahead of the connection

Almost no highway authority will accept an unrestricted connection. Expect a restriction to a stated rate, usually derived from the greenfield run-off rate for the contributing area, with a practical minimum below which a flow control becomes prone to blockage.

Two design consequences follow. You need attenuation volume for the design event, typically the 1 per cent annual exceedance probability storm with a climate change allowance applied, and you need to show the exceedance route for anything larger. And you need a treatment train, because the authority will not accept hydrocarbons and silt from a car park into its system. A sequence of permeable paving, a filter or bioretention feature and a silt trap or interceptor ahead of the control chamber is the usual answer, designed to the pollution mitigation approach in the recognised drainage guidance.

The flood risk evidence that sits behind these figures belongs in the Flood Risk Assessment, and the two documents should share one set of rates and volumes rather than quoting slightly different numbers, which happens more than it should.

The agreements, and who maintains what

Three instruments usually appear, and they do different jobs.

Works in the public highway, including the connection itself, the chamber and any reinstatement, are carried out under an agreement under section 278 of the Highways Act 1980, with a design approval, an inspection fee and a bond. Where new estate roads are to be adopted, a section 38 agreement covers the road and its drainage, and the drainage design has to meet adoption standards from the outset rather than being retrofitted.

A commuted sum is common. The authority is taking on a flow it did not design for, and it will often ask for a capital payment towards future maintenance of the length affected. Ask early what the basis of the calculation is, because the figure can be material on a scheme with a large impermeable area.

Then there is the demarcation. Fix the ownership boundary at a defined chamber, draw it on the drainage plan, and record it in both the section 278 agreement and the management company's documents. The commonest dispute five years later is about who clears a blocked control chamber that sits half in the verge.

Schedule 3 of the Flood and Water Management Act 2010, which would create a separate SuDS approval regime, is still awaiting commencement in England, so for now adoption and maintenance are settled through these agreements and through planning conditions rather than through a statutory approval body. Verify the commencement position before relying on it.

When the connection is refused

A refusal is not the end of the scheme, and it is better received at feasibility stage than at the discharge of conditions.

The routes back, in rough order of preference:

• Re-test infiltration properly, at depth and in a different part of the site. Permeability varies enormously across a single field, and a competent ground investigation sometimes finds a layer the first test missed.
• Find the watercourse. Works to an ordinary watercourse need land drainage consent from the lead local flood authority, and the process is set out in land drainage consent for works to an ordinary watercourse. An easement across third party land is often the real obstacle rather than the consent.
• Negotiate a connection to the sewerage undertaker's surface water sewer under section 106 of the Water Industry Act 1991, where one exists, since that right does exist and the undertaker's conditions are at least predictable.
• Reduce the discharge. Rainwater harvesting, green roofs, extra attenuation and permeable surfacing can cut the rate and volume to something the authority will accept.
• Reduce the impermeable area, which is the only option that changes the layout and the one clients resist until the alternatives have been priced.

Start the highway drainage conversation in the first month of a project, not after the layout is fixed. A connection point dictates the levels of the whole site, and levels dictate everything else.

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