The surface water discharge hierarchy and how to evidence it

The surface water discharge hierarchy sets the order in which you have to consider getting rainwater off a site: into the ground first, then to a surface water body, then to a surface water sewer or highway drain, and only as a last resort into a combined sewer. Councils and lead local flood authorities apply it strictly, and the reason applications get stuck is almost never the drainage design. It is the absence of evidence that the higher options were properly tested.

A sentence saying the ground is clay does not discharge the first step. Neither does a phone call to the water company.

Where the order comes from

The hierarchy appears in Part H of the Building Regulations for the building control side, and it runs through national planning guidance on flood risk and through the non-statutory technical standards for sustainable drainage systems. Most lead local flood authorities publish their own version with local additions, and that local document is the one your drainage strategy will be checked against.

The principle behind it is simple. Water that soaks into the ground where it falls behaves as it did before the site was developed. Water in a pipe has to go somewhere else, and the further down the hierarchy you go, the more of somebody else's capacity you are using.

You have to work down the list in order and show why each step was rejected before moving to the next. Skipping to the answer you wanted at the outset is the pattern reviewers spot immediately.

Discharge to ground

Infiltration is the first option and it needs real testing.

Commission in-situ soakage tests at the location and depth of the proposed feature, carried out to a recognised method with repeat fills at each pit, and report the actual infiltration rates. Tests in the wrong place, or at 1 metre when the feature will be at 2.5 metres, prove nothing about the design.

Groundwater is the other half of the question. Infiltration features need a clearance above the highest groundwater level, so a single dip in August is not enough. Monitoring through the wetter months gives the number a reviewer will accept.

Then the constraints. Infiltration is ruled out where it would mobilise contamination, where the site sits in a Source Protection Zone that restricts it, where made ground of unknown composition sits under the feature, where slope stability is an issue, or where the feature would be too close to a building or a retaining structure. Each of those is a defensible reason, and each needs stating with the evidence behind it.

Put the results in a Geotechnical Assessment with the test locations shown on a plan and the raw data appended. That is what allows the lead local flood authority to accept step one as closed.

Discharge to a watercourse

Second in the hierarchy, and often the most practical answer on a rural site.

Start with ownership and access. You need a physical route to the watercourse and the legal right to use it, which usually means an easement across third party land unless the site fronts the channel. Riparian ownership commonly extends to the centre line of the channel, and that affects who you negotiate with.

Then consent. Discharging to an ordinary watercourse requires land drainage consent from the lead local flood authority under the Land Drainage Act 1991, or from the internal drainage board where the site falls within a drainage district. Where the receiving water is a main river, an Environment Agency permit is likely to be needed instead.

The discharge rate will be limited. On a greenfield site the starting point is the calculated greenfield runoff rate, and on a brownfield site the authority will normally expect a reduction against the existing rate rather than a like for like replacement. Design the outfall properly as well: a headwall, erosion protection at the point of discharge, and a non-return arrangement where the receiving water can back up.

Discharge to a surface water sewer

Third, and the option most self-builders assume is first.

Make a formal pre-development enquiry to the water and sewerage undertaker, in writing, describing the proposed discharge point, rate and volume. The response tells you whether a surface water sewer exists in the right place, whether it has capacity, and what discharge rate would be permitted.

The undertaker will normally require evidence that infiltration and watercourse discharge have been ruled out, so the enquiry works best once the soakage testing is done. It will also usually impose a flow control, and it may require attenuation to hold the difference.

Where the undertaker offers a connection subject to conditions, get those conditions in writing and reflect them in the Drainage Strategy rather than paraphrasing them.

The combined sewer

Last resort, and treated as such. Undertakers are actively reducing surface water flows into combined systems, because every litre of rainwater entering a combined sewer is a litre that has to be treated and a contribution to storm overflow discharges.

Expect a refusal unless the earlier steps have been tested and documented. Where a connection is permitted, expect a tightly restricted rate and a requirement for attenuation, and expect the authority to ask again whether any part of the site could infiltrate.

Documenting a refusal so it counts

This is the part that separates a strategy approved first time from one that goes round twice.

• Keep written correspondence with dates, reference numbers and the name of the organisation, and put it in the appendix.
• Record verbal advice in a written note and send it back to the other party for confirmation.
• Include the soakage test data, the test pit locations and the groundwater monitoring record.
• Include the pre-development enquiry and the undertaker's full response, not a summary of it.
• Where a landowner refused an easement, record the request and the refusal.
• Date everything, because a response about sewer capacity from four years ago will be challenged.

A refusal you cannot evidence is a refusal that did not happen, as far as the reviewer is concerned.

Presenting the hierarchy in the strategy

Set it out as a table near the front of the document, before the calculations.

Cross-refer the conclusion into the Flood Risk Assessment so that the flood and drainage positions are consistent, and deal with foul drainage separately in a Foul Sewage Assessment, since the foul hierarchy and the consents are different.

One honest uncertainty. How much groundwater monitoring an authority requires varies widely, from a single winter reading to a full seasonal record. Ask the lead local flood authority what it expects at pre-application, because that answer changes both the programme and the cost.

This is general guidance, not legal advice. Drainage design should be prepared or checked by a suitably qualified engineer.

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