Proving the surface water discharge hierarchy

Almost every drainage strategy says the discharge hierarchy was followed. Far fewer show it. The surface water discharge hierarchy evidence a lead local flood authority actually wants is a set of dated documents: percolation test results at the right depth and location, a letter to the riparian owner with the reply, and the water undertaker's written position with a reference number. Assertion that infiltration is not feasible because the ground is clay will be sent back, and the resubmission costs a month.

Collect the evidence before you write the strategy, not after the officer asks.

The order the hierarchy sets out

Surface water should be discharged to the ground by infiltration where that is practicable. Where it is not, the next option is a surface water body, which in most cases means a watercourse. Then a surface water sewer, highway drain or other drainage system. A combined sewer is the last resort.

The hierarchy appears in Approved Document H of the Building Regulations for rainwater drainage, in national flood risk guidance, and in the standards the lead local flood authority applies to a drainage strategy as a statutory consultee. The order is not advisory. Moving down a step requires a reason, and the reason has to be evidenced rather than stated.

One point that gets missed. The hierarchy applies to the site, not to the easiest corner of it. A site that drains to clay in the north and to sand in the south should be using infiltration in the south.

Evidence that infiltration was properly tested

Infiltration testing follows the soakaway design method in BRE Digest 365. The test is not complicated, and it is routinely done badly.

What a credible test record contains:

• A plan showing each test location, with the proposed infiltration features on the same drawing.
• The depth of each trial pit, which must be at the depth the feature will work at rather than 1 m down because that is what the digger reached.
• Three successive fillings per pit, with timings recorded for the fall from 75 per cent to 25 per cent of the effective storage depth.
• The calculated soil infiltration rate for each pit, and the value adopted in the design with the factor of safety stated.
• Observed groundwater level in each pit, with the date, and an allowance for seasonal variation.
• A soil description by horizon.

Where infiltration really cannot work, say which of the real constraints applies: a measured infiltration rate too low to drain the feature within the required time, groundwater within the required separation of the base, made ground or contamination that would create a pathway, a source protection zone, shallow rock or a slope stability concern.

A test carried out in July on a site with a winter water table two metres higher tells you very little. Note the limitation in the report, because an officer who spots it before you do will ask for a winter monitoring round.

Approaching a watercourse owner and recording the answer

Where there is a watercourse within reach, the authority will expect you to have pursued it. That means identifying the owner, writing to them, and keeping the reply.

Riparian ownership usually runs to the centre line of the channel for the owner of each bank, and a new outfall needs the owner's consent as a matter of property law regardless of drainage approval. On an ordinary watercourse, works in, over or under the channel also need land drainage consent from the lead local flood authority under the Land Drainage Act 1991. On a main river, the Environment Agency's permitting regime applies instead.

Record the whole exchange: who you wrote to, the date, what you asked, the response and the date. A refusal from a landowner is perfectly good evidence for stepping down the hierarchy. An absence of any attempt is not.

Where consent is given, the outfall itself needs designing properly, and bank stability and scour protection are the details that attract conditions.

Why water company refusal needs to be in writing

Phone calls do not count. Ask the sewerage undertaker for a formal position on a surface water connection, keep the reference number, and put the correspondence in the appendix.

Undertakers often accept a connection subject to a restricted discharge rate. That is useful information, because the restricted rate becomes the design constraint for attenuation volume, and the volume is where the cost sits. Get the rate in writing early enough to influence the layout.

Combining methods across a site

The best strategies rarely use one method. A scheme can infiltrate roof water through soakaways in the permeable part of the site, attenuate highway runoff in a basin, and discharge the balance to a watercourse at a restricted rate.

That approach also answers the hierarchy properly, because each catchment has been dealt with at the highest step available to it. Set it out catchment by catchment in a table, with areas, the route chosen and the reason, and the justification writes itself.

Mind the maintenance consequence. Four different drainage systems mean four maintenance regimes, and a management company or a householder has to be able to run them. A strategy that is elegant hydraulically and unmaintainable in practice will attract a condition requiring a maintenance plan you then have to invent.

Common reasons a strategy is sent back

In no particular order, these are the recurring ones.

No test results at all, or results from a single pit used to characterise a whole site. Pits dug to a convenient depth rather than the design depth. No groundwater observations. An infiltration rate quoted without the factor of safety. No evidence of any approach to a riparian owner. Water company correspondence absent or undated. A discharge rate taken from a rule of thumb rather than calculated. Rainfall data from the wrong source, which is a problem in itself because the rainfall dataset you cite changes the volumes materially. And no maintenance plan.

Most of these are evidence failures rather than design failures. The design was probably fine. The file just did not prove it.

Structuring the justification section

Write the hierarchy section as a sequence of findings, each with the document that supports it.

1. State the site areas and the catchments, with a plan.
2. Report the infiltration testing, with the results, the adopted rate and the limitations.
3. Give the conclusion on infiltration for each catchment, and the reason where it is rejected.
4. Identify watercourses within reach, the ownership position and the correspondence, with the outcome.
5. Give the undertaker's written position and any rate restriction, with the reference.
6. Set out the chosen route for each catchment and the resulting discharge rate and attenuation volume.
7. Cross-refer to the maintenance plan and say who holds the duty.

Put the correspondence in an appendix in date order. An officer who can follow the paper trail in five minutes will approve a strategy that an officer who cannot will refuse, even where the engineering is identical.

Pull it together in a Drainage Strategy that reads as a decision record rather than a calculation printout, and cross-reference it to the Flood Risk Assessment so the two documents use the same levels and the same areas.

For specialist technical reports, qualified professional input may be required. This is general guidance rather than legal advice, so check the position with your local planning authority or a qualified professional before relying on it.

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