A worked example of a drainage hierarchy decision

The drainage hierarchy has to be worked through in order, and each step you reject needs evidence rather than a sentence. For a single new dwelling or a small infill plot, that usually means an infiltration test in the ground where the soakaway would actually go, a calculation from the result, and a written record of why the next option down was taken if infiltration fails. Councils and lead local flood authorities reject small site drainage strategies for one reason above all others: the applicant asserted that infiltration was unsuitable without testing it.

The hierarchy and the order it must be tested in

Surface water should be managed as close to its source as possible, working down a preference order.

1. Discharge into the ground by infiltration.
2. Discharge to a surface water body, such as a watercourse.
3. Discharge to a surface water sewer or highway drain.
4. Discharge to a combined sewer, as a last resort.

Rainwater reuse sits above all of these where it can take a share of the volume, and green roofs, permeable surfacing and rain gardens can be used at any level to slow and clean what runs off.

The order matters legally as well as technically. A water company has the right to refuse a surface water connection where a higher option is available, and the lead local flood authority, as statutory consultee on major applications and as adviser on smaller ones, will ask which steps were tested.

A Drainage Strategy is the document that records this, even on a small site where it may run to three or four pages.

Infiltration testing and reading the results

Infiltration is tested in trial pits dug at the proposed soakaway location and to the proposed depth, following the method in BRE Digest 365. Three tests in the same pit is the normal requirement, each filling the pit and timing the fall in water level.

The design figure is taken from the slowest of the three, which is the conservative result. Record the pit dimensions, the date, the depth to groundwater if encountered, the soil description at each horizon and the time readings, because the calculation cannot be checked without them.

Timing matters too. A test in August on clay can give a flattering answer through desiccation cracks that close in winter. Sensible practice is to note the antecedent conditions and, where the result is marginal, to test again in a wetter period.

An illustrative worked calculation

The following is illustrative and shows the method rather than describing a real site.

A trial pit is dug 1.5 m long, 0.6 m wide and 2.0 m deep. The effective storage depth is taken between 75 per cent and 25 per cent of the pit depth, which is between 1.5 m and 0.5 m below ground, a fall of 1.0 m.

The volume of water lost over that fall is 1.5 × 0.6 × 1.0, which is 0.9 m³.

The infiltrating surface is taken as the internal side area of the pit up to 50 per cent of the effective depth, excluding the base. That is 2 × (1.5 + 0.6) × 1.0, which is 4.2 m².

If the water level takes 150 minutes, or 9,000 seconds, to fall through that range, the soil infiltration rate is 0.9 divided by (4.2 × 9,000), which gives approximately 2.4 × 10⁻⁵ m/s.

A rate of that order supports infiltration. A rate two orders of magnitude lower, around 10⁻⁷ m/s, generally does not, and at that point the hierarchy moves down a step.

The storage volume the soakaway then needs comes from the impermeable area draining to it and the rainfall depth for the design event at your location, including the climate change allowance current at the time of design. Take the rainfall figures for the site rather than from any worked example, and check the current Environment Agency climate change allowances, because they are revised.

When infiltration must be ruled out

Test results are not the only reason to reject infiltration.

Groundwater too close to the base of the proposed soakaway, usually requiring a clear vertical separation, rules it out. So does a site within a groundwater source protection zone at the most sensitive level, and so does known or suspected contamination, because infiltrating water can mobilise contaminants into the aquifer. That question belongs with the Contaminated Land Assessment rather than being decided by the drainage designer alone.

Ground stability is the fourth reason. Soakaways near foundations, on made ground, on shrinkable clay or in areas of historic mining need input from a Geotechnical Assessment, and the usual minimum separation from a building foundation is 5 m.

Document each of these with the evidence behind it. A borehole log, a groundwater level, a source protection zone extract or a desk study conclusion, each referenced.

Watercourse and sewer discharge options

If infiltration is out, the next step is discharge to a watercourse. You need a hydraulic connection you have the right to use, consent from the lead local flood authority for an ordinary watercourse or a permit from the Environment Agency for a main river, and a restricted discharge rate.

Rates are normally limited to the greenfield runoff rate for the site, or to an agreed betterment figure on brownfield land. On very small sites the calculated greenfield rate can be a fraction of a litre per second, which no practical flow control device will deliver reliably, so a minimum rate is usually agreed instead.

Sewer discharge comes next and needs the water company's agreement to connect. That is a separate application with its own timescale, and capacity in the receiving sewer is not guaranteed. Start it early, because a planning condition requiring an approved drainage scheme cannot be discharged without it.

Where the site is in Flood Zone 2 or 3, or over one hectare, a Flood Risk Assessment is required alongside the drainage work, and the two documents must use the same areas and rates.

Treatment and water quality

Discharging clean water matters as much as discharging slowly. The standard approach indexes the pollution hazard of the surfaces draining to a point against the mitigation the drainage components provide.

For a single house, roof water is low hazard and needs little treatment. Driveway and parking runoff carries hydrocarbons and sediment, so it wants a filter strip, a permeable surface with a sub-base, or a small rain garden before it reaches the ground or the sewer.

Two components in series usually satisfy the index where one does not. Draw them on the drainage layout and label what each is doing.

Documenting the decision

The record the authority wants is short and specific: the hierarchy in order, what was tested at each level, the evidence, and the conclusion.

Attach the trial pit logs with dates and photographs, the infiltration calculation with its inputs, the impermeable areas schedule, the discharge rate calculation and its method, the flow control device proposed, the maintenance regime and who is responsible for it, and any correspondence with the water company or lead local flood authority.

Maintenance is the item most often missing on small sites. A soakaway with no access for inspection, or a permeable driveway with no statement about not sealing it later, will attract a condition anyway. Write it in and the condition gets discharged first time.

This is general guidance, not legal advice. Check the position with your local planning authority, the lead local flood authority or a qualified professional before relying on it.

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