Drainage Strategy outline with a worked runoff example

A drainage strategy outline that a lead local flood authority will accept follows the same order every time: what is there now, where the water can go in order of preference, how much of it there is, where the excess is held, how it is cleaned, what happens when the design event is exceeded, and who maintains the result. Get the order wrong and the document reads as a set of assertions. Get it right and the reviewer can follow the arithmetic from the site plan to the attenuation volume.

The sections below are the structure, with one worked runoff calculation. Every figure in that example is illustrative and exists to show the method, not to be copied.

Existing drainage and ground conditions

Start with the site as found, because everything downstream depends on it. State the site area to two decimal places, the existing impermeable area, the topography and the direction of overland flow, and the existing positive drainage if any.

Then the ground. Infiltration potential is the single input that decides the shape of the whole strategy, and it has to come from field testing rather than a geological map. Soakaway testing to BRE Digest 365 is the normal method, with at least three trial pits and three fillings of each pit. Record the depth to groundwater and the seasonal variation, because an infiltration system needs clear separation between the base of the device and the highest expected water table.

Record the constraints that rule options out: source protection zones, made ground, shallow rock, contamination that would make infiltration unacceptable, and any culverted watercourse crossing the site. A Flood Risk Assessment will cover flood zone and flood risk from other sources, and the two documents should share one set of levels.

The discharge hierarchy applied in order

The hierarchy in Approved Document H and the non-statutory technical standards for sustainable drainage runs: infiltration to ground, then discharge to a surface water body, then discharge to a surface water sewer or highway drain, then, as a last resort, a combined sewer.

Work down it and evidence each rejection.

1. Infiltration, supported by the soakaway test results and the groundwater record.
2. Watercourse, supported by a plan showing the nearest watercourse, the route and the consent position with the lead local flood authority or the Environment Agency for an ordinary watercourse or main river.
3. Surface water sewer, supported by the water undertaker's sewer records and a pre-development enquiry response.
4. Combined sewer, supported by the undertaker's written position that no alternative is available.

A strategy that jumps to a combined sewer connection with a sentence saying infiltration is not viable will come back. The reviewer wants the test results.

Runoff rates and an illustrative calculation

The design aim on a brownfield site is normally betterment against the existing rate, and on a greenfield site it is the greenfield runoff rate for the equivalent return periods. Rates should be derived using an accepted method and agreed with the lead local flood authority before the volumes are modelled.

The following worked example is illustrative only.

A site of 0.80 ha, currently rough grass, is to be developed with 0.55 ha of impermeable surface. Infiltration testing gave no usable rate, so discharge is to the surface water sewer.

Three things about that table matter more than the numbers. The greenfield rate has to be derived by a published method for the site's own rainfall and soil characteristics, not lifted from a rate per hectare used on a previous job. The climate change allowance comes from the Environment Agency's published peak rainfall allowances, which are revised, so state the version you used and the date. And the critical duration is an output of the model, not an assumption, so report which duration governed.

Where the permitted rate falls below about 5 l/s the discharge control becomes the practical problem, because small orifices block. Say how the control will be kept clear and who checks it.

Attenuation volume and how it is stored

Having established the volume, say where it goes and show it on a plan with levels. Geocellular tanks, oversized pipes, basins, ponds, permeable paving sub-base and blue roofs all work, and they differ in maintenance burden and in how much amenity and biodiversity value they deliver.

Give the storage schedule: device, plan area, depth, void ratio where relevant, invert and cover levels, and volume. The volumes in that schedule must add up to the figure your calculation demanded. Reviewers check this, and mismatches between the calculation sheet and the drawing are the commonest reason a Drainage Strategy is returned.

Half-drain time is worth stating too. A basin that has not emptied before the next storm is not providing the storage the model assumed.

Treatment stages and water quality

Water quality is handled through the management train and the simple index approach in the SuDS Manual, which compares a pollution hazard index for the contributing surface against the mitigation indices of the proposed components.

Residential roofs need little. Residential parking, access roads and commercial yards need more, and a single geocellular tank under a car park provides almost no treatment on its own. Permeable paving, filter strips, swales and a shallow wet basin each contribute. Set out the train for each catchment on the site rather than for the site as a whole, because the roof and the yard need different answers.

Where the site drains to a sensitive receptor, expect the lead local flood authority to ask for treatment above the minimum the index calculation gives.

Exceedance routes

Every drainage system is exceeded eventually, and the strategy has to say where the water goes when it is. Show overland flow routes on a plan with arrows and spot levels, with flow kept away from buildings and away from the site boundary where it would flow onto a neighbour.

Finished floor levels sit here. A freeboard above the modelled flood level and above adjacent ground is the usual expectation, and the figure should come from the flood risk assessment rather than being invented in the drainage document. Where levels cannot be raised, say so and explain the alternative.

Do not describe exceedance as a residual risk and leave it there. Draw it.

Adoption and maintenance responsibilities

The last section is the one most often left thin, and it is the one that generates conditions.

State for each component who will own it, who will maintain it, and under what mechanism: adoption by the water undertaker under a section 104 agreement, adoption by the highway authority, a management company funded through a service charge, or private ownership with a maintenance obligation in a section 106 agreement. Then give the maintenance schedule: the task, the frequency and the access arrangement, including how a vehicle reaches a control chamber in ten years' time.

Schedule 3 to the Flood and Water Management Act 2010 would change the approval route for sustainable drainage, and its commencement has been long anticipated. Check the current position rather than assuming either way.

A Foul Sewage Assessment covers the separate question of foul capacity, and the two documents should agree on which connections are proposed.

Draft your drainage strategy in minutes and then send it for professional review, or read the minor residential worked example for a smaller scheme. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. 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.