Departing from the drainage hierarchy: evidence that works

Every lead local flood authority asks the same question of a drainage strategy that discharges to a sewer: why not higher up the hierarchy. A drainage hierarchy justification succeeds when each tier above the proposed one has been tested and the test is documented. It fails when the strategy asserts that the ground is clay, that there is no watercourse nearby, or that infiltration is unsuitable, without a trial pit log, a level survey or a letter behind any of it. The work is evidential rather than argumentative.

The order the hierarchy sets

National guidance and Building Regulations Part H set a preference order that most authorities reproduce in their own standards: capture and reuse first, then infiltration to ground, then a discharge at a controlled rate to a surface water body, then to a surface water sewer or other drainage system, and only then to a combined sewer.

Two points about the order are worth stating in the strategy itself, because reviewers see them misunderstood weekly.

Reuse is not attenuation. A rainwater harvesting tank only provides storage when it happens to be empty at the start of a storm, so any credit claimed against the design event needs a yield and demand analysis, and the approach that reviewers accept is set out in rainwater harvesting and attenuation credit. Reuse belongs in the strategy for water efficiency reasons regardless.

And the hierarchy is site wide rather than single choice. A scheme can infiltrate from permeable parking, discharge roof water to a watercourse and route a small area to a sewer, and a mixed answer is usually the strongest one because it demonstrates that each tier was actually considered.

What counts as evidence that a tier is unavailable

The standard is proportionate to the scale of the scheme, but the categories of evidence do not change.

Note what is not on that list. Professional opinion, geological mapping alone, a previous scheme in the area, and the absence of a watercourse on a small scale plan are all starting points rather than evidence. Authorities do not disbelieve consultants as a matter of course, but the officer signing off the strategy needs something in the file.

Infiltration: the tests and the disqualifying conditions

Infiltration is the tier most often dismissed without evidence, and it is the one with the clearest method.

Soakage testing to the recognised digest method requires trial pits at the proposed soakaway locations, excavated to the proposed depth, with three separate test runs in each pit so that a rate can be derived rather than estimated. Testing one pit in the corner of a site at 1 metre depth tells a reviewer very little about a system proposed at 2.5 metres under a car park.

Conditions that properly rule infiltration out, each of which has to be demonstrated rather than asserted:

1. A measured infiltration rate too low to empty the storage within the required period.
2. Groundwater or a perched water table too close to the base of the proposed system, which needs monitoring over a period rather than a single dip on a dry day.
3. Contaminated ground, where infiltration would mobilise a pollutant through the unsaturated zone.
4. A groundwater Source Protection Zone or other aquifer sensitivity where the Environment Agency objects.
5. Slope stability, where infiltrating water would load a slope or a retaining structure.
6. Proximity to building foundations, basements or highway subgrade, where saturation would cause damage.
7. Made ground of variable composition and unknown behaviour.

Seasonality matters for the second of those. Tests carried out in August on a site with a winter water table three metres higher will be challenged, and a groundwater monitoring record across a wet period is the evidence that settles it. Where flooding from groundwater is part of the picture, the same monitoring evidence serves both the flood risk assessment and the drainage case.

Get the geotechnical and contamination work to talk to the drainage work. A Geotechnical Assessment that reports permeability and a contamination assessment that identifies a pathway to groundwater are the two documents an infiltration argument stands or falls on.

Watercourse discharge and the barrier of third party land

Discharge to a watercourse is preferred over a sewer, and it is defeated by practicalities more often than by hydraulics.

Levels come first. The outfall invert has to sit above the water level in the receiving channel for a usable proportion of the time, and a site lower than the watercourse needs pumping, which pushes the argument elsewhere. Survey the levels rather than interpolating them from contours.

Then ownership. Reaching a watercourse across land you do not own requires an easement, and the evidence that this route is unavailable is correspondence: a dated approach to the landowner, the response, and any terms sought. An unsupported statement that third party land is not available is the single most common weakness in these justifications, because it is also the easiest thing to fabricate and officers know it.

Consents follow. Works in an ordinary watercourse need land drainage consent from the lead local flood authority, and works on or near a main river or defence need an environmental permit. Neither is usually refused for a well designed outfall, but both take time, and a programme that cannot accommodate them is not a reason to move down the hierarchy.

Where the watercourse has been culverted through the site, resist the instinct to treat it as a sewer. It remains a watercourse, the presumption against burying it further applies, and the presumption against burying a watercourse governs what can be done to it.

Cost as a justification and its limits

The hierarchy is applied so far as reasonably practicable, and cost is part of what is reasonable. It is also the argument most often overstated.

Cost works as a justification when it is quantified, comparative and proportionate to the scheme. A costed comparison showing that a 180 metre gravity connection across two ownerships to reach a watercourse costs several times the cost of a sewer connection and attenuation, on a scheme of nine dwellings, is an argument an officer can weigh. The same argument on a 400 unit scheme is not, because the disproportion disappears.

Cost does not work as a bare assertion, and it does not work where the expense arises from a design choice made for other reasons, such as a layout that puts a building over the only viable outfall route. Reviewers will say so.

Two related points. Land take is a legitimate cost, and the developable area lost to open storage can be quantified. And maintenance liability over the design life is a legitimate consideration, particularly where a component would fall to a management company with no realistic funding.

Contamination, groundwater and where infiltration is prohibited

Contaminated land turns the hierarchy on its head, because the preferred tier becomes the prohibited one.

Where a pathway exists between made ground or a contaminant source and an aquifer, infiltration creates the pathway that risk assessment is trying to break. The Environment Agency's groundwater protection position statements set out where infiltration through contaminated ground is unacceptable, and a Source Protection Zone adds sensitivity around a public supply abstraction.

The strategy should therefore sit alongside a Contaminated Land Assessment, with the conceptual site model referenced directly. Land contamination reports must be prepared in accordance with the relevant British Standard by suitably qualified and experienced persons, and the local planning authority will expect that, so the drainage engineer should cite the contamination report rather than reaching a view on contamination independently.

Where remediation will remove the pathway, say whether infiltration becomes viable afterwards and whether the strategy depends on that outcome. A strategy contingent on a remediation result should say so and propose a condition covering both cases.

Structuring the justification in the strategy

Put it in one place, early in the document, in a form that can be checked.

Set out the tiers in order. For each, state whether it was tested, what was done, what the result was, which appendix holds the evidence, and the conclusion in a sentence. Then state the proposed solution and identify which tier or tiers it uses. A single table of six rows with cross references does more than four pages of narrative, and it gives the reviewer somewhere to record their agreement.

Two habits improve the outcome further. Date and reference every piece of evidence, including the pre-development enquiry response, because responses obtained before the layout changed will be challenged. And where a tier remains open but was not chosen, say why, rather than implying it was unavailable; a strategy that overstates its constraints loses credibility on the points where the constraints are real.

Smaller schemes are held to a proportionate version of the same discipline, and the expectations for a single plot are set out in drainage evidence for a single new dwelling. The Drainage Strategy is the document that carries all of this, and it should read as though written for someone who will disagree with it.

PlanScope drafts that document structured to UK planning policy, with test results, levels and correspondence flagged as inputs to be supplied rather than assumed, which leaves the engineer to do the testing and the design. Reports should be reviewed before submission, and PlanScope does not guarantee planning approval. For specialist technical reports, qualified professional input may be required.

This is general guidance, not legal advice. Check your lead local flood authority's own standards and the current national guidance before relying on it.