The sustainable drainage hierarchy in London

The London Plan drainage hierarchy asks for something stricter than the national position. It is not enough to show that your chosen drainage solution works. Each step above the one you have adopted has to be discounted with evidence, in order, and the borough drainage officer will read the strategy looking for those rejections rather than for the hydraulic calculations. Development is expected to aim for greenfield run-off rates where feasible, which on a dense urban site with London clay beneath it is a demanding target and the reason so many strategies come back with comments.

The order of preference

The hierarchy runs from most to least preferred, and every London borough applies it.

1. Store rainwater for later use.
2. Infiltrate to the ground, using infiltration techniques such as porous surfaces in non-clay areas.
3. Attenuate rainwater in ponds or open water features for gradual release.
4. Attenuate rainwater by storing in tanks or sealed water features for gradual release.
5. Discharge rainwater direct to a watercourse.
6. Discharge rainwater to a surface water drain.
7. Discharge rainwater to the combined sewer.

Note where attenuation in open features sits. Above tanks, and well above any discharge. A strategy that jumps from a paragraph dismissing infiltration straight to an underground crate tank has skipped the step boroughs care about most, because open features do more for amenity, biodiversity and water quality than a crate does.

Targeting greenfield run-off rates

The aspiration is to achieve greenfield run-off rates and to manage as much rainfall as possible on site, with the calculated rate derived from the site area using a recognised method and stated in the strategy.

On a redeveloped urban site the practical outcome is usually a substantial reduction against the existing rate rather than a true greenfield figure. Where that is the case, say what the existing rate is, what greenfield would be, what you are achieving, and why the gap exists. A number presented without that comparison tells the officer nothing.

Very small calculated rates create a practical problem. A flow control device below a certain discharge rate blocks, so a minimum workable rate is normally agreed with the borough and the water company rather than pursued to a theoretical figure.

Rainwater harvesting at the top

Reuse sits at the head of the hierarchy and is the step most often dismissed in a sentence.

Take it seriously on schemes with a demand that matches the supply: irrigation of communal landscaping, green wall systems, toilet flushing in commercial floorspace, wash-down water for refuse stores. State the roof area collected, the estimated yield, the demand profile and the resulting tank size.

Then be honest about the limits. Harvested water contributes little to attenuation during a design storm, because the tank is as likely to be full as empty when the rain arrives, and any attenuation credit has to come from dedicated storage above the reuse volume. Boroughs know this. A strategy claiming a large attenuation benefit from a harvesting tank alone will be queried.

Infiltration and the clay problem

Much of London sits on London Clay, and infiltration rates in it are generally too low for conventional soakaways. That does not remove the obligation to test.

Where the site sits on river terrace gravels, on the Thames floodplain deposits or on the sand and gravel of parts of north and south London, infiltration can work well. The geology varies over short distances, and a desk assessment from a geological map is a starting point rather than an answer.

Test where the drainage would go, record the results, and reject infiltration on the evidence. A strategy that says the site is underlain by London Clay and therefore infiltration is not viable, with no trial pit behind it, is the single most common reason for a drainage comment on a London application.

Basements complicate this further. Where a scheme includes a basement, the drainage strategy and the Basement Impact Assessment have to agree about groundwater levels, and many boroughs require them to be read together.

Attenuation in open features

Step three of the hierarchy is where design quality enters the drainage conversation.

Rain gardens, swales, detention basins, permeable paving with attenuating sub-base, blue roofs and green roofs all deliver storage while doing something else at the same time. On a constrained site, blue roofs and green roofs are often the only open storage available, and they should be quantified rather than mentioned.

Coordinate this with the planting proposals and with the Green Infrastructure Plan, because the same podium area is usually being counted for urban greening, play space and drainage storage at once. The three commitments need to be compatible on a single drawing.

Where tanked storage is unavoidable, explain why the open options were rejected on each part of the site rather than in a single sentence about space constraints.

Blue roofs deserve particular care in the drawings. They need a flow restrictor at each outlet, a maintenance access route, and a structural allowance for the retained water depth, and all three have to appear in documents other than the drainage strategy. Where the roof is also carrying photovoltaic panels or a biodiverse substrate, show the layers and the falls on a section rather than describing them.

Evidencing each step that was rejected

Set the strategy out as a sequence of tested and rejected options. For each step, give the test, the evidence and the conclusion.

• Reuse: demand calculation, yield, tank size, conclusion on the proportion of demand met.
• Infiltration: trial pit results with dates, groundwater levels, contamination and source protection zone position.
• Open attenuation: areas available, storage achievable, coordination with planting and amenity requirements.
• Tanked attenuation: volume, location, access for maintenance.
• Watercourse discharge: whether a hydraulic connection exists and who consents to it.
• Sewer discharge: correspondence with the water company on capacity and connection.

The water company conversation should start early. Capacity is not guaranteed, agreement to connect is a separate process from planning, and a condition requiring an approved drainage scheme cannot be discharged without it.

What a borough drainage officer checks

Five things, in roughly this order.

Whether the impermeable area schedule matches the submitted layout. Whether the existing and proposed discharge rates are stated with the method used. Whether each step of the hierarchy has been rejected on evidence. Whether exceedance flows above the design event have been considered, so that water overtops towards a safe route rather than into a basement entrance. And whether there is a maintenance regime with a named responsible party and a funding route.

The exceedance question is the one that catches otherwise competent strategies. Drawing the overland flow route for an event beyond the design standard takes one plan and answers a question the officer would otherwise raise at week five.

A Drainage Strategy covering all of this sits alongside the Flood Risk Assessment where the site is in Flood Zone 2 or 3 or over a hectare, and the two must use the same areas, levels and rates.

This is general guidance, not legal advice. Check the current London Plan guidance and your borough's drainage requirements before relying on it.

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