What the lead local flood authority checks in a drainage strategy

The lead local flood authority is a statutory consultee on surface water drainage for major development, which means a scheme of 10 or more dwellings, 1,000 m² or more of floorspace, or a site of one hectare or more. Its comments carry real weight, and a holding objection from the LLFA will stop a recommendation reaching committee however comfortable the rest of the case is.

LLFAs audit rather than design. They are checking whether your calculations stand up, whether the drawings match them, and whether anyone will maintain the result in 30 years. Most of the objections I see arise from the third question.

The checklist most authorities apply

The wording varies between county and unitary authorities, but the substance rarely does. A drainage strategy is expected to show:

• the discharge hierarchy applied in order, with evidence for each rejection
• the proposed discharge rate, with the method used to derive the greenfield equivalent stated
• attenuation volumes for the design events including climate change and urban creep
• infiltration testing results, or a reasoned explanation of why infiltration is not viable
• groundwater levels, including seasonal maxima, with the source
• exceedance flow routes for events beyond the design standard
• water quality treatment stages appropriate to the land use
• a maintenance plan naming the party responsible for each component
• confirmation from the sewerage undertaker where a connection is proposed

Miss any one of these and the response is usually a request for further information rather than an outright objection. Miss three and it becomes an objection with a list.

The hierarchy, and why it is not a formality

Surface water should discharge to the ground first, then to a watercourse, then to a surface water sewer, and only last to a combined sewer. Officers know that most submissions reach for the sewer, and they read the justification carefully.

Rejecting infiltration needs evidence. Percolation testing carried out to the BRE Digest 365 method, at the depth and location of the proposed feature, with the groundwater level recorded, is what an LLFA expects. A statement that the underlying geology is clay, unsupported by a trial pit, is not evidence. Neither is a single test carried out in the driest month of a dry year.

Where infiltration really does fail, say so with numbers and move to the next tier. The strategy reads better for having tested the option properly.

Calculations they will audit

Expect the LLFA to open the model output rather than the summary table.

Greenfield runoff rate is the first figure checked. State the method, the catchment area used, the SAAR and SPR values and the resulting QBAR or Qmed, and be consistent between the report text, the calculations and the drawings. A discrepancy between the rate quoted in the Drainage Strategy narrative and the rate in the model is the single most common reason for a further information request.

Climate change allowances come next. The Environment Agency publishes peak rainfall intensity allowances by river basin district and epoch, and they are revised, so cite the version used and the date you took it. Applying an allowance from a superseded table is an avoidable error.

Urban creep is the one people forget. An allowance for future paving over of gardens is expected on residential schemes, and the LLFA will state the percentage it wants if the strategy is silent.

Then the design events themselves: the 1 in 1 year event for the first flush and treatment, the 1 in 30 year event for no surface flooding, and the 1 in 100 year event plus climate change for no flooding of property, with flood extents shown where storage surcharges.

On brownfield sites the question is betterment. National policy expects surface water runoff to be reduced where practicable, and many authorities look for a reduction towards greenfield rates rather than simply matching the existing positive drained rate. Establish the existing rate with evidence, not assertion, since historic connections are frequently smaller than assumed.

Maintenance and adoption questions

This is where strategies fail late. The LLFA will ask who owns each component, who maintains it, how it is funded, and what access exists for the vehicle that will do the work.

A maintenance plan should list each feature, the tasks, the frequency and the responsible party. Naming a management company is acceptable. Naming a management company that does not yet exist, with no funding mechanism and no access strip to the basin, is not.

Where adoption by the sewerage undertaker is proposed, the components have to be adoptable in their design, and that constrains the choice. Where the highway authority will adopt road drainage, the interface between adopted and private systems needs a drawn boundary rather than a sentence.

Where the strategy meets the flood risk work

For a site in Flood Zone 2 or 3, or of one hectare or more, the Flood Risk Assessment deals with flooding to and from the site from all sources, while the drainage strategy deals with the water the development itself generates. The two documents should share a levels drawing and a set of finished floor levels, and they frequently do not.

Where the sequential approach applies, the Sequential Test and, for more vulnerable uses in higher-risk zones, the Exception Test are separate exercises that come earlier in the process. An excellent drainage strategy does not cure a failed sequential test.

Responding to comments without redesigning

Most LLFA responses ask for clarification rather than a different scheme. Handle them in that spirit.

1. Answer each numbered point in a separate short note, cross-referenced to the original comment.
2. Supply the model files if asked. Refusing looks like something to hide, and the officer will assume the worst.
3. Where a comment rests on a misreading, say so politely and point to the drawing reference rather than restating the argument at length.
4. Where a comment is right, change the drawing and issue it. A revised general arrangement is faster than three rounds of correspondence.
5. Ask for a call. Fifteen minutes with a drainage engineer resolves what a fortnight of email will not.

Conditions and what they commit you to

The usual outcome is approval with conditions requiring detailed drainage design before development starts, a verification report before occupation confirming the system was built as approved, and a maintenance plan secured for the lifetime of the development.

Read the verification condition closely. It requires as-built evidence, including photographs of buried components during construction, and a contractor who has not been told will not have taken them. That is a discharge problem that appears at the worst possible moment.

Note that Schedule 3 to the Flood and Water Management Act 2010, which would introduce a separate SuDS approval regime, has not been brought into force in England. Check the current position before assuming an approving body exists.

Illustrative example: a 45-unit scheme on a former depot with clay at 1.2 metres and a combined sewer in the adopted road. Infiltration testing across three trial pits confirmed rates too low to use. The strategy discharged to the combined sewer at a rate below the existing drained rate, used a below-ground tank with a hydrobrake, provided treatment through permeable paving in the parking courts, routed exceedance flows along the estate road to a shallow swale, and named the management company with a funded schedule. The LLFA response ran to two clarification points.

Enter your postcode on PlanScope to see which flood and drainage reports your site is likely to need.

This is general guidance, not legal advice; check the position with your lead local flood authority, the Environment Agency or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.