Flood Risk Assessment or Drainage Strategy: what goes where

A flood risk assessment tells the council what could flood the site. A drainage strategy tells the council what the site will do to everywhere downstream of it. They answer to different consultees, they use different standards, and on most minor applications one of them gets written and the other gets a paragraph, which is why so many schemes end up with a pre-commencement condition nobody budgeted for.

The split is easier to hold once you know who reads each document.

Two documents, two consultees

The Environment Agency is the statutory consultee on flood risk from main rivers and the sea, and it comments on development in Flood Zones 2 and 3. Its interest is whether the development is safe, whether it displaces floodwater, and whether people can get out.

The lead local flood authority, which is the county or unitary authority, is the statutory consultee on surface water drainage for major development. Its interest is run-off rates, storage volumes, water quality and who will maintain the system in thirty years.

Water and sewerage undertakers comment on connections and capacity, foul and surface water alike. The council's own drainage engineer often comments on minor schemes where the lead local flood authority does not.

Four readers, three documents, one site. Writing a single combined report and hoping each reader finds their part is how contradictions get published.

What belongs in the flood risk assessment

The flood risk assessment is about risk to and from the development, from every source:

• fluvial and tidal risk, with the flood zone taken from the published map for planning and refined by any modelling held by the Environment Agency or the council's strategic flood risk assessment
• surface water, groundwater, sewer and reservoir or other artificial sources, each addressed rather than dismissed in one line
• design flood levels for the relevant return periods, with the climate change allowance applied and the allowance stated by name and epoch
• finished floor levels, with the freeboard above design flood level agreed rather than assumed
• safe access and egress, described as a route to land above the design level, not as a statement that the road floods only briefly
• floodplain compensation where any part of the development occupies functional floodplain, level for level and hydraulically connected
• flood resistance and resilience measures, and the residual risk that remains after them
• flood warning and evacuation arrangements where occupants could be at risk

It should also cross-reference the sequential and exception tests rather than reproduce them, because those are separate documents on most validation lists.

What belongs in the drainage strategy

The drainage strategy is about water leaving the site:

• existing drainage arrangements, ground conditions and any infiltration testing, carried out to the recognised soakaway method and at the depth and season that give a fair answer
• the discharge hierarchy worked through in order, with evidence for each rejection
• the permitted discharge rate, whether that is a greenfield equivalent rate for a previously undeveloped site or a betterment on the existing rate for a brownfield one
• attenuation storage volume for the design event including climate change and an allowance for urban creep, with the calculation shown
• the sustainable drainage management train, including water quality treatment appropriate to the land use
• exceedance routes for events beyond the design standard, shown on a plan
• adoption and maintenance, naming who will hold and fund the system

Foul drainage is a separate question with its own document. Where the site is not connecting to the public sewer, or where capacity is uncertain, a foul sewage assessment is what the undertaker and the council will expect.

Greenfield run-off and the discharge hierarchy

The hierarchy is the spine of any drainage strategy. Discharge to the ground by infiltration first. If that is not feasible, to a surface water body. If not, to a surface water sewer or highway drain. Only then to a combined sewer, and undertakers now resist that route in most circumstances.

Each step down needs evidence. "No infiltration" means percolation tests with results, not an assertion about clay. Groundwater levels, contamination and slope stability are all legitimate reasons to reject infiltration, and all need a sentence with a source.

Greenfield run-off rates are estimated using the recognised hydrological methods, and the mean annual maximum rate is the usual benchmark, with lower rates sometimes required for smaller return periods. A practical floor applies in most authorities because very small orifices block, so a minimum controlled discharge rate is normally accepted even where the calculated greenfield rate is lower.

State the method, the catchment characteristics used and the resulting rates in a table. A reviewer who cannot reproduce your number will ask for the model.

Illustrative example: a 0.4 hectare site restricted to 5 litres per second

Take a 0.4 hectare greenfield site with 0.25 hectares of proposed impermeable area, in a clay catchment where infiltration testing has failed and the only outfall is a surface water sewer. The lead local flood authority sets a discharge rate of 5 litres per second.

The arithmetic that follows is straightforward and it drives the layout. The storage volume needed for the critical duration event, with the climate change allowance and an allowance for urban creep applied to the impermeable area, has to be found somewhere on site. At that discharge rate the critical duration is usually a long one, often several hours, and the volume is larger than people expect.

Where that volume goes changes the scheme. A basin needs a low point and space. Cellular storage under a car park needs cover depth and a maintenance access. Permeable paving contributes storage but not on clay with no infiltration. This is why the drainage engineer needs to see the layout at concept stage rather than at planning submission, and it is the single most common cause of a late redesign. The figures in this example are illustrative.

Overlaps that cause duplication and contradiction

Three things appear in both documents and have to say the same thing.

Site levels. If the topographic survey used by the flood modeller differs from the one used by the drainage engineer, the finished floor levels and the drainage invert levels will not reconcile, and a consultee will spot it.

Climate change allowances. Both documents apply allowances, but they are different allowances: peak river flow for the flood assessment, peak rainfall intensity for the drainage design. Naming them precisely stops a reviewer assuming one has been used for both.

Surface water flood risk. It is a source of flooding in the flood risk assessment and a design constraint in the drainage strategy. Say it once properly in the first and cross-reference from the second.

Contradictions between the two documents are read as carelessness, and they justify a holding objection while the applicant reconciles them. Agree the levels datum, the survey and the allowances before either document is drafted.

Conditions each consultee tends to ask for

The Environment Agency typically asks for a condition requiring development to be carried out in accordance with the approved flood risk assessment, with finished floor levels specified, and sometimes a condition securing the flood warning and evacuation plan.

The lead local flood authority typically asks for a detailed surface water drainage scheme to be submitted and approved before commencement, a verification report confirming the system was built as designed before occupation, and a maintenance plan with a named responsible body.

Both are dischargeable, both cost a fee, and both take weeks. Where the evidence is available at application stage, submitting the detail up front and asking for the condition to be worded accordingly saves a month later. Where it is not, budget for the discharge applications rather than treating them as a formality.

Flood and drainage documents are usually expected from a suitably qualified author, and professional review before submission is strongly advised. PlanScope drafts the structure and the policy framing of both documents and flags where site data is missing rather than inventing it.

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.

Browse the 66 PlanScope report types to see how the flood and drainage documents sit alongside the rest of an application pack. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.