Flood Risk Assessment outline for a Flood Zone 2 scheme

A Flood Risk Assessment outline for a minor scheme in Flood Zone 2 should run to seven sections and perhaps a dozen pages with appendices, and it should answer the reviewer's questions in the order they arise. The commonest fault in small-site assessments is not a lack of modelling. It is a document that never states the flood level, never states the finished floor level, and leaves the officer to work out whether the house will be dry.

The section order below follows what national guidance asks a site-specific assessment to demonstrate. It works for an infill plot, a barn conversion or a pair of houses on a village edge.

Site description and flood zone classification

Open with the facts a reviewer needs to locate the site and fix its risk. Address, national grid reference, site area, existing use and existing built form. Topography from a levels survey, with spot levels rather than adjectives, related to Ordnance Datum. The proposal in one paragraph, including the number of units, their vulnerability classification and whether any part is below ground.

Then the classification. State the zone from the flood map for planning, the date you checked it, and the source of the flood risk, naming the watercourse. State whether the site is defended, and by what. Include the map extract as a figure with the site boundary drawn on it.

Where the site straddles a boundary, say which parts sit in which zone and give the areas. That single sentence often decides the layout.

Sources of flooding assessed

Deal with each source in turn, and say so explicitly even where the answer is that the source is not relevant:

• fluvial, from the named main river or ordinary watercourse, with the modelled levels if available
• tidal, where the site is within the influence of a tidal reach
• surface water, from the long-term flood risk mapping and from the local topography
• groundwater, from the council's strategic flood risk assessment and any borehole evidence
• sewers, from the water company's records and from any reported flooding history
• artificial sources, such as reservoirs, canals and attenuation ponds upstream

Ask the Environment Agency for modelled level data where a model exists. On small schemes the product request is straightforward and the levels transform the quality of the assessment. Where no model exists, say so and explain what you used instead.

Climate change allowances applied

State the river basin district, the epoch you assessed against, and the allowance percentage you applied, with the source. The allowances are revised, so cite the current guidance and the date you consulted it rather than reproducing a table from an older report. The climate change allowances section sets out how to structure that part in more detail.

For a residential scheme the epoch to assess is normally the one covering the expected lifetime of the development, commonly taken as a hundred years from completion for housing. Say which lifetime you assumed, because the choice drives the allowance and the allowance drives the floor level.

Peak rainfall allowances apply to the surface water calculations separately from the peak river flow allowances. Keep the two clearly labelled, since mixing them is a frequent source of reviewer queries.

Sequential test where required

Minor development and most changes of use are outside the sequential test. A new dwelling in Zone 2 that is not minor development is not, and the test cannot be satisfied by assertion.

What a test that survives scrutiny contains: the search area, agreed with the council and justified by the nature of the proposal. The sources of site availability evidence, typically the council's housing land availability assessment, the brownfield register and its allocations. The sites considered, with reasons for discounting each. And a conclusion on whether reasonably available sites exist in a lower risk zone.

If the site is allocated in the development plan, say so, because the plan-making sequential approach carries weight in the site-specific test.

Finished floor levels and access

This is the section reviewers turn to first. Give the numbers:

1. The design flood level for the relevant event including the climate change allowance, in metres above Ordnance Datum.
2. The proposed finished floor level, with the freeboard stated.
3. The existing ground levels around the building, so the relationship is visible.
4. The access route, with levels along it, and the depth and hazard rating of water on that route in the design event.
5. Whether the access is dry, safe or neither, using the terms the guidance uses.

Where a dry route cannot be achieved, say what the strategy is: safe refuge above the flood level, a flood plan, registration with the flood warning service. Do not claim safe access where the levels do not support it. An officer with a levels drawing will check.

Illustrative example: an infill plot in Zone 2, one dwelling, design flood level 12.40 metres above Ordnance Datum including the higher central allowance, existing ground 12.35 to 12.60, finished floor level set at 13.00 with 600 millimetres freeboard, access along a driveway rising to 12.70 with shallow flooding of under 100 millimetres in the design event. Illustrative only, and the levels on any real site come from survey and modelling rather than from an example.

Surface water management

Keep this proportionate but numerical. Existing run-off, the discharge hierarchy applied in order with reasons, infiltration test results, the proposed rate and attenuation volume, treatment for run-off from parking, exceedance routing and maintenance responsibility. Where the site sits in a critical drainage area the council will have specific requirements, so check its strategic flood risk assessment.

On many small sites this section is really a Drainage Strategy in miniature, and there is no harm in producing it as a separate document cross-referenced from the assessment. Councils often prefer that, because the drainage engineer and the flood risk reviewer are different people.

Residual risk and emergency planning

Residual risk is what remains after the design measures work as intended, plus what happens when they do not. Cover the failure of a defence or a blockage, an event exceeding the design standard, the flood warning service and how occupants receive warnings, the evacuation or refuge strategy, and flood resilient construction to a stated level with materials specified.

Then close with a short statement of the assessment's conclusion, which should say in plain terms that the development is safe for its lifetime without increasing flood risk elsewhere, and set out the measures that deliver that. Residual risk and finished floor levels goes further into how that section is written.

A Flood Risk Assessment draft can give you this structure and flag the site facts it cannot know, including levels, modelled data and infiltration results, which have to come from survey and from the Environment Agency. The rest of the evidence base sits across all 66 report types.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and check the current climate change allowances at the time of writing.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.