Assessing surface water flood risk from the national map

Surface water flood risk is assessed from a different dataset than fluvial risk, and confusing the two is the commonest error in a small practice. The flood map for planning shows Flood Zones 2 and 3, which are about rivers and the sea and which deliberately ignore defences. Surface water risk, sometimes called pluvial risk, comes from rain falling faster than the ground and the drains can take it, and it is shown on a separate national map published by the Environment Agency.

Plenty of sites sit in Flood Zone 1 and still have a band of high surface water risk running diagonally across the middle of them. That band, not the zone, is what will shape the layout.

What the national map models

The map is the output of a national scale hydraulic model. Rain is applied across a terrain surface, an allowance is made for what soaks in and what the sewers take, and the water is routed downhill.

That approach produces good regional screening and unreliable site detail, because the model does not know the things that matter on a plot. It does not know where your gullies are, whether the culvert under the neighbouring garden is blocked, how high the kerbs are, or that the ground was resurfaced in 2019. Its terrain data has a grid size that smooths out exactly the features which control shallow flow.

So read it as a question rather than an answer. If it shows water crossing the site, something makes it cross the site, and your job is to find out what.

The Environment Agency has revised its national flood risk information, including the surface water outputs, so state which dataset and which date you used in the assessment. A reviewer comparing your figures against a newer version needs to know why they differ.

Reading the outputs properly

Three outputs are published and they answer different questions.

Extent tells you where water is predicted to sit or flow for a given annual chance of flooding, in bands from high down to very low. Extent alone is close to useless for design, because it does not distinguish a puddle from a hazard.

Depth is the one that matters for floor levels. Look at depth for the more extreme events as well as the frequent ones, because that is what sets the finished floor level and the threshold detail.

Velocity, combined with depth, tells you about hazard to people. Shallow fast water in a channelled route between buildings is more dangerous than deeper still water on open ground, and it is the combination that determines whether a route is safe for access and egress.

Look at all three across the site, not at the single worst point. A site with a narrow high risk corridor along one boundary and nothing elsewhere is a design opportunity. A site with medium risk over the whole footprint is a different proposition.

Flow routes and where the water goes next

The most useful thing the national map gives you is a picture of overland flow. Water arrives from somewhere and leaves somewhere.

Trace the route on and off the site using the map together with contours, then walk it. Look for the low point in the boundary wall, the dip in the lane, the gully that always silts up, the point where a field drain outfalls into a ditch. Ask the neighbours, who will tell you exactly where the water goes because they have watched it.

Two design rules follow.

Do not build in the flow route. Sounds obvious, and schemes do it constantly by placing a terrace across a natural low line and then relying on a drainage system to compensate.

Do not make it worse downstream. Blocking a route that currently crosses the site sends water to the property next door, which is a civil liability as well as a planning objection.

Critical drainage areas and local designations

The national map is not the only screening layer. Councils identify critical drainage areas in their strategic flood risk assessments, where surface water problems are known and where policy requires more of an applicant.

Check the strategic flood risk assessment, the local plan policies map and any surface water management plan for the catchment. In some boroughs a critical drainage area designation triggers a full Flood Risk Assessment for a single dwelling.

While you are there, check whether a Sequential Test is engaged. The test is usually associated with fluvial zones. Surface water risk is a source of flooding too, and councils apply sequential thinking to it when steering development within a site.

What the lead local flood authority expects

For major development the lead local flood authority is a statutory consultee on surface water drainage, and its comments carry weight. What it wants to see is consistent between authorities.

1. Existing conditions described from the national map, from ground investigation, and from a walkover, with photographs.
2. The discharge hierarchy worked through in order: infiltration first, then a watercourse, then a surface water sewer, then a combined sewer as a last resort, with evidence for each rejection.
3. Discharge rates limited to the greenfield rate where the site is greenfield, or reduced substantially where it is previously developed.
4. Storage sized for events including an allowance for climate change, with a table of volumes and a plan showing where the water sits.
5. Exceedance routes drawn for events beyond the design storm, showing where water goes when the system is full.
6. A maintenance schedule naming who does what and how often, for the whole design life.

Put all of that in the Drainage Strategy rather than scattering it between the flood risk assessment and the engineer's drawings. Reviewers work from one document and mark it against a checklist.

Ground conditions and the infiltration question

The discharge hierarchy starts with infiltration, so the first thing a reviewer looks for is whether soakaways were tested and, if rejected, why.

Test properly. Soakage testing to a recognised method, in trial pits at the depth and position of the proposed feature, with three fills, and with groundwater levels recorded. A single percolation test in a garden in August tells you nothing about February.

There are places where infiltration is unacceptable regardless of the rate: within a source protection zone where the ground is contaminated, over shallow groundwater, on made ground of unknown composition, or where it would destabilise a slope or a neighbouring basement. Say which of those applies rather than reporting a poor rate and moving on.

Where infiltration is ruled out, the next step is a watercourse. That needs the consent of the riparian owner and, for a main river or an ordinary watercourse, land drainage or flood risk activity consent from the relevant authority. Those consents take time and are not a planning matter, so start them in parallel.

Recording the assessment

Write the surface water section so someone can repeat it. Name the dataset and the date it was downloaded. Include a screenshot with the site boundary overlaid and a legend that is legible. State the depth and hazard figures you took from it and where on the site they apply. Then say what you did about them.

The honest uncertainty is worth stating too. National modelling of shallow flooding is approximate, and where the outputs matter to floor levels a site specific model is the right answer rather than a national screening product. Saying that in the report is better than being asked.

If you want to see which flood and drainage documents a site is likely to need, run a postcode check or look through the flood and drainage report types.

This is general guidance rather than legal advice. Check the position with your local planning authority 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.