Reading the surface water flood risk map for a site

The surface water flood risk map shows where rainfall running over the ground is likely to pond or flow, and it is a completely separate dataset from the Flood Map for Planning. The Flood Map for Planning shows Flood Zones 2 and 3 from rivers and the sea. It does not show surface water at all. A site can sit comfortably in Flood Zone 1 and still have a modelled surface water flow path running through the middle of it.

That is the single most useful thing to know before checking a site, because most householders and a fair number of applicants assume one map covers everything.

What the national surface water map shows

The Environment Agency publishes a national assessment of the risk of flooding from surface water, presented through the public service for checking long term flood risk in England. It models rainfall falling on the ground surface and routing across it, using national terrain data.

Risk is expressed in bands based on the annual chance of flooding in a given location. The bands are defined by the chance of flooding in any year, running from high, through medium and low, to very low, and the service explains the percentage attached to each. Use the wording on the service rather than a remembered figure, because the presentation has been revised as the underlying national assessment has been updated.

The dataset is a screening tool designed for strategic and awareness purposes. It is not a property-level prediction, and the service says so.

Depth, extent and velocity

Three layers are available and they answer different questions.

Extent tells you where water goes. It is the layer everyone looks at, and on its own it overstates the problem, because a modelled extent includes shallow sheet flow of a few centimetres.

Depth tells you how bad it is. A 100 mm ponding depth in a car park is a maintenance issue. A 600 mm depth against a proposed doorway is a design constraint, and above about 300 mm at a vehicular access you are into a safe access and egress argument.

Velocity tells you whether it is dangerous. Depth multiplied by velocity is the usual basis for judging hazard to people, and a fast shallow flow down a steep road can be more dangerous than deeper standing water.

Read all three before concluding anything. A proposal refused on surface water grounds on the strength of an extent map alone is a proposal that was assessed lazily.

Why the Flood Map for Planning does not cover surface water

The two maps exist for different statutory purposes. The Flood Map for Planning supports the sequential and exception tests by showing the probability of flooding from rivers and the sea, ignoring the presence of defences, and it drives the flood zone classification used in national policy.

Surface water is dealt with separately, because it is managed by lead local flood authorities under the Flood and Water Management Act 2010 rather than by the Environment Agency as a fluvial matter. The lead local flood authority is the county or unitary council, and it is the body that holds local knowledge, investigates significant flooding under section 19 of that Act, and is consulted on surface water drainage for major development.

The consequence for an application is that checking one map is checking half the question. A Flood Risk Assessment is required for sites in Flood Zones 2 and 3, for sites of one hectare or more in Flood Zone 1, and for sites in Flood Zone 1 where there are other sources of flooding to consider, which includes surface water.

Checking historic records with the council

Modelled data and lived experience often disagree, and the disagreement is informative.

Ask the lead local flood authority for its records of historic flooding in the area, any section 19 investigation reports, whether the site lies in a critical drainage area identified in the strategic flood risk assessment, and whether any surface water management plan covers it. Ask the sewerage undertaker separately about recorded sewer flooding, which the surface water map does not include.

Local sources that repay ten minutes: the parish council, the strategic flood risk assessment for the district, and neighbours who have lived on the street for twenty years. A flow path that appears on the map and has never produced a flood may reflect a culvert the national model does not know about. A road that floods every winter and shows nothing on the map probably has a drainage asset problem.

None of this should be presented as an argument that the map is wrong. It is context that explains what the map shows.

When a site specific model is needed

A national screening map is enough for many small applications. A site specific model becomes necessary when:

1. A modelled flow path crosses the developable area and the layout depends on where it goes.
2. The site is in a critical drainage area or a surface water management plan area.
3. The proposal changes levels materially, by raising the site, building a basement or adding a substantial retaining structure.
4. The lead local flood authority has asked for one, which usually means it has local knowledge the map does not carry.
5. The scheme proposes to divert or culvert an existing flow route, which needs to be modelled rather than asserted.

Site specific modelling normally means a linked one and two dimensional model using a detailed topographic survey rather than national terrain data, and it should be run by a suitably qualified engineer. The difference in resolution is often the whole answer, because a 2 m grid national model cannot see a 400 mm kerb upstand that actually controls where water goes.

Presenting the finding in a drainage strategy

Whatever the outcome, write it up so a reader can follow it without opening the map themselves.

Include the map extract with a north point, a scale and the date it was accessed. State the risk band at the site and, separately, at the proposed access. Show the depth and velocity layers as well as extent where the site is affected. Describe the flow route in words: where water enters the site, where it leaves and what it does in between.

Then say what the design does about it. Finished floor levels set above the modelled level with a stated freeboard, flow routes kept open through the layout, exceedance routes shown on a drawing, and attenuation sized on the basis set out in the Drainage Strategy. Where the lead local flood authority will check the calculations, the approach set out in what the lead local flood authority checks is the standard to write to.

Keep the flood levels consistent between documents. A drainage strategy quoting one figure and a flood risk assessment quoting another is the fastest way to a request for further information.

Surface water as a reason for refusal

It is a real refusal reason, and it has become more common as the maps have improved.

The grounds usually given are that the development would be at unacceptable risk itself, that it would increase risk elsewhere by displacing flows or increasing run-off, or that safe access and egress cannot be achieved during a flood event. The third is the one applicants underestimate, because a dry building reached by a road under 400 mm of fast-moving water has not solved the problem.

Where an objection is raised late, a topographic survey and a short technical note showing the actual levels will often resolve it, because the national model's terrain data is the usual source of the disagreement. And check flood resistance and resilience measures conditioned before offering measures, since an offer that does not match what the authority conditions elsewhere tends to be refused as inadequate.

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