Obtaining modelled flood levels from the Environment Agency

The flood map for planning tells you which zone a site sits in. It does not tell you the level the water reaches, and for anything that needs a finished floor level, a freeboard or a safe access route, the level is the number you need. That comes from a separate request to the Environment Agency for its detailed modelled flood data, and it takes weeks rather than minutes.

Request it at the start of the project. Applicants who leave it until the flood risk assessment is being written lose a month they did not plan for.

What a detailed flood data request returns

Where a hydraulic model exists for the watercourse, the Environment Agency can supply the modelled outputs behind the published map.

A typical response includes flood levels at model node points, given in metres above Ordnance Datum, for a range of return periods and for defined climate change scenarios. It usually also includes flood extents, the location of the nodes relative to the site, and information about the model itself: when it was built, what it covers, and any known limitations.

The last part matters more than people expect. A model built for a strategic study fifteen years ago, upstream of a scheme that has since changed the channel, is data with a caveat attached, and the caveat belongs in your report.

Some information is published free under the Open Government Licence and some involves a charge for staff time. Check the current position and the published turnaround before you build a programme around it.

Choosing the right event and return period

Ask for what the assessment needs rather than everything available, and say what the site is.

For fluvial sites the design event for most development is the one per cent annual exceedance probability flood, commonly called the one in a hundred year event, with the climate change allowance applied. For tidal and coastal sites the equivalent is the half per cent event. The extreme one tenth of one per cent event is used to test residual risk and the consequences of defence failure or overtopping.

Ask for the undefended as well as the defended case where defences exist, because the assessment has to consider what happens if they fail. And be specific about where you want node data, giving a grid reference and the red line boundary, because a level at a node 800 metres downstream of your site is not the level at your site.

Climate change allowances applied to the output

Allowances are published by the Environment Agency for peak river flow and for peak rainfall intensity, varying by management catchment, by the period being considered and by the allowance category.

Two things follow. The allowance to use depends on the vulnerability of the development and the lifetime you are assessing, which for residential is conventionally taken as a long horizon. And the allowances are revised periodically, so a level taken from a report written three years ago may be based on a superseded figure.

State in the assessment which allowance category you applied, for which period, and from which published version. A reviewer checks that before anything else.

Turnaround, fees and programme

Treat the request as a long lead item.

The published service standard varies and requests involving bespoke work take longer than those answered from existing outputs. There is no realistic way to accelerate it, and a request made a fortnight before a target submission date will not be answered in time.

Two practical habits help. Send the request as soon as the site is under offer rather than after planning fees are committed. And ask the lead local flood authority at the same time for anything it holds, because surface water and ordinary watercourse flooding sit with that authority rather than with the Environment Agency, as does the strategic flood risk assessment that may already contain the level you need.

Reading the output against site survey levels

This is where the arithmetic gets done, and where errors are expensive.

Modelled levels are given to Ordnance Datum. Your site levels have to be on the same datum, from a measured topographical survey tied to a benchmark, not from remotely sensed national data. Comparing a modelled level against an approximate ground level produces a finished floor level you cannot defend.

Where the nearest node is some distance away, interpolating between nodes along the channel is normal practice, but the method has to be stated and it has to account for the gradient of the water surface. Do not average two nodes either side of a structure.

Then add freeboard. A margin above the design level is standard practice, commonly a few hundred millimetres, with the precise figure set by the local planning authority or the Environment Agency in its consultation response. Our note on setting finished floor levels and freeboard on a flood site works through the calculation in full.

When no model exists

Plenty of watercourses have never been modelled, particularly ordinary watercourses and small catchments, and a request will come back saying so.

Three routes remain. Commission hydraulic modelling, which is proportionate for a scheme of any size but heavy for a single dwelling. Use a simplified hydrological estimate of flows and a conservative approach to levels, agreed in advance with the authority. Or design out the problem by raising levels well clear of any plausible flood level and demonstrating safe access, which is often the practical answer on a small site.

Whichever route, agree the approach with the Environment Agency or the lead local flood authority before doing the work. An unagreed methodology is the most common reason a flood risk assessment comes back.

One habit worth adopting on any site with defences. Ask what standard the defences provide, who maintains them, and whether they are formal Environment Agency assets or privately held. A scheme relying on a privately maintained embankment with no maintenance agreement is relying on something that may not be there in thirty years, and a consultee will say so.

Using the data in a flood risk assessment

The levels feed almost every part of the document.

They set the finished floor levels and the freeboard. They determine whether safe access and egress can be achieved above the design level. They inform the resistance and resilience measures for the residual risk case. They underpin the Sequential Test where one is required, and they establish the baseline against which any compensatory storage is designed.

Put the data request, its reference and its date in an appendix to the Flood Risk Assessment, along with the model information and the caveats supplied with it. An assessment that quotes a level without saying where it came from will be queried, and the query costs more time than the appendix would have.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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