Finding the right climate change allowance for a catchment

Climate change allowances in planning come from one place: the Environment Agency's guidance on climate change allowances for flood risk assessments, published on gov.uk. It gives uplifts for peak river flow and peak rainfall intensity by management catchment, for defined epochs, in bands from central to upper end, plus sea level rise allowances by region. Everything a flood risk assessment says about climate change should trace back to a named table in that guidance, on a stated date.

The figures are revised as the national climate projections are updated, and they have changed materially more than once. So the version matters as much as the number, and an assessment that does not record which version it used cannot be checked by anyone.

Where the allowances are published and how often they change

The guidance sits as a single page with a set of tables, and it carries a "last updated" date at the top. Read that date before you read anything else. Substantial revisions followed the publication of new national projections, and the structure changed too: peak river flow allowances were once given by river basin district and are now given by management catchment, which is a much finer grain.

Do not take the figures from an earlier flood risk assessment on a neighbouring site, from a consultant's internal template, or from a software default. Each of those is a snapshot, and on a site where the allowance decides the finished floor level, an out-of-date uplift is an expensive error to find at the consultation stage.

Alongside the river and rainfall allowances, the guidance covers sea level rise by region as a cumulative figure over the period, and allowances for offshore wind speed and extreme wave height for coastal work.

Identifying the management catchment

The allowances are keyed to management catchments, which are the units the Environment Agency uses for water management planning. There are far more of them than river basin districts, and neighbouring catchments can carry noticeably different uplifts.

Find the catchment for the site from the Environment Agency's catchment data, then record the name exactly as the guidance spells it. Getting the wrong catchment is the commonest mechanical error in this part of a flood risk assessment, and it usually happens because the site is near a boundary or because the assessor picked the catchment of the nearest large river rather than the one the site drains to.

Two edge cases. Where the site straddles a catchment boundary, use the more onerous allowance unless the Environment Agency agrees otherwise in writing. And where the flood risk at the site comes from a watercourse whose catchment lies mostly in a different management catchment, the allowance follows the source of the flooding rather than the site's postcode.

Choosing the epoch to match the development lifetime

The guidance gives allowances for defined future periods, and the period you assess has to cover the intended lifetime of the development from the date it is expected to be completed.

In practice that means residential development is normally assessed over 100 years, so the longest epoch applies. Commercial and industrial development is commonly assessed over a shorter design life, often around 60 years, and temporary or short-life uses shorter still. Minerals and waste sites are assessed over the period of the operation plus any aftercare.

State the lifetime you assumed and the reason. An assessment that quietly uses a mid-century epoch for a housing scheme will be challenged by the Environment Agency, and the correction will change the finished floor levels after the layout has been fixed.

Where a scheme is phased over many years, the lifetime runs from the completion of each phase, which can mean different epochs for different parts of a large site. Say so rather than averaging.

Central, higher central and upper end

The allowances are given in bands representing different percentiles of the projections. Which band or bands you assess is not a judgement for the assessor: the guidance contains a table linking the flood zone and the flood risk vulnerability classification of the development to the allowances that have to be tested.

The pattern is that more vulnerable development in a higher flood zone has to be assessed against the more onerous bands, often with two bands tested so the sensitivity of the design is visible. Less vulnerable development in a lower zone may need only the central allowance.

Read that table before commissioning any modelling, because it determines how many model runs you are paying for. And present the results band by band rather than reporting only the band that gives the answer you want. The purpose of testing two allowances is to show what happens if the climate response is at the more severe end, so the assessment should say what the residual risk looks like under each, and how the design responds.

Where the upper end allowance puts water over the finished floor level, the honest options are raising levels, changing the layout, flood resilient construction below the design level, or accepting a residual risk with a warning and evacuation plan. That analysis belongs in the exception test where one is required.

Peak rainfall allowances for drainage design

The peak rainfall intensity allowances are a separate set of tables, also by management catchment, and they apply to surface water drainage design rather than to river flooding.

Three things to get right in a drainage strategy.

1. Use the rainfall allowance for the correct catchment and epoch, and state which annual exceedance probability events you applied it to.
2. Apply an urban creep allowance as well, to account for future increases in impermeable area within the site. The figure is normally set by the lead local flood authority or the sewerage undertaker, so ask rather than assume.
3. Check whether the lead local flood authority has its own requirement that goes beyond the national allowance. Some do, particularly in critical drainage areas, and a strategy built on the national figure alone will be returned.

The output the drainage officer is looking for is the attenuation volume and the discharge rate with climate change applied, compared against the greenfield or existing rate, with the calculation sheet attached. The allowance is an input to that calculation, so it has to be visible in the calculation rather than only mentioned in the text.

Recording the version you used

Write a short paragraph in the assessment that a reviewer can check in two minutes. It needs six things.

• The title of the guidance and the "last updated" date shown on it.
• The date you accessed it.
• The management catchment name.
• The epoch, with the development lifetime it corresponds to and the assumed completion date.
• The allowance band or bands tested, with the percentage uplift for each.
• The table in the guidance each figure came from.

Put it in the climate change section rather than in an appendix, and repeat the figures in the drainage calculations. The structure of that section is set out in more detail under the climate change allowances section of an FRA.

This matters for a reason beyond good practice. Where an application is refused and appealed, the inspector will be shown both the applicant's and the Environment Agency's figures, and the party who can show exactly which published table it used is in a much stronger position.

When the allowances change mid-application

Applications get determined against the policy and guidance current at the time of the decision, so a revision published while your application is pending can reopen a settled position. On a 13-week major application that is unlikely. On a scheme that takes a year, with a section 106 agreement to negotiate, it happens.

Three ways to reduce the exposure.

Run a sensitivity test against the next band up at the outset. If the design works under a more onerous allowance, a revision to the guidance is unlikely to change the conclusion and you can say so in one paragraph.

Build headroom into the finished floor levels rather than designing exactly to the threshold. The cost of an extra 150 mm at design stage is small against the cost of a redesign.

And keep the model rather than only the report. Where the allowance changes, re-running an existing model with a new uplift is a short exercise, while rebuilding a model from a PDF is not. Make that a contractual requirement when you commission the work, along with the format the model will be handed over in.

Councils normally expect flood risk work to be prepared by a suitably qualified person, and professional review before submission is strongly advised. PlanScope drafts the report structure, the methodology narrative and the policy framing, and marks every figure it cannot source as to be confirmed, so the modeller supplies the numbers. Enter your postcode to see which reports your site is likely to need.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. For specialist technical reports, qualified professional input may be required.