Applying Environment Agency climate change allowances

Applying climate change allowances flood risk guidance correctly is mostly a matter of recording your choices well enough that a reviewer can repeat them. The Environment Agency publishes allowances for peak river flow, peak rainfall intensity and sea level rise, organised by geography, by time period and by allowance category. Pick the wrong combination and the modelling is wrong. Pick the right one and fail to say which you picked, and the reviewer has to assume the worst.

How the allowances are structured

Three separate sets of figures do three different jobs.

Peak river flow allowances are percentage uplifts applied to modelled flows in watercourses. They are published by management catchment, in categories that run from central through higher central to upper end, for a series of future time periods.

Peak rainfall intensity allowances are percentage uplifts applied to design storms in surface water and drainage modelling. They are also published by catchment, for stated storm probabilities, with central and upper end categories.

Sea level rise allowances are cumulative figures by region, used for tidal and coastal modelling, and they are applied year by year rather than as a single uplift.

Do not mix them. Using a peak river flow figure in a drainage calculation is a common error in submitted reports, and an experienced reviewer spots it immediately because the number looks wrong for the context.

Choosing the catchment and the period

The geography is the first choice. Allowances are published at management catchment level, so identify the catchment that contains the site rather than the river basin district it sits in. Where a site straddles a boundary, or where the receiving watercourse drains to a different catchment than the site, say which you used and why.

The time period follows the lifetime of the development. Residential development is normally assessed over a lifetime of around 100 years. Commercial development is commonly assessed over a shorter period, often around 60 years, unless the authority says otherwise.

So the working sequence is: establish the lifetime, convert it to an end date from the expected date of completion, and select the time period covering that date. State all three in the report.

An illustrative example. A residential scheme expected to complete in the late 2020s, assessed over a 100-year lifetime, reaches into the last published period, so that is the one to model. A commercial shed over 60 years lands in an earlier period. The dates here are illustrative and your programme will differ.

Selecting the allowance category

The category depends on the flood zone and on the vulnerability classification of the development, which comes from the flood risk tables in the Planning Practice Guidance. Essential infrastructure and highly vulnerable uses are treated more cautiously than less vulnerable ones.

The guidance sets out which category to apply in which circumstances, and it also expects sensitivity testing. Running the central allowance as the design case and the upper end as a sensitivity is the pattern that satisfies most reviewers, because it shows what happens if the more pessimistic projection is realised.

Where the results diverge sharply between categories, say so and explain what it means for the design. A scheme that works under the central allowance and floods under the upper end needs either a design change or a clear statement about residual risk and how it is managed. Hiding the divergence in an appendix is the fastest way to a request for further information.

Recording the selection so it can be checked

This is the part that separates a report that passes review from one that generates correspondence.

• The management catchment name, as published.
• The allowance table version and the date you accessed the guidance.
• The development lifetime, the assumed completion date and the resulting end date.
• The time period selected.
• The allowance category used for the design case and for each sensitivity test.
• The actual percentage or figure applied, written out rather than implied.
• For tidal sites, the sea level rise figures and the base year used.

Put that in a single table near the front of the Flood Risk Assessment rather than scattering it through the modelling appendix. Reviewers at the lead local flood authority and the Environment Agency check these first, and a clear table shortens their response considerably.

Keep the source. A dated copy of the allowance page as you used it belongs in the project file, because the published figures change and a reader in two years will want to know what you were working from. The related note on finding the right climate change allowance for a catchment covers the lookup itself in more detail.

Sea level rise and coastal sites

Tidal and coastal assessments work differently. Sea level rise allowances are given as rates by region, accumulating over the assessment period, and they feed into extreme water level calculations alongside surge and wave effects.

Two points cause trouble. The base year matters, because the allowance accumulates from it, and reports sometimes apply the full period allowance to a level that already includes part of the rise. And coastal sites frequently sit behind defences with their own design life and maintenance regime, so the residual risk case has to consider what happens when those defences reach the end of their life or are overtopped.

Where a site falls within a coastal change management area, the policy tests are different again and the local plan will set them out.

Get the tidal boundary condition agreed with the Environment Agency before the modelling runs. Extreme water levels, joint probability with fluvial flows and the treatment of wave overtopping are all matters where the Agency has a settled position for a given stretch of coast, and adopting it at the start avoids a rebuild of the model later.

When the guidance is updated

The allowances are revised as the underlying projections are updated, and a revision during a live application is a real possibility on a scheme with a long determination period.

Three practical responses. Check the guidance page at submission and note the date. Check it again before any committee date. And where a revision lands mid-determination, write a short addendum stating whether the change affects the conclusions, rather than reissuing the whole assessment.

Consultees will usually accept an assessment carried out under the guidance current at the time of submission where the difference is immaterial. They will not accept silence, and an objection from the lead local flood authority on that basis is easy to avoid and tedious to resolve.

Where drainage design is affected, the revised rainfall figures flow through to storage volumes, so the Drainage Strategy and the flood risk work need to move together. The underlying mapping has its own limitations, set out in the note on reading the Flood Map for Planning.

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

Draft the flood risk and drainage documents for a site on PlanScope, then have the allowances and modelling checked by your flood risk consultant.

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