Climate change allowances: choosing epoch and percentile

Two decisions settle most of a climate change allowance flood risk assessment: which epoch you model against, and which allowance category you apply within it. Both fall out of one prior question, the lifetime of the development. Fix that honestly and the rest is arithmetic and presentation. Get it wrong and the Environment Agency will say so in its consultation response, usually in a sentence that costs you six weeks.

The allowances themselves are published data, not judgement. The judgement sits in how you pair them with the proposal.

What the allowances are and where they live

The Environment Agency publishes peak river flow, peak rainfall, sea level rise, offshore wind speed and extreme wave height allowances on gov.uk under "Flood risk assessments: climate change allowances". They are derived from the UK Climate Projections and they are revised. The 2022 revision moved peak river flow allowances from river basin district to management catchment and rebuilt the rainfall figures on the same geography, which is why older consultants' templates still quote numbers that no longer exist.

Peak river flow allowances are given against three epochs, the 2020s, the 2050s and the 2080s, with the last running to 2125. Within each epoch the figures come in categories: central, higher central and upper end. Peak rainfall allowances are published against their own epochs and their own categories, and they do not map one to one onto the river flow set. Read both tables. Do not assume the structure of one from the other.

None of this is policy on its own. National planning policy requires development to be safe for its lifetime without increasing flood risk elsewhere, and the Planning Practice Guidance chapter on flood risk and coastal change is what sends you to the allowances. The allowances are the evidence you use to answer the policy question in a Flood Risk Assessment.

Fixing the lifetime of the development

Residential development is taken to have a lifetime of 100 years unless the permission is time limited. Non-residential is commonly assessed over 60 years, though that figure is a convention rather than a rule, and a council can reasonably ask why you chose it for a distribution shed with a 30-year lease behind it.

The clock starts at anticipated completion, not at the date you submit. On a scheme with a two-year build and a year in determination, that is a three-year shift, and it is occasionally enough to push the end date across an epoch boundary.

Time-limited uses are the genuine exception. A caravan site with a ten-year permission, a temporary classroom block, a meanwhile use on a cleared site: each of those can justify a shorter lifetime and therefore an earlier epoch, provided the application itself is framed that way and the condition is accepted. Do not claim a short lifetime for a building that will plainly still be there in 2090.

Matching the epoch to that lifetime

Once the end year is settled, the epoch is the one that contains it. Where a lifetime straddles two epochs, you model against the later one, because the development has to be safe at the end of its life and not merely in the middle.

The following is illustrative only.

Where the end year runs beyond 2125, you use the 2080s allowances and say plainly in the assessment that these are the furthest published figures. That is not a weakness in the submission. It is the limit of the dataset and reviewers know it.

Choosing the allowance category by vulnerability

The category is set by what you are putting on the site, not by how much attenuation you would like to build. The Environment Agency guidance pairs the flood zone with the flood risk vulnerability classification and tells you which allowance categories to assess, and for most residential proposals in Flood Zone 3a that means running more than one.

The pattern to hold in your head is this. Water compatible and less vulnerable uses in lower zones sit at the central allowance. More vulnerable uses in Flood Zone 3a step up, with the higher central figure as the design case and the upper end run as a test of what happens when the design case is exceeded. Highly vulnerable uses and essential infrastructure, where they are permitted at all, are assessed at the upper end. Verify the current pairing against the published table rather than a previous job, because the pairings have moved.

Vulnerability is a property of the use, so a basement flat and a ground floor flat in the same building are classified differently. That single point causes more arguments than any other part of the exercise, and it sits behind the way basement schemes attract flood risk objections even outside Zone 3.

Where the sequential and exception tests are in play, the allowance category feeds the second limb of the exception test directly. If the development is not safe for its lifetime at the allowance the guidance sets, the exception test evidence fails on its own terms, whatever the wider benefits.

Rainfall allowances and river flow allowances do different jobs

Peak river flow allowances uplift the flows going into a fluvial hydraulic model. Peak rainfall allowances uplift design storms in the surface water calculation. They are separate inputs to separate pieces of work and they are not interchangeable.

The error I see most often in a submitted Drainage Strategy is a single uplift applied across the whole assessment, usually a river flow figure carried into the microdrainage run because it appeared at the top of the page. The reviewer spots it immediately, because the storage volume comes out wrong in a way that has a recognisable shape.

Two further points on rainfall. First, the allowance applies to the design event you have chosen, so state the annual exceedance probability you modelled and the allowance applied to it, in the same sentence. Second, an allowance is not a substitute for testing what happens beyond the design event, which is where exceedance routing and above-ground flow paths earn their place. Whether measures such as rainwater reuse count towards the volume is a separate question, and the position on rainwater harvesting and attenuation credit is narrower than most applicants expect.

Sensitivity testing and how to present it

Model the baseline, then each allowance you are required to assess, and put the results in one table. Flood level, depth on the site, and where relevant hazard rating, for each scenario. A reviewer should be able to read across a single row and see what the site does.

Then show the design response at the upper end run as well as at the design case. Finished floor levels with the freeboard stated and justified, access and egress routes with depths on them, and the consequences if the upper end scenario occurs. Label this as residual risk and keep it short.

Where you have made an assumption that materially moves the answer, such as a defence assumed to remain in place or a channel assumed to be maintained, name it in the same section rather than burying it in an appendix. Uncertainty stated up front is a much easier conversation than uncertainty found by a consultee.

When the allowances change mid-application

They will, eventually. The practical protection is to record the version and the date in the assessment: which allowances page, which management catchment, which figures, accessed on which date. If a revision lands during determination, that record turns a rebuild into a check.

If the revision is upward and material, expect to re-run before committee rather than after. Councils are reluctant to resolve to grant on superseded evidence, and an officer who has to explain that in a committee report will usually ask for the update first.

Enter your postcode on PlanScope to see which reports your site is likely to need, including whether a flood risk assessment is triggered. 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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