Climate change allowances in a drainage strategy

The climate change allowance drainage strategy question has one right answer and several plausible wrong ones. Surface water drainage design uses the Environment Agency's peak rainfall intensity allowances, chosen by management catchment and epoch. River modelling uses the peak river flow allowances, which are different numbers for different purposes. Applying a river flow uplift to a rainfall calculation, or the reverse, produces storage volumes that are wrong by a wide margin and a reviewer who stops reading.

So the first line of the calculations section should name the allowance, the catchment and the epoch.

Which allowance belongs to which calculation

The Environment Agency publishes climate change allowances as a set of tables serving distinct purposes.

The allowances are stated as percentage uplifts applied to the rainfall depth or the flow, and they are catchment specific rather than national. The 2022 revision moved peak rainfall intensity from a single national table to values by management catchment, which means a national figure quoted in an older report is no longer the right starting point even if it happens to be close.

Peak rainfall against peak river flow

These get muddled because both appear in the same guidance page and both are expressed as percentages.

Peak rainfall intensity allowances apply to short duration, high intensity rainfall over small and urban catchments. That is what a drainage design responds to: the storm that fills a basin, surcharges a pipe or floods a car park. The uplift is applied to the rainfall depth for each design duration before the hydraulic model is run.

Peak river flow allowances apply to flows in watercourses and to the modelling that produces flood levels and extents. They belong in the flood risk chapter, where they set the design flood level used for finished floor levels and for safe access. They have nothing to do with sizing a tank.

A single scheme often needs both, and the two documents have to be consistent. Where the Flood Risk Assessment uses one epoch and the Drainage Strategy another, the reviewer's first question is which one is right, and the second is what else was not checked.

Choosing the epoch for the development lifetime

The epoch is chosen from the lifetime of the development, measured from completion. National guidance treats the lifetime of residential development as 100 years unless there is specific justification for something shorter, and commercial development is generally assessed over a shorter period.

That creates an awkwardness worth stating plainly in the report. A residential scheme completing this decade runs well past the latest epoch for which allowances are published, so the convention is to use the latest available epoch and say so. Nobody is expected to extrapolate the tables, and a reviewer who sees the reasoning stated will accept it. A reviewer who sees an unexplained choice of the nearer epoch will not.

For temporary development the shorter epoch is legitimate, but the justification has to be the permission rather than the client's intention. A scheme with a 25 year temporary consent can reasonably use the nearer epoch. A permanent building whose owner expects to redevelop in 30 years cannot.

Central and upper end allowances

Each epoch has more than one allowance, describing different points in the range of projections. The choice between them is not free.

For river flow, national guidance sets out which allowance to assess according to the flood zone and the vulnerability of the development, and it commonly requires more than one to be tested so that the sensitivity is understood. For rainfall in drainage design, practice is set locally. Many lead local flood authorities require the upper end allowance for the design of attenuation, some require the central allowance with a sensitivity test at the upper end, and some ask for both to be reported with the storage sized on the higher.

Read the local surface water guidance before running the model, because rerunning it later is the expensive way to find out. Where the authority has published nothing, ask, and record the answer in the report with the date and the name of the officer's advice. That single sentence prevents an argument at determination.

Urban creep as a separate uplift

Urban creep is not a climate allowance, though it is routinely applied in the same calculation. It accounts for the gradual increase in impermeable area within a completed development: front gardens paved for parking, rear gardens decked, side returns concreted, extensions built under permitted development rights.

The allowance is a percentage added to the impermeable area rather than to the rainfall, and the figure comes from local guidance. Ten per cent for houses with private gardens is common, with lower figures for flats where there is no private curtilage to pave and higher figures for low density housing with large plots.

Two points get missed. Urban creep applies to the developed impermeable area, so it compounds with the rainfall uplift rather than replacing it. And it applies to the areas that can realistically be paved, so a scheme of flats around a shared courtyard has a weaker case for a high figure than a scheme of detached houses, which is an argument worth making where storage space is tight.

Recording the allowance so it can be audited

A reviewer should be able to reproduce your numbers from the report without asking a question. That takes one short table and one paragraph.

1. The guidance used, with its title and the date of the version consulted.
2. The management catchment name, as the guidance gives it, with the date checked.
3. The epoch selected, and the development lifetime that led to it.
4. The allowance applied, as a percentage, and whether it is the central or upper end figure.
5. The urban creep allowance, the percentage, its source and the areas it was applied to.
6. The rainfall dataset used for the baseline depths, named and dated separately from the uplift.
7. Which calculations each uplift was applied to, so nothing is uplifted twice.

Illustrative example: a strategy for a 60 unit scheme states the management catchment, the 2070s epoch on a 100 year residential lifetime, the upper end peak rainfall allowance as required by the authority's published guidance, a 10 per cent urban creep allowance applied to the plot areas but not to the adopted road, and the rainfall dataset with its version. The reviewer checks four figures against the published tables and moves on. That is the outcome the section exists to produce.

Point 6 deserves emphasis. The baseline rainfall dataset and the climate change uplift are separate choices, and updating one does not update the other. Reports that conflate them tend to double count or to miss the uplift entirely.

What to do when the guidance is reissued

Allowances have been revised more than once, and they will be again. Three situations come up.

Where the guidance changes before submission, use the new version and say so. Where it changes while an application is undetermined, expect the lead local flood authority to ask whether the design still works, and answer with a short addendum rather than a reissued report if the storage is adequate under the new figures. Where it changes after permission but before a drainage condition is discharged, the condition is normally discharged against the current guidance, not the guidance in force when permission was granted, so build a margin into the design if the programme is long.

The practical defence is to design with some headroom and to record the sensitivity. A strategy that reports storage volumes at the central and upper end allowances has already answered the question a revision will prompt. One that reports a single number has to be redone.

PlanScope drafts a Drainage Strategy structured to UK planning policy from your site data and guided answers, and flags catchment-specific figures such as the applicable allowance as still to be confirmed rather than inventing them. Hydraulic design needs a suitably qualified author, and for specialist technical reports, qualified professional input may be required.

If you are working out which reports a site needs before commissioning the modelling, enter your postcode on PlanScope to see the likely set. This is general guidance rather than legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.