Climate change allowances section of an FRA

The climate change allowances section of a flood risk assessment is where the design flood level is fixed, which means it decides the finished floor level, the mitigation and often whether the layout works at all. Everything downstream in the report depends on getting three inputs right: the lifetime of the development, the epoch that follows from it, and the allowance band the vulnerability classification requires.

Written properly it runs to a page and a half. Written badly it produces a floor level that a lead local flood authority rejects at consultation stage, three months in.

This is general guidance rather than legal advice, and the allowances are updated periodically, so work from the current published tables.

Why this section decides the finished floor level

The chain is short. Modelled flood level, plus the climate change uplift, gives the design flood level for the lifetime of the development. Add freeboard and you have the finished floor level. Compare that to existing ground and you know whether the scheme needs to be raised, redesigned or abandoned.

Because the chain is short, an error at the first link travels straight through. A Flood Risk Assessment that quotes an allowance without saying which epoch, which band and which river basin district it came from cannot be checked, and reviewers treat unverifiable numbers as absent.

Put the section near the front, immediately after the flood zone classification and the sources of flooding. Reviewers read it first anyway.

Choosing the allowance band for the lifetime of the development

Lifetime sets the epoch. National guidance takes the lifetime of residential development as 100 years unless there is specific justification for something shorter, which for a scheme designed today reaches into the latest epoch published. Most commercial and non-residential development is assessed over a shorter period, commonly in the region of 60 years.

Say what you assumed and why, in one sentence. Where a temporary permission or a short design life is proposed, justify it rather than asserting it, because a shorter lifetime produces a lower allowance and reviewers notice the convenience.

Two traps. A residential scheme with a commercial ground floor takes the residential lifetime for the building. And a phased scheme takes its lifetime from completion of the last phase, not the first.

Peak river flow against peak rainfall intensity

These are different allowances doing different jobs, and reports conflate them regularly.

Peak river flow allowances uplift modelled river flows. They are published by river basin district, by epoch, in bands, and they feed the fluvial flood level. If your site is in Flood Zone 2 or 3 from a river, this is the allowance that sets the design flood level.

Peak rainfall intensity allowances uplift short duration rainfall. They are published by management catchment and feed surface water modelling and drainage design, so they belong in the Drainage Strategy as much as in the flood assessment. Attenuation volumes and the storm events tested come from here.

Sea level rise allowances apply on tidal and coastal sites and are published separately by region and epoch.

A site near a river with a large impermeable area needs both of the first two. Saying so in the report, and cross-referring between the two documents, saves a round of consultee comments.

Vulnerability classification and the allowance you must test

National guidance classifies development by vulnerability: essential infrastructure, highly vulnerable, more vulnerable, less vulnerable and water compatible. Housing sits in the more vulnerable class. Most retail, offices and light industry sit in less vulnerable.

The classification, combined with the flood zone, determines which allowance bands you have to assess. The general pattern is that the more vulnerable the use and the higher the flood risk, the more likely you are to have to test an upper end allowance alongside the central one, and to report the consequences of both.

State the classification explicitly with the use it applies to. On a mixed use scheme, classify each use and apply the most demanding requirement to the shared parts, including basements and car parks, which are frequently the lowest point in the building.

An illustrative comparison of central and upper end results

The following figures are illustrative and are used only to show the method. Real allowances must come from the published tables for your river basin district.

Take a modelled one in one hundred annual chance river level of 12.40 metres above ordnance datum at the site. Applying a central allowance produces one design level. Applying an upper end allowance to the same model produces a level several hundred millimetres higher. Ground level across the plot is 12.60 metres.

Under the central allowance the finished floor level, with 300mm freeboard, sits a little above existing ground and the scheme works with a modest raise. Under the upper end allowance the same freeboard puts the floor level well above the pavement, which changes the access, the step arrangement, the drainage connection and the appearance from the street.

Present both. Say which one the design adopts, why, and what would change if the higher figure were required. Reviewers who can see the sensitivity rarely ask for more modelling. Reviewers shown a single number usually do.

Saying where the allowance came from

Four lines of provenance prevent most queries.

• The publication you used and the date you retrieved it.
• The river basin district or management catchment name.
• The epoch and the lifetime assumption behind it.
• The band applied, and any band tested as a sensitivity.

Add a sentence saying that allowances are revised periodically and that the figures should be checked against the current published schedule before the assessment is relied upon. That is honest, and it protects a report that sits in a planning file for two years before anyone builds.

How the section feeds the exception test

Where the exception test applies, the climate change section is the evidence that the development will be safe for its lifetime without increasing flood risk elsewhere. That phrase is doing the work, and the word lifetime is why the epoch choice matters so much.

The Exception Test then combines that safety case with the wider sustainability benefits of the scheme. Both parts have to be passed, and the flood safety half fails most often on residual risk: access and egress in a flood, the depth on the escape route, and what happens in an event larger than the design event.

If you are unclear which of the two tests applies to your site, what the sequential and exception tests must prove sets out the difference before you commit to a modelling scope.

Get the flood evidence scoped before the layout is fixed. You can enter your postcode on PlanScope to see which reports your site is likely to need.

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