What's in a Site-Specific Flood Risk Assessment?

A site-specific Flood Risk Assessment is more than a copy of the flood map. A decision-ready FRA has to assess every relevant source of flooding, apply the right climate change allowances, and show exactly how the finished development will stay safe and dry. Here is what a thorough FRA contains.

All sources of flooding — not just rivers

A good FRA considers fluvial (river), tidal and coastal, surface water (pluvial), groundwater, sewer and, where relevant, reservoir or canal flooding. Surface water in particular is often the real risk on sites that sit comfortably in Flood Zone 1 for rivers.

Climate change allowances

Assessments have to look ahead across the lifetime of the development — typically 100 years for residential schemes. That means applying the Environment Agency's climate change allowances for peak river flow and sea level rise, which vary by river basin district and time period. Using the correct allowance for your location and timescale is one of the most common things reviewers check.

Hydraulic modelling and flood levels

Where detailed river modelling exists, an FRA uses it to establish design flood levels at the site; where it does not, modelling or a suitably conservative estimate may be needed. These levels drive the key design outputs that follow.

Finished floor levels, access and egress

From the design flood level, the FRA sets finished floor levels — usually with a freeboard allowance above the modelled level — and demonstrates safe access and escape during a flood event. For more vulnerable uses, showing a safe, dry route in and out is essential.

Surface water and SuDS

Almost every FRA is accompanied by a surface water drainage strategy using sustainable drainage systems (SuDS). The aim is to manage runoff so that the development does not increase flood risk elsewhere — ideally restricting discharge to greenfield rates. Right-sizing attenuation storage, rather than over-engineering it, is where proportionate design keeps costs down. See our SuDS and drainage strategy service.

Residual risk and resilience

Finally, the FRA addresses residual risk — what happens in an event larger than the design standard, or if defences fail — and may recommend flood-resilient construction, flood warnings and evacuation planning.

Read next: Do I need an FRA? or send us your site for a fixed-fee quote.

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