Flood Risk & Drainage

Flood Zones

The Environment Agency classifies land into flood zones: Zone 1 (low probability — less than 1 in 1,000 annual probability), Zone 2 (medium probability — between 1 in 100 and 1 in 1,000), Zone 3a (high probability — 1 in 100 or greater), and Zone 3b (functional floodplain).

Sequential Test

Development should be directed to areas with the lowest risk of flooding. The sequential test aims to steer new development to Flood Zone 1. If there are no reasonably available sites in Zone 1, then Zone 2, and so on. For allocated sites, the sequential test is applied at plan-making stage.

Exception Test

Where the sequential test is passed but the site is in Zone 2 or 3, the exception test may be required. It has two parts: the development must provide wider sustainability benefits that outweigh the flood risk, and a site-specific FRA must demonstrate that the development will be safe for its lifetime.

Site-Specific Flood Risk Assessment

All development in Zones 2 and 3, and development over 1 hectare in Zone 1, requires a site-specific FRA. The FRA must assess all sources of flooding, apply climate change allowances, and demonstrate the development is safe without increasing flood risk elsewhere.

Sustainable Drainage (SuDS)

Major developments must incorporate SuDS unless demonstrated to be inappropriate. The SuDS hierarchy prioritises infiltration, then attenuation and controlled discharge. Discharge rates should be restricted to greenfield runoff rates. The LLFA is a statutory consultee on surface water drainage for major applications.

Related Topics: Environment Agency, LLFA, Climate Change