West London SFRA: Essential Flood Risk Assessment Guidance
Six West London boroughs operate under a distinctive SFRA that places surface water flooding on equal footing with river and tidal flooding, with unique compensatory storage requirements.
Flood risk assessments represent a crucial component of planning applications across six West London Boroughs: Barnet, Brent, Ealing, Harrow, Hillingdon, and Hounslow. These areas operate under the West London Strategic Flood Risk Assessment (SFRA), which establishes distinctive requirements that differ significantly from other London regions.
Key Distinction: Surface Water Treatment
A fundamental difference in these boroughs involves how surface water (pluvial) flooding is addressed. Rather than treating it as secondary to fluvial flooding, these councils place surface water flooding on equal footing with river and tidal flooding. Consequently, developers must implement comparable mitigation measures and compensatory storage solutions.
Flood Zone Definitions
The West London SFRA delineates flood zones using the Environment Agency’s Risk of Flooding from Surface Water (RoFSW) mapping:
- Pluvial Flood Zone 3a: Defined by the 1 in 100-year surface water event
- Pluvial Flood Zone 3b (Functional Floodplain): Defined by the 1 in 30-year surface water event
This differs from standard practice elsewhere in England, where these zones typically reflect fluvial and tidal risk.
FRA Requirements and Implications
A significant practical consequence emerges: a site classified as Flood Zone 1 according to the EA Flood Map for Planning might still require an FRA if surface water risk exists within the 1 in 30-year event based on RoFSW mapping.
The SFRA explicitly requires that developments demonstrate “no net loss of floodplain storage for all proposed developments in Flood Zone 3b and Flood Zone 3a.” This obligation applies to both fluvial and pluvial flood zones, mandating that any lost flood storage must be compensated through equivalent or greater replacement storage.
The compensatory storage calculations must account for predicted flood depths from either the 1 in 30-year and 1 in 100-year RoFSW mapping or site-specific modelling results.
Aegaea has supported over 100 developments across these boroughs, specialising in compensatory flood storage exercises using industry-standard GIS software.