Flood risk assessments in the west London Boroughs of Barnet, Brent, Ealing, Harrow, Hillingdon and Hounslow will need special reference to the West London SFRA. The West London SFRA covers additional requirements for flood risk assessments that may not be in place in other London Boroughs.

What is the West London SFRA?

The West London Strategic Flood Risk Assessment (SFRA) has been produced to cover the London Boroughs of Barnet, Brent, Ealing, Harrow, Hillingdon and Hounslow. It is an online portal with web based mapping and supporting text sections.

What is different in the West London SFRA Boroughs?

While the guidance in the West London SFRA stems from the NPPF, these Boroughs have specific requirements for Flood Risk Assessments (FRAs) for planning relating to surface water flooding. In these Boroughs, surface water flooding is treated as equal to fluvial flooding and thus the requirements for mitigation measures are on par with what could be seen for fluvial flooding.

While for most Local Planning Authorities across England, fluvial and tidal flooding is usually used to define the Functional Floodplain (and the 1 in 20 year event typically used), the West London Boroughs also delineate Flood Zones and the Functional Floodplain using pluvial flooding. In the West London Boroughs, pluvial Flood Zone 3a is defined by the 1 in 100 year event and pluvial Flood Zone 3b (Functional Floodplain) is defined by the 1 in 30 year event, as modelled by the EA Risk of Flooding from Surface Water (RoFSW) mapping.

Flood Risk Assessments in West London

Therefore, while a site in these Boroughs may be in Flood Zone 1 from fluvial or tidal flooding, if the site could be affected in the 1 in 30 year event based on the EA RoFSW mapping, it would be considered Flood Zone 3b by the councils. As a result, even if your site is in Flood Zone 1 according to the EA Flood Map for Planning, if your site is at risk of flooding from surface water, an FRA could be required in support of your planning application. If you are unsure whether an FRA is required for your planning application, please get in touch with the team.

Further to the classification of Flood Zone 3a and 3b in the West London Boroughs and the requirement for an FRA that comes with it, the planning authorities also require an assessment of the loss of pluvial flood storage as a result of increased footprint. Under the NPPF, FRAs should consider the impact of a proposed development on flood risk elsewhere; usually with regards to fluvial flooding.

The West London SFRA states that an FRA for planning should include:

evidence demonstrating that there will be no net loss of floodplain storage for all proposed developments in Flood Zone 3b and Flood Zone 3a.

This applies to both fluvial and pluvial Flood Zones 3a and 3b, as the SFRA continues:

If permissible development decreases the volume of a fluvial floodplain or surface water flood area, flood storage compensation needs to be provided. The compensatory storage provided must equal or exceed the storage lost to ensure there will be no net loss of flood storage. Where developments are proposed within Flood Zone 3a (surface water), floodplain compensation must account for predicted flood depths for the 1 in 30yr and 1 in 100yr RoFSW mapping or depths predicted by site specific modelling

We at Aegaea have extensive experience in producing Compensatory Flood Storage (CFS) exercises to meet the pluvial requirements of the West London SFRA using industry standard GIS software. The team have supported over 100 developments across these Boroughs and we encourage you to get in touch via the details below to discuss how we can support your development also from an early stage. Get in touch with Aegaea today to see how we can support your flood risk assessment in West London.