Risk of Flooding from Surface Water - Hazard

Risk of Flooding from Surface Water (RoFSW) map is an assessment of where surface water flooding may occur when rainwater does not drain away through the normal drainage systems or soak into the ground, but lies on or flows over the ground instead. It is produced using national scale modelling and enhanced with compatible, locally produced modelling from lead local flood authorities (LLFAs). The RoFSW datasets include information about flooding extents, depths, speed and hazards.

This dataset shows information about flood hazard ratings. Flood hazard ratings are a measure of how dangerous a flood might be. They are calculated using a combination of flood depth and speed – higher depths and speed mean more dangerous flooding and a higher flood hazard rating.

RoFSW is a probabilistic product, meaning that it shows the overall risk, rather than the risk associated with a specific event or scenario. In externally published versions of this dataset, risk is displayed as one of three likelihood bandings:

High - greater than or equal to 3.3% (1 in 30) chance in any given year; Medium - less than 3.3% (1 in 30) but greater than or equal to 1% (1 in 100) chance in any given year; Low - less than 1% (1 in 100) but greater than or equal to 0.1% (1 in 1000) chance in any given year.

Note that all locations outside of mapped extents should be considered at ‘Very Low’ risk. ‘Very Low’ extents in the dataset have been explicitly modelled as below 1 in 1000 chance.

This hazard dataset shows the likelihood of a flood occurring with a given flood hazard rating (or higher). There are separate layers for hazard ratings of 0 (i.e. flooding extent), 0.25, 0.5, 0.75, 1.25 and 2.