Flood Map for Surface Water

The Environment Agency’s surface water flood maps give an indication of the broad areas likely to be at risk of surface water flooding, i.e. areas where surface water would be expected to flow or pond.

In 2010 the Flood and Water Management Act defined ‘surface runoff’. Generally, the type of flooding shown by the Flood Map for Surface Water (FMfSW) fits with the definition in the Act and shows the flooding that takes place from the 'surface runoff' generated by rainwater (including snow and other precipitation) which:

is on the surface of the ground (whether or not it is moving), and
has not yet entered a watercourse, drainage system or public sewer.

The FMfSW will pick out natural drainage channels, rivers, low areas in floodplains, and flow paths between buildings. But it will only indicate flooding caused by local rainfall. It does not show flooding that occurs from overflowing watercourses, drainage systems or public sewers caused by catchment-wide rainfall events or river flow. It is therefore very important that users apply local knowledge and in particular the 'locally agreed surface water information' held by the lead local flood authority to assess how suitable the Flood Map for Surface Water is for their needs.

Note: Environment Agency surface water flood maps are not suitable for identifying whether an individual property will flood. This is because the modelling only gives an indication of broad areas at risk, and because we do not hold information on floor levels, construction characteristics or designs of properties. We would need this and other detailed information to be able to say whether flooding of certain depth would enter into an individual property and cause damage.

FMfSW may be suitable for identifying where properties are in areas at risk of flooding for locations where surface water flooding is strongly influenced by topography.

Two rainfall events, one with a 1 in 30 and the other with a 1 in 200 chance of occurring in any year, are modelled and mapped. However, users must note that this is the chance of this rainfall, and not of the resulting flood extent occurring. Consequently it only provides a general indication of areas which may be more likely to suffer from surface water flooding in these rainfall probabilities.

For each rainfall probability, the map provides two layers of information which can be used individually to indicate:

'Surface Water Flooding' (flooding greater than 0.1m deep);
'Deeper Surface Water Flooding' (flooding greater than 0.3m deep).

When the information for each rainfall probability are shown together (with ‘deeper’ displayed on top) the lighter colour will indicate:

Flooding between 0.1 – 0.3m deep.

The 0.3m threshold is chosen as it represents a typical value for the onset of significant property damages when property flooding may start (above doorstep level) and because it is at around this depth that moving through floodwater (driving or walking) may become more difficult; both of which may lead users to consider the need to close roads or evacuate areas.

Given the assumptions and the lack of comprehensive validation of the results, it is important that users:

do not rely on the Flood Map for Surface Water alone to show expected areas of surface water flooding;
interpret the Flood Map for Surface Water as defining the flood extent only approximately for the given probability event;
interpret the Flood Map for Surface Water bands as only approximate predictions of specific depths;
interpret the 1 in 30 rainfall probability Flood Map for Surface Water as being less representative than the 1 in 200 rainfall probability Flood Map for Surface Water.