Flood Zones Explained
Many people living in England will be surprised to learn that their home is situated in an Environment Agency Flood Zone. In fact, everybody in England is living in a Flood Zone, as defined by the Environment Agency for planning purposes.
Flood Zones are defined by the Environment Agency in the document Technical Guidance to the National Planning Policy Framework, published by the Department of Communities and Local Government in March 2012. 
Flood Zones refer to the annual probability of river and sea flooding, ignoring the presence of defences. There are four Flood Zones:
- Flood Zone 1 – low probability
- Flood Zone 2 – medium probability
- Flood Zone 3a – high probability
- Flood Zone 3b – the functional floodplain (i.e. land where water has to flow or be stored in times of flood)
The Environment Agency publishes a publicly-accessible flood map for planning purposes which uses colour coding to display Flood Zones 2 and 3. Flood Zone 1 is not displayed but comprises all the land that falls outside Flood Zones 2 and 3. In short, everybody in England lives in an Environment Agency Flood Zone, though many will be situated in Flood Zone 1 and with a low probability of river and sea flooding. 
The purpose of the Environment Agency’s definition of Flood Zones
The Environment Agency’s Flood Zones are defined for planning purposes. Local Authorities have a statutory duty to produce a Local Plan, which identifies sites that may be suitable for new development. In developing its Local Plan, a Local Authority is advised to carry out a Strategic Flood Risk Assessment, which gathers together information on all known sources of flooding that may affect existing or future development in its area. The Environment Agency’s Flood Zones are intended to guide a Local Authority in carrying out a Strategic Flood Risk Assessment and identifying sites for new development, the intention being to steer development away from areas at the highest risk of flooding to areas at the lowest risk of flooding. The Strategic Flood Risk Assessment identifies and maps areas that have a ‘low’, ‘medium’ and ‘high’ risk of flooding.
The Environment Agency defines an “area at risk of flooding” as being land within Flood Zones 2 and 3, or land in Flood Zone 1 that has critical drainage problems and has been notified as such to the Local Planning Authority. However, “flood risk” means risk from all sources of flooding, including river and sea flooding but also “directly from rainfall on the ground surface and rising groundwater, overwhelmed sewers and drainage systems, and from reservoirs, canals and lakes and other artificial sources.” A Strategic Flood Risk Assessment takes all these sources into account, with the Environment Agency’s Flood Zones comprising a significant contribution.
Flood Zones and Flood Risk Assessments
A Local Planning Authority uses the Environment Agency’s Flood Zones, together with its Strategic Flood Risk Assessment (if available), to identify if a site-specific flood risk assessment is required when it considers a planning application. The requirements for a Flood Risk Assessment, and the level of detail it should contain, vary according to the Flood Zone a site is located in. All development proposals in Flood Zones 2 and 3 should be accompanied with a Flood Risk Assessment. For sites in Flood Zone 1, the Environment Agency’s guidelines are as follows:
“For development proposals on sites comprising one hectare or above, the vulnerability to flooding from other sources as well as from river and sea flooding, and the potential to increase flood risk elsewhere through the addition of hard surfaces and the effect of the new development on surface water run-off, should be incorporated in a flood risk assessment. This need only be brief unless the factors above or other local considerations require particular attention.”
The Environment Agency also states with regard to Flood Zone 1: “In this zone, developers and local authorities should seek opportunities to reduce the overall level of flood risk in the area and beyond through the layout and form of the development, and the appropriate application of sustainable drainage systems. Sustainable drainage systems cover the whole range of sustainable approaches to surface drainage management. They are designed to control surface water run off close to where it falls and mimic natural drainage as closely as possible.” There is a similar policy aim for Flood Zones 2 and 3 but with the additional aims of relocating existing development to land with lower risk of flooding, and of safeguarding open space for flood storage.
The Environment Agency’s designation of Flood Zones is also a factor in determining whether a proposed development is appropriate for the Flood Zone it is located in. It has produced a ‘Flood Risk Vulnerability’ classification table for this purpose, comprising five classes:
- Essential infrastructure (such as essential utility infrastructure which has to be located in a flood risk area for operational reasons)
- Highly vulnerable uses (such as basement dwellings, mobile homes, and police and fire stations)
- More vulnerable uses (such as hospitals, residential care homes, student halls of residence)
- Less vulnerable uses (such as buildings used as shops, restaurants or offices)
- Water-compatible development (such as flood control infrastructure)
With regard to the Flood Zones, only essential infrastructure and water-compatible development should be permitted in Flood Zone 3b, for instance, whilst ‘highly vulnerable’ uses should not be permitted in Flood Zone 3a. The end use of a development will also have a consequence for the level of detail a Flood Risk Assessment should contain, with ‘less vulnerable’ uses generally requiring less detail.
Planning Guidance: Flood Risk Assessments
It can be seen from the above that there is no “one size fits all” when it comes to assessing flood risk. The government’s planning portal website states that “site-specific flood risk assessments should always be proportionate to the degree of flood risk.” A flood risk assessment should also “be appropriate to the scale, nature and location of the development. For example, where the development is an extension to an existing house (for which planning permission is required) which would not significantly increase the number of people present in an area at risk of flooding, the local planning authority would generally need a less detailed assessment to be able to reach an informed decision on the planning application. For a new development comprising a greater number of houses in a similar location, or one where the flood risk is greater, the local planning authority would need a more detailed assessment.”
 The DCLG’s Technical Guidance to the National Planning Policy Framework, containing the Environment Agency’s definition of Flood Zones, is available as a PDF document from the GOV.UK website.
 Flood Zones are based on the annual probability of river and sea flooding, ignoring the presence of defences. For example, it is unlikely, but possible, that a flood with an annual probability of 1% will occur two years running. The definitions are set out in the National Planning Policy Guidance, as follows:
- Flood Zone 1 – Land assessed as having a less than 1 in 1,000 annual probability of river or sea flooding (<0.1%) in any year.
- Flood Zone 2 – Land assessed as having between a 1 in 100 and 1 in 1,000 annual probability of river flooding (0.1% – 1%), or between a 1 in 200 and 1 in 1,000 annual probability of sea flooding (0.5% – 0.1%) in any year.
- Flood Zone 3 – Land assessed as having a 1 in 100 or greater annual probability of river flooding (>1%), or a 1 in 200 or greater annual probability of flooding from the sea (>0.5%) in any year.
Ford near Clun © Copyright Anthony Bloor and licensed for reuse under a Creative Commons Attribution-ShareAlike 4.0 International License.