Flood Zones Explained

If you live in England, it may come as a surprise to learn that you are living in a Flood Zone, as defined by the Environment Agency for planning purposes

The Environment Agency has mapped the whole of England according to the annual probability of river and sea flooding

April 20th 2015

The Environment Agency’s definition of Flood Zones is intended for planning purposes and is documented in the Technical Guidance to the National Planning Policy Framework, published in March 2012. [1]

Local Authorities have a statutory duty to produce a Local Plan, which identifies sites that may be suitable for new development. They are also advised to produce a Strategic Flood Risk Assessment, which identifies and maps areas that have a ‘low’, ‘medium, and ‘high’ risk of flooding. The Environment Agency’s Flood Zones are intended to help Local Authorities in this forward planning, the intention being to steer new development away from areas at a high risk of flooding to areas with a lower risk.

The Agency has divided England into four Flood Zones for this purpose, defined as follows:

  • 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)

It has published 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 includes all the land that falls outside Flood Zones 2 and 3. In other words, everybody in England lives in an Environment Agency Flood Zone, though many will be located in Flood Zone 1 and with a low probability of river and sea flooding. To find out what Flood Zone your site is located in, input your postcode into the map. [2]

Flood Zones and Flood Risk Assessments

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, not simply river and sea flooding. All sources of flooding would include surface water flooding, groundwater flooding, and flooding from overwhelmed sewers and drainage systems. A Strategic Flood Risk Assessment takes all these sources into account, with the Environment Agency’s Flood Zones making a significant contribution.

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. In summary:

“A site-specific flood risk assessment is required for proposals of 1 hectare or greater in Flood Zone 1; all proposals for new development (including minor development and change of use) in Flood Zones 2 and 3, or in an area within Flood Zone 1 which has critical drainage problems (as notified to the local planning authority by the Environment Agency); and where proposed development or a change of use to a more vulnerable class may be subject to other sources of flooding.”

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. With this end in mind, it has classified land use according to ‘Flood Risk Vulnerability’, the uses ranging from “water compatible development” (such as flood control infrastructure), through “less vulnerable”, “more vulnerable” and “highly vulnerable” uses, to “essential infrastructure.” A Local Planning Authority uses this classification not only as a guide to decide whether a proposed development is appropriate for its Flood Zone area, but also to decide what level of detail a Flood Risk Assessment should contain. For example, ‘less vulnerable’ uses (which include buildings used as shops, restaurants or offices) generally require less detail than ‘highly vulnerable’ uses (which include basement dwellings, mobile homes, and police and fire stations).

The conclusion from all of the above is that there is no “one size fits all” when it comes to assessing flood risk. The government’s planning portal website states that a site-specific flood risk assessment 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 property or development. [3]

Notes

[1] 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.

[2] 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.

[3] For a more detailed explanation of Flood Zones, see our web page “Do I live in a Flood Zone?”.

Photograph:

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Ford near Clun © Copyright Anthony Bloor and licensed for reuse under a Creative Commons Attribution-ShareAlike 4.0 International License.

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