Flood zones explained

The Environment Agency (EA) has devised a set of flood zones for guidance by developers, councils and communities to explain the probability of river and sea flooding, ignoring the presence of flood defences.

Flood zones are shown on the EA’s Flood Map for Planning (Rivers and Sea), as indicated below:

Flood Zone 1: Low probability

Land having a less than 1 in 1,000 annual probability of river or sea flooding.

(Shown as ‘clear’ on the Flood Map – all land outside Zones 2 and 3)

Flood Zone 2: Medium probability

Land having between a 1 in 100 and 1 in 1,000 annual probability of river flooding;

or

Land having between a 1 in 200 and 1 in 1,000 annual probability of sea flooding.

(Land shown in light blue on the Flood Map)

Flood Zone 3a: High probability

Land having a 1 in 100 or greater annual probability of river flooding;

or

Land having a 1 in 200 or greater annual probability of sea flooding.

(Land shown in dark blue on the Flood Map)

Flood Zone 3b: The functional floodplain

This zone comprises land where water has to flow or be stored in times of flood.

Local planning authorities should identify in their Strategic Flood Risk Assessments areas of functional floodplain and its boundaries accordingly, in agreement with the Environment Agency.

(Not separately distinguished from Zone 3a on the Flood Map)

Flood zones and climate change allowances

The flood zones do not take account of the possible impacts of climate change and consequent changes in the future probability of flooding. Reference should therefore also be made to the Strategic Flood Risk Assessment when considering location and potential future flood risks to developments and land uses.

The EA made revisions in March 2016 to the peak flow rate of rivers in various catchments across the country based on climate change predictions. This guidance is available here.

This translates to new guidance for councils and developers on the relevant peak rate to use in each Flood Zone:

Zone 2

• essential infrastructure – use the higher central and upper end to assess a range of allowances
• highly vulnerable – use the higher central and upper end to assess a range of allowances
• more vulnerable – use the central and higher central to assess a range of allowances
• less vulnerable – use the central allowance
• water-compatible – use none of the allowances

Zone 3a

• essential infrastructure – use the upper-end allowance
• highly vulnerable – development should not be permitted
• more vulnerable – use the higher central and upper end to assess a range of allowances
• less vulnerable – use the central and higher central to assess a range of allowances
• water-compatible – use the central allowance

Zone 3b

• essential infrastructure – use the upper-end allowance
• highly vulnerable – development should not be permitted
• more vulnerable – development should not be permitted
• less vulnerable – development should not be permitted
• water-compatible – use the central allowance

If (exceptionally) development is considered appropriate when not in accordance with flood zone vulnerability categories, then it would be appropriate to use the upper-end allowance.

We’ve collected Strategic Flood Risk Assessments from different local authorities and you can find further information in the GeoSmart Knowledge Hub.

Get a Clearer Picture

GeoSmart understands how river, surface and groundwater flooding can affect property and development sites.  Our expert flood risk consultants will advise on flood zones and constraints, as well as potential vulnerabilities that could impact on your client’s permitted development project.

For more information about our flood risk assessments, please call 01743 581415 or email enquiries@geosmartinfo.co.uk

Tags: flood risk, flood zones