Flood Zones Explained

Contents 

• What are the different types of flood zones?
• How do I know if my property or development is in a flood risk area?
• How will proposed developments be impacted by flood zones? 
• What are the flood risk vulnerability classifications?
• Flood zones and climate change allowances
• Get a clearer picture

Flood zones are a guidance tool, formed by the Environment Agency, to demonstrate the probability of river and sea flooding in areas across England. 

The flood zones were produced to help developers, councils and communities understand the flood risks present in specific locations or regions, taking into account the presence of flood defences.

Despite being a very useful indicator of an area’s flood risk, the zones cannot tell you whether a location will definitely flood or to what severity. 

What are the different types of flood zones? 

There are four different flood zones which the Environment Agency uses to specify the flood risk of different areas in England. 

Flood zone 1: low probability

Locations which are determined to be in flood zone 1 have a low probability of flooding – less than 0.1% annual probability of river or sea flooding. 

Areas in flood zone 1 can also be described as: 

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

(Shown as ‘clear’ on the flood zone map – all land outside Zones 2 and 3)

Flood zone 2: medium probability

Areas situated in flood zone 2 have a medium probability of flooding and have an annual probability of river flooding between 1.0% and 0.1% and annual probability of sea flooding between 0.5% and 0.1%. 

Areas in flood zone 2 can also be described as: 

• 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 zone map)

Flood zone 3a: high probability

Unlike the previous two zones, flood zone 3 is divided into two sections: 3a and 3b. 

Flood zone 3a is distinguished as land which has a 1% or greater annual probability of river flooding or a 0.5% or greater annual probability of sea flooding. 

Areas in flood zone 3a can also be described as: 

• 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 zone map)

Flood zone 3b: The Functional Floodplain

Flood zone 3b is also called the functional floodplain and refers to land which either stores water from rivers or the sea during flooding, or which allows such water to flow through in periods of flood. 

The government’s flood risk and coastal change planning guidance states that the ‘identification of functional floodplain should take account of local circumstances and not be defined solely on rigid probability parameters.’ 

Flood zone 3b usually refers to land that has a 3.3% or greater annual probability of flooding, regardless of the effective operation of flood risk management measures that are in place. 

This zone also refers to land which is ‘designed to flood’, which is the case for land that might only flood during more severe weather events. 

(Not separately distinguished from Zone 3a on the flood zone map)

How do I know if my property or development is in a flood risk area?

If you are worried about your area’s flood risk or want to know whether the location of a proposed development could be vulnerable to flooding, you can find out by using the Environment Agency’s flood map for planning. 

This flood map enables users to type in an area’s postcode and discover the level of long term flood risk for a specific address. 

The flood risk areas displayed in your search will be divided into risks from river and sea flooding and surface water flooding. 

The Lead Local Flood Authority (LLFA) for the searched area will be shown under the surface water section; the Environment Agency is responsible for the management of river and sea flood risks.  

It is important to note that this flood map of England does not account for the potential impact which climate change may have upon future flood events. 

Individuals or companies wishing to know the UK’s flood risk of areas outside of England can find out using the equivalent tools for Northern Ireland, Scotland and Wales. 

How will proposed developments be impacted by flood zones? 

According to the National Planning Policy Framework (NPPF), certain proposals for developments in flood zone 1 should be accompanied by a site-specific flood risk assessment. 

Such proposals include: 

• projects where the site is more than 1 hectare.
• projects where the land has been identified as having critical drainage problems by the Environment Agency.
• projects where a strategic flood risk assessment has identified the site being at an increased risk of flooding in future.
• projects where the site could be susceptible to other sources of flooding and where development would increase such vulnerability.

The NPPF also states that all proposals for developments in flood zones 2 and 3 should be accompanied by a site-specific flood risk assessment. 

What are the flood risk vulnerability classifications? 

The flood risk vulnerability classification table is a system which dictates where proposed developments should and should not be built and if an Exception Test is required. 

Across the top of the table are five categories that are considered when a development’s vulnerability to flooding is assessed. These appear as follows: essential infrastructure, highly vulnerable, more vulnerable, less vulnerable and water compatible. 

If developments are proposed to be built in flood zone 1, an Exception Test is not required to approve the build and are the most likely to be approved. 

For developments proposed in flood zone 2, an Exception Test is only required if the development is deemed to be highly vulnerable to flooding. 

Exception Tests are also required for essential infrastructure proposed to be built in flood zones 3a and 3b and for developments in 3a if they would be more vulnerable to flooding. 

Proposed developments cannot be permitted in flood zone 3a if they would be classed as highly vulnerable to flooding, nor can they be approved if they would be located in flood zone 3b and classed as highly vulnerable, more vulnerable or less vulnerable to flooding. 

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. Read their guidance online.

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

Flood 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

Flood 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

Flood 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) a 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 get in touch today.