EA Flood Zones Policy (Part)
Provided by Duncan Monroe, Regional Modelling Team Yorkshire & North East Region on 7
October 2013
Environment Agency, Lateral, 8 City Walk, Leeds, LS11 9AT
Tel: 01904 825810, E-mail: duncan.munro@environment-agency.gov.uk
Flood Zones for Development Planning in England
Flood Zones are designed for strategic spatial and development planning purposes in England. The
definition of each Flood Zone originates from Planning Policy Statement 25 (PPS25) on Development and
Flood Risk (DCLG, December 2006). The Flood Zones are defined as follows:
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Flood Zone 1. Low Probability. This zone comprises land assessed as having a less than 1
in 1000 annual probability of river or sea flooding in any year (<0.1%).
Flood Zone 2. Medium Probability. This zone comprises land assessed as having between a
1 in 100 and 1 in 1000 annual probability of river flooding (1% – 0.1%) or between a 1 in 200
and 1 in 1000 annual probability of sea flooding (0.5% – 0.1%) in any year;
Flood Zone 3a. High Probability. This zone comprises 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
Flood Zone 3b. The Functional Floodplain. This zone comprises land where water has to
flow or be stored in times of flood.
We are not required to map the outer boundary of the extent of Flood Zone 3b: the functional
floodplain. Flood Zone 3b is included within our mapped extent of a flood with an annual
probability of 1% (1 in 100) fluvial / 0.5% (1 in 200) tidal.
Background
Our Flood Mapping Strategy (June 2003) identifies the need to produce Flood Zones to support planning
policy in England and Wales.
Flood Zones are required to identify the extents over which flooding could occur, from rivers and the sea,
ignoring the presence of flood defences. The way in which different types of flood defences are considered
is explained below:
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We interpret PPS25 to mean that flooding is not constrained by formal raised flood defences.
Therefore the Flood Zones ignore the effect of defences in reducing the probability of flooding but do
not underestimate the extents of flooding where defences increase the area potentially at risk.
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The definition of Flood Zone 3b (Functional Floodplain) in PPS25 includes land which ‘is designed to
flood in an extreme (0.1%) flood’. This means that our mapped extent of a flood with an annual
probability of 1% (1 in 100) fluvial / 0.5% (1 in 200) tidal will include areas that are designed to flood
due to the operation of flood storage areas.
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Other types of flood defences or infrastructure (whether or not their primary purpose is flood
alleviation) such as engineered river channels, bypass channels, culverts and bridges are considered
as existing infrastructure for the purpose of Flood Zones. In principle this means they are included
when modelling and mapping Flood Zones. This principle also applies to embankments that are not
flood defences, although any pathways through the embankment should be taken into account.
Flood Zones are combined with other data layers showing the location of flood defences and the
areas that benefit from them, to form the Flood Map. The Flood Map is then used to raise public
awareness of flood risk.
Flood Zones have been derived from detailed modelled flood outlines, supplemented by national generalised
modelling. Because of its scale, the national generalised modelling necessarily simplifies the channel and
floodplain topography and flow in many areas, but any more detailed mapping and modelling that is carried
out will take account of this infrastructure. This is necessary to enable stable, consistent modelling of the
whole of England and Wales. Any improved data (see below) used to replace the national generalised model
data will incorporate the effects of such structures and may therefore show a different extent of flooding in
some locations.
The national generalised modelling covers all rivers with catchments with an area of 3km 2 or greater, and the
sea. Where we do have information on flood risk from smaller watercourses we will include it in our Flood
Zones.
Inclusion of defence breach scenarios does not fit with this policy and must not be incorporated into the
Flood Zones.
Flood Zone Improvements
We will take a risk-based approach to prioritising and updating our data. This means we will only
improve our data where the flood risks to people, property or the environment justify additional
investment in flood mapping to improve the understanding of flood risk.
Our aim is to deliver ‘fit for purpose’ Flood Zones. This means that we will incorporate the best
data that we have access to, but we will not produce new data solely to improve Flood Zones
where Flood Zones are already considered fit for purpose.
Data used to improve the Flood Zones must always be the best available information, and it must
comply with the Acceptable Data Requirements set out below.
Acceptable Data Requirements
Annual probability of 1% (1 in 100) fluvial / 0.5% (1 in 200) tidal
These data will generally be derived from detailed modelling and mapping studies. In lower risk
locations, for example in sparsely populated low risk areas, other analytical methods may be used
to determine the area of this flood extent.
All modelling and data for this flood extent must meet all of the following requirements:
 Based on 1% fluvial / 0.5% tidal present-day probability
 Not constrained by formal raised flood defences
 Includes areas designed to flood
 Based on surveyed ground levels
 No allowance for in-channel blockages or temporary obstructions
 Based on predicted actual flood water levels for the specified return periods (including where
those predicted flood water levels are raised due to flood defences upstream or downstream).
Exceptionally, historic flood event data may be used to define this flood extent. This applies only
over reaches where all of the following circumstances apply:
 No blockage has affected the flooding
 The flood extent has a 1% fluvial / 0.5% tidal present-day probability
 Based on flood extents without being constrained by formal raised flood defences
 We are confident that the outline represents the peak of the flood event
 We are confident that the outline has been recorded accurately.
This will generally rule out historic extents that are more than 5 years old, or that have not been
recently reassessed. The number of circumstances where historic flood data are used should be
limited.
Historic flood event data that is not suitable for inclusion in Flood Zones may still be suitable for
inclusion in the Historic Flood Map subject to acceptable levels of confidence in the data
The following data are not appropriate for inclusion in this flood extent:
 Data relating to sources of flooding other than fluvial or tidal
 Flood outlines mapped on the basis of subjective or unquantified historic data
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Flood outlines produced using contours that are derived from a DTM with a lower vertical
accuracy than NextMap SAR data (for example, at the date of publishing, Ordnance Survey
contours are not suitable)
Flood outlines that are not supported by numerical analysis
Flood reports from the public that have not been confirmed or checked by Environment Agency
staff
Data from the IoH 130 report.
Historic flood data from an event with unknown probability, or with other than a 1% fluvial /
0.5% tidal probability based on present-day probability, using FEH methods of calculation
Historic or modelled flood data incorporating a channel or structure blockage, or breach of a
flood defence
Acceptable Data Requirements
Annual probability of 0.1% fluvial / tidal
Data to determine the extent of the extreme flood with this probability must meet all of the following
requirements:
 Based on 0.1% fluvial or tidal present-day probability
 Based on flood extents without being constrained by formal raised flood defences
 Based on surveyed ground levels
 Based on predicted actual flood water levels for the specified return periods (including where
those predicted flood water levels are raised due to flood defences).
 No allowance for blockages
OR
 Be based on historic fluvial or tidal flood extents (which may include those where blockages
may have affected flood extents or where the flood probability is unknown) – where this
extends beyond any 0.1% outline produced by modelling. Fluvial and tidal historic flood
extents which are greater than the modelled 0.1% extent should always be used with the
following exception. Where the cause of historic flooding is known to have been removed
through any means other than construction of flood defences, such that the historic flooding
could not recur, Area staff may choose not to incorporate this historic flood data in this flood
zone. An example is where land has been raised above the flood levels which occurred (Flood
extents for such events should still be recorded on the Historic Flood Map).
It is rare that data to this standard exists for the extent of a flood with an annual probability of 0.1%.
However, if data to this standard for the extent of the 1% fluvial / 0.5% tidal chance flood extends
beyond the national generalised modelling of the 0.1% chance flood, and the extent of the 0.1%
chance flood has not been reassessed, then the extent of the 0.1% chance flood will be moved to
match the good quality data for the 1% fluvial / 0.5% tidal chance flood.
The following data are not appropriate for inclusion in this flood extent:
 Data relating to sources of flooding other than fluvial or tidal;
 Flood outlines mapped on the basis of historic data where we cannot be confident that the
outline has been recorded accurately;
 Flood outlines produced using contours that are derived from a DTM with a lower vertical
accuracy than NextMap SAR data (for example, at the date of publishing, Ordnance Survey
contours are not suitable);
 Flood outlines that are not supported by numerical analysis or historic evidence;
 Flood reports from the public that have not been confirmed or checked by Environment Agency
staff;
 Data from the IoH 130 report.
Other suitable data.
Exceptionally, survey data alone can be used to improve Flood Zones. For example, where validated data
shows that flood extents are incorrect because the DTM is in error, there is scope to adjust the outline. In
effect this process amends the DTM and not the water levels and so this approach should only be used
where the user is confident that water levels will not be sensitive to the change in DTM. Changes required to
the DTM should be documented.