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					          Flood Map


National Dataset User Guide


          Version 1.1.5
         20th April 2006




  Copyright  Environment Agency   1
                                                                                  Contents

1.0 Record of amendment................................................................................................................................................... 3
2.0 Introduction .................................................................................................................................................................. 4
    2.1 Description of the Flood Map datasets ......................................................................................4
             2.1.1 Flood Zones 2 & 3 (2 layers)....................................................................................................................... 4
             2.1.2 Flood defences............................................................................................................................................. 5
             2.1.3 Flood Storage areas (FSA) .......................................................................................................................... 5
             2.1.4 Areas benefiting from flood defences ......................................................................................................... 5
    2.2 Related data sources...................................................................................................................6
             3.0 Public Register and non Public Register Data................................................................................................ 7
    3.1 National Security issues .............................................................................................................7
    3.2 Legislation relevant to the Flood Map datasets .........................................................................7
4.0 Data structure................................................................................................................................................................ 8
5.0 Data Quality.................................................................................................................................................................. 9
    5.1 Data standards ............................................................................................................................9
             5.1.1 Containment of Flood Zone 2...................................................................................................................... 9
    5.2 Reporting errors to the NDU......................................................................................................9
    5.3 NDU Validation rules and completeness checks .......................................................................9
    5.4 Validation checks specific to the Flood Map data .....................................................................9
             5. 4.1 Individual verification of data .................................................................................................................... 9
             5.4.2 National verification of data ........................................................................................................................ 9
    5.5 Known quality issues ...............................................................................................................10
6.0 Frequently Asked Questions (FAQ) ........................................................................................................................... 11
7.0 Useful information when using the data ..................................................................................................................... 13
    7.1 What is not included in the data...............................................................................................13
    7.2 Recommendations on how to use or analyse the data..............................................................13
             7.2.1 Use of Flood Zones in England ................................................................................................................. 13
    7.3 Changes in the data ..................................................................................................................13
8.0 Glossary ...................................................................................................................................................................... 14
9.0 Feedback form ............................................................................................................................................................ 15
10.0 Appendix 1 ............................................................................................................................................................... 16




                                                        Copyright  Environment Agency                                                                                             2
                          1.0 Record of amendment

Version   Section number Amendment description         Date
number
1.1.5     1.0            Minor formatting change       20/04/06
1.1.5     10.0           Minor formatting change       20/04/06




                      Copyright  Environment Agency              3
                                           2.0 Introduction

This document provides information to accompany the Environment Agency’s Flood Map dataset
supplied to external users. It can be used to gain a better understanding of the Flood Map data when
used in conjunction with the current data release. The Product Sheet accompanying this user guide
provides a brief introduction to the dataset, but for information relating to national datasets in
general, please refer to the National Dataset Generic Text document.

Details of the Environment Agency’s distribution policy can be found in the National Dataset
Generic Text document, along with contact details for the National Data Unit (NDU).

Information about the Environment Agency’s web site can also be found in the National Dataset
Generic document. The Flood web site is located at:

http://www.environment-agency.gov.uk/subjects/flood/

This site includes data analysis, reports and links to other web pages of interest.


2.1 Description of the Flood Map datasets

The Flood Map is a multi-layered product aimed primarily at increasing awareness of flood risk. It
provides information on the floodplain for flooding from the rivers and sea for floods with a 1% and
0.5% chance of happening in any year respectively. The extreme flood outline (EFO) 0.1% year
outline is also shown for both river and tidal flooding. The Flood Map also provides information on
flood defences and areas that benefit from them.

Although the other layers of information on the Flood Map will be useful to feed into Strategic
Flood Risk Assessments and for community awareness of flooding issues, only the Flood Zones and
their updates should be used as planning constraint maps in England. The Flood Map should not be
used as a planning constraint map for Wales.

Following are descriptions of each of the five layers supplied in the Flood Map national dataset.


2.1.1 Flood Zones 2 & 3 (2 layers)

Flood Zones are defined by the Government’s Planning Policy Guidance 25 on ‘Development and
flood Risk’ for England (PPG25) dated July 2001. The Flood Zones illustrate the probability of
flooding across England for planning consultation. The Flood Zones have been identified using the
best available data held by the Environment Agency in June 2004, ignoring the presence of flood
defences (as required by PPG25, reference table 1 note (a)).

Flood Zones or flood extents included in the Flood Map are the Environment Agency’s best
estimate of the areas of land, ignoring the presence of defences. There are 2 layers, based on
flooding probability:

•   Flood zone 2: With an annual probability of flooding of 0.1% (1 in 1000) or greater from rivers
    and the sea, but with an annual probability of flooding of less than 1% from rivers or 0.5% from
    the sea (identical to Flood Zone 2 in England).


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•   Flood Zone 3: With an annual probability of flooding of 1.0% (1 in 100) or greater from rivers,
    and 0.5% (1 in 200) or greater from the sea (identical to Flood Zone 3 in England).

These definitions of Flood Zones 2 and 3 apply only in England. Equivalent data is also provided
for Wales, although in Wales the data for Flood Zones 2 and 3 of the Flood Map does not have the
same relationship to Planning Guidance and is not referred to as Flood Zones.


2.1.2 Flood defences

All flood defences constructed during the last five years and with a standard of protection equal to
or better than 1% for rivers and 0.5% from the sea are shown. Some additional defences, which
may be older or may have been designed to provide a lower standard of protection, are also shown
where the information is currently available. This layer comprises linear flood defences, for
example flood embankments and walls. Flood Storage Areas are also flood defences, but are shown
on a separate data layer. Flood defences that are not yet shown, and the areas that benefit from
them, will be gradually added.


2.1.3 Flood Storage areas (FSA)

 A flood storage area may also be referred to as a balancing reservoir, storage basin or balancing
pond. Its purpose is to attenuate an incoming flood peak to a flow level that can be accepted by the
downstream channel. It may also delay the timing of a flood peak so that its volume is discharged
over a longer time interval.

A flood storage area may take the form of a wet or dry reservoir. A wet reservoir is a water storage
facility in which storage can be effected by allowing water levels to rise during flood times. A dry
reservoir is typically adjacent to a river and comprises an enclosed area that accepts water only at
flood peaks.

A storage facility may be “on-line” (i.e. on-stream), causing backup of water above the control
mechanism, or “off-line”, where water is diverted to a storage area adjacent to a (typically
embanked) river. Both types of storage allow excess water to drain away at a controlled rate once a
flood event is over. Depending on whether a flood storage area is wet or dry, on-line or off-line, its
development may involve the construction of a dam on a watercourse, embankments surrounding a
storage area adjacent to a river, excavation of a storage area adjacent to a river, and/or associated
control and pumping facilities.


2.1.4 Areas benefiting from flood defences

Those areas benefiting from the flood defences shown in a 1% fluvial / 0.5% tidal flood event. If
the flood defences were not present, these areas would flood in a 1% fluvial / 0.5% tidal flood
event; however, because the flood defences are present, these areas would not flood in this event
(provided that the defences do not breach).

The area benefiting from a defence can usually only be derived from a more detailed model than the
national generalised modelling (JFlow or HYDROF). In some cases the area defined as benefiting
from a flood defence may extend beyond the flood extents defined by the national modelling, but as
it is the output from higher quality modelling the data is included without amendment.

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2.2 Related data sources

To support the production of the flood zones digital surface model (DSM) and Inferometric
Synthetic Aperture Radar (SAR) data was captured. A Digital Terrain Model (DTM) that can be
used for flood modelling with reference to OS mapping included a full drainage network.

National flood risk assessment (NaFRA) or Risk assessment for Flood Defence Systems for
Strategic Planning (RASP) is based on the extent of flood zone 2 with high, intermediate and
detailed risk assessment applied.




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                         3.0 Public Register and non Public Register Data

For a general description of what constitutes public register and non-public register data please refer
to the National Dataset Generic Text document.

The flood map data is not from a public register.


3.1 National Security issues

A discussion of national security issues can be found in the National Dataset Generic Text
document. There have been no exclusions from the Flood Maps dataset due to national security.


3.2 Legislation relevant to the Flood Map datasets

Flood Zones are defined by the Government’s Planning Policy Guidance 25 on ‘Development and
flood Risk’ for England (PPG25) dated July 2001. The Flood Zones illustrate the probability of
flooding across England for planning consultation. The Flood Zones have been identified using the
best available data held by the Environment Agency in June 2004, ignoring the presence of flood
defences (as required by PPG25, reference table 1 note (a)).




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                                         4.0 Data structure


The detailed data structure of the Flood Map data is given in the Flood Maps National Dataset Field
Attributes document.

The Flood Maps dataset is supplied as five layer files:

   Flood Zone 2 (nat_floodzone2_vn_n)
   Flood Zone 3 (nat_floodzone3_vn_n)
   Flood Defences (nat_defence_vn_n)
   Flood Storage Areas (nat_FSA_vn_n)
   Areas Benefiting from flood defences (nat_areabenefit_vn_n)

There are no relationships between these files.




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                                          5.0 Data Quality

5.1 Data standards

For data standards applicable to a number of national datasets please consult the National Dataset
Generic Text document. For standards specific to the Flood Map data Please refer to the briefing
note as supplied on the CD.


5.1.1 Containment of Flood Zone 2

Flood zone two must be completely within flood zone 3.

5.2 Reporting errors to the NDU

Methods of reporting errors and the recording of data queries by the NDU, are included in the
National Dataset Generic Text document, whilst a feedback form can be found at the end of this
user guide.


5.3 NDU Validation rules and completeness checks

Every effort is made to minimise data errors occurring in the Flood Map data release. For
background information on the data validation process and general completeness checks carried out
by the NDU, please refer to the National Dataset Generic Text document.


5.4 Validation checks specific to the Flood Map data

Following is an outline of the validation checks specific to the Flood Map dataset.


5. 4.1 Individual verification of data

Our local staff reviewed the national scale model outputs and used them to fill any gaps between
locations with better quality local data from more detailed model studies, for example those
locations covered by our earlier Section 105 Project mapping work. The flood zones were therefore
produced using the best available data meeting the requirements of the flood zones as defined in
PPG25, ensuring that the final flood zones were nationally consistent in terms of interpretation and
data content. To be used in the flood zones, the existing better quality local data had to fit specific
criteria to maintain consistency and meet the requirements of PPG25. In some locations the
Environment Agency holds good quality mapped and modelled data which does not all meet the
requirements for flood zones, for example because it has been modelled taking account of the
presence of flood defences, or by simulating a breach in an embankment. This is still valuable data
and should be taken into account by developers in preparation of Flood Risk Assessments for
individual developments and by planning authorities in preparing Strategic Flood Risk
Assessments.


5.4.2 National verification of data

The NDU checks the format, structure, number of records and topology of the flood map layers.
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5.5 Known quality issues

Please refer to the briefing note as supplied on the CD for examples of acceptable and unacceptable
data for inclusion in each zone.




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                            6.0 Frequently Asked Questions (FAQ)

1. What is the flood map, and what information will be available?

      The Flood map replaces the IFM and shows the floodplain for flooding from the rivers and
      sea for floods with a 1% and 0.5% chance of happening in any year respectively. The
      extreme flood outline (EFO) 0.1% year outline is also shown for both river and tidal
      flooding.

      The Flood Map also displays flood defences. Currently it chiefly shows those defences
      which are less than 5 years old and offer a standard of protection have of at least 1.0% for
      fluvial flooding (from rivers) and 0.5% for tidal flooding (from the sea), and the areas that
      benefit from them.

      By ‘benefit from defences’ we mean that, if the defences were not there and this flood
      occurred, the area behind would flood. But because the defence is there, flooding does not
      occur (It is not used to mean that flooding still occurs but is shallower than it would
      otherwise have been.)

2. Is the Flood Map the same as Flood Zones?

      The Flood Zones data forms the base layers for the Flood Map. The data is continuously
      being improved and updated. The Flood Zones provide more extensive coverage than the
      Indicative Floodplain Map (IFM) which they replace, showing the floodplain on 80,000 km
      of rivers in England and Wales compared to 50,000km on the IFM, and modelling
      catchments down to 3sq km.

      Unlike Flood Zones, which distinguish between flooding from the rivers and sea, the Flood
      Map does not show the source of flooding. This is to ensure clarity when displayed on the
      internet. The attributes of the base zones are available and are held in NFCDD.

3. What is not shown on the Flood Map?

      The Flood Map does not show sources of flooding other than from the rivers or sea and
      therefore groundwater, surface water or flooding from surcharged drainage systems will not
      be represented.

      The Flood Map shows the extent of flooding but does not give depths or levels.


4. How often will the Flood Map be updated?

      The Flood Outlines and the locations of defences with associated area of benefit will be
      updated quarterly in January, April, July and October. In making a commitment to updating
      the Flood Map on a quarterly basis the Agency will be making the best available
      information on Flood risk available to the public and key stakeholders.




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5. The Flood Map shows defences and areas benefiting from them, which were not on the
   IFM. What are the implications of this?

      Eventually the intention is to show all of the defences in our ownership or those that are
      registered as formal flood defence assets on NFCDD. However, initially for the first release
      of the Flood Map, only defences which are less than 5years old, with a standard of
      protection of at least 1.0% fluvial and 0.5% tidal, need to be shown. As the Flood Map is
      updated over the next 3 years other defences and areas benefiting from defences will be
      added. Many of these will need detailed modelling in order to identify the areas benefiting.
      Detailed modelling may also be required for groups of defences that interact to provide a
      given standard of protection.

      Adopting the standard return periods of 1.0% fluvial and 0.5% tidal when showing areas of
      benefit will mean that areas defended to a lower standard may be shown as at risk of
      flooding. In some cases property adjacent to a recent defence will be shown as at risk of
      flooding. For example, an area behind a fluvial defence with a SoP of 2% (1:50), will not be
      shown in full as an area benefiting from a defence in a flood with a 1% chance, even though
      it would be defended against a lesser a flood with a 2% (1:50) chance of happening in any
      year.

      The information on defences and Area of Benefit will be updated quarterly as better data
      becomes available.

      The provision of information on the location of defences on the Flood Map and their area of
      benefit will assist the public, insurers and planners to better understand the level flood risk
      in an area.

      The area benefiting from a defence can only be derived from a more detailed model than the
      national generalised modelling (JFlow or HYDROF). In some cases the area defined as
      benefiting from a flood defence may extend beyond the flood extents defined by the national
      modelling, but as it is the output from higher quality modelling the data is included without
      amendment.




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                            7.0 Useful information when using the data

7.1 What is not included in the data

The main river centre lines are not included with the flood map.


7.2 Recommendations on how to use or analyse the data

To view the flood map data properly order the data in the following way: defences, fsa, areas of
benefit, flood zone 2 and flood zone 3. This order helps view the separate floodmap data layers
together as one, which is vital for any form of analysis.


7.2.1 Use of Flood Zones in England

Flood Zones have been produced to support the implementation of Government Planning Policy
and to prevent inappropriate development in flood risk areas. This information is indicative rather
than specific. Local planning authorities will need to consult the Environment Agency on strategic
and individual land use proposals. The Environment Agency will update this data, as better
information becomes available, for example from changes in hydrological river response; from
observations following flood events; or from improved modelling techniques. It should be noted
that locations adjacent to rivers, not included at this stage of the survey, might be in the floodplain
and at some risk of flooding. When in doubt the Environment Agency should be consulted. The
Flood Zones may also be used by Local Authority Emergency Planners when producing Major
Incident Flood Response Plans. The limits of the floodplain are indicative of the area which could
be affected by the flood events and are largely based on modelled data. Due to differences in
guidance and legislation between England and Wales this terminology, derived from PPG 25, will
not be used for the Flood Map on the Internet.


7.3 Changes in the data

Any major changes identified in the data will be highlighted in this section. An explanation of why
the change had occurred is also provided and if you have any questions relating to these changes
please contact the NDU.

There have been no changes to the structure of the dataset. However, over time the data is being
revised as new and improved techniques become available (see section 7.2.1).




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                    8.0 Glossary


Defra               Department for the Environment, Food & Rural Affairs

DSM                 Digital Surface Model
DTM                 Digital Terrain Model
EFO                 Extreme Flood Outline
FSA                 Flood Storage Areas
IFM                 Indicative Flood Maps
NaFRA               National Flood Risk Assessment

NDU                 National Data Unit
NFCDD               National Flooding and Coastal Defence Database
PPG 25              Government’s Planning Policy Guidance 25 on
                    Development and Flood Risk.
RASP                Risk assessment for Flood Defence Systems for
                    Strategic Planning
SAR                 and Inferometric Synthetic Aperture Radar




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                                            9.0 Feedback form

The NDU welcomes feedback on its databases and if you have any comments please complete the
form below and return to the National Data Unit:

Name:                                                   Organisation:

Phone:                                                  Email:

Fax:

National Dataset …………………………………………………………….
(Please enter dataset to which your feedback relates)

Comments:


Feedback on contents of documentation:




Feedback on data content:




General feedback:




Environmental Data Team                      Data Team Leader
Environment Agency                           Tel: 01225 487609
Lower Bristol Road                           E-mail: susan.macleay@environment-agency.gov.uk
Rivers House
BATH BA2 9ES




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                                        10.0 Appendix 1

Complete list of all amendments ever made to the document

Version        Section number    Amendment description                      Date
number
1.1.4          2.0               Text revised                               20/01/2006
1.1.4          2.2               Text revised                               20/01/2006
1.1.4          5.1               Text revised                               20/01/2006
1.1.4          5.1.1             Section added                              20/01/2006
1.1.4          5.2               Minor text revision                        20/01/2006
1.1.4          5.3               Minor text revision                        20/01/2006
1.1.4          5.4               Text added                                 20/01/2006
1.1.4          5.4.1             Section added                              20/01/2006
1.1.4          5.4.2             Section added                              20/01/2006
1.1.4          5.4               Text revised                               26/01/2006
1.1.4          5.5               Text revised                               26/01/2006
1.1.4          7.2.1             Minor text revision                        20/01/2006
1.1.4          7.3               Text revised                               20/01/2006
1.1.4          8.0               Text revised                               26/01/2006
1.1.3          1.0               Table updated                              21/07/05
1.1.3          2.0               Text revised                               21/07/05
1.1.3          2.1               Text revised                               21/07/05
1.1.3          2.1.1             Text revised                               21/07/05
1.1.3          2.1.2             Text revised                               21/07/05
1.1.3          2.1.4             Text revised                               21/07/05
1.1.3          2.2               Text revised                               21/07/05
1.1.3          3.1               Text revised                               21/05/05
1.1.3          4.0               Text revised                               21/07/05
1.1.3          5.1.1             Section removed                            21/07/05
1.1.3          5.3               Section name changed                       21/07/05
1.1.3          5.3.1.1           Section removed                            21/07/05
1.1.3          5.3.1.2           Section removed                            21/07/05
1.1.3          5.3.1.3           Section removed                            21/07/05
1.1.3          5.4               Section number changed from 5.3.1 to 5.4   21/0705
1.1.3          5.4               Text added                                 21/07/05
1.1.3          5.5               Section number changed from 5.4 to 5.5     21/07/05
1.1.3          7.2               Text revised                               21/07/05
1.1.3          7.2.1             Table updated                              21/07/05
1.1.3          7.4               Section removed.                           21/07/05
1.1.3          9.0               Text revised                               21/07/05
1.1.3          10.0              Table updated                              21/07/05
1.1.2          1.0               Table updated                              22/04/05
1.1.2          2.2               Text updated                               22/04/05
1.1.2          3.2               Text updated                               22/04/05
1.1.2          4.0               Text updated                               22/04/05
1.1.2          7.2               Text updated                               22/04/05
1.1.2          8.0               Table updated                              22/04/05




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