Flood Map. National Dataset User Guide

Transcription

1 Flood Map National Dataset User Guide Version th April 2006 Copyright Environment Agency 1

2 Contents 1.0 Record of amendment Introduction Description of the Flood Map datasets Flood Zones 2 & 3 (2 layers) Flood defences Flood Storage areas (FSA) Areas benefiting from flood defences Related data sources Public Register and non Public Register Data National Security issues Legislation relevant to the Flood Map datasets Data structure Data Quality Data standards Containment of Flood Zone Reporting errors to the NDU NDU Validation rules and completeness checks Validation checks specific to the Flood Map data Individual verification of data National verification of data Known quality issues Frequently Asked Questions (FAQ) Useful information when using the data What is not included in the data Recommendations on how to use or analyse the data Use of Flood Zones in England Changes in the data Glossary Feedback form Appendix Copyright Environment Agency 2

3 1.0 Record of amendment Version number Section number Amendment description Date Minor formatting change 20/04/ Minor formatting change 20/04/06 Copyright Environment Agency 3

4 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: 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 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 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). Copyright Environment Agency 4

5 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 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 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 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. Copyright Environment Agency 5

6 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. Copyright Environment Agency 6

7 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 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)). Copyright Environment Agency 7

8 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. Copyright Environment Agency 8

9 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 Containment of Flood Zone 2 Flood zone two must be completely within flood zone 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 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 National verification of data The NDU checks the format, structure, number of records and topology of the flood map layers. Copyright Environment Agency 9

10 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. Copyright Environment Agency 10

11 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. Copyright Environment Agency 11

12 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. Copyright Environment Agency 12

13 7.1 What is not included in the data 7.0 Useful information when using 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 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). Copyright Environment Agency 13

14 8.0 Glossary Defra DSM DTM EFO FSA IFM NaFRA NDU NFCDD PPG 25 RASP SAR Department for the Environment, Food & Rural Affairs Digital Surface Model Digital Terrain Model Extreme Flood Outline Flood Storage Areas Indicative Flood Maps National Flood Risk Assessment National Data Unit National Flooding and Coastal Defence Database Government s Planning Policy Guidance 25 on Development and Flood Risk. Risk assessment for Flood Defence Systems for Strategic Planning and Inferometric Synthetic Aperture Radar Copyright Environment Agency 14

15 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: Phone: Organisation: 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: Lower Bristol Road Rivers House BATH BA2 9ES Copyright Environment Agency 15

16 10.0 Appendix 1 Complete list of all amendments ever made to the document Version Section number Amendment description Date number Text revised 20/01/ Text revised 20/01/ Text revised 20/01/ Section added 20/01/ Minor text revision 20/01/ Minor text revision 20/01/ Text added 20/01/ Section added 20/01/ Section added 20/01/ Text revised 26/01/ Text revised 26/01/ Minor text revision 20/01/ Text revised 20/01/ Text revised 26/01/ Table updated 21/07/ Text revised 21/07/ Text revised 21/07/ Text revised 21/07/ Text revised 21/07/ Text revised 21/07/ Text revised 21/07/ Text revised 21/05/ Text revised 21/07/ Section removed 21/07/ Section name changed 21/07/ Section removed 21/07/ Section removed 21/07/ Section removed 21/07/ Section number changed from to / Text added 21/07/ Section number changed from 5.4 to /07/ Text revised 21/07/ Table updated 21/07/ Section removed. 21/07/ Text revised 21/07/ Table updated 21/07/ Table updated 22/04/ Text updated 22/04/ Text updated 22/04/ Text updated 22/04/ Text updated 22/04/ Table updated 22/04/05 Copyright Environment Agency 16