Created: 03 October 2019 Updated: 05 August 2020Further details on how the data were derived are provided in the project technical report. Practical guidance on how to use the new datasets, including a worked example, is given in the user guide. Extreme swell wave conditions are no longer available online but can be requested from national authorities if required. For further information about the update of Coastal Flood Boundaries project please contact the Environment Agency: <a class="external-link" href="mailto:enquiries@environment-agency.gov.uk" rel="nofollow">enquiries@environment-agency.gov.uk</a> – marked for the attention of Coastal Modelling and Forecasting team.For regional queries contact:<a class="external-link" href="mailto:advice@sepa.org.uk" rel="nofollow">advice@sepa.org.uk</a> (SEPA)<a class="external-link" href="mailto:enquiries@naturalresourceswales.gov.uk" rel="nofollow">enquiries@naturalresourceswales.gov.uk</a> (NRW)<a class="external-link" href="mailto:dcu@infrastructure-ni.gov.uk" rel="nofollow">dcu@infrastructure-ni.gov.uk</a> (DfINI)

Coastal Flood Boundaries - additional help and further information

Created: 03 October 2019 Updated: 05 August 2020The Coastal Flood Boundaries (CFB) data and guidance are available for download here: <a class="external-link" href="https://environment.data.gov.uk/dataset/5f903624-8b23-4bb9-a799-f4e1d99b4319" rel="nofollow">Administrative Boundaries - Regional Flood and Coastal Committees</a>Detailed results from the update are given in a series of geographical information system shapefiles. A search for <a class="external-link" href="https://environment.data.gov.uk/searchresults;query=Coastal%20Design%20Sea%20Levels%E2%80%99;page=1;pagesize=20;orderby=Relevancy" rel="nofollow">‘Coastal Design Sea Levels’</a> on the website should find links to the latest version of the datasets and documentation.The technical and user reports are downloaded with the data and contained in a folder called “doc”. A separate link to the documentation is provided if the data itself is not required.

Where can I access the Coastal Flood Boundaries data and guidance documents?

Created: 03 October 2019 Updated: 03 October 2019UK National Tide Gauge Network data was de-trended to a base year of 2017. This ensures that all historic events are comparable in magnitude to present day events. Extreme sea levels provided are also to base year 2017 therefore are for probabilities of events that could happen now.Sea level rise should be taken into account when predicting flood risk in future years beyond 2017. Information on this is provided in the supporting documentation: <a class="external-link" href="https://www.gov.uk/government/publications/coastal-flood-boundary-conditions-for-uk-mainland-and-islands-design-sea-levels" rel="nofollow">Coastal flood boundary conditions for the UK: update 2018, User report</a>.

How is sea level rise accounted for in the Coastal Flood Boundaries?

Created: 03 October 2019 Updated: 03 October 2019Improved methods and longer observed gauge data series have been used to produce the new extreme sea level information. This has led to differences compared to previous estimates.Differences of less than 0.1m are not considered significant as these are within the accuracy of ESL estimation. Gauge locations with changes in level larger than 0.1m for the 200-year return period include Mumbles, Newport, Barmouth (Wales), Felixstowe and Cromer (England). Areas of greater change between gauge sites include the Severn Estuary, Solent, Solway Firth, Ornkey Islands, Pentland Firth and Liverpool.The new levels have been validated against other statistical methods and extensively checked against local knowledge and information. The new levels provide the best available source of information extreme sea level information and so should be used in all flood risk management and decision making.Lower and upper confidence levels are provided with all extreme sea levels and may be used to inform flood risk where there remains uncertainty.

I am concerned about the large differences in values in some locations and the effects on my project

Created : 03 October 2019 Updated : 03 October 2019Sea level rise was included to ensure the levels are relevant to base year 2017 and equates to increases in the order of approximately 2cm at most UK locations. Had the previous CFB levels not changed, base year 2017 levels would all be bigger than the 2011 published levels due to sea level rise differences between base years. However, improvements to the methodology and data has offset small changes in extreme sea levels due to sea level rise. These include:<ul><li>Significantly longer gauge records available in some areas than for the previous CFB. This can affect the magnitude of any given design level. (also slope of projections may be flatter – therefore low AEP events lower than previous CFB).</li><li>Different methodology or process used to derive the levels.</li><li>Detailed local study carried out since previous update.</li></ul>More significant changes due to sea level rise are expected over longer return periods, for example to 2050 or 2100. Sea level rise should be taken into account when predicting flood risk in future years beyond 2017. Information on this is provided in the supporting documentation: <a class="external-link" href="https://www.gov.uk/government/publications/coastal-flood-boundary-conditions-for-uk-mainland-and-islands-design-sea-levels" rel="nofollow">Coastal flood boundary conditions for the UK: update 2018, User report</a>.

Levels have fallen in my area of interest since the previous Coastal Flood Boundaries update. How can this be when sea levels are known to be rising?

Created: 03 October 2019 Updated: 03 October 2019The ESLs in the 2018 update have not changed significantly for most UK NTGN gauge sites. Differences of less than 0.1m are not considered significant as these are within the accuracy of ESL estimation. Gauge locations with changes in level larger than 0.1m for the 200-year return period include Mumbles, Newport, Barmouth (Wales), Felixstowe and Cromer (England).Larger changes in ESLs from CFB 2011 to CFB 2018 are present at some locations in the 2km return level dataset despite smaller changes at UK NTGN gauge locations. This is due to improvements in the interpolation of levels between gauge locations. These have been validated against predicted high tides and local gauge data. Areas of greater change include the Severn Estuary, Solent, Solway Firth, Ornkey Islands, Pentland Firth and Liverpool.

What has changed in Coastal Flood Boundaries 2018?

Created: 03 October 2019 Updated: 03 October 2019The 2018 updated dataset includes HAT and MHWS tide levels. These are based on interpolated levels at locations between NTGN sites and should be used for flood risk management purposes only. These levels should not be used for navigation purposes.

I’ve been given mean high water springs (MHWS) and highest astronomical tide (HAT) predicted Tide Levels by another source. They don’t match up to those in the Coastal Flood Boundaries. Which should I use?

Created: 03 October 2019 Updated: 03 October 2019The inland extensions (from here-on ‘estuaries’) are calculated using the following methods. A shapefile is available from coastal modelling teams to find out which method has been used to derive levels for each point within estuaries:<ol start="1"><li>No data. Where no observed data were available an estuary relationship was established by borrowing a relationship from a similar estuary or assuming that the gradient is flat (Scotland only).</li><li>Observed data. Upstream of the downstream boundary levels were derived using an estuary relationship based on observed data expressed as a gradient.</li><li>Modelled data. In some cases levels were taken directly from existing coastal modelling studies where these studies were deemed appropriate. Results were taken from the modelling of extreme coastal events only. A background fluvial flow may have been included such as the index flood QMED, but the results do not include any joint probability of extreme fluvial and coastal event modelling. All levels derived using this method are labelled ‘ESTUARY_’ in the Location field of the shapefile CFB_Extreme_Sea_Levels_Estuary_2018.shp available on <a class="external-link" data-card-appearance="inline" href="http://data.gov.uk" rel="nofollow">http://data.gov.uk</a>.</li></ol>The CFB 2018 estuary and tidal river levels are based on the interpolation of modelled levels including defences and so do not necessarily represent the scenario in which there are no flood defences. Flood defences can constrain coastal flood waters, resulting in elevated water levels upstream. Similarly, levels based on gauge analysis at upstream locations represent the scenario in which defences exist.As improved modelling becomes available following this 2018 update, the CFB ‘ESTUARY’ levels may be subject to review and further updates. The models in this 2018 update also do not include all models available at the time of this report was written. Detailed models have been included as a priority in regions of particular interest. Further locations will be added in the future.

How are levels within ‘Inland Extensions’ or ‘Estuaries’ derived?

Created: 03 October 2019 Updated: 03 October 2019For inland flood risk assessments where water courses may be influenced by tide, the CFB can provide the downstream tidal boundary. Levels included in tidal rivers, estuaries and harbours only take account of risk due to extreme coastal events. They don’t include risk due to extreme fluvial or combined fluival-coastal events, which requires additional modelling and analysis. Guidance may be sought from national coastal modelling teams on the appropriate use of levels in these areas. Additional information is also available on the methods and model information used to derive levels in estuaries from modelling teams.

How should I use Coastal Flood Boundaries levels in estuaries and tidal rivers?

Created: 03 October 2019 Updated: 03 October 2019The following were updated or added to the dataset in 2018:<ul><li>ESLs of annual exceedance probability ranging from 1-year to 10,000-year return period</li><li>highest astronomical tide and mean high water spring tide conditions for some locations</li><li>ESLs for the full study area coastline at a spacing of about 2km along the open coast or less in estuaries and harbours (enabling rapid selection of appropriate levels without any need for further interpolation)</li><li>advice on generating appropriate total storm tide curves for use with the extreme sea levels</li><li>extension of the dataset in several key estuaries</li></ul>The following were not updated in 2018:<ul><li>extreme swell wave conditions were supplied for CFB 2011 but were not updated for CFB 2018</li></ul>

What are the key outputs of Coastal Flood Boundaries 2018?

Created: 03 October 2019 Updated: 03 October 2019The CFB data must be maintained to ensure it reflects the best available information and sea level science. This 2018 update is the first carried out since the project first publication in 2011.Work was undertaken to:<ul><li>apply new sea level science and improvements to statistical methods to update existing ESLs</li><li>make use of nearly 10 years of additional observational data recorded at National Tide Gauge Network (NTGN) sites since the original study, supplementary data available at NTGN sites and gauge data provided by other organisations for non-NTGN sites</li><li>extend the locations for which ESLs are provided to include Northern Ireland, Jersey, additional Scottish Islands and along priority tidal rivers and estuaries</li></ul>The significant enhancements achieved by the project included improved tidal analysis, improved de-trending, an improved statistical model, improved confidence intervals and improved extremal index estimation.

What has been done to update the Coastal Flood Boundaries in 2018?

Created: 03 October 2019 Updated: 03 October 2019The CFB represents the best available information on extreme sea levels within the UK. The methods used in the extreme value analysis are internationally pioneering and are best on the best available tide gauge data. To maintain this high standard, regular updates are required to reflect the most recent improvements in sea level science and newly available gauge information.Even if there are no significant tidal flood events in the eight year update period, the results are expected to change if they are re-analysed. Changes are primarily due to differences in data, improvements to the statistical methods and understanding of spatial variation between tide gauges.Changes in levels may be problematic to some users and decisions makers. To make these changes manageable, an eight year period of stability is considered practical for most purposes.

Why are regular updates, and therefore likely change in the estimates, necessary?

Created: 03 October 2019 Updated: 05 August 2020 The open coast Coastal Flood Boundaries (CFB) data will be updated approximately every eight years. Inland extensions (estuaries, tidal rivers, harbours, sea lochs and firths) will be updated every one to two years.

How often is the Coastal Flood Boundaries dataset updated?

Created: 03 October 2019 Updated: 03 October 2019The method for generating time series design tide curves is given within the user guide contained within the supporting documentation: Coastal flood boundary conditions for the UK: update 2018, User report. A link is provided with the data on <a class="external-link" href="http://environment.data.gov.uk" rel="nofollow">Defra Data Services Platform</a> or can be found in the “doc” folder of downloaded data. Extreme sea levels and surge profiles are provided by the CFB. The UK Hydrographic Office, Admiralty and the National Oceanographic Centre supply astronomical tidal time series data for a range of tide gauges throughout the UK. There are also software packages available to derive astronomical tidal data from Admiralty and NOC.

How can I generate my Base Tide Curves?

Created: 03 October 2019 Updated: 03 October 2019Impacts due to wave action, such as wave setup, run-up or wave overtopping are not included in the Coastal Flood Boundaries (CFB) dataset and would need to be considered in addition to extreme sea level risk where waves produce flood or erosion risk. Wave and overtopping data may be included from other sources such as State of the Nation (England only). Contact your national authority’s modelling teams for more information.Extreme swell wave conditions were supplied for CFB 2011 but were not updated for CFB 2018.

Are waves included in the Coastal Flood Boundaries?

Created: 03 October 2019 Updated: 03 October 2019Both the CFB_Extreme_Sea_Levels_2018.shp and CFB_Extreme_Sea_Levels_Estuary_2018.shp shapefiles contain 16 extreme sea levels (prefixed ‘T’). They also include 16 lower (2.5%) and upper (97.5%) confidence levels (prefixed ‘C_T1’ and ‘C_T2’ respectively).Each extreme sea level value represents the peak total sea level that occurs during a tidal cycle associated with particular extreme probability. Predicted tidal levels combine with storm surge to produce the extreme peak sea level. Information on how to generate a design time series tide curve using CFB extreme sea level and surge information is provided in the supporting documentation: Coastal flood boundary conditions for the UK: update 2018, User report.The confidence levels provided account for statistical uncertainty in the skew surge joint probability method. They do not take account of uncertainty due to interpolation using model data or predicted tidal variation between gauge sites. Locations of gauge sites used in the statistical analysis are given in CFB_Gauge_Data_2018.shp.The shapefiles also include information on base year and Highest Astronomical Tide (HAT) and Mean High Water Spring (MHWS) predicted tide levels (not suitable for navigation). The location and chainage of each point is given.Location chainage information included in the shapefiles is unchanged from CFB 2011 where the same location has been included in CFB 2018.

What information is included in the extreme sea levels shapefiles?

Created: 03 October 2019 Updated: 03 October 2019ESLs provided by this project can be considered accurate to one decimal place. Two decimal places have been provided to differentiate between nodes on the chainage. This does not infer greater accuracy and the user should be mindful of this when selecting a node for an ESL.

What is the accuracy of the Coastal Flood Boundaries extreme sea levels?

Created: 03 October 2019 Updated: 03 October 2019 The return levels presented in this report are generally relative to the main tide gauge benchmark, which has a fixed height relative to Ordnance Datum Newlyn (ODN) and Ordnance Datum Belfast (in Northern Ireland). Sites relative to local datums include Stornoway, Lerwick and St Mary’s.ODN is referred to here as the height above mean sea level at Newlyn from 1950 to 1968. Comparisons of the return levels can only be made with other data relative to the same datum (that is, ODN). Heights measured using a GPS can be converted to ODN via the spatial surface of the transformation model OSGM15, or previously OSGM02.

What is the vertical datum of the Coastal Flood Boundaries Level?

Created: 03 October 2019 Updated: 03 October 2019The levels are to a base year of 2017. The base year refers to the year for which the levels are valid and takes account of the mean sea level for the year. Modelling required for years other than 2017 should apply corrections for sea level rise.

What is the base year for Coastal Flood Boundaries levels?

Created: 03 October 2019 Updated: 03 October 2019The Coastal Flood Boundaries (CFB) includes 16 probabilities of extreme sea level ranging from 100% to 0.001% AEP (or 1 to 10,000 year return periods).Annual Exceedance Probability (AEP) describes the likelihood of being exceeded in any given year. For instance, an AEP of 1% has a chance of being exceeded 1 in 100 in any given year. In coastal design the reciprocal of an AEP is often termed as a ‘return period’. An AEP of 1% is equivalent to a return period of 100 years.

What is meant by Annual Exceedance Probability (AEP) and which extreme sea level probabilities are included?

Created: 03 October 2019 Updated: 03 October 2019 The Coastal Flood Boundaries (CFB) dataset is used as input for a range of coastal management applications and decision making, including flood risk mapping, flood risk assessments, spatial planning, coastal flood defence design, flood warning, port operations, infrastructure decisions, coastal erosion management, climate change assessments and informing emergency planning.Extreme sea level data can be used as input to numerical models to assess the depth and extent of inundation to coastal communities or other vulnerable areas.

How does the Coastal Flood Boundaries inform us about flood risk around the UK coast?

Created: 03 October 2019 Updated: 03 October 2019The Coastal Flood Boundaries (CFB) dataset provides the industry standard for coastal water level extremes across the UK and is used universally in flood risk assessments and modelling both internally and externally. There are a huge number of users of the data across the UK including public bodies, engineering consultants, coastal analysts, hydrologists, climatologists and academics. The CFB is used to inform coastal defence strategy, flood mapping and forecasting, and to support policy, implementation and operational decision-making.

Who should use the Coastal Flood Boundaries (CFB) data?

Created: 03 October 2019 Updated: 03 October 2019Successful management of coastal flood risk and coastal protection work requires the best available information on extreme sea-levels (ESLs) and other important drivers of coastal flood and erosion risk. The Coastal Flood Boundary Conditions for UK Mainland and Islands (CFB), first published in 2011, supports this work with a robust and consistently derived evidence base. The CFB dataset includes:<ul><li>a continuous and spatially consistent set of ESLs around the coasts of England, Wales, Scotland, Northern Ireland, Isle of Man and Jersey</li><li>a means of generating appropriate total storm tide curves for use with the extreme sea levels</li><li>best practice guidance on how to use these new datasets</li></ul>This 2018 CFB update was carried out for the UK Coastal Flood Forecasting partnership, which includes the Environment Agency, Scottish Environment Protection Agency, Natural Resources Wales and the Department for Infrastructure Northern Ireland. The work was conducted by a project team including JBA Consulting, Professor Jonathan Tawn of Lancaster University and the National Oceanography Centre. The project also included extensive consultation with UK practitioners and subject matter experts.

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Where can I access the Coastal Flood Boundaries data and guidance documents?