Community-Scale Coastal Flood Extents - High-End Future Scenario

Abstract: This Flood Hazard data is produced to meet the requirements of Article 6 of the European Communities (Assessment and Management of Flood Risks) Regulations 2010 to 2015 (the “Regulations”) (implementing Directive 2007/60/EC). This dataset shows the modelled extent of land that might be flooded by the sea (coastal flooding) during a theoretical or ‘design’ flood event with an estimated probability of occurrence, rather than information for actual floods that have occurred in the past. The flood extents have been developed taking account of effective flood defences. Areas that are not shown as being flooded may still be at risk of flooding from unmodelled coastal or river reaches, as well as from other flood sources.

The purpose of the OPW's Flood Maps is not to designate individual properties at risk of flooding. They are community-based maps, suitable for community-level assessments.

Annual Exceedance Probability - Flood event probabilities are referred to in terms of a percentage Annual Exceedance Probability, or ‘AEP’. This represents the probability of an event of this, or greater, severity occurring in any given year. These probabilities may also be expressed as odds (e.g. 100 to 1) of the event occurring in any given year. They are also commonly referred to in terms of a return period (e.g. the 100-year flood), although this period is not the length of time that will elapse between two such events occurring, as, although unlikely, two very severe events may occur within a short space of time. The following outlines common flood event probabilities used for fluvial and coastal flood maps, expressed as Annual Exceedance Probability (AEP) and their equivalent forms. - 10% (High Probability) Annual Exceedance Probability - This can also be expressed as the 10 Year Return Period and as a 10:1 odds of occurrence in any given year. - 1% (Medium Probability - Fluvial/River Flood Maps) Annual Exceedance Probability - This can also be expressed as the 100 Year Return Period and as 100:1 odds of occurrence in any given year. -0.5% (Medium Probability - Coastal Flood Maps) Annual Exceedance Probability - This can also be expressed as the 200 Year Return Period and as 200:1 odds of occurrence in any given year. - 0.1% (Low Probability) Annual Exceedance Probability, which can also be expressed as the 1000 Year Return Period and as 1000:1 odds of occurrence in any given year.

Climate Scenarios - The following outlines common climate scenarios used for fluvial and coastal flood maps.

The Present Day Scenario - This is also referred to as the Current Scenario. Present Day Scenario data was generated using methodologies based on historic flood data, without taking account of potential changes due to climate change.

The Mid-Range Future Scenario (MRFS) - This scenario is generated taking in the potential effects of climate change using an increase in rainfall of 20% and sea level rise of 500mm (20 inches).

The High-End Future Scenario (HEFS) - This scenario is generated taking in the potential effects of climate change using an increase in rainfall of 30% and sea level rise of 1,000 mm (40 inches).

Lineage: Fluvial and coastal flood hazard data is developed using hydrodynamic modelling. This is based on calculated design river flows and extreme sea levels, surveyed channel cross-sections, river / coastal structures, Digital Terrain Models, and other relevant datasets such as land use, data on past floods for model calibration etc. The data has been developed taking account of effective flood defences. Areas that are not shown as being flooded may still be at risk of flooding from unmodelled coastal or river reaches, as well as from other flood sources.

The flood depths are calculated by subtracting the ground levels from the predicted water level. The flood depths are mapped as constant depths over grid squares, whereas in reality depths may vary within a given square. The flood depth data originates from a significant number of individual projects as described below and the process may vary for particular areas or maps. Individual Technical Hydrology and Hydraulics Reports set out full technical details on the derivation of the flood maps for each location.

The primary flood depth data source is the original project raster data, suitable for analysis purposes. For national-scale display, the individual depth datasets are reprocessed into a banded vector product to harmonise resolutions and grid origins and to remove dataset overlaps. The resulting depth vector data product is suitable for display purposes only at a national scale.

Flood extents are derived from raster flood depth maps and vectorised to produce the final vector outputs. The national flood extents dataset is compiled from the original source data and is based on the alignment of the original raster datasets, rather than on the banded national vector depth product.

The CFRAM data was the first set of community-scale maps delivered under the EU 'Floods' Directive [2007/60/EC] and is updated as required under subsequent cycles of this Directive.

The OPW's updated Community-scale Flood Maps are effectively a combination of the following datasets:

• CFRAM 2018 - This data was produced for the 300 'Areas of Further Assessment' (AFAs), as identified under the first cycle of the EU 'Floods' Directive [2007/60/EC] and designated under the Preliminary Flood Risk Assessment. Data was also produced for other reaches between the AFAs and down to the sea that are referred to as 'Medium Priority Watercourses' (MPWs). River reaches that have been modelled are indicated by the CFRAM Modelled River Centrelines dataset.

• National Flood Map Reviews - Updates and revisions take place to the community-scale flood maps on an ongoing basis, where evidence is presented of a change in physical conditions, a detection of errors or based on other specific evidence. More information can be found on the OPW's Flood Map Review Page, https://www.floodinfo.ie/map-review/.

• Completed Flood Relief Schemes - Updates and revisions take place to the community-scale flood maps where a Flood Relief Scheme has been completed. More information can be found on the OPW's Flood Relief Scheme Information Summary Page, https://www.floodinfo.ie/scheme-info/.

v101 (March 2025) The section of map near Oranmore Galway updated following a map review process see https://www.floodinfo.ie/map-review/ for further information, Map Review Code: MR019.

Purpose: The data has been developed to comply with the requirements of the European Communities (Assessment and Management of Flood Risks) Regulations 2010 to 2015 (the “Regulations”) (implementing Directive 2007/60/EC) for the purposes of establishing a framework for the assessment and management of flood risks, aiming at the reduction of adverse consequences for human health, the environment, cultural heritage and economic activity associated with floods.

The purpose of the OPW's Flood Maps is not to designate individual properties at risk of flooding. They are community-based maps, suitable for community-level assessments.