National Coastal Flood Depths 2021 - High End Future Scenario

Abstract: This data shows the extent of land that might be flooded by the sea (coastal flooding) and the associated flood depths 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. This represents the worst case scenario as any flood defences potentially protecting the coastal floodplain are not taken into account.

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 the chance or odds (e.g. 200 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 200-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 sets out the range of flood event probabilities for which coastal flood extent maps were developed, expressed in terms of Annual Exceedance Probability (AEP), and identifies their parallels under other forms of expression.

50% AEP can also be expressed as the 2 Year Return Period and as the 2:1 odds of occurrence in any given year. 20% AEP can also be expressed as the 5 Year Return Period and as the 5:1 odds of occurrence in any given year. 10% AEP can also be expressed as the 10 Year Return Period and as the 10:1 odds of occurrence in any given year. 5% AEP can also be expressed as the 20 Year Return Period and as the 20:1 odds of occurrence in any given year. 2% AEP can also be expressed as the 50 Year Return Period and as the 50:1 odds of occurrence in any given year. 1% AEP can also be expressed as the 100 Year Return Period and as the 100:1 odds of occurrence in any given year. 0.5% AEP can also be expressed as the 200 Year Return Period and as the 200:1 odds of occurrence in any given year. 0.1% AEP can also be expressed as the 1000 Year Return Period and as the 1000:1 odds of occurrence in any given year.

The High End Future Scenario (HEFS) maps represent a projected future scenario for the end of century (circa 2100) and include allowances for projected future changes in sea levels and glacial isostatic adjustment (GIA). The maps include an increase of 1000mm in sea levels above the current scenario estimations. An allowance of -0.5mm/year for GIA was included for the southern part of the national coastline only (Dublin to Galway and south of this).

Flooding from other sources may occur and areas that are not shown as being within a flood extent may therefore be at risk of flooding from other sources. The flood extent and depth maps are suitable for the assessment of flood risk at a strategic scale only, and should not be used to assess the flood hazard and risk associated with individual properties or point locations, or to replace a detailed flood risk assessment. Lineage: The National Coastal Flood Hazard Maps (NCFHM) 2021 are ‘predictive’ flood maps, as they provide predicted flood extent and depth information for a ‘design’ flood event that has an estimated probability of occurrence (e.g. the 0.5% AEP event), rather than information for floods that have occurred in the past.

The maps have been produced at a strategic level to provide an overview of coastal flood hazard in Ireland, and minor or local features may not have been included in their preparation. A Digital Terrain Model (DTM) was used to generate the maps, which is a ‘bare-earth’ model of the ground surface with the digital removal of human-made and natural landscape features such as vegetation, buildings and bridges. This methodology can result in some of these human-made features, such as bridges and embankments, being shown within a flood extent, when in reality they do not flood.

It should be noted that the flood extent maps indicate the predicted maximum extent of flooding, and flooding in some areas, such as near the edge of the floodplain area, might be very shallow. The predicted depth of flooding at a given location is indicated on the flood depth maps. The flood depth is displayed as a constant depth over grid squares with a 5m resolution, whereas in reality depths may vary within a given grid square.

No post-processing of the flood extent and depth map datasets has been undertaken to remove small areas of flooding that are remote and isolated, small islands within the flooded area, etc. Local factors such as flood defence schemes, structures in or around river channels (e.g. bridges), buildings and other local influences, which might affect coastal flooding, have not been accounted for.

Detailed explanations of the methods of derivation, data used, etc. is provided in the NCFHM 2021 Flood Mapping Methodology Report. Users of the maps should familiarise themselves fully with the contents of this report in advance of the use of the maps.

Purpose: The data has been developed to inform a national assessment of flood risk that in turn will inform a review of the Preliminary Flood Risk Assessment required 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.