Indicative flood maps
The purpose of the interactive Indicative Flood Maps is to illustrate the general areas that have flooded in the past and those which are considered to be at a medium risk of flood now and in the future.
Areas which are known to have flooded in the past are depicted in the Historical Map layer. The areas which are estimated to be at risk from a medium probability event have been derived using predictive computer modelling techniques.
Government has determined that within Northern Ireland the areas at ‘medium’ risk are those which are predicted to be inundated during floods of the following magnitude:
|Source||Flood event probability||Predictive flood model types|
|Rivers||1% annual exceedance probability (AEP)*||Strategic and detailed|
|Sea||0.5% AEP||Strategic and detailed|
|Surface water||0.5% AEP||Strategic|
The table above shows that the flood outlines illustrated within the Indicative Flood Maps for each of the flood sources are based, at least in part, on strategic flood models which use fairly coarse modelling techniques and data.
As there is a degree of uncertainty in regard to the extents of the flood plains derived from strategic models, the Indicative Flood Maps cannot be used to predict the flood risk to a particular property or specific point location.
To reinforce the fact that the Indicative Flood Maps should be used only to identify ‘general areas’ that may be prone to flooding from medium probability flood events, users are unable to view the map at a scale that would enable them to identify individual properties.
Indicative flood maps explained
The extents of the floodplains for all of the flood sources have been estimated using predictive computer modelling techniques that are commonly used as decision support tools by flood defence authorities throughout the UK, Ireland and beyond.
Present day and climate change
Each of the indicative flood maps for rivers, sea and surface water are available for the Present Day and Climate Change (2030 year) epochs.
The Present Day maps illustrate the floodplains and inundation areas that have been predicted by the predictive models using meteorological input data that is representative of the current climate conditions.
It is widely accepted that our climate is changing and that with the passage of time, extreme rainfall events may become more frequent and intense and the sea around our coastline will rise and become stormier.
The Climate Change maps have been produced to highlight the estimated floodplains for the year 2030 and are based on the best available predictions for the meteorological conditions and sea levels for that time.
River floodplain map
The River Floodplain Map provides an illustration of river floodplains, which are the relatively flat areas of land adjacent to rivers that are subject to periodic flooding. The outlines of the floodplains highlighted in the map identify the areas that in any year have a 1% AEP (1 in 100) or greater chance of flooding from rivers.
Users should note that floodplain outlines are not available for every river in Northern Ireland as there are many small rivers for which sufficient data was not available to enable hydraulic modelling to be undertaken.
These small unmodelled rivers, which typically have a catchment less than three square kilometres, are highlighted in the map by a dashed blue line.
Sea (coastal) floodplain map
The Sea Floodplain map provides an illustration of the coastal floodplains which are the relatively flat areas of land around the shoreline that are subject to periodic coverage by the sea.
The outlines of the floodplains highlighted in the map identify the areas around the coastline that in any year have a 0.5% (1 in 200) or greater chance of flooding from the sea.
Surface water flood map
Surface water flooding is usually related to short duration high intensity rainfall. The flooding occurs when the ground is unable to absorb the rainwater, causing it to flow over the surface and fill depressions and low spots in the landscape where local natural and engineered drainage systems are overwhelmed.
The Surface Water Flood Map illustrates the low lying areas and hollows that are estimated to be prone to flooding from an extreme 1 in 200yr rainfall event.
The surface water flood map does not specifically illustrate areas that are at risk from flooding due to local deficiencies in sewerage or drainage systems which are at best, designed to cope with a 1 in 30 year rainfall event.
Historical flood map
The Historical Flood Map illustrates areas that are known to have flooded in the past. The flooded area outlines depicted on this map have been generated from archived field data and aerial photographs that were collected by DfI Rivers at the time of the actual flood events.
DfI Rivers shall continue to capture data for all significant future flood events and this map shall be updated periodically as this new information becomes available.
However, we only have records for a relatively short period of time and many of the past floods cannot be linked to flood events with any particular return period. Furthermore, flood alleviation schemes will have been undertaken at some of the flood prone locations which are highlighted on this map and this work will have significantly reduced the likelihood of future flooding at these areas.
As a consequence of these limitations this information does not provide a sound basis by which to estimate how likely it is that flooding will re-occur at a particular location in the future or for identifying flood prone areas that haven’t yet flooded.
In fact, it would require historical records that span many hundreds of years to produce reliable estimates of the magnitude and return period of potential future floods. However, the Historical Flood Map is useful insofar as it can serve as a strong visual reminder that the risk of flooding is very real. Therefore to identify potential flood risk areas in a consistent and comprehensive manner across NI, it is necessary to employ predictive flood modelling.
As explained above, a range of predictive models have been used to estimate the extent of river and coastal flood plains and those areas that are likely to be susceptible to inundation by surface water.
Accuracy and use of flood hazard maps and flood risk maps
The hazard and risk maps for the rivers and sea have been produced using the best available modelling techniques, tools and data and as a result there is a high degree of confidence in their accuracy.
Consequently, these maps are considered to be suitable for determining the likelihood of flooding to individual properties and specific point locations. Therefore they are published at a scale (1 in 5,000) that will enable users to identify individual properties.
However, there was no option but to produce the hazard and risk maps for surface water flooding using a strategic flood model that covers the whole of the province. There are clear limitations with methodology and data used in the development of this broad scale model.
Due to this uncertainty the maps derived from the model should be treated with caution and viewed as indicative rather than accurate or precise. Therefore, the published Flood Hazard Maps for surface water should be used to identify general areas and communities that may be at risk of flooding from surface water.
Because of the indicative nature of these maps they have been published at a scale (1 in 10,000) to prevent the identification of individual properties.
Flood hazard maps
A suite of detailed Flood Hazard Maps which effectively cover the named Significant Flood Risk Areas and Areas for Further Study are available as PDFs.
The detailed Flood Hazard Maps for rivers and the sea have been derived from flood models produced using the best available flood modelling techniques, tools and data. Consequently, the Flood Hazard Maps for rivers and the sea are considered suitable for predicting the level of flood risk to individual properties or point locations and as a consequence the maps are available at a scale that will enable users to identify individual properties.
However, due to the inherent complexities in the production of detailed integrated urban drainage models for surface water flooding, it is not practical to undertake a broad scale programme of detailed modelling for this flood source.
Therefore, strategic modelling is the only viable option available to estimate the areas that may be prone to surface water flooding. As the Flood Hazard Maps for surface water are produced using a strategic model they are not suitable for predicting the risk to individual properties or point locations and this limitation is clearly stated on the relevant PDF maps.
The detailed Flood Hazard Maps for each of the flood sources are available for high medium and low probability flood events. These maps essentially provide a range of important information to describe the characteristics of the flooding, including for example the:
- geographical extents of the flood inundation area
- water depth and height
- the flow and velocity of the floodwater
Flood hazard maps explained
The preparation of Flood Hazard Maps and Flood Risk Maps are a specific requirement of the EU Floods Directive.
Under the Directive government is obliged to produce Flood Hazard Maps and Flood Risk Maps for each of the twenty named Significant Flood Risk Areas (SFRA) which it reported to the European Commission subsequent to the completion of the Preliminary Flood Risk Assessment for NI in December 2011.
In addition to the legal requirement to produce maps for the SFRA, government determined that it would produce hazard and risk maps for another 49 settlements named Areas for Further Study (AFS). The locations of the SFRA and AFS can be viewed within the Flood maps (NI) interactive viewer. As required by the Directive, the Flood Hazard and Flood Risk Maps were produced by December 2013.
The suite of Flood Hazard Maps for each of the SFRA and AFS are currently available as PDFs within Flood Maps (NI). A similar suite of Flood Risk Maps has been produced, the information illustrated within this range of maps is most suitable for flood risk management practitioners and is unlikely to be of interest to the general public.
The Directive requires that for each of the main flood sources, hazard and risk maps at an appropriate scale shall be prepared for high, medium and low probability flood event scenarios. The specific requirements for each of the map types are as follows:
Flood hazard maps
Flood Hazard Maps essentially describe the characteristics of the predicted flood for each of the flood event scenarios and includes information such as:
- the geographical extent of the flood inundation areas
- the water depth or height
- where appropriate the velocity or flow of the floodwater
Flood risk maps
The Flood Risk Maps essentially describe the main adverse consequences of the predicted flood for each of the flood event scenarios. The data used to describe the adverse consequences is collated and displayed on a 250m grid and includes information such as:
- the number of inhabitants who could be adversely affected
- the affect on economic activity (in terms of the monetary damage)
- the locations of flood prone industrial sites that might cause accidental pollution and in particular where this could impact nationally important environmental areas