Production and Purpose of the Strategic Flood Map FAQs

   Return to the Strategic Flood Map Frequently Asked Questions

The Strategic Flood Map has been developed to provide a general overview of the flood risk in Northern Ireland. The flood map is designed to increase awareness among the general public, local authorities and other organisations of the likelihood of flooding and to encourage them to take appropriate action to manage the risk.

The production of the Strategic Flood Map is an important first step in a process that, in line with the EU Floods Directive, will lead to the development of flood risk management plans for all areas across NI that are estimated to be at potential significant risk of flooding. Rivers Agency shall be actively involving and consulting with all stakeholders in the development of these catchment-based plans which are programmed for completion by end of 2015. The shared challenge will be to produce plans that contain appropriate, affordable solutions that reduce flood risk and protect and enhance the environment.

The Strategic Flood Map is used by DOE to inform the planning process in terms of controlling development and in the preparation of development plans. DOE will also use other sources of information to inform planning decisions such as historical evidence and where available detailed local flood study maps.

The publication of the Strategic Flood Map for Northern Ireland is consistent with flood risk management taken in Scotland, England and Wales where similar flood maps are available through the Environment Agency and SEPA websites.

The main purpose of the Strategic Flood Map is to highlight areas throughout Northern Ireland that are estimated to be at risk of flooding from rivers, the sea and surface water. For the purposes of clarity and ease of use the Strategic Flood Map contains five separate map layers. A description of the data contained within each of the map layers is as follows:

The ‘Historical’ flood map layer shows the outlines of areas that are known to have been inundated by flood water in the past. The outlines are generated from data collected by Rivers Agency following actual flood events.

The ‘River’ flood map layer illustrates the estimated extents of river plains for a flood event with a 1 in 100yr return period (1% AEP).  The flood plains ignore the presence of existing flood defences and the areas benefiting from the defences are highlighted in the ‘Flood Defences/ Defended Area’ map.  Flood plains for both the present day and future (2030yr) climate are shown on separate layers.

The ‘Sea’ flood map illustrates the estimated extents of coastal flood plains which are the areas of land around the shoreline which may be inundated by the sea as a consequence of a coastal storm surge with a 1 in 200 yr return period (0.5% AEP).  The flood plains ignore the presence of existing flood defences and the areas benefiting from the defences are highlighted in the ‘Flood Defences/ Defended Area’ map.  Flood Plains for both the present day and future (2030yr) climate are shown on separate layers.

The ‘Surface Water’ map illustrates the low lying areas and hollows that are prone to flooding from the ‘ponding’ of overland flow arising from a 1 in 200 yr rainfall event.

The ‘Flood Defences/ Defended Areas’ map layer identifies the location of existing flood defences such as embankments and walls and shows the areas which benefit from a reduction in the likelihood of flooding as a consequence of their presence.

For a definition and further explanation of a ‘flood plain’ read the answer to Question 1 under Flood Risk Information and its Uses. What do the highlighted flood plain outlines in the Strategic Flood Map mean?

3. How has the Strategic Flood Map been created?

In the case of the Historical flood map, the outlines are based on records (i.e. maps, sketches, photographs, level measurements) taken or collected by Rivers Agency at the time of actual flood events.  This is in contrast with the outlines in the other maps which have been derived from predictive models.   These regional scale models have been developed using a streamlined approach that uses computers to integrate rainfall, hydrological, coastal and digital terrain models to produce flood inundation maps for flood events with various return periods.  The methodologies used are consistent with those applied by other flood authorities in GB.  If you would like more technical information about the methodologies used to create these maps, please click here

The Strategic Flood Map was developed to provide an indication of the general areas throughout the whole of Northern Ireland that may be prone to flooding and the methodology used to estimate the location and extents of these areas is consistent with that purpose.  There are clear limitations associated with the methodology and data used that inevitably have an impact on the accuracy of the estimated flood plain outlines and inundation areas.  For that reason the Strategic Flood Map is not suitable for determining the flood risk to a particular property or specific point location. Information suitable for this purpose can only be obtained through a detailed local study. As studies of this nature are very expensive and time consuming they are not appropriate for a national strategic assessment of flood risk. However, where an area of potential significant flood risk has been identified through the strategic assessment, Rivers Agency shall be undertaking detailed local studies as appropriate. The revised flood plain outlines that are generated through the detailed local studies will be displayed within the on-line map viewer as they become available. In accordance with the Floods Directive, the detailed flood maps for all areas of significant risk shall be completed by December end of 2013.

Within the Strategic Flood Map there is a ‘Historical’ flood map layer which highlights the areas that have been inundated during actual flood events in the past. This map has been generated using data that was collected by Rivers Agency at the time of the flood event and from aerial photographs. The Historical flood map will continue to be updated as new information floods becomes available.  Although this information is useful insofar as it can serve as a reminder that the risk of flooding is very real it does not, of itself, provide clear insight to the likelihood of flooding now or in the future. The likelihood of flooding can only be established, with any degree of confidence, using predictive modelling techniques similar to those used to determine the extent of the flood plains and inundation areas for the Rivers, Sea and Surface Water maps.  However, the Historical flood data does have an important role to play in the validation of the predictive models by ensuring that their outputs are credible.

6. When and how often will the Strategic Flood Map be updated?

Rivers Agency is committed to providing the best available flood risk information to as wide an audience as possible and therefore updates and revisions to the Strategic Flood Map will be provided as appropriate. For example, Rivers Agency shall continue to collect data on actual flood events as they occur and, if these are considered to be significant, the Historical flood map layer shall be updated. Similarly, as we develop more detailed flood risk maps to meet the requirements of the EU Floods Directive, these shall be hosted on the web-viewer. The detailed flood risk maps shall be available for all areas estimated to be at significant risk of flooding by the end of 2013.

The Strategic Flood Map does not show the floodplain for any watercourse where the catchment area is less than 3km2. As the map was developed to provide a general overview of flooding from rivers, it would not have been practical or appropriate to undertake the hydraulic modelling of every sheugh or ditch.

Within the River Flood Map and the Sea Flood Map there are two distinct layers which are titled ‘Present Day’ and ‘Climate Change’.  The Present Day map layer was established through modelling techniques which use the meteorological, river flow and sea level data which is relevant at this point in time and to that extent this has taken account of any changes to our climate that have already occurred. It is however widely accepted that our climate is changing and that with the passage of time sea levels around our coastline will rise and the extreme rainfall events may become more frequent and intense. Therefore with this in mind, the Climate Change map layer has been produced to reflect the flood plain outlines for the year 2030 and this is based on the best available predictions for the meteorological conditions and sea levels at that time.

Rivers Agency has decided not to publish a Climate Change map layer for the Surface Water Map.  The reason for this is that the pluvial flood outlines have been generated using a relatively new and largely untested surface water modelling technique.  Although the surface water model used is currently the best available for a broad scale strategic assessment and similar to the models used by the other flood authorities within GB and Ireland, the results should be treated with a degree of caution due to uncertainties in the methodology and data.  Given the known limitations of the Surface Water Map it would not be appropriate to add to this uncertainty by publishing a climate change version at this time.