Production and Purpose of the Strategic Flood Map FAQs
- Why has Rivers Agency produced the Strategic Flood Map (NI) - Rivers & Sea?
- What is the purpose of the Strategic Flood Map and what does it show?
- Will the Strategic Flood Map illustrate the likelihood of flooding from sources other than from rivers and the sea at some time in the future?
- How has the Strategic Flood Map been created?
- How accurate is the Strategic Flood Map?
- Does the Strategic Flood Map take into account knowledge from previous floods?
- When and how often will the Strategic Flood Map be updated?
- Does the Strategic Flood Map cover small streams and burns?
- Does the Strategic Flood Map take the effects of climate change into account?
– How the Map was produced and its purpose –
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 will be 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 study maps.
The publication of the Strategic Flood Map will bring Northern Ireland into line with Scotland, England and Wales where similar 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 and the sea. For the purposes of clarity and ease of use the Strategic Flood Map contains four 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 from rivers and/or the sea in the past. The outlines are generated from data collected by Rivers Agency following actual flood events.
The ‘Present Day’ flood map layer illustrates the estimated extents of the river and coastal flood plains under current climatic conditions. Areas that benefit from the presence of existing flood defences can be viewed on the ‘Flood Defence’ map layer.
The ‘Climate Change’ flood map layer illustrates the estimated extents of the river and coastal flood plains and takes account of predicted changes to climatic conditions and sea-levels for the year 2030. Areas that benefit from the presence of existing flood defences can be viewed on the ‘Flood Defence’ map layer.
The ‘Flood Defences’ 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. Will the Strategic Flood Map illustrate the likelihood of flooding from sources other than from rivers and the sea at some time in the future?
Rivers Agency is currently developing a map to illustrate the areas throughout Northern Ireland that may be prone to flooding from pluvial flows. Pluvial flow (also known as surface water or overland flow) can occur as a result of an intense rainfall event and this can cause flooding in low lying areas and hollows before reaching a watercourse or sewer. It is anticipated that this information may be uploaded to the Strategic Flood Map as a map layer at some time in the future.
The flood plains illustrated in the Present Day and Climate Change flood map layers have been developed using industry standard methodologies that have been applied consistently throughout the United Kingdom. The methodology involves the use of computers to combine a digital model of the ground surface with hydraulic models for all rivers with catchments greater than 3km2. An estimated extreme sea level was projected onto the ground surface model to calculate the extent to which the sea inundated the coastal zone. If you would like more technical information about the methodologies used to create these maps, please click here.
The flooded area outlines illustrated in the Historical flood map has been produced using information captured at the time of actual flood events. The details illustrated within the Flood Defences map (i.e. the location of existing flood defences and the areas which they benefit) are taken from the Rivers Agency’s records of its flood alleviation schemes.
The Strategic Flood Map was developed to provide an indication of the general areas that may be prone to flooding from rivers and the sea for the whole of Northern Ireland. For that reason the methodology used 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 flood plain outlines. 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 was generated using data that was collected by Rivers Agency at the time of the flood event and from aerial photographs. 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 much guidance as 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 in the Present Day and Climate Change map layers.
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 undertaken were this is appropriate. For example, Rivers Agency shall continue to collect data on actual flood events that occur in the future and the Historical flood map layer shall be updated to reflect this information. 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 chance of flooding from any watercourse where the catchment area draining to it is less than 3km2. As the map was developed to provide a general overview of flooding from rivers and the sea, it would not have been practical or appropriate to undertake modelling of every sheugh or ditch.
The flood plain illustrated within the Present Day map was established through modelling techniques which used 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 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 (that lead to river flooding) shall become more frequent and intense. Therefore with this in mind, the Climate Change map has been produced to reflect the flood plain outlines for the year 2030. This is based on the best available predictions for the meteorological conditions and sea levels at that time.
