River (Fluvial) Flood Map

The River Flood Map provides an illustration of river flood plains which are the relatively flat areas of land adjacent to rivers that are subject to periodic coverage by floodwater. The outlines of the floodplains highlighted in the map identify the areas that in any year have a 1 in 100 or greater chance (1% Annual Exceedance Probability (AEP)) of flooding from a river. The extents of the floodplains have been estimated using predictive computer modelling techniques that are commonly used as a decision support tool by flood defence authorities throughout the UK, Ireland and beyond. As the predictive models cover the whole of Northern Ireland there are clear limitations associated with the methodology and data used. These inevitably have an impact on the accuracy of the flood plain outlines. For that reason, the flood outlines should regarded as ‘indicative’ of the general areas that are estimated to be prone to flooding and are not suitable for determining the flood risk to a particular property or specific point location.

Users should note that flood plain outlines are not available for every river in Northern Ireland as there are many small rivers and sections of small rivers for which sufficient data was not readily available to enable hydraulic modelling to be undertaken. These un-modelled rivers, which typically have a catchment area of less than three square kilometres are highlighted in the map by a dashed blue line.

The River Flood Map contains two distinct layers namely Present Day and Climate Change. The Present Day map layer illustrates the river flood plains that have been estimated by the predictive model using meteorological input data that is representative of the current climate conditions. It is widely accepted that the climate is changing and that with the passage of time extreme rainfall events will become more frequent and intense. Therefore, the Climate Change map layer has been produced to reflect the flood plain outlines for the year 2030 and are based on the best available predictions for the meteorological conditions for that time.