PPS 15: Planning and Flood Risk
FLD 1: Definition of a Flood Plain
8.2 Flood plains are the generally flat areas adjacent to watercourses or the sea where water flows in time of flood or would flow but for the presence of flood defences. For planning purposes, and taking into account present scientific evidence, the limits of flood plains are currently defined as follows:
- Rivers - the extent of a flood event with a 1% annual probability of exceeding the peak floodwater level.
- The Coast - the extent of a flood event with a 0.5% annual probability of exceeding the peak floodwater level 6.
8.3 The Rivers Agency advises the Department on the extent of riverine and coastal flood plains in Northern Ireland. However, because of the uncertainty inherent in predicting the magnitude of flood events, arising from climate change, and the improvement in the quality and coverage of information on flood risk expected to derive from the Rivers Agency ongoing Flood Mapping Strategy 2005 - 2010, the extent of flood plains may change over time.
6 A lower annual probability of occurrence is appropriate in defining coastal flood plains where flooding can mean rapid inundation by fast flowing water and a consequently greater risk to life