PPS 15: Planning and Flood Risk
Annex A: River Flooding
A1 River flooding is typically the result of heavy or prolonged rainfall causing high volumes of run-off into a river system. When run-off exceeds the capacity of the receiving watercourse to convey it downstream then water spills out of the river channel onto the flood plain. Flood plains are generally flat low-lying areas adjacent to the watercourse and their flooding plays an important role in moderating flood conditions within a catchment by conveying and/or storing flood water.
A2 Northern Ireland has one of the largest run-off per unit areas in the British Isles, soils of low permeability and a substantial number of river systems with flat lower reaches. River flooding, though limited in scale when compared to the rest of the UK, is thus likely to continue as a primary source of flood risk affecting the Region. Although rivers are prominent in the public's perception, as a primary source of flooding there are a number of non-riverine mechanisms that can cause serious flood events.