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PPS 15: Planning and Flood Risk
Annex C Sustainable Drainage Systems

C1 Development changes the natural drainage regime, it reduces the amount of water infiltrating into the ground by replacing fields with buildings and hard surfaces and contributing to the compaction of other areas by vehicular movements. This increases the volume and speed of surface water run off and requires built up areas to be drained to remove excess water. Traditionally this has been done by installing underground pipes to convey water away as quickly as possible. Although this approach may prevent local flooding it can simply transfer flood risk to other parts of a catchment. The extension of built development alters natural flow patterns both in terms of quantity and the speed with which peak flows occur. The most obvious result may be downstream flooding but the increased flows from new development can also cause damage to property through erosion and ecological damage to streams and streamside habitats.
C2 While the disposal of surface water has long been a material consideration in determining planning applications, amenity, ecology and water resource issues have historically had limited influence on drainage system design and the determination of development decisions. The commitment to a sustainable approach to building and the use of land underlined in the Regional Development Strategy for Northern Ireland. In addition, the water quality improvements required by the EC Water Framework Directive means that continuing to drain built up areas without taking these wider issues into consideration is no longer an option. Flood risk and the environmental damage associated with flood events can be managed by minimising changes in the volume and rate of surface run-off from development sites through the use of sustainable drainage systems.
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