PPS 15: Planning and Flood Risk
Policy FLD 4: Flooding and Land Drainage
Justification and Amplification
8.29 Culverting and/or canalisation of watercourses, whether undertaken as an operation in its own right, or as works associated with the development of land requires planning permission.
8.30 The modification or disturbance of natural drainage systems can increase flood risk. The continuous culverting of watercourses is normally accompanied by the installation of protective grilles at inlets. The blockage of such inlets is a common cause of localised flooding particularly during periods of high rainfall intensity. Similarly, the canalisation of a natural watercourse can contribute to flooding where, for example it speeds up the flow of water through a catchment leading to surcharging and inundation downstream.
8.31 Culverting and canalisation can also impair amenity in the built environment and damage the landscape quality, ecological integrity and biodiversity of watercourses. Such impacts are generally inconsistent with a commitment to sustainable development and the adoption of a precautionary approach to flood risk. Culverting and/or canalisation will therefore only be acceptable in exceptional cases.
8.32 It is also important to recognise that each new culvert adds to an already substantial network of covered drains throughout Northern Ireland, much of which is now aged and in need of attention. Continuing to permit culverting in association with development therefore not only increases the potential of flood risk and adverse impacts on the environment but serves to increase the future burden of maintenance commitments.
8.33 Accordingly, new development should aim to be in harmony with the water environment and not attempt to work against it. Adopting a sustainable approach to building development requires the recognition of the importance of accommodating natural features such as open watercourses. Good layout and design will avoid relegating open watercourses to the backs of properties where they are difficult to maintain, can become a dumping ground that contributes to the risk of flooding and can cause other amenity problems.
8.34 Consistent with the requirements of PPS 7 on Quality Residential Environments, incorporating existing watercourses into the open space requirements for new residential development will be preferred to locating them to the rear of properties or culverting them. Where it is necessary to achieve a quality design solution to a development problem the diversion of a watercourse will be acceptable on planning grounds where it keeps the watercourse open and it is demonstrated that it will not adversely affect the drainage of the area9.
8.35 It is acknowledged however that, exceptionally, culverting may be unavoidable. This may occur where the proximity of existing development has caused the degradation of a watercourse and where an attendant risk of flooding could be alleviated by culverting the affected section, or where it is necessary to provide access to a development site. In the latter circumstances permission will only be granted to culvert when it is demonstrated that alternative measures such as open span bridges or diversion of the watercourse cannot be employed and where the length of culvert involved is the minimum necessary to facilitate the development proposed.
8.36 For the purpose of this PPS a flood relief scheme is defined as works undertaken by the Rivers Agency in pursuance of it statutory responsibilities under the Drainage (NI) Order 1973 intended to prevent or reduce the incidence of flooding.
9 Works that affect a watercourse require prior written consent from Rivers Agency under Schedule 6 of the Drainage (Northern Ireland) Order, 1973