Policy FRD3 Management of Development in regard to Surface Water Flood Risk
A proposal for development (except minor development) outside of floodplains will be permitted where the applicant has demonstrated that the risk of surface water flooding (pluvial) to the development, or elsewhere as a result of the proposed development, can be effectively managed through adequate drainage arrangements.
In order to demonstrate this, a Drainage Assessment (DA) will be required for any of the following types of development:
A residential development comprising of 10 or more dwelling units.
A development site in excess of one hectare.
A change of use involving new buildings and/or hardsurfacing exceeding 1000 square metres in area.
Where a proposed development is located in an area where there is evidence of a history of surface water flooding.
Where surface water run-off from the development may adversely impact upon other development or features of importance to nature conservation or the historic environment.
Justification and Amplification
Surface water or pluvial flooding occurs as a result of high intensity rainfall which overwhelms natural or man-made drainage systems resulting in water flowing overland and ponding in depressions in the ground. It has increased steadily with the expansion of urban areas dominated by non-permeable surfaces, the infilling of green spaces and the cumulative effects of minor development such as house extensions and the paving of gardens to provide for patios and car parking. All of these factors have combined to intensify surface water runoff and place additional pressures on the drainage network, particularly during prolonged periods of high intensity rainfall. It is not uncommon for drainage systems to be overwhelmed during such rainfall events, particularly where blockages occur.
This policy seeks to ensure that development proposals likely to be at risk from surface water flooding or to exacerbate the risk elsewhere, are only permitted subject to a satisfactory drainage assessment. This must demonstrate that the flood risk can be effectively controlled and mitigated and that the development will not create greater potential for surface water flooding elsewhere. In addition, applicants must use sustainable drainage systems (SuDS) as the preferred drainage solution where appropriate/feasible (see Policy FRD4 Sustainable Drainage).
Technical details and methodology relating to drainage assessments can be found in Appendix I Part A.
To assist applicants, areas where surface water flooding could be a potential problem are identified on the Strategic Flood Maps NI website at: https://www.infrastructure-ni.gov.uk/topics/rivers-and-flooding/flood-maps-ni or alternatively by contacting DfI Rivers.