DM10 — On Site Water Management and Surface Water Attenuation.

Policy DM 10: On Site Water Management and Surface Water Attenuation A. Proposals for new development will be required to make provision for the installation and management of measures for the efficient use of mains water and for the control and reduction of surface water run off. Substantial weight will be afforded to the target for mains water consumption of 105 litres or less per person per day and to the achievement of greenfield run off rates. Where greenfield run- off rates cannot be achieved this should be clearly justified by the applicant; however the fact that a site is previously developed and has an existing high run-off rate will not constitute justification. B. The design and layout of major development proposals will be required to: a. use appropriate sustainable drainage measures to control the rate and volume of surface water run-off; b. ensure separation of surface and foul water systems; c. make reasonable provision for the safe storage and passage of flood water in excessive events; and d. demonstrate adequate arrangements for the management and maintenance of the measures used. Examples include Headstone Manor Recreation Ground, Harrow Recreation Ground, Pinner Park Farm, Newton Park (east) and Queensbury Recreation Ground. Some open spaces have been adapted specifically to function as flood storage areas, such as the open space at Stanmore Park, Whitchurch Playing Fields and Prince Edward Playing Fields, whilst the Core Strategy recognises the potential of Kenton Recreation Ground to provide a sustainable flood management solution for more urban areas up and downstream of the Wealdstone Brook. Harrow’s Preliminary Flood Risk Assessment and Surface Water Management Plan. The Thames Catchment Flood Management Plan (Environment Agency, 2009) includes Harrow in sub area 9: London catchments where it is accepted that the most sustainable approach to managing flood risk is through adaptation of the urban environment Development Management POLICIES Some of the Borough’s open spaces contain river corridors and form a part of the functional flood plain(11). The modelled flood extents illustrated on Harrow’s flood maps represents the best available information about the strategic risk of flooding. Due regard must be given to areas shown as falling within the functional floodplain, which represents the most frequent and serious risk of flooding. However for the purposes of applying the National Planning Framework (paragraphs 100-104) and associated Technical Guidance, a distinction will be made between Greenfield Zone 3b and Developed Zone 3b. Undeveloped floodplain, defined as zone 3b flood extents within areas designated as Green Belt, Metropolitan Open Land or open space on the Policies Map, is of particularly high value within the urban context. It provides unimpeded space where water is able to flow or be stored during times of flood. However, as recognised in flood management plans(12), much of the Borough is already urbanised with many rivers culverted, changing their behaviour in response to rainfall and flooding. Previously developed land within the floodplain, defined as Developed Zone 3B flood extents on land not designated as Green Belt, Metropolitan Open Land or open space on the Policies Map, provides the opportunity through redevelopment to realise the flood risk management and reduction objectives of Harrow’s Core Strategy and the Thames Catchment Flood Management Plan. However, Zone 3B represents land where water has to flow or be stored in times of flood and it therefore remains essential that development in this zone is designed to be safe and does not simply displace flood risk to land elsewhere.

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