A sustainable drainage strategy will outline how you plan to manage onsite water runoff to lessen the amount of water that ends up in the local sewage system, as well as improve its quality. The sustainable element comes from its ability to mimic natural systems, and process rainwater in a holistic way.
Requirements
A sustainable drainage strategy is required for:
- all new buildings (except for residential out-buildings)
- non-residential extensions; and
- the provision of additional hard surfacing covering a ground area of 20 square metres or more;
which are located within:
- flood zone 2
- flood zone 3 or
- surface water priority area
A sustainable drainage strategy is not required for extensions to single dwellings.
The drainage strategy can form part of the flood risk assessment or be included as a separate document. The strategy should normally include:
- a definition of the natural drainage characteristics within and hydrologically linked to the site, showing that the proposals will integrate with and not compromise the natural drainage system
- a strategic approach to managing on-site flood risk from all sources, and outline the implications on existing flood risk of the proposed sustainable drainage
- an outline assessment of existing geology and ground conditions. Infiltration tests should be carried out at this stage where possible
- confirmation of the discharge points and identify any environmental requirements of the receiving water bodies for runoff, e.g. Water Framework Directive, or ground water protection zones
- layout plans demonstrating that all watercourses are open with a 5 metre maintenance corridor (SPD Biodiversity and Trees Section 8.4)
- an explanation of why the chosen type of sustainable drainage has been decided, and why other forms have not been used
- conceptual design including interception, pollution control treatment train, conveyance, peak flow and volume control, storage and exceedance routes and components
- initial adoption and maintenance arrangements
- details of any offsite works required
At outline application stage
For outline applications, the strategy should outline a concept for a sustainable drainage design using the criteria set out within the ‘West of England Sustainable Drainage Developer Guide’ sections 1 and 2 below.