3.1 Surface water run-off should be controlled as near to its source as possible through a sustainable drainage approach to surface water management. SuDS seek to mimic natural drainage systems and retain water on or near to the site, when rain falls, in contrast to traditional drainage approaches, which tend to pipe water off-site as quickly as possible. SuDS therefore offer significant advantages over conventional piped drainage systems.
3.2 In accordance with paragraph 080 of the NPPG, applicants must follow the hierarchy for drainage options. Where it is not possible to achieve the first level in the hierarchy (discharge through the ground – infiltration which is also supported by Approved Document Part H of the Building Regulations 2010), applicants must demonstrate in sequence why the subsequent drainage option (discharge destination) has been selected.
3.3 Where the intention is to dispose to soakaway, these should be shown to work through an appropriate assessment carried out under Building Research Establishment (BRE) Digest 365. All designs shall be based on actual infiltration figures obtained through percolation tests, carried out in accordance to BRE Digest 365. Where such tests cannot be undertaken prior to the start of construction, desktop study will be accepted, based on anticipated geology of the site and/or data from the British Geological Survey. Those will then have to be confirmed through site test at construction phase. House or rubble soakaways are not acceptable.
3.4 Defra/EA have published guidance on Rainfall Runoff Management, and a storage calculator is available at various websites, including UK Sustainable Drainage
3.5 In accordance with CIRIA Report 156, Infiltration Drainage and SuDS Manual (C697 or latest amended version C753), an adequate factor of safety must be applied to the observed infiltration value. The minimum factor of safety acceptable is 2 and that must be increased to reflect the consequences of failure of the system, the topography of the site and the likelihood of flooding.
3.6 Infiltration units must stand the test of half-emptying the provided storage within 24hrs for up to the 1 in 10yr return period storm (and that is for all rainfall duration events).
3.7 Where disposal of surface water into the ground is not practicable, nor into a surface water body, subject to the evidence being provided to support the choice of discharge destination, proposals to dispose of surface water into a surface water sewer, highway drain or another drainage system, should be accompanied by evidence of the system having spare capacity downstream. Developers need to contact the relevant Water Company to discuss this option.
3.8 Where an application is part of a larger site which already has planning permission, or in the case of detailed applications following an outline master plan, it is essential that the new proposal does not compromise the drainage scheme already approved. The Sustainable Drainage Strategy should explain how all the elements are related and linked. RBC SuDS Advice note v1 3
3.9 During construction on large application sites, full details of individual development plot discharge and storage constraints, as well as full details of responsibility for controlling the overall surface water management of the site prior to final phase completion, must be also supplied at the application stage.
3.10 As required by the NPPG and the Non Statutory Technical Standards, SuDS must be properly designed to ensure that the maintenance and operation costs are proportionate and sustainable for the lifetime of the development. Full details are required of how the SuDS scheme has been designed so that the capacity takes account of likely changes in impermeable area within the development over its lifetime, for example by way of extensions to buildings, increased paving within gardens etc.
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