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Policy SI13 Sustainable drainage

SI13

  1. Lead Local Flood Authorities should identify – through their Local Flood Risk Management Strategies and Surface Water Management Plans – areas where there are particular surface water management issues and aim to reduce these risks.
  2. Development proposals should aim to achieve greenfield run-off rates and ensure that surface water run-off is managed as close to its source as possible in line with the following drainage hierarchy:
    1. rainwater harvesting (including a combination of green and blue roofs)
    2. infiltration techniques and green roofs
    3. rainwater attenuation in open water features for gradual release
    4. rainwater discharge direct to a watercourse (unless not appropriate)
    5. rainwater attenuation above ground (including blue roofs)
    6. rainwater attenuation below ground[136]
    7. rainwater discharge to a surface water sewer or drain
    8. rainwater discharge to a combined sewer.
  3. Development proposals for impermeable paving should be refused where appropriate, including on small surfaces such as front gardens and driveways.
  4. Drainage should be designed and implemented in ways that address issues of water use efficiency, river water quality, biodiversity, amenity and recreation.

[136] The benefit of attenuation above compared to below ground or in a basement is that pumping is normally not required to empty the attenuation tank.

London is at particular risk from surface water flooding, mainly due to the large extent of impermeable surfaces. Lead Local Flood Authorities have responsibility for managing surface water drainage through the planning system, as well as maintenance arrangements. Local Flood Risk Management Strategies and Surface Water Management Plans should ensure they address flooding from sewers, drains and groundwater, and run-off from land and small watercourses that occurs as a result of heavy rainfall.

Development proposals should aim to get as close to greenfield run-off rates[137] as possible depending on site conditions. The well-established drainage hierarchy set out in this policy helps to reduce the rate and volume of surface water run-off. Rainwater should be managed as close to the top of the hierarchy as possible and there should be a preference for green over grey features. A blue roof is an attenuation tank at roof or podium level; the combination of a blue and green roof is particularly beneficial as the attenuated water is used to irrigate the green roof.

[137] The runoff that would occur from a site in undeveloped state.

For many sites, it may be appropriate to use more than one form of drainage, for example a proportion of rainwater can be managed by more sustainable methods, with residual rainwater managed lower down the hierarchy. In some cases, direct discharge into the watercourse is an appropriate approach, for example rainwater discharge into the tidal Thames or a dock. This should include suitable pollution prevention measures. However, in other cases direct discharge will not be appropriate, for example discharge into a small stream at the headwaters of a catchment, which may cause flooding. This will need to be assessed on a case-by-case basis, taking into account the location, scale and quality of the discharge and the receiving watercourse. The maintenance of identified drainage measures should also be considered in development proposals.

The London Sustainable Drainage Action Plan complements this policy. It contains a series of actions to make the drainage system work in a more natural way with a particular emphasis on retrofitting.