Vision By 2040, London will manage its rainwater sustainably to reduce flood risk and improve water security, maximising the benefits for people, the environment and the economy
Target To achieve a 1% reduction in surface water flows in the sewer network each year for 25 years, resulting in a 25% reduction in flows by 2040.
Sustainable drainage is an important element of Water Sensitive Urban Design (WSUD). The aim of WSUD is to reduce the demand for water, improve water quality and manage flood risk through taking an integrated approach. These wider principles are supported through the London Plan, but this Action Plan is focused specifically on the sustainable drainage element of WSUD, although some measures such as rainwater harvesting do have direct benefits in terms of saving water. The WSUD approach is also important in ensuring that sustainable drainage measures are functional through periods of drought as well as flood.
The Government is committed to increasing the use of sustainable drainage via the Flood and Water Management Act 2010 and recent changes to the National Planning Policy Framework (NPPF), which have strengthened the emphasis on sustainable drainage for new development
Rainwater harvesting Capture of rainwater into a tank(s) for use (usually non potable) such as irrigation, toilet flushing, vehicle or plant cleansing. For some premises the rainwater harvesting can generate significant cost savings on water bills. Care is needed to prevent the development of bacteria, algae and insect infestation.
Detention basin/ponds Landscape features designed to store and in some cases infiltrate rainwater. Detentions basins are usually dry, whereas a pond should retain water. These features need areas of open space but can often be combined with other sustainable drainage techniques.
Discharge to tidal river/dock Although not normally considered a sustainable drainage technique, this action plan recognises that discharging clean rainwater to tidal rivers or docks is generally more sustainable than discharging to the combined or surface drainage systems. Other more productive techniques should be used first but residual surface water can be discharged to tidal/large waterbodies, in some cases with no limitation on volumes. Care is needed to prevent scour in the receiving waterbody and potentially to prevent pollution.
Design for exceedance This involves designing areas such that they will flood and hold water during rare storm events (typically with a frequency of once in 10 years or longer)
National Planning Policy Proposals to increase the use of sustainable drainage techniques are in line with England’s planning policies, as set out in the National Planning Policy Framework (NPPF). Sustainable drainage is seen as a means by which developments can avoid increasing the risk of flooding elsewhere. Following the Government’s consultation “Delivering Sustainable Drainage” (Defra & DCLG 2014), the Guidance that supports the NPPF has been strengthened with respect to sustainable drainage. Since April 2015, Planning Authorities have been responsible for the approval of sustainable drainage designs for all major planning applications in consultation with the Lead Local Flood Authority (LLFA). In London, the relevant borough is the Local Planning Authority and the LLFA. The London Plan The London Plan’s Policy 5.13 (Box 1) is aimed at ensuring that sustainable drainage techniques are utilised wherever practical for new developments. This policy has remained broadly unchanged since 2004 and is understood to be working well for large-scale development. It is anticipated that in any future London Plan the broad thrust of the policy will remain.