LB Merton Surface Water Drainage and SuDS From 6th April 2015, the London Borough of Merton, as a Lead Local Flood Authority (LLFA), is a statutory consultee for surface water drainage matters relating to decisions on planning applications for major development i.e. developments of 10 dwellings or more; or equivalent non-residential or mixed development (as set out in Article 2(1) of the Town and Country Planning (Development Management Procedure) (England) Order 2010) .

Planning applications for major development are required to submit a Sustainable Drainage Strategy that demonstrates how Sustainable Drainage Systems (SuDS) will be used to appropriately manage rainfall runoff and water quality. Developments in areas of flooding, i.e. such as those located within Environment Agency flood zones, will continue to require a Flood Risk Assessment (FRA). Please refer to the Environment Agency’s online flood maps for information regarding the extent of flood risk. The Sustainable Drainage Strategy can be submitted as part of site specific Flood Risk Assessment. Please refer to Merton’s Local Flood Risk Management Strategy.

What is a Sustainable Drainage Strategy? A Sustainable Drainage Strategy details how surface water runoff and waste (foul) water arising from the development of a site will be managed in line with related policies and requirements in the National Planning Policy Framework (NPPF), the National, Planning Practice Guidance (NPPG), Defra’s National Standards for Sustainable Drainage and the London Plan (including Supplementary Planning Guidance to the London Plan).

All developments proposals in Merton are required to have consideration to the above listed documents and Merton's Local Plan policies.

What should a Sustainable Drainage Strategy include?

  • A plan of the existing site
  • A topographical plan of the area
  • Plans and drawings of the proposed site layout identifying the footprint of the area being drained (including all buildings, access roads and car parks)
  • The existing and proposed controlled discharge rate for a 1 in 1 year event and a 1 in 100 year event (with an allowance for climate change), this should be based on the estimated greenfield runoff rate.
  • The proposed storage volume (attenuation)
  • Information on proposed SuDS measures with a design statement describing how the proposed measures manage surface water as close to its source as possible and follow the drainage hierarchy in the London Plan
  • Geological information including borehole logs, depth to water table and/or infiltration test results
  • Details of overland flow routes for exceedance events
  • A management plan for future maintenance and adoption of drainage system for the lifetime of the development.