LB Merton Surface Water Drainage and SuDS 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 located within Environment Agency flood zones or shown to be at other sources of flooding will continue to require a Flood Risk Assessment (FRA).

Please refer to the Environment Agency's online flood maps to establish if you or the site you are assessing is at risk of flooding.

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 and Strategic Flood Risk Assessment for further advice.

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 planning policies and requirements contained in the National Planning Policy Framework (NPPF), the national Planning Practice Guidance (PPG), 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.

Sustainable Drainage (SUDS) Design and Evaluation Guide

Merton Council and 15 other local authorities across England have worked with Robert Bray Associates and McCloy Consulting to produce a Sustainable Drainage Guide. The Guide provides a new approach to the design and evaluation of SuDS with easy to understand and practical information for all those involved with the development process.

SuDS provide a way of managing rainwater by mimicking natural drainage and are a requirement for all new major developments. To ensure successful and affordable Sustainable Drainage Systems, they should be fully integrated from the start of the design process along with other aspects of development design. The guide is for developers, architects, landscape architects or anyone who plans to build or redevelop (both residential and commercial) in Merton. It creates a shared vision of SuDS for all involved in the development process, enabling design and evaluation to meet agreed standards and ensuring that SuDS are maintainable now and in the future.

The guidance follows the design process from concept, through outline, to detailed design and provides the reader with an easy to follow, step by step, process. The guidance is in conformity with statutory requirements such as the National Planning Policy Framework (NPPF); as well as CIRIA's 2015 SuDS Manual and other recognised guidance.

The council plans to adopt the SuDS guide as a Supplementary Planning Guidance (SPD) in the autumn 2018 after a public consultation.