Local Requirements - Surface Water Drainage Details

Types of application or development that require this information

• All major planning applications 

What information is required?

A Surface Water Drainage Strategy should outline the principles that are to be followed to ensure that proposed surface water drainage system satisfied the requirements of the non-statutory technical standards for sustainable drainage and should include:

1. An existing drainage plan which demonstrates the existing site drainage characteristics. This should include the location of nearest watercourse (open and culverted), public sewer (surface and/ or combined water) and highway drainage network. Any existing flood sources or flood extents on or near to the site derived from the flood risk assessment of the site should be illustrated on the plan. A calculation to determine the existing sites peak runoff rates for the 1:2, 1:30 and 1:100 plus climate change rainfall events is required.
2. Proposed surface water drainage principles and arrangements including the proposed finished levels of the site and how these incorporate any flood mitigation measure informed by the flood risk assessment.  A site drainage layout of the main surface water drainage infrastructure with estimated sizing and location of each component is required.  An overview of the design parameters in concluding these details should also be included. The strategy should delineate the agreed discharge rate and receptor (including location) and prove its suitability in receiving the designed flow rate and volume.  Detailed calculations showing that the proposed design will achieve the agreed requirements are required. This will need to be supported with a discussion about the selection of suitable SuDS methods including in the drainage scheme.
3. A geological assessment of the site which details the surface levels of the existing site (topographical survey), underlying geology and infiltration rates of the sub strata in supporting the use of: infiltrating drainage components, providing an indication of groundwater levels and migration, indicating the location of nearest source protection zone (from Environment Agency Source Protection Zone maps) and establishing if soil and groundwater contamination exists within the site boundaries.
4. A plan or strategy showing how the development will manage and control exceedance events with predicted flow paths will be necessary.
5. A plan or strategy setting out how the drainage design will integrate within the wider public realm or green infrastructure environment.
6. An indication of proposals relating to the future management and maintenance of surface water features over the lifetime of the development in the form of a detailed maintenance management plan which includes full details of who the responsible parties for the various activities and structures are.
7. A drainage strategy for dealing with each phase of the development scheme (where relevant) which indicates the flow rates, storage structures and filter components for each stage of the scheme.

Further information

• Sustainable drainage systems: non-statutory technical standards

Policy driver

• National Planning Policy Framework - Chapter 14: Meeting the challenge of climate change, flooding and coastal change
• National Planning Practice Guidance: Flood risk and coastal change
• Flood & Water Management Act 2014