When is a Surface Water Drainage Strategy required?
A surface water drainage strategy is needed in the following circumstances:
- Properties with more than 10 dwellings
- Development site area over 1ha
- Floor space over 1000m2
- Minor development of less than 10 dwellings located within an area at risk of flooding, or within an area identified as having critical drainage problems or at risk from surface water flooding
- Any development where there is an increase in impermeable areas and the Local Planning Authority is concerned about the cumulative impact of extensions
Surface Water Drainage Strategies are also frequently conditioned as part of planning permission.
It is becoming increasingly important to demonstrate to the Local Planning Authority, the Lead Local Flood Authority and the Environment Agency that surface water drainage has been considered.
What is a Surface Water Drainage Strategy?
A surface water drainage strategy investigates how surface water runoff from the site can be managed to comply with the requirements of the NPPF, the Environment Agency or local planning policy (for example the London Plan or Local Planning Authority Flood Risk and Drainage Supplementary Planning Documents).
If the development is on a previously undeveloped ‘greenfield’ site, the strategy will firstly consider the existing topography, ground conditions and the proximity to watercourses and surface water public sewers with respect to surface water drainage.
If the development is on a previously developed ‘brownfield’ site, the strategy will seek to utilise the existing discharge disposal arrangement to watercourses, soakaways or public sewers.
What is SuDS?
SuDS or Sustainable Drainage Systems are a natural approach to managing drainage, which works by slowing and holding back the water that runs off from the site.
Planning policy dictates that a development should utilise SuDs unless there are practical reasons for not doing so, and should aim to manage surface water runoff as close to its source as possible in line with the drainage hierarchy:
- store rainwater for later use
- use infiltration techniques, such as porous surfaces in non-clay areas
- attenuate rainwater in ponds or open water features for gradual release
- attenuate rainwater by storing in tanks or sealed water features for gradual release
- discharge rainwater direct to a watercourse
- discharge rainwater to a surface water sewer/drain
- discharge rainwater to the combined sewer.
What do the drainage strategy reports contain?
- Describe the existing and proposed drainage arrangements for the site.
- Calculate the existing and proposed surface water runoff for the site using industry standard MicroDrainage software.
- Assess the suitability of SuDS options and propose appropriate elements (e.g. soakaways, ponds, attenuation tanks, rainwater harvesting) to meet the runoff rate requirements for the site based on national and local planning policy.
- Provide an indicative drainage layout for the site, showing the location of the proposed drainage measures.
- Complete the report as a standalone document, or incorporated within a Flood Risk Assessment report for the site.
Drainage Strategy vs Detailed Drainage Design?
A surface water drainage strategy is often sufficient for a planning application, however the detailed design of the drainage system can sometimes be required, or made a condition of planning. Whether a detailed drainage design is required depends on the size and nature of the development, as well as local planning policy and the view of the Lead Local Flood Authority.
Detailed drainage design provides detailed drainage layouts and construction details such as pipe sizes, layouts and connections to existing systems.
What is a Foul Water Drainage Strategy?
A foul water Drainage Strategy considers the proximity and capacity of existing foul water public sewers and the site topography. A local water authority capacity check will determine if there is sufficient capacity in the existing sewer system or if an upgrade of the existing foul water public sewer network is required.
How we can help:
- Professional advice about surface water drainage and SuDS.
- Provide a cost effective drainage strategy to meet the required national and local policy.
- Give an early indication of development drainage constraints and sustainable drainage options.
- Soil soakage testing to determine infiltration potential (e.g. for soakaways or other infiltration SuDS).
- Provide the information needed by Local Planning Authorities to discharge planning conditions relating to drainage.
- Prepare foul water and surface water Drainage Strategies.
- Prepare detailed foul water and surface water Drainage Design.
Our report provides initial strategy options and an outline strategy that will offset the increase in surface water run-off from a new development using best practice and using a methodology approved by the regulators.
Our report is prepared and written in a clear concise style and includes the important calculations relating to any increased run-off volumes or increased flow rates. It also provides the volumes required for attenuation (storage of increased water flow) to support the approval of the drainage scheme.
All Unda’s Surface Water Drainage Strategy (SuDS report) recommendations are provided in accordance with the latest industry standards. We use industry-standard MicroDrainage software to ensure approval with local authorities.
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If you need a quote or to discuss your requirements in more detail contact Jackie Stone, one of our experienced Flood Risk and Drainage Consultants.
Call 01293 214444