Introduction to Sustainable Drainage Systems

Sustainable Drainage Systems (SuDS) are strategies that manage surface water, accounting for water quantity (flooding), water quality (pollution), biodiversity (wildlife and plants) and amenity. They aim to mimic nature and usually manage rainfall near where it falls.

Where rain can’t soak into surfaces such as roads and driveways, a significant amount of it becomes Surface Water runoff, which can cause flooding, pollution and erosion problems. Climate change projections show that it is likely that heavy rainfall and flooding will become more frequent in addition to an increase in water shortages.

Surface Water Management can be integrated into the design of urban developments by using SuDS to mitigate potential issues and provide additional benefits such as an increase in biodiversity and improvement to water quality.

Philosophy, principles and concepts

Sustainable drainage is transitioning from the traditional mindset of designing only to mitigate flood risk, where runoff is seen as a nuisance, to a philosophy that regards surface water as a valuable resource that should be managed to provide maximum benefit. This philosophy aims to reproduce as similarly as possible the natural drainage from a site prior to development.

There are some key principles that influence the planning and design process enabling sustainable drainage systems to mimic natural drainage.

These are:

• Storing runoff and releasing it slowly otherwise known as attenuation.
• Harvesting and using the rain close to where it falls.
• Allowing water to soak into the ground by infiltration.
• Slowly transporting or conveying water on the surface.
• Filtering out pollutants.
•  Allowing sediment to settle out by controlling the flow of the water.

In the same way as a natural catchment, drainage techniques can be used in series to change the flow and quality characteristics of the runoff in stages. This is called the Management Train.

Discharge hierarchy

When deciding on which SuDS to use, the discharge hierarchy should be applied. It is the order of priority that developers should use and each option should be discounted before the next is considered.

The Hierarchy:  

1. Collection for reuse.
2. Infiltration.
3. Watercourse or other surface water body.
4. Surface water sewer, highway drain or other drainage system.
5. Combined sewer.

Where a developer proposes to connect surface water to the existing sewer system, they should provide evidence showing how the surface water hierarchy has been applied to the site and why connection to the sewer is the most practical solution.

Sustainable drainage systems and planning

There are 14 non-statutory technical standards for SuDS Sustainable drainage systems that developers should adhere to when submitting planning to the Local Planning Authority.  

The Local Planning Authority (LPA) consults the Lead Local Flood Authority (LLFA) on planning applications in relation to surface water. For matters relating to main rivers and fluvial flood risk the LPA consults the EA (Environment Agency). For matters relating to Foul Water and sewer capacity the LPA consults with the sewerage undertaker which, for the Royal Borough of Windsor and Maidenhead, is usually Thames Water.

On receipt of planning applications for major and some minor developments the LPA asks LLFA to check that the developer has adequately met each of the non-statutory technical standards, that they have followed the SuDS hierarchy and that they are compliant with the NPPF (National Planning Policy Framework) requirements and its accompanying PPG (Planning Practice Guidance). 

The LLFA will then make a recommendation on whether the non-statutory technical standards and the NPPF have been met. 

Where insufficient information has been provided, the LLFA will provide justification on what has led to this decision including further information required. 

If the LLFA is then satisfied that the surface water will be managed in accordance with national and local policy they may recommend conditions are applied to any decision notice should planning permission be granted.  The condition may require the provision of a more detailed drainage strategy before construction of the development could commence.