Sustainable Drainage Systems (SuDS): guidance for developers

Sustainable drainage systems (SuDS) aim to reduce the impact of development by replicating the natural processes through which rainwater is captured, stored, and transported within a development.

Developing a greenfield area increases the amount of impermeable area, which increases the amount of surface water runoff. This can lead to an increased risk of surface water flooding.

Development can also lead to:

• a decrease in water quality due to contaminants (e.g., fuel/oil) being picked up by water as it flows across impermeable surfaces
• loss in biodiversity if habitat areas are cleared, this can also lead to a decrease in natural visual amenity

Traditional piped drainage systems remove runoff from a site as quickly as possible, however SuDS slow flow and store runoff on a site before slowly releasing it offsite.

The utilisation of SuDS allows for the:

• quantity of runoff to be managed
• treatment of surface water (for example via filtration)

When designing a drainage scheme, priority should be given to green, above ground SuDS which can help meet the 4 pillars SuDS:

1. Water Quantity – Controlling the quantity of runoff
2. Water Quality – Managing the quality of the runoff in order to prevent pollution
3. Amenity – Create and sustain better places for people
4. Biodiversity – Creating and sustain better places for nature

All sites will be able to accommodate some form of SuDS, even if the site has constraints; site constraints affect the choice of SuDS not the use of SuDS.

Constraints can relate to:

• topography
• infiltration potential of the underlying geology
• the size of the site

For example, it is common to not see any infiltration SuDS proposed due to clayey soils with low infiltration potential.

However, there are a number of SuDS that can be utilised that do not rely on infiltration to the ground to provide benefits. These include storage and conveyance

In summary, sustainable drainage in Buckinghamshire has the potential to:

• protect people and property from increased flood risk resulting from the development
• create attractive places where people want to live, work, and play through integration of water and green spaces in the built environment
• improve people’s understanding of how runoff from their development is being managed and used, and the benefits of more sustainable approaches
• support the creation of developments that are more able to cope with changes in climate
• deliver cost-effective infrastructure that uses fewer natural resources and has a smaller whole-life carbon footprint than conventional drainage
• protect the quality of groundwater and surface waters from polluted runoff from development
• protect natural flow regimes (and thus the morphology and associated ecology) in rivers, lakes and streams
• support local natural habitats and associated ecosystems by encouraging greater biodiversity and linking habitats
• improve soil moisture and replenishing depleted groundwater levels
• provide society with a valuable supply of water

The drainage hierarchy is taken from paragraph 056 the Planning Practice Guidance, and outlines where discharge should be directed within the drainage design.

To comply with paragraph 080 of the Planning Practice Guidance (PPG) ‘the aim should be to discharge surface run off as high up the following hierarchy of drainage options as reasonably practicable:

• into the ground (infiltration)
• to a surface water body
• to a surface water sewer, highway drain, or another drainage system
• to a combined sewer

It should be noted that while rainwater harvesting (both active and passive) is not mentioned in the drainage hierarchy, we consider it to be top of the hierarchy above infiltration. Any overflows that are part of rainwater harvesting schemes must also follow the drainage hierarchy.

If you are proposing to discharge to a water body, evidence as to why infiltration is not viable is required, and if it is proposed to discharge to a sewer, evidence for why infiltration and discharge to a water body are not viable must be provided. Details about ground investigations required as evidence can be found in the “Supporting Detail for Each Method of Surface Water Disposal” section.

Select an option below to view different examples of sustainable drainage systems.