Appendix 9: General principles for Sustainable Drainage Systems.

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SuDS are expected to:

• Drain surface water and be integrated into the green infrastructure provision of the development . Preference should be given to the installation of blue-green surface infrastructure, as opposed to hardscape (artificial engineered solutions) or underground solutions, which should be a last resort, due to the wider benefits attained from a landscaped SuDS scheme;
• Integrate SuDS into the design of streets, public open spaces and parking as visually appealing features that contribute to creating distinctive character to development.
• Where possible, incorporate 'blue corridors' to create a network that facilitates natural hydrological processes that help to minimise flooding; this approach is also helpful for the movement of wildlife;
• Restrict run-off rates to as close to the greenfield rates, or at the very minimum a betterment of at least 30% over the brownfield run-off rates ;
• Contribute to the design quality of the scheme;
• Deliver sufficient and appropriate water quality and aquatic biodiversity improvements wherever possible. This should be complementary of any local designations such as Source Protection Zones;
• Where practical incorporate green roofs and/or rainwater harvesting systems (which could be used on their own for the collection of rainwater in water butts, or integrated into a water recycling system for uses not requiring potable water, such as flushing toilets);
• Should be designed to be safe to access. Where restrictions to access are required due to an unavoidable safety concern, low impact barriers such as landscaping and an appropriate planting scheme should be used instead of fencing, if possible.
• When deciding whether to incorporate a retention element (e.g. a pond with a permanent minimum level of water) or detention element (e.g. a basin which is dry most of the time, having been designed to completely drain within a maximum of 48 hours after rainfall), the amenity benefits of either option should be carefully assessed within the overall site context. For example, is there a publicly accessible blue infrastructure within a walkable radius of the site or is the community currently under served in this regard. Is a detention basin more suited to the public open space to be provided;
• Should take account of any impacts on the historic environment, where applicable;
• Deliver environmental improvements including improvements to water quality, biodiversity and flood risk;
• Create a more varied natural environment within the site. A SuDS scheme that contrasts but has relationship with other green/blue infrastructure elements on site such as using a ‘rock garden’ approach to add variety to what is otherwise a wooded network of green features on site;
• Not connect to the foul system and should only connect to the combined or surface water system in exceptional circumstances where there are no feasible alternatives. Foul and surface water flows should also be separated.
• Contribute to the creation of green space, and improving the overall aesthetic quality and enjoyment of the public space on site;
• Contribute to the overall habitat, food source and breeding spaces provided for ecosystems to thrive on the site through the use of supportive flora;
• Improve biodiversity: increasing the amount of biodiversity able to be supported by the site, potentially whilst also prioritising the needs of native and/or priority species, through the use of supportive flora for new habitat creation;
• Inform and educate residents and visitors. This may be through the use of interpretation / signage and/or, on larger sites, the establishment of nature trails (particularly where the SuDS scheme is an integral part of a larger green/blue infrastructure plan for the site);
• Lead to an improvement in the water quality of the surface run-off water (environmental net gain);
• Provide overall space for play and leisure experiences by ensuring the landscaping design is safe to access (most relevant to detention elements);
• Support the physical and mental health of future occupants by being integrated into and having a relationship with other health and wellbeing amenities on site, for example, creating a natural setting to encourage use of the site’s nearby outdoor gym equipment and trim trail/equipped area of play.
• Reduce the heat island effect of urban environments: creating urban cooling, particularly where nature-based SuDS schemes include trees with significant canopy cover; the latter is also linked to improving air quality;
• Enable easy site maintenance by being intentionally designed to be low-maintenance and any necessary artificial elements (if relevant) easy to access and repair. Where maintenance is required, integrate access and buffer zones into the wider landscape design.
• Use surface level SuDS systems rather than below ground attenuation tanks or storage.
• Design SuDS to be multifunctional, for example as wildlife habitats, for formal or informal recreation, for parking, and/or supporting community educational learning.
• Integrate SuDS into building design through including green, brown or blue roofs.
• Maximise the benefits to the sense of place, amenity and biodiversity;
• Provide attractive, biodiverse and non-buried systems;
• Maximise the amount of permeable and absorbent surfaces on the site.
• SuDS can also be used to harvest rainwater for use by on-site or adjacent community green infrastructure such as allotments, community gardens, orchards, school gardens or, where applicable to the mix of land uses present on the site, on-site land-based enterprises (e.g. plant nurseries).

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