B2.4.1 Integrating Sustainable urban Drainage Systems (SuDS)

All new developments must integrate Sustainable urban Drainage Systems (SuDS) using a hierarchical, multi-functional approach that prioritises natural solutions over engineered drainage, in full compliance with the National Standards for Sustainable urban Drainage Systems. SuDS must be designed to manage surface water quantity, quality, amenity and biodiversity, with clear arrangements for long-term maintenance and adoption where required.

This approach seeks to maximise opportunities to enhance green spaces, habitats, wildlife and water quality, whilst reducing flood risk. Examples include rainwater harvesting, permeable or pervious paving, rain gardens, filter strips, green roofs and walls, swales, bio-retention areas, infiltration, and measures to slow, store, convey and treat runoff.

Applicants must submit an assessment of the feasibility of using SuDS with their proposals for development (this could be included within the drainage strategy).

Above-ground SuDS attenuation features must be incorporated wherever possible, prioritising nature-based solutions in line with the SuDS hierarchy. Where this is not feasible, alternative SuDS measures must be provided, with interception storage for the first 5mm of rainfall wherever practicable to improve water quality and reduce runoff.

Where below-ground SuDS are necessary, the associated open spaces must be thoughtfully designed to avoid large, featureless grass areas, instead creating attractive, functional environments that deliver biodiversity, amenity and/or recreational value.

SuDS must be fully integrated into the wider green infrastructure network strategy of a development. This includes aligning SuDS with open spaces, biodiversity features and active travel routes to maximise their multifunctional value, delivering benefits for flood management, water quality, amenity, biodiversity and community wellbeing.

Exception: SuDS components designed solely to manage run off for storm events greater than 1 in 30 years (3.3% probability in any year) may not require full integration with multifunctional green infrastructure where this is impractical; however, they must still meet the relevant performance standards for surface water quantity and quality.

In all cases, applications must demonstrate how SuDS have been designed to deliver maximum benefit on site and must justify any instances where full integration with green infrastructure cannot be achieved.

Development proposals should be designed to meet the following water standards set out within ‘Building with Nature Standards Framework’ to ensure that sustainable water management contributes to climate resilience, community wellbeing and nature enhancement:

Water Standards

• Standard 9: Delivers Climate Resilient Water Management

• Standard 10: Brings Water Closer to People

Surface feature SuDS should be designed to complement and enhance ecological value by creating diverse habitats that support a wide range of species. Key measures include:

• incorporating a mix of wet and dry areas, varied vegetation types, and a range of water depths

• using locally appropriate native planting to maximise habitat value and ecological resilience

• designing swales with broad, vegetated side slopes with gradients no steeper than 1:4

• designing ponds and basins with naturalised edges, marginal and emergent vegetation, and gently sloping shelves and shallow zones that transition to deeper areas

Where feasible, driveways, footpaths and parking areas should use permeable surfaces to manage rainwater at source. Materials such as porous asphalt, permeable block paving and resin-bound gravel allow water to filter through and either infiltrate into the ground or be conveyed to an appropriate SuDS feature.

Designs should follow the principles set out in the Construction Industry Research and Information Association (CIRIA) SuDS Manual (C753), ensuring an appropriate sub-base, adequate infiltration capacity, and clear connectivity to the wider surface water drainage system.