Drainage strategy, Surface Water Drainage Scheme, Sustainable Drainage System (SuDS)
Last Updated: 24 August 2020
Used to ensure a satisfactory standard of surface water drainage for development and to minimise the risk of flooding.
A surface water strategy must demonstrate a full investigation of the surface water hierarchy as described in the National Planning Practice Guidance on Flood Risk and Coastal Change.
It must highlight options that are preferred to the public combined sewer for the discharge of surface water. Applicants should provide clear evidence when demonstrating why more preferable options within the hierarchy have been discounted. United Utilities are happy to open dialogue regarding this, and would we also welcome such discussions.
An appropriate drainage strategy should be provided with applications.
For major developments (10 dwellings or more, 1.0 hectare or more, 1000 square metres or more) a site specific drainage strategy is required.
In other cases, applicants should specify where this is not considered necessary, taking into account of national and local guidelines (Development Design Guide).
Applications for development on sites which form part of a wider development must demonstrate how the proposed drainage system for the individual site relates to a wider master drainage strategy for the whole development.
Surface water drainage
All residential, commercial and industrial development should include an appropriate Sustainable Drainage System (SuDS)
All major applications and all applications in areas at risk from flooding (Flood Zones 2 or 3) or sites within Flood Zone 1 in an Area with Critical Drainage Problems (ACDA) as notified by the Environment Agency or sites adjacent to areas at risk of flooding and applications where development adjoins a highway.
In an ACDA we expect new development to actually reduce flood risks downstream, rather than having just neutral impact.
Applications for development on sites which are part of a wider development proposal will be expected to demonstrate how the proposed drainage system for the individual site relates to a wider master drainage strategy for the whole site.
The Lead Local Flood Authority in South Lakeland is Cumbria County Council.
Cumbria County Council's development design guide includes information about surface water drainage.
Sustainable drainage systems (SuDS)
Drainage systems can contribute to sustainable development and improve the places and spaces where we live by balancing the different opportunities and challenges that influence urban design and the development of land.
Approaches to manage surface water that take account of water quantity (flooding), water quality (pollution) biodiversity (wildlife and plants) and amenity are collectively referred to as Sustainable Drainage Systems (SuDS).
SuDS mimic nature and usually manage rainfall close to where it falls. SuDS can be designed to transport (convey) surface water, slow runoff down (attenuate) before it enters watercourses, they provide areas to store water in natural contours and can be used to allow water to soak (infiltrate) into the ground or evaporated from surface water and lost or transpired from vegetation (known as evapotranspiration).
SuDS are drainage systems that are environmentally beneficial, causing minimal or no long-term damage. They are often regarded as a sequence of management practices, control structures and strategies designed to efficiently and sustainably drain surface water, while minimising pollution and managing the impact on water quality of local water bodies.
Contents of a surface water drainage scheme
A surface water drainage scheme should include the following information:
- a metric scaled plan of the existing site
- a metric scaled topographical level survey of the area to metres above ordnance datum
- metric scaled plans and drawings of the proposed site layout identifying the footprint of the area being drained (including all buildings, access roads and car parks and any green spaces draining into the drainage system)
- proposed controlled discharge rate for a 1 in 1 year event and a 1 in 100 year event (with an allowance for climate change), this should be based on the estimated greenfield runoff rate
- the proposed storage volume (attenuation)
- information on proposed SuDS measures with a design statement describing how the proposed measures manage surface water as close to its source as possible
- geological information including borehole logs, depth to water table and/or infiltration test results and interpretive report describing the suitability of the site for infiltration (especially on steeply sloping sites)
- detailed flood and drainage design drawings
- hydraulic calculations for the proposed drainage design
- evidence of third party agreement for discharge to their system (in principle/consent to discharge)
- details of overland flow routes if drainage capacity is exceeded
- a management plan for future maintenance and adoption of drainage system for the lifetime of the development
- construction phasing plan. The strategy should demonstrate access to/from interconnecting phases
- capacity and discharge rate of the current drainage system
- details of adoption
- finished floor levels of proposed buildings