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Planning technical zone

Drainage, Flood Risk and Sustainable Drainage Systems

How water is managed as part of a proposed development is an important planning consideration, in particular in Somerset due to large areas being especially sensitive to flood events. On the lower ground in and around the Somerset Levels or on steeper, rapid rise catchment areas, it is important that the planning process is able to fully consider flood and water management implications of a development proposal.

The National Planning Policy Framework requires flood risk to be taken into account at all stages of the planning process. All types of flooding and their impact on the natural and built environment need to be considered.  Certain developments will need to include appropriate measures to ensure it is not susceptible to flooding and that flood risk is not increased elsewhere.

  1. Flood Risk Assessment

    Flood Risk Assessment- when we require one as part of an application
    Householder Developments Change of Use Minor Developments (full or outline) Major Developments (full, outline or reserved matters)

    Development in Flood Zone 2 and 3 or works within 8m of a main river, this document must be completed.

    Applicants should refer to the National Flood Risk Standing Advice and Local Flood Risk Standing Advice, here

    A shortcut to advice on householder and minor Extensions is available, here

    Essential infrastructure / Highly vulnerable / More vulnerable uses within Flood Zone 2 and 3

    Applicants should refer to the National Flood Risk Standing Advice and Local Flood Risk Standing Advice, here

    Development over 1 hectare in Flood Zone 1 and all development in Flood Zone 2 and 3

    Works within 8m of a main river

    Applicants should refer to the National Flood Risk Standing Advice and Local Flood Risk Standing Advice, here

    Development over 1 hectare in Flood Zone 1 and all development in Flood Zone 2 and 3

    Works within 8m of a main river

    Applicants should refer to the National Flood Risk Standing Advice and Local Flood Risk Standing Advice, here

    A Flood Risk Assessment (FRA) is a report which assesses the flood risk of a development from a range of possible sources, classifies its vulnerability to flooding both now and under future conditions, taking climate change into account. A FRA will outline measures that may be needed to increase resilience including the requirement for safe access/exit from a development in an area at risk of flooding, and identify opportunities to reduce the chance and impact of flooding.

    Depending on the level of flood risk, a FRA can be a short report or could require hydraulic modelling in areas where the risk is high. If a site is in Flood Zone 2 or 3 a Sequential Test assessment may be required to demonstrate that there are no reasonably available sites in areas with a lower probability of flooding that would be appropriate to the type of development or land use proposed.

    A FRA should use the Environment Agency’s Flood Maps as a basis – click here, this data takes in account of flooding from rivers and the sea.

    Other data is available including surface water flooding from within the site and from adjacent land; groundwater flooding; sewers and other infrastructure; man-made structures reservoirs or canals. https://flood-warning-information.service.gov.uk/long-term-flood-risk

    For more information on Flood Risk Assessments please click here.

  2. Drainage Strategy and Sustainable Drainage Systems (SUDS)

    Drainage Strategy and SUDS- when we require one as part of an application
    Householder Developments Change of Use Minor Developments (full or outline) Major Developments (full, outline or reserved matters)

    Not required

    Required where the change of use proposed could have implications on sensitivity to flood risk e.g. where the change increases the vulnerability of the use

    The plan should detail the surface water drainage strategy and should include Sustainable Urban Drainage principles.

    The plan should detail the surface water drainage strategy and should include Sustainable Urban Drainage principles.

     Reserved matters submissions will be required to demonstrate how they comply with the drainage details provided in the outline permission

    A Drainage Strategy will need to set out how surface water will be dealt with, and how Sustainable Drainage Systems will be incorporated into a development.

    In natural environments rain falls on permeable surfaces and soaks into the ground; a process called infiltration. When land becomes developed surfaces may become sealed by buildings and paving and therefore natural infiltration becomes limited. The modern approach to dealing with this water is to ensure a sustainable drainage system is provided as part of a development, which aims to alleviate these problems by storing or re-using surface water at source, by decreasing flow rates to watercourses and by improving water quality.

    SUDS manage water through methods that try to mimic natural drainage processes using measures such as source control which decrease the flow of water entering the system; use of pre-treatment steps, such as swales or filter trenches; retention systems such as ponds or retention basins; infiltration systems, such as trenches and soakaways which allow water to soak into the ground.

    A Drainage Strategy will demonstrate how multiple benefits will be delivered in terms of flood and water management; water quality; biodiversity/habitat; amenity; and water resources. Ideally a Strategy will show how betterment will be achieved through implementation of a scheme. For major developments the Strategy should set out how it will facilitate community/health and wellbeing benefits through attractive, well designed public open space that incorporate SUDS; how, as part of wider open space, SUDS can act as sports/play areas; how SUDS in schools provide learning opportunities whilst also providing additional recreational space; how SUDS can free up capacity in already established drainage networks and therefore possibly enable future development.

    A Drainage Strategy should contain:

    • Relevant drainage design drawings, supporting runoff and volume calculations and design justifications to demonstrate SuDS scheme is in accordance with NPPF and best practice to limit to agreed rates.
    • Detailed development and landscaping layouts including all sustainable drainage features, their role (storage, conveyance, treatment) and details of any residual risks associated with such features.
    • Exceedance routes and how these will be managed – including the management of any existing flow routes through the site.
    • Viable point of discharge and where relevant an assessment of risk of surcharged outfall.
    • Any third party access, works or consent agreements including easement requirements.
    • Full Structural, hydraulic & ground investigation supporting reports, including infiltration testing.

    More information is available on SUDS here.

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