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Flood risk and drainage

The effects of climate change, population growth and urbanisation are placing significant pressure on the effective management of surface water. Changing patterns of rainfall and a growing area of impermeable development are giving rise to high rates and volumes of surface water run-off. Conventional drainage systems constructed from constrained networks of underground pipes and storage tanks are struggling to cope with overwhelming quantities of surface water run-off, resulting in problems such as flooding and pollution.

 Planning Policy Statement 25 (PDF 4.7MB) sets out Government policy on development and flood risk. Its aims are to ensure that flood risk is taken into account at all stages in the planning process to avoid inappropriate development in areas at risk of flooding, and to direct development away from areas of highest risk. Where new development is necessary in such areas, policy aims to make it safe, without increasing flood risk elsewhere, and, where possible, reducing flood risk overall. For more information, please click on the links below.

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  • Flood risk in Dacorum

    Dacorum's geographical information system (GIS) provides up-to-date floodplain data for the borough in terms of Flood Zones 1, 2 and 3. Our policy on development in areas of flood risk can be found in Policy CS25 of the Dacorum Borough Core Strategy (September 2013) (PDF 7.66MB).

    The Environment Agency website includes an online flood map that provides the most comprehensive picture of flood risk. The flood map shows the floodplain for England and Wales - areas that could flood from rivers or the sea. It lets businesses know whether they are at risk, what they can do about it, and how the Environment Agency can help.

    The online flood map is low resolution. For more detailed advice about which flood zone a site falls within, please contact the Environment Agency External Relations Team on 01707 632302.

  • Flood Risk Assessment (FRA)

    In order for the Local Planning Authority (LPA) to determine whether development within a flood plain is acceptable you may be asked to submit a Flood Risk Assessment (FRA). The Environment Agency has produced a matrix that sets out the FRA requirements for development based upon the size of site and the location within the floodplain.

    Please consult the technical guidance notes on the Environment Agency's standing advice on development and flood risk - England for details on how to carry out an FRA. Where an FRA is required, it should be submitted with your application.

     
  • Management of surface water drainage - major development

    On 24 March 2015, the Government laid a statutory instrument making the Lead Local Flood Authority (Hertfordshire County Council) a statutory consultee in planning for all major development in relation to the management of surface water drainage from 15 April 2015. Please refer to the National Planning Policy Framework and the planning practice guidance and non-statutory technical standards, which now reflect these changes.

    The Flood and Water Management Act 2010 established Hertfordshire County Council as a Lead Local Flood Authority covering the area of Hertfordshire. In February 2013, the first Local Flood Risk Management Strategy for Hertfordshire was published and this included a specific policy relating to the approval and delivery of Sustainable Drainage Systems (SuDS) in Hertfordshire.

  • Sustainable Urban Drainage Systems

    Sustainable Drainage Systems (SuDS) manage surface water based on an understanding of the ‘hydrologic cycle’ – the movement of water within the natural environment. They are an environmentally-friendly and cost-effective approach to drainage that avoid the problems associated with conventional drainage systems by managing water at the surface, replicating the flow routes and stopping places that water finds in the natural landscape. This integrated approach to SuDS and the landscape is referred to as the SuDS philosophy, and provides an opportunity for the delivery of multiple environmental, economic and social benefits.

    When should a sustainable drainage system be considered?

    Whether a sustainable drainage system should be considered will depend on the proposed development and its location - for example, whether there are concerns about flooding. Sustainable drainage systems may not be practicable for some forms of development (for example, mineral extraction). New development should only be considered appropriate in areas at risk of flooding if priority has been given to the use of sustainable drainage systems (see paragraph 103). Additionally, and more widely, when considering major development, as defined in the Town and Country Planning (Development Management Procedure) (England) Order 2015, sustainable drainage systems should be provided unless demonstrated to be inappropriate.

    Where to go for advice on surface water drainage?

  • Management of surface water drainage - minor development

    Make sure your proposal is designed to cater for the existing drainage patterns on the site as well as the new ones you will create with your development.

    Surface water should not be discharged into foul drains or storm water system because this can:

    • reduce the effectiveness of the sewage treatment plant;
    • lead to overloading of sewers and pollution at times of peak flows;
    • result in water being returned to the river system too quickly.

    Only use soakaways where:

    • ground conditions are suitable;
    • water tables are low enough that they will work properly;
    • they will not receive effluent from septic tank outfalls.

    Try to slow down the rate at which water runs away from your site using:

    • ponds, lakes, grass swales or wetland habitats;
    • permeable surfaces for car parking and other paved areas where ground conditions are suitable, and the risk of pollution is low;
    • French drains (gravel filled trenches) rather than pipes;
    • hydro-brakes at pond or reservoir outfalls where the downstream drain capacity is limited.

    Avoid pollution - If there is a risk of pollution from oil or chemicals from roads and industrial and parking areas, consider the following remedies:

    • silt and oil traps;
    • biological filters in layered French drains or hard surface construction.

    Reduce maintenance requirements by:

    • making sure pipes are laid at gradients which promote self-cleansing;
    • placing traps, rodding points, inspection chambers and manholes where access for cleaning or emptying is easy.
  • Water conservation

    Our policy on water conservation and Sustainable Drainage is set out in Policy 124 of the Dacorum Borough Local Plan 1991-2011.

    Try to design your scheme so that it reduces the amount of mains water you use. This will reduce the amount of water being returned to the river system.

    In the home, install devices such as showers, low-flush cisterns and spray taps.

    Collect rainwater for outdoor use:

    • in butts or pools for use in the garden;
    • in ponds or lakes for use in agriculture. These usually need planning permission. Please ask for advice.

    We have produced a document on water conservation (PDF 395KB) which provides further information on this subject.