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PPS 15: Planning and Flood Risk
Annex D Assessing Flood Risk & Drainage Impact

Assessing Flood Risk

D1 Where an assessment of flood risk is required to facilitate the proper consideration of a planning application it shall be the responsibility of the applicant to provide the necessary information.
D2 Such an assessment may be of a relatively minor nature, for example where the development proposed is small, or it may comprise a major basin-wide study, where a large infrastructure development is proposed. Preliminary or scoping studies may assist the preparation of a full report and developers should make early contact with the Planning Service and Rivers Agency Opens link in a new browser window to discuss proposals.
D3 The detail and technical complexity of an assessment will vary depending on the scale and nature of the development proposed and the significance of the study. In all cases however applicants will be expected as a minimum to provide the following baseline information:
  1. a location plan which clearly illustrates geographical features and built development including, where appropriate, street names. The plan should identify all watercourses, water bodies and other drainage infrastructure in the vicinity including drainage outfalls;
  2. a plan(and where appropriate, cross sections) of the site showing existing and post development levels related to Ordnance Datum;
  3. details of any existing flood alleviation measures or flood defence works that may influence the site;
  4. the identification of all potential sources of flooding;
  5. a plan of the site showing the extent and depth of flood events or flood predictions. (Information may include anecdotal or photographic evidence or where necessary, survey results and/or model estimates.); and
  6. proposals for the mitigation of any increases in flood risk that may arise as a result of the development proposed.
D4 The range and variety of local conditions will be such that each assessment will normally be an individually tailored report describing the implications of a proposed development. While it will be necessary to consider all the factors identified above, the depth of assessment necessary may vary greatly from case to case.
D5 Where it is anticipated that the effects of a proposal may require a basin wide study either by virtue of its size or its nature, applicants may be required to provide additional information on its impact(s) on flood risk. This may include:
  • previous flood event data supported by information on rainfall, flood return periods and the probability of storm surge occurrences where appropriate. (Evidence on trends in flood occurrences is particularly valuable and should be included where available). Changes in the environment of the locality that have occurred since the last event where these may be material. Where this information is relevant its assessment will be an important aspect of the report;
  • a plan and description of structures which may influence local hydraulics. For example, bridges, pipes/ducts crossing watercourses, culverts, embankments and walls;
  • an assessment of hydraulics of all drains and sewers at the site or in the location, both existing and proposed, during a flood event. The methodology for assessment must be clearly identified;
  • an estimate of the volume of run-off likely to be generated by the proposed development. Where appropriate the maximum flows that would be displaced from the site by the proposed development should also be estimated and the consequences of that displacement on neighbouring or other locations assessed;
  • an assessment of :
    1. the likely speed with which flooding might occur;
    2. the sequence in which various parts of the site or location may flood;
    3. the likely duration of a flood event; and
    4. the potential consequences/impacts of a flood event.
D6 Because of the uncertainties inherent in flood estimation and expected climate change impacts, the application of the precautionary approach to hydrological analysis of flood flows and the determination of flood event return periods requires that any assessment of flood risk to incorporate the necessary allowances for increased rainfall, storminess and sea-level rise specified in UKCIP 0219.
D7 All assessments should also acknowledge that there are no circumstances in which the risk of flooding can be removed entirely. In defended areas therefore consideration should always be given to the potential impacts of extreme events on defences and the minimising of risks to life in such cases.

Drainage Assessment

D8 Where a drainage assessment is required to facilitate the proper consideration of a planning application it shall be the responsibility of the applicant to provide the necessary information.
D9 Drainage assessments will normally be required to accompany development proposals where:
  • the development comprises of 10 new dwelling houses or more;
  • the development site exceeds one hectare;
  • changes of use involving new buildings and/or hardsurfacing which exceed 1000 square metres in area; or
  • surface water run-off from the development may adversely impact upon a sensitive area20.
19 United Kingdom Climate Impacts Programme (2002 Report)
20 Sensitive areas will include areas within, or upstream of, a conservation site designated under national or international legislation, for e.g. Areas of Special Scientific Interest (ASSI)
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