Guidance on Environmental Flow Releases from Impoundments to Implement The Water Framework Directive
March 2007

Executive Summary

Implementation of the EC Water Framework Directive (WFD) requires the development of procedures to ensure the adequate mitigation of the negative impacts created by water abstraction and impoundments. Flow regime releases from impoundments, such as reservoirdams, will be included in the licence conditions to allow the proper mitigation of negative impacts caused by their construction and operation.

This report provides general background on WFD and the potential impacts on the river
ecosystem together with guidance on the setting of environmental flow release regimes from impoundments. The guidance is aimed at environmental regulators, dam operators and other stakeholders and it is equally applicable in all UK member countries.

The guidance is divided into three parts:
  1. A step by step guide, supported by a flow chart, that describes the process of defining the target river ecosystem status, setting flow regime releases from impoundments and monitoring their effectiveness in achieving that status. The steps include revision of the flow release regime according to impoundment capability, purpose and designation.
  2. A method for initial assessment of whether a water body is likely to fail to meet Good Ecological Status because of changes to the flow regime (indexed by simple flow regime statistics), which can be used where appropriate biological assessment tools are not adequate. Annex A provides some examples of pre and post impoundment regimes from the UK that illustrate the degree of alteration to the flow regime hydrographs and to key flow regime statistics.
  3. A procedure for defining an environmental flow regime release based on the requirements of riverine species for basic elements (building blocks) of the natural flow regime. Three levels of assessment (desk-top flow assessment, hydraulic assessment and biological assessment) provide a risk-based approach in which greater investment in the assessment yields lower uncertainty in results. In all three levels, assessments should be carried out by a team of experts that normally includes physical scientists, such as a hydrologist, hydrogeologist and geomorphologist, and biological scientists, such as an macro-invertebrate ecologist, a freshwater botanist and a fish biologist.
All guidance is provided within the limits of current knowledge of the flow regime requirements of river ecosystems and the likely impacts of flow alterations of ecosystem

Copies of this report are available from the Foundation, in electronic format on CDRom at 20.00 + VAT or hard copy at 15.00, less 20% to FWR members.

N.B. The report is available for download from the SNIFFER Website.