Guidance on Environmental Flow
Releases from Impoundments to Implement The Water Framework Directive
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:
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
- 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
- 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.
- 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.
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.
The report is available for download from the SNIFFER Website.