The increasing requirements of environmental conservation and efficient use of natural resources have represented challenging demands for legislators, governments and society. At the same time, the new context creates favourable opportunities for conflict resolution, enhanced performance and equitable allocation of resources. The Water Framework Directive (WFD) is presently the most important piece of European legislation for the regulation of water use and conservation. The fundamental principle of the directive is the achievement of a good ecological condition of surface, coastal and ground water bodies, taking into account the socioeconomic context and the demands of different sectors of stakeholders. Specifically in terms of water quantity, it means that the abstraction and storage of water need to reconcile other ecological and social demands upstream and downstream in the same catchment.

The implementation of the WFD requires the development of structured approaches and adequate methodologies. One of these requirements involves developing procedures to ensure the adequate mitigation of the negative impacts created by the construction, operation and decommissioning of impoundments. The term ‘impoundment’ includes a range of structures, particularly dams and weirs, that are built to store water, control water level, or divert water flow in order to satisfy many different social and economic purposes.

Depending on the location, scale and the operational practice of each scheme, they can create distinct and potentially very significant environmental impacts. The most common impacts are due to changes in the hydrological flow regime and to natural geomorphological processes within a catchment. These can alter the volume, velocity and depth of flows, the passage of biota and sediment transport. They may also cause a degradation of riparian areas and alter the interaction between an affected river and its natural floodplain.

The mitigation of the environmental impact of an impoundment must reconcile the protection of the local aquatic ecology with the benefits derived from its operation, as well as must fitting within the overall river basin management planning process. Any mitigation methodology must be consistent with the overarching objectives of the new directive and have a straightforward use by dam operators and other stakeholders. Considering those criteria, the Scotland and Northern Ireland Forum for Environmental Research (SNIFFER) was commissioned by the Scottish Environment Protection Agency (SEPA) and the Environment Agency (EA) to develop a guidance document that presents a user friendly guide for identifying best practice and cost-effective management strategies and mitigation measures. This was identified as ‘SNIFFER PROJECT WDF29: Management Strategies and Mitigation Measures required to deliver the Water Framework Directive for Impoundments’. SNIFFER was charged with the selection and management of a pool of companies and scientists involved in the project (Posford Haskoning, Royal Haskoning and the Centre for Ecology and Hydrology, Dorset).

The initial phase of the project comprised an international review of impoundment management practices to provided a legislative, planning, management and regulatory context for impoundments across five countries (Norway, France, Germany, Australia and US) and identified typical and good practice mitigation measures. A preliminary analysis of the mitigation techniques tried and tested by the five different countries showed that a substantial number of them are commonly used by all of the countries – such as compensation flows, fish passes, fish screens and measures to improve dissolved oxygen levels and maintain natural temperatures. Novel approaches were also identified, such as salt interception systems close to the coast in Australia, fish friendly turbines to prevent fish damage in US, fish passes specifically for eels and a retaining weir dam to maintain water levels in an isolated area of the impounded water in Norway. Using the outcome of the first phase of the project, international approaches can now be compared with similar measures adopted in the UK and hopefully facilitate the exchange of information between organisations (a list of contacts was included in the final report of the project).

The second phase of the project involved the elaboration and testing of a specific approach for the selection of cost-effective mitigation measures. The adoption of cost-effectiveness analysis is a tool of environmental economics that aims to evaluate project alternatives. The most cost-effective is the option that has the lowest discounted social costs (i.e. the most cost-effective from the point of view of society as a whole). In other words, the cost-effectiveness analysis goes beyond simply the outcomes to consider also the forms of achieving those outcomes. It is relevant to note that the analysis of cost-effectiveness is a complex process, so the resulting guidance document was intended to break it down into eleven key steps, which provide a logical, clearly understandable and transparent procedure. The approach developed in this project is for potential application to impoundments for hydro power, flow regulation and water supply, and may be later expanded to cover small dams and flood defence schemes.

This guidance document was prepared in the light of technical experts on the project team from a wide range of disciplines and a workshop with key stakeholders (including impoundment managers, environmental consults, British Dam Society and SEPA/EA licensing officers). The guidance document has been amended in light of recommendations made following trials by licensing officers and private sector impoundment managers on the river Dee (Wales) and the Glendevon (Scotland). The final document can be used when considering what conditions to include when licensing impoundment structures under the new licensing regime (in Scotland this is anticipated at the end of 2005) and when undertaking reviews of existing impoundment licenses under the requirements of the WFD.

The guidance document to select cost-effective mitigation measures proposed in this project can be summarised by:

• A step-by-step approach – from defining the drivers and pressures on a system to identifying a potential solution or combination of cost-effective solutions.

• A set of ten Guidance Sheets, which cover the main environmental/management issues facing impoundment management (including topics such as hydro-peaking flows, fish migration etc.), cross-referenced to the mitigation measures database and the conceptual models.

• A spreadsheet of more than 100 practical mitigation measures and management strategies that could be considered.

• A spreadsheet of the potential indicative cost of those measures and strategies (where known).

• Consideration of the broader environmental and technical issues associated with the implementation of the mitigation measures and management strategies (e.g. flood management, social, recreational and navigation implications etc.).

• Conceptual models of impoundments for hydro power, flow regulation and water supply in the UK.

• A comprehensive reference list for further information on individual techniques and strategies.

• Additional discussion on the construction and decommissioning of impoundments; the impacts of impoundments on sustainable flood management practices; and considerations for impoundments associated with coastal and transitional waters.

The methodology suggested in this guidance document hopefully enriches the discussion of alternatives in public decision-making. Following this procedure will ensure that the final selection of appropriate cost-effective measures is based on the specific characteristics of, and problems identified in, the water system being considered. In order to keep the guidance document as brief as possible, a set of technical appendices and additional references to supporting information are given where appropriate. When following the proposed approach, it is important that the user is aware of the background to the process and understands the implications of assumptions and decisions made. It is also vital to ensure that local knowledge and experience of the impoundment managers and other users is sought out and taken into account.

Finally, it must be emphasised that the final documents of this project contain the best recommendations of the consultants and does not necessarily represent the official views of SEPAand the EA. Because further tests and additional methodologies will complement this Guidance, feedback from dam operators and other water stakeholders are always welcome and should be directed to the respective environmental regulator.

Author Info:

The author is policy development officer (SEPA) & Manager of the Project SNIFFER WFD29. Tel: 01786 452594. Email:

The final report of this project comprises two volumes (methodology and technical annexes), which is available at request from SNIFFER, First Floor, Greenside House, 25 Greenside Place, Edinburgh EH1 3AA, Tel: +44 (0) 131 557 2140,,