Environmental flows from dams: the water framework directive

ion and impoundments. It is widely acknowledged that alterations to flow regime impact on riverine ecosystems. As a result, hydromorphology, which includes the hydrological regime, is embedded within the WFD as a supporting element to achieve good ecological status (GES). Environmental flow releases from impoundments such as reservoir dams will need to be implemented to mitigate impacts from their construction and operation. This paper outlines the process involved in the analysis of available scientific information and the development of guidance criteria for the setting of environmental flow release regimes for UK rivers. The paper describes two methods—developed by round table expert knowledge and discussions and supported by available data—for implementation of the WFD for rivers subject to impoundments. The first is a method for preliminary assessment of a water body to determine if it is likely to fail to achieve GES because of changes to the flow regime (indexed by simple flow regime statistics) in systems where appropriate biological assessment methods are limited or currently unavailable. The second is a method for defining an environmental flow regime release based on the requirements of riverine ecological communities and indicator organisms for basic elements (building blocks) of the natural flow regime. 1. BACKGROUND The Bruntland report, Our Common Future, and Agenda 21 of the 1992 United Nations Earth Summit in Rio marked a turning point in modern thinking. A central principle of Agenda 21 and Caring for the Earth is that the lives of people and the environment are profoundly inter-linked. Ecosystem processes keep the planet fit for life, providing our food, air to breathe, medicines and much of what we call ‘quality of life’. Successive international meetings (The Hague 2000, Johannesburg 2002, Kyoto 2003) have highlighted the need to ensure the integrity of ecosystems through sustainable water resources management. It has been recognised that, whilst people need direct access to water to drink, grow food and support industry, providing water to the environment means using water indirectly through support ecosystem services. Sustainable water management thus requires knowledge of freshwater ecosystems and their interaction with hydrological processes. Many physical factors determine and influence the character of riverine ecosystems, including channel structure, temperature, oxygen, light, suspended sediment concentrations and river discharge/flow. Flow volume (discharge volume per unit time) is important in dilution of chemical elements and, through interactions with channel structure, in determining water depth and flow velocity. When combined with other factors (e.g. composition of bed material) they can collectively help define instream physical habitat. All elements of the flow regime may have a role in structuring a river ecosystem, including floods, average and low flows and all have a direct influence upon fish, macroinvertebrates and macrophytic plant communities as well as riparian and associated terrestrial ecosystems. Impoundments such as dams are constructed for a range of purposes, including water supply management, hydropower generation and flood control. The objective of most impoundments is to divert water or store it temporarily for later use or release, thus smoothing out natural variations in flow regimes. Consequently, river flow regimes downstream of impoundments will be fundamentally different from their natural state. Given that large dams can effectively store all of the flow from the upstream catchment (though large floods may pass the spillway), the flow regime downstream may be totally controlled by operation of the impoundment. Active management is therefore required to generate an appropriate flow regime downstream in order to protect and maintain instream communities. This contrasts with direct abstraction from rivers, where many of the elements of the flow regime are left broadly untouched (e.g. timing, variability, high flows) and management is required to restrict the volume of water abstraction. Proceedings of the Institution of Civil Engineers esu700043.3d 15/12/08 18:47:31 The Charlesworth Group, Wakefield +44(0)1924 369598 Rev 7.51n/W (Jan 2