ENVIRONMENTAL FLOW ASSESSMENT FOR A HYDROPOWER PROJECT ON A HIMALAYAN RIVER

Abstract

The flows of the world's rivers are increasingly being modified through impoundments, abstractions, return flows, inter-basin diversions, and flood control structures (Dyson et al., 2003; Postel and Richter, 2003). It is estimated that more than 60% of the world's rivers are fragmented by hydrological alterations (Ravenga et al., 2000). This has led to widespread degradation of aquatic ecosystems (Millennium Ecosystem Assessment, 2005). Tropical monsoon hydrology in India necessitates development of storage and flow diversion schemes for multipurpose utilization of water. A large number of hydropower schemes on the Himalayan rivers are in different stages of development. These river valley schemes will cause flow related impacts due to storage, flow diversion, tunnelling, muck disposal. etc. There may be critical reaches in which altered flows are not able to sustain the riverbed ecology and riparian environment existing prior to implementation of the storage and diversion schemes. Environmental flows (EF) are the water that is left in a river eco-system or released into it for the specific purpose of managing the condition of that ecosystem. A wide range of outcomes, from environmental protection to serving the needs of people, are to be considered for the setting of an environmental flow. There is no simple figure that can be given for the environmental flow requirements of rivers and environmental water requirements of catchments. Much depends on stakeholders' decisions about the future character and health status of these ecosystems. RESEARCH GAPS Review of available literature on environmental flow assessment (EFA) shows that: 1. The status of EF research in India may be characterized as being in its infancy because of very limited knowledge base (NCIWRDP, 1999). EF in India has usually been understood as the minimum flow to be released downstream from a dam as compensation for riparian right without considering impacts on river ecosystem. 2. Efforts made by scientists in different parts of the world on EFA (methods, methodologies, approaches) vary in terms of knowledge base. Further, the EF studies and guidelines are region specific. 3. Socio-economic and water quality aspects of environmental flows also need to be considered in Indian context because of social and religious significance of rivers. Only a few methodologies consider these aspects in EFA.