A STUDY ON ENVIRONMENTAL FLOW ESTIMATION FOR A CASCADE TYPE HYDROPOWER DEVELOPMENT

Abstract

11 can be seen from the current trend that there is a rise for clean, cheap and more effective energy resource throughout the world. The usage and purpose of cascade type hydropower has also risen in the last ten years. Most hydropower structures are designed as multipurpose projects which include water supply, agriculture and flood mitigation and helps in sustaining the development. This type of energy resource has been considered, through research, as the major source of renewable energy in the world and accounts for 18% of total energy supply. This important type of energy which is readily available due to naturally occurrence of water may be a breakthrough in delivering different household services, driving engines and providing heat to billions of people who are currently in short of access, out of which the majority is found in Africa. This form of energy can stimulate tremendous economic and environmental benefits to the poorest countries. There are two types of cascade type hydropower schemes namely, storage and run-off river. The storage type hydropower involves construction of a storage reservoir for energy generation in which an estimated amount of flow is released as minimum flow. Under run-off river scheme, only estimated amount of flow required for energy generation is diverted and the rest is allowed to flow downstream as environmental flow. Despite the advantages and benefits of cascade type hydropower, the diversions and storage alters the flow regime downstream of the river and this is destructive to the ecosystem. This calls for a need to estimate an amount of flow which must be available all the time in the river. This amount of flow can either be released or allowed to flow downstream to maintain the ecosystem and is known as minimum or environmental flow. 1herefore, more than 200 different ways of estimating minimum flows are used worldwide. Most hydropower projects have still been carried out worldwide without accurately estimating environmental flows. Most countries base the environmental flow estimation on 90% dependable year which is not reliable as it has no justification on factors such as groundwater, sediments, channel characteristics, temperature, water quality etc. The problem is that there are no fixed guide lines to decide how much water is required to optimally maintain the ecosystem. Therefore, the objective of this study is to review the existing methods in detail and thoroughly study their applicability. In this study, four approaches are reviewed namely Hydrologic, Hydraulic, habitat and Holistic. The hydrologic and 1-lydraulic approaches arc briefly discussed while the more reliable and data intensive ones, Habitat Simulation and 1-lolistic are discussed in detail. Amount of data required, steps for estimations and worked examples are outlined for each methodology. In conclusion, each method has conditions of application which have been outlined together with their advantages and disadvantages.