Analysis of Water and Sediment Flow in Desilting Basin of a Run-of-River Hydroelectric Project

Analysis of Water and Sediment Flow in Desilting Basin of a Run-of-River Hydroelectric Project,T. P. Singh,J. Chandrashekhar,A. K. Agrawal

Analysis of Water and Sediment Flow in Desilting Basin of a Run-of-River Hydroelectric Project  
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The paper deals with the analysis of water and sediment flow carried out in respect of an underground desilting basin of a large run-of-river hydroelectric project under operation on a heavily sediment-laden river in Indian Himalayas. The basic hydraulic design of the desilting basin done during planning stage is studied on a physical model to assess the settling efficiency of the basin. The flow of water and sediment is also calculated using a three dimensional numerical model. Since commissioning of the project, the project authorities are systematically monitoring the sediment in the river, at intake and draft tube. A preliminary attempt has been made by the authors to analyze the available sediment data and evaluate the performance of the desilting basin with reference to its settling efficiency. The results of the physical model, numerical model and prototype have been compared and briefly presented in this paper. 1 PROJECT In planning run-of-river hydropower projects in heavily sediment-laden rivers, elaborate sediment handling techniques are to be employed for diverting relatively sediment free water for power generation. Undesirable sediment will cause significant damage to the hydro-mechanical and electro-mechanical parts of t he power station. Settling basin (also called desander, desilting basin) is one of the sediment control measure commonly adopted in such small or large projects. In this paper, the case study of the underground desilting basin of Nathpa Jhakri hydroelectric project, a run-of-river project under operation in the Indian Himalayas is discussed. The project has an installed capacity of 1500 MW through six Francis units of 250 MW each, design discharge of 405 m³/s and design head of 425m. Satluj Jal Vidyut Nigam Ltd. (SJVNL), formerly Nathpa Jhakri Power Corporation Ltd., executed the project and is now operating and maintaining the power station. The project comprises of a concrete gravity diversion dam, an underground desilting complex comprising of four basins, 10.15 m diameter and 27.3 km long headrace tunnel, 21.6 m diameter and 301 m deep surge shaft, three 4.9 m diameter steel lined pressure shafts, an underground power house complex and 10.15 m diameter and 982 m long tailrace tunnel. The river on whi ch the project is located is known to carry heavy sediment load during monsoon and snowmelt. A series of sediment handling measures have therefore been incorporated in the planning and design of the headworks of the project. The operation strategy for reservoir flushing, desilting basins and power house units for various conditions attaches importance to minimizing entry of undesirable sediment into the power house and preventing damages to the machines and other underwater parts. For controlling sediment in the water diverted towards the powerhouse to the desired level, underground desilting basins have been provided.
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