Siltation and Desilting Practices in A Runoff Reservoir on Heavy Sediment-Laden River

Sediment deposition in the reservoir of run-of-the-river power station is severe, in this paper we take upper Marsyangdi reservoir as an example to analyze sedimentation and desilting process according field data measured from September 2016 when the reservoir had just been impounded in order to find strategy for managing reservoir sedimentation. The ratio of Upper Marsyangdi reservoir capacity and volume of sediment into the reservoir is about 0.2. The reservoir arrived silt-stable in a year after impoundment with a depth of 12m deposition at the dam site. Most of sediment deposit in the periods that at the initial and the end of flood reason and it is found the flow rate is the key factor influencing trap efficiency because that due to damming velocity of medium flow decreased significantly compared to natural condition which caused numerous deposition. Based on result of analysis of deposition the desilting condition is determined. Empty flushing is proposed to release deposition after flood season when flow rate is greater than 100m3/s and the new capacity will last to next flood season. In order to reduce sediment concentration into diversion channel a desilting should be done in flood season when flow rate is larger than 200m3/s and flow rate for impound should not be more than 1/10 of that into reservoir which can avoid deposition during impoundment near dam site. 1 Research background It is usually necessary to build reservoirs in order to generate electricity in hydropower stations. However, during the process of reserving river water, reservoirs also intercept large amounts of sediment. According to the statistical data of 2001, the annual average amount of silt in the world’s larger reservoirs accounted for 0.5% to 1% of the reservoirs’ remaining storage capacities[1]. These sediment accumulations not only occupy large amounts of the storage capacities, but also cause abrasions to the turbine blades of the hydropower stations[2, 3]. Therefore, the reservoirs which have been constructed on sandy rivers generally have larger capacities, in order to include sufficient space for containing silt and siltation, and to reduce deposition by reservoir regulation for the purpose of maintaining the long-term operations of both the reservoirs and the power stations[4]. However, due to limited development conditions, some runoff power plants have been gradually constructed on sandy rivers in recent years. For these power plant reservoirs, silt and sediment can be easily deposited, which have negative effects on the normal operations of the power stations. The existing research findings have mainly focused on the siltation processes and treatment measures of large reservoirs[5-10]. At the current time, studies regarding the siltation processes and causes of silt accumulations in the runoff power station reservoirs are lacking. The Upper Marsyangdi Power Plant in Nepal is a typical runoff power plant located on a sandy river. Since its completion, sediment and silt deposits have rapidly accumulated in the reservoir area. In this research study, the actual measured water and sediment data from the Marsyangdi Power Plant were used for analyzing the processes and causes of the siltation in the reservoir. The key factors affecting the siltation processes were determined, and desilting suggestions for this particular reservoir were proposed according to the analysis results. The Upper Marsyangdi Reservoir is located in the upper reaches of the Marsyangdi River of Nepal. The normal water level is 902.25. The back-water height at the front of the dam is approximately 14 m. The total storage capacity under normal water level conditions is approximately 500,000 m3. The reservoir barrage is located on the Marsyangdi River. The Nardi River flows as a tributary to the Marsyangdi River at an upstream position 150 m from the dam. In September of 2016, the Marsyangdi Power Station had begun to officially store water and started to generating power. Also, a systematic observation of the sediment transport in the reservoir was carried out from March 27, 2017 to November 20, 2017, in order to obtain comparatively complete data of the landforms; water; silt and sediment; reservoir inflow process; sediment concentration process; sediment grading; and water levels in front of the dam. The measured section position is shown in Fig. 1. , 0 (2018) MATEC Web of Conferences https://doi.org/10.1051/matecconf/201824601062 246 10 ISWSO 2018 62 © The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0 (http://creativecommons.org/licenses/by/4.0/).