Sedimentation and sediment handling in Himalayan reservoirs

Abstract

Dams have been promoted as an important means of meeting needs for water and energy. Due to increasing population in the earth, more dams are needed to be built in the future to meet people’s increased demand for water, food and energy. In contrast to ever increasing demand, global water storage volume is reduced by approximately one percent annually due to reservoir sedimentation. A more accurate reservoir sedimentation survey is one of the useful means to monitor reservoir sedimentation rate. However, numerous factors affect the accuracy of surveyed data from the reservoir sedimentation survey. Even a small scale error may lead to an erroneous estimate in the total sediment volume, which ultimately may result to an inaccurate computation of reservoir sedimentation rate. In general, reservoir sedimentation survey in large reservoirs is being used to determine the total capacity of the reservoir, the sediment deposition rate, the capacity-elevation curves and the sediment yield from the watershed. However, in daily peaking reservoirs designed to regulate only for the peak hours of a day, the reservoir surveys is carried out to determine the sediment deposition pattern, the amount of flushed sediment after flushing, the shift in the sediment deposition area and the capacity-elevation curves. To assess the measurement accuracy, a bathymetric survey was carried out in Øvre Leirfoss tailrace pond and Selbusjøen Lake close to its outlet. Further, similar survey was carried out in Kulekhani and Kaligandaki A reservoirs in Nepal with an aim to determine the effects of survey track lines and survey point density in DGPS bathymetric survey. The main factors that effect on the accuracy of survey data and volume computations from the bathymetric survey are terrain irregularity, data density and track lines of the survey. These effects can be minimized by increasing survey point data density with the coverage of the entire reservoir area. The sediment yield, the capacity reduction rate and sediment deposition thickness should be assessed with respect to the accuracy of survey technique used to determine the schedule and the frequency of bathymetric survey of the reservoir. At present, Kaligandaki A and Middle Marsyangdi hydropower plants supply about one third of total hydropower generation in Nepal. Hence, the sustainability of these reservoirs is a key issue. In this respect, bathymetric surveys were conducted in December 2010 in order to prepare base line maps of the reservoir and to monitor the sedimentation process in these reservoirs. The Kaligandaki A and the Middle Marsyangdi reservoirs have lost 51% and 65% of their total volume, respectively. The total losses within the live storage capacity are in the range of 6.7 percent and 14.1 percent of the total live storage capacity, respectively. Still, both power plants are capable in generating with their full capacity during the peak load demand in Nepal. Hence, reservoir capacity loss within the dead volume in the peaking reservoir does not affect in generation capacity. Reservoir sedimentation surveys in the Kulekhani reservoir were also carried out using Differential Global Positioning Systems (DGPS) method in 2009 and 2010 by this author with a support received from Norwegian University of Science and Technology (NTNU) Norway and Hydro Lab Nepal. Bathymetric maps were prepared from the survey data. The deviation (variation) in depths between survey data of 2009 and 2004, 2010 and 2004 and 2010 and 2009 are computed using software program “Surfer”. The deviation maps are also plotted. The reservoir volume based on 2010 survey is 64.89 mill m3. The Kulekhani reservoir has lost 20.4 mill. m3 in total and 14 mill. m3 in it’s live storage capacity during last 27 years of operation. It gives an average annual loss rate of about 1 percent of the original reservoir capacity. The annual average thickness of sediment deposition is 0.16 meter since 1996. Similarly, the annual average sediment deposition volume is 0.56 percent of the total reservoir capacity. The annual average thickness and the sediment deposition volume in the Kulekhani reservoir are less than the average error of the bathymetric survey with the DGPS method. Based on the current sediment yield, capacity reduction rate, sediment deposition thickness and the accuracy of the DGPS bathymetric survey it is concluded that the bathymetric survey in this reservoir should be conducted at an interval of at least five years. It is noted that the peak energy demand is growing annually by about 10 percent in Nepal, creating acute electricity shortage in the country. Demand is exceeding supply every year. The greatest challenge is to bridge the gap between supply and demand of electricity. At present, Nepal is facing load shedding up to 16 hours a day during the dry season. The Kulekhani reservoir is the only reservoir project in operation in Nepal, which facilitates seasonal water storage for energy production. The load shedding hours during dry season depends mainly on the availability of water in this reservoir. There is power deficit even during the wet season, resulting in the use of Kulekhani power plants to generate electricity during the wet season. This is due to the fact that the run of river (RoR) plants have generated with lower capacity (up to 50% of the total capacity). The Kulekhani reservoir is therefore not filled to its full capacity during the wet seasons since 2006. If RoR plants were capable in generating their maximum capacity, the Kulekhani power plant would not be needed to operate during the wet season. In this respect, an excel model is developed for the optimum operation of the Kulekhani reservoir and for some key peaking run of river (PRoR) power plants. The model is based on historic data records. The model focuses on water availability, generation and storage capacities and the power demand. The transmission part of the system is not considered to be a bottleneck in this context. The model shows that if Nepalese hydropower plants are operated and maintained properly, they can generate their full capacity and the load shedding can be reduced significantly. The Kulekhani reservoir can also be operated at higher reservoir levels. The large amount of load shedding in Nepal is due to the capacity deficiency during the peak hours from October-November to March-April. Development of PRoR instead of RoR helps to reduce the load shedding during this time. The RESCON (REServoir CONservation) model is a spreadsheet-based program written in Visual Basic programming language and works with macros. This model is used to review the sustainability of the Kulekhani reservoir. Some sensitivity analyses are also carried out. Based on the RESCON analysis both the flushing and Hydrosuction Sediment Removal System (HSRS) sediment management options seem to be feasible for the Kulekhani reservoir. However, HSRS is found to be best sediment management option in regards with economy. In RESCON model, the calculated sediment concentration through hydrosuction pipe of HSRS is very low compared to the sediment concentration achieved during the field test in Nepal. Quantity of sediment removed by HSRS method could be higher in comparison with as calculated by RESCON method. Hence, calculation of sediment transport and sediment concentration in RESCON method should be reviewed and updated. Field tests of the Hydrosuction Sediment Removal System (HSRS) with the Modified Double Layer Sediment Sluicer (MDLSS) were also performed at the settling basin of Sunkoshi small hydropower plant and at the peaking pond of the Sunkoshi hydropower plant in July 2009. The field tests of HSRS indicates that it is a promising technology and has relatively low-cost and low-power requirement system for the sediment removal from the reservoirs and peaking ponds. The field test study shows that the efficiency of the hydrosuction sediment removal can be increased if it is combined with water jetting to break consolidated sediment deposits. The efficiency and performance of the HSRS can also be improved with the adjustment of the opening between inner and outer pipe according to the consolidation level of sediment deposit. The study of the sediment management in the Kulekhani reservoir showed that HSRS is best suited for removing sediment deposit from the lower portion of the reservoir. The field test also proved that it is possible to remove even consolidated sediment deposit from the bottom of reservoir. The HSRS can evacuate sediment deposits from the downstream area of the reservoir (about two km upstream from the dam) and sediment from the upstream parts will be transported from live storage to the downstream. In principle, it is possible to evacuate the total incoming average annual sediment load from the reservoir every year with HSRS, which can help to establish sustainable reservoir with present storage capacity. Hence, the Hydrosuction Sediment Removal System (HSRS) is economically attractive and viable for sediment handling in the Kulekhani reservoir. However, HSRS is one of the long-term sedimentation removal solutions for the sustainability of the Kulekhani reservoir.