Problems of Nepalese hydropower projects due to suspended sediments

Abstract

Nepal has high potential for hydropower due to glaciers in the Himalayas, regular monsoon rain and local topography. Himalayan Rivers contain large quantities of sediment with hard abrasive particles, which is a hurdle for the economic exploitation of hydropower resources. Erosion of the earth's surface takes place because of influence of climatic, tectonic and human activities. Almost all the power plants in the Himalayan region are affected by excessive sediment in rivers that reduce the life of plants either by filling reservoirs or by erosion of turbine components. The design and operation of headwork components in hydropower plants can only manipulate sediment particle size and hence only sediment concentration in the water. The erosion of hydraulic machinery depends on eroding particles, that is, their size, shape and hardness; on substrates, their chemistry, elastic properties, surface hardness and surface morphology; and on operating conditions, velocity, impingement angle and concentration. The shape, size and mineral content of sediment vary at different locations of the same river system depending on distance traversed by particles, gradient of the river and the geological formation of the river course and catchment areas. The Khimti hydropower project in Nepal represents a typical high head power plant in the Himalayan River that is affected by river sediment. Erosion tests of turbine material carried out at Kathmandu University revealed the dependence of erosion rate on mineral content. The erosion tests have shown trends relative to the quartz content present in samples from river basins in different parts of the country. There was some indication that the shape of particles had some effect. Sediments in the hydropower projects are normally considered as a burden, but there is scope for industrial use of these sediments.

Analysis of sediment abrasion potential in hydro turbines by studying fine sediments from the Budhi Gandaki-Trishuli River of Nepal

“…Bastola (2014) also studied sediment size 125 mirons-600 microns of the Marsyangdi River and found that abrasion rate was 16 mg/g per hour to 24 mg/g per hour. Both those hydropowers were facing severe abrasion of turbine requiring frequent maintainance (Thapa 2005). Mineral composition of the Marsyangdi River sediment is 61%-56% quartz and10% feldspar.…”

Comparative study of sediment erosion on alternative designs of Francis runner blade

“…These sand particles are mainly responsible for the erosion in turbine components. The hydraulic turbines of several other Asian and South American hydropower projects are also facing this problem [1]. The problems of sediment erosion have also been reported in hydropower plants with comparatively lower sediment concentration as in Norway [2].…”

“…This in turn decreases the efficiency, reliability and operating life of the turbines and creates a serious challenge in operation of the hydropower project [2]. Although, sand traps and settling basins are installed in hydropower plants to filter out sands from the river water before introducing to turbines, the complete removal of sediments is not possible [1]. Very fine sand particles (less than 0.2 mm in diameter) are less likely to settle down in the settling basins and thus escape into the turbines.…”

“…The rivers in this region contain high percentage of hard abrasive minerals like quartz, which causes rapid erosion of turbine components and affects the performance of turbines [1]. This in turn decreases the efficiency, reliability and operating life of the turbines and creates a serious challenge in operation of the hydropower project [2].…”

Assessment of Sediment Load of Langtang River in Rasuwa District, Nepal

“…Natural disasters on the other hand are very frequent in the Nepalese Himalaya, which creates a tremendous volume of sediment load [16] and is influenced by the extreme relief and high monsoon rainfall. This becomes a big challenge for operation and maintenance of hydropower facilities which in Nepal are mostly located on rivers that are fed by runoff [17]. In the Langtang River catchment glaciers are retreating steadily with an estimation of 32% reduction in glacier area by 2035, which will cause further exposure of rocks and soil and thus increase the amount of sediment runoff [18].…”