ASSESSMENT OF SEDIMENT ABRASIVE POTENTIAL IN HIMALAYAN RIVERS

Abstract

In geologically young mountains like the Himalayas, rivers transport large amount of suspended sediment during the snow melt and monsoon seasons. The Himalayas are characterized by a steep topography, which plays a vital role in the spatial distribution of rainfall. High-intensity rainfall in the catchment results in stream flow events, causing extensive slope instabilities, floods, and high sediment transport. During such events, the Himalayan rivers pose high temporal variation in suspended sediment concentration (SSC), and under extreme circumstances, the SSC may reach up to ~50,000 mg/l. The Himalayas are the source of several perennial rivers with significant hydropower potential; many hydropower projects have been already commissioned, and many more are being built in the region. Some Himalayan rivers flowing in India, having a lot of untapped hydropower potential, are Beas, Ravi, Sutlej, Alaknanda, Bhagirathi, and Brahmaputra. One of the major challenges in these river basins is high sediment transport, which impacts the operation of hydropower plants (HPPs). For the development of new HPPs, comprehensive information on sediment properties is required by the project developers. To design suitable desilting structures, the developers require knowledge of SSC and particle size distribution (PSD). On the other hand, to design suitable erosion resistive material for hydraulic turbines, the developers require knowledge of sediment properties such as suspended sediment concentration, particle size distribution, shape, and mineral hardness. However, the major challenge for such design is the lack of availability of sediment details. In HPPs, the suspended sediment properties considerably affect various processes such as reservoir sedimentation, abrasive erosion of underwater parts, and clogging of the cooling water system. Sediments with high proportion of hard minerals and angular shapes accelerate erosion. The problem of abrasive erosion is quite common in the Himalayas, the Andes, the European Alps, and the Pacific Coast Mountain Ranges. Due to abrasive erosion, frequent and high maintenance, repair, and replacement of eroded underwater parts are required. Further, on account of reduced efficiency and downtime, there are losses in electricity generation. The problem of abrasive erosion become more severe in the future due to higher suspended sediment availability from retreating glaciers and intense rainfalls caused by climate change. Due to the continuous development of the hydropower potential of the Asian and South American continents, the number of hydro turbines facing abrasive erosion shall increase. In high head run of river HPPs, abrasive erosion is a serious issue, especially in Pelton turbines, due to high impact on account of high velocity. For the optimization of a hydropower plant with respect to suspended sediment and hydro-abrasive erosion, information is required on the costs of various mitigation measures and losses in electricity generation. Currently, only limited information is available due to lack of practically proven measurement techniques as well as reliable data on suspended sediment properties and turbine erosion with efficiency losses. Further, the correlations between suspended sediment properties, especially sediment hardness, with turbine erosion are scarce in the literature. It is evident from the above discussions that abrasive erosion of hydraulic turbines is an important subject in the Indian Himalayas and thus, pertinent to assess the sediment abrasive potential in the Indian Himalayan rivers. The assessment is required by both field and laboratory investigations. In the present study, it is proposed to identify the sediment characteristics of several Himalayan rivers and to investigate experimentally by using the obtained information. The objectives for the proposed research study are as follows: • To identify the sampling locations and collect sediment samples from different river basins of the Indian Himalayas. • To investigate the spatial variation of suspended sediment properties (SSC, PSD, and shape) in the river basins. • To investigate the source and occurrence of sediment mineral composition in the river basins. • To determine IEC 62364 (2019) based sediment shape and hardness factors in the river basins. • To investigate the correlations of Pelton bucket erosion with suspended sediment concentration (SSC), sediment size, erosion velocity, and exposure operating time based on tests of commonly used grades of steel and coatings and different types of sediment particles, in the laboratory test rig.To achieve the proposed objectives, the methodology adopted is as follows: • Identification and selection of Himalayan river basins on the basis of existing HPPs and available untapped hydropower potential. • Identification and selection of sampling locations on the basis of existing and proposed diversion head works of HPPs and ease of access in the selected river basins. • Sampling of suspended and bed sediment from all the selected locations. • Application of several recent sediment measurement techniques, including laser diffraction, dynamic imaging, X-ray diffraction (XRD), and scanning electron microscope (SEM), for measuring sediment samples. • Analysis of obtained suspended sediment measurements (SSC, PSD, and shape) and investigation of their spatial variation. • Analysis of obtained XRD based mineral quantifications and identification of their source and occurrence in the river basins. • Determination of IEC 62364 (2019) based sediment shape and hardness factors. • Selection of suitable sediment minerals for experimental investigations of erosion in the model Pelton buckets. The selection of minerals was carried out based on hardness on Moh’s hardness scale and abundance in river basins. • Selection of parameters for experimental investigations based on available literature and field observations. • Determination of the correlation of erosion with various sediment properties based on the experimental investigations. • Uncertainty analysis for parameters involved in the experimental investigations. To investigate the spatial variation of suspended sediment properties, two major Himalayan river basins carrying high suspended sediment, namely the Alaknanda river basin (ARB) and Sutlej river basin (SRB), were selected, and sediment sampling was carried out at all the selected locations. The sites were selected based on the location of the diversion intake works of HPPs. The outcomes from the above work are as follows: 1. There is wide spatial variation of SSC in both ARB and SRB. In both the basins, the SSC was found to be decreasing from upstream to downstream location mainly due to change in river gradient from steeper at higher elevation (upstream) to milder at lower elevation (downstream). 2. No significant trend was observed for mean particle size (d50) from upstream to downstream in both ARB and SRB. In the ARB, the d50 varies from 15.54μm to 35.24μm (6.17 Φ (phi scale) to 4.60 Φ), whereas in the SRB, the d50 varies from 6.00 μm to 25.36 μm (5.53 Φ to 7.53 Φ). 3. In the ARB, the sediments are characterized by moderate sorting, coarse skewness, and leptokurtic distribution, whereas in the SRB, the sediments are characterized by very poorly sorting, coarse to nearly symmetrical skewness and platykurtic to leptokurtic distribution. 4. In both the basins, no significant correlation was observed between mean particle size with either sorting, skewness, or kurtosis, as their behavior exists with a wide range of d50. 5. In both basins, there is no significant spatial variation in sediment shape. The sediments exhibited high degree of sphericity and slight elongation. To investigate the source and occurrence of sediment mineral composition, five major Himalayan river basins having high hard mineral content, namely Beas river basin (BERB), Ravi river basin (RRB), Alaknanda river basin (ARB), Bhagirathi river basin (BRB), and Sutlej river basin (SRB), were selected and sediment sampling was carried out at all the selected locations. The minerals in the sediment samples were identified using the X-ray diffraction (XRD) technique and quantified using the matrix flushing approach. The principal component analysis (PCA) was applied to identify the interrelationship of source and occurrence between different minerals. The outcomes from the above work are as follows: 1. Wide variety of minerals were observed in the different river basins. These minerals include quartz, feldspar, muscovite, biotite, diopside, garnet, dolomite, diopside, chamosite, illite, laumontite, silicates, clays, oxides, carbonates, and zeolites. 2. There is no significant spatial variation of minerals from upstream to downstream in the river basins. The minerals were observed to be dependent on the river catchment geological formation.3. In all the river basins, the sediments are dominated by quartz, which signifies the presence of quartzite, gneisses, and granite rocks. The weathering of these rocks results in the release of high quartz concentration in the rivers. The quartz content varies from 61% to 85%. 4. A strong interrelation was observed between quartz, feldspar, muscovite, biotite, diopside, silicate, and clay minerals. Based on the principal component loadings, the sediment mineral composition was observed similar in both BERB and RRB. 5. A strong interrelation was observed between feldspar, muscovite, silicate, and clay minerals. Geologically, the weathering of igneous and metamorphic rocks releases feldspar, and the alteration of feldspar result in the release of muscovite, clay, and silicate minerals To determine the International Standard IEC 62364 (2019) specified sediment shape and hardness factors, the obtained sediment shape and mineral hardness were used for each river basin. The shape factors were determined by considering the mean sphericity (MS) and mean aspect ratio (MAS) for each river basin. The hardness factors were determined by considering those sediment particles having Mohs hardness greater than 4.5. The outcomes from the above work are as follows: 1. In all five river basins, the hardness factor for the main river stem is influenced by the sediment inputs from their respective tributaries. 2. From the sediment shape perspective, as per the obtained average shape factor, the most erosive potential river basin is ARB (1.38), followed by SRB (1.36), RRB (1.36), BRB (1.29), and BERB (1.23). 3. From the sediment hardness perspective, as per the obtained average hardness factor, the most erosion potential river basin is BRB (0.90), followed by the SRB (0.85), ARB (0.84), RRB (0.84), BERB (0.84). To investigate the correlations of Pelton bucket erosion with suspended sediment and operating parameters, test runs were conducted using a model Pelton jet test rig. A total of 20 Pelton buckets of 11 different materials, with detachable features for ease of dismantling, to measure erosion were fabricated and supplied as well as supported by Andritz Hydro Pvt Ltd. The bucket materials used are given in Table A1 The erosion after each test was measured with weight loss of buckets using a high precision weighing balance. Based on the investigations, the outcomes are as follows: 1. 13Cr4Ni with WC-CO-Cr coating was found to have the best erosion resistance among the materials tested. 2. A linear relationship was observed between SSC and erosion, whereas no significant pattern was observed between PSD and erosion. Among operating and sediment parameters, the erosion velocity was found contributing maximum to the erosion of the buckets. 3. The maximum erosion was observed for quartz particles, followed by garnet, feldspar, and muscovite. 4. For muscovite particles, the computed exponents for SSC, PSD, erosion velocity, and exposure time vary from 0.27 to 0.91, 0.25 to 0.74, 2.29 to 3.14, and 0.86 to 1.14, respectively.5. For feldspar particles, the computed exponents for SSC, PSD, erosion velocity, and exposure time vary from 1.05 to 1.34, 0.21 to 1.00, 2.61 to 3.78, and 0.84 to 1.21, respectively. 6. For garnet particles, the computed exponents for SSC, PSD, erosion velocity, and exposure time vary from 1.10 to 1.36, 0.21 to 0.93, 2.94 to 3.71, and 0.98 to 1.16, respectively. 7. For quartz particles, the computed exponents for SSC, PSD, erosion velocity and exposure time vary from 1.15 to 1.36, 0.21 to 0.92, 3.11 to 4.00, and 0.98 to 1.19, respectively. This study is significant in the context of sediment abrasive potential assessment and mitigation for HPPs in Himalayan river basins. The comprehensive information on suspended sediment properties obtained will be useful for HPP operators, equipment manufacturers, civil contractors, and researchers to develop strategies for handling erosion issues effectively and operation of HPPs optimally. The mineral composition information, along with suspended sediment properties, will allow the designers and researchers to predict abrasive erosion in underwater turbine components in accordance with IEC 62364 (2019). The proposed correlations between the suspended sediment properties and turbine erosion will be useful for the manufacturers in selecting suitable materials for hydraulic turbine.