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International Conference on Small Hydropower - Hydro Sri Lanka, 22-24 October 2007

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Analysis of Water and Sediment Flow in Desilting Basin of a Run-of-River Hydroelectric Project T.P.Singh, J.Chandrashekhar and A.K.Agrawal Designs (NW&S) Organization, Central Water Commission (CWC), New Delhi, India E-mail: [email protected] ABSTRACT 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 the 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 which 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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2 UNDERGROUND DESILTING BASINS The underground desilting basin complex (Ranjodh Singh, et.al., 1997) comprises of four parallel oval-shaped desilting basins, each 525 m long, 16.31 m maximum width and 27.5 m maximum depth with a flow through velocity of 0.3 m/s. The inflow discharge into each desilting chamber is 121.50 m³/s, which includes flushing discharge of 20.25 m³/s. This is 20 % of the outlet discharge to the headrace tunnel, which is 101.25 m³/s. Four independent intakes feed four basins separately through a full flowing inlet tunnel that joins the desilting basin through a diffuser. At the downstream end of every basin, an exit tunnel at the basin overt leads the desilted water into the 27.3 km long headrace tunnel. The hopper shaped bottom of the basin has a 3 m wide settling trench with inlet holes of varying sizes, discharging into a flushing conduit underneath the basin, which extends along the entire length of the basin. Beyond the gates, the free flowing silt flushing tunnel leads the heavily sediment laden water back to the river. Continuous silt flushing is envisaged when the discharge in the river during monsoon exceeds the diversion requirement. In the remaining period intermittent flushing is planned. The details of the desilting basins are shown at Fig.-1. The river carries significant amount of bed and suspended sediment load. The historical data on suspended sediment distribution during planning stage of the project, averaged over 15 years indicated percentage of coarse (0.2 to 2 mm), medium (0.075 to 0.2 mm) and fine fractions (less than 0.075 mm) as around 17 %, 25 % and 58 % respectively. With the data inputs coming for 10 more years, these figures averaged over 25 years works out to around 16 %, 22 % and 62 % respectively. The petrographic analysis of sediment indicated a very high content of Quartz, Garnet, Zircon and other hard minerals. The desilting basin has been designed for 90 % settlement of 0.2 mm size sediment and above with maximum inflow sediment concentration of 5000 ppm by volume at intake. Beyond this value, the powerhouse is to be shut down. Basic design study carried during planning stage indicated the theoretical settling efficiency of the desilting basin as 92 % for removal of 0.2 mm size sediment. The settling efficiency of the basin to settle medium and fine sediment, both of which together constitute about 84 % is expected to be much lower. 3 MODEL STUDIES The desilting basin arrangement of the project is one of the largest in the world. Detailed physical and numerical model studies have been carried out to study the various aspects of the desilting basin. Physical model study of the desilting basin has been carried out at Central Water and Power Research Station, Pune (CWPRS, 1990). The study is conducted in a 1:30 scale model of the entire unit of a desilting basin from the inlet to headrace tunnel, including the flushing conduits. The particle size distribution of the suspended sediment inflow adopted is d0 = 0.001 mm, d58 = 0.075 mm, d83 = 0.2 mm, d100 = 1.0 mm. For a design discharge of 405 cumecs, flushing discharge of 81 cumecs and overall inflow sediment concentration of 5000 ppm by volume, the physical model study indicated that the 525 m length of the desilting basin is adequate for 90 % settlement of sediment coarser than 0.2 mm. The overall settling efficiency of the basin is expected to be about 36 % for the above gradation of the sediment considered.


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Numerical model study (Chandrashekhar, 1994) to simulate one unit of the desilting basin has been carried out using a three-dimensional numerical modeling software (Olsen, 1991) ‘SSIIM’, an abbreviation for “Simulation of Sediment movements In water Intakes with Multiblock option” developed by Dr. Nils Reidar B Olsen (Olsen, 2006), Norwegian University of Science and Technology, Trondheim. The numerical model uses a finite volume method to solve the Navier-Stokes equations for three dimensions on a non-orthogonal grid. The k-ε turbulence model is used to solve the Reynold’s stress term. The diffusion/convection equation for the sediment concentration is solved which gives the sediment concentration in the desilting basin. The numerical model is used to simulate one unit of the desilting basin including 10 m of inlet tunnel (Olsen et.al., 1995). The model uses a grid of 25 x 7 x 5 cells in the streamwise, cross-streamwise and vertical direction, respectively. The. settling trench at the bottom of the basin and the flushing system has been excluded from the grid, and a 3m wide flat bed is assumed to be a closed boundary. The inflow sediment concentration and size distribution is equivalent to the values in the physical model study. As per the study, the proposed desilting basin is adequate for 90-95 % removal of sediment up to 0.2 mm size for an overall inflow concentration of 5000 ppm by volume. Considering the inflow sediment size distribution, the overall settling efficiency of the basin is expected to be about 37 %. The results of settling efficiency from numerical model and physical model studies is compared and presented in Table: -1.The calculated values from the numerical model and the results from the physical model study showed good agreement. The numerical model study gave settling efficiency values for a wide range of particle sizes besides providing other information regarding velocity and sediment concentration for the entire domain of the basin. Table :1 Comparison of settling efficiency between numerical and physical model study

Sediment Size (mm) SSIIM ( in % ) Physical Model ( in % ) 0.20 93.9 90 0.15 82.0 83 0.10 52.5 48 0.075 34.1 27

It is to mention here that for a similar underground desilting basin in Tala hydroelectric project, Bhutan (Agrawal, 2005), this numerical model was used wherein the results of settling efficiency calculated using the numerical model and the results obtained from the physical model study of the basin carried out by CWPRS showed good agreement. 4 PERFORMANCE OF THE DESILTING BASINS The commercial operation of the first generating unit commenced in October 2003 and all other units were put on commercial use by May 2004. The project has since been generating the much needed peaking power for the country. During this period, the project has encountered certain sediment related challenges for which Expert Committees have been constituted which have offered short-term and long-term recommendations. SJVNL is systematically monitoring sediment data at different locations in the project through manual and automatic data collection systems. Daily observations are being taken in the river, at the intake and at the draft tube. The information being collected by SJVNL is very valuable for carrying out various analyses, facilitating decision-making and also in incorporating any mid-way corrections as required during operation of the project. A preliminary attempt has been made by the authors to analyze the available sediment data at the intake and the draft tube to evaluate the performance of the desilting basin with particular reference to its settling efficiency. In this river, the suspended sediment concentration normally varies from a few ppm to 12000 ppm. During extreme events like landslides, blockades, flash floods, breaches etc., the concentration has gone as high as 30000 ppm and in some cases even crossed 1,00,000 ppm. It is observed that the percentage of coarse, medium and fines in the suspended sediment before and after any extreme event varies significantly. Analysis of the measured sediment data reveals that the sediment concentration measured at intake is less than that measured in the river at entry to the reservoir. From this it can be inferred that the small reservoir


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formed by the diversion dam is itself acting as a settling basin and trapping a part of the incoming sediment in the river. With the gradual increase in deposition in front of the dam and intake, the likelihood of sediment entering the intake also increases. Hence, timely and effective reservoir flushing through dam sluices is important in managing the incoming sediment and the sediment already deposited in the reservoir. A broad comparison of the settling efficiency of the prototype and estimation of the physical and numerical model studies is tabulated in Table:-2. Table: 2 Comparison of the settling efficiency in the prototype and estimation of model studies

Sediment Size Physical Model ( in % ) SSIIM ( in % ) Prototype ( in % )

Greater than 0.2 mm 90 94 90 – 100 Overall 36 37 40 – 60

On analysis of the available data, it is observed that for same concentration but different gradation of sediment, the efficiency of the settling basin varies. Similarly, it is observed that with increasing inflow concentration of fine sediment, the overall settling efficiency is showing a reducing trend. The desilting basins of the project are functioning efficiently and the settling efficiency of the basin for the sediment size 0.2 mm and above is found to range between 90 to 100 %. 5 OBSERVATIONS i) The real time performance of the desilting basin with reference to the settling efficiency is found to

be better than that estimated by the physical and numerical model studies. ii) The design criteria initially stipulated as safe upper limit of sediment concentration at intake for

closing the power house was 5000 ppm. For this limit it was observed that sediment of the order of 3000 ppm was passing the turbines. This is understandable from the fact that the percentage of fines in the suspended sediment in the river averages around 62 % and overall settling efficiency of the basin is around 36 %. The petrographic analysis of the sediment carried out revealed the content of quartz and other hard minerals to be more than estimated. As a result, the underwater parts of the station experienced severe wear and tear. SJVNL therefore constituted an expert committee for advice on the matter which recommended reduction of the safe upper limit of sediment concentration at intake in the water diverted for power generation from 5000 ppm to 4000 ppm. The gradation of sediment in river water and the petrographic analysis are important in evolving the design criteria for desilting basin, machines and coating of the underwater parts.

iii) Both, physical as well as numerical model studies are good design tools to estimate the settling

efficiency of desilting basin. Their inherent limitations are however to be kept in view. For small hydropower projects, where time is essence, numerical model studies may be preferable for firming up the design of desilting basin as they can provide results in much less time and at comparatively less cost covering all range of sediment sizes.

ACKNOWLEDGEMENT The authors would express their sincere thanks to Chairman, Central Water Commission, New Delhi for granting permission to publish this paper. The authors would like to thank Satluj Jal Vidyut Nigam Ltd., Shimla, India and Central Water and Power Research Station, Pune, India whose basic data have been used/referred in this paper. The authors would thank Norwegian University of Science and Technology, Trondheim, Norway whose model has been used for the studies.


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REFERENCES Agrawal, A. K. (2005) “Numerical Modelling of Sediment Flow in Tala Desilting Chamber” M.S. Thesis,

Department of Hydraulic and Environmental Engineering, NTNU, Norwegian University of Science and Technology, Trondheim, Norway.

Chandrashekhar, J. (1994) “Numerical Simulation of Sediment Movement in Desilting Basin using SSIIM – Nathpa Jhakri Hydroelectric Project – A case study”, M.S. Thesis, Division of Hydraulic and Environmental Engineering, The Norwegian Institute of Technology, Trondheim, Norway.

CWPRS (1990) “Specific note on model studies for desilt ing basin for Nathpa Jhakri Project” Olsen, N. R. B. (1991) “A three-dimensional numerical model for simulation of sediment movements in

water intakes” Dissertation for the Dr. Ing. Degree, The Norwegian Institute of Technology, Trondheim.

Olsen, N. R. B. (2006) “A three-dimensional numerical model for simulation of sediment movements in water intakes with multiblock option” User’s Manual

Olsen, N. R. B., Chandrashekhar J. (1995) “Calculation of water and sediment flow in desilting basins” International Symposium on ‘River Sedimentation’, New Delhi, India.

Ranjodh Singh, Chauhan,R.S., Ravi Uppal (1997) “Hydraulic Design for large underground desilting arrangement for 1500 MW hydro-electric station” Seminar on “Silting problems in hydropower stations”, Roorkee, India.

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