SEDIMENT MANAGEMENT CASE STUDY NATHPA JHAKRI, INDIA Key project features Name: Nathpa Jhakri Country: India Category: reforestation/revegetation; upstream sediment trapping; bypass channel/tunnel; reservoir drawdown and sluicing; pressure flushing; empty flushing; improve operational efficiency Reservoir volume (original): 343 Mm3 Installed capacity: 1,500 MW Date of commissioning: 2003 The 1,500 MW Nathpa Jhakri run-of-river project in northern India uses water from the Sutlej river to generate electricity. The first sediment problems at the project were experienced a year after commissioning in 2004. High levels of soil erosion during the snowmelt in the Himalayas results in high sediment loads in the Sutlej river, in particular quartz particles that cause serious damage to the plant s technical equipment. Protecting the hydraulic machinery from abrasion and preserving future reservoir storage required effective sediment management strategies. The Nathpa Jhakri plant is the largest hydroelectric project on the Sutlej river, in the state of Himachal Pradesh in northern India. The Sutlej river rises in the Tibetan Plateau to an elevation of about 4,570 masl, and meets the Chenab river, forming the Panjnad river before the confluence with the Indus river. The project is owned by SJVN Ltd., a joint venture between the government of India and the government of Himachal Pradesh. The government of India envisages about 30 projects in total on the Sutlej river. International Hydropower Association Chancery House, St Nicholas Way, Sutton, London SM1 IJB, United Kingdom T: +44 20 8652 5290 F: +44 20 8643 5600 E: iha@ The USD 2 billion Nathpa Jhakri project features a concrete gravity dam of 62.50 m in height, a large underground desilting complex, and a hydropower plant with an installed capacity of 1,500 MW. Water flow is diverted through the intake structure, which August 2017 is composed of four intakes and a has a capacity of 486 m3/s, to the desilting complex which comprises four chambers each of dimensions 525 m long, 16.31 m wide and 27.5 m deep. Then the 27.4 km long headrace tunnel conveys water to the underground power station where six 250 MW Francis turbines generate about 6,778 GWh annually. Water from the turbines is released back into the Sutlej river through the 982 m tailrace tunnel. The reservoir is 2.5 km long and located in a very narrow valley. It has a total live capacity of 343 Mm3 and a live storage capacity of 303 Mm3. In addition, the flood control pool capacity is 5,660 m3. The maximum operating level is 1,495.5 masl and the minimum drawdown level is 1,474 masl. The reservoir is normally operated at full regime to benefit from the 493 m design head.
Nathpa Jhakri dam and instake structure on the left bank Hydrology and sediment The Sutlej river basin represents a major part of Indus river basin. It is considered a perennial river because the upper catchment contains a large glacierised area. The catchment has a high rate of soil erosion as a result of its fragile geology and a steep topography. At the Nathpa Jhakri site, more than 40 per cent of the suspended sediment load has been recorded as fine fraction sediment, while only about 20 per cent is coarse fraction. This latter figure rises up to 33 per cent during floods. Often, landslides sharply increase the sediment concentration in the river flow. The total catchment tributary to Nathpa Jhakri dam is 49,820 km 2, from which 74 per cent is unregulated area. The river flow is comprised of glacier melt, snowmelt (estimated to make up about 50 per cent of the runoff) and rainfall, and is subjected to a high hydrological seasonal variability. The mean annual flow varies from 700 to 2,500 m 3 /s during snowmelt and monsoon season, and drops to 100-150 m 3 /s during winter months, considering that the mean annual inflow from inter-basin transfers accounts for 40 per cent. During the winter season, all the water inflow is required for power generation. The average annual total runoff of Sutlej river is about 16,000 million m 3. Sediment issues The Sutlej river is known for its high sediment loads, and the current mean annual suspended sediment load is in the order of 700,000 tonnes per year. However, the design sediment load was estimated at about 430,000 tonnes per year, due to a lack of historical records and monitoring in the catchment. In 2005, a flash flood caused by an artificial lake breach increased silt load up to over 25,000 ppm because of the destabilisation of river banks. Twenty days after the flood, the silt load was
5,000 ppm, which is the limit for power generation. As a result, the power plant had to be shut down for several weeks. After the first year of operation, all the technical equipment (runners, guide vanes, plates and sealing rings) were severely damaged, despite their plasma coating, due to the high sediment load passing through the turbines. In the monsoon season of 2004, the average monthly sediment load was 213,000 tonnes. The quartz content in the silt is on average 38 per cent, according to the historical data, but later observations have reported it at over 50 per cent quartz content. In 2005, the annual silt load that passed the turbines was 850,000 tonnes, although the design sediment load was estimated at around half of that figure - about 430,000 tonnes per year. Sediment management strategies In the second year of operation, all units were refurbished with HVOF type Tungston carbide coating and therefore the performance of the components improved by up to 90 per cent. In addition, the upper limit of 5,000 ppm at the intake was decreased to 3,500 ppm. For coarser particles above 0.2 mm, the upper limit was established at 600 ppm at the intake. Despite the HVOF coating, the limit for total silt load passing through the turbines was set to 700,000 tonnes. The limit for uncoated runners was 400,000 tonnes. The underground desilting complex was part of the original Nathpa Jhakri project. It was designed to remove particles above 0.2 mm up to 60 mm. Hydraulic model studies showed that the overall trapping efficiency of the prototype was about 40 per cent. In order to minimise silt entry into the intakes and maintain the live storage capacity, flushing operations were adopted. Sediment is flushed effectively every year through the low-level gates. The hydraulic model estimates that 2.5 Mm 3 of sediment can be flushed out in one day, with a discharge of 1,500 m 3 /s. The dams planned upstream of Nathpa Jhakri will trap sediment, thus reducing sediment inflow into Nathpa Jhakri. For example, the Khab dam will be 275 m high, with a gross storage of 625 million m 3, which will be able to store silt particles for about 28 years for a storage of 340 million m 3. Khab is mainly being constructed to deliver power generation, control floods, and to reduce silt damage and increase the lifespan of projects downstream. Watershed management to reduce bank erosion and a new diversion tunnel are also envisaged. The diversion tunnel could divert up to 1,000 million m 3 of excess water from about 750 m upstream of the downstream reach. These measures would reduce the number of flushing operations. Catchment treatment includes reforestation by the regional forestry department, retaining walls to prevent bank erosion, and checking dams on tributaries with the help of governmental regional entities (from the Himachal Pradesh region). Graphs and figures >
Graphs and figures Figure 1: Nathpa Jhakri hydroelectric project layout Figure 2: Nathpa Jhakri surroundings with fragile geology Graphs and figures cont. >
Figure 3: turbine damage at Nathpa Jhakri Figure 4: Nathpa Jhakri desilting complex This is part of a series of sediment management case studies collated by International Hydropower Association with support from the South Asia Water Initiative (SAWI), trust funds to the World Bank. www./sediment-management