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Nathpa Jhakri hydropower plant suffered its first sediment problems a year after commissioning, in 2004. The high soil erosion during the snowmelt in the Himalayas results in large sediment loads, in particular quartz particles that can severely damage the plant’s technical equipment. Protecting the hydraulic machinery from abrasion and preserving the reservoir storage in the future called for effective sediment management strategies.

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At 1,500 MW capacity, Nathpa Jhakri is the largest hydropower project on the Sutlej River in the state of Himachal Pradesh in northern India. The Sutlej River rises in the Tibetan Plateau at an elevation of about 4,570 masl, meeting the Chenab River to form the Panjnad River before the confluence with the Indus River.

The project is owned by the SJVN Ltd, a joint venture between the government of India and the government of Himachal Pradesh. The government of India envisages a total of around 30 projects on the Sutlej River.

The USD 2 billion Nathpa Jhakri project includes a 62.5 m-high concrete gravity dam, a large underground desilting complex and a hydropower plant with an installed capacity of 1,500 MW.

The water flow is diverted through the intake structure, composed of four intakes and 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 headrace tunnel of 27.4 km long conveys the water to the underground power station, where six 250 MW Francis turbines generate about 6,778 GWh annually. The water from the turbines is released back to the Sutlej River through the 982 m-long tailrace tunnel. The scheme is shown in figure 1.

The 2.5 km-long reservoir is located in a very narrow valley, and 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.

Hydrology and sediment

The Sutlej River basin represents a major part of the Indus River basin. It is considered a perennial river because the upper catchment contains a large glacierised area. The catchment has a high soil erosion rate as a result of its fragile geology and a steep topography, as shown in figure 2.

At the location of the Nathpa Jhakri project, more than 40 per cent of the suspended sediment load has been observed as corresponding to fine fraction sediment, while only about 20 per cent corresponds to coarse fraction sediment, rising to 33 per cent during floods. Often, landslides heavily increase the sediment concentration in the river flow.

The total catchment tributary to the Nathpa Jhakri dam is 49,820 km2, of which 74 per cent is unregulated area. The river flow is comprised of glacier melt, snowmelt (estimated to be about 50 per cent of the runoff) and rainfall, and is subject to a high hydrological seasonal variability. The mean annual flow varies from 700 to 2,500 m3/s during snowmelt and the monsoon season, and drops to 100-150 m3/s during the winter months, with the mean annual inflow from inter-basin transfers accounting for 40 per cent. During the winter season, all the water inflow is required for power generation. The average total annual runoff of the Sutlej River is about 16,000 Mm3.

Sediment problems

The Sutlej River is known for its high sediment loads, and the current mean annual suspended sediment load is on the order of 700,000 tons per year. However, the design sediment load was estimated at about 430,000 tons per year due to a lack of historical records and monitoring in the catchment.

In the second year of operation, all units were refurbished with HVOF-type Tungsten carbide coating and the performance of the components improved by up to 90 per cent."

In 2005, a flash flood caused by an artificial lake breach increased silt load to over 25,000 ppm because of 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 of the technical equipment (runners, guide vanes, plates and sealing rings) were severely damaged despite their plasma coating, due to the high sediment load that passed through the turbines. Some of this damage is shown in figure 3. In the monsoon season in 2004, the average monthly sediment load was 213,000 tons. The quartz content in the silt is on average 38 per cent according to historical data, but later observations have reported over 50 per cent quartz content. In 2005, the annual silt load that passed through the turbines was 850,000 tons, although the design sediment load was estimated at around half of that, at 430,000 tons per year.

Sediment management strategies

In the second year of operation, all units were refurbished with HVOF-type Tungsten carbide coating and 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 tons. The limit for uncoated runners was 400,000 tons.

The underground desilting complex, shown in figure 3, was part of the original Nathpa Jhakri project. It was designed to remove particles above 0.2 mm and up to 60 mm. Hydraulic model studies showed that the overall trapping efficiency of the prototype was about 40 per cent. The inside of the desilting chamber number IV is shown in figure 4.

In order to minimise silt entry into the intakes and maintain the live storage capacity, flushing operations were adopted at the dam. Sediment is flushed effectively every year through the low-level gates at the dam. The hydraulic model estimates that 2.5 Mm3 of sediment can be flushed out in one day with a discharge of 1,500 m3/s.

The dams envisaged upstream of Nathpa Jhakri will trap sediment, therefore reducing the sediment inflow into Nathpa Jhakri. For example, the Khab dam will be 275 m high with a gross storage of 625 million m3 that will be able to store silt particles for about 28 years for a storage of 340 Mm3. Khab is mainly built for power generation, control flooding and to reduce silt damage and increase the lifespan of projects downstream.

Watershed management to reduce bank erosion and the creation of a new diversion tunnel are also envisaged. The diversion tunnel could divert up to 1000 Mm3 of excess water from about 750 metres upstream to the downstream reach. These measures would reduce the number of flushing operations needed. Catchment treatment includes reforestation by the regional forestry department, maintaining walls to prevent bank erosion and checking dams on tributaries with the help of government regional entities of the Himachal Pradesh region.

Graphs and figures

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