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Rangit Dam
Rangit Dam forms the headworks of the Rangit Hydroelectric Power Project Stage III, is a run-of-the-river hydroelectric power project on the Ranjit River, a major tributary of the Teesta River in the South Sikkim district of the Northeastern Indian state of Sikkim. The project's construction was completed in 1999. The project is became functional in 2000. The project was built at a cost of Rs 4922.6 million (Rs 492.26 crores) (at US$1 = Rs 45, this is US$109.39 million). The average annual power generation from the 60 MW (3x20 MW) project is 340 GWh with firm power of 39 MW.
The Ranjit River, on which the Rangit Dam is located, is a major right-bank tributary of the Teesta River in Sikkim. The river arises from the Talung glacier, and it meets the Teesta river at Melli after a travel of 61 kilometres (38 mi) from its source. At the dam site, the catchment area drained is 979.02 square kilometres (378.00 mi2) (rain-fed catchment is 712 square kilometres (275 mi2) and the balance area is snow-fed, above 4,570 metres (14,990 ft)); the elevation of the catchment area varies from about 600 metres (2,000 ft) to about 7,338 metres (24,075 ft) at North Kabru Peak and is delimited between 27°16′30″N 88°00′51″E / 27.275°N 88.0141°E and 27°37′10″N 88°25′12″E / 27.6195°N 88.42°E. A number of perennial streams originate in glacial fields of the river basin; important snow-fed rivers which constitute the Rangit basin above the dam site are the Rathong Chu, Rimbi Chu, Prek Chu, Ralli Chu, Rongdon Chu, and Kayam Chu. The drainage pattern is sub-dendentric.
The dam is located at a distance of 130 kilometres (81 mi) from Siliguri and 70 kilometres (43 mi) from Gangtok. The dam is downstream of the confluence of Rathong Chu and Rangit Rivers near the Legship town, and the powerhouse of the project is near Sagbari village.
The annual inflow in the river at the location of the dam has been estimated as 696000 m3. The maximum flood discharge has been adopted as 3,395 m3/s (119,900 cu ft/s), while the design flood discharge adopted for the spillway is 2,725 m3/s (96,200 cu ft/s). The dependable discharge adopted for diversion from the reservoir for power generation is 1,717.8 m3/s (60,660 cu ft/s) (without considering contribution from the upstream Stage II project, which is yet to be implemented).
The climate of the Rangit River basin is cold and humid. The climatic seasons of the basin represented in the project area are: spring season–late February, summer season–March, premonsoon showers–April and May; monsoon season–May to September, sometimes extending to October. Snow, at higher elevations of the catchment, falls between December and February. Winters are very cold with mist and fog lasting from November to February.
Precambrian formations of the Daling series of quartzites and phyllites dominate the area. This rock type is overlain by crystalline Darjeeling Gneiss comprising gneisses and granitoides. Recent alluvium of sandy loam, silty loam, and clayey material of varying thickness overlay the rock formations. The banks of the Rangit River have silty clay material with large rock blocks. Many landslides are observed in the catchment, which add to the siltation problems of the reservoir.
The Rangit Dam is a 45-metre-high (148 ft) concrete gravity structure, 100 metres (330 ft) long. The reservoir created behind the dam has a storage capacity of 1.175×106 m3. The storage created is used for hydropower generation at a surface powerhouse on the left bank of the Rangit River. The diversion of flow from the reservoir to the surface powerhouse is effected through an intake leading to a concrete-lined headrace tunnel (HRT) of 4.5 metres (15 ft) diameter (horseshoe-shaped and concrete-lined) of 3 kilometres (1.9 mi) length, a surge shaft (14 m × 60 m; 46 ft × 197 ft) at the end of the HRT with control arrangement followed by one main penstock pipe (3.5 m × 270 m; 11 ft × 886 ft) trifurcating into three lines of 2 metres (6.6 ft) diameter each to connect to the three Francis turbine generators of 20 MW capacity each, through the MIVs. The tailwaters from the turbines are fed back into the river through a combined short tailrace channel. The firm power generation is on the order of 39 MW, corresponding to annual energy generation of 340 GWh (in a 90% dependable year).
The ruling levels for power generation are:
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Rangit Dam
Rangit Dam forms the headworks of the Rangit Hydroelectric Power Project Stage III, is a run-of-the-river hydroelectric power project on the Ranjit River, a major tributary of the Teesta River in the South Sikkim district of the Northeastern Indian state of Sikkim. The project's construction was completed in 1999. The project is became functional in 2000. The project was built at a cost of Rs 4922.6 million (Rs 492.26 crores) (at US$1 = Rs 45, this is US$109.39 million). The average annual power generation from the 60 MW (3x20 MW) project is 340 GWh with firm power of 39 MW.
The Ranjit River, on which the Rangit Dam is located, is a major right-bank tributary of the Teesta River in Sikkim. The river arises from the Talung glacier, and it meets the Teesta river at Melli after a travel of 61 kilometres (38 mi) from its source. At the dam site, the catchment area drained is 979.02 square kilometres (378.00 mi2) (rain-fed catchment is 712 square kilometres (275 mi2) and the balance area is snow-fed, above 4,570 metres (14,990 ft)); the elevation of the catchment area varies from about 600 metres (2,000 ft) to about 7,338 metres (24,075 ft) at North Kabru Peak and is delimited between 27°16′30″N 88°00′51″E / 27.275°N 88.0141°E and 27°37′10″N 88°25′12″E / 27.6195°N 88.42°E. A number of perennial streams originate in glacial fields of the river basin; important snow-fed rivers which constitute the Rangit basin above the dam site are the Rathong Chu, Rimbi Chu, Prek Chu, Ralli Chu, Rongdon Chu, and Kayam Chu. The drainage pattern is sub-dendentric.
The dam is located at a distance of 130 kilometres (81 mi) from Siliguri and 70 kilometres (43 mi) from Gangtok. The dam is downstream of the confluence of Rathong Chu and Rangit Rivers near the Legship town, and the powerhouse of the project is near Sagbari village.
The annual inflow in the river at the location of the dam has been estimated as 696000 m3. The maximum flood discharge has been adopted as 3,395 m3/s (119,900 cu ft/s), while the design flood discharge adopted for the spillway is 2,725 m3/s (96,200 cu ft/s). The dependable discharge adopted for diversion from the reservoir for power generation is 1,717.8 m3/s (60,660 cu ft/s) (without considering contribution from the upstream Stage II project, which is yet to be implemented).
The climate of the Rangit River basin is cold and humid. The climatic seasons of the basin represented in the project area are: spring season–late February, summer season–March, premonsoon showers–April and May; monsoon season–May to September, sometimes extending to October. Snow, at higher elevations of the catchment, falls between December and February. Winters are very cold with mist and fog lasting from November to February.
Precambrian formations of the Daling series of quartzites and phyllites dominate the area. This rock type is overlain by crystalline Darjeeling Gneiss comprising gneisses and granitoides. Recent alluvium of sandy loam, silty loam, and clayey material of varying thickness overlay the rock formations. The banks of the Rangit River have silty clay material with large rock blocks. Many landslides are observed in the catchment, which add to the siltation problems of the reservoir.
The Rangit Dam is a 45-metre-high (148 ft) concrete gravity structure, 100 metres (330 ft) long. The reservoir created behind the dam has a storage capacity of 1.175×106 m3. The storage created is used for hydropower generation at a surface powerhouse on the left bank of the Rangit River. The diversion of flow from the reservoir to the surface powerhouse is effected through an intake leading to a concrete-lined headrace tunnel (HRT) of 4.5 metres (15 ft) diameter (horseshoe-shaped and concrete-lined) of 3 kilometres (1.9 mi) length, a surge shaft (14 m × 60 m; 46 ft × 197 ft) at the end of the HRT with control arrangement followed by one main penstock pipe (3.5 m × 270 m; 11 ft × 886 ft) trifurcating into three lines of 2 metres (6.6 ft) diameter each to connect to the three Francis turbine generators of 20 MW capacity each, through the MIVs. The tailwaters from the turbines are fed back into the river through a combined short tailrace channel. The firm power generation is on the order of 39 MW, corresponding to annual energy generation of 340 GWh (in a 90% dependable year).
The ruling levels for power generation are: