Rangit Dam
Rangit Dam, which 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 . The average annual power generation from the 60 MW project is 340 GWh with firm power of 39 MW.
Topography
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 from its source. At the dam site, the catchment area drained is ; the elevation of the catchment area varies from about to about at North Kabru Peak and is delimited between and. 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 from Siliguri and 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.
Hydrology
The annual inflow in the river at the location of the dam has been estimated as. The maximum flood discharge has been adopted as, while the design flood discharge adopted for the spillway is. The dependable discharge adopted for diversion from the reservoir for power generation is .Climate
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.Geology
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.Project features
The Rangit Dam is a concrete gravity structure, long. The reservoir created behind the dam has a storage capacity of. 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 of diameter of length, a surge shaft at the end of the HRT with control arrangement followed by one main penstock pipe trifurcating into three lines of 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.The ruling levels for power generation are:
- Full reservoir level :
- Minimum draw-down level:
- Normal tail-water level in the tailrace channel from the powerhouse:
- Operating gross head:
The project was funded by the Government of India and built by the National Hydroelectric Power Corporation. The construction of the project was completed in December 1999, and operation started in January 2000. Operation and maintenance of the project is also with the NHPC.
This power project was the third stage of the five-stage cascade development conceived on the main stem of the Rangit River, and was the first to be built in the series of Rangit Stage I to IV initially conceived by the Central Water Commission. Three other projects on the Rangit River planned and under development are the Rangit Stage II, Rangit Stage IV, and Jorethong HEP ; the last two projects are now under construction.
Environmental aspects
In river-valley reservoir projects, the gravity of the siltation problem induced by to catchment degradation is serious and needs to be suitably addressed. For this purpose, the Ministry of Environment and Forests of the Government of India have made it obligatory for the project authorities to implement physical engineering and biological measures in the catchment area of the project to be taken up concurrently with the implementation of the project. This activity involves several works defined under the Catchment Area Treatment plan.For evolving the CAT plan, the status of the reservoir catchment was analysed. The reservoir catchment consists of five types of forests namely, the East Himalayan Sub-Tropical wet hill forests, East Himalayan wet temperate forests, oak and rhododendron forests, mixed coniferous forests, and alpine scrubs/pastures. There are 35 reserve forests in the catchment, out of which 29 are in West Sikkim district and six are in South Sikkim district. The entire catchment area was analysed in detail to assess the degraded areas to be treated under the CAT plan to reduce siltation problems. An area of was identified for implementing engineering and biological treatment measures. These measures were implemented, starting in 1995–96, by the Forest Department of the Government of Sikkim involving engineering treatment measures and biological treatment measures. 15 nurseries were established covering an area of to provide saplings/seedlings for plantation in the sub-watersheds of the catchment identified for treatment.