DESIGN OF DESILTING BASIN. FOR A HYDRO-ELECTRIC PROJECT ON A HIMALAYAN RIVER

Abstract

Streams in hilly areas, particularly in the Himalayas, carry huge amount of sediment as, suspended load and bed load. The problem of sediment control therefore involves two approaches. First is to prevent the entry of sediments into the primary water conductor system by suitable preventive measures. The second approach is to eject the sediment after it enters the canal or tunnel and before it reaches the turbine. In any case, a part of the suspended sediment load however enters the power channel / tunnel even after several preventive measures are taken at the head works to prevent sediment entry into the canal. These sediments not only reduce the conveyance capacity of the channel but also damage turbine runners, valves, and its other components. As a result, maintenance costs as well as loss of power generation increases after a few years of powerhouse operation. There exist various types of desilting basins or desedimentation chambers for sediment removal from the power channel before its entering into the powerhouse, such as underground desilting basin, hopper type desilting basin, surface desilting basin, and so on. In this study, a surface desilting basin is proposed upstream of the barrage axis of Rongni Chu tributary of Teesta river in Sikkim (India). Notably, surface type desilting basins have been rarely constructed in India. It is however provided in the Rongni Chu project because river upstream of the barrage axis takes a convex bend towards the right bank leading to the formation of a terrace of about 100 m x 30 m x 40 m on the left bank of the upstream of the proposed barrage axis and this terraced space is found adequate for construction of a desilting basin. This (proposed) scheme is to operate as a run-of-the-river scheme providing a daily maximum of 6 hours of firm peak energy during the non-monsoon season and during the monsoon season. It is to operate continuously for several weeks. Its performance for the design of collection chamber and height of the barrage is evaluated through a model study carried out at Irrigation Research Institute, Roorkee, and found to be generally satisfactory.