Guwahati: The Subansiri Lower Hydroelectric Project (SLHEP) is expected to be completed by March next year and will incorporate a flood cushion designed to help moderate normal flood flows.
At present, four of the project‘s eight generating units are operational. The fifth unit is scheduled to be commissioned in August, while the sixth is expected to begin operations within the following two months.
The reservoir has a total storage capacity of 1,365 million cubic metres. During the monsoon, the upper 15 metres of the reservoir will be kept vacant as a flood cushion, allowing it to temporarily store excess inflows during periods of normal rainfall and help regulate downstream water releases.
Officials said the Subansiri River can safely carry about 7,000 cubic metres of water per second. Water discharged from the reservoir during the rainy season will be managed keeping this limit in view to reduce the risk of flooding downstream.
They, however, clarified that the flood cushion is designed to handle normal flood conditions and cannot prevent flooding caused by exceptionally heavy rainfall. In such rare events, which historical records suggest occur roughly once every decade, downstream flooding may still take place despite regulated releases from the project.
During the winter, the reservoir will be maintained at full capacity as rainfall is minimal and inflows remain low. To optimise water use during the lean season, most generating units will remain shut during the day, allowing water to accumulate in the reservoir. Power generation will then be ramped up during evening peak demand hours, typically between 5 pm and 10 pm.
Responding to concerns over flooding caused by water released from the Ranganadi dam and Bhutan’s Kurichu dam, officials said both projects have comparatively small reservoirs with limited storage capacity. As a result, prolonged or intense rainfall quickly fills the reservoirs, forcing excess water to spill over the dams and increasing the risk of flooding in downstream areas.