With the International Commission on Large Dams (ICOLD) observing that lifespan of large dams could go down by 50 per cent in tropical areas, serious doubts are being raised about the survival of the world’s longest earthen dam, the Hirakud, India. Built to survive over river Mahanadi for about a century, the first post-independence multi-crore project in India is now shrouded in uncertainty. Researchers have said, siltation will take its toll on the dam by 2020, 37 years earlier than its planned life. It was formally inaugurated on January 13, 1957 by the then Prime Minister Jawaharlal Nehru. Chief engineer and basin manager, upper Mahanadi basin, Burla Sudhakar Patri allays the apprehensions but admits that the utility of the dam will be reduced in flood control and power generation in non-monsoon months. The reservoir irrigates 1,59,100 hectares in kharif and 1,08,385 hectares in rabi besides generating 956 million units of power annually.
A remote sensing survey conducted by Central Water Commission (CWC) in 1995 and another by the dam authorities in 1996-97 over water holding capacity reveals that the total water storage capacity of the dam has come down by 27.25 per cent due to siltation. Even a state government study reveals that every year 12,000 acre feet silt gets deposited. Of the 53-km upstream spread of the reservoir, 50 km has been silted up.
Maintaining that there has been 17 percent reduction in the storage capacity of the dam, Patri said siltation in the dam has grown 2.5 times faster and hoped it would come down if Chhattisgarh went ahead with its plan of building dams upstream. Patri said ‘‘trap efficiency’’ was responsible for faster sedimentation. The original report had suggested desilting during the rainy season and maintaining reservoir level at 590 feet. ‘‘But that will mean no irrigation and power. No government can afford it and hence we keep it at 630 feet till September,’’ he added.
Prof Artabandhu Mishra of School of Life Sciences, Sambalpur University attributes the siltation to massive deforestation in the catchment areas and lack of soil conservation measures. But silt deposit being part of any dam, the sedimentation capacity of the dead storage was planned for its life span of 100 years at 2,262 cubic metres which is contrary to remote sensing survey that puts the total amount of silt deposit at 2,209 million cubic metres. With the state government cash-strapped, desiltation at more than Rs 12,000 crore, seems a distant dream.