|Water being released from Hirakud reservoir. Telegraph picture
Bhubaneswar, Aug. 17: As thousands of flood-hit people cry out for help, the focus has turned once more on the 4.8-km dam-cum-reservoir over the Mahanadi river at Hirakud near Sambalpur.
Reckoned among the biggest earthen dams of the world, Hirakud is a victim of heavy siltation, which has taken a toll on its storage capacity raising questions about its efficacy during floods since it receives heavy inflow from the Mahanadi’s upper catchment area, the bulk of which falls in Chhattisgarh.
There is no formal water-sharing arrangement between Odisha and Chhattisgarh on the Mahanadi. Since the two states depend on sharing of information about rainfall and discharge from their respective dams and reservoirs over the river, Hirakud’s reduced storage capacity could make it suspect in the event of any miscommunication or lack of it between the two states.
This is what happened in September 2011 when Odisha witnessed one of its worst floods in the Mahanadi system following a rainfall of 201.97mm in the river’s upper catchment area, lying mostly in Chhattisgarh, for four days. To make things worse, the Bango dam over the Hansdeo, one of the Mahanadi tributaries in Chhattisgarh, released huge amount of water without any warning forcing Hirakud dam authorities to make heavy discharge causing floods in the downstream areas.
Former chief engineer and manager (Hirakud basin) Sudhakar Patri admitted that siltation was a problem. He said the live storage capacity of the reservoir commissioned in 1957 came down from 4.72 million acre feet (MAF) to 3.91MAF due to this factor and little could be done about it.
“Dredging is not possible. The best one can do to slow down the process of siltation is to undertake plantation and soil conservation measures in the upper catchment area of the river,” he said.
Hirakud’s superintending engineer Rabindra Kumar Panda, though not alarmed by siltation, admits that desiltation is a stupendous task. “Identifying the heavy siltation areas itself is a difficult job. Besides, no amount of dredging can restore the reservoir’s original storage capacity,” he said, recalling that a small attempt was made in 2008 to desilt the Sason canal of the reservoir. The state government had sanctioned Rs 32 crore for the project of which Rs 20 crore was spent. But, never ever has an attempt been made to desilt the reservoir itself.
With regard to discharge from the Chhattisgarh side, experts agree that there has to be proper exchange of information for effective flood control — which would avoid disasters such as the one that happened in 2011. “Around 26 million acre feet of water flows into the reservoir annually from the upper catchment area, whereas its gross storage capacity is around 4.8 million acre feet. So even if the Chhattisgarh government builds a few more small dams or reservoirs on the Mahanadi, it should not be a problem. But, flood management and exchange of information on both sides has to be proper,” said Patri.
However, upper Mahanadi basin chief engineer Biswajit Mohanty feels that Chhattisgarh, which already has five dams and reservoirs on the Mahanadi and its tributaries, including the Ravi Shankar Sagar project near Raipur, would find it hard to build another.
“The cumulative capacity of their existing dams and reservoirs on the river and its tributaries is less than Hirakud. But, the topography of the state will not permit them to construct another,” he said, adding that rainfall and flood information sharing arrangement between the two states was working fine.
Since there is about 58,000sqkm of uncontrolled catchment area of the Mahanadi below the Hirakud reservoir where its tributaries Ong and Tel often cause havoc, there is a debate over whether there should be more dams or reservoirs on the river downstream. Sources said A.N. Khosla, former chairman of the Central Water Commission, who was also the governor of Odisha, in 1945 recommended unified development of the Mahanadi basin through storage dams at Hirakud, Tikarpara and Naraj to mitigate floods in the river delta.
However, except Hirakud, others did not come up for the fear of large-scale displacement and submergence. Today, Odisha has a barrage at Naraj near Cuttack to prevent flooding.
While Patri argued in favour of dams and reservoirs downstream on the Mahandi even at the cost of displacement and submergence, asserting that this was the only solution to resolve the perennial flood problem in the river, Panda shot down the idea.
“A reservoir at Tikarpara, for example, would not only cause submergence at Athmallick and Boudh, but also affect Sonepur. Similar damage can also be expected from such ventures at Naraj and Manibhadra, the latter being in Nayagarh district upwards of Tikarpara that falls in Angul. That would be too heavy a price to pay,” he said.
A much better idea, according to Panda, would be building small storage facilities on Mahanadi tributaries such as Tel and Ong.
“Of the 20 floods in the Mahanadi system, nine have been caused by these two rivers, eight by the Mahanadi and them together and only three by the bigger river on its own. So, building manageable storage facilities over these tributaries and managing them through proper communication between the officials controlling them makes much more sense,” said Panda.
Mohanty, too, agreed that reining in the Mahanadi tributaries had become imperative.