Siltation in reservoirs deprives Karnataka of 10% storage capacity

The loss of storage because of accumulating sediments is seen primarily in reservoirs of north Karnataka; problem acute in Tungabhadra dam

In focus: The Central Water Commission’s compendium on silting of reservoirs in India in 2015 had shown that Tungabhadra dam is the second most affected dam in the country after Bhakra dam in Punjab.

Imagine a reservoir the size of the Hemavati dam in Hassan district completely filled with sand and mud and unable to retain water. With silt accumulating in the State’s 11 major reservoirs, nearly 103.95 tmcft (thousand metric cubic feet) of water or 10% of the storage capacity is being lost. This “lost storage” could annually cater to at least five cities as large as Bengaluru.

The loss of storage because of accumulating sediments — carried as the river flows — is seen primarily in reservoirs of north Karnataka. Basavasagar dam and Malaprabha dam across the Krishna have seen more than a fourth of their storage being eaten up by accumulating silt. Officials, however, said that as the dams have not been pushed to their design limit, the loss of storage was “yet to be felt”.

At the Tungabhadra dam, the situation is acute and manifests itself as overflowing water during periods of intense rains. On an average, nearly 17 days of overflow is observed.

The Central Water Commission’s compendium on silting of reservoirs in India in 2015 had shown that Tungabhadra dam was the second most affected dam in the country after the massive Bhakra dam in Punjab.

While the last survey of the silt in the dam was conducted in 2008, extrapolation by technical experts based on the high siltation levels in the TB dam (of around 0.5 tmcft a year) pegs the loss in storage at more than 36 tmcft over the past 65 years.

“The problem is acute because the catchment area of the Tungabhadra spreads across areas which are highly susceptible to soil erosion,” said Aravind Galagali, director of the Krishna Bhagya Jala Nigam Ltd. (KBJNL) who last year looked at alternatives to recover this lost storage. The technical report on the dam pursued various options to tackle the issue — dredging, increasing the height of the dam or of existing reservoirs, interlinking of rivers, and construction of a new reservoir at Navali in Koppal district.

Of these, Mr. Galagali said the reservoir at Navali was the best alternative. “The rest of it was too expensive or would lead into problems of acquisition and environmental clearances. This proposal has been put forth before the Tungabhadra Board (which comprises members from riparian States), but a final decision has to be taken on this,” he said.

The proposal involves riparian States sharing the cost on a pro-rata basis to construct the dam that will hold 30 tmcft of water.