Silt eating into storage capacity of State's dams

Accumulation of silt in major reservoirs over the years has started to severely affect water storage in the State.

Hydrological surveys reveal that seven reservoirs, including Tungabhadra and Almatti, have accumulated silt that has reduced the storage capacity of these dams by a staggering 49.1 tmc ft.

To get a feel of the quantum of water that goes into one tmc ft (tmc ft is an abbreviation for one thousand million cubic feet, a unit of measure used to report volumes of water), imagine a huge outlet that is pouring out 33,000 litres of water per second.

Keep this outlet open for 24 hours and the total water collected will amount to one tmc ft. That’s indeed a lot of water. The water supplied by Bangalore Water Supply and Sewerage Board to Bangalore City for the entire year amounts to 12 tmc ft.

Tungabhadra dam has been the worst hit from the accumulation of silt. According to hydrological survey conducted by the Water Resources Development Organisation, the loss in storage capacity of Tungabhadra dam (constructed in 1953) is as high as 28 tmc ft, Narayanapura dam, constructed in 1982, has already accumulated silt that has resulted in loss of storage capacity by 10 tmc ft, reducing the original dam capacity by 27 per cent.

What is alarming is the high rate of siltation witnessed during recent years. In Tungabhadra for instance, the loss of capacity between 2004 and 2008 is 0.871 tmc/year which is two times the assumed rate of siltation (0.427 tmc ft/year). Loss of forest cover in catchment area, erosion due to mining, industrialisation and urbanisation have resulted in the siltation rate increasing over the years.

The government-appointed ‘Task Force for Quality Assurance in Public Construction’ has now come forward with a proposal to desilt the reservoirs on a long-term basis.

“We have proposed a long-term, but economically viable solution to the problem. It is observed that in any reservoir there is very high sedimentation at the river confluence and moderate sedimentation at the periphery of the reservoir. The idea, simply put, is to remove the silt from the interior of the reservoir and dump it scientifically and in methodical stages at the boundaries,” Task Force member secretary I Ravindranath told Deccan Herald.

Ravindranath, an engineer himself and who has been working on the idea for several years, said the proposal is technically viable and has been approved by experts. “We have also interacted with Chief Minister D V Sadananda Gowda, who has given us a positive response,” he said.

The silt removed can be used by farmers as it is rich in soil nutrients and fertile for agriculture. The Task Force has proposed to take up Doopadal, downstream of Hidakal dam, as pilot project.

“The reservoir has 85 per cent silt accumulation. We are in talks with the Cauvery Niravari Nigam (CNN) to take up the project at the earliest,” Ravindranath said. 

Construction of barrages across rivers, making water user charges mandatory and a legislation to manage catchment area are other suggestions mooted by the Task Force.