Irrigation experts are jittery about the condition of the Srisailam dam, which had to handle huge volumes of water in recent times.
The crest gates of Srisailam dam had to be lifted eight times in the last five and a half months to release flood water. Every time the gates are lifted, thousands of cusecs of water drops from a height of 250 feet to pound the apron. The scouring or erosion caused by the water was enormous.
In 2009, the dam, designed for a maximum flood of 19 lakh cusecs, endured a flood of 25.5 lakh cusecs.
According to a survey conducted last summer (2018), the scouring resulted in the formation of a huge pit in the apron downstream the dam. And a structure protecting the ‘toe’ and foundation of the main dam has weakened.
Repairs need to be taken up on a war-footing to prevent irreparable damage.
Former Irrigation Superintending Engineer K. Haranath told The Hindu that “protection works” and repairs should be undertaken every year, but in the case of the Srisailam dam no such effort has been made since 2003.
Engineers who undertook an inspection last year found the steel cylinders of the coffer dam that protected the toe and foundation of the main dam lay exposed, Mr. Haranath said.
No effort was made to replace the concrete apron after its erosion. The rock below the apron was relatively soft “quartzite interbedded with a shale band” that was getting scoured easily, he explained.
A cavity of 4,500 cubic metres (CuM) was found in blocks 8 to 11 of the apron in 2002, but only 143 CuM could be filled, Mr. Haranath said.
On October 3, 2009, the dam was subjected to maximum stresses for the first time after its commissioning in 1983-84. The dam tilted towards downstream by 4 mm and the tilt reached a maximum of 8.8 mm on October 8, 2009. Fortunately, it returned to normal after the flood receded.
With climate change, it was becoming harder to predict the intensity and interval between floods.
The National Institute of Oceanography, Goa and Visakhapatnam, conducted a detailed survey last summer, but the report is yet to be made public, Mr. Haranath said.
In the event of the Srisailam dam bursting, the Nagarjuna Sagar and Pulichintala dams, Prakasam Barrage and half of Vijayawada and Amaravati would be washed away, and Rayalaseema would become a desert, he added.