Large-scale deposits of silt caused by the recent floods have severely affected the storage capacity of many reservoirs, including the Idukki reservoir that has the highest dead storage level. Dead storage is for storing siltation, which is 26% of its full-storage level. Smaller dams have no dead storage level.
Dams in hilly areas with loose soil have more chances of high silt concentration in the reservoirs. Landslips added to the problems of Idukki as the desilting process had not been done there annually unlike in reservoirs located in hilly areas in developed countries.
A senior engineer of the Kerala State Electricity Board (KSEB) told The Hindu on Monday that there were high chances of silt even making some reservoirs unworthy and an immediate survey should be done on the silt level in reservoirs, including Idukki, which has the largest catchment area in the State.
A study conducted earlier in the catchment area of Idukki dividing it into eight zones showed that the Upputhara-Ayyappancoil stretch and Kulamavu area were highly prone to silt deposits due to large human habitations and loose soil.
No study in 10 years
“However, for nearly 10 years, no study on silting was undertaken,” the official said adding a behavioural study on the dam should be conducted in the wake of the flooding that resulted with the opening of the vents of the Cheruthoni dam.
In 1979, a panel appointed to look into the safety of dams had pointed out that the Idukki dam was not free from risk by way of silt and other factors if the Mullaperiyar dam opens its 13 vents or if there is a major inflow to the dam when it was full.
Two capacity surveys were held in the past — by RITES (Rail India Technical and Economic Services) using global positioning and National Remote Sensing Agency, Hyderabad, using satellite images on silt concentration.
Though an attempt to mine sands by a corporate agency was started sometime back, it did not materialise.
As per the directions of SNC Shawinglam, the Canadian consultant for the construction of the dam, silt prevention measures were to be taken up annually.
In developed countries, when a river is diverted to a main reservoir, filter dams to prevent silting are constructed. In the case of Idukki, no filter dams are in place though four rivers are diverted to the reservoir. This also causes more silting in the reservoir.