Getting sedimental

Reservoirs lost 6% of storage capacity due to accumulated sediments shows latest survey

The four Cauvery basin reservoirs - Krishna Raja Sagara (KRS), Harangi, Hemavati, and Kabini - have lost nearly 6% of their total storage due to accumulated sediments between the first and most recent years of surveys.

While the cumulative gross capacity of these reservoirs was 3,244.26 Mcum during their first survey year, the last survey results show that it has now gone down to 3,027.59 Mcum.

Harangi has witnessed a loss of 23.57 million cubic metres (Mcum) in its gross capacity over 27 years. The total volume of Harangi was first surveyed in 1982, and it was estimated to be 240.69 Mcum. However, the most recent survey (2009) shows it has gone down to 217.12 Mcum. Similarly, Hemavati has lost 76.13 Mcum in 30 years, and Kabini’s gross capacity has dropped by 23.06 Mcum in 36 years.

The highest drop in volume (93.91 Mcum) is reported at KRS. While the total capacity of the reservoir was 1,400.31 Mcum in 1932, it dropped to 1306.4 Mcum in 2009.

These are some of the highlights of the capacity survey report by the Central Water Commission (CWC). Sedimentation has impacted the storage capacity of these reservoirs, affecting their performance. Damages to turbines and loss of hydropower production are some other probable impacts of sedimentation.

The volumes of these reservoirs were estimated using hydrographic survey and Satellite Remote Sensing (SRS) techniques.

Experts say estimation of dams’ capacity is an important exercise to note the rate of sedimentation and thus analyse the implication of annual loss of storage over a period of time.
It also helps in the proper estimation of loss of storage at the planning stage.

“Apart from the drop in storage capacity, sedimentation reports also show impact of soil conservation measures taken in the catchment area of the reservoirs, if any. Sediment accumulation in reservoirs is tricky as de-silting of dams is costly. The extracted silt has environmental impacts too. The only sustainable solution is to manage soil erosion in the catchment area,” said a hydrologist.