Silt eats into storage capacity of KRS, Kabini, Harangi Dams

If silt is cleared from three Reservoirs, storage capacity can be increased by up to 8 tmcft

By Bapu Lingaraj Urs

Mysore/Mysuru: The live storage capacity of Krishna Raja Sagar (KRS), Harangi and Kabini Reservoirs along the Cauvery Basin is decreasing due to siltation. Surprisingly, successive Governments have not taken any action to address this issue, despite the alarming rate at which silt is accumulating.

Numerous studies have been conducted over the years to assess silt deposition in these Dams, but no practical solutions have been implemented on the ground. While Governments allocate substantial funds for various irrigation and water resource projects, they have yet to allocate even a fraction of that amount to de-silt these reservoirs.

Reservoirs are essential for storing water for use during non-monsoon months. However, these reservoirs, created by damming rivers, also accumulate silt from the river water. A significant portion of this silt settles within the reservoir, reducing its storage capacity.

Studies have shown that silt accumulates both in the dead storage (the area below the Minimum Draw Down Level, not normally used) and the live storage. This process of silt accumulation is known as siltation, and it significantly diminishes the benefits of these expensive Dams. Siltation can also lead to increased evaporation losses, backwater flooding and damage to power house turbines.

2.34 tmcft of silt in KRS Dam

For instance, the KRS Dam, with a capacity of 49.45 tmcft (thousand million cubic feet) of water, has accumulated silt across an area of 131 square kilometres since its completion in 1932, without any silt removal efforts.

To assess the extent of siltation in the KRS Dam, the Karnataka Engineering Research Station (KERS) conducted an Integrated Bathymetric Survey (IBS), which collects data on reservoir depth and bottom topology. The survey revealed that approximately 2.34 tmcft of silt has accumulated in the Dam.

Interestingly, the silt accumulation in the KRS Dam is relatively lower than in Dams like Tungabhadra and Almatti, attributed to the rocky terrain in the upstream region of the Cauvery river, which reduces silt transport.

However, it’s worth noting that the silt in the KRS Dam also includes rocky debris left to act as wave-breakers for safety. In the case of Kabini Dam, located at Beechanahalli in H.D. Kote, silt accumulation is estimated at 1.35 tmcft, which is lower due to the forested and mountainous areas from which water flows.

Harangi Dam

In the Harangi Dam in Kodagu, which is the primary Cauvery water supply reservoir to the KRS Dam, over 1 tmcft of silt has accumulated, with an increase after natural disasters in 2018.

The KERS report stating this accumulation prompted the Cauvery Neeravari Nigam Limited (CNNL) to prepare a Detailed Project Report (DPR) worth Rs. 130 crore for silt removal, check Dam construction, retaining walls and other development work. However, the State Government has not taken any action thus far.

Clearing the silt deposits at KRS, Kabini and Harangi Dams could increase storage capacity by up to 8 tmcft, benefiting both water storage and agriculture.

Scientists and geologists suggest removing silt from the interior of the reservoirs and disposing of it scientifically and methodically at the boundaries, presenting an economically viable solution.