16,000 truckloads of silt to be removed from Hesaraghatta dam

BENGALURU: Revival of Hesaraghatta reservoir, located on the city’s outskirts, will be a mammoth exercise — it involves removing around 16,000 truckloads of silt, according to the detailed project report (DPR).
The government is all set to give a green signal to the project, which also entails the revamp of Thippagondanahalli (TG Halli) dam. The two reservoirs are vital for augmenting drinking water supply to Bengaluru city by getting water from the Yettinahole diversion project.
TOI has accessed a copy of the DPR, which has been prepared by Tata Consulting Engineers Limited and submitted to the Bangalore Water Supply and Sewerage Board (BWSSB).It will be placed before the cabinet for approval in the coming weeks. The total project cost has been pegged at Rs 393 crore and the time frame for its completion has been estimated at 50 months.
The project envisages enhancement of storage volume at Hesaraghatta dam, which can be achieved by removal of excess silt via open dredging. Around 102 million cubic feet, which translates into 16,000 truckloads, will be removed through dredging. In fact, water will flow from Hesaraghatta Lake to TG Halli reservoir before being pumped to Bengaluru.
The DPR says: “Hesaraghatta reservoir is completely dried up with no water, the storage area is filled with silt and trees are growing over the reservoir area. The last time the reservoir was filled with water was in 1994.” Ditto with TG Halli, where the water is considered too polluted for consumption.
KC Subhash Chandra, hydro-geologist and former member of the expert panel in the Karnataka Groundwater Authority, said sediment settlement was a very big problem in catchment areas.“Silt from the catchment area consists of laterite (soil), which comprises a large component of clay. This is what gives the silt an impervious texture, preventing the percolation of water below the ground and preventing groundwater recharge. Desilting must be undertaken periodically,” he said.
Reviving the two reservoirs will ensure that Bengaluru will get an additional supply of 2.5 tmcft of water per year. The DPR also suggests fencing the reservoir area, strengthening earthen bunds, replacing old pipelines, decommissioning the valve tower at Hesaraghatta, revamping the 124-year-old Soladevanahalli pump house complex, repairing sluice gates and upgrading pumping stations.
Former BWSSB chief engineer PN Ravindra said: “The Hesaraghatta bund has to be repaired as it is over 200 years old. Additionally, the quality of water at TG Halli is very poor and loaded with benthic sediment, which is deposition of organic material over time. This will also contaminate any fresh water entering the reservoir, which is why it must be cleaned through dredging. By 2020, atleast 100 mld can be pumped to the city using the existing pumping stations,” he said, adding that more alum and chemicals would be added to enhance the purification.
Box: Tech to cleanse water
To ensure the water is suitable for drinking , the DPR has suggested deployment of special filtration technologies. Ozone with granular activated carbon (dual media filters) will be used to treat raw water. “A 30mld-capacity plant will come up at Hesaraghatta and one with 170 mld capacity will treat water at TG Halli,” said Ravindra.
‘Existing infra outdated’
According to Tushar Girinath, BWSSB chairman, the unexpected growth of the city has rendered the existing infrastructure at the two reservoirs obsolete. “We need water treatment plants, local distribution networks, new tanks and pumping machinery. The scale of the rejuvenation effort is akin to taking up a new pumping project altogether,” he said.