Chennai's reservoirs not desilted in over a decade

A PWD official however assures TNM that the work will be carried out before the North East monsoons.
Chennai's reservoirs not desilted in over a decade
Chennai's reservoirs not desilted in over a decade

Storage levels of Chennai's four reservoirs stands at 0.19% of their capacity as of Friday. Ground water levels have dropped drastically by over 300 feet this summer. Borewell installation charges have increased by 150% as demand grows.

These alarming issues are merely the tip of the iceberg for Tamil Nadu's capital city, which is reeling under an acute water crisis. And yet the state government, which remains in denial about the scarcity, allegedly failed to even prepare for the possibility of a drought this summer. Sources in the Chennai Metropolitan Water Supply and Sewerage Board (CMWSSB) reveal to TNM that the city's main reservoirs have not been desilted in over 10 years. Further, according to an RTI reply from December 2018 in connection to the city's water bodies, completion of desilting would have resulted in an increase of surface water by close to 2 tmcft.

Desilting is the process of removing deposited silt from the beds of water bodies. Silt is defined as a sedimentary material consisting of grains or particles of disintegrated rock, smaller than sand and larger than clay. According to experts, desilting will help recharge the ground water table, increase capacity of water bodies and reduces incidences of flood.

According to a senior officer in CMWSSB, the last time desilting work was carried out was in 2008. And even then only two of the four reservoirs were desilted.

"Back then, Puzhal Lake's capacity was increased from 2800-3100 mcft and desilted mud was deposited on the bank to increase the level. After that, no action was taken," he says. "Desilting can be done when the reservoir is dry so this would mean only once in three-four years. But it is important for the health of the water body to do it regularly. Every monsoon, at least 10 cm of silt gets deposited in a reservoir," he adds.

The RTI reply from December admits that the water capacity of the city's main reservoirs was being affected due to silt. It further added that desilting works were being planned for Poondi at a cost of Rs.13.80 crore, for Chembarambakkam at Rs.6.55 crore, for Puzhal at Rs 9.90 crore and Cholavaram at Rs.5.43 crore. A total of Rs.35.68 crore was to be spent on the project and this would have increased the total capacity of reservoirs by 1.904 tmcft. The earth that is removed from the reservoirs would have earned the government Rs.647.01 crore.

Multiple media reports in March and April stated that government orders were given to begin desiltation work. However, the work was not completed as planned, says CMWSSB.

A source in the Public Works department told TNM that it would take three more months to finish the desiltation work.

"We have already strengthened the bunds around the reservoirs. Now, desiltation work will begin and will be completed in three months, before the next monsoon,” he claims.

The CMWSSB official however pointed out that there are challenges involved in desilting. He also contradicted the RTI reply given by the government with regard to the purchase of earth removed from the reservoirs.

"Multiple departments have to cooperate with Public Works to get this done,” says the Metrowater official. “And most importantly, there is no demand for the earth that we remove. If we use it to create bunds, those who have encroached areas near the reservoirs will remove it.

In addition to this Metrowater officials fear the percolation of water into the ground after desilting, which will reduce the levels in the reservoir.

Hydrogeologist J Saravanan, however, points out that even if there is percolation, it will only recharge the ground water supply.

"There will definitely be a great amount of leakage based on the soil condition," he tells TNM. "But it will stabilise in two-three years and will ultimately help wells and borewells around the area."

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