If the silt accumulated in the Tansa and Modak Sagar lakes is removed, it will increase water stocks by 18,000 million litres (ML) in both the lakes, an additional 12 days of supply without the current 15 per cent cut.
This is part of the findings of a report by Nashik-based Maharashtra Engineering Research Institute (MERI), which surveyed silt levels in these lakes for a year.
The report says 10 per cent of the total capacity of each of the two lakes is silt — by volume, that amounts to 32 days of water drawn every day from Tansa, or 550 million litres daily (MLD), and 30 days of water drawn every day from Modak Sagar, which is 610 MLD.
The survey was carried out over a year, with a budget of Rs 5 crore.
“This is the first survey to estimate the amount of silt that has deposited over time,” said V. Deshpande, chief hydraulic engineer.
Considering the existing water crisis, the Brihanmumbai Municipal Corporation is taking steps to increase the capacity of these two lakes.
“Siltation is a common phenomenon that any dam or reservoir goes through. Because of the continuous accumulation of sediment in the dams, its capacity has shrunk,” said Additional Municipal Commissioner Anil Diggikar.
“We will take all possible steps to restore the capacity of the lakes and tap more water from them,” he added.
Tansa dam is over 100 years old, Modak Sagar dam is 54.
“The BMC should take immediately steps to desilt the lakes — if done in time, it will help increase their water accommodation capacity,” said a MERI source.
Deshpande added that the BMC will have another meeting with MERI, to find out the exact locations where the silt has accumulated, and learn how it can remove the silt without disturbing the water.
“If the silt is heavily disturbed while removing it, the turbidity of the water could increase — it will then become very difficult to purify the water, which is why we need to complete the desilting without disturbing the water too much,” Deshpande said.
A separate survey of the Vihar and Tulsi lakes is also underway. Their reports are expected soon.