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BMC reaches 'new low' in Modak Sagar to solve water woes

Mumbai civic body, with help of engineers from the state water department, 'tapped' the Modak Sagar reservoir to further reduce the lowest drawable levels; a new inlet was created to ensure supply for nine more days

The Brihanmumbai Municipal Corporation became the first municipal body in the country to carry out a lake tapping experiment successfully.

Mayor Sunil Prabhu, Sena chief Uddhav Thackeray, civic chief Sitaram Kunte are present as the button is pressed to trigger dynamite blasts. Pics/Atul Kamble
Mayor Sunil Prabhu, Sena chief Uddhav Thackeray, civic chief Sitaram Kunte are present as the button is pressed to trigger dynamite blasts. Pics/Atul Kamble

Yesterday, the civic officials, with help from engineers in the state government’s water resources department, started operating a new outlet which can withdraw water from the Modak Sagar reservoir for nine more days than its current operating capacity.

The hexagonal tunnel shaft where the three inlets empty out water they get from the lake
The hexagonal tunnel shaft where the three inlets empty out water they get from the lake

In the tapping process, officials connected a third inlet to the reservoir that supplies water towards the pumping system. This inlet is at a further lower level inside the lake, ensuring that the lowest drawable level falls further and more water can be extracted from the lake.

As a result of the new inlet at a lower level from THD, more water can be extracted from the Modak Sagar lake, which is situated near Shahpur, Thane district
As a result of the new inlet at a lower level from THD, more water can be extracted from the Modak Sagar lake, which is situated near Shahpur, Thane district

The project was executed under the technical guidance of Dr Deepak Modak, chief engineer (civil) and Rajneesh Shukla, superintendent engineer, from the water resources department of the state government. The duo have been working extensively on the project for the past three months.

In 2012, tapping was done in the Koyna dam. “At that time, the state’s engineers were helped by consultants from Norway. This time, we haven’t taken any foreign consultant’s help. Tapping Modak Sagar dam was challenging,” said Modak.

How it’s done
The Modak Sagar reservoir already had two inlets the first at 156 metres from Town Hall datum (THD), a reference level used to determine levels of drainage, road and other civic amenities. The second is at a height of 146 metres THD, which was excavated this year on March 29. The new inlet is at 136 metres THD, the lowest of the three, thereby making a large volume of earlier unused water now available. Each inlet is 195 metres long.

Tapping involves blasting away the layer of protective rock between the lake and the inlet. The rock plug, as it is called, is drilled with reamer holes; its size and location is determined after a detailed geological study of rock mass below the lake. A ‘muck pit’ is created below the blast site, where the rock can fall in after it has been blown away. The quantity of explosives is just enough to remove the rock plug and create a hole to let the water flow in.

Around 1 pm yesterday, the button to detonate the 180 kg of dynamite was pressed and water rushed in with great force. Civic chief Sitaram Kunte, Mayor Sunil Prabhu, Shiv Sena chief Uddhav Thackeray were present for the occasion. All three inlets empty into a hexagonal intake shaft tunnel 104 metres deep.

From here, water is diverted to Bell Nallah and further on to the Bhandup pumping station. Since the diameter of the inlet is only 3 metres, rock fragments and silt from the lake bottom settled at the entrance of the inlet and the flow is not to its full capacity. These impediments will be cleared after monsoon. The new inlet will be able to supply 36,000 million litres of water, which will last the city for an additional nine days.

The project is part of the Rs 222-crore Middle Vaitarna Project, which seeks to augment water supply to the city by 455 million litres daily (MLD). 35 per cent of this project is funded by the Centre, 15 per cent by the state and the rest by the BMC. The city currently uses 3,750 MLD.

Official speak
“With this experiment, the lowest water drawable limit of Modak Sagar has reduced. Now, we can use the water four metres below the lake level that was earlier unused,” said civic chief Sitaram Kunte.

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