As Mumbai’s lakes overflow, why BMC can’t store the excess water

Of the seven lakes, Tulsi and Vihar are located within Mumbai, while the others are in the satellite districts of Palghar, Thane and Nashik. The full quantum of water stored in these lakes stands at 14.47 lakh million litres. Last year, on this day, the water stock in all seven lakes stood at 99.09 per cent, as compared to 2023, when the stock stood at 99.33 per cent.

Daily, the BMC supplies water amounting to 3,850 million litres daily (MLD). As a result, the current stock is expected to last for nearly 12 months.

The civic body’s data also show that the overall stock has seen a dip by 2 percentage points in the past 24 hours, even though the catchment areas of each lake recorded good rainfall.

In the last 24 hours, the catchment area of Middle Vaitarna Lake recorded 275 mm of rainfall, followed by 273 mm at Modak Sagar Lake, 195 mm at Tulsi Lake, 173 mm at Bhatsa, 91 mm at Upper Vaitarna, and 55 mm around the catchment area of Vihar Lake.

“The water stock cannot touch 100 per cent because it is a live storage stock, which means the existing quantum of water is being used and supplied daily. It is normal for the water stock to hover between 97 per cent and 99 per cent during the months of September and October, considering the lakes start to get filled during the months of June and July, and by this time, the water bodies reach their full capacity,” a senior civic official said.

“At present, the water stock is adequate to last us till next monsoon. However, owing to the increasing summer temperatures, we are witnessing a faster dip in water levels due to rapid evaporation. This is a concern. However, this issue got automatically resolved this year since monsoon onset happened very early,” the official added.

After intense rains picked up during the middle of September, the lakes neared full capacity by September 20, and since then, the water stock has been hovering between the 96 and 99 per cent mark.

Meanwhile, civic officials maintained that as all the lakes are full now, the surplus water that is coming from the recent rains is getting washed away.

Why is the BMC unable to catch the surplus water?

Civic officials said that the administration is unable to hold the water because the quantum of surplus water that gets washed away is massive. Of the seven dams, the Upper Vaitarna has a capacity to hold 2.25 lakh million litres of water, while Middle Vaitarna holds 1.86 lakh million litres, and the Modak Sagar has a quantity of 1.25 lakh million litres.

“Usually, Upper Vaitarna holds the maximum water and gets filled first. Following this, the surplus water gets channelled to the Middle Vaitarna, and after this lake gets filled, the water gets washed out to the rivers. Similarly, after the Modak Sagar Lake reaches full capacity, the excess water gets channelled to the sea,” said a senior civic official.

The official said that while the BMC does not have a record of the excess water being supplied, the total volume of surplus water is almost the same as each of these lakes.

“Therefore, to store the excessive water, we need to build new dams which will be as big as the facilities, but financially it is not viable, as the rainfall quantity varies every year, and it is also another mammoth task to lay down fresh pipelines to get the excess water supplied to Mumbai,” the official added.

What is being done?

Civic officials said that at present, the BMC has taken up a project to catch 200 MLD of additional water from Vihar Lake, which is located within the periphery of Mumbai. This lake has a capacity to hold 27,698 million litres of water.

Every year, during the monsoon, Vihar Lake overflows after reaching its peak capacity, following which the surplus water gets channelled out into the Mithi River, which, upon increased water level, leads to flooding in the suburban pockets of Santacruz, Kalina, Kurla and Ghatkopar. As a result, the civic body had now planned to create a pumping station for holding and reusing the surplus water.

“The surplus water will be captured in a holding tank or sump pit, from which it will be further channelled to the Bhandup pumping station. After the water is filtered, it could be sent to residential and commercial units for their daily usage,” said an official.

Officials said that since the distance between Vihar Lake and the Bhandup pumping station is not much, therefore, it is economically viable to take up this project. At present, the project is underway, and the facility is set to become operational by 2027.