The civic body will pull down a concrete water reservoir near Navina cinema after two Jadavpur University professors declared it “unsafe and precarious”, the first instance in the city of a concrete structure being pulled down while still in use.

The Calcutta Municipal Corporation (CMC) had decided to conduct an audit of the condition of reservoirs following the collapse of a part of the Ultadanga flyover.

Since the tank on Prince Anwar Shah Road is 40 years old and the life of a concrete structure is generally assumed to be 50 years, the CMC bosses decided to start the audit with it.

“The JU report warns of the possibility of a catastrophe and calls for immediate intervention,” said a civic official.

The two professors submitted their report in the third week of May. The decision to demolish the tank was taken at a meeting between mayor Sovan Chatterjee and municipal commissioner Khalil Ahmed.

Metro accessed the JU report, which states “(the) condition of this particular overhead reservoir is so distressed that it can lead to a catastrophe at any time if not intervened immediately”.

At a height of 59ft, the tank stores water weighing about 5,625 quintal (562,500kg). If it crashed on the busy road below, it could crush vehicles and people.

“This is the first instance of a concrete structure in use being demolished to prevent a collapse. Buildings that have been demolished are the ones built illegally,” said a CMC official.

The 1.25 million-gallon capacity tank, built in 1973 by the CMDA, supplies water to about 10,000 households in surrounding areas.

The two JU teachers — Biswajit Som, associate faculty and consultant, and associate professor Gokul Mondal — found no option other than to suggest demolition of the tank following three days of test.

“It can collapse on the adjacent residential buildings and lead to fatalities,” said Mondal.

The CMC had approached the JU duo about 45 days ago.

Residents said concrete chunks, some possibly as heavy as 50kg, frequently fall off the structure. The falling chunks have damaged at least two cars parked in an adjoining lane in the past two months.

At the meeting between the mayor and the municipal commissioner, it was also decided that no cylindrical overhead water reservoir, like the one to be pulled down, would ever be constructed in the city.

Unlike the Tallah water tank, which is a square, the others do not have inspection platforms around them. The CMC will allow only rectangular/square overhead reservoirs with a provision for inspection platforms.

The tank off Navina cinema cannot be demolished before four months, till an underground reservoir with a similar capacity is built to ensure water supply in the area remains unaffected. The tank will then be dismantled part by part under the supervision of engineers.

The CMC authorities decided to demolish the tank after they calculated that the cost of repairing it would be too high.

“The bill for repairs will be nearly Rs 34 lakh while building a tank will cost Rs 95 lakh,” said Bibhas Maity, director general (water supply), CMC. The pump that is now used to fill the overhead tank will be used to distribute water from the underground reservoir into the network.

Ahmed, an IIT engineer, said all RCC overhead reservoirs built 15-30 years ago would be inspected. He called the JU report an “eye-opener”. There are 24 overhead water tanks in the city, of which 15 supply filtered water, including the one on Anwar Shah Road.

The city consumes 320 million gallons of water daily, of which 35 million gallons is unfiltered water.