Once again, Chembarambakkam lake is the talk of the town for the last couple of days. Though the lake is one of the major sources of drinking water for Chennai, it gets attention only when there is heavy rainfall or a cyclone, like the one now – Nivar.
Learning a lesson from the 2015 floods, the Tamil Nadu government has decided to release 1,000 cusecs of water from Chembarambakkam lake from Wednesday afternoon (November 25).
Spread across 15 sq km, the lake has a total capacity of 24 feet and is able to store water up to 3,645 mc ft. Located near Poonamallee, on the outskirts of Chennai, the lake is believed to be built during the Pallava era and is around 500 years old.
In the past, the level of the lake was increased many times. Centuries ago, it was 19.5 ft and during the British rule, it was increased to 22 ft and in the Telugu Ganga project in the 1980s, it went up to 24 ft.
The lake has an ability to discharge about 50,000 cusecs of water. In 2015, due to the AIADMK government’s procrastination over the releasing of the water, the lake was at its full capacity and started overflowing. Following this, about 30,000 cusecs of water was released in one go, that resulted in the flooding.
“Two decades before, the lake’s water was used for irrigation and helped about 13,500 acres of land in 39 villages. Over the years, due to rapid urbanisation, the agricultural lands have gone into the hands of realtors and now the lake is just providing drinking water,” said K Sivasubramaniyan, professor, Madras Institute of Development Studies.
In Tamil Nadu, agriculture is mainly dependent on irrigation. There are three types of irrigation practised here, through canal, tank and well.
“Across the state, we have 41,127 tanks. The original capacity of Chembarambakkam lake was 3.1 TMC. It has been increased to the present capacity of 3.65 TMC. This was done with a budget of ₹130 crore budget in 2011. We can say it is one of the modernised tanks in the state. If such attention was given to remaining tanks, the state would have never faced water shortage,” he added.
According to Chennai-based renowned water expert S Janakarajan, in one of his interviews to a YouTube channel, the encroachments on water bodies was the main reason why the city witnessed floods in 2015.
The water discharged from Chembarambakkam lake caused flooding in river Adyar, which originates near to the lake in Kanchipuram district.
“The flood plains on either side of river Adyar have been encroached. People are dumping waste into the river. The river once had a gentle slope. But due to the dumping of waste along the river, at some places, it has risen and in some other places, it is low. Because of this the capacity of the river is shrinking. The original capacity of the river was 20,000 cusecs of water,” Janakarajan said.
“In north Chennai, we have Arani and Kosasthalaiyar rivers, in central Chennai we have Cooum, and in south Chennai, we have Adayar and Buckingham Canal. Besides these, we have 16 major drains. There is no possibility of flooding with all these facilities. There is no other city like Chennai, which has such drainage facilities,” he explained.
“Clearing the encroachments and restoring the river is the only solution to avoid flooding in the future. Building concrete embankments on both sides of the river is not a solution. If desilting is done in Chembarambakkam lake, it can store more water,” he added.
In some quarters, it is alleged that the state government is waiting for the lake to attain its full capacity and after that water may be released. Instead, the water should have been released in small quantities every now and then ahead of the heavy rainfall, so that the inflow of water could be controlled and there would be an elbow room to clear stagnating water.
But the engineering experts have varied views on this argument. Ensuing the 2015 floods, a team from IIT Madras, IIT Bombay and IISc Bangalore has done a rapid assessment and a paper was published. While dealing with the above-said opinion, the authors of the paper felt that the reservoirs like Chembarambakkam are conservation reservoirs and they are regulated differently.
“Being conservation reservoirs, without a reliable weather forecast (forecasts with 70-80% false alarms) and a reservoir inflow forecast, decisions on timely release of floodwaters are rather difficult. Further, all water bodies were completely full from the above normal after November rainfall and the catchment was completely saturated, resulting in the heavy runoff. Hence, the reservoir release alone cannot be blamed for the huge deluge in Chennai,” the report by IISc Bangalore’s Pradeep Majumdar and team in 2016, said.
The rapid assessment report also has added, nonetheless, it is high time to have a comprehensive flood forecast system in place for assisting the authorities in making informed decisions about operating the reservoirs for managing flood in addition to making sure that the storage for drinking water supply is not compromised.
To this, an official from the Public Works Department (PWD) told The Federal that, along with consultants, the department is carrying real-time forecasting now.
“We have completed a detailed project report worth ₹50 crore under ‘Chennai City Partnership’, a measure to manage drinking water programme in the city. If this gets implemented, we can have gauging stations to measure continuous inflow data from other lakes in the city. But as of now, we have desilted the water bodies in the city and we expect there will be no need to release more than 10,000 cusecs of water,” the official said.
Meanwhile, a former PWD engineer has said that if the PWD engineers are allowed to carry out their work without any political or bureaucratic intervention, some of the disasters could be prevented.
“Each dam and reservoir has its own register. It has details such as who should be responsible for supervising the dam, at what level the water should be released in case of flood alerts given, etc. With Chembarambakkam lake, the responsibility of releasing the water was entrusted with a junior engineer in 2015. He could have released water promptly according to the rainfall. But the higher officials prevented him from releasing the water. That has caused a bigger damage,” he said.