Deep dive: Why does Chennai flood every time it rains heavily?

The city recorded over 900mm excess rainfall this November.
Chennai recorded over 900mm excess rainfall this November. (Agencies)
Chennai recorded over 900mm excess rainfall this November. (Agencies)
Updated on Dec 01, 2021 07:24 AM IST
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In the 1920s, a large reservoir called the Long Tank was filled up to build a residential colony in erstwhile Madras. It is now known as Thyagaraya Nagar, or T Nagar, a symbol of many things right and wrong in Chennai.

Located in the heart of the city, it has rows of homes and a shopping hub where stores range from jewellery and clothing showrooms to street vendors. It was T Nagar that was picked to implement the Chennai “smart city” project.

But now, it is synonymous with another Chennai image: Roads that turn into rivers, boats for vehicles, and people stranded in homes at the first hint of heavy rainfall.

When Chennai received over 900mm excess rainfall in November, the roads of T Nagar, and indeed other parts of Chennai, were completely submerged. It was the second highest rainfall recorded in the city in November since 2015, when it suffered massive floods.

In November 2015, Chennai received 1,049mm of rain, compared with 915mm of rainfall till November 29 this year. There have been particularly heavy rains between November 6 and 7 (210 mm), November 10 (22mm), November 19 (32mm), and November 26 (29mm). Overall, Tamil Nadu and the neighbouring Union territory of Puducherry together received 630mm of rainfall from October 1to November 29 as against the average for these months of 350mm. “During the same time period the rainfall received in Chennai is 1,130mm, which is 83% higher than the average,” said V Puviarasan, deputy director of the Regional Meteorological Centre (RMC), Chennai.

Across the state, 72 people were killed in rain-related incidents, 15,164 people were evacuated from low-lying areas and accommodated in 182 relief camps, and crops spread over 50,000 hectares were damaged.

Low flood limit

In the 2015 tragedy, more than 400 people lost their lives across the state. The situation was compounded when water from the Chembarambakkam lake was released, flooding Chennai. Since then, the Tamil Nadu government has said that a 5,000 crore integrated stormwater project is being implemented, and in T Nagar alone, 9.3 crore has been spent on a drain under the “smart city” mission, and was completed in 2020.

This expenditure includes work to construct collector, feeder and arterial drains, canals for the Adyar and Cooum basins, which were completed last year, and ongoing work for canals for the Kosasthalaiyar basin in northern Chennai and Kovalam in the south. And yet, Chennai floods.

The first reason is Chennai’s long history of vanishing lakes and water bodies, captured by rampant and unfettered urbanisation. Only 15% of Chennai’s wetlands are left, according to a study by Chennai-based Care Earth Trust, a biodiversity research organisation. Their findings show that Chennai’s built-up area grew from 47 sq. km in 1980 to 402 sq. km in 2012, while wetlands declined from 186 sq. km to 71 sq. km.

Between 2017 and 2018, about 48km of drains were built under the smart cities mission. Wherever these new drains have been laid, there was little to no flooding, said an official of Greater Chennai Corporation. “Places like T Nagar flood all the time mostly because there are old drains, which we are looking to reconstruct,” he said, requesting anonymity.

In T Nagar, particularly, there was another problem. Contractors engaged in improvement work for the Mambalam canal, where all the water from T Nagar drains, had dumped debris all along a length of 1.7km that blocked inlets.

An indication that the drains in the city failed to work during this monsoon comes from the overburdening of another department, the Chennai Metropolitan Water Supply and Sewerage Board, which manages the city’s water supply and sewage network.

“We usually pump out 585 mld (million litres per daily) of sewage everyday, which reaches the treatment plants, but since heavy rains began, we are pumping out 920 mld,” an official from the department said, but declined to be named. “The additional load is because the corporation is using heavy motors to drain rainwater from waterlogged streets into the sewerage pipes.”

“We also stop our system for about four to five hours every night for maintenance,” he said. “But this entire month, the pumping system and treatment plants have been functioning 24x7.”

According to municipality records, there are more than 30,000 interior roads and 471 bus route roads in Chennai that add up to close to 5,000km and 300km, respectively, but the city’s drainage network is present in just around 2,000km. Many of these drains were built at least three decades ago.

“The old drains were constructed for a rainfall capacity of 20mm per hour, whereas the new drains are being built for 70mm per hour. The old drains are built of brick and mortar so they are dilapidated and break easily,” said the municipality official cited above said.

“They don’t have the capacity to withstand the kind of rains we have received this year,” he said. “We have been using reinforced concrete for the new drains since 2015.”

Many areas in the city were flooded although they had roads with drains on both sides, found a field audit by Chennai-based Arappor Iyakkam, an anti-corruption NGO.

“Why are they spending crores of money to build these drains which aren’t working? They may as well spend less to buy pumps to drain water via sewage pipes, which is what they have been doing every monsoon,” said Jayaram Venkatesan, convener of Arappor Iyakkam.

Corruption in awarding tenders as well as improper design where the drain slope is higher than the height of the road or the gradient not matching the water body to which it is linked are primary problems, Venkatesan said.

After years of demands by activists and environmentalists, the municipal corporation his week began releasing maps on its website on where drains are located, a crucial information gap till now. “Only with these maps will we be able to understand where the drains are present, how they are connected and in which direction they drain,” said Venkatesan.

These maps show, for instance, a ward in T Nagar where the height of a manhole on the street is at 5m but the drain is at a height of 5.7m, impeding the flow of water, said Raj Bhagat, senior manager of Geoanalytics at research organisation World Resources Institute India, who is studying these maps.

There has also been no way of verifying if these drains have been designed in tune with the rate of rainfall, he said. “Even if the drain is designed for a particular rainfall rate, it is connected to another drain so the second drain has to have more capacity as well because it accumulates water from both streets,” Bhagat said.

The city’s drainage network is designed only to carry average rainfall, and that it cannot handle excess rainfall, according to KP Subramanian, former professor of urban engineering at Chennai’s Anna University. “And it gets clogged further with solid waste and drain slits that are not cleaned regularly.”

There is also a clear lack of coordination between various civic departments ranging from the Chennai Metropolitan Development Authority, which approves building constructions, the public works department that is in charge of de-silting water bodies, the municipal corporation which builds drains and Metro Water that lays pipelines under roads for water and sewage connections.

“We have a common problem where the corporation lays a new bitumen road and almost immediately it is dug up by other state agencies such as the Metro Water, electricity board, telephone and internet providers to lay utility cables underground,” a government official said on condition of anonymity. “Similarly, in some places, Metro Water pipelines run perpendicular to the drains underground, which reduces the capacity of the drains.”

Moving forward

Based on the November rains, municipal authorities have identified pockets where drainage capacity has to be augmented, said GCC commissioner Gagandeep Singh Bedi, who took over the civic body in May.

He also said drain construction will now follow prescribed standards. “The World Bank’s standard, for example, is for bigger drains to take 70mm of rainfall per hour. But if we receive 100mm rainfall and that has to be taken care of, the stormwater drains will be bigger and the roads will become smaller,” Bedi said. “Drains are constructed for optimum rainfall and not heavy rainfall because there has to be a balance between the road width and the drain.”

Given this tenuous balance, Bedi said the corporation was looking at ways to bolster drainage even if there is stagnant water, like permanent pumps. “We are looking at how fast we can drain out the water even if it stagnates. As the city expands, many drains’ capacity has to be increased. And it’s not just the case in Chennai. It is the same for Bengaluru or Mumbai,” he said.

Another peculiar issue that GCC engineers identified during these rains were so-called saucer-type streets, where the constant relaying of roads resulted in them being at a higher level than homes in densely populated areas. “You can’t put up drains in these places so we have decided to have permanent pumps there too,” said Bedi.

Ironically, another conundrum that the city faces is a drought-like situation a few months after the northeast monsoon, which experts say can be addressed if the urban run-off is stored. Water from drains should be diverted to existing water bodies, particularly those located outside Chennai, which will help in mitigating flooding as well as recharging aquifers, experts said.

“We thought they would learn lessons from the 2015 floods, but they haven’t,” said Sekhar Raghavan, director of the city based Rain Centre, a non-profit. “You cannot build a stormwater drain over a lake or a pond and let that water go into the sea. Drains work only in specific locations.”

“The government needs to look into the potential of water bodies, protect, reclaim and rejuvenate them,” Raghavan said. “This will rescue flooding and, if these water bodies can be made into recharge ponds and reservoirs, the local community can use it for their water needs.”

All said, the city will continue to flood, former corporation officials said. “It’s a complex issue. One, we have geographical disadvantages of being a flat city close to sea level. And since for more than 70 years, we have encroached on all the natural water bodies,” a former official said, asking not to be named. “It’s a collective failure of officials, real estate and civil society.”

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  • ABOUT THE AUTHOR

    Divya Chandrababu is an award-winning political and human rights journalist based in Chennai, India. Divya is presently Assistant Editor of the Hindustan Times where she covers Tamil Nadu & Puducherry. She started her career as a broadcast journalist at NDTV-Hindu where she anchored and wrote prime time news bulletins. Later, she covered politics, development, mental health, child and disability rights for The Times of India. Divya has been a journalism fellow for several programs including the Asia Journalism Fellowship at Singapore and the KAS Media Asia- The Caravan for narrative journalism. Divya has a master's in politics and international studies from the University of Warwick, UK. As an independent journalist Divya has written for Indian and foreign publications on domestic and international affairs.

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Thursday, January 20, 2022