Why Chennai floods

Unlike most cities, Chennai has three rivers — Kosasthalaiyar, Cooum, and Adyar — and the Buckingham Canal to carry floodwaters to the sea. Yet, the city floods every time it rains heavily. Thousands of crores of rupees have been spent on building drains, but these cannot compensate for the loss of lakes, floodplains and channels to unchecked encroachments.

The British protected the city’s waterways with strict laws. The Tamil Nadu Rivers Conservancy Act of 1884, for example, ensured no development for at least 80metres from both banks of rivers and around waterbodies. Even before that conservation was practised through kudimaramathu (community involvement in waterbody upkeep) and setting apart of floodplains around rivers (aathu poramboke), lakes (eri poramboke) and canals (odai poramboke). Most of these have vanished.

In the ‘tank memoirs’, the British recorded details of waterbodies in Chennai in the 18th and 19th centuries. “It was intended to levy cess on landowners for use of water for irrigation, but it protected the waterbodies,” says S Janakarajan in his book Water Management in Rural South India and Sri Lanka.
From the 1960s, however, rapid urbanisation and unplanned growth played havoc with the waterbodies. “The rules were diluted; encroachments on river banks got pattas,” said professor A Ramachandran, professor emeritus, Centre for Climate Change and Disaster Management, Anna University.

All the unapproved house layouts before 1989 were regularised by the Tamil Nadu government. After the 2015 flood, a CAG audit report said that between 1989 and 2016, the Chennai Metropolitan Development Authority approved 195 colonies on 175 hectares that were actually part of 54 waterbodies. The report said the number of colonies on water bodies could be higher as the CMDA had no monitoring mechanism.

Chennai’s biggest tragedy is what it has done to the Pallikaranai marshland, which was recently designated a Ramsar site. The marsh is now just a little more than 10% of its original 5,500 hectares. A 2017 CAG report blamed the development of the IT corridor on OMR for the marshland’s demise.
The marshland is a sponge that soaks up water, but its nature is changing because of encroachments and influx of sewage. The government has also set up an 80.9hectare garbage dump in the marsh.
The residential colonies in Velachery, Madipakkam, Perungudi, Perumbakkam and Pallikaranai that went under in early December have all come up on the marsh or in its vicinity.

“We have not learned lessons from the 2015 floods. Encroachments on water bodies have not been removed. The state government should have retrieved more areas of Pallikaranai marshland and revived its connections with 32 tanks in south Chennai. This could have reduced the floods in Madipakkam, Pallikaranai and Velachery. Buckingham Canal also has not been deepened before the monsoon.

It could have increased the floodwater carrying capacity,” said professor L Venkatachalam, officiating director, Madras Institute of Development Studies. The depth of the Buckingham Canal was six feet below mean sea level when it was built; the current depth is three feet above sea level due to siltation, according to WRD. “Each lake would have a kalangal (surplus channel) through which water flowed to the next tank. We must recover the kalangals. The bunds of lakes must be strengthened and the waterbodies deepened to hold more water,” said Sekhar Raghavan, director of Rain Centre.


What has happened to the marsh – encroachments reducing its ability to hold water – has also happened to the Buckingham Canal, which flows across Chennai parallel to the coast before emptying into the sea at Kovalam. Now the Greater Chennai Corporation and the water resources department plan to lay pipes at 15 locations to drain water from Buckingham Canal to the sea. “We will create a pipe-based drainage system from B-canal directly into the sea with shutters and permanent pumps. This will reduce the water-load going into the four basins, and ease flooding in the core city,” said GCC chief engineer S Rajendiran.

However, he said, high-power pumps would have to be set up at various places to push out water. These include Kasimedu, Napier Bridge, Loop Road and outfalls draining from Pallikaranai. “Shutters will prevent seawater from coming in,” he said.

Areas in north Chennai such as Manali and Thiruvottiyur faced inundation because the Kosasthalaiyar breached near Sadaiyankuppam. Water at 45,000 cusecs (cubic feet per second) was discharged from Poondi reservoir on December 4. Vyasarpadi, Pulianthope and Korukkupet flooded due to poor gradient of stormwater drains as the water did not flow even after three days.

While this was water from ‘outside’ flooding the city, there were also instances where the city’s ‘own’ water inundated the areas, said Sekhar Raghavan. “People fail to use the water from shallow aquifers because many have closed their open wells, resorting to borewells that tap water from deep aquifers,” he said. If people use open wells, flood waters will recede faster. People can install borewells with slotted casing instead of straight casing to connect both the shallow and deep aquifers.

Retired professor of urban engineering, Anna University, K P Subramanian said the government should come up with a classification of objectionable and nonobjectionable lands, as per the 2004 guidelines given by the ministry of housing and urban affairs in 2004. “The guidelines said floodplains, waterways, parks, playgrounds, and open spaces should be noconstruction zones. The state has to identify settlements in such areas and create policies to rehabilitate people,” he said.

A 2021 CAG report, released this year, said the number of untenable slums in water bodies increased by 91% between 2014 and 2018, from 235 to 451. The numbers reflected on the state’s poor housing policies, it said.

“Decentralization of the city is the only way forward. People come here for jobs and labour. Chennai is a ticking time-bomb and it cannot accommodate more houses,” said Subramanian. Extreme rainfall events will be more frequent and intense, exacerbated by climate change, said Ramachandran.

“The east coast of the country will face more rain because of the El Nino effect,” he said. With climate change and the temperature of the sea surface rising, the intensity of cyclones will be greater, he added. “LiDAR instrument can give a more accurate weather forecast than the current model used by IMD. But it is more expensive,” said Ramachandran.

The Union ministry’s Central Water Commission has recommended that the WRD set up dam inflow forecasting systems to ensure better decision-making on water release from reservoirs. “Usually, there is indecisiveness on when to open and close floodgates as Chembarambakkam is a drinking water source. Understanding inflow from catchments can help in releasing water in small amounts in a phased manner,” said Ramachandran.
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