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Although cyclones and heavy rainfalls are normal, their increasing frequency and intensity as well as urban flooding is not.
In the past, Chennai has been witness to many floods and their frequency only seems to be increasing. Some catastrophic years for Chennai vis a vis floods were 1943, 1978, 1985, 2002, 2005, 2015, 2021. According to experts, there are several reasons for rainfall that cause flooding in the city ranging from Chennai’s location itself to climate change.
To understand just why the city floods so frequently, it is important to first understand the city’s ecosystem.
Chennai has a total of three rivers, five wetlands, and six forest areas. The three rivers are Kosathalaiyar, Cooum and Adyar. The five wetlands are Pallikaranai Marsh, Pulicat Lake, Kattupalli Island, Madhavaram & Manali Jheels, Adyar Estuary Creek.
The six forest areas are Huzur Gardens, Vandalur, Guindy National Park, the forests in IIT Madras, Madras Christian College, and Theosophical Society. However, over the course of the years, damage to many parts of this ecosystem, especially the wetlands and rivers, has meant that the city’s natural capabilities to drain water has been compromised.
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Low-lying areas are places that are essentially just above or at sea level. These areas are more prone to floods. Elevation maps are usually used to map the elevation of locations. Although different areas in Chennai have different elevation levels, the average level of the land in the city is just 2 m above the Mean Sea Level (MSL). Low-lying areas and coastal areas are usually the most affected by flooding due to heavy rainfalls or cyclones. Since Chennai is on flat terrain with several low-lying areas, the water doesn’t get drained easily.
After the 2015 floods, the Tamil Nadu State Disaster Management Authority (TNSDMA) mapped vulnerable areas in Tamil Nadu. They identified 306 flood-prone areas in Chennai and classified them according to their vulnerability.
A monsoon means changes in the wind pattern. Monsoon can be accompanied by heavy rainfalls, thunderstorms, and even cyclones. The northeast monsoon essentially represents the direction; it travels from the northeast to the southwest. The northeast monsoon in India usually has its effects in the southern states – Tamil Nadu, Kerala, Andhra Pradesh, some parts of Telangana, and Karnataka during the months of October, November, and December.
Chennai receives most of its rainfall during the northeast monsoon. The northeast monsoon accounts for 63% of the annual rainfall in Chennai. The city’s normal rain from the entire northeast monsoon is about 87 cm. However, this year, Chennai has already recorded 100 cm of rainfall by November itself. Pradeep John, a weather blogger, who goes by the name Tamil Nadu Weatherman, says 2021 had one of the wettest Novembers for Chennai.
Stormwater drains are drainage systems that are designed and built to drain the excess water from heavy rainfalls and carry them to lakes, ponds, or rivers. It is vital in cities, especially in flood-prone areas to avoid flooding. A good well-connected storm drain system collects the excess water at a low point where water naturally accumulates to avoid flooding. Stormwater drains are normally built on roads of width 7m and above.
A river basin is a piece of land that is dried by the river. Chennai has 4 such river basins – Kosasthalaiyar basin, Kovalam basin, Adyar, and Coovum basin. The Greater Chennai Corporation says that although the city has integrated stormwater drains, the areas consisting of the river basins don’t have integrated stormwater drains.
Satta Panchayat Iyakkam (SPI), an NGO found out that there was only 1894 km of SWDs against the city’s road stretch of about 5500 km in an RTI reply from October 2018. This essentially means that only 34% of the roads in Chennai are capable of draining water.
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As of January 2021, the SWDs coverage increased to 2,071 km, which is inadequate to drain the city’s water. Of the SWDs present, there were many complaints. Dayanand Krishnan, a Geographic Information System (GIS) consultant told The News Minute that the alignment was improper, leading to an uphill flow (SWDs must be built in such a way that the water flows downhill) after studying the SWDs in T Nagar and Ashok Nagar. He also pointed out that some drains were way too narrow to actually have any meaningful impact.
The Greater Chennai Corporation (GCC) published a study in January 2021 covering select zones. It’s estimated about 45 km of SWDs are missing. Some have to be constructed again, others demolished and reconstructed. Of the SWD maps published by GCC, 86 out of the 200 wards were missing, which includes the areas Manali, Thiruvotriyur, Sholinganallur, and Perungudi.
In the GCC maps published, many experts pointed out that more details were required. Jayaraman Venkatesan, Convenor of Arappor Iyakkam stated that more information like the gradients, disposal points, year of construction would be required to identify gaps and perform a detailed analysis.
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The rampant urbanization eventually led to encroachment near water bodies. Up until 1990, the build-up area stayed clear from waterbodies. However, the later build-ups were in areas near water bodies, in low-lying areas, leading to flooding and waterlogging.
The Pallikaranai marshland shrunk from 13,500 acres in 1972 to 1,500 acres in 2020. Ramsar site is a wetland of international significance. Tamil Nadu is calling for the declaration of the Pallikaranai as a Ramsar site. Pulicat Lake, the second-largest coastal lagoon in India was measured 3.8 m in depth at 1610. By 2017, it was reduced to just 0.8 m. The Adyar and Coovum river has been reduced to dumping waste and carrying sewage through the city.
There have been restoration efforts. The Chennai River Restoration Trust (CRRT) has been coordinating with various departments to restore the river. It showed substantial results as the water spread of the creek increased from 5% to 59%.
Nityanand Jayaraman is an activist and writer from Chennai who pointed out the encroachment via a tweet that showed the encroachment using a map of Velachery.
The scroll compared the maps from the 1900s to the present day maps to show that some water bodies have completely disappeared
The Council on Energy, Environment and Water (CEEW) performed a first-of-its-kind district-level climate vulnerability assessment. They analyzed 640 districts to assess their vulnerabilities to extreme weather events like cyclones, floods, heatwaves, droughts, etc. Chennai was ranked seventh in their vulnerability assessment because the city is prone to extreme weather conditions like cyclones and floods.
A modeling study by the researchers of the Indian Institute of Technology (IIT) found correlations between the increasing carbon emissions and extreme rainfall. The model made projections for the year 2075, that the precipitation on a peak rainy day can increase by 17.37%.
The 2015 Chennai floods caught the attention of people worldwide. French Foreign Minister Laurent Fabius expressed his solidarity for the people in Chennai and called for “concrete and urgent action against climate disruption.”