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The extreme heavy monsoon rain in Chennai this November highlights the city’s vulnerability to climate change and also the skewed development that has upset the fragile environmental balance and made it prone to floods

File image of floods in Chennai (Image by Dharma Chandru Photography)

File image of floods in Chennai (Image by Dharma Chandru Photography)

Life in Chennai, the bustling south Indian metropolis, ground to a halt mid-November as the heaviest rainfall in more than a decade led to floods and boats being taken out to rescue people stranded in their waterlogged homes. The extreme weather event exposes the climate change vulnerability of the fast changing city and untrammelled development that has encroached upon floodplains and traditional wetlands.

On November 16 this year, the India Meteorological Department (IMD) observatory in Chennai recorded rainfall of 236 mm. It had rained copiously the whole of the previous day as well. In the past decade, the highest rainfall in a day in November was 150 mm in 2009.

The coastal city receives an average annual rainfall of 1,468 mm, with November being  the month of the heaviest showers with a monthly average of 374 mm. This year, the torrential rainfall on November 15-16 contributed to 63% of the monthly average and 16% of the annual average, and exceeded the season’s average by 80%.

The Tamil Nadu capital is built over flat coastal plains, which runs into a sand bank along the coast. Four rivers – Cooum, Adayar, Araniyar and Kortaliyar – run from west to east across the city. These rivers normally don’t carry water since supply from their catchments is intercepted and stored in four reservoirs in the west. The floodplains of the rivers have been built upon and sand bars keep forming across their mouths. Thus, the city floods when it rains.

It is not as if residents are not used to flooding. Chennai receives predominantly the northeast monsoon in short, sharp bursts of rain between October and December. It is usually a string of depressions, cyclonic storms and severe cyclonic storms emanating from the Bay of Bengal that bring rains to the city.

However, the mid-November rains this year were different. It was not just more intense than what most citizens of the city could remember but deadly too with more than 70 people killed in rain-related incidents.

The National Disaster Response Force and the state police brought out boats to evacuate those stranded in waterlogged locations. Helicopters from the Indian Coast Guard and the Indian Air Force flew sorties to rescue people. Trains and flights were cancelled and supply of essential food items was disrupted.

According to a senior IMD official, the city received a rainfall of 452.4 mm in November 1976. But much has changed over the decades and the city’s ability to withstand a heavy downpour has declined.

Cause and effect

In the past decades, Chennai and its surrounding municipalities and panchayats, which constitute the Chennai ‘urban agglomeration’ (UA), have seen amongst the highest growth in population in Indian cities. The population registered during the 2011 census for Chennai UA was 8.65 million, a 31.8% growth from the 6.56 million registered in 2001.

A significant part of this population growth was contributed by the influx of the workforce for the information technology (IT) sector. Tidel Park, the first IT hub that was constructed in the city by the state government to promote the IT industry, was inaugurated in July 2000.

The Old Mahabalipuram Road, linking Chennai to the historical town of Mamallapuram, was reclaimed from the adjoining wetlands and turned into a six- and four-lane expressway.  More office buildings sprang along the corridor. Multi-storeyed residential apartments, housing colonies, shopping malls, schools and hospitals followed. The city stretched along a north-south axis. New arterial roads linking this part of the city with other parts came in the past years, and construction spread along the sides of these roads.

There was a problem though. Much of this was built over a complex of marsh and wetlands constituting the Pallikarnai Swamp. To make matters worse, at Perungudi on the north-eastern edge of the swamp, the city corporation started dumping municipal solid waste. According to Chennai Corporation figures, the city generates 4,500 tonnes of garbage and 700 tonnes of building debris every day. Around half of this is deposited at Perungudi.

After much protests from environmental groups in the city, in 2007 the Tamil Nadu government declared 317 hectares of the Pallikarnai Swamp a reserve forest, thereby restricting unplanned development at least in this area.

The eating away of the Pallikarnai Swamp has been the last among a series of systematic encroachment and reclamation of the wetlands in the city. Over the decades, the series of local tanks and water bodies in many parts of Chennai city have either been reclaimed for development or left to die a death of disuse with rubbish dumped into them.

Upsetting the balance

Considering the fact that Chennai is a topographically flat city – with altitude ranging from 2 metres to 15 m above sea level – it was these water bodies that gave the city its sense of water balance. They held excess overflow water during torrential rains and recharged the aquifer in the dry season, which could stretch for as long as 10 months in some years.

With these balancing structures gone, the city either floods or precious rainwater runs into the sea. The rivers that would have otherwise taken the floodwaters into the sea have lost their carrying capacity. Housing colonies built on the floodplains of the Adayar and Cooum rivers do not come in the path of water during the dry months. But when it floods and the rivers swell across their banks, these buildings block the passage of water into the sea. The floodwater instead finds space between buildings and spreads into the city.

The protection walls built into the sea for the Chennai Port since the 1960s have been adding an additional dimension to floods in the city. The city coast south of the port walls has been accreting with sand taken away from the northern side. Thus, Chennai has one of the widest beaches in the country – the Marina Beach.

However, while sand on the beach adds to its beauty, sand bars across the Cooum and Adayar river mouths obstruct the flow of floodwater. Keeping the river mouth free of the sand bar and cleaning of the storm water drain network are two activities that require much investment of time, human resources and money.

Extreme weather

The combined effect of these factors have made Chennai city prone to flooding, and in turn vulnerable to climate change. According the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, extreme weather events (such as the mid-November rains) are expected to increase in the coming years.

However, it is not as if the annual number of depressions, cyclonic storms and severe cyclonic storms in the Bay of Bengal region have increased over the years. A review of IMD data from 1950 to 2014 shows that the highest number of 16 events occurred in 1966, with the next year 1967 recording 14 events. In comparison, in the years between 2005 and 2014, the highest number was 10 events, which was recorded in 2006.

According to the Tamil Nadu State Action Plan on Climate Change, even though the number of cyclones may decrease in the future, their intensity and wind velocity could increase. Also, the adverse impact of flooding in coastal cities such as Chennai will be exaggerated by a rise in the sea level. The sea along Tamil Nadu coast is expected to rise between 0.19 m to 0.73 m by 2100.

A community’s success in dealing with current weather uncertainties indicates its ability to adapt to future climate change. Chennai did not fare very well in dealing with the recent floods. The administration and the people will need to get their act together to ensure at least such flooding does not happen even with severe rains. Only this will reduce Chennai’s vulnerability to climate change.

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