Expansion a major cause of flooding in Chennai: Experts
Even as the heavy rainfall, lashing Chennai and its neighbouring districts since Saturday, subsided on Monday, experts said flooding would remain a perennial problem in the city due to its expansion taking over natural water drainage systems and flood plains.
Most of the roads and bylanes in city are covered under sheets of water while there was water up to two feet in low lying localities. Overnight, Chennai recorded 21 cm of rainfall between November 6 and 7, causing severe inundation. This was the city’s highest single-day rainfall for November since 2015. The intensity of the rains reduced on Monday, with Chennai’s Nungambakkam station registering 14.5 mm of rainfall up to 5.30 p.m.
Following the floods in 2015, the state as well as the Greater Chennai Corporation (GCC) in their budgets had announced several projects to improve the stormwater drain network. Chennai corporation in its 2016-17 budget allocated ₹415 crores for the drainage system.
Between October 2011 and June 2016, the corporation laid around 565 km of new drains. Chennai, as of today, has 2,071 km of stormwater drain network. Civic authorities at that time had also found that a crucial factor that links the drain network was largely either absent or broken, officials said.
‘Missing links’ were identified for 33 locations in Chennai’s central city and 278 locations in the extended areas, they added. Projects to build drains and links receive funding from the World Bank, Asian Development Bank and German-based KfW bank to the tune of more than ₹5,000 crores, spanning across the past few years, they said.
A massive integrated stormwater drain network project announced in 2014 by the city corporation to connect them to the four river basins Adyar, Cooum, Kosasthalaiyar, and Kovalam is still in progress.
The area under the Chennai Corporation was expanded from 176 sq km to 426 sq km in 2011, following which the basic road infrastructure for extended areas is still not in place. Based on topography, the corporation and the Public Works Department have divided Chennai into four watershed basins, comprising 12 major catchments. Work for about 406 km of drains for the basins in Adyar and Cooum combined was completed last year and civic officials say that this has helped decrease flooding in the south and central parts of Chennai in the present rains.
“In 2015, water was stagnant in localities such as Valasaravakkam for a week. Due to the new drains, there has been no stagnation,” said a senior civic official not wishing to be named.
The work includes the building of collector drains, feeder drains, arterial drains and macro and major micro canals to prevent flooding. Work is ongoing in the ISWD network to cover 763km for Kosasthalaiyar Basin in north Chennai and 360-km for Kovalam in the south. “It will take two years to complete,” the civic official said.
“The ISWDs are designed in a scientific way to prevent a worst-case scenario, like the 2015 floods.” In 2015, the local body had identified 588 spots where flooding probability was high. However, after the unprecedented rainfall and flooding, the corporation revised this number to around 820.
The coastal city of Chennai is essentially a flat terrain dotted with wetlands where excess water can drain into, but it has been occupied for residential, commercial and industrial projects. A study by Chennai-based biodiversity research organisation Earth Trust states that only 15% of the city’s wetlands are left. Their findings show that Chennai’s built-up area grew from 47 sq km in 1980 to 402 sq km in 2012 - while the area under wetlands declined from 186 sq km to 71sq km during the same period. The study also found that most of the water bodies affected were in the southern and western suburbs.
This can be attributed to the Information Technology boom in Chennai in the 1990s where IT parks came up in the south which was accompanied by increased residential projects.
“Due to overall dense urbanisation where there is too much concrete and tar and no natural buffers like lakes and open spaces, rainwater can’t move anywhere so it stagnates,” said Raj Bhagat, senior manager, GeoAnalytics, World Resources Institute-India.
“Chennai’s terrain is close to the sea level, so you require larger conduits. The stormwater drains are not scientifically designed according to rainfall data though officials may claim that. So when there are no natural buffers and inadequate road infrastructure, Chennai will continue to flood.”