Opportune moment to rediscover Chennai’s hydrology

• Unusually heavy rainfall years have been becoming more frequent in recent decades in India.

Key Points

• As a result, people are also experiencing more frequent occurrences of floods in several parts of the country including Chennai.
• These are considered climate change-induced floods/disasters.

Are we hiding behind climate change for all the blunders made so far?

• Chennai suffered the serious impacts of floods in 2005, 2015 and, again, in 2023.
• The flood in 2023 is considered the worst in the past 47 years.

Have we resorted to corrective measures?

Upstream and downstream watersheds

• Chennai city and the adjoining districts are richly endowed with wonderful watersheds.
• There are 3,588 irrigation tanks (some are very large) in Kancheepuram, Chengalpattu and Tiruvallur districts, respectively, as highlighted in tank memoirs.
• These are man-made but magnificent watersheds created through a series of earthen embankments, constructed across streams which carried heavy flows during the monsoon months.
• Unfortunately, these tanks are neglected, silted up with broken bunds and control structures.
• In addition, catchment areas, flood plains, feeder and supply channels and even the water spread area in many of these tanks are heavily silted and encroached.
• Since these districts are already substantially urban, there is a dire need to protect these water bodies from encroachments.
• Not just tanks, but catchment areas, inlet and surplus channels, foreshore areas (tank flood plains) and tank bunds are equally important.

Tackling Chennai’s flood problem

• A comprehensive hydro-elevation (drainage) mapping needs to be drawn up covering the upstream-downstream watersheds with Chennai and the sea.
• Chennai is in fact geographically very uniquely placed, which is a blessing.
• It has three waterways (rivers) that run through the city, something which no other city in the country and in South Asia can boast of.
• The Kosasthalaiyar river runs through the northern part of Chennai, the Cooum which takes care of central Chennai, the Adyar which caters to southern Chennai, and further south, the Palar which carries the flow.
• Each of these rivers also feeds numerous tanks before reaching the Bay of Bengal.
• There is the Buckingham canal which cuts across all the four rivers in close proximity to the sea.
• Unfortunately, these major drainage systems are in pretty bad shape due to heavy encroachments, more so on the flood plains.
• These rivers have also lost their gravity and velocity due to sludge and silt deposits.
• These drains as well as the 2,900 kilometre long Storm Water Drain network constructed in the GCC area also deserve year-long attention and maintenance

Why Chennai went under water again

• Rising urban expansion
• it is important to recognise that the urban expansion process is irreversible and can be disastrous if not regulated.
• In the process, Chennai has lost many water bodies (lakes and ponds) and much of the Pallikaranai marsh land and coastal wetlands.

Why was Chennai so badly flooded?

• Chennai city and the CMA can be permanently saved from floods while, at the same time, get round the clock water supply even in a drought year
  • provided the measures indicated above are followed truthfully and scientifically.
• This is what is called converting disaster into an opportunity.