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Ecology & Environment
Mahesh

16/12/23 07:48 AM IST

Chennai’s hydrology

In News
  • Unusually heavy rainfall years have been becoming more frequent in recent decades in India.
  • As a result, people are also experiencing more frequent occurrences of floods in several parts of the country including Chennai.
Chennai urban hydrology
  • 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.
  • These tanks were constructed in such a way that surplus from an upstream tank served as a feeder to a downstream tank. 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.
  • The net result is a double disadvantage: Water storage in these tanks is very little and run-off is very high (over 80%) which caused heavy damage to the city of Chennai.
Solutions
  • Study the urban and peri-urban water dynamics — interconnected hydrological conditions; map water bodies (around 4,000) located in the 5,904 sq.km area of the proposed CMA (spread over Tiruvallur, Chengalpattu, Kancheepuram and parts of Ranipet districts).
  • 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.
  • The missing links between these tanks need to be restored.
  • These water bodies need to be restored to their original capacity or, where possible, even double the capacity so that excess water can be saved in these water bodies which will contribute to a substantial saving of run-off water.
  • A comprehensive hydro-elevation (drainage) mapping needs to be drawn up covering the upstream-downstream watersheds with Chennai and the sea.
Chennai geography
  • 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. Several efforts have been made to restore these rivers as well as the Buckingham Canal, but the conditions remain far from satisfactory.
  • There are many macro and micro drains such as Okkiam maduvu, Mambalam canal, Velachery canal, Kodungaiyur drain, Otteri nallah, Virugambakkam/Arumbakkam canal, Veerangal Odai, Captain Cotton canal and the Villivakkam canal.
  • 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.
Urban Expansion
  • Chennai has lost many water bodies (lakes and ponds) and much of the Pallikaranai marsh land and coastal wetlands.
  • At present, the CMA is to be expanded from the existing 1,189 sq.km to 5,904 sq.km covering the entire districts of Tiruvallur, Chengalpattu, Kancheepuram and parts of Ranipet district as part of the Master Plan III.
Source- The Hindu

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