CHENNAI : The changing land use pattern in Chennai and its peripheral areas and shrinkage natural water drains are posing new challenges to disaster management and minimising of flooding risk and so, a comprehensive flood protection is required, according to the State Disaster Management Perspective Plan 2018-30 (DMPP), unveiled by Chief Minister Edappadi K Palaniswami.Giving a detailed account of the topography of the Greater Chennai and its peri-urban areas, the perspective plan says these areas are spread over an almost flat terrain along the coastal plains, having an average altitude of only 4 to 6 metres above the Mean Sea Level.
The entire surface runoff drains into the Bay of Bengal only through the outlets - Ennore creek in the north, Cooum and Adyar river mouths in the centre and Kovalam creek in the south.“The capacities of these discharging outlets are also under severe stress due to shrinking of the width of the creeks/river mouths and inlets that pass through the marshlands,” the plan says.
Various reasons have been underlined including man-made ones like encroachments, siltation, change in land use pattern and accumulation of garbage along the waterways. Some natural factors such as existence of naturally formed ecologically sensitive shoals within the creek, sedimentation due to littoral movements, and increased tide level during extreme weather condition are also cited as reasons for preventing draining of flood waters into the sea in case of a repeat of the 2015 deluge.
These pose challenges not just to urban planning but also to disaster management.
The Ennore and Kovalam creeks are highly sensitive ecosystems which play a very important part in the ecological, hydrological and social security of the local inhabitants. Due to the burgeoning population in the Chennai Metropolitan Area, the Ennore and Kovalam creeks are highly degraded by the disposal of waste water from the surrounding ULBs and industries. In this connection, a detailed project report for restoration of Ennore creek and Kovalam creek has also been initiated, the DMPP says.
The Chennai city and its urban neighbourhoods are spread over the river basins of Kosasthalaiyar, Cooum, Adyar and Kovalam which are connected through the Buckingham canal formed parallel to the coast in the tidal flats. This system also functions as a unique system for draining surface run-off from the isolated watersheds sandwiched in the four basins through the river mouths viz., Ennore creek, Cooum river mouth, Adyar river mouth and Kovalam creek.
“All these systems will be treated holistically to mitigate the risks for which detailed estimates are being prepared and taken up in a phased manner. The financial requirement is expected to be about `2,800 crore,” the plan says.To ensure that the river restoration is holistic and comprehensive, three separate studies have been started for the preparation of detailed project reports for plugging of outfalls, interception and diversion of sewerage, solid waste management and R&R in the major drains draining into the Adyar river and the Buckingham Canal.
proposed measures
Delineation of the flood-prone areas in the fringes of flood bowl and in the flood plains and evolving suitable method for draining the run-off from these areas by constructing flood walls on the boundary of flood bowl /rivers and collection wells for stormwater with suitable pumping mechanism to drain the water into the river system
Forming link canals and diversion canals for discharging surplus of tanks into the river system wherever feasible. Where roads have been formed in the erstwhile waterways, cut and cover conduit system will be adopted to ensure proper drainage
Installing decentralised incinerators to dispose of the generated garbage effectively instead of the routine practice of dumping those in the water bodies. The 2015 December floods had revealed that the stretch of the Adyar river within the Chennai urban limits was incapable of discharging the flood of magnitude exceeding 10-year return period
To minimise the risk due to the floods of higher return period, the possibility is being examined of diverting water from upper reaches of Adyar through a carrier canal with regulatory arrangements from surplus channels of the Sriperumbudur to Palar river