As the flood water recedes in Chennai, serious questions are being raised about reservoir management in the city. Much of the flooding and subsequent waterlogging was a consequence of the outflows from major reservoirs into swollen rivers and into the city following heavy rains. The release of waters from the Chembarambakkam reservoir in particular has received much attention.
Official data from the Chennai Metropolitan Water Supply and Sewerage Board for the last 20 days suggest that the high precipitation and reservoir outflows on November 16 and December 1 respectively were primarily responsible for swelling the rivers. On November 17, 18,000 cusecs was released from the reservoir in Chembarambakkam.
However, from November 24 to November 30, when the city experienced minimal rainfall, the outflows from the reservoir were limited, even as the storage levels were maintained at almost 85-88% of the total capacity of 3645 mcft. When heavy rains to the tune of nearly 48 cm fell on the reservoir on December 1, 29,000 cusecs was released over 12 hours.
Experts raise the question as to why storage was set at such high levels. Professor Janakarajan of the Madras Institute of Development Studies also argues that the outflow management from the reservoir was improper. If storage levels were set at less than 75% and sluice gates were opened to allow for outflow from the reservoir in a way that it exceeded the inflows in the last week of November, the blow in the form of the high outflow on December 1 could have been softened, he asserts.
Officials formerly with the PWD say if the risk of releasing a particular volume prior to the heavy rainfall day had been taken, there would have been lesser floods in places like K.K.Nagar.
It is reliably learnt that the government is considering a move to further reduce the set storage level in the reservoir. Now the level is at two feet below the brim, and discussions are under way to change this to four feet.
Reservoir managers ignored warnings of heavy downpour
On November 17, 18,000 cusecs was released from the reservoir in Chembarambakkam, causing massive flooding in areas such as Mudichur, West Tambaram, and Manapakkam among others.
However, from November 24 to November 30, when the city experienced minimal rainfall, the outflows from the reservoir were limited. Officials in the Public Works Department asserted that the inflow of 31,000 cusecs (26,000 before 6:00 p.m. owing to the high rainfall) resulted in the high outflow of 29,000 cusecs from the reservoir on December 1 and that they couldn’t hold the reservoir beyond a point.
Former officials of the department, however, said if they had taken the risk of releasing a particular volume from the reservoir prior to the heavy rainfall day, water received from some tanks like Porur and Mudichur could have been stored.
The decision to conservatively hold storage despite early warnings from various meteorological agencies about heavy rainfall in the offing in late November and early December, clearly exacerbated the disaster that followed.
This was in addition to the structural issues in Chennai with floodplains and drainage systems being constrained by unplanned urbanisation.
Storage levels in Chembarambakkam and other reservoirs are decided based on drinking water requirements for the city and other areas in summer.
That the authorities should have hedged on the side of flood management over concerns on drinking water is now evident in hindsight.
(With inputs from K. Lakshmi and Ramya Kannan)