Perils of ignoring a new arithmetic of incessant rain

But these figures have played out a new arithmetic, largely explaining the sudden floods in many parts of Tamil Nadu, says Natarajan, a noted groundwater expert, member of the working group of state planning commission and a former Director of the Centre for Climate Change, Periyar Maniammai University.

As a constant researcher-tracker of the conditions in various river basins in the state, Natarajan, currently in the US, sharing some of his insights with Deccan Chronicle, said 2015 could be termed “an abnormal flood year” for Tamil Nadu, like it was in 1997, 2005 and 2007, barring the December 2004 Tsunami which was a natural catastrophe of a totally different order and magnitude. Excess rain is usually hugged with gusto in a water-starved state like Tamil Nadu. But what happened in recent weeks, according to him, is the rain runoff for the period of 394.30 mm, “works out to 452.79 tmcft (thousand million cubic feet) which is almost half the surface potential of the state.”

Hence, there is now floods in Thenpennai and Palar rivers after two decades, he said. Further, almost all rivers in the state, “except the Vellar river in Pudukkottai district are in spate”. The latest to join the flood discourse on Tuesday is the Tamiraparani river down South, inundating Tirunelveli and Thoothukudi districts. Even jungle streams are “flowing to their maximum capacity.”

Stating that this roughly translates into two lakh cusecs flowing into the sea daily from all rivers with discharge working out to 17.28 tmcft per day, Natarajan said with more rains predicted, for at least another five days, the discharge would swell to 86.40 tmcft. Eventually, by a conservative estimate, about 100 tmcft of flood waters “is being let into the sea during this short period”, he has assessed.

Significantly, this quantity, going by World Health Organisation’s per capita water requirement, could meet Tamil Nadu’s present population’s domestic water requirements for one whole year, Natarajan pointed out.

The value of this ‘wasted water’, in terms of the paddy it can help grow, is a staggering Rs 5,000 crore (at current prices). He feared that if the low pressure systems persist, Tamil Nadu could lose over 500 tmcft of water this year.

Citing the findings of T.S. Vijayaraghavan committee, the water expert said, the average flood flow “let into the sea in Tamil Nadu in every normal monsoon year is 76.94 tmcft (Palar – 12.50 tmcft, Then Pennaiyar 9.09, Vellar-21.47, Vaigai-3.26, Vaippar-4.73, Tamiraparani 11.39, and Kodaiyar-14.50). But in the Coleroon arm of the Cauvery river alone, which is a flood carrier, about 100 tmcft of flood waters is let into the sea ‘every normal monsoon year’, the expert said indicating that it should also be factored into the total average flood flows.

Leave alone ambitious schemes like Ganga-Cauvery link, the only way to avert further floods in Tamil Nadu “is to utilise these excess flows in abnormal years through inter-basin transfer within the state that can arrest the endemic water scarcity in many other parts of the state,” he argued. Construction of Mayanur barrage across Cauvery was a good step in that direction, but there must be water release linkages to the GA (Grand Anicut) canal, if ironies like irrigation tanks in Pudukkottai district being dry even now is to be averted, he said.

Analysing the causes of the 2015 floods in greater Chennai, Natarajan said while the full capacity of the reservoirs supplying water to the metropolis is 11.06 tmcft, the discharge in the last few days due to continuous rain could go up 16.20 tmcft this year, if their embankments are not to be breached.

In the last three weeks alone, the rain generated a runoff of 9.68 tmcft from greater Chennai metropolitan area of 1189 sq km. But the enormous buildings in the city, cement roads blocking percolation and very poor drainage are among the multiple factors that has led to roads and streets ‘becoming rivers’, once the ‘safe runoff coefficient of 0.25’ was exceeded, he pointed out