NEW DELHI: On the eve of torrential rains that caused widespread flooding in

, the state’s major dams, barring Idukki, were already full to capacity leaving little scope of containing cumulative runoff totalling more than 200% of the storage capacity of all big and small reservoirs in the state.

The finding is part of a detailed study on Kerala floods carried out by the Central Water Commission. It submitted its report to the water resources ministry last week. Kerala received 12 billion cubic metre (BCM) of water in three days of exceptionally high rainfall during August 15-17, more than double the capacity of all reservoirs (5.8 BCM) in the state, presenting an impossible scenario as waters began to rise in several districts leading to mass evacuation.

‘Kerala dams didn’t release extra water’

In view of the state’s experience, the report recommended revisiting the ‘rule curves’ — strategic water level for planning operations of a

— of all reservoirs. The report said the dams neither added to the flood nor helped in reducing the flow. It said “above normal” rainfall in June and July and exceptionally high rainfall in August left the state helpless despite following standard operating procedures on reservoirs.

The report will be submitted to the Kerala government on Monday. “The dams did not release anything extra of what they received. The authorities had released water in a very controlled manner. The commission has come to the conclusion after computing and analysing step by step inflow and outflow of water during the entire season,” CWC director (

) N N Rai said. He told TOI on Sunday that the report would recommend “revisiting rule curves of all reservoirs” in view of shrinking of their water carrying capacity over the years.

A rule curve specifies storage or empty space to be maintained in a

during different times of the year. “The rule curve as such does not give the amount of water to be released from the reservoir. This amount will depend on inflow to the reservoir, or sometimes it is specified in addition to rule curves,” said Sharad Jain, director of National Institute of Hydrology,

. Rule curves are derived by studying the historical data. Though it helps in maintaining water levels, certain reports suggested that a tendency to keep back water for leaner seasons may have led to high reservoir levels.

Kerala had, in fact, last seen such high intensity rainfall during a short duration of time in 1924. "The catchment area, tapped by the dams, alone could see cumulative runoff of 2.19 BCM out of the total 12 BCM received by the state during those three days," Rai said.

Besides listing extreme weather events, the CWC is learnt to have referred to other reasons including encroachment of flood plains and other activities in catchment areas behind the disastrous consequences of such intense rainfall.

The CWC's role, however, is only advisory in nature. All the dams are maintained by state governments with the help of experts in management bodies, taking into account alerts sent by different central agencies.