New Delhi, Sep 14: Central Water Commission (CWC) of Union water resources development ministry in its report has suggested that Kerala floods occurred due to huge runoff generated within a short period of three days which was beyond the carrying capacity of most of the rivers in the state. This has resulted into over bank flows from most of the rivers. However, it has recommended that it is necessary to review the rule curves of all the reservoirs in Kerala as they should be formulated for both conservation and operations during floods.
As per the report, the total average rainfall that Kerala receives between June 1 and August 19 is 1649.5 mm but this year it has been 2346.6 mm which is 42 per cent more and if it is compared from August 1 to 15 the average rainfall is 287.6 mm which has gone up to 758.6 mm which is 164 per cent more. The report tells that it was identical floods to worst ever floods of Kerala that had happened in 1924.
The report says that release of water from reservoir have very minor role to play in the flood augmentation. In fact Idukki reservoir absorbed a flood volume of almost 60 MCM during August 15-17, 2018. Even with 75 per cent filed reservoir conditions, the current flood could not have been mitigated. The CWC report says, "Dams in Kerala neither added to the floods nor helped reduction of floods as most of the dams were already at FLR or very close to FLR on August 14, 2018 when the maximum rainfall took place in the state. However, had the reservoir been a few feet below FRL the flood condition would not have changed much as severe storm continued for 3-4 days at majority of places in the state and in many cases release from the reservoirs was necessary on the day one itself."
But CWC report has suggested that reservoirs having capacity of storing more than 200 MCM in order to create dynamic flood cushion for moderating the floods to the lower return periods particularly in the early period of monsoon. Actually runoff generated from Pamba, Manimala, Achenkovil and Meenachil Rivers due to heavy rains during August 15-17, 2018 was about 1.63 BCM against 0.6 BCM carrying capacity of Vemband Lake. Further discharging capacity of 630 cumec of Thottappally spillway was the other major constraint for the disposal of runoff.
The report lays down that considering the high rainfall and the absence of appreciable storage reservoir of rivers in the area with shrinkage of carrying capacity of Vembanad Lake and reduction of the capacity of Thottappally spillway worsened the situation and the backwater flow to the low lying areas in the upper reaches of the lake.
The report concludes that August 2018 flood in Kerala was due to severe storm occurrence during August 8-9, 2018 and August 15-17, 2018. The storm of August 15-17, 2018 resulted in heavy flooding in Periyar, Pamba, Chalakudi and Bharatpuzha sub-basin of Kerala. The rainfall during August 15-17, 2018 was almost comparable to the historical July 16-18, 1924 rainfall in Kerala particularly in the above mentioned sub-basin.