Drastic reduction in the carrying capacity of Vembanad Lake and the poor discharge capacity of Thottappally spillway, coupled with the absence of storage reservoirs in the upstream reaches of the Pampa, Manimala, Achencoil, and Meenachil rivers, had worsened the flood situation in Kuttanad and surrounding areas, according to a study by the Central Water Commission (CWC).
The study says that “the dams in Kerala neither added to the flood nor helped reduce flood, as most dams were already at their Full Reservoir Level (FRL) or very close to the FRL on August 14 due to more than normal rainfall in the months of June and July.” The flood water run-off generated from the Pampa, Manimala, Achencoil, and Meenachil rivers during the August 15 to 17 rainfall was about 1.63 billion cubic metres (BCM) against the 0.6 BCM carrying capacity of Vembanad Lake.
The discharge capacity of the Thottappally spillway is 630 cumecs and that of Thanneermukkom barrage, 1,706 cumecs.
It has been concluded that out of 1.63 BCM run-off, only about 0.605 BCM could be drained out of Vembanad Lake.
The remaining run-off of about 1 BCM raised the water level in Vembanad Lake and nearby areas.
This continuous rising of lake water could be a reason for the overall change in the hydrodynamics of the Pampa, Manimala, Meenachil, and Achencoil, leading to the unprecedented flood situation in these rivers, says the study report.
Land reclamation
The CWC report further recalls the observation made in the Planning Commission report of July 2008 that the water carrying capacity of Vembanad Lake has come down to 0.6 BCM from 2.4 BCM owing to land reclamation.The rainstorm of August 15 to 17 had resulted in heavy flooding in the Periyar, Pampa, Chalakudy, and Bharathapuzha sub-basins.
The rainfall during these three days was comparable to the rainfall of July 16 to 18 in 1924, particularly in the Periyar, Pampa, Chalakudy and the Bharathapuzha sub-basins, according to the CWC study.
42% more rainfall
As per IMD data, Kerala received 2,346.6 mm of rainfall from June 1 to August 19, in contrast to the expected rainfall of 1,649.5 mm.
This rainfall was about 42% above normal. Further, the rainfall over Kerala during June, July and August 1 to 19 was 15%, 18% and 164% respectively, above normal.
According to CWC experts, the approach channels to the Thottappally spillway and the passage of the Thaneermukkom barrage should be widened, taking into consideration the lake hydrology, ecology, saline water intrusion, etc., based on scientific and engineering inputs, for an efficient discharge of flood run-off from Vembanad Lake.
