With the water level continuing to rise, one shutter of the Cheruthoni Dam, which is part of the Idukki Reservoir, was opened on Sunday.
The shutter has been raised y 70 cm to release 50 cusecs (about 50,000 litres) of water.
The gushing water will reach Kavalangad in Ernakulam district in 1 hour and 10 minutes.
The water is estimated to reach Kaladi in 4 hours, Aluva in 9 hours and Varappuzha in 12 hours.
The reservoir is bound on three sides by the Idukki, Cheruthoni and Kulamavu dams. It is the shutter(s) of the Cheruthoni dam that is opened to release water from the reservoir.
The released water enters Lower Periyar before emptying into the Arabian Sea.
The decision to open the shutter and release water was taken on Saturday after the water level exceeded the permitted capacity as per the rule curve.
A red alert was sounded yesterday. The water level reached 2383.30 feet at 8 pm on Saturday, though it is at 2,403 ft the reservoir reaches full capacity.
Though there is no emergent situation to raise the dam shutters, it has been decided to release water as a precaution after ensuring all the necessary security measures, said Roshy Augustine, Minister for Water resources.
The level in the Idukki dam has risen despite a lull in the rains in the catchment areas, owing to the increase in inflow and the water release from the Mullaperiyar dam.
As many as 10 shutters of Mullapperiyar Dam were raised by noon on Friday, releasing 2122 cusecs of water. The water level at Mullapperiyar is 138.20 feet as of now.
Though the rain has abated at most places on Saturday, the India Meteorological Department (IMD) inform that more rain is on the way.
According to the national weather agency, Kerala is likely to see widespread rain and isolated heavy rain till August 9.
In view of all this, district authorities have urged those staying on the banks of the Periyar River to be cautious. Relief camps have been set up at various places.
Thirty-four camps have been set up in Ernakulam alone and 1,286 people have been moved here.
Most camps are set up in Paravur region (16), while seven has been set up in Aluva and 6 in Muvattupuzha.
Authorities are constantly monitoring the situation and have allayed fears of widespread damage like seen in 2018.
The Idukki Reservoir is one of the highest arch dams in Asia. It began operation in 1973 and is managed Kerala State Electricity Board. It was opened four times last year.
The Cheruthoni Dam has five shutters. The shutter at the centre is opened first. Then one shutter each on the left and right sides will be opened, if required. Then two shutters will be opened in the same manner. About 10-15 cm of each shutter is usually opened. All the shutters are operated by electricity.
At 168.91 metres the Idukki reservoir is one of the highest arch dams in Asia. Its construction is so unique that the pressure of the water inside the double curvature dam is eased to both sides.
The dam connects Kuravan and Kurathi hills.
It was after 26 years that the Cheruthoni dam was opened in August 2018.
The likely path and impact
When the shutter is opened, the water will enter the Cheruthoni river first. It will travel through the spillway and reach Vellakayam where it will merge with the Periyar river. If the Chapathu in Thozhupuzha Pulliyanmala State Highway is submerged in water, the traffic on the Idukki-Kattappana route will be affected.
Then the water will wind its way through Thadiyampad, Karimpan Causeway, Chelachuvadu, Keerithodu, Plamba forest area, Lower Periyar Dam, Neriamangalam, Bhoothankettu, Idamalayar, Malayatoor, Kalady, Aluva, Chengamanad, Eloor, Varapuzha, Manjaly and Chendamangalam.
This water also goes to areas in western Kochi such as Vallarpadam, Panambukad, Mulavukadu and Ponnarimangalam.
The water will finally enter the sea through the Vembanad Backwaters on one side and through the Kottapuram Backwaters near Kodungalloor on the other side.