The Idamalayar Dam is located on the Idamalayar River, a tributary of the Periyar River in Kerala. Edamalayar originates in the Anamala Hills at elevation of. The river is a perennial source draining a catchment area of with a topography of high relief. The catchment receives annual rainfall of, 90% of which occurs during the monsoon months of June to September. The dam is located from Ernakulam in Ernakulam district and from the Bhoothathankettu dam on the Periyar River, which takes its name from local myths attributing its construction to demons who built it with the intention of submerging Thrikkariyoor temple.
Water resources
Based on detailed hydrological analysis, with observed data supplemented by rainfall-runoff correlation and prediction of low flows during dry months, the mean annual runoff was estimated as about 1,207 million cubic meters and that at 95% availability as 730 million cubic metres. Therefore, the live storage in the reservoir was fixed at 1,032.3 million cubic metres. However, subsequent to the commissioning of the project in 1987, availability of the flows was reassessed. It was noted that flows reduced to 1,178 million cubic metres in dry season and reached 4,361 million cubic metres during the monsoon. The total annual flow was assessed as 5,539 million cubic metres. As irrigation requirements are not critical during monsoon season the assessment of dry weather flows is relevant for irrigation. Further, Idamalayar storage is being supplemented from diversion of flows from the Peringalkuthu Reservoir during the monsoon season through the Vachumaram diversion canal constructed during 1997. The water stored in the reservoir is used for energy generation during the monsoon season by adopting a suitable reservoir operational schedule so that the water diverted from the diversion canal is not spilled out.
Structural details
The dam built across the Idamalayar River is a gravity concrete structure with a total height of above the deepest foundation level with a length of at the crest level. The full reservoir level is elevation and the minimum draw down level is. The dam has created a reservoir with a water spread of. The reservoir has a gross storage capacity of 1,089.0 million cubic metres of which the effective storage is 1,017.8 million cubic metres. The spillway has been designed to pass a design flood discharge of. The storage in the reservoir created behind the high concrete gravity dam is utilized for power generation by diversion through a water conductor system comprising a long power tunnel, surge shaft and two lines of penstocks connected to the turbines installed in a surface power station for generation of 75 MW of power with two units of 37.5 MW capacity each. Instrumentation is provided in the Idamalayar Dam and the observations carried out are analysed regularly to monitor the safety of the structure. However, a rehabilitation project envisaging improving the safety and operational performance of selected structures such as dams, barrages and regulators, which covers 19 projects under the Irrigation Department and 12 projects of the Kerala State Electricity Board, has been approved under the World Bank funded "Dam Rehabilitation and Improvement Project " to be implemented from January 2011. Idamalayar Dam is one of the projects included in this programme. However, the rehabilitation work on this dam is proposed to be taken up in the second stage.
Irrigation benefits
Irrigation benefits from the water stored in the Idamalayar Reservoir are proposed by utilizing the tail-race water released from the Idamalayar Power house. For this purpose, the project titled "The Idamalayar Irrigation Project" is under implementation. Under this project, the tail race discharge from the Idamalayar powerhouse is picked up at the barrage constructed across the Periyar River at Bhoothathankettu and diverted through a canal system on the right bank of the barrage. The irrigation benefits envisaged covers an area of of wet and dry agricultural lands, out of which the cultivable command area is. The project is under implementation since 1981. The cost of the project was initially Rs 18.5 crores, which as per latest reports of 2007 is stated to be Rs. 539.50 crores. According to the Annual Plan 2010–2011 of the State Government, substantial work of the main canal, the low level canal and the link canal still needs to be completed to derive full planned benefits of irrigation. Apart from power and irrigation benefits, the project situated in scenic forest area also provides recreational benefits of boating in the reservoir, bird watching and trekking.
Construction delays
The Idamalayar Dam project was started in 1970 and completed after 17 years. In an analysis carried out in 2001 on the reasons for the delay in completion of Hydropower Projects resulting in immense cost overruns in Kerala, it was noted that the Idamalayar project could be commissioned only in 1987 with a time overrun of 9 years and consequent cost overrun of 285% resulting in a capital cost of Rs 2.81 per kWh of generation. While technical reasons for the delays were fully documented, the delay highlighted, however, was attributed to the "irrational behaviour of the organized militant labour". A High Level Committee set up by the Government of Kerala, which examined this issue, observed that the Idamalayar Dam project has been the "victim of recurring and long inertial periods of labour unrest". There was also controversy over the contract awards. The government appointed Justice K. Sukumaran to inquire into allegations related to the Idamalayar and Kallada dam construction contracts. Based on his report, Minister R. Balakrishna Pillai and others were prosecuted by a Special Court. The Kerala high court later acquitted them, but in February 2011 the Supreme Court of India sentenced R. Balakrishna Pillai and two others to a year in prison for abusing their positions when awarding the contracts.
