The beauty of Idukki dam explained well ..
Some musings on Idukki hydro electric project (IHEP)
Sreekumar.S,
Chief Engineer( Retd),KSEB
Of late Idukki project especially
Cheruthony dam with all the five radial crest gates partially opened and discharging up to 8 lakh litres per second has been stealing the thunder and the entire electronic and print media vying each other to beam the grand and rare spectacle across the world. Of course Cheruthony dam do deserve the publicity it is getting but as an engineer associated with all the three dams including its spillway gate operations , Idukki arch dam situated quite nearby should not feel alienated and let down by any means and I felt obliged to pen a few words .
It was early 1983 , a year or so after my my joining service at KSEB , I was posted at Idukki project in charge of the dam monitoring section. It was an obligatory posting after undergoing 5 months of intensive training at Montreal, Canada in Dam Monitoring & Instrumentation as part of the transfer of technology programme between Canada and India under CIDA ( Canadian International Development Agency) who aided the whole project.
The Idukki , Cheruthoni and Kulamavu dams forming part of the Idukki HEP were still in their infancy ( project was commissioned about 7 years back in late 1975) and constantly being monitored through the observation of the various parameters through the embedded instrumentation like electronic stress and strain gauges , base meters, suspended and inverted pendulums etc.
Idukki dam has several firsts to its credit. Simply speaking it is the first double curvature thin shell parabolic arch dam in India or in Asia. The Canadian consultancy firm M/s SNC who assisted in designing this unique structure had by that time completed the design and execution of a much bigger multiple arch dam namely Daniel Johnson across Manicougan river in Hudson bay , north of Quebec province in Canada and I am privileged to see that super dam ( 702 feet high with a volume 5 times of Idukki) considered to be the parent dam of our Idukki.
Idukki dam's statistics are astounding and features jaw dropping. It is almost 555 feet high from the deepest foundation ( It is hard to forget this figure as I had linked it to the famous cigarette brand State Express 555!) .
Double curvature means it is built with curves in both vertical and horizontal axis. Thin shell denotes a portion of hen's egg shell known for its phenomenal strength.Eggs have a reputation for being quite strong under compressional loads. You won't believe your eyes when you see how strong they actually are — an egg can support a person!As compared to an egg shell having shell thickness of 0.6 mm , the Idukki dam bottom is more than 30,000 times thick at about 19 m and it tapers to about 6 m at the dam top.
Shells structures derive their strength from curvature and so the geometry of the structure matters a lot.When the load applied is uniform/distributed, the forces experienced by the shell are transmitted as in-plane shear forces and the stress in the direction of thickness is negligible. Most importantly the principle of arch action (Instead of pushing straight down, the load of an arch is carried outward along the curve of the arch to the supports at each end. The weight is transferred to the supports at either end. These supports, called the abutments, carry the load and keep the ends of the spreading out) has been incorporated in the Idukki dam by the ingenious concept of an INVERTED ARCH. Here the huge thrust from the water is transmitted to the natural abutments viz. Kuravan and Kurathy Mala(hillocks) of igneous granite and charnockite formed millions of years back.
The major advantage derived from Arch dam is minimal amount of concrete material required as the stresses of the thrust of water is taken care by both arch action and cantilever action requiring considerably small width at the bottom. Had Idukki been constructed a conventional Gravity type dam , it would have consumed about 4 to 5 times as much concrete . With Idukki dam having 4.5lakh m^3 volume , one can calculate how much concrete has been saved by this clever design vis-a-vis Cheruthony gravity dam despite being shorter by 100 feet at 450 feet high having a volume of 17 lakh m^3.
As mentioned earlier my job was to monitor the performance of the dams due to the fluctuating water levels in the reservoir . Though these dams appear to be solid monolithic impervious structures from outside , actually it is not so. In order to facilitate inspection a labyrinth of galleries have been provided in the core of the dam at different levels all connected through inclined galleries with steps . So also a maze of formed drains have also been provided through which whatever water seeping through the body of the dams are collected and safely let out . Lifts have been provided for gaining access to these galleries from dam top. While the lift machine room is visible for Cheruthony and travels through the dam body itself for Idukki due to the thin shell and parabolic arch section , the lift well has been provided inside the right abutment which is Kurathymala. It travels a vertical distance of almost 50 floors ( +500 feet) from gallery @ 1900 feet( all levels above Mean Sea Level in short MSL) to dam top +2410 with two intermediate stoppage at +2100 & +2300 feet galleries) . That time it was one of the tallest lifts (OTIS) in India which even now I am sure is true for our state . As the AE in charge It was part of my duty to maintain the trouble free operation of the lifts and a privilege to accompany VIP's and high ranking officials during their visit to the dams and for the travel in to galleries through the lift.
I still recollect while descending with VIP's from top suddenly the lift got stuck about 250 feet( 25 floors )below, and I had to climb all the way up through the slippery rung ladder provided in the vertical lift shaft with a free fall of about 50 stories so as to reach the machine room and meddle with some control relays to bring the lift up. That was one frightening experience and even now I get cold sweat recollecting it.
Similarly my duties very often took me to the dam galleries which are dimly lit ,cold eerily silent and lonely ( except the company of 2 or 3 staff to carry torch and small equipments for taking readings) environment and spent hours together. At times during such inspections power failure would happen and in such cases would have to climb the treacherous steps to the exit with the faint glow of a torch in pitch dark. Mind that hardly few meters away huge reservoir with 2 Thousand Million Cubic meter (2000TMC) of water is
thrusting against the dam body. But we had absolute faith that the dam will not cheat . And for the next five years or so the galleries of Idukki , Cheruthoni and Kulamavu dams were practically like a second home for me. Such intimate was my association , I believe even now the dams may perhaps remember me if I go near it!
Last week the spillways of Cheruthoni were opened with a maximum regulated discharge of 8 lakh litres per second. At least few people had a doubt what would happen if more inflow to the reservoir had occurred demanding higher spillage. To that query , Cheruthoni spillways can handle even 5 times that ( 42.5lakhs/ sec)spill , but pray that nature will not test us to such extreme level.
To sum up I can confidently state that the KSEB dams are built to last and they will withstand the vagaries of nature and act as our guardian Angeles for a long time to come from deluge.
