TEHRI DAM

Purushottam Gupta Int MSc Earth Sciences

4 semester

Tehri Dam Project, a prestigious hydropower cum irrigation project, is the first majorattempt to harness vast hydro potential of Bhagirathi river which is fed by Gangotri Glacier. Thestorage project in the Bhagirathi valley in Uttarakhand Himalaya, envisages impounding of surplus monsoon water of the river for utilizing it in regulated manner for hydropower generation and irrigation. It was conceived by the officers of Geological Survey of India in 1949 and investigations were initiated in 1961.

It envisages construction of 260.5m high earth and rock-fill dam across river Bhagirathi, an underground powerhouse with 1000 MW installation and appurtenant works. The water impounded in the reservoir will be utilized to generate 3029GWH of power in a year of 90% availability. It will also provide irrigation to an additional area of 270,000 hectares of land and stabilize irrigation in the command area in Ganga Yamuna doab.

This mega project of 2000MW installed capacity, envisaged construction in two stages.The stage I, is termed as Hydropower Plant (HPP) and Stage II comprising a Pump Storage Plant‐ ‐(PSP), have an installed capacity of 1000 MW each. From inception to the year 1988, the projectwas executed by Irrigation Department of Uttar Pradesh and afterwards by Tehri HydroDevelopment Corpn. Ltd. (THDC). After commissioning of both the stages, the project will facilitate

addition to the installed generating capacity in the northern grid by 2400MW which means availability of annual energy (Peaking) to the tune of 6200Million Units,

ii)additional Irrigation Facilities for 2.70 Lac Hectare of land and stabilisation of existing irrigation facilities (besides above) of 6.04 Lac Hectare of land,

iii) availability of 300 Cusecs (162 million gallons per day) of drinking water to meet therequirement of 40 Lac population of NCR of Delhi in addition 200 cusecs (108 million gallons perday) of drinking water for about 30 Lac population of the towns and villages of U.P.

Additional benefits accruing out of the project area

development of Garhwal Region, relocation and construction of New Tehri town with the provision of all modern civic

amenities, improved communication, education and health, development of horticulture, fisheries and afforestation development of local economy through promotion of ecotourism and water sports

THE DAMThe main Tehri Dam is engineered earth and rock material in a unique height of 261 meter, making it the 8th tallest in the world (Figure 2). The crest width of the dam is 20 m with adequate fl atter slopes on either side giving the width of 1,128 meters at the

bottom. The length of the dam at the top is 575 meters, which is walled up in narrow gorge section of the river Bhagirathi. The dam body is filled withearth and rock material in well compacted layers. In order to retain the water, there is an impervious clay core wall on the dam body along its length, which is compacted to an unprecedented high density. In Tehri Dam this earthen core is centrally located with fl atter sloping sections and is separated from the rock-material with well-graded fi lterson both sides. Earth and rock-fi ll dams are naturally more earthquake resistant than concrete dams due to their large inertia, high degree of fl exibility, and stability. In addition, a fine sand layer is provisioned on the upstream face of the core wall that, in the unlikely event of cracking of core, sand will be washed into the cracks to seal them. The dam is safeguarded with a chute spillway and four vertical shaft spillways to discharge the probable maximum fl ood of 15,540 cumecs, which is equivalent to 10,000 year return period fl ood. .The Morning Glory type vertical shaft spillways are unique from engineering perspective that is engineered at varied elevations in addition to normal chute spillway. In case of extreme events like a monsoon cloudburst, these spillways can drain the incoming fl ood fl ow without hindrance. The Tehri Dam has an underground powerhouse with four units of Francis turbines of 250 MW each, three transformers of 306 MVA capacity and a computerized control system. The generated power fromTehri Dam is being transmitted to the national grid system through 765 kV transmission lines The religious concerns of the Tehri Dam Project werealso vital in addressing challenges to uninterrupted flow of Bhagirathi river. According to Hindu mythology, the Bhagirathi river is considered as the actual Ganga, which has to fl ow continuously from Gangotri to the Ganga Sagar. To address this religious issue a team of experts was constituted, which recommended providing uninterrupted flow of Bhagirathi across the dam body. As per the decisions of expert team, the THDC made provision of continuous flow of water from upstream to downstream which is knownas Aviral Dhara. A piped intake with 40 cm diameter pipe allows 35 cusecs of Bhagirathi water to fl ow across the dam body and exit at the toe of the Tehri Dam, up to Meerut

With a view to provide maximum assurance of safety, the dam has been designedadopting most stringent design criteria, incorporating certain features which would ensure itssafety, in an unforeseen major seismic event.i) A very conservative design slope, with U/S slope of 2.5:1 and D/S slope of 2.0:1, as against relatively steeper slopes in some recent dams built/proposed in region of very high seismicity.ii) A very wide crest of 20m, which increases to 25m at its contact with abutments, has been provided.(iii) A very liberal free board of 9.5m above FRL has been provided to take care of anysettlement, slumping due to earthquake and wave action.(iv) The D/S filter as designed is capable of preventing migration of finest particles (clay flocks) in the event of its cracking and would not permit any piping. A zone of fine (sand) filter has been provided on the U/S face, which in the unlikely event of cracking of core would get washed into cracks and seal them.(v) The dam shell material is being compacted to unprecedented high density of minimum2.36 tons/m3 to ensure little settlement, so that no pore pressure is built up during earthquakes.(vi) The dam embankment is founded directly on bedrock, after removal of all loose, semicompact overburden, thus eliminating possibility of foundation liquifaction during slaking in the event of any earthquake.

(vii) The provision of access galleries at three different levels, on abutments, below dam seat and within the core to enable physical monitoring during

operation, would help taking timely measures,

through these galleries, in the event of any distress .Diversion Tunnel - Tehri Project Upstream

River BhagirathiRiver Bhagirathi originates from Gangotri glacier at an altitudeof 7,010 meters and traverses about 200 kilometers to its confl uence with Bhilganga, where Asia’s tallest Tehri Dam is walled up. The average annual fl ow of the Bhagirathi is 258 cumecs(similar to the Sunkoshi river at Pachuwaghat and Kaligandaki river at Seti Beni, in Nepal). After Tehri, the Bhagirathi traverses 30 km through Koteshwar to its confl uence with the Alaknanda at Devprayag; henceforth, the river is named the Ganga and fl owsthrough Rishikesh and Hardwar, major holy places for Hindus. Of the total annual water yield of 8,135 million cubic meters of Bhagirathi, about one third (i.e., 2,615 million cubic meters) is being stored in the Tehri main reservoir. The Tehri reservoir is designed to reservethe monsoon fl ow from July to September when river stages are high enough, and to utilize it to generate 6,200 GWh of energy

1Diversion Tunnel - Tehri Project Upstream

2 Gated chute spillway and ungated shaft spillway a unique feature of Tehri Dam ‐Project

Geology of the areaThe rocks exposed in the study belong to the Damtha, Tejam and Jaunsar Groups The Chandpur Formation is restricted towards the north by the well-defined North Almora Thrust, trending roughly northwest-southeast and dipping towards the southwest. The Deoban and the Nagthat-Bearing Formations have a thrusted contact (the Pratapnagar Thrust), trending parallel to the North Alomra Thrust and dipping steeply towards northeast. The rocks in the intra-thrust zone and are highly fractured in nature.The reservoir area has been mapped on 1:50 000 scale to establish the lithological and structural setting. The major rock types observed in the reservoir area are phyllites, quartzites and quartzitic phyllites of the Chandpur Formation, overlain by recent colluvial and alluvial materials. Fluvial terraces are present in abundance on both sides of the River Bhagirathi and form fertile agricultural land of this region. Phyllites are exposed on each bank close to the River Bhagirathi. On the left bank, phyllites are generally weathered close to the surface and have a thin soil cover. On the right bank, old terraces are present at lower levels, and thick eluvial materials and colluvial soil cover are present on the upper levels.

The most important tectonic feature in the vicinity of the Tehri dam site is moderate tosteep dipping Srinagar thrust (N60oW S600E), exposed at a distance of about 5km N N E of the‐dam site. This tectonic surface which, at places, is displaced by transverse faults, has a regionalcontinuity of more than 100km. Srinagar thrust is displaced for about 500m by a transverse fault, Dewal tear (N60oE S600W), near Dewal village in Bhilangana valley‐ .

GEOTECHNICAL APPRAISSAL OF TEHRI DAM SITE:

The entire catchment area of the dam is mountainous terrain and forms parts of the Himalayan mountain ranges. It measures about 7691 km2 out of which about 2735 km2

is snow bound. The area is hilly with heights varying from 350 m – 7000m.

Due to weak formations of rocks (owing to the presence of folds, fractures etc.) generally steep slopes and erosion of toe destabilize causing landslides. These are mostly susceptible to erosion and the surface runoff, on its may to plains have made deep gullies.

The catchment area has steep valley slopes. Due to topographical features of the catchment development activities, deforestation, overgrazing and terrace cultivation, the rate of soil erosion in the area is more, which will lead to a higher rate of siltation

Even though the Tehri dam may be regarded to be located in a moderate region as regards to the seismic activity is concerned, the area 20 km NE has been registering appreciable seismicity. This may not only result in decrease of the storage capacity created but also may lead to loss of stability of numerous villages. If the reservoir level happens to be at its maximum, very high surges are likely which may result in endangering the stability of the Tehri dam. The oceanic apron of the Indian plate continued to dive beneath Tibet until the collision of its continental crust with the later and which now behaves like a rigid zone. Because' of this, the entire region is under compression and effect of pore pressure due to impounding of water would be to reduce the effective compressive stress. Impounding of water in this region would have the effect of delaying fracture and the apprehension of the RIS is NOT real.

Controversial Aspects The soil on the upper slopes is unstable and grouting can only take care of

surface problems. The volume of water envisaged (maximum of 15540 cusecs) would further weaken the slopes and affect the dam’s life span.THDC has assumed the dam’s life span to be 150 years, while independent experts say 50 years.

The catchment area management by the forest department has beenentrusted with planting trees to bind the soil. The department is alleged to have failed in doing its job despite huge amounts of money given to them. Shoor Vir Singh Sajwan, Minister of Irrigation agrees with this allegation.However the Principal Conservator of Forests, Uttaranchal, M M Harbola claims that thought the catchment area is about 7 lakh hectares, the forest department was entrusted with the task of greening 52000 odd hectares of which 44000 hectares had already been covered. 39 villages of 3355 families will be totally submerged along with Tehri town while 72 villages with about 2074 families will be partially submerged. According to an NGO Matu, the project affects around 125 villages of which 86 will be partially submerged. This number of fully and partially affected villages may increase since a fresh survey of the rim area has beencommissioned. Many villages are located in the unstable rim area, including villages like Khola, Kangsali and Jawalgaon.