Seminar on case history of Tehri dam Geologist Planet

Content What is dam and its purpose ? Terminologies related with dam Structural classification of dam TEHRI DAM- Introduction Major Components of Projects Engineering aspect of Tehri dam Case Study Failure of dams due to earthquakes Precaution against earthquake Issues and concerns Environmental impacts of Tehri dam Conclusion

Dam is a solid barrier constructed at a suitable location across a river valley to store flowing water. Storage of water is utilized for following objectives: Hydropower Irrigation Water for domestic consumption Drought and flood control For navigational facilities Other additional utilization is to develop fisheries

Structure of Dam Crest Free board Sluice way Down stream Upstream Spillway (inside dam) MWL Max. level NWL Normal water level Free board Sluice way Gallery Heel Toe

Heel: contact with the ground on the upstream side Toe: contact on the downstream side Abutment: Sides of the valley on which the structure of the dam rest Galleries: small rooms like structure left within the dam for checking operations. Diversion tunnel: Tunnels are constructed for diverting water before the construction of dam. This helps in keeping the river bed dry. Spillways: It is the arrangement near the top to release the excess water of the reservoir to downstream side Sluice way: An opening in the dam near the ground level, which is used to clear the silt accumulation in the reservoir side.

Gravity Dams: These dams are heavy and massive wall-like structures of concrete in which the whole weight acts vertically downwards Reservoir Force Gravity (weight of dam) As the entire load is transmitted on the small area of foundation, such dams are constructed where rocks are competent and stable. Example- Bhakra Dam on sutlej river in district Hoshiarpur of Himachal Pradesh.

Buttress Dam: Buttress Dam – Is a gravity dam reinforced by structural supports Buttress - a support that transmits a force from a roof or wall to another supporting structure Google.com This type of structure can be considered even if the foundation rocks are little weaker Example- Roseland Dam in France

Arch Dams: These type of dams are concrete or masonry dams which are curved or convex upstream in plan This shape helps to transmit the major part of the water load to the abutments Arch dams are built across narrow, deep river gorges, but now in recent years they have been considered even for little wider valleys. Example- Iddukki dam in kerala on periyar river Google.com

EMBANKMENTDAMS (Rock Fill or Earth Fill Dams) They are trapezoidal in shape Earth dams are constructed where the foundation or the underlying material or rocks are weak to support the masonry dam or where the suitable competent rocks are at greater depth. Earthen dams are relatively smaller in height and broad at the base They are mainly built with clay, sand and gravel, hence they are also known as Earth fill dam or Rock fill dam Example- Tehri Dam 0n Bhagirathi river. Koldam Dam on Ravi Google.com

TEHRI DAM A most controversy-ridden Tehri High Dam is in reality an engineering marvel of modern India that provides new horizons for implementing high dams in Himalayan region. Construction of this dam was completed in 2006 while the second part of the project ‘the Koteshwar dam’ completed in 2012.

Top view of Tehri Dam Reservoir Google.com

Cross-section of Tehri Dam

Tehri Dam is located at Tehri town, Uttarakhand , and is constructed upon river Bhagirathi. The dam is primarily meant for hydropower in the seismically active Himalayas, a region that is expected to experience an earthquake of magnitude 8.4 . The salient data of the project are as follows: Tehri dam is an earth and rockfill dam and it is constructed on slightly jointed phyllites of different grades. Maximum height of the dam is 260.5 m, and the length of the dam is 575m. Total area of the reservoir in Full Reservoir Level (FRL) condition is 42 km2, and the gross storage capacity is 3539 Mm3

View of Tehri

Municipal water supply purposes irrigation Municipal water supply Hydro electricity

Foundation rock type The folded meta sedimentary rocks exposed at the Tehri dam site form an uninterrupted sequence of Chandpur phyllites (Proterozoic‐III) having variable proportions of argillaceous and arenaceous constituents. Considering the rhythmicity of intercalated bands of arenaceous and argillaceous material and varied degree of tectonic effects in them, the phyllites at the dam site have been classified into mainly four lithological variants. These are, i) Phyllitic quartzite massive (grade I) ii) Phyllitic quartzite thinly bedded (grade II) iii) Quartzitic phyllite and iv) Sheared/schistose phyllite (grade III)

Salient features Type of dam: Embankment, earth and rock-fill Height: 260.5 m (855 ft) Length: 575 m (1,886 ft) Crest width: 20 m (66 ft) Base width 1,128 m (3,701 ft) Impounds Bhagirathi River and Bhilangna river Type of spillway: Gate controlled Spillway capacity: 15,540 m3/s (549,000 cu ft/s)

Tehri Reservoir Water Spread : 42 SQ KM Gross Storage : 3540 Miliion Cum Live Storage : 2615 Million Cum Power House Power House : Under ground Cavern Size : 197mx24mx63m Type of Turbines : Francis Rated Head : 188 M Speed : 214.3 RPM Installed Capacity : 4x250MW Annual Energy : 3568 MUs

Tehri Hydro Power Complex (2400 MW), comprises the following components: This mega project of 2000MW installed capacity, envisaged construction in two stages. The stage‐I, is termed as Hydropower Plant and Stage‐II comprising a Pump Storage Plant have an installed capacity of 1000 MW each. Koteshwar Hydro Electric Project (400 MW)

Sketch of Tehri and Koteshwar Dams

Benefits from the Tehri Hydro Power Complex Addition to the installed generating capacity in the Northern Region (1000 MW on completion of Tehri Stage-I) 2400 MW Annual energy availability (Peaking) (3568 MU on completion of Tehri Stage-I) 6200 MU Irrigation (additional) 2.70 Lac.ha. Stabilisation of existing irrigation (besides above) 6.04 Lac.ha. Additional Generation in downstream Projects 200 MU 300 Cusecs (162 million gallons per day) of drinking water for Delhi which will meet the requirements of about 40 lac people. In addition, 200 Cusecs (108 million gallons per day) of drinking water for towns and villages of . Flood Moderation rovision of all civic facilities; improved communication, education, health, tourism, development of horticultures, fisheries, and afforestation of the region.

UPSTREAM VIEW OF DAM D/S VIEW OF DAM & POWERHOUSE

Koteshwar H.E. Project is an integral part of the 2400 MW Tehri Hydro Power Complex. Koteshwar Project comprises a 97.5 m. high concrete Dam and Surface Power House, housing 4 units of 100 MW each and is located around 22 Km. downstream of Tehri Dam. Koteshwar Project is a run-off-river scheme with minimum diurnal storage. The Koteshwar Project will regulate water releases from Tehri Reservoir for irrigation purposes. The reservoir created by Koteshwar Dam shall also function as balancing reservoir for Tehri Pumped Storage Plant. The Govt. of India has approved the execution of Koteshwar H.E Project (4X100 MW), in April’2000, at a cost of Rs.1301.56 cr. including IDC of Rs.190.04 cr. at Oct.’99 price level (with debt equity ratio of 3:1)

KOTESHWAR HYDRO POWER PROJECT

FAILURE OF EARTH DAMS DUE TO EARTHQUAKES (a) Failure due to disruption of the dam by major fault movement in the foundation (b) Slope failure induced by ground motions (c) Loss of freeboard due to differential tectonic ground movement (d) Loss of freeboard due to slope failure or soil compaction induced by ground motions (e) Piping failure through cracks due to ground motions (f) Overtopping of dams due to slides or rock-falls into the reservoir (g) Sliding of dams on weak foundation materials, and (h) Failure of spillway or outlet works.

Issues The Tehri dam project area lies within the Main Himalayan Block (MHB), in the midlands of Lesser Himalaya, bounded to the north and south by regional tectonic lineaments‐the Main Central Thrust (MCT) and Main Boundary Fault (MBF) respectively. seismically active Himalayas, a region that is expected to experience an earthquake of magnitude 8.4.

Concerns i) A very conservative design slope, 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 any settlement, slumping due to earthquake and wave action.

Continue…. (iv) The D/S filter as designed is capable of preventing migration of finest particles (clayflocks) 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 minimum 2.36 tons/m3 to ensure little settlement, so that no pore pressure is built up during earthquakes.

ENVIRONMENTAL IMPACTS OF TEHRI DAM The Project has identified both positive and negative impacts (i) water chemistry, especially with respect to dissolved oxygen and (ii) turbidity of water. b. Likely impact on biodiversity, i.e., flora and fauna of the area. c. Likely obstruction of movements of migrating fish species during breeding season

Continue… d. Since 109 villages (full or partial) and Tehri town (full) were affected and the residents were to vacate their ancestral homes and agricultural fields, a scheme was prepared, to resettle these people, with the idea to improve their living standard, keeping their social bonds intact. e. Likely problem of water-logging and salinity of the land in the command area.  

conclusion Risk… Risk ????? Impact of every project has two faces 1)positive and 2)Negative Prospect of positive face is much more vivid and prosperous than the negative face. Therefore positive face should be taken in to the consideration for the welfare and development of the country . There is no reward without risk!

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