TEHRI DAM

INTRODUCTION AND VARIOUS DAM TERMINOLOGY

Planned exploitation of various water resources of the Perennial Himalayan Rivers is essential for sustained development of our society and improving the standard of living of our vast population. For harnessing this important resource development schemes are being framed.

India is the second most populous country in the world. To raise the standard of living of this vast population, the national emphasis on industrialization and enhancing agricultural product will continue. For sustained development of industrial and agricultural sector of our economy, the importance of two basic inputs viz: Water and power can't be overemphasized. At present, agriculture sector is being allocated most of the water available. This is essential to keep up the production of enough nutritious food. With development, the need of water for industrial use has also become substantial. The requirement of water for urban population for domestic use also has swelled at a fast rate. Thus, a keen competition has developed amongst agriculture, industry and domestic uses for allocation of available water resources.

The bountiful nature has provided the country with enormous water resources in the form of seasonal rains and perennial snow fed rivers emanating from the Himalayas. But the distribution of rain in time and space is unfavorable. Most of the annual rainfall is confined to the monsoon period extending over three months only. It is therefore essential that surplus monsoon flow is stored in reservoirs for its proper management and optimal utilization.

For exploitation of water and power potential of the Ganges river system several hydropower and multipurpose projects have been proposed/are being constructed on the tributaries of the river.

Tehri Dam Project is one of such major schemes. 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 Yumna doab.

THE DAM

LOCATION, PROJECT WORKS & CONSTRUCTION PROGRAMME:

The Planning Commission approved Tehri Dam Project in 1972. Due to paucity of funds and resistance to the construction of dam by local people. Only the infrastructure works alone could be carried out till 1978. The construction of civil works was started in right earnest since 1979. Two rights bank diversion tunnels and two left bank diversion tunnels were expected to be completed by 1985. Tehri Dam Project involving the construction of a 260.5m high earth and rock-fill dam and an underground power house of 2000MW (8x250MW) installed capacity is one of the most ambitious projects of the country, in the Tehri District of Uttaranchal. The project had a long construction period of over 10 yrs.

Tehri Dam is located about 1.5km downstream of the confluence of Bhagrathi river and its tributary Bhilangana, is a clay-core rock-fill dam. The diversion works were initially designed for a flood discharge of 12,850 cumecs (routed discharge 8120 cumecs) which corresponds to a flood of 1000 year return period. Two tunnels (T₁ and T₂) of 11.0m diameters (standard horse-shoe) section off-take from Bhillangana and are located on left bank of the river. Two more diversion tunnels (T₃ & T₄) of 11.0m diameter each are provided on right bank of the river. The invert level at intake of T1 and T2 is E1. 632m while the intake levels of T3 and T4 are E1. 606m and E1. 609m respectively. These tunnels pass the design-routed discharge of 8120 cumecs at a reservoir level of E1. 694.5m. The top of the cofferdam which ultimately becomes a part of the main dam was planned at E1. 694.5m. The total fill quantity of 260.5 m. high main dam is 26.53 million cubic meters including a quantity of 2.28 million cubic meters in the first stage cofferdam. The chute spillway with two SW cisterns is provided on the right bank for a probable maximum flood value of 15,540 cumecs which gets routed to 11800 cumecs. The spillway crest is at E1. 816.5m.

Four head race tunnels of 8.5m diameters each off-take from left bank at E1. 720m.The average length of one headrace tunnel is about 1.085m. The tunnels end in differential surge tanks of 26m in diameter. Two penstocks of 5.25m in diameter take off from each surge shafts leading the water to 250MW vertical axis Franchis machine units. In the first stage development of the power station, conventional machines of 250MW each were installed and in second stage development pump-turbine units of 250MW each is installed in a separate cavity.

Water Availability

River Bhagirathi originating from Gomukh glacier has a catchment area of 7511sq.km. Based on the discharge data computed from observed discharges at Raiwala for the period 1941-1963 and the observed discharge at dam site for the period 1964-1978, the available values of runoff are given below:

Maximum annual runoff – 1.12 x 10¹⁰m³.

Mean annual runoff – 0.82 x 10¹⁰m³.

Minimum annual runoff – 0.55 x 10¹⁰m³.

The average discharge in various months as computed from 36 year’s discharge is as per table 1.

Month	Cumec	Month	Cumec	Month	Cumec
January	62	May	145	September	489
February	62	June	296	October	187
March	72	July	642	November	94
April	95	August	890	December	74

GEOTECHNICAL PROBLEMS AT TEHRI DAM SITE

GEOLOGY

The dam is located in middle Himalayas, the mountains of this region being of recent origin. The rocks exposed in the vicinity of the dam and towards north and east of the dam site are in contact with younger dolomites and quartzites of the Garhwal group. The rock formations of the Tehri Dam Site are phyllites of Chandpur series.

Very detailed and extensive surface and sub-surface investigations comprising surface mapping, drilling and drifting have been carried out. Magnetic survey of the entire region has also been conducted to carefully locate various geological features of the area. All lineaments and features have thus been identified. The investigations have shown that there are several faults in the area. The region comprises of two important tectonic dislocations namely the North Almora Thrust, also called the Srinagar Thrust, five Kms from the dam site and Tons thrust. The Tons thrust is marked by mylonization and intensive crushing of rocks along the S-W contact terrain in the southern parts.

The results of initial drilling in the area gave indications of a river bed fault at dam site. Subsequently, inclined drilling, however, ruled out any such fault. To draw definite conclusion on the point about 65mts long under river drift has been excavated at dam site. This has conclusively ruled out presence of any riverbed fault. The permeability of the rock encountered in the under river drift is quite low.

Having located all the major and minor lineament of the region, it can be asserted that the geological conditions at Tehri dam site are NOT worse than those of high earth dams constructed in recent past world over.

SEISMICITY

On the basis of past history and future possibilities from point of view of seismicity, the country has been divided into five zones. The Tehri dam is located at the border of zone IV and V (seismically prone area).

Considering all the aspects, the standing committee in Central Water Commission (constituted for deciding the value of seismic coefficient of each river valley project) decided that the value of horizontal acceleration for preliminary design might be taken as 0.15g. The design may be modified by three dimensional dynamical analysis and vibration table test, if necessary.

In pursuance of the decision of the committee for evaluating the rational parameters for developing earthquake resistant design for Tehri Dam and other important works, the Deptt is carrying out seismological studies of Earthquake engineering, University of Roorkee. To facilitate seismic studies, Tehri dam site seismological observatory has been setup for collecting data and is operating since 1970.

As reported in 'Bhagirathi' a periodical from the Central Water Commission, Ministry of Water Resources, Sewa Bhawan, R.K. Puram, New Delhi – 110 066 in the year 1992, Tehri Dam has been found safe and adequate by an independent assessment by Soviet experts, retained as consultants by Tehri Hydro Power Development Corporation (THDC). THDC informed that seismic status of the Project was of unique importance in view of the high seismicity of the area in which the dam was located. To ensure the safest dam even in the event of an earthquake of the sizes reportedly encountered in the region, the aspects reviewed by Soviets included site specific assessment of seismicity based on:

· Review of historical occurrence of earthquake.

· Identification of tectonic features around Tehri area based on studies of satellite imageries and ground surveys.

· Field surveys for Paleo-seismological investigations.

· Micro-earthquake recording for establishing seismo-genicity of tectonic features.

· Assignment of magnitudes of earthquakes for various tectonic lineaments.

· Assessment of peak ground acceleration using distance-magnitude-acceleration relations.

The Hydro Project Institute, Moscow had checked the dam section for levels of peak ground acceleration corresponding to a magnitude of earthquake and found safe using dynamic material parameters and using elasto-plastic models based on tests carried out in Soviet Unit. The analysis as concluded cover, interalia, and dynamic stress analysis by finite element method. Plastic displacement analysis using basic approach of Goodman and Sead and a 2D dynamic stress analysis by Finite Different method using elasto-plastic model. The soviet design confirmed that the dam section is safe, no plastic zones are formed in the dam and the settlement of dam works out to only 0.6m due to severe earthquake. This study should set at rest the vague fear psychosis that is deliberately spread by anti-dam lobbyists. The general insertion in press media is not sparing even a seismic design aspects (which should have been left to the design engineer, for being purely technical) as one of the alleged reasons for dam work to stop, are a motivated campaign, perhaps on large dams.

FAILURE OF DAMS DUE TO EARTHQUAKES

Man has always been keen on improving his material well-being and his standard of living. In the process, construction of dams and reservoirs has become essential for storage of water for irrigation, water supply and power development and to tame of fury of the river at the times of flood. These reservoirs are good "servants" as along as they are under control, but are a menace of public safety in case of any failure, causing considerable loss of life and property. Nature, often interferes with any change caused in the environmental factors by its so called "acts of Gods" in the form of unprecedented floods or unpredictable earthquakes. Recent catastrophic seismic disturbances in Koyna in India, in Kariba in South Africa, and in Kresmasta in Greece have opened up the possibility of a new type of disaster – MAN MADE DISASTER

The earthquake may occur due to natural or artificial causes. The natural causes could be tectonic, plutonic or volcanic, depending upon the nature of the disturbances caused at the source, which may be structural in origin to volcanic activity. The artificial earthquakes, on the other hand, may be due to blasting operations or from other causes as described later. Also, slipping of certain rocks in the earth's crust against other rocks along faults causes an earthquake. When this occurs, waves are transmitted from the point of disturbance in all directions through the rocks of the earths crust. The intensity of vibration decreases with the distance from the source and are finally imperceptible at a very large distance. These waves or vibrations, produce, as they travel through the rocks, similar vibrations information and past records, furnished by seismographs or similar instruments. The form of these waves can be assumed, with reasonable accuracy, to be simple harmonic. The maximum acceleration (a), which is produced in the vibrating rocks or soils, depends upon the rigidity of the vibration and its amplitude. The acceleration increases with shorter periods and larger amplitudes. In a region, where a severe earthquake is anticipated, the probable amplitude of movement in the rock may be of the order of 6mm. and damage to structure and property would be produced when the period of waves is of the order of a second or a second and a half. It is usual to express the maximum acceleration due to earthquake as a fraction of the acceleration due to gravity. The acceleration of the order of 0.30g to 0.50g is considered very severe.

DESIGN OF DAM FOR EARTHQUAKES FORCES

Under the influence of seismic acceleration, the dams are subjected to greater inertial forces and additional water thrust, apart from these, it is necessary to consider the consequences, if "synchronism" or "resonance" occurs.

Natural period of the vibration of a structure is the period with which a structure will vibrate when subjected to a single shock and allowed to vibrate freely until the inertial effects completely dampen the vibrations. This period depends upon the weight, the dimensions, the elastic coefficients of the material of the structure and the external loads. If the natural period of vibration of the structure and period of forced vibration are the same, a condition known as "synchronism" or "resonance" occurs, maximum stresses are then produced in the structure. Also, on the ratio of these two periods depend on the relative displacement, acceleration etc. of the top of the structure with respect to the corresponding quantities at the base of the structure.

As a result of vibrations caused by an earthquake, the rock under the dam moves. If slipping or rupture between the foundation and the dam should be avoided, the dam should follow the movement of the foundations. Forces, must therefore, be applied to overcome the inertia of the structure and produce such motions. Additional stresses, are therefore, develop in the dam and its foundations. The forces ‘P_e' required to accelerate the mass of the dam is given by:

P_e = sW ---- (i)

Where

P_e = Additional earthquake force in Kilogram

s = Ratio of the seismic acceleration to the acceleration due to gravity.

And W = Weight of the dam (in Kg).

In general, the intensity of the inertial force due to earthquake at any depth where the width of the dam is 'b' meters is given by the equation :-

P_e = s b rCos q ---- (ii)

Where

P_e = Intensity of earthquake force in square Kilogram

b = Width of the dam at the point considered.

r = Specific gravity of the material of which the dam is built.

q = Angle between the assumed direction of the earthquake motion and normal to the face of the dam.

s = Ratio of the seismic acceleration to the acceleration due to gravity.

Now, in order to simplify the calculation, earthquake acceleration on dams founded on rocks is usually assumed to act at the center of gravity of the mass. It should be noted, however, in the case of equation (ii), that the total force due to earthquake is obtained by computing the area of the force diagram and should be assumed to act at the center of gravity of the force diagram as is done while computing the water thrust on a dam. Seismic acceleration may act in any direction and guidance, with regard to the direction of acceleration of magnitude (s), which has to be taken from official reports. Generally, in regions of mild seismicity, the value of s is taken as 0.10 and in severe earthquake regions both horizontal and vertical accelerations of magnitude 0.15 and 0.10 respectively should be considered. The horizontal acceleration is considered to be positive, if it acts in the downstream direction and is considered negative if it acts in upstream direction. For the condition when the reservoir is full, positive earthquake condition should be considered and when the reservoir is empty negative earthquake conditions should be considered in the design. The upward acceleration, if it is to be considered in the design, opposes the acceleration due to gravity and reduces, momentarily, the effective weight of the structure.

Inertia of the mass of the stored water in the reservoir when subjected to seismic acceleration produces an additional water thrust. According to Prof. Westergaard, this additional water pressure is assumed to be a parabola and the intensity (f_e) due to an earthquake of acceleration sg at a depth 'y' meters below the water surface in a straight dam having a vertical face is given by: -

f_e = C_e . s Öh.y. ----- (iii)

Where,

h = Max depth of water in metres stored in the reservoir.

C_e = a coefficient having dimensions of specific weight and its computed from the equation 816.

h. ²

C_e = 1 – 4.72 -------------------- ------ (iv)

1000 t_e

t_e in equation (iv) is the period of earthquake in seconds usually assumed as one second. It has also been pointed out by Brahtz and Heilbron that the magnitude of additional water pressure is significant only in the case of reservoirs having a fetch more than three times the depth. Vol Karman has shown that this force could be computed without introducing much error, by simpler equation of the form f_e = 0.555 s w y², whereas the exact equation is

f_e = 0.543 s w y², where w is the specific weight of water. This force is applied at a height of 4y/3P above the base of dam.

On the basis of investigations, carried out on dams having vertical or sloping faces, Zangar has proposed the following equations in dimensionless form for computing the intensity of additional water pressure on dams storing water to a depth of 'h' meters. It is as follows:

F_e = C.s.W.h. ----- (v)

C is a dimensionless constant giving the distribution and magnitude of pressure and is given by:

C = Cm/2 [y/2 (2-y/h) ^- + Ö y/h (2-y/h)] --- (vi)

The values of 'Cm' and 'C' could be read directly from the curves.

The total force (Fe) above any elevation 'y', assuming parabolic distribution is, therefore, given by:

F_e = 0.726f_e^.y ----- (vii)

And the overturning moment about that elevation is given by :-

M = 0.299.fe.y² ----- (viii)

These equations are more accurate for low dams.

The influence of curvature of the dam on the inertial forces acting on the body of water has not been clearly understood, however, equations (i) – (viii) can be tentatively applied for the arch dams also. Nevertheless, these equations are true and correct only if the plane on which the force is acting is normal to the line of earth movement.

WHAT IS RESERVOIR INDUCED SEISMICITY (RIS)?

The seismicity induced on impoundment of water in storages created in dams, which imposes new stresses in the rock strata below the reservoir, not only on account of dead loads, but also due to pore water pressure build up, is known as reservoir induced seismicity (RIS). This situation is likely to prevail on some of the larger dams under construction in India (Tehri, Sardar Sarovar and Ranjit Sagar) :

Do the impounding of water in reservoirs created by dams induce seismic disturbances leading to earthquakes? The above question is NO LONGER the ‘million dollar' question most difficult to be answered. It is well accepted now that the impounding of water stimulates an increase in seismicity that, in the case of previously faulted rocks below the reservoir may trigger the earthquake if the rock foundation is favorable to inducement. To monitor the seismic activity around the reservoir, in number of cases, sensitive seismographs have shown alarmingly high ‘Swarms’, which need to be carefully considered.

At the initial stage, understanding of the mechanism of RIS was inhibited due to non-availability of data relating pre-impoundment period seismicity in the area. Two major contributors to RIS are:

(i) The load resulting form the impoundment of water in the reservoir and

(ii) Buildup of pore water pressure in the rocks due to hydraulic continuity of the rock formation.

There are a number of promoters and retarders which lead to attenuation of RIS or its subsidence.

EXTRACTS FROM VARIOUS SOURCES REGARDING RIS:

(1) There is enough statistics to support the idea that due to massive load of water impounded behind the dam, and on the reservoir floor, in hither to unloaded, impresses new strains in the rock strata below the dam and the reservoir.

(2) If soft rocks without fractures are present, then there will be accumulation of stress and the deformation is gradual and continuous.

(3) If the rock strata is having the character of being brittle and hard (eg. Trap, quartzite, granite, basalt), there is accumulation of strains till break or rupture point is reached and a sudden disturbance or tremor occurs due to rupture of rocks. Among soft rocks not developing accumulation of stresses fall sandstone, mudstone and clay stone etc.

(4) But, even a small earthquake needs hundreds of cubic Kms of rocks to rupture and the weight of both lake and dam is too small to cause such a rupture. Thus, a dam causes earthquakes only in such areas where shifting of rocks takes place even without dams and new loads can trigger off such pending earthquakes somewhat earlier than due.

Next promoter of the RIS is the water seeping along fault planes, cracks and other discontinuities, which lead to building up of pore-pressure. This leads to changes in the geological environment prevailing below the reservoir at some depth. Seeping water also tend to lower rock strength of the rock mass as a whole because the pore-water pressure buildup leads to down in principal stresses acting along the fault planes. This may tend to permit movement along fault planes, thus contributing to the RIS. Hydraulic continuity to deeper larger rock bodies appears to be an essential condition for this promoter.

SUMMING UP ALL THESE ABOUT RIS:

1. RIS may be confined to an area of 25-30 km. around the lake normally.

2. The onset of RIS occurs mostly immediately after fist filling of the reservoir but in some cases there is a time lag of a few years noticed between onset of RIS and first filling.

3. The most probable maximum magnitude of shocks due to RIS is between the ranges of 3-5.9 (Richter's scale) after the first filling. Generally, an increase in seismicity of the area due to RIS is confined to increase in frequency of small magnitude earthquakes (3-4). But if two small shocks come at close intervals they have an accumulated effect of a much bigger shock.

4. Enhanced seismicity has been following faster loading in a few cases.

5. No direct relationship between volume of water of reservoirs and the maximum magnitude of the tremors could be established.

6. RIS activity decreases with time and terminates after a couple of years.

7. The most favorable tectonic regime seems to be with moderate tectonic stress and the vicinity of active normal or strike-slip faulting.

8. RIS events are characterized by relatively shallow foci and the moderate magnitude in most cases.

9. Presence of stronger and brittle rocks promotes RIS activity whereas soft homogenous rocks act as retarder as far as RIS is concerned.

10. The maximum value of magnitude and intensity of earthquakes due to natural seismicity of the region is NOT increased above the natural seismicity level.

11. Hydraulic continuity to deeper layers is one of the essential conditions for inducing RIS.

GEOTECHNICAL APPRAISSAL OF TEHRI DAM SITE:

A 260.5 m high earth-cum-rock fill dam is to be built across the river some distance below the confluence of the river Bhagirathi and the river Bhilangana. It provides a reservoir extending 69 km upstream with stored water volume of about 3550 million m³. The site is located in the middle Himalayan mountain region where rocks are of recent origin. The area is traversed by faults and the geological conditions encountered are rather complex relating tectonic features and lineaments. The main central thrust (MCT) lies between the worst affected area of Oct. 1991 earthquake of Uttarkashi, whereas the Main Boundary Fault (MBF) passes near Rishikesh. As both MCT and MBF are quite near Tehri, the dam is regarded belonging to seismically active area (Category V).

A seismological observatory has been established along with a number of recording sites (32 in number) around the reservoir to monitor pre-impoundment seismic activity as well as the post-impoundment disturbance if its takes place. Historical data relating to the site have been collected and analyzed to obtain synthesized pattern of probable oscillations likely to occur. The evolved pattern will also be used to check the safety of the design evolved.

Most recent earthquake, which occurred in the area, was on 20^th October 1991 and that of Tehri – Garhwal earthquake of year 1999 (of magnitude 7.6 in the Richter scale). The exact location of epicenter of this earthquake, which caused widespread destruction, leaves a subject of controversy among experts and the government agencies.

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. Also, according to Shri P S Nautiyal, Former Director General, G.S.I., the hillsides bounding the reservoir have got steep slopes and are liable to fall into the reservoir even on small disturbance. 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 available data has raised serious doubts about the reservoir induced seismicity (RIS) activity is likely to trigger an earthquake in the area after impoundment of water in the reservoir. A question which is being asked is – if a small reservoir of the Maneri – Bhali hydroelectric Project could cause an earthquake of 6.5 as experienced on 20.10.1991 what could happen when Tehri reservoir is filled? Some people cite example of Koyna dam in India (on Koyna river) where major earthquake occurred soon after filling of Koyna reservoir and some other dams in different parts of the world in support of their apprehension regarding induced seismic activity.

In this regard it may be pointed out that Koyna dam is located in a region, which is believed to form the oldest part of the earth crust (being in the peninsular India), has very sound geological conditions and lies in ZONE I of seismic map of India. But in 1967, an earthquake of magnitude 6.5 on Richter scale devastated the area. On the other hand, no such phenomenon has been observed on any of the several dams constructed in the Himalayas, a region so well known for seismic activity.

According to some eminent geologists, 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. This is one of the latest opinions given by Dr. Devendra Kumar, Superintending Engineer, Tehri dam circle IV, Government of Uttaranchal, Rishikesh.

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.

It is being apprehended that introduction of intensive surface irrigation in command area of site may lead to rise of the water level which may, in turn, cause of the site may lead to rise of the water level which may, in turn, cause the reservoir rim slippage causing large scale movement of soil mass. Because the fluctuation in water level in the lake may cause adverse effects on the saturation of rocks and the slopes along the reservoir rim.

Tehri Dam Dams Around the World Dams Around the World II World Rivers Review, June 1996