In the small hours of February 18, 1911, the Central Pamirs - in what is now the Gorno-Badakhshan Autonomous Area of the Republic of Tajikistan - was hit by a violent earthquake which touched off a giant landslide in the Murgab river valley. It came to be known as the "Usoi Obstruction" in memory of kishlak (hamlet) Usoi wiped out with all its residents. That obstruction dammed the river valley where a large lake was formed. A small village, Sarez, found itself under its water, hence the name - Lake Sarez. As much as 17 cubic kilometers of water have accumulated there, 3,250 meters above sea level, in these nine decades. Every year the level of the lake's water rises by another 0.2m. Should the water burst the natural dam formed by the obstruction, it will cause terrible havoc in a large territory of Central Asia with a population of six million. That tragedy can break out anytime.

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by Leonid PAPYRIN, head of the GEON geophysical and geoecological research division, Ministry for Natural Resources of the Russian Federation

A MONSTER BIDING ITS TIME

Back in 1967 research teams of the TAJIKGEOLOGIA R&D Association and the National Institute of Engineering Geology and Hydrogeology (VSEGINGEO) were carrying out aerial surveys to assess the hazard of mud and rock torrents coming down from the high-mountain lakes of the Pamirs. Surveying the right slope of the Lake Sarez basin, just between the Usoi Obstruction (rock dam) and the sai (gorge) Biromband (4,000 to 4,300 meters above sea level), they discovered crevices (fissures) extending for something like two kilometers - what came to be known as the Right-Shore Landslip. Should it slide into Lake Sarez, its water would destroy the lower northern part of the rock dam and set off an immense torrent of mud and rock rushing down the valleys of the rivers Bartang, Pyanzh and Amu Darya.

The possible disaster may be very much like one that occurred in 1963 on northern Italy's river Piave where a giant landslide, Tocc (0.24 km ³ in size) precipitated into a nearby water reservoir, Vaiont, filling its bowl in split seconds with rock 175 meters above the lake level over a stretch of two kilometers. The rock "squeezed" some of the water across the opposite shore edge 260 m high; this water, passing around the arch dam (265.5 m), swept down the valley in a destructive 90-meter tall wave, killing thousands of people within a few minutes. Yet Lake Sarez, mind you, has a hundred-fold as large volume of water, and the feared landsline is far more massive than Tocc. If we compare the absolute water-line marks in Lake Sarez (3,250 m) and in Vaiont (680 m), it will become clear: the possible calamity in Central Asia may be hundreds and even thousands of times as destructive as that of the Piave.

Meanwhile rocked-dammed lakes are a common phenomenon in the Pamirs. Such lakes come and go, spilling their waters in violent cataracts. Now we know: over the past 10 thousand years there used to be at least fifteen large lakes along the rivers Pyanzh, Gunt, Shakhdara and Kudara, but only two-Lakes Sarez and Yashilkul - have survived, while the others have drained their water with the destruction of rock dams. Incidentally, the obstruction at Lake Yashilkul is partway damaged - the upper lip of this rock dam is eroded by water.

Two parent lakes were thought to be in existence there - the ancestor Sarez lake and its predecessor, both formed by two successive landslips in the Murgab river valley. Geologists have discovered yet another seven earlier dammings of the Murgab over 400 thousand years. So the present Lake Sarez is the tenth in geological count. We can-

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not tell how in particular the rock dams were washed out, causing the old lakes to drain their water. Yet one thing is clear: unless man interferes in this natural process, a breaking of the Usoi Obstruction and a consequent draining of Lake Sarez will be inevitable. Unfortunately a reliable forecasting of earthquakes that could trigger off disaster is yet impossible. It's a real cliff-hanger indeed. Verbally.

Way back in 1975 the Soviet Union's government decided on a package of measures aimed at preventing a debacle and utilizing Lake Sarez water for irrigation and power engineering. Accordingly, the national ministries in charge of geology and water management were to undertake comprehensive feasibility studies to see what could be done to ensure a proper level of safety.

Yet it is quite a job of work to carry out any studies at Usoi. Plodding through a pile of blocks in its central part, even a well-trained field worker can cover no more than one kilometer an hour. The Right-Shore Landslip is even more forbidding, with its slope of 30 to 34 degrees and the upper edge hanging 1,500 m above the water surface. Certain sites are inaccessible, they provide no footing and can be reached by helicopter or by boat only. And over some stretches you must tote instruments and equipment.

Still and all, geological and geodetic work was started. It included land and aerial surveys as well as a variety of measurements, in particular, of lake water seepage through the rock dam. Maps were made of the topography of the lake's shores and of the hydrography of its bottom.

In 1988, after a panel of experts from the national State Planning Committee (Gosplan) considered the situation at Sarez, the government decided to keep up the work. Research institutes involved with it worked out urgent practical measures to ensure safety, namely: building up the Usoi Obstruction to a height forbidding water spillage in case the Right-Shore Landslip is down; channeling some of the water through a canal in the lower part of the rock dam or its drainage through siphons down to a safety level; construction of a buffer dam to span the river Bar-tang 80 km downstream from the Usoi Obstruction; and last, bringing the water level down by means of tunnels built in the valley's left slope.

Yet none of these projects was endorsed by the government. In 1989 all the work at Sarez was delegated to a national institute of water management. Soon after, with the Soviet Union's disintegration, this work was stopped altogether...

USOI

Early in the 20th century it was Ivan Preobrazhensky who offered the first ever geological description

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of this natural rock dam. Subsequently Arkady Sheko and Anatoly Lekhanov of the national VSEGINGEO engineering geology and hydrogeology research center, proceeding from topographical and aerial survey data, mapped the dynamics of the Usoi Obstruction on a 1:5,000 scale. This map was revised and amended by Viktor Lim, Yusuf Akdodov and Yuri Kazakov of the TAJIKGEOLOGIA R&D Association. According to their evidence, Usoi's displacement occurred in two stages-that of landslip and that of downfall cum slip. The former was characterized by the displacement of the solid mass bulk about 2 cubic kilometers in size moving at a terrific velocity of 24-25 m/s and thus giving rise to zones of frontal massifs, extrusions, protrusions, intensive compression and reverse displacements, and zones of rear blocks. The other stage was responsible for the fall-and-slide reworking of the separation niche where, throughout its extension, the displaced mass of rock ins composed of blocks, lumps, and debris of limestone, shale and sandstone overlying the rear blocks formed during the first stage.

The mud-and-rock torrent Usoidar has been active in the separation niche ever since the descent of the landslide. In the northwestern part of the rock dam the depth of Usoidar's cover deposits has built up to 20-30 meters over these ninety years, and to as much as 100 meters in the northwest. A chain of slump holes and open fissures has appeared between these two sites. Working with a field party there, Yuri Kazakov discovered permafrost. Most likely, its deposits came down on top of the landslide to be buried then under hillside waste and debris carried by mud-and-rock torrents.

In length the Sarez lake rock dam measures 3,750 meters from the upper pool to the tail-water. Its maximum width from the top of the separation niche to the left original edge makes up 5,200 m, and along the upper pool - 3,150 m. The total volume of the obstruction is 2.2 cubit kilometers, the area of its above- water part - 9.2 square kilometers, and the total area, including the subwater part, 12 km ² . The lower part of the crest of the

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dam rises 45 to 55 meters above water; but this value changes in the course of seasonal fluctuations of the lake's level.

In 1968 the author of the present article reconnoitered in the district to see whether it was essentially possible to carry out geophysical work at the Usoi Obstruction. A year after, we made the first electromagnetic survey in the northwestern part of the rock dam, right in the "mud-and-rock field". And in 1975-1976 we conducted a comprehensive series of geophysical investigations, including seismic studies, gravimetric and magnetic surveys as well as high- precision measurements of temperature and electric resistance of the lake Sarez water.

And thus two zones of low coherence were identified in the body of the rock dam: first, the zone of loose downfall-and-slip and mud-and-rock deposits - washed out readily. The other zone was detected in the central part, a zone of large crevices and hollows which channel the lake's water through the rock dam. We also pinpointed the position of thalwegs (the lowest parts of the flow bed) buried by Usoi deposits in the Murgab and Shadau river valleys. The maximum depth of the Usoi Obstruction is 550 m of the Sarez lake level, and it is 650 to 700 m around the crest of the watershed ridge. No wells were sunk at Usoi at all, which means that geophysical data are the only objective information about the obstruction's in-depth structure.

HOW TO DRAIN THE LAKE?

In area Lake Sarez is 80 square kilometers large; the body of its water varies from 15.5 to 17 cubic kilometers, and its maximum depth changes from 480 to 490 meters depending on the water level. Some of the water seeps through Usoi deposits and issues in springs which have formed a two-kilometer-long canyon, 30 to 35 meters deep, in the tail-water pool of the rock dam. Let us recall that the mean level of Sarez water has been gaining 0.2 m annually over many years. However, seasonal fluctuations mar the general picture. Throughout the year such fluctuations attain to 10-12 meters: in the autumn freshet (September) the water level rises 5 to 6 m above the mean annual, and it drops as much to the normal level in May.

Many years ago a hydrological station was set up at Barchadiv 17 kilometers away from Usoi, down-

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stream the Murgab, where, from 1939 to 1988, yearly measurements were made of the water flowing out of Lake Sarez. During this stretch of time the water expenditure level remained constant at 44.6 m ³ /s. Judging by some random measurements, it was about the same in the 1920s and 1930s. In 1988, however, the maximum level of the lake was up 45 meters over the year 1926, and as much as 50 m compared with 1922. This rise is caused by the colmatage of the inflow zone, that is the clogging of pores and cracks with clay material contributed by the Usoidar mud-and-rock torrent, abrasion of the shoreline, and local subwater landslides in the upper pool of the rock dam.

It is common knowledge that one of the ways of boosting a mountain lake's safety is to bring down its level. In 1977 yours truly suggested a simple and nonexpensive method of solving the Sarez problem by lowering the water level by 40 to 50 meters - in a natural way, mind you. Indeed: considering that the mean annual expenditure of water through Usoi was 44.6 m ³ /s, and its maximum expenditure in freshets being in a range of 70 to 80 m ³ /s, we could enhance the hydraulic connection of the upper water pool with the filtration (seepage) pathways across the obstruction and thus boost the amount of water leaving Lake Sarez. Much later, in 1990, Dr. Hermann Postoyev of the VSEGINGEO Institute and I applied to a head organization with a request to consider our project. And so in 1992 we obtained a national patent - our plan of bringing down the water level of the obstructed mountain lake was approved. The gist of our idea is this. The rock dam is cut by transverse fissures - some of them before the descent of the landslide, and other - during its direct and reverse displacement. Yet on the upper slope of the dam these fissures have come to be closed by landslip and mud-and-rock debris which do not let water through. This debris can be cleared by overburden mining and explosives. Then the rock dam would let out more water, and the level of Sarez decrease as a result.

THE RIGHT-SHORE LANDSLIP

Its exploration is a formidable job indeed. We planned to carry out comprehensive studies way back in

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1975-1976. However, none of the organizations involved with engineering geophysics would turn to this job because of the forbidding terrain. Seismic surveying with the use of explosives was rather hazardous, though this work was done at Usoi in 1976 (with measurements made with the aid of the analog seismic station SMOV-24). Yet the hazard was minimized with the appearance of portable seismic stations sensible to elastic vibrations caused by crow-bars and like tools.

In 1983 we succeeded at long last in one seismic profile, and as many as six two years after. So we could identify three horizons corresponding to strata characterized by a different degree of bedrock destruction. The total thickness of landslide deposits proved to be half of what we had thought, that is it was 230 to 240 meters. And so we could make do with a rather small drilling rig for hole sinking. It took us four months to sink a 240-meter-deep well. The data thus obtained fully confirmed the results of our seismic studies. Like it was at Usoi, a roof of intact dense bedrock underlay the Right-Shore Landslip; magnetometric surveys indicated a low-coherence zone, the same as at Usoi. This zone had visible cracks which came as a danger signal back in 1967 to call attention to the Sarez problem.

Casting a balance of the work done, we saw that the overall volume of loose deposits on the Right-Shore Slip was 1.25 cubic kilometers, and their mean vertical thickness - 250 m. In the event of an earthquake, the landslide will be shifting toward the partition between Lakes Sarez and Shadau at azimuth 240- 250.

ANTICIPATED CONSEQUENCES

Proceeding from the available geophysical data, we can predict the scenario of the imminent disaster. A seismic shock will cause the Right-Shore Landslip to precipitate into Lake Sarez. A 200-250 meter tidal wave will sweep above the lower northern part of the rock dam. Coming next will be a mud-and-rock torrent, thick with solid material, raging below Usoi. The water level in the Bartang will rise 50 to 100 meters, engulfing loose clastic (fragmental) material from the slopes and activating landslips on low-coherence sites. The mud and water will combine into a violent stream rushing down the Murgab river valley in the northwest and moving in a turbid How of mud into

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Lake Sarez in the northeast. The width of the rock dam will contract to 150 to 200 meters, and subsequent tides will clear the clogged crevices and hollows between the packets and blocks of the obstruction's deposits. The seeping water will wash the dam underneath to a depth of 100 m from the present-day level of the lake.

The next, second series of mud-and-rock torrents will take a longer time compared with the first. Moving even a larger mass of loose and fractal material, they will come to a stop at the confluence of the Bartang and the Pyanzh, since the Bartang's bed is much flattened out, and then the flows will be hitting against the left steep bank of the Pyanzh in Afghanistan. The solid material carried there will form an obstruction which will dam the river for a time. A lake that will appear as a result will take in water from Sarez and the Pyanzh. The ultimate destruction of the natural dam will give rise to a disastrous flood of mud and rock that will rush as far as the lower reaches of the Amu Darya and destroy population centers and enterprises in Afghanistan, Tajikistan, Turkmenia and Uzbekistan. These states of Central Asia will sustain colossal economic damage. And it will take decades to repair the ravages...

CAUSES OF FAILURE

As Colonel Grigory Spilko of the Russian Army General Staff said in an article published in the journal of the Russian Geographical Society in 1915: "The enormity of the Usoi phenomenon makes one stop and think of how weak human forces are."

Although ninety years have passed since the birth of this terrible dragon of Central Asia, human forces are not strong enough yet to render it harmless. The point is that the interest in this problem has been sporadic, off and on. It flared in the wake of disastrous earthquakes in other regions and slackened afterwards. One of the causes thereof was the financial angle. As we have said above, this mountainous area is hard of access and too difficult for exploration. Conventional techniques and procedures of geological and engineering studies are a very costly business there.

SAREZ: INTERNATIONAL PROBLEM

The main objective of geological and engineering studies at Sarez is to find ways of taming the lake monster. Even though a significant

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amount of work was done in years past, its evidence is known only to a narrow group of experts. Meanwhile, our findings are of particular value today. For years the author of this article has been collecting information on Lake Sarez and its basin; he has collected a wealth of historical, geological, geophysical and hydrological material. Its evaluation and publication could help find rather simple and nonexpensive ways of making the deadly lake harmless, and detailing a program for further research. Needless to say, this is a difficult job that will involve huge expenses. Neither Tajikistan nor any other country of Central Asia will be able to cope on its own. What they need is to pool efforts with the assistance of Russia and the world community at large.

An international expedition should be set up for work in the area. And its engagement should be financed adequately. Since the possibilities of conventional research methods of surveying have been all but exhausted, and a large amount mining and drilling work is all too expensive and laborious, we must zero in on advanced geophysical and engineering methods.

To begin with, it is urgent to use seismic and electromagnetic survey methods to explore the structure and the strength potential of the northern and central zones of weak coherence of the Usoi Obstruction and the subwater part of the Right-Shore Landslip. We must pinpoint the pathways of water seepage and the sites of inflow in the upper pool of the rock dam. We must investigate the in- depth structure of the lake's western part so as to make a seismic hazard forecast when the water level is down. And next, we should check geophysical research data by sinking wells and doing further geophysical work in the wells.

All that will enable us to find a rather simple, nonexpensive and effective way of making Sarez a safe lake.

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