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Usoi Dam & Lake Sarez, Tajikistan: Complex Mega-Flood Hazard
On February 18, 1911, an earthquake 7.4 in magnitude violently jarred the territory of the Gorno-Badakhshan area of the Pamir mountain area of Tajikistan in Central Asia, triggering a massive seismogenic rockslide that discharged about 0.5 cubic miles of rock into the Murgab River valley. The rockslide completely filled that valley and completely obstructed the flow of the Murgab River, which is a headwater tributary to the Amu Darya River (Oxus River of Greek times), the largest river in Central Asia. The rockslide formed a natural dam, which at 1,970 feet, is the highest (natural or manmade) in the world. (1-2) By comparison, the manmade Hoover Dam in the states of Nevada and Arizona, USA, is a “mere” 726 feet high. (3)
O.E. Agakhanianz reported in 1989 eyewitness reports of the 1911 earthquake: “After the collapse, tremors continued for a few more days; dust settled only in three days and rocks kept falling from the slopes for fifteen more years. (4) During the four years after the 1911 earthquake, the so-called Usoi (Usoy) Dam—named after the village the seismogenic rockslide buried—impounded a freshwater lake (Lake Sarez) 37 miles long, with a maximum depth of 1,800 feet and a total volume of about 4 cubic miles of water. Lake Sarez received its name from the town drowned by its rising waters. (1) The surface of Lake Sarez sits at an altitude of 10,700 feet above sea level. (2) Lake Sarez is situated practically in the center of the Pamirs. (2) Note that there is a smaller tributary impoundment of water off Lake Sarez, which goes by the name of Lake Shadau.
Consider, in comparison, prehistoric Glacial Lake Missoula in the USA’s Pacific Northwest. During the Pleistocene, Lake Missoula contained about 530 cubic miles of water (about half the volume of Lake Michigan, another glacial lake). A 30-mile long ice dam impounded this huge amount of water. The ice dam, which gave way and reformed multiple times, released the impounded water at up to 9 cubic miles per hour. The treacherous water flow scoured away 50 cubic miles of soil and basalt from the Columbia Basin, leaving channeled scablands, coulees, dry falls, deep canyons, exposed rock, braided channels, huge potholes and ripple marks 30-feet high. All of these topographic structures remain visible today. (5)
Consider a second comparison--the reservoir behind manmade concrete Vajont Dam in northern Italy. It contained about .03 cubic miles of water when a mountain landslide of about 0.06 cubic miles of rock disastrously slid into the reservoir on October 9, 1963, displacing about 0.02 cubic miles of water over the top of the Vajont dam. The water rushed downstream to drown five towns, killing more than 2,000 people. One cubic yard of water weighs about 2,000 pounds (one ton). There are 5½ billion cubic yards in one cubic mile. A wind that preceded the water’s forward rush during the Vajont disaster had a force or pressure two times that of the bombs dropped on Hiroshima. The wall of air pressure blew off people’s clothes and skin and caused blast injuries to their internal organs. The rockslide took between 20 and 30 seconds to displace the reservoir’s water, which rose straight up in a mushroom shape reaching 820 feet high. The entire scouring of the downstream valley (Piave River Valley) took less than seven minutes. (6)
Many landslide dams have failed, producing catastrophic floods. (7) Understandably, the potential for catastrophic drainage of Lake Sarez has concerned both “the governments of the riparian republics of the Amu Darya River basin (Tajikistan, Afghanistan, and Uzbekistan) and to the international development assistance agencies and banks that are the source of most of the available economic aid in the region today.” The Greeks, as described elsewhere knew the Amu Darya as the Oxus River. (8) The Central Asian republics themselves do not have the necessary resources for even the most modest remedial engineering efforts on the dam, the lake, and the draining basin,” note Schuster and Alford. (1) Before the Soviet Union dissolved in 1991, Soviet engineers and geologists had carried out “a great amount of field investigations.” (2) “Among the early investigators of Lake Sarez were P. Zaimkin, G. A. Schpilko, D.D. Bukinich, I. A. Preobrazhenski, V. S. Kolesnikov, O. K. Lange, V. A. Afanasiev, V. I. Razek and many others. (4) Schuster and Alford say that most of the Soviet-era studies of the Usoi landslide dam and Lake Sarez are unpublished or are available only in Russian. (1)
Currently, there are two points of view on the stability of Usoi Dam. The first says the dam is unstable and a catastrophic flood from the lake is a probability involving all the imaginable consequences. The second is that the Usoi Dam is a natural stable formation and the lake will exist for quite a long time similarly to other conformable lakes: Yashirkul Lake in the Pamirs, Iskanderkul Lake in Central Tajikistan and many others. (4) An example of scientists who sit on the fence is evident in the following quote: “The stability of the dam with regard to sliding as a whole is guaranteed with a comfortable factor of safety (the upstream-downstream base line of the dam is about 5 km long). The surface morphology of the dam however shows that adjustment movements have taken place, and are perhaps still active.” (9) The Pamir plateau is widely known as one of the most seismically active regions in the world. “Many earthquakes with [a magnitude] greater than 7 have occurred in the Pamir during the twentieth century. A zone of intermediate earthquakes dips south-southeast beneath the Pamir. This is generally taken as evidence for subduction of the cold Tianshan lithosphere beneath the Pamir,” note Zhang, et al. (10) They continue: We have divided continental Asia into 26 seismic belts. The delineation of seismic belts is based on regional patterns of seismicity, seismotectonics, geological structure, and regional geophysics. The rate of seismicity in each belt is computed according to [a statistical method described in the paper]. The Pamir, Western Sichuan and Yunnan, Myanmar, Himalayan frontal arc, Southern Tianshan, and Tibetan Plateau are the seismic belts with the highest rates of earthquake activity. We have delineated 425 seismic sources that are distributed through the 26 different seismic belts. Among these seismic sources, 38 have maximum magnitude earthquakes ³ 8, 192 have maximum magnitude earthquakes between 7.0 and 7.9, 159 have maximum magnitude earthquakes between 6.0 and 6.9, and 36 have maximum magnitude earthquakes between 5.0 and 5.9. Most of the seismic sources with the largest maximum magnitude earthquakes are distributed along the Himalayan range front, the Myanmar (Burma) arc, Sichuan and Yunnan, the North China basin, Taiwan, the Tianshan mountains, the Pamir mountains, and the Tibetan Plateau. This seismic source distribution coincides with known active tectonic processes as well as the largest historical seismicity. (10) (Emphasis added)
Lake Sarez, according to a Tajik seismic zoning map of 1981, is situated in a 9.0 scale zone. Tremors of this scale occur in the area once in 2000 years, while 7.0 scale earthquakes occur once in 100 years, says the study. (11) The rock that comprises the Usoi Dam is carboniferous quartzites and schists and Permian-Triassic marbles and shales. (1) Its volume is about 0.5 cubic miles, as noted above, which means it qualifies as a “massive rock slide,” according to the classification of Cruden and Varnes. (1,12) The mass of the dam is 6 billion tons. (2)
Landslide dams, according to Schuster and Alford, are commonly much wider (dimension parallel to the stream) than engineered embankment dams (e.g., earthen dams) of similar height. Above water, the Usoi Dam has a surface area of 3.8 square miles. Its total area (above and below water) is approximately 4.6 square miles. (1) Schuster and Alford continue: The landslide originated on a very steep slope (inclination, 35-40 degrees or greater). The crest of the head scarp was at an elevation of approximately 14,700 feet, and the rock mass fell as much as 5,900 feet to an elevation of 8,860 feet in the valley of the Murgab River. (1) Soviet scientists Viktor Lim, Jusuf Akdodov, and Uri Kazakov estimated the velocity of the Usoi rockslide was around 55 miles per hour. (1) (Emphasis added) Some water from Lake Sarez does get through the dam to form the Bartang River, although very little is known about the nature of the flow pattern through the dam itself. (1) The elevation difference between the lake level and the water exit point through the dam into the canyon is 486 feet. Fifty-seven springs leak from dam, according to one source (more below). (2) “Filtration” through Usoi dam began in 1914, says one source. (4) Lake Sarez rose rapidly after the Murgab River was dammed in 1911, say Schuster and Alford. “By October 1913 the lake was 17 miles long and 920 feet deep. As the lake widened as the surface rose, the rate of rise slowed…From 1949 to 1988, the average annual increase in lake level was 7.3 inches per year. [In 2003] the lake was rising at an average rate of approximately 4 inches per year. Apparently, this trend is continuing; however, it is difficult to predict what effects global warming may have on the future water regime of the lake.” (1) Indeed, there are 879 glaciers in the Murgab basin and there total volume is 8 cubic miles of ice. (13)
Soviet records indicate that inflow to Lake Sarez averages 1,630 cubic feet/second. “Lake Sarez drains by discharge through the dam; thus, the dam has never been overtopped,” say Schuster and Alford. The measured average flow of the downstream “lower Murgab River” (Bartang River) is 1,635 cubic feet/second, “or approximately the same as the outflow from Lake Sarez.” “The flow exits the dam as a series of approximately 45 large springs that regroup to form the lower Murgab River.” (1) (If outflow and inflow are approximately the same, why is the lake still rising after all these years? MRO) (Note the number of springs varies according to observer, i.e., 56 v. 45; the point is, “springs” exist, which “filter” the dam, according to language used by scientists familiar with the Usoi.) The water that gets through the Usoi Dam forms the Bartang River, which courses down its valley to join the Panj River. The Panj River subsequently joins the Vaksh River to form the mighty Amu Darya, or Oxus River, the largest river in Central Asia. The total length of the Amu Darya from its source (there controversy over what is its source) is 1,500 miles (the Mississippi River, by comparison, is 2,320 miles long) and its drainage basin covers 206,464 square miles, mostly in Turkmenistan. The Amu Darya is a “mad river” to some, because it shifts its course from time to time. For example, “[i]n the 3rd and 4th millennia bc the Amu Darya flowed westward from the Khorezm Oasis into Lake Sarykamysh, and from there to the Caspian Sea. From the 17th century until the 1950s the Amu Darya emptied exclusively into the Aral Sea, except during periods of intense flooding, when overflows went into Lake Sarykamysh.” (14)
The Amu Darya is the largest source of irrigation and water supply in Central Asia. It is also the main “donor” of the Aral Sea. Intensive irrigation of the Kara Kum desert along the course of the Amu Darya and Soviet construction of the 870-mile Karakum Canal, which takes Amu Darya water to Ashgabat, the capital of Turkmenistan, has caused a change in the lower course of the river, i.e., at present, the waters of the Amu Darya do not reach the Aral Sea. (15) Some five million people live in the Amu Darya basin. Theoretically, a Lake Sarez outburst would decimate everything in its path, somewhat like the Glacial Lake Missoula flood did millions of years ago, as noted above. For flood scenarios, see below. A topic of controversy is the so-called “Right Bank Landslide” on Lake Sarez, about 2½ miles northeast of the dam. Its volume is between 0.1 and 0.5 cubic miles (similar to the 0.5 cubic mile volume of the Usoi Dam). (1) Soviet geologists characterize it as a “dormant” landslide. The data collected on this landslide is open to many different interpretations. Schuster and Alford suggest that a worst-case catastrophic failure scenario could be similar to the 1963 Vajont landslide disaster in Italy, as described above, which means overtopping of the Usoi Dam and forward movement of a huge wall of water down the valleys, all the way to the desert. (1)
The Usoi Dam has been stable for almost 100 years, which in geologic time frames, is a hardly worth noting. However, it is not an engineered structure, and it impounds a very large volume of water upstream from populated valleys downstream. What are the scenarios for its failure? Schuster and Alford mention seven, as follows: Schuster and Alford wrote their assessment before the 9/11 terrorist attack. Theoretically, terrorists could blow up the dam with the right kind of bomb, delivered on foot, by missile, or in an airplane. Schuster and Alford suggest two kinds of outburst-flood initiation from failure of the Usoi dam: breach flood and seiche flood. A breach flood would result from overtopping and downcutting of the dam. A seiche flood would result from a wave generated by a large landslide, most probably on the right bank slope issuing into Lake Sarez (similar to the Vajont disaster, noted above). The wall of released water, depending on breach or seiche types, could range from 650 feet in height immediately downstream from the dam to 180 feet 91 miles downstream from the dam. Alford noted that a major flood from Lake Sarez probably would become a debris flow within a distance of a mile or less, thus altering flood hydraulics unpredictably. A debris flow contains all the stuff scraped off the floor and sides of the valleys as the floodwaters roar downhill. The population of the villages in the Murgab/Bartang/Panh valleys that would be affected by an outburst probably does not exceed 25,000, say Schuster and Alford. (1) Soviet scientists and engineers gave considerable thought to installing engineered remedial measures to reduce the risk of dam failure resulting from any of the above-mentioned factors. They noted two methods of protecting the dam: The problem with the engineering solutions is the remoteness of the site in a highly mountainous region with complicated engineering geology. The cost is prohibitive. “Even preliminary studies necessary to assess the feasibility of such engineering measures would be difficult without heavy vehicles being able to access the dam and the lake by road.” (1) The Tajik government installed a meteorological observatory on Lake Sarez at the south end of Usoi dam. The idea is to monitor the dam and the right-bank landslide and, if certain signs appear, to provide a warning to government officials in Dushanbe by short-wave radio, and thence to villagers, by telephone. This system has three shortcomings, according to Schuster and Alford. First, the harsh winter climate precludes onsite observations. Second, the warning time under any system probably will not be great enough to allow people in the downstream villages closest to the dam to escape a possible flood. Recall that the Vajont flood was over in seven minutes. In 2000, residents of Barchidiv took about two hours to evacuate to higher ground. Third, the warning system in 2000 lacked reliability. (1) The foregoing information is somewhat dated, and work to improve the systems is ongoing.
A Sarez Lake mega-flood seems probable sometime in the future, with a Vajont-dam-disaster-type scenario (another landslide falls into the lake, displacing water over the dam) instead of an outright dam failure scenario, say, because of piping. Human interventions to prevent or mitigate a mega-flood seem limited because of the inaccessibility of the region and its challenging geology. Work continues to try to find solutions. The best one right now is to monitor the lake and dam, and to improve the reliability of early warning systems to move people out of the nearest valleys in an emergency, before the water wind and wall arrive. Notes:
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