2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 4
Presentation Time: 8:45 AM


SCHUSTER, Robert L., Engineering Geology/Geotechnical Engineering Consultant, Golden, CO 80401 and ALFORD, Donald, Focus Humanitarian Assistance, 1831 Poly Drive, Billings, MT 59102, rschuster@usgs.gov

In 1911, a huge earthquake-triggered rock slide (volume: ~2 km3) dammed the Murgab River in the Pamir Range of southeastern Tajikistan. The still-existing natural dam is about 600 m high, the highest stream blockage, natural or man-made, in the world. Lake Sarez, impounded by this blockage, is 53 km long, with a maximum depth of approximately 550 m and a volume of about 17 km3. The lake has never overtopped the dam; instead, it exits the downstream face as several large springs that regroup to form the Murgab River. The freeboard between the lake surface and the lowest point on the dam crest is currently about 50 m, and the lake is rising at about 20 cm/yr.

If this natural dam were to fail, a worst-case scenario could endanger hundreds of thousands of people in the Murgab, Bartang, Panj, and Amu Darya valleys downstream. There is a slight possibility of dam failure due to: (1) seismic shaking, (2) catastrophic overtopping caused by a landslide entering the lake at high velocity from the valley wall, (3) surface erosion due to natural overtopping by the slowly rising lake, (4) internal erosion (piping), (5) instability caused by pressure of the lake against the dam, or (6) instability of the slopes that form the dam faces. The occurrence of an overtopping wave as a result of reactivation of the large, currently “dormant” landslide high on the right bank of Lake Sarez seems the most realistic of these slight failure possibilities. Because of the high cost of installing physical remediation to the dam in this rugged mountain area (there are no roads to the dam), the main protective measures now being undertaken in response to the slight possibility of dam failure are hydrological monitoring at the dam and installation of a flood early-warning system downstream.

Recent studies of the Usoi landslide/natural dam and Lake Sarez, which have been funded mostly by the World Bank, the Swiss government, the Government of Tajikistan, and USAID, with cooperation from Focus Humanitarian Assistance, have been carried out mainly by Stucky Consulting Engineers of Lausanne, Switzerland. Until recently, field studies at Usoi/Sarez have been “on hold” because of the situation in nearby Afghanistan, but Stucky geologists and engineers returned to the site in early June 2002 and are planning on future field work.