Usoi Dam wave overtopping and flood routing in the Bartang and Panj Rivers, Tajikistan

The Usoi dam was created in the winter of 1911 after an enormous seismogenic rock slide completely blocked the valley of the Bartang River in the Pamir Mountains of southeastern Tajikistan. At present the dam impounds 17 million cubic meters of water in Lake Sarez.

Flood volume and discharge estimates were made for several landslide generated floods that could overtop the dam. For landslide volumes of 200, 500, and 1,000 million cubic meters, estimated overtopping flood volumes were 2, 22, and 87 million cubic meters of water, respectively. Estimated peak discharge at the dam for these three flood scenarios were 57,000, 490,000, and 1,580,000 cubic meters per second, based on triangular hydrographs of 70-, 90-, and 110-second durations, respectively.

Flood-routing simulations were made for the three landslide-induced overtopping floods over a 530-kilometer reach of the Bartang and Panj Rivers below the Usoi dam. A one-dimensional flow model using a Riemann numerical solution technique was selected for the study. A constant 50-meter wide rectangular channel, which represented the mean channel width, was used for the entire reach. A roughness coefficient of 0.038, appropriate for steep mountainous streams, also was used for the entire reach.

For the 87 million cubic meter volume overtopping flood scenario, the peak flows were approximately 1,100, 800, and 550 cubic meters per second at locations 50, 100, and 150 kilometers downstream of the dam, respectively.

The model was also used to simulate the less likely scenario of an instantaneous dam breach and draining of the total volume of the lake. Simulated peak flows were approximately 64,000, 52,000, 40,000, and 20,000 cubic meters per second at locations 50, 100, 150, and 530 kilometers downstream of the Usoi dam.