The Modern Activity of the Buzulgan Landslide and Its Influence on the Debris Flow Hazard for the Tyrnyauz Town (Northern Caucasus, Russia)

Abstract

AbstractThe valley of the Gerkhozhan-Su River (a tributary of the Baksan River) in the Norther Caucasus is one of the most debris flow-prone areas in Russia. Large-scale debris flows here have occurred in 1937, 1960, 1961, 1962, 1977, 1999, 2000, 2002, 2011, 2017. The Tyrnyauz town is located in the impact zone of the debris flows. A significant impact on the debris flow hazard in the Gerkhozhan-Su River valley is exerted by the Buzulgan landslide, which was formed on the right side of the valley with a height of about 350 m and an average steepness of 23–25°. The area where the Buzulgan landslide was formed is located within the Pshekish-Tyrnyauz suture zone, which is a 2–3 km wide belt of the intensive tectonic fragmentation. The modern landslide activity began in March 2020. The main phase of the landslide displacement occurred in August 2020. The total volume of the landslide is estimated as 7.0–7.2 × 106 m3. It formed a dam that blocked the Gerkhozhan-Su River valley. No large lake was impounded upstream, as the river has eroded a new channel near the left side of the valley in a short time. A landslide dam, composed of disintegrated deposits, increases the debris flow hazard. The occurrence of debris flows in the river valleys upstream the dam site will rise the landslide-damned lake level, which rapid breakthrough may cause a catastrophic debris flow surge downstream. In this study the influence of Buzulgan landslide on the debris flows passage was identified. The method of complex modeling was applied: the transport-shift model (the FLOVI program) for the zone of the potential debris flow—landslide interaction and the FLO-2D hydrodynamic model for the valley zoning. Modeling of the alluvial fan inundation was carried out for the case of the channel blockage at the Tyrnyauz bridge by debris flow deposits or bridge collapse. The difference between inundation zones using the various rheological parameters was ~10%. The distinction between inundation zones for this case and in the absence of the bridge blockage was about 26%. Residential buildings, buildings of the Elbrus district administration, several schools, and a kindergarten are located in the flood prone zone.