Keywords

1 Introduction

The Greater Caucasus form the north-eastern sector of the Alpine-Mediterranean belt (Fig. 1), bounding the Eurasian Plate from the south and formed by continental collision of interacting Eurasian and Arab Plates (Khain 1998).

Fig. 1
A map of the Greater Caucasus. On the left is the Black Sea, and on the right is the Caspian Sea. The study site in the Caucasus is labeled by a star.

Position of the study site in the Greater Caucasus

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The Gerkhozhan-Su River valley is located in the Northern Caucasus and is one of the most debris flow-prone territories in Russia. Catastrophic debris flows have repeatedly occurred in the basin of the Gerkhozhan-Su river. They caused significant damage to the Tyrnyauz town. Large-scale debris flows took place here in 1937, 1960, 1961, 1962, 1977, 1999, 2000, 2002, 2011, 2017 (Chernomorets 2005; Baranovsky 2004; Gerasimov 1980; Zaporozhchenko 2002; Seinova et al. 2003). One of the factors that increase the level of debris flow hazard in the Gerkhozhan-Su River basin is the river damming cased by the Buzulgan landslide movements. In this case, the debris flows acquire additional material and impulse. The role of the Buzulgan landslide is described by an eyewitness of the July 19, 2000 debris flow Mr. S.D. Dzhubuev: “The main wave of the debris flow caught up with us ..., which destroyed a 9-storey building before our eyes... There was no water in the riverbed before the wave of the debris flow… Apparently, there was a jam in the area of the landslide, after which there was a breakthrough, which is why a wave was formed.” (Chernomorets 2005).

2 Natural Conditions

The research site is located 38 km east of the central part of volcano Elbrus, the highest peak in Europe (the western cone is 5642.7 m, the eastern cone is 5620 m). According to radiocarbon dating, the last volcanic eruptions occurred 1330 ± 80 and 990 ± 60 years ago (Laverov 2005).

2.1 Geomorphological Conditions

The Buzulgan landslide formed on the right-bank slope of the 4 km long Gerkhozhan-Su River valley—a right tributary of the Baksan River. The Tyrnyauz town is located at their confluence (Fig. 2).

Fig. 2
A photograph of the Tyrnyauz town. The labeled regions are Tyrnyauz, Baksan River, and Gerkhozhan-Su River. The town has buildings, houses, and trees. Mountains and clouds are in the background.

Central part of the Tyrnyauz town

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The Gerkhozhan-Su River is formed by the confluence of the Kayarty-Su and the Sakashili-Su Rivers sourced from the glaciers of the Main Caucasian Ridge located at altitudes of more than 4000 m. The Buzulgan landslide was formed almost immediately downstream from their confluence. The average annual water flow in the Gerkhozhan-Su River is 1.1 m3/s, but in summer the flow rate increases up to 5 m3/s (Chernomorets 2005).

The relief of the area is of the alpine type. The Gerkhozhan-Su River valley is V-shaped. The width of the valley at the watershed level (at elevations ~ 2000 m) is about 1.5 km, while that of the valley floor is less than 100 m. The height of the right west-facing slope of the valley at the Buzulgan landslide site is about 350 m with an average steepness of 23–25o.

2.2 Geological and Tectonic Conditions

The area where the Buzulgan landslide occurred is located within the Pshekish-Tyrnyauz suture zone (Fig. 3)—a narrow (2–3 km wide) tectonic zone affected by a fault system, traceable for more than 150 km. Within the Pshekish-Tyrnyauz suture zone, almost all geological boundaries have a tectonic nature. This zone started forming in the Early Variscan tectonic subepoch and was rejuvenated in the Alpine-Himalayan tectonic epoch. South of Tyrnyauz town, the neointrusive granite massif of the Tyrnyauzsky complex (γN2tr2) is confined to the Pshekish-Tyrnyauz suture zone.

Fig. 3
A map of the Pshekish-Tyrnyauz suture zone in the Greater Caucasus. It presents 14 geological regions in different color gradients. It presents Buzulgan landslides in 2000, 2020, and 2021, the Northern fault, the Sakashil-Kardansky fault, and the Southern fault, towns, and rivers.

A detailed fragment of the State Geological Map (sheet K-38-I, VII) (by Pysmenny et al. 2013, with modified). Geological Units: γN2tr2—tyrnyauzsky complex (leucocratic granites); J1-2dž—dzhigiatskaya formation (sandstones, aleurolites, argillites); J1bz—bezengijskaya formation (conglomerates, sandstones, aleurolites); Р1gm—gimaldykskaya formation (conglomerates, sandstones, aleurolites, argillites, tuffites); С3kr+gr—karadzhelmasskaya and gremuchinskaya formations (interbedding sandstones, aleurolites, argillites with coaly interbed); D2el—elmeztjubinskaya formation (tuff conglomerates, tuffstones, phyllite-like argillites); D1-2kz—kyzylkolskaya formation (interbedding tuff conglomerates, tuffstones, spilites, phyllite-like argillites with jasper); γPZ3u1—ullukamsky complex (granites); gsPR2šk—shaukolsky complex (gneiss, crystalline schists); ksPR2tb—tuballykulakskycomplex (quartzites, crystalline schists); sPR2kk—kamyksky complex (crystalline schists); sPR2kt—ktiteberdinsky complex (crystalline schists); γPR2vb—verhnebalkarsky complex (migmatite-granites); υσPR2ut—ullutallykolsky ultrabasite complex (pyroxenites)

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Currently, the Pshekish-Tyrnyauz suture zone separates the northern limb of the Northern Caucasus megaanticlinorium (tectonic block) from the horst-anticlinorium (tectonic block) of the Main Caucasus ridge. The geological map is shown in Fig. 3. Tectonic wedges and tectonic slabs trace Paleozoic thrust faults.

In the modern relief, the Pshekish-Tyrnyauz suture zone is expressed by a system of sublatitudinal faults separating the blocks with different altitude. Currently, the tectonic structures of the Pshekish-Tyrnyauz suture are deformed by the Sakashil-Kordonsky right strike-slip fault traced south of the Buzulgan landslide site.

The territory of the Caucasus is seismically active. The possible intensity of earthquakes in the area of the Pshekish-Tyrnyauz suture is estimated to be up to 10 points (Seismic 2018), though no one historical earthquake reported in this part of the Greater Caucasus had so high intensity.

3 The Buzulgan Landslide

3.1 History of Study

The Buzulgan landslide is located on the right bank of the Gerkhozhan-Su River valley (Figs. 3 and 4). Though the exact time of the landslide formation is unknown, its influence on debris flow activity in the Gerkhozhan-Su River valley has been repeatedly noted when describing debris flows that occurred in 1937, 1960–1962, 1978, 1999, 2000, 2011 and 2017 (Gerasimov 1967, 1980; Zolotarev et al. 1979; Seinova et al. 2003; Chernomorets 2005; Dokukin et al. 2012; Dokukin et al. 2018; Bekkiev et al. 2020). The Buzulgan landslide partially dammed and displace the riverbed of the Gerkhozhan-Su River.

Fig. 4
A photograph of the Buzulgan landslide. It has mountains on both sides covered with dense trees. An arrow points to the Gerkhozhan-Su River.

General view on the Buzulgan landslide (2020) (Photo of O.V. Zerkal)

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It caused the congestion during the passage of debris flows and the formation of catastrophic debris flow surges. The main reason for the activation of the Buzulgan landslide is usually considered due to the erosive activity of debris flows that undercut the frontal part of the landslide. However, the activation of the Buzulgan landslide in August 2020 was not caused by such effect.

Before 1999–2000, slope deformations occurred in the lower part of the slope. Devonian deposits of the Kyzylkolskaya formation (D1-2kz), distributed south of the fault crossing the slope (Fig. 3), were involved in the displacement. According to the geodetic observations, the maximum velocity of the lower part of the landslide in 1965 was 0.75 m/day, and in 1977 was 0.5 m/day (Zolotarev et al. 1979; Gerasimov 1980). The greatest total shift of the riverbed (20–25 m) during the movement of the landslide, previously estimated by the results of the analysis of multi-time remote sensing data, occurred after the 1977 debris flow (Bekkiev et al. 2020). A comparable displacement of the Gerkhozhan-Su riverbed was also observed during debris flows in 1999 and 2000 (Chernomorets 2005). However, after them slope deformations were detected in the middle part of the slope. During the debris flows of 2011 and 2017, the banks of the Gerkhozhan-Su River remained almost unchanged. In 2019, signs of landslide activity were recorded in the upper part of the slope (Bekkiev et al. 2020).

3.2 Method for Assessing the Current Landslide Activity

To assess the current state of the Buzulgan landslide, a field description of landslide activity was carried out and a survey of the slope deformations development area and the adjacent territory was carried out using the MavicPro Platinum quadcopter equipped with an FC220 camera. As a result of the survey, an orthophotoplan and a DEM were obtained that were analyzed later, along with field observation data.

3.3 Modern Activity of the Buzulgan Landslide

Destabilization of the slope massif—local rock falls in the frontal part of the old (2000) landslide—began to be noted from March 2020 (Bekkiev et al. 2020). According to M.D.Dokukin’s observations, the first stage of landslide displacements occurred from the end of May to the beginning of August 2020. During this period, the movement of the landslide was about 35 m at an average speed of 0.4–0.5 m/day. The main phase of the landslide displacement occurred from 07.08.2020 to 14.08.2020 (Bekkiev et al. 2020). The peak activity of the landslide was on 12–14.08.2020. During this time, the movement in the frontal part of the landslide was up to 120 m at an average speed of 40 m/day (~1.67 m/h). The movement in the upper part of the landslide during this time was 290 m at an average speed of up to 97 m/day (~4 m/h). In total, the upper part of the landslide (V landslide phase) shifted by ~430 m from 07.08.2020 to 14.08.2020. Unlike earlier displacements, the main slope deformations in 2020 involved not Devonian (D1-2kz), but Carboniferous deposits (С3kr+gr) that outcrops north of the fault crossing the slope (Figs. 3 and 5). As a result of active slope displacements, a landslide array of complex structure was formed (Figs. 4 and 5). At present, Buzulgan landslide body can be divided into several parts.

Fig. 5
A map of the Buzulgan landslide structure. It presents 11 landslide units with phases 1 to 5, secondary landslides, and other units. It also presents major and minor scarps, boundary landslide phases and elements, fault zone, modern river channel, new river channel, lake, and pre-landslide channel.

Map of a structure of Buzulgan landslide (2020) (made by O.V. Zerkal)

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The oldest part of the Buzulgan landslide (that was activated last time in 1999–2000) is the deformation zone in the lower and middle parts of the slope located south of the fault (the southeastern part of the landslide massif). The total length of this one is about 290–300 m with a width of the sliding surface up to 250 m. The excess over the riverbed of the Gerkhozhan-Su River is ~160 m (H/L = 0.55, α ≈ 33.5o). The volume of displaced masses is estimated at 1.2–1.5 mln m3. The tuffstones, spilites, phyllite-like argillites of the kyzylkolskaya formation (D1-2kz) were involved in the displacement. The lower part of this “old landslide” experienced some reactivation in 2020. Local rock falls were observed here. The upper part of the “old landslide” was overlaid by the material of the 2020 landslide when a significant expansion of the area of slope deformations took place.

In 2020 the sandstones, aleurolites, argillites with coal interbed of the karadzhelmasskaya and gremuchinskaya formations (С3kr+gr) of Carboniferous age were involved in the displacements. These formations lie to the north of the fault crossing the slope. The total length of the 2020 landslide deformation zone is about 1160 m, including 270 m in the segment along the riverbed. Width of the sliding surface is up to 200 m. The excess over the riverbed of the Gerkhozhan-Su River is ~320 m (H/L = 0.36, α ≈ 21o). The length of the transit zone within the landslide headscarp is ~400 m. The total length of displaced masses is about 780 m, and their volume is estimated at 5.8–5.9 mln m3.

The displacement of the landslide in 2020 occurred in five phases, which deposits consistently overlapped the landslide masses of earlier phases, forming ramparts and steps on the landslide surface (Figs. 5 and 6). In the lower part, the landslide reached the bed of the Gerkhozhan-Su River, where it turned and continued moving down the valley floor at a distance of up to 480 m. However, the river quickly eroded a new channel in a landslide dam composed of disintegrated deposits (Fig. 7). As a result, a large-scale landslide-dammed lake was not formed (Fig. 8). The new riverbed was shifted 95–100 m towards the left side of the valley.

Fig. 6
A photograph of the central part of the Buzulgan landslide. It has rocky land with trees, shrubs, and dried trees. The labeled regions are Gerkhozhan-Su River, landslide phase 3, landslide phase 4, and landslide phase 5.

The central part of the Buzulgan landslide (Photo of O.V. Zerkal)

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Fig. 7
A photograph of the New Gerkhozhan-Su River channel in a mountainous region. There are trees and grass on the elevated lands. The river flows between the elevated lands.

New Gerkhozhan-Su River channel (Photo of O.V. Zerkal)

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Fig. 8
A photograph of the upstream part of the landslide dam. The elevated lands have small and large rocks and a few shrubs. A small, landslide-dammed lake is between the elevated lands.

The upstream part of the landslide dam formed in 2020 (Photo of O.V. Zerkal)

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Currently, the total width of the Buzulgan landslide headscarp is 350 m. In the upper part of the slope it narrows up to 200–250 m. In the axial part of the headscarp there is a fault that is a part of the system of the Pshekish-Tyrnyauz suture. The total volume of the Buzulgan landslide is estimated at 7.0–7.2 mln m3.

Thus, the modern Buzulgan landslide is an active polyphase retrogressing landslide (Varnes 1978; Cruden and Varnes 1996; WP/WLI, 1993a, b; Hungr et al. 2014). J.N. Hutchinson defined landslides of this type as multiple rotation slides (Hutchinson 1988).

4 Debris Flows Activity and Hazard

Since the dynamics of the Buzulgan landslide directly affect the debris flow passing along the Gerkhozhan-Su River, we performed a simulation to assess the hazard for buildings and structures in the Tyrnyauz town.

4.1 Materials and Methods

In this study, the method of complex modeling was applied: the transport-shift model (the FLOVI program) for the zone of potential debris flow origination site in this case and the FLO-2D hydrodynamic model for the valley zoning. Modeling on the alluvial fan inundation was carried out for the case of the channel blockage in the bridge area by debris flow deposits or as a result of the bridge collapse (Fig. 9).

Fig. 9
An aerial photograph of a town with a road bridge on the Gerkhozhan-Su River. The town has roads, buildings, houses, and trees.

The road bridge on the Gerkhozhan-Su River alluvial fan. Photo by S.S. Chernomorets (2020)

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4.2 Transport–Shift Model (FLOVI)

The FLOVI computer program was applied to calculate the debris flow characteristics at the Buzulgan landslide site where significant entrainment takes place. The program is based on the equations of the transport-shift model of debris flow formation (Vinogradov 1980; Vinogradov and Vinogradova 2010). This model took into the account the measurements obtained during the Chemolgan experiments (Vinogradova and Vinogradov 2017). The calculated and observed values were also compared, the difference was from 9 to 38%. It is assumed that the increment in the solid flow while crossing the landslide body is directly proportional to the instability coefficient of the PDFB (potential debris flow body), the elementary potential flow capacity and the index of debris flow mass mobility (Vinogradova and Vinogradov 2017). Previously, the model was improved and used to simulate both debris flows and outburst floods (Kurovskaia et al. 2022).

The area of potential debris flow increment was identified where it crosses the landslide body. The total length was 332 m, the average slope was 12.4°. The input discharge before the landslide was assumed to be 700 m3/s, which is equal to that of the 2017 debris flow. The parameters of a potential debris flow massif, such as initial moisture content, internal friction angle, and density were selected based on measurement data (Zaporozhchenko 2002). Thus, the internal friction angle was set as 32o, and the density of the massif was assumed to be 2500 kg/m3.

Due to the fact that there are a large number of springs in the landslide, the maximum value of the initial moisture content was increased to 0.4, which was obtained during measurements (Zaporozhchenko 2002). The simulation results were the debris flow mass increment, the change in density and the debris flow wave hydrograph along the entire length of the valley section where it crosses the landslide.

4.3 Hydrodynamic Model FLO-2D

The hydrodynamic model FLO-2D (O’Brien et al. 1993) was applied to calculate the flow transformation in the valley. The model is based on the solution of the Saint-Venant equations, in which the flow characteristics are averaged over depth (the so-called “shallow water” equations) (Cunge et al. 1980). The motion of debris flow in FLO-2D model is simulated under the assumption that it moves as Bingham (viscoplastic) liquid (O’Brien et al. 1993). If the detail rheological analysis of debris flow and deposits cannot be made, the authors of the model propose the following parameters from Table 1.

Table 1 Parameters for calculating the stress of plastic flow and viscosity as a function of sediment concentration (O’Brien and Julien 1988)
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For identification of the Buzulgan rockslide dynamics, route surveys have been carried out since 2020, a month after the movement. The relief data was obtained using a UAV DJI Mavic Pro Platinum. The average spatial resolution of the surveys was 12.2 cm/pixel. The images were processed using the Agisoft Metashape program software [https://www.geoscan.aero/ru/software/agisoft/metashape_pro]. In areas where a large number of trees were identified, automatic classification was performed for a dense point cloud for the “high vegetation” class. Carrying out this operation made it possible to avoid the creation of a “false dam” in forested areas. Thus, a high-resolution digital surface model (DSM) was created, which took into account the location of the tray and buildings. After correction, all relief data were interpolated into the 3 × 3 m computational grid of the model.

4.4 Results and Discussion

The discharge of a possible debris flow at the outlet of the landslide site according to UAV data from 2020 was 1019 m3/s, with an input of 700 m3/s. At the top of the alluvial fan, the flow discharges vary approximately from 1 to 18% depending on the parameters set of the rheological block in the FLO-2D model (Table 2).

Table 2 Debris flow modeling results
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The maximum flow velocities on the alluvial fan were observed in the channel of the Gerkhozhan-Su River and changed for different variants of rheological parameters: a) −9.3 m/s, b) −10.7 m/s, c) −11.5 m/s, d) −10.7 m/s. The obtained values turned out to be slightly lower than the values if there was no blockage of the channel at the road bridge site (Kurovskaia et al. 2022).

The flow velocity on the fan itself varied from 1.5 to 5 m/s and higher. The flow depth at the alluvial fan, on average, for all variants of rheological parameters changed from 1 to 9 m, which was higher than for the modeling without the channel blockage by debris flows (Kurovskaia et al. 2022). The average inundation zone on the alluvial fan was 0.385 km2. The largest zone was obtained using the variant of parameters a), the smallest—c), the difference between them was about 10%. The distinction between inundation zones for this case and in the absence of blockage of the bridge was about 26%. At the same time, the debris flow covered both along the left bank of the Gerkhozhan-Su River and along the right bank. According to the simulation results, residential buildings, buildings of the of the Elbrus region administration, several schools, and a kindergarten were located in the flood prone zone, despite the debris flow observed in August 2022 passed exclusively through the flume. In 2021, the flume was cleared; therefore, the inundation hazard was reduced for some time. However, after debris flow in August 2022, the flume was partially filled with deposits again. Thus, the simulation reproduced the worst situation (Fig. 10).

Fig. 10
2 satellite maps of the region surrounding the Gerkhozhan-Su River with contours of debris flow velocity. 1 and 2. Most of the alluvial fan has flow velocity values between 0 and 1.5, a channel has a greater than 5 value, and other small regions have values between 1.5 and 3, and 3 and 5.

Planar distribution of debris flow velocities on the alluvial fan of the Gerkhozhan-Su River using the FLO-2D model 1) calculation variant a), 2) calculation variant c)

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5 Conclusion

The valley of the Gerkhozhan-Su River (a tributary of the Baksan River) in the Northern Caucasus is one of the most debris flow-prone areas in Russia. One of the factors of catastrophic debris flows in the valley of the river Gerkhozhan-Su is the Buzulgan ladslide. Earlier, as a result of the landslide movement, the debris flows entrained additional solid material and impulse.

The Buzulgan landslide was formed within the Pshekish-Tyrnyauzskaya suture, which is a narrow (2–3 km) tectonic zone affected by derives of faults.

The modern destabilization of the slope massif has been observed since March 2020. The peak of the landslide activity was in August 2020. At this time, the movement in the frontal part of the landslide was up to 120 m at an average speed of 40 m/day (~1.67 m/h). The movement in the upper part of the landslide during this time was 290 m at an average speed of up to 97 m/day (~4 m/h). In total, the upper part of the landslide shifted by ~430 m from 07.08.2020 to 14.08.2020. The total volume of the Buzulgan landslide is estimated at 7.0–7.2 mln m3.

As a result of the movement of the landslide in August 2020, the bed of the Gerkhozhan-Su River was blocked for 480 m. However, the river quickly eroded a new channel in a landslide dam, composed of disintegrated debris. As a result, no large-scale landslide-dammed lake was formed in 2020. The new river channel was shifted 95–100 m towards the left side of the valley.

Currently, the role of the Buzulgan landslide in increasing the debris flow hazard is the possible formation of a new landslide-dammed lake when a debris flow originate sup stream in the valley. The subsequent breakthrough of such a landslide-dammed lake may be accompanied by the formation of a high debris flow surge. Such a scenario could be catastrophic for the Tyrnyauz town, which is located just 2 km down the valley. Thus, the continuation of the study of the Buzulgan landslide and organization of monitoring at this site are critically important.

The paper presents the results of modeling based on the FLOVI and FLO-2D PRO software package. The FLOVI program, which is based on the equations of the transport-shift model, was used to calculate the flow characteristics at the site where debris flow crosses the Busulgan landslide. Modeling of flow transformation in the valley was performed using a two-dimensional hydrodynamic model FLO-2D. The sensitivity of the hydrodynamic model to the rheological parameters was shown, so the difference in debris flow discharges at the alluvial fan varied from 1 to 18%, the difference of inundation zone was about 10%. Modeling on the cone itself was carried out for the case of the channel blockage in the bridge area by debris flow deposits or because of the collapse of the bridge. In this case, the area of the flood zone is 26% larger. The results obtained show the effectiveness of the applied complex of models.