2. Study area
The Kuibyshev reservoir, built on the Volga River, is the largest in Eurasia and the third-largest in the world, after Volta (Ghana) and Smallwood (Canada) reservoirs. It is located in the central part of the Middle Volga basin at the intersection of the forest and forest-steppe landscape zones of the Volga uplands and the Low Volga (Yermolaev, 2017) (Figure 1). The geographic coordinates of the reservoir’s borders are 56°10’ - 53°30’ N, 47°30’ - 49°30’ E. There are three orographic regions within its basin: Volga Upland, Vyatka Predkamye, Low Volga. The research was conducted within the Volga Upland at the right side of the reservoir; its banks are steep and precipitous. Absolute heights are 175-215 m in the north and 250-270 m in the south. The maximum altitudes are confined to the Zhigulevskiy Mountains (349 m).
The reservoir was formed on October 31, 1955 following the Volga River damming by Kuibyshev hydroelectric complex. The reservoir reached its full reservoir level (FRL) of 53 m a.s.l. during the 1957 high waters. Its total capacity is 57.3 km3, its water surface area equals 6150 km2 and its total length along the Volga river is 510 km and 280 km along the Kama river. Its width varies between 2 and 27 km with a maximum of 38 km at Kamskoe Ustye. Average water depth is 9.4 m, maximum – 41 m. The length of the coastline is 2604 km, minimum navigation level is 49.00 m. The reservoir is a seasonal flow regulation storage: the average annual conditional coefficient of water exchange is 4.3.
The inflow and outflow discharges are asynchronous, and their ratio determines the reservoir filling and drawdown. The reservoir fills up to the maximum level during spring floods, while in autumn and winter the level is at its lowest position. The annual amplitude of level fluctuations is about 6 m. The average runoff velocity in the reservoir is 2-10 cm/sec, depending on the value of transit flow and the live cross-sectional area. The morphological structure of the reservoir is a system with lake-like expansions. This reservoir serves several economic sectors: i.e. energy, water transport, agriculture, fisheries, industrial and municipal water supply.
Two sites on the right bank of the Kuibyshev reservoir near Kamskoe Ustye village (Republic of Tatarstan) were selected to evaluate the intensity of shoreline abrasion and landsliding. The sites were selected based on their representativeness in terms of geological-geomorphological and landscape conditions for the study area. In addition, the economic aspect was taken into account since there is a threat of partial destruction of the buildings and infrastructure facilities according to the state authorities of the Russian Federation.
At site 1 not only landsliding but also abrasion processes at the shoreline have developed. The site is located on the right bank 30 m upstream of the Volga river pier (Figure 2).
The landslide and abrasion process acting on the bank are observed here. In the upper part of the slope, composed of heavy and middle deluvial-solifluction loams, collapse and sliding of earth blocks of different volumes towards the shoreline occur. At the foot of the abrasion ledge in some places, there are outcrops of the Kazanian age deposits, i.e. clayey-melty gray pack of rocks of the upper Permian system.
The landslide has a frontal shape, its length is 32.0 m and its width is 55 m (total area is 1760 m²). An abrasion scarp has formed in the lower part of the slope with traces of landslides and washout. In aslope profile, the landslide body itself is well identified in the upper part with a head scarp and in the lower part a terrace-shape section. The beach is 2.5-5.0 m wide and is composed of sand-and-shingle material. The abrasion section at the lower part of the slope erodes and collapses; soil blocks slide through cracks to the shoreline where they are eroded by water. Wavebreaking caves are practically absent. Other exogenous processes are acting on the slope in addition to the dominant landslide process. There are rills and ephemeral gullies in the upper part of the slope and gravitational processes (collapse and crumbling) in the middle and lower part. However, they are of subordinate importance in terms of active slope processes.
At site 2, a large landslide cirque has formed as a result of mass movement processes. Here, the landslide-abrasion type of coastal escarpment is observed. Due to changes in groundwater outflow caused by water level rise, a sliding landslide develops (Figure 3), forming a large landslide cirque.
The upper part of the landslide processes led to the destruction of the old cemetery. Landslide length is 173 m and its width is 110 m (total area is 13900 m²). The height of the landslide edge is 14,7 m, the steepness is 90 degrees. The body of the landslide is hilly and canopy-shaped. The upper landslide scarp is close to the buildings.
The lower northern part of the landslide body is affected by very active slope subsidence of the earth block type. The slope is composed of deluvial-solifluction loams with the vertical type of clastic-block structure, blocks with shrub and woody vegetation are collapsing along the cracks.
The height of the blocks is 5-6 m, width up to 7-8 m. The soil surface of the old cemetery is deformed due to cracking and subsidence. The landslide bodies are located further down the slope, moving towards the Volga at low speed; their hilly surfaces are overgrown with willow, american maple, saltbush, common nettle and coltsfoot. Behind the edge of this landslide body is the landslide cirque. Its southwestern cliff destroys the old cemetery. Numerous human remains emerge from a depth of 1.5-2 m in the landslide escarpment. In the lower part of the slope, an abrasion scarp with traces of breakwater caves can be observed.
In 2018 to the south of the study Site 2, a shore reconstruction was carried out to organize a recreational area near the camping base (Figure 4). As a result of excavation works and re-organization of access roads to the camping base pier, the south-eastern part of the studied landslide cirque and landslide body was leveled and sodded (Figure 4b). As a result, this fragment of the landslide ledge was excluded from further analysis. However, the works carried out to improve the sloping area did not reduce the intensity of geomorphic processes, and the works on slope stabilization had to continue.