Bedia landslide site in eastern Abkhazia

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Abstract

The article presents the results of geomorphological studies of the area of widespread landslide processes in the upper reaches of the Okhodzha River basin (right tributary of the Okum River, Black Sea basin). The characteristics of the geological and geomorphological conditions and factors that contributed to the occurrence of a large landslide in January 2021 are given. It has been established that the main type of landslides within the Bedia site are landslide flows developing along the roof of Eocene and Oligocene (Khadum horizon and Maikop Formation) claystones. The thickness of landslide bodies ranges from to 34–78 m. The slope cover is mainly subject to displacement, including the bodies of previous landslides. It is shown that the main condition determining the development of landslide processes is the position of the Bedia landslide site at the junction of the water-logged carbonate rocks of Cretaceous age exposed along the Southern macroslope of the Greater Caucasus, and denuded foothills composed of the Paleogene clayey rocks. The landslides are triggered by prolonged precipitation and facilitated by the coincidence of the slope and the bedrock dip direction on the left slope of the Okhodzha River valley. Due to the large-scale development of landslide processes accompanied by erosion, an erosion-landslide badland was formed on the left bank of the Okhodzha River. The most unstable areas, not recommended for land-use due to periodic landslide movements, are the slopes of small valleys – left tributaries of the Okhodzha River, as well as their wide bottoms filled with landslide deposits. According to the main features of the geological and geomorphological structure, the studied area is similar to the Novoafonsky, Eshersky and Macharsky landslide areas previously described in scientific literature. Economic development of these territories should include not only constructive measures to protect against landslides (retaining walls, artificial terracing, etc.), but also the installation of drainage systems to reduce the water saturation of slope deposits and the water content at the base of the slope.

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About the authors

E. A. Eremenko

Lomonosov Moscow State University, Faculty of Geography

Author for correspondence.
Email: eremenkoeaig@gmail.com
Russian Federation, Moscow

R. Yu. Zhiba

Institute of Ecology, Academy of Sciences of Abkhazia

Email: eremenkoeaig@gmail.com
Abkhazia, Sukhum

Yu. N. Fuzeina

Lomonosov Moscow State University, Faculty of Geography

Email: eremenkoeaig@gmail.com
Russian Federation, Moscow

I. P. Neshenko

Institute of Ecology, Academy of Sciences of Abkhazia

Email: eremenkoeaig@gmail.com
Abkhazia, Sukhum

N. P. Zaraiskiy

Lomonosov Moscow State University, Faculty of Geography

Email: eremenkoeaig@gmail.com
Russian Federation, Moscow

R. S. Dbar

Institute of Ecology, Academy of Sciences of Abkhazia

Email: eremenkoeaig@gmail.com
Abkhazia, Sukhum

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2. Fig. 1. Location of the research area: (a) – on the overview map of the Black Sea region, (б) – within the Abkhazia.

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3. Fig. 2. Geological and geomorphological structure of the upper reaches of the Okhodzha River basin: (a) – fragment of the geological map (Bukiya, Abamelik, 1971); (б)– geomorphological structure; (в) – transverse profile of the Okhodzha River valley along line A–Б. 1 – landslide deposits that make up the body of the 2021 landslide-flow; deposits of Quaternary age: 2 – alluvial deposits, 3 – deluvial-colluvial deposits, 4 – deluvial-landslide deposits; bedrocks of the Neogene system: 5 – sandstones and conglomerates of the Maeotian Stage, 6 – clays, sandstones, marls, limestones and conglomerates of the middle and lower subarcs of the Sarmatian Stage, 7 – sandstones, clays and marls of the Konk horizon, 8 – sandy clays, sandstones and marls of the Karagan horizon, 9 – clays, marls, sandstones, limestones and conglomerates of the Chokrak horizon, 10 – sandstones, sandy clays and marls of the Kotsakhur and Tarkhan horizons, 11 – sandstones and conglomerates with interlayers of clays of the Sakaraul horizon; bedrock of the Palaeogene system: 12 – gypsiferous sheet-bearing bituminous clays and sandstones of the Khadum horizon and Maikop Formation of the Oligocene, 13 – bituminous marls of the Upper Eocene, 14 – limestones, marls and marly limestones of the Middle and Lower Eocene, 15 – limestones and marly limestones of the Paleocene; bedrocks of the Upper Cretaceous system: 16 – thickly stratified limestones of the Danish Stage, 17 – stratified limestones of the Maastrichtian Stage, 18 – limestones and marly limestones of the Santonian and Campanian stages, 19 – limestones of the Turonian and Konyakian stages, 20 – sandstones and clays of the Cenomanian Stage; bedrock of the lower section of the Cretaceous system: 21 – clays, marly clays, marls and sandstones of the Albian Stage, 22 – marls and marly limestones of the Aptian Stage, 23 – limestones (including dolomiticised ones) of the Barremian Stage, 24 – limestones (including dolomiticised ones), sandstones, conglomerates and breccias of the Valanginian and Goterivian stages; 25 – elements of the occurrence of bedrock layers; 26 – fault position, direction and dip angle; 27 – geological boundaries (a – conformable bedding of layers, б – unconformable bedding of layers); 28 – structural-denudation summit surfaces (steepness up to 8°); slopes created by complex denudation: 29 – 8–35°, 30 – more than 35°; 31 – rounded ridge crests; 32 – accumulative alluvial levels in river valley bottoms; 33 – basement and erosional river terraces; 34 – colluvial and deluvial-landslide trains, 35 – edges of landslide-talus slopes; 36 – landslide of the 2021 (a – on the geological map, б – on the geomorphological diagram); 37 – the area of the most widespread development of landslides; 38 – river beds; 39 – thalwegs of temporary watercourses; 40 – paved roads; 41 – settlements.

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4. Fig. 3. Landslide on the left side of the Okhodzha River valley (the shooting was carried out from a UAV in February 2021): (a) – general view of the left side of the Okhodzha River valley and the position of the landslide body; (б) – the marginal part of the landslide that shifted the bed of the Okhodzha River 25–30 m to the west. 1 – watercourses (a – Okhodzha River, б – temporary watercourses-tributaries of the Okhodzha River); 2 – roads (a – asphalt, б – dirt); landslide body: 3 – boundary of the landslide body, 4 – cracks on the surface of the landslide body; steep slope: 5 – edge of the steep slope, 6 – foot of the steep slope.

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5. Fig. 4. Geomorphological structure of the middle part of the Okhodzha River valley. (a) – DEM constructed based on the results of the 2014 lidar survey (contours are drawn every 10 m); (б) – geomorphological structure; (в) – geomorphological profile along line A–Б (relief – according to the 2014 DEM). Structural-denudation slopes: 1 – with a steepness of up to 35°, 2 – with a steepness of more than 35°; erosional slopes: 3 – with a steepness of more than 35°, significantly transformed by landslide processes, 4 – with a steepness of less than 35°, in places transformed by landslide processes; 5 – alluvial levels in the bottom of the Okhodzhi river valley; 6 – deluvial-colluvial plume; 7 – deluvial-landslide plume; 8 – landslides, dissected by gullies; 9 – steep slopes of landslide failure; watercourses: 10 – permanent watercourses, 11 – temporary watercourses; boundaries: 12 – boundaries of the landslide body on the DEM, 13 – boundaries of the landslide body on the map; 14 – tectonic fault; 15 – area of distribution of Oligocene clay rocks.

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6. Fig. 5. Landslide sites identified in the junction zone of mountains and hills (geological basis according to Bukiya, Abamelik, 1971). Sedimentary rocks: 1 – of the Quaternary system, 2 – of the Neogene system, 3 – of the Oligocene division of the Paleogene system (Khadum horizon and Maikop suite), 4 – of the Mekhadyr suite of the Eocene and Oligocene divisions of the Paleogene system, 5 – of the Paleocene and Eocene divisions of the Paleogene system, 6 – of the Cretaceous system, 7 – sedimentary-volcanogenic and metamorphic rocks of the Jurassic system; 8 – metamorphic rocks of the Cambrian age; igneous rocks: 9 – of the Jurassic age, 10 – of the Paleozoic; 11 – metamorphic rocks Paleozoic; 12 – faults; 13 – landslide areas (1 – Khashupsinsky, 2 – Bzybsky, 3 – Kaldakhvarsky, 4 – Primorsky, 5 – Novoafonsky, 6 – Eshersky, 7 – Macharsky, 8 – Akhutsinsky, 9 – Adzhampazransky, 10 – Bedia, 11 – Dikhazurgsky); 14 – the foothills of the mountain system of the Southern macroslope of the Greater Caucasus (bergstriches are directed towards the hills).

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