Technical report ■Landslides in Bulgaria Boyko BEROV Geological Institute, Bulgarian Academy of Sciences, Bulgaria, (STA Fellow NIED) Nikolay DOBREV Geological Institute, Bulgarian Academy of Sciences, Bulgaria, (STA Fellow PWRI) Ilia BROUCHEV Geological Institute, Bulgarian Academy of Sciences, Bulgaria Teruki FUKUZONO National Research Institute for Earth Science and Disaster Prevention, Japan Key words: Iandslide, Bulgaria, conditions, distribution, causes 1. Introduction rise to man-provoked slope movements, the numbers Bulgaria, a Southeast European country, is situated of which are on a big increase. in the central part of the Balkan Peninsula and lies at northern latitude of 42-44 degrees. Neighbouring coun- 2. Engineering-geological conditions for landslides tries are Romania to the north, Greece to the south, Engineering-geological conditions in Bulgaria are Turkey to the southeast, Yugoslavia and Macedonia to various and complex due to the variety of geological the west. The Black Sea is the eastern frontier of Bul- units in the country's territory, the presence of many garia. Various types of destructive geological processes different tectonic structures and the genetic and li- assail Bulgarian territory as a result of complex geo- thological abundance of rocks. From the geotechnical logical, tectonic and topographical features. More than point of view, the lithological variety of rocks has been 60 types of natural and man-made processes and phe- organized into 4 main groups of engineering-geological nomena are registered on the Map of Geological Haz- types of rocks and soils (Fig. 1). The types of solid ards in Bulgaria (Iliev-Broutchev, ed. 1994). rocks include hard and dense magmatic, metamorphic The landslide hazard is one of the most important in and sediment rocks with strong structural bonds and the country. Landslides are widely spread with uneven high strength parameters (such as granites, diorites, territorial distribution. The number of slope move- gneisses etc.) When these kinds of rocks are tectoni- ments in Bulgaria is not as great as in Japan, but al- cally disturbed, broken, weathered or somehow most all of the types recognized by Varnes (1978) are changed, their strength parameters are lower, which manifested. Cases of complex landslide movements oc- characterize types of soft rock. Clayey and cohesion- cur very often. The variety in kinds of mechanism, less soils are the youngest lithological formations (Neo- rate of movement, different size and shape in landslide gene and Quaternary). Considering mainly geological, manifestation is due to the diversity of the geological, tectonical, morphological and geotechnical criteria, five geomorphologic, hydrogeological and engineering geo- large regions and some inside zones have been distin- logical conditions in Bulgaria. The main natural factors guished in the territory of Bulgaria (Kamenov & Iliev, that contribute to landslide activity in the country are 1963). The names of these engineering-geological re- endogenic: slow tectonic movements and earth- gions and zones as well as the main geological proc- quakes ; and exogenic: erosion, sea erosion, precipita- esses are given in Fig. 1. tion, melting snow and variations in the ground water Of the geological conditions, which contribute to the tables. Nowadays, the part played by and the variety landslide manifestations, the presence of clayey and of man-made works or human factors is considered to sandy-clayey material in the structure of slopes is the be as strong as natural ones. The reason is that con- most important, as well as some weak interbeds and struction and mining activities in Bulgaria have given surfaces. These kinds of geological conditions are espe- 34 J. of the Jpn. Landslide Soc., Vol.38, No. 4 334 (2002) Berov, B. et al.: Landslides in Bulgaria Fig. 1 Engineering geological regions in Bulgaria (according to Kamenov & They, 1963) cially characteristic for the structures formed by Terti- 3. Topography ary and Quaternary sediments. About 90% of the Bul- Considering its small size -approximately 111,000 garian population lives in such sediment terrains. The square kilometers, Bulgaria has a large variety of to- hilly parts of these basins, the basin boundaries and pographical features. The relief is characterized by di- the river slopes inside the basins are the areas vulner- versity including high and low mountains, plateaus, ele- able to landslides. There are many places in Bulgaria vated lands, hilly regions, vast low lands, valleys, Qua- where the inversion geomechanical model (heavy and ternary river basins, gorges and deep cut defiles. More brittle rocks situated over soft and plastic ones) cre- than half of Bulgarian land is hilly and mountainous. ates the possibility for continuous and complex land- The average altitude of the country is about 480m. slides. The thickness of weathering materials, the large Plains (up to 200m) make up 30% of the territory, pla- tectonic zones with intensively cracked rocks and the teaus, elevated lands and hills (200-600m)-40%, low influence of ground and surface water all provide fa- mountains (600-1000m) -16%, medium high mountains vourable landslide conditions that also exist in the (1000-1500m)-10%, and high mountains (over 1500m) country's territory. There have been many investiga- -4% . The highest peak in the country and also in the tions on the conditions and factors for landslide activ- Balkan Peninsula is Mussala -2925m in the Rila Moun- ity in the country (Kamenov & Iliev 1963; Kamenov tains. The main characteristics of the topography are et al. 1973; Iliev 1973; Kamenov et al. 1977; Frangov alternating bands of low and high terrains that extend et al. 1996; Brouchev 1996). The general conclusion is in an east-west direction across the country. From that in 2/3 of the territory of Bulgaria, the geologic , north to south, these bands are the Danube Plain, the geomorphologic and geotechnic conditions create the Fore Balkan, the Balkan Range (or Stara Planina), the possibility for the existence and reactivation of land- Southern Balkan Valleys, the Sredna Gora Range, the slide processes. But it is difficult and perhaps incorrect Upper-Thracian Plain and the Rila-Rhodope Massif to generalize the engineering-geological conditions as Mountains (Fig. 2). The contact zones between high and every landslide manifestation has its own specific par- low terrains are prone to landslide movements. ticularities that are discussed in more detail in "Land- The border between Bulgaria and Romania is the slide distribution". Danube River. The riverbank on Bulgarian territory is J. of the Jpn. Landslide Soc., Vol.38, No.4 335 (2002) 35 Berov, B. et al.: Landslides in Bulgaria age altitude is more than 700m and the highest point is Botev Peak (2376m) in the central part of the Range. The Balkans then continues at a lower altitude to the cliffs of the Black Sea. The Southern Balkan Valleys are a relatively nar- row band of lowlands that lie to the south of the Bal- kan Range. In fact, the Southern Balkan Valleys repre- sent a number of relatively small or bigger valleys separated from each other by hilly areas. The Sofia Valley (or Sofia basin) is the biggest amongst these val- leys. Debris fans cover the zones at the foot of the Bal- Fig. 2 Situation and main characteristics of the topogra- kans. These areas are prone to slow creeping and con- phy of Bulgaria (according to Rekacewicz, 1997 with struction works must be conducted very carefully, oth- supplements) erwise active landslide movement may be provoked. To the south of the Southern Balkan Valleys lies the high while on the opposite side on Romanian territory Sredna Gora Range. This range is about 300km long. there is vast lowland. This topographical feature cre- Its medium altitude is about 600m and its highest ele- ates the possibility for the permanent action of river vation is more than 2000m. The contact zones be- erosion and slow cutting of the high Bulgarian coast- tween the Southern Balkan Valleys and the Sredna line. The result is periodically activated landslides due Gora Range are also the territories prone to creeping to erosion of the Danube River. The river coastline on and potential landslides. Bulgarian territory is about 470km long and many The Upper-Thracian Plain lies to the south of the landslides have been observed along it. To the south of Sredna Gora Range. The plain includes the Maritsa the Danube River lies the Danube Plain-the largest River valley and the lowlands that extend to the east hilly plane region, covered by loess formation in Bul- from the river to the Black Sea. Situated approxi- garia. The Shoumen Plateau (about 500m) is the high- mately in the central part of the Upper-Thracian Plain est inner region of the Danube Plain. The rivers (Iskar, is the East Maritsa coal basin-the biggest coal basin Vit, Osum, Yantra etc.) passing through the Plain from in the country's territory. As a result of open pit min- the south to the north, cut the elevated lands of loess ing exploitation in this basin, many cases of man-made and create favourable zones for landslide manifestation. landslides have been manifested. Other landslide prone zones are the peripheries of the The Rila-Rhodope Massif Mountains occupy the plateaus. Southern and South-western Bulgarian areas. The To the south of the Danube Plain lies the Fore Bal- massif includes the mountains Rila, Pirin, Rhodopes kan - one of the fore mountain ranges of the Balkan and the mountain group of Osogovo-Belasitsa. Many Range. The Fore Balkan's medium altitude is about valleys have been developed between and inside the 370m and its highest elevation about 1500m. Many mountains. The Rila and Pirin Mountains are the most valleys and defiles divide the longitudinal mountains of outstanding topographical features in Bulgaria and the the Fore Balkan. Landslides occur in river slope zones. Balkan Peninsula. The average altitude of the moun- The Balkan Range (Stara Planina) is situated ap- tains is more than 1000m.