Causes of debris flow formation in flysch area of North Istria, Croatia

Abstract

The central and northern parts of the Istrian Peninsula situated in the northwestern part of the Croatian Adriatic coast are formed within the Paleogene flysch rock complex. The flysch zone in the northeastern part of the peninsula borders with the Ćićarija Mountain Range, which mostly consists of carbonate rocks. The occurrences of debris flows that cause huge damage to transport and other constructions are quite frequent. In this paper, we present the causes and triggering mechanisms of debris flows sporadically occurring in the Krbavčići village area near the city of Buzet. The potentially unstable soil mass formed by the erosion of the flysch bedrock fills the narrow and long ravines cut into the flysch bedrock all the way to the limestone slopes of the Ćićarija Mountain Range. This paper describes in detail the occurrence of a particular debris flow along with its repetitive appearances in the period from the winter of 1979 until today. The debris flow mass damaged the road embankment and caused its downslope slide in the length of about 300 m in one night in the winter of 1979. The movement of the debris mass triggered in the ravine put into danger the bridge constructed over an old unstable zone in the winter of 2003, however fast intervention measures prevented the possible damage.

Influence of Carbonate-Flysch Contact and Groundwater Dynamics on the Occurrence of Geohazards in Istria, Croatia

“…The first major mass movements on slopes in the village of Krbavčići, near Buzet were recorded in 1961. The multiple retrogressive translational landslide in this area was reactivated in January 1979 [33] after a long rainy period on a slope with an average inclination of 15-20%. The driven landslide mass (59,000 m 3 ) crossed the road and buried the vineyards located on the slopes southwest of the road.…”

“…Instabilities are mainly rotational and translational landslides in which the material parameters vary greatly [13]. The geological cross-section usually consists of two characteristic layers: colluvium and/or residual soil, which form the superficial deposits, and the flysch bedrock of different degrees of weathering [12,13,32,33]. In most cases, the slip surface is formed at the contact between the bedrock and the cover layer, which may vary between the residual soil and the completely weathered flysch rock.…”

Geotechnical Study of Raspadalica Cliff Rockfall, Croatia

“…For this reason, the more ductile flysch rock mass is deformed by the relatively rigid limestone rock blocks. Recent gravitational slides of huge carbonate rock blocks over the flysch bedrock are also visible [7]. The flysch bedrock complex formed by turbidite sedimentations is lithologically very heterogeneous due to the frequent vertical and lateral alternations of different lithological sequences: marls, siltstones, fine-grained sandstones, and very distinct layers of calcarenites.…”

“…In contrast to the karstified limestone rocks of the cliffs and the hill, the flysch bedrock is covered by Quaternary deposits. At the foot of the cliffs, the coarse-grained fragments originating from the cliffs have mixed with the silty clay from the flysch weathering zones, forming slope deposits of a few meters in length at the foot of the carbonate rock complex of the Ćićarija Mountain [7].…”

“…The Raspadalica Cliff is an almost vertical 100 m high limestone slope (Figure 1) located at the contact of two geomorphological units, namely, the hilly Paleogene Flysch Basin and the elevated Ćićarija Mountain Range in the northern part of Istrian Peninsula, Croatia [7][8][9]. The wider Raspadalica location is known for numerous instabilities occurring as rockfalls from the Raspadalica Cliff, as well as landslides [10], that have previously caused significant damage to the railway line located on the slope made of flysch succession at the foot of the cliff.…”