Vladimir Jurak

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

The middle and northern part of the Istrian Peninsula situated in the northwestern part of the Croatian Adriatic coast were formed in Paleogene flysch rock complex. Flysch zone on the northeastern part of the peninsula border with the mountain chain of Cicarija consists mostly of carbonate rocks. On the southwestern slopes of Cicarija Mountain, where the fault contact between carbonate and flysch rock mass exists, thick bodies of slope deposits are sporadically present. These slope formations were formed by mixtures of coarse-grained limestone fragments and silty-clay as the result of weathering and eroding of flysch bedrock. The described slope deposits are potentially unstable due to unfavourable hydrology conditions, and mass movements are frequent. The phenomena of debris flows, which caused the biggest damages on transport and other constructions, are largely present. In this paper is it presented the causes and triggering mechanisms of debris flow triggering, sporadically occurring in the Krbavcici village area near Buzet city. Potentially unstable soil mass do flysch bedrock erosions ranging to the limestone slopes of the Cicarija Mountain form filling narrow and length ravines. As a rule, the debris flows appear after hard rainy periods, when water content in clayish sediments reaches liquid limit and soft materials in ravines transform in liquid consistency mass. On the slope where these phenomena are occurring, two roads and a railway are present and the debris flows have been causing numerous damages and traffic interruptions. The occurrence of a particular debris flow was described in detail in this paper, along with its repetitive appearances covering the period from winter 1979 until today. The debris flow mass damaged the road embankment and caused their sliding about 300 m down the slope in one night in winter 1979. Debris mass movements triggered in ravine put into danger the bridge constructed over an old unstable zone in winter 2003, but fast intervention measures prevented the possible damage.