Sanja Dugonjić

A complex landslide in the Rječina Valley: results of monitoring 1998-2010

Results of landslide monitoring undertaken between 1998–2010 on the north-eastern slope of the central part of Rjecina Valley in front of Grohovo village (north-eastern coastal part of Adriatic Sea, Croatia) are presented. This is the largest regional active landslide. The slopes around the Rjecina riverbed are formed in siliciclastic sedimentary rocks with flysch characteristics. The bedrock is mostly covered with unstable slope formations. A limestone rock mass is visible on the cliffs around the top of the river valley. The landslide is complex and retrogressive, with 13 sliding bodies. It occurred in December 1996 by displacement of an initial landslide body where movement had been registered in the 19th century. The limestone mega-blocks and separated rocky towers on the top of the slope have also moved, which is an atypical phenomenon of the flysch slopes in the area of Rijeka. After initial sliding ceased, and major movements subsided, monitoring of benchmark movements from 1998 until 2010 determined further maximum displacements on the upper part of the slope, and minimum movement in the lower part. The area of the Rjecina Valley from the Valici Dam to the Pasac Bridge was selected as a pilot area in the framework of the Croatian-Japanese bilateral joint research project. Monitoring results provided the basis of establishing an early warning system for possible landslide occurrence and estimating the degree of landslide risk.

Causes of Small Scale Landslides in Flysch Deposits of Istria, Croatia

In this paper we will present the overview of recent landslides in the Istria Peninsula, Croatia. The central area of the Istrian Peninsula is situated from the Trieste Bay in the west to the Ucka Mountain in the east and is called Gray Istria according to gray colour of the Paleogene flysch sediments. Flysch rocks mostly consist of claystones, siltstones with intercalated calcareous sandstones and breccia-conglomerates. During the last two decades many small scale landslides occurred in the area. Based on field investigations conducted during remedial works designing, a data base of investigated landslides was established. Landslides in flysch slopes of Gray Istria are mostly rotational slides, falls and debris flows of small scale. The sliding surface is typically on the contact of the superficial deposits and flysch bedrock which is almost impermeable below the superficial deposits with low permeability. Due to the inconvenient geological conditions, long term intensive rainfall and anthropogenic influence mostly during road construction, the natural balance in the slope was disturbed, and instabilities occurred.