Čedomir Benac

Coastal cliff geometry derived from structure-from-motion photogrammetry at Stara Baška, Krk Island, Croatia

The aim of this study was to examine the capability of structure-from-motion photogrammetry in defining the geometry of cliffs and undercuts in rocks of complex geomorphology. A case site was chosen along pocket beaches near the village of Stara Baška on the Adriatic Sea island of Krk, Gulf of Kvarner, Croatia, where cliff erosion of 5 m in breccias was identified by comparison of aerial photographs from 1960 and 2004. The 3D point cloud was derived from approx. 800 photos taken on 9 January 2014 by a single camera from various elevations and angles, and processed using the online software ReCap (Autodesk). Data acquisition was found to be quick and the method easy to implement. The difference between the georeferenced 3D cloud points and an RTK-GPS survey was 7 cm, i.e. within the limits of RTK-GPS precision. Quantifying the spatial variation in undercut geometries revealed that the deepest and largest (17 m3) undercut was in the south-eastern sector of the beach. Reconstructing the detailed geomorphology of this 3.8-m-deep undercut convincingly demonstrates the high efficiency of the method. Such assessments of spatiotemporal changes in undercut and overhang volumes can prove useful for evaluations of cliff erosion risk. Coupled with the low cost and relatively simple application, this is evidently an attractive technique for meaningful geotechnical and coastal engineering monitoring in the future on the island of Krk and, for that matter, also on other Adriatic islands and in similar settings worldwide.

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.

Landslide Inventory in the Area of Dubračina River Basin (Croatia)

Systematic landslide mapping was performed for an area of 9.35 km2 of the geomorphological unit of hills in the Dubracina River Basin. Based on the visual interpretation of LiDAR imagery, supplemented by field reconnaissance mapping of damaged roads and structures, 48 landslides were identified. In this first phase of landslide inventory preparation for the area of the Dubracina River Basin, a preliminary landslide inventory map presents data on locations of all identified landslides. Three examples of identification of landslide contours are given to show different possibilities in visual interpretation of airborne LiDAR imagery.

Seabed and Surface Sediment Map of the Kvarner Region, Adriatic Sea, Croatia (Lithological Map, 1:500,000)

A lithological map of recent marine sediments and the seabed of the Kvarner region (Adriatic Sea, Croatia) is presented on a scale of 1:500 000. The map was compiled from existing published and unpublished data. This is an area characterised by a number of islands located between the Istrian Peninsula and the Vinodol-Velebit coast. Water depths in channels between the islands reach up to 125 m, compared to depths of 40-50 m in the open waters of the adjacent northern Adriatic. Over most of the Kvarner area, muddy and sandy sediments cover the seafloor. Mud (M) is found on the bottom in Rijeka Bay, the northern part of the Kvarner, in Kvarneric, and in the Vinodol and Velebit channels, whereas over the rest of the Kvarner region seafloor sandy mud (sM) prevails, with subordinate occurence of gravelly mud (gM). Sandy sediments, i.e. muddy sands (mS) dominate in the SW part to the open Adriatic, and west of Rab Island, in the Pag and Velebit Channels. Previous investigations indicate that the fine grained particles that are found in deeper parts of Rijeka Bay, Kvarneric, and Vinodol Channel are of recent origin, and are deposited at water depths below the wave base. Sources of these particles are local permanent and temporary streams and the direct input from weathering processes, along with input by submarine springs (vrulje) near the coast. The large areas of the bottom not covered with sediments, or covered with gravelly and sandy sediments are found above the wave base, i.e. in the erosional wave zone. Coarse-grained material is lithic and/or biogenic. However, due to the very rapid Late glacial-Holocene transgression, when sea-level rose more than a hundred metres, coarse sandy sediments are found below the recent wave base in the SW part towards the open Adriatic, and west of Rab Island, in the Pag and Velebit Channels. Therefore the sediment distribution in the Kvarner region is only partly in dynamic equilibrium with modern hydrodynamic conditions.

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.

Geotechnical properties in relation to grain-size and mineral composition: The Grohovo landslide case study (Croatia)

The Grohovo landslide is the largest active slope movement along the Croatian coast, situated on the north-eastern slope in the central part of the Rjecina River Valley (north-eastern coastal part of Adriatic Sea, Croatia). Slopes in this valley are formed of siliciclastic rocks (i.e., flysch), while limestone rock mass is visible on the cliffs around the top of the river valley. The slopes are at the limit of a stable equilibrium state, and slope movement phenomena have been recorded since 19th century. Samples for laboratory testing were taken from the flysch bedrock, weathered zone and slope deposits to provide specimens for determination of their mineralogical, physical and geotechnical properties. Correlation between mineralogical and geotechnical properties and their influence on sliding processes are presented in this paper. Clay fraction in samples ranges from 17 % to 38 %. Clay activity of the tested samples is from 0.45 to 0.89, and the residual friction angle varies from 13.0° to 17.7°. These results correspond to the presence of kaolinite and illite groups of clay mineral. Mineral composition and decrease in strength of fine- grained soil materials, due to the increase of pore water pressures, contributes to the slope movements.

Landslides in the Čabranka River valley, Croatia

Cabranka River is a bordering river between the Republic of Croatia and Slovenia, with its channel deeply cut into the rock mass of Gorski kotar region. A reconstruction works on a road on the Croatian side, right next to the Cabranka channel, required a widening which could have been made only by undercutting into the very steep slopes of heterogeneous geological composition. Due to a covered terrain, and very unfavorable hydrological conditions during works execution, on a relatively short section, several instabilities with different mechanisms occurred: planar and wedge failure in rock mass, and soil sliding in soil deposits on steep bedrock. This paper presents instabilities mentioned above and description of their formation mechanisms. In order to determine the situation and make the remediation design, field investigations were performed: boring, geophysical surveying, engineering geological and geotechnical investigations. Results of investigations, as well as the slope stability analysis and designed remediation measures for the largest landslide are also presented.

Coastal Erosion and Instability Phenomena on the Coast of Krk Island (NE Adriatic Sea)

This paper presents the influence of the marine erosion on slope stability in the south eastern coastal area of the Krk Island (north-eastern part of the Adriatic Sea). The bedrock (Paleogene marls and flysch) is occasionally covered with talus breccia from Quaternary period. The coast is strongly exposed to wave attack and thereby to marine erosion. Comparison of few orthophoto map generations shows significant coastal retreat during the fifty-year period. This phenomenon has been a fundamental trigger off different instability phenomena. The type of instabilities is a consequence of local geological fabric and resistivity of rock mass to marine erosion. In the investigated area, rock falls and slumps prevail in cliffs formed in talus breccias. Extremely high tides from 2008 and 2012 have caused significant coastal erosion. This is obviously an indicator of the possible higher hazard degree caused by the sea-level rise.

Marine Erosion and Slope Movements: SE Coast of the Krk Island

This paper presents the interaction of the marine erosion and slope movements on the south eastern coastal area of the Krk Island, Croatia. Larger parts of the coastal bedrock are carbonate rocks, and smaller parts are marls and flysch. The bedrock is occasionally covered with Quaternary sediments, which are characterized in terms of engineering soil properties. Complex geological fabric of the coastal area around Stara Baska settlement, in the south western part of the Krk Island, caused different movements on the coast. Due to the different grade of fissuring and karstification, as well as different weathering grade, carbonate and siliciclastic rock mass have wide range of resistance to the sea effects including wave attack. Exposure to the wave attack generated due to winds from the south directions contributes to the decreased resistivity. Additionally, occasional torrent flows and intensive erosion cause sporadic higher slope instability. The, effect of the previously registered extremely high tides, as well as the possible hazard increase due to the estimated sea level rise, is also analysed in the paper.

Coastal erosion and instability phenomena at Stara Baška, Krk Island, Croatia

The paper describes the influence of the marine erosion and slope instabilities on cliff retreat in the coast near Stara Baška settlement on Krk Island (northeastern channel part of the Adriatic Sea). The bedrock is occasionally covered by breccia from Quaternary period. The coast around Stara Baška is exposed to wave attack and thereby to marine erosion. By comparison of orthophoto maps from 1966 and 2004, the significant coastal retreat was determined. This phenomenon has been a fundamental trigger off different instability phenomena. The type of instabilities is a consequence of the local geological fabric and rock mass resistivity marine erosion. In the investigated area, rock falls and slumps prevail in cliffs formed in breccia. Extremely high tides with the combination of strong waves from southern direction have a significant influence on the coastal erosion. This is an indicator of the possible higher hazard degree caused by the sea-level rise.