Snježana Mihalić Arbanas

Review of Monitoring Parameters of the Kostanjek Landslide (Zagreb, Croatia)

Since 2011, in the framework of the Croatian-Japanese SATREPS FY2008 Project, scientists have been working on the establishment of the Kostanjek landslide monitoring system in the City of Zagreb (Croatia). External triggers at Kostanjek landslide are measured with rain gauge and accelerometers. Displacements at the surface are measured by GNSS sensors and extensometers, while subsurface displacement is measured by vertical extensometers and inclinometer. Hydrological measurements consist of groundwater level measurements, discharge measurements, chemical and isotope analysis. Monitoring sensors recorded landslide reactivation due to external triggers in the winter period of 2012/2013. During the period from September 2012 to March 2013 the total cumulative precipitation was 793.7 mm and horizontal displacements were in the range of 9–20 cm. The installed monitoring sensor network proved to provide reliable data for the establishment of relations between landslide causal factors and landslide displacement rates aimed at establishing threshold values for early warning system.

The ICL Adriatic-Balkan Network: analysis of current state and planned activities

International Consortium on Landslides (ICL) Adriatic-Balkan Network was established in January 2012 as one of eight regional and thematic ICL networks to promote activities of the International Consortium on Landslides and the International Programme on Landslides. This paper presents the annual report of the ICL Adriatic-Balkan Network for the year 2012. The main activity of the regional network was to complete an overview of publicly available data and sources about landslides in the region, scientific and professional practices related to evaluation and mitigation of landslide hazard, as well as related legislative framework. Recommendations for the discussion and endorsement in the course of the ICL Adriatic-Balkan Network activities will be derived from the analysis of strengths, weaknesses, opportunities, and threats related to landslide issues which are present at the national level in Croatia, Slovenia, and Serbia and at the regional level. Discussion and endorsement of the recommendations are planned as a further activity which will be held in March 2013 in Zagreb (Croatia), during the planned first regional symposium on landslides in the Adriatic-Balkan Region.

Study of Landslides in Flysch Deposits of North Istria, Croatia: Landslide Data Collection and Recent Landslide Occurrences

The northeastern part of the Istrian Peninsula in Croatia is built of Palaeogene flysch deposits, in which instability is common and where a large number of landslides, with significant consequences, have been recorded. Study of landslides in flysch deposits of North Istria will be conducted as one of the main activities of ongoing IPL-184 Project. It includes analyses and recognition of sliding mechanisms to use in landslide modelling and to determine landslide susceptibility and hazard in flysch rock mass deposits. Several landslide types, mechanisms and conditions occur in the study area. The majority of studied landslides occurred during the spring and winter. Generally, the landslides are of rotational and translational sliding type, and rarely rock falls and debris flows. Local roads and rarely other structures and facilities suffered major damage as a consequence of the landslides. Landslide inventories, as well as landslide susceptibility maps, have never been carried out in the study area. Today’s knowledge about landslides in this area is based on investigations of individual landslides and partial scientific research. After the project started, some of the activities from all stages of the project have been initiated. The proposed first stage of the project includes supplementation of the existing database, and field investigations of recent and existing landslides in the study area, as well as soil sampling for laboratory testing. Moreover, the methods used for landslide susceptibility and landslide hazard assessment are presented. This paper presents the current state of investigations and research in the initial stage of the IPL-184 Project.

Influence of weathering processes on the shear strength of siltstones from a flysch rock mass along the northern Adriatic coast of Croatia

Weathering processes cause significant changes in the engineering properties of rocks. Slope instability in flysch rock formations along the northern Adriatic coast of Croatia is related to the effects of weathering on the shear strength of siltstones from the flysch rock mass. Therefore, changes in geotechnical properties according to weathering grade are of immense importance in relation to instability processes. In this work, we investigated siltstones from flysch rock masses in the study area, and evaluated changes in engineering properties due to weathering. The research began with field observations and determination of the strength of different weathering grades of siltstones in the area. Mineralogical and laboratory studies were subsequently conducted, and mineral content was determined for siltstones of different weathering grades. We also performed a series of drying–wetting cycles to simulate natural conditions of the weathering process involved in the disintegration of the rock material into sand-sized and smaller particles. This weathering process resulted in disintegration of the siltstone rock mass into smaller particles that were not a unique rock block, with the soil-like material consisting of unbound particles of rock. Laboratory tests were also carried out on the soil-like material to determine the specific gravity, grain size distribution, Atterberg limits and residual shear strength for the different weathering grades of siltstones. Based on this research, we determined the changes in engineering properties for different weathering grades. Our results underscore the significant influence of the weathering process on mineral content, cation exchange capacity, liquid limit and residual shear strength, thus affecting slope stability in siltstones in flysch rock masses.

Landslides Triggered in the Continental Part of Croatia by Extreme Precipitation in 2013

The subject of this paper are landslides triggered by the extreme precipitation during winter and spring of 2013. More than 900 landslides were (re)activated in the continental part of Croatia during the analyzed period. This paper presents preliminary analysis results of the relationship between 55 slide-type occurrences and precipitation conditions. The preliminary analysis is based on the sliding data from the hilly area of Medvednica Mt. in the City of Zagreb. The exact landslide location, the slope failure time, information on the precipitation duration D and the mean intensity I, which have resulted in landslide activation, are known for each analyzed landslide event. A comparison of precipitation series and landslides occurrence dates with global rainfall intensity-duration (ID) thresholds showed that precipitation conditions exceeded threshold values and were likely to trigger a landslide. The results will be used for further study of the empirical threshold of landslides in the area of NW Croatia.

Inventory of precipitation triggered landslides in the winter of 2013 in Zagreb (Croatia, Europe)

The subject of this paper is inventory of precipitation induced landslides in Zagreb in the period of extreme weather conditions in the winter of 2013. Long-lasting heavy rainfall and a thick snow cover triggered more than 60 landslides in the hilly area of the Medvednica Mt. which belongs to the City of Zagreb. Basic data about landslide event locations, received from records of the City Office of Emergency Management (OEM), are analyzed. We present the results of preliminary temporal distribution analysis of landslide occurrence during the following extremes: cumulative monthly precipitation in January, February and March in 2013 was 130-190% higher than the average monthly values for the period from 1862 to 2012 ; and cumulative precipitation for a 3-month period in 2013 has the highest value in the last 150 years. Spatial distribution analysis shows that most of the analyzed landslide events were triggered in engineering soils of Pontian age (approximately 1 landslide per 2 km2). Presented landslide inventory map provide a solid basis for organizing further research aimed at the analysis of landslide triggering rainfall thresholds as well as development of landslide inventory map which will depict landslide contours.

Preliminary Investigations and Numerical Simulations of a Landslide Reactivation

Open image in new window A large landslide occurred near the Grohovo Village in outback of the City of Rijeka, Croatia, on 13 February 2014 after long term period of heavy rain. The preliminary surface observation was carried out immediately after sliding appearance enabled estimation of site condition, dimension of the landslide so as an assessment of a hazard of further landslide movements. The estimated dimension of landslide body are length of 350 m, width of 135 and 20–30 m of depth to the slip surface. The movement of approximately 12–15 m down the slope caused the complete damage of the local road over the landslide body. The toe of the landslide reached the bank of the Valici Reservoir 250 m away from the downstream located Valici Dam. Based on hazard assessment of further landslide movements, a lowering of the water level in reservoir and surface drainage from the landslide body were conducted as emergency landslide mitigation measures but the main reason for further landslide movements reduction was the end of heavy rain. Although the detailed filed investigations were not carried out, the analysis of possible further development of sliding was conducted. To establish the engineering geological model of the landslide, the analyses of existing LiDAR imagery and engineering geological mapping were carried out. It was identified that the recent landslide is a reactivation of the dormant landslide. Since a slip surface position was not identified by field investigation, a numerical analysis of a slope using the strength reduction method was introduced to determine the shape of a zone of rupture which would be used in landslide simulation. As a main hazard of further landslide movements, the filling of the Valici Reservoir, forming a landslide dam so as possible overflow of the dam and forming the wave that can reach the center of the City of Rijeka downstream the Rjecina River were identified. To determine possible scenarios those could be realized in case of new long term rainy period and raising of ground water level in the landslide body, a numerical simulations of further landslide development were conducted using LS-Rapid simulation software. In case of high reservoir water levels correspondent to the dam overflow, the sliding mass would significantly fill the reservoir and cause the landslide dam, while the water level rising and landslide caused waves (tsunamis) would overflow the Valici Dam and cause significant damage downstream the Rjecina River channel. Conducted LS-Rapid simulation results enabled a selection of relatively safe reservoir water level at which, in case of further landslide movements, no current harmful consequences would be realized. The reservoir filling by sliding mass would cause significant reduction of reservoir volume, disturbances in the Rjecina River and Valici Reservoir flow regimes so as long term disruption of hydro power plant work that imply on necessary landslide remediation before further landslide reactivation.

The Kostanjek Landslide in the City of Zagreb: Forecasting and Protective Monitoring

Since 2011 the scientists have been working on the establishment of the Kostanjek landslide monitoring system in Zagreb (Croatia) in the frame of the Croatian-Japanese SATREPS FY2008 Project. The system consists of sensors for measuring displacement, hydrological properties and external triggers. The total rain quantity of 455.2 mm, which fell in the winter period of 2012/2013, caused a reactivation of the landslide. Displacements in the range of 9–20 cm have been recorded by GNSS sensors and extensometers in various parts of the landslide surface (an area of 1 km2). This paper briefly presents measured displacements which pointed out the necessity to establish an early warning system (EWS). Monitored data will be used for the prediction of landslide failure and for the establishment of the threshold values for an early warning system. The final user of the early warning system will be the City’s administration for civil protection.

Identification and mapping of shallow landslides in the City of Zagreb (Croatia) using the LiDAR-based terrain model

Landslides in the hilly zone of Medvednica Mt. were identified visually using LiDAR DTM with a spatial resolution of 15 × 15 cm. Acquisition of the LiDAR data was performed in December 2013, following an extreme precipitation period that resulted in numerous landslides. Topographic derivative datasets for interpreting landslide morphology were computed from the LiDAR DTM: hillshade maps, degree of slope, contour lines, curvature, and surface roughness. Visual interpretation of LiDAR DTM derivatives was taken for the pilot area of 21 km2, which represents 12% of the hilly area in Zagreb City. This resulted in a landslide inventory map, indicating the contours of 676 landslides. Seventy-five percent of the landslide bodies showed a size between 159 and 2018 m2. The area of the smallest identified landslide in the test area is 43 m2. The majority of mapped landslides are located in a forested area. Each mapped landslide was assigned a level of confidence based on the LiDAR characteristics and it could be concluded that the LiDAR-based terrain model is a valuable tool for preparation of landslide inventories in heavily vegetated regions such as the hilly area of Medvednica Mt. The analysis of morphological properties of the landslides will be implemented to perform a semi-automated landslide mapping in the entire hilly area of the City of Zagreb (total area is 180 km2).

Landslides in the Dinarides and Pannonian Basin — from the largest historical and recent landslides in Croatia to catastrophic landslides caused by Cyclone Tamara (2014) in Bosnia and Herzegovina

Two neighboring countries in the southeastern region of Europe, Croatia, and Bosnia and Herzegovina (BIH), belong to the same geotectonic units of the Dinarides and to the Pannonian Basin, which influence relief types, lithology, and types of slope movements, i.e., landslides. The Dinarides are a mountain chain with a northwest-southeast direction that span from Slovenia through Croatia, BIH and Montenegro to Albania. The Pannonian Basin is situated within the Alpine, Carpathian, and Dinaric mountain belts at the boundary between Central and Southeastern Europe. The paper describes the general geological and geomorphological conditions in the Dinarides and the European Pannonian Basin in Croatia and BIH that are preparatory causal factors for landslides in the following environments: the hills of the Istrian Peninsula and Rječina River Valley; hills, low- and mid-altitude mountains in the Dinarides; and lowlands and hills in the Pannonian Basin. Landslide types, dimensions, and activities in the described areas are related to natural conditions primarily influenced by tectonic evolution and by recent anthropogenic processes, e.g., urbanization. More detailed descriptions are provided for the following selected phenomena, which are interesting because of the associated damage and potential risk: translational block landslide Brus and erosion phenomena on the Istrian Penninsula; relict and historical large, deep-seated landslides in the Rječina River Valley, including the recently reactivated Grohovo and Valići Landslides; catastrophic landslides triggered by precipitation during Cyclone Tamara in BIH (2014) (the Mačkovac-Šerići Landslide, Mjestova Ravan Landslide, Kosova Landslide, and Lukavica Landslide); and a large, deep-seated landslide in urban area of Zagreb, the Kostanjek Landslide. Recent rainfall triggering conditions of landslides in Croatia (2013) and BIH (2014) are also specified to emphasize the landslide risk and necessity of landslide risk management. The conclusions of the paper also note historical and potential damage due to landslide reactivations together with the spatial distribution of landslide-prone areas, which requires landslide mapping in the form of landslide inventory, susceptibility, hazard, and risk maps.