Author: Associate professor Martin Krkač
Landslides, generally defined as the movement of a mass of rock, debris, or earth down a slope (Cruden, 1991), play an important role in the evolution of a landscape (Crozier, 2010). Landslides include different materials, various types of movements, states of activity and different velocities (Cruden and Varnes, 1996; Hungr et al., 2014). Knowledge of kinematics helps to reveal temporally and spatially variable stresses acting within landslides, their boundary geometries, mechanical properties of materials, external forcing conditions, and characteristics of future landslide movement (Schulz et al., 2017). Slow-moving landslides sometimes accelerate rapidly and fail catastrophically, causing widespread destruction and casualties (Lacroix et al., 2020), so knowledge about kinematics is important for those responsible for managing the risks posed by landslides (Glastonbury and Fell, 2008). Monitoring is one of the most important tools to understand landslide kinematics and dynamics (Angeli et al., 2000). One of the landslides which is monitored by scientists from the Faculty of Mining, Geology and Petroleum Engineering (RGNF) is the Kostanjek landslide, the biggest landslide in the Republic of Croatia. The Kostanjek landslide area is 1 km2, which is approximately 1,000 times bigger than the area of an average-sized landslide (730 m2) in the City of Zagreb (Bernat Gazibara et al., 2019). The displaced mass consists of the topmost part of the Sarmatian deposits and Lower and Upper Pannonian deposits. Generally, dominating rocks of the Sarmatian and Pannonian ages, in the area of landslide, are very weak to weak marls. The landslide was triggered due to the excavation of marl, in the open marl pit, which was used for cement production in the nearby cement factory 'Sloboda' (Stanić and Nonveiller, 1996).
Kostanjek landslide monitoring observatory (Figure 1), one of the laboratories of the RGNF, was established in the frame of the Croatian-Japanese SATREPS project 'Risk identification and land-use planning for disaster mitigation of landslides and floods in Croatia' in the period from 2009 to 2014. The objectives of the monitoring system were civil protection of the residents (approx. 300 single-family houses and infrastructure networks are placed on the moving landslide mass), scientific research and education. Protection of residents from the consequences of sliding is done through continuous monitoring of movement and its causes, and through the development of an early warning system, which includes: 1) definition of empirical thresholds for landslide velocities, precipitations and groundwater levels and definition of different emergency levels; 2) development of statistical models for prediction of landslide velocities. The scientific research activity related to the Observatory is carried out in the form of numerous projects, for example, the ongoing PRI-MJER project (Applied landslide research for development of risk mi') financed by the European Regional Development Fund (KK.05.1.1.02.0020) and co-financed by the Environmental Protection and Energy Efficiency Fund (https://pri-mjer.hr/). Education of students within the Observatory is carried out for graduate students of Mining Engineering (Geotechnical Engineering) and graduate students of Geological Engineering (Hydrogeology and Engineering Geology) from the Faculty of Mining, Geology and Petroleum Engineering.
Figure 1 Central measuring station of the Kostanjek landslide with the GNSS antenna in the foreground.
The monitoring system at the Kostanjek landslide consists of multiple sensor networks for observations of (Krkač et al., 2019): (1) external triggers (rain gauge and meteorological station); (2) hydrological properties (pore pressure gauges and water level sensors); (3) movement/activity. All sensors measure in almost real-time, and the measured data is transmitted via the Internet to the application/data server at the RGN faculty. The movement of the Kostanjek landslide is measured with 15 GNSS (Global Navigation Satellite System) sensors. GNSS is a system of satellites and earth stations used for precise positioning on the Earth. Satellites orbit the Earth twice a day in very precisely defined orbits and continuously transmit signals with information about the time of signal transmission and its position at the time of signal transmission. Earth stations consisting of antennas and receivers receive satellite signals and determine the distance of satellites based on the difference in the time of transmission and reception of signals. From the distances between the antenna and a minimum of four satellites and the position of these satellites, the receivers accurately calculate the position on the Earth. GNSS receivers use GPS and GLONASS satellite signals, and the system operates continuously 24 hours a day, in different weather conditions, and does not require optical visibility between measurement sensors (Ghiliani and Wolf, 2012).
The precision of GNSS measurements at the Kostanjek landslide, calculated as the root mean square error on the 24-h post-processing position (at 2σ, 95 % confidence), is 3.2–4.6 mm in planimetry and 6.1–10.5 mm in altimetry (Krkač et al., 2017). During the monitoring period (2013-2019) all GNSS sensors showed a significant displacement, i.e. the measured displacements were greater than the measurement errors, except for GNSS 01 which is located outside the landslide (see Figure 3). The largest measured horizontal displacement is 65 cm and the largest vertical displacement is +41 cm. Since 2013., a total of eight periods of faster movement have been measured (Figure 2). During the periods of faster movement more than 90% of the total displacement of the Kostanjek landslide occurred. The maximum observed velocity was 4.5 mm/day and it was measured in the first week of April 2013. The highest velocities were measured in the central part of the landslide (Figure 3).
Figure 2 Cumulative horizontal displacements at the Kostanjek landslide, measured with GNSS sensors during. The grey areas represent periods of faster movements (Krkač et al., 2020a).
Figure 3 Spatial distribution of average yearly velocities at the Kostanjek landslide, and vectors of horizontal displacements measured by GNSS sensors (Krkač et al., 2020a).
All periods of faster movement occurred during the periods of high groundwater levels (Figure 4), which occurred after periods of intensive precipitation and snow melting. The data records from water level sensors in the central part of the landslide show groundwater levels (GWL) oscillations up to 8.5 m, between the depths of 19 and 10.5 m, corresponding to the variation of the pore pressures at the sliding surface from 425 to 510 kPa. The cumulative precipitations that caused GWL rise at the central part of the Kostanjek landslide ranged from 21 mm to 180 mm, depending on initial GWL depth, i.e. depending on a season. During the summer months, due to high evapotranspiration and surface runoff, there was no measured increase in groundwater levels that would affect landslide movement. Compared with the average annual precipitation (889 mm) in City of Zagreb, the meteorological conditions during 2013–2014, when the greatest movement occurred, can be considered very wet. In 2013, the total precipitation (rainfall and snow) was 1092 mm, and in 2014 it was 1234 mm. Also, during that period of two years, several extremes occurred. The highest daily precipitation (55.2 mm) was recorded in February 2013 and the maximal monthly precipitation (208 mm) was recorded in September 2014. One of the most significant triggers of landslide movement is an earthquake. The Zagreb earthquake, magnitude 5.5 (March 22, 2020), and the earthquake near Petrinja, magnitude 6.4 (December 29, 2020), resulted in a Kostanjek landslide displacement of a totally 2 cm.
Figure 4 Cumulative horizontal displacements and groundwater level depths measured at the central monitoring station of the Kostanjek landslide and 3-day antecedent precipitations measured at the Zagreb-Grič meteorological station (Krkač et al., 2020b).
Reference:
Angeli M.-G., Pasuto A., Silvano S. (2000): A critical review of landslide monitoring experiences. Engineering Geology, 55, 3, 133-147. https://doi.org/10.1016/S0013-7952 (99)00122-2.
Bernat Gazibara S., Krkač M., Mihalić Arbanas S. (2019): Verification of historical landslide inventory maps for the Podsljeme area in the City of Zagreb using LiDAR-based landslide inventory. The Mining-Geology-Petroleum Engineering Bulletin, 34, 1, 45-58. DOI: 10.17794/rgn.2019.1.5
Crozier M.J. (2010): Landslide geomorphology: An argument for recognition, with examples from New Zealand. Geomorphology, 120, 3-15. https://doi.org/10.1016/j.geomorph.2009.09.010
Cruden D.M. (1991): A simple definition of a landslide. Bulletin of the International Association of Engineering Geology, 43, 27-29. doi:10.1007/BF02590167
Cruden D.M., Varnes D.J. (1996): Landslide types and processes. In: Turner, A.K., Schuster, R.L. (eds.): Landslides, Investigation and Mitigation. Transportation Research Board, Special Report 247, Washington D.C., USA, 36–75, 673 p.
Ghiliani C.D., Wolf P.R. (2012): Elementary Surveying: An Introduction to Geomatics (Thirteenth Edition). Pearson Education, Inc., New Jersey. 984 p.
Hungr O., Leroueil S., Picarelli L. (2014): The Varnes classification of landslide types, an update. Landslides, 11, 2, 167–194. https://doi.org/10.1007/s10346-013-0436-y
Krkač M., Špoljarić D., Bernat S., Mihalić Arbanas S. (2017): Method for prediction of landslide movements based on random forests. Landslides, 14, 3, 947–960. https://doi.org/10.1007/s10346-016-0761-z
Krkač M., Bernat Gazibara, S., Sečanj, M., Arbanas, Ž., Mihalić Arbanas, S. (2019): Continuous monitoring of the Kostanjek landslide. Proceedings of the 4th Regional Symposium on Landslides in the Adriatic-Balkan Region. Sarajevo: Geotechnical Society of Bosnia and Herzegovina, 43-48
Krkač M., Bernat Gazibara S., Sečanj M., Sinčić M., Mihalić Arbanas S. (2020a): Kinematic model of the slow-moving Kostanjek landslide in Zagreb, Croatia. Rudarsko-geološko-naftni zbornik, 36/2, 59-68. doi:10.17794/rgn.2021.2.6
Krkač M., Bernat Gazibara S., Arbanas Ž., Sečanj M., Mihalić Arbanas S. (2020b): A comparative study of random forests and multiple linear regression in the prediction of landslide velocity. Landslides, 2515–2531. https://doi.org/10.1007/ s10346-020-01476-6
Lacroix P., Handwerger A.L., Grégory G. (2020): Life and death of slow-moving landslides. Nature Reviews Earth & Environment, 1, 404–419. https://doi.org/10.1038/s43017-020-0072-8
Schulz W.H., Coe J.A., Ricci P.P., Smoczyk G.M., Shurtleff B.L., Panosky J. (2017): Landslide kinematics and their potential controls from hourly to decadal timescales: Insights from integrating ground-based InSAR measurements with structural maps and long-term monitoring data. Geomorphology, 285, 121-136. https://doi.org/10.1016/j.geomorph.2017.02.011.
Stanić B., Nonveiller E. (1996): The Kostanjek landslide in Zagreb. Engineering Geology, 42, 269-283.
Martin Krkač is Assistant Professor of Engineering Geology at the University of Zagreb, Faculty of Mining, Geology and Petroleum Engineering. His research interests are landslides, especially landslide monitoring and remote sensing. He is a member Croatian Landslide Group, and he is the author of more than 70 scientific works in international journals, books and conference proceedings. He participated in more than 40 engineering geology / geotechnical studies in the field of landslide research and stabilization.
E-portfolio Link
ResearchGate Link
Google Scholar Link
CROSBI Link