© Author(s) 2022. CC Atribution 4.0 License Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) Pregled raziskav in nastanek plazov v zaledju naselja Koroška Bela (SZ Slovenija) Tina PETERNEL1, Ela ŠEGINA1, Jernej JEŽ1, Mateja JEMEC AUFLIČ1, Mitja JANŽA1, Janko LOGAR2, Matjaž MIKOŠ3 & Miloš BAVEC1 1Geological Survey of Slovenia, Dimičeva ulica 14, SI-1000 Ljubljana, Slovenia; e-mails: tina.peternel@geo-zs.si, jernej.jez@geo-zs.si, mateja.jemec@geo-zs.si, mitja.janza@geo-zs.si, milos.bavec@geo-zs.si 2Chair of Geotechnical Engineering, Faculty of Civil and Geodetic Engineering, University of Ljubljana, Ljubljana, Slovenia; e-mail: janko.logar@fgg.uni-lj.si 3Chair of Hydrology and Hydraulic Engineering, Faculty of Civil and Geodetic Engineering, University of Ljubljana, Ljubljana, Slovenia; e-mail: matjaz.mikos@fgg.uni-lj.si Prejeto / Received 30. 5. 2022; Sprejeto / Accepted 22. 9. 2022; Objavljeno na spletu / Published online 30. 9. 2022 Key words: landslide, debris flow, research, monitoring, landslide evolution, Koroška Bela Ključne besede: plaz, drobirski tok, raziskava, monitoring, nastanek plazov, Koroška Bela Abstract This paper gives an overview of landslide research and the activity of landslides located above the Koroška Bela settlement in Northwest Slovenia. There are several landslides in this area and they pose a direct threat to the settlement below. The settlement is very densely populated (about 2,100 inhabitants) and has well-developed industry and infrastructure. It is built on deposits from past debris flows, indicating that large slope mass movements have occurred in the past. In this regard, the hinterland of Koroška Bela has been investigated since 2006, within the framework of various research, technical and European projects. The most extensive geological and geotechnical investigations were carried out after April 2017, when part of the Čikla landslide collapsed and mobilised into a debris flow. All of the investigations which have been carried out over the years revealed that the hinterland of Koroška Bela is characterised by high landslide activity due to geological, hydrogeological and tectonic conditions. In order to protect people and their property, it is essential to implement a holistic mitigation measure which includes remediation works (drainage works, debris flow breaker, etc.) and non- structural measures (monitoring system, early warning system, risk management, etc.). Regular and continuous monitoring of all landslides is also crucial to observe the landslide dynamics and evaluate the effectiveness of structural mitigation measures. Izvleček Članek predstavlja pregled raziskav in aktivnosti plazov, ki se nahajajo nad naseljem Koroška Bela v severozahodni Sloveniji. Obravnavano območje je podvrženo plazovom, ki predstavljajo neposredno nevarnost za spodaj ležeče naselje. Naselje je gosto poseljeno (s približno 2.100 prebivalci) in ima dobro razvito industrijo ter infrastrukturo. Zgrajeno je na sedimentih preteklih drobirskih tokov, kar tudi dokazuje, da so se obsežni pobočni masni premiki prožili tudi v preteklosti. Zaradi naštetih dejstev na tem območju potekajo raziskave že od leta 2006, ki so se izvajale v okviru različnih raziskovalnih, tehničnih in evropskih projektov. Najobsežnejše geološke in geotehnične preiskave so bile izvedene po aprilu 2017, in sicer po sprožitvi manjšega drobirskega toka na Čikli. Vse raziskave in študije, ki so bile izvedene v vseh teh letih, so pokazale, da je zaledje Koroške Bele podvrženo aktivnim plazovom, ki so se formirali predvsem zaradi danih geoloških, hidrogeoloških in tektonskih razmer. Za zaščito ljudi in njihovega imetja je nujna celostna izvedba varovalnih ukrepov, ki bodo vključevali tako preventivne (npr. sistem za opazovanje, opozorilni sistem, načrt obvladovanja ogroženosti, itd.), kot tudi gradbene ukrepe (npr. drenažni sistem, pregrade, itd.). Prav tako je nujno izvajanje rednega in kontinuiranega spremljanja vseh aktivnih plazov v zaledju, ki bo omogočal prepoznavanje dinamike plazov in meril učinkovitost izvedenih gradbenih ukrepov. GEOLOGIJA 65/2, 129-147, Ljubljana 2022 https://doi.org/10.5474/geologija.2022.008 130 T. PETERNEL, E. ŠEGINA, J. JEŽ, M. JEMEC AUFLIČ, M. JANŽA, J. LOGAR, M. MIKOŠ & M. BAVEC Introduction Catastrophic landslides are usually the result of rapid collapse of soil, rock, and fluids triggered by heavy rainfall, snowmelt, earthquakes, or an- thropogenic activities (Gariano & Guzzetti, 2016; Lacroix et al., 2020). In Slovenia, landslides are fairly common and are related to active tectonics, diverse geological settings and climatic condi- tions. Landslides frequently occur in clastic rocks located under steep slopes composed of highly permeable carbonate rocks (Jemec Auflič et al., 2017a). An example of this is the slope morphology of the Vipava Valley, which is primarily influenced by the different lithology of the thrust units and is characterised by steep carbonate cliffs and gentle lower slopes formed in the underlying flysch (Ver- bovšek et al., 2017; Popit et al., 2022). In recent decades, four major landslides have occurred in Slovenia, with a volume of approxi- mately 1 × 106 m3. In November 2002, the Stože de- bris flow formed in the catchment area above the village of Log pod Mangartom. The debris flow caused seven casualties and destroyed residential and farm buildings (Mikoš, 2020). In the same period, reactivation of the Slano Blato landslide occurred above the village of Lokavec (Fifer Biz- jak & Zupančič, 2009; Mikoš et al., 2009; Maček et al., 2016) and, one year later (2001), the Strug landslide occurred above the village of Koseč (Mikoš et al., 2006). A large landslide area is also located in Rebernice in the Vipava valley, where deep-seated landslides have formed in complex geological settings (Popit et al., 2014; Popit, 2017). This area is crossed by a motorway, where inves- tigations and remediation works are constantly carried out due to road subsidence. These events, and the fact that alpine and perialpine regions are highly susceptible to land- slides, have revealed that, on the whole, Slovenia needs to pay more attention to landslide preven- tion measures, to reduce the impact of landslide activity and to protect people and infrastructure (Mikoš, 2021). This paper focuses on historical, as well as current, landslide activity on the mountain slopes above the settlement of Koroška Bela in Northwest Slovenia (Fig. 1). This territory is one of the most active landslide-prone areas in Slovenia. It attracts additional attention due to historical evidence of past debris flows in recent geological history. The first registered event oc- curred in the 18th century and it caused partial or complete destruction of more than 40 buildings and devastated cultivated areas in the village of Koroška Bela (Lavtižar, 1897; Zupan, 1937). The most recent event occurred in April 2017, when part of the Čikla landslide collapsed and turned into a debris flow (Jež et al., 2019a). Even though this particular debris flow did not reach the set- tlement, it was perceived to be an additional warning sign that slope mass movements in the hinterland of Koroška Bela are still a source of potential debris flows. The area of interest is also prone to other types of landslides, such as slides, flows, falls, and combinations thereof. Fig. 1. Location of Koroška Bela and the extent of the landslide-prone area. 131Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) Koroška Bela is a densely populated urban settlement in the alpine Upper Sava River valley. It is built on a typical torrential fan with an es- timated area of 1.02 km2, formed by past debris flows. Currently, it has about 2,100 inhabitants, a well-developed steel works and an important infrastructure connection between the capital of Slovenia and Northwest Slovenia. In this respect, the landslide-prone area in the hinterland of Koroška Bela has been under in- vestigation since 2006. The research started within the framework of various national and international projects (Fig. 2; Table 1). All of the investigations can be roughly divided into two stages: before and after the Čikla debris flow in April 2017. The first set of studies, carried out before April 2017, was primarily scientific. Their main objective was to identify and understand the characteristics and kinematics of landslides. Studies carried out after April 2017 were primar- ily conducted with the goal of developing and im- plementing mitigation measures (Fig. 2; Table 1). Morphological and geological settings The hilly and mountainous hinterland of Ko- roška Bela is a part of the Belščica slope (Kar- avanke mountain ridge). It covers an area of approximately 6 km2 and extends from 520 to 2,100 m a.s.l. It is characterised by medium to steep slopes, sloping to the southwest. Since the slopes are exposed to the sun, they experience relatively large temperature oscillations. The area is characterised by very rugged terrain, dominated by concave slopes. The prevalence of concave slopes may have an indirect influence on the potential occurrence of landslides since, in these areas, the velocity of water flow usually de- creases, resulting in water accumulation that in- creases soil water saturation and slope instability (Hengl & Reuter (2009) cited in Romer & Feren- tinou (2016)). Komac (2005) also observed that a greater number of landslides in Slovenia occur in concave areas. The land use map shows that the Koroška Bela hinterland is almost completely Fig. 2. Timeline of landslide activity and research since 1789. The coloured background defines the type of project (green – research projects funded by Slovenian Research Agency; blue – projects cofunded by EU Programmes; orange – technical projects funded by the Ministry of the Environment and Spatial Planning and the Municipality of Jesenice). This paper gives an overview of landslide re- search above the Koroška Bela settlement. The first part consists of a detailed description of the study area, the investigations conducted and their main outcomes. The second part deals with the evaluation of recent landslide activity using multitemporal digital orthophoto imagery and digital elevation models (DEMs). This paper summarises the extensive succes- sive research carried out in the Koroška Bela landslide area and provides an example of good practice in landslide risk management. vegetated and covered with forest (MKGP, 2016; Peternel, 2017). In this case, it is assumed that the forest has a rather limited protective function against slope instabilities, since most of the for- est consists of larch and spruce, which are char- acterised by shallow root systems. Conversely, large unstable trees on steep slopes can increase the risk of triggering landslides, as windstorms may cause uprooting, toppling and falling of trees and accelerate the weathering and erosion of slopes (Jakša & Kolšek, 2009). In addition, fall- en trees and other vegetation accumulate in the 132 T. PETERNEL, E. ŠEGINA, J. JEŽ, M. JEMEC AUFLIČ, M. JANŽA, J. LOGAR, M. MIKOŠ & M. BAVEC Bela stream as additional unstable material that could be mobilised during torrential floods. The Koroška Bela hinterland is defined by complex geological and tectonic conditions. For geological maps and detailed geological descrip- tions of the Koroška Bela hinterland, the reader is referred to previous studies (Jež et al., 2008; Peternel et al., 2017, 2018). A general, geological description of the wider Koroška Bela area has been presented by Buser (1978), Buser (1980) and Jež et al. (2008). The study area mainly consists of Upper Carboniferous and Permian clastic rocks – Permian carbonates and Triassic to Lower Ju- rassic carbonate rocks (Jež et al., 2008). The main slope instabilities are related to tectonic contacts between the clastic Upper Carboniferous to Per- mian rocks (claystone, siltstone, sandstone and conglomerate) and various Permian and Triassic carbonate and clastic rocks. (Jež et al., 2008). Tectonically, the area belongs to the South- ern Alps (Placer, 2008). It is a part of the Košuta fault zone and is dissected by numerous NW-SE faults connecting two major fault zones (the Sava and Periadriatic fault zones) (Jež et al., 2008). The rocks are heavily deformed and, therefore, very prone to rapid and deep weathering. Carbonate rocks in the uppermost parts of the Belščica slope are also subject to intense physical and chemical weathering, resulting in large quantities of talus and scree material. Prominent morphological features in the study area are relatively long ridges interrupted by long, deep, narrow and irregular mountain wa- tercourses (Brenčič & Poltnig, 2008; Janža et al., 2018). In general, three types of aquifers char- acterise the regional hydrogeological settings, which are determined by geological conditions: intergranular aquifers in clayey carbonate grav- els, karst-fissured aquifers in carbonate rocks and small local aquifers, typically occurring in clastic rocks. The morphology of the slopes, unfavourable geological and tectonic conditions, and climatic diversity contribute to the fact that the region above Koroška Bela is prone to landslides and torrents and represents the source of potential debris flows that could pose a threat to the dense- ly populated settlement below. Landslide activity is evidenced by an irregular and hummocky ter- rain comprised of protrusions and depressions of various sizes, curved trees, tension cracks, ero- sion slumps and wetlands on the surface, as well as widespread subsidence of local roads. The first warnings of landslide activity in the area, that attracted the attention of our research group, were reports of sliding by the Slovenian Forest Service. They reported the subsidence of a local road, the presence of curved trees and ero- sion flanks in the wider catchment area of the Ur- bas water reservoir (Fig. 2). Climatic conditions The study area is characterised by an alpine climate, ranging from a low mountain to a high mountain climate (Brenčič & Poltnig, 2008). The climate is highly variable due to the effects of al- ternating high and low atmospheric pressure and alternating atmospheric fronts. The variable cli- matic conditions are also related to the great dif- ferences in altitude, the rugged terrain with deep and narrow valleys, and the slope orientation (Rakovec et al., 2000). It is typical for this part of the Karavanke Mountains that the mean annu- al precipitation ranges from 1,600 to 2,000 mm, while the maximum 24-hour precipitation with a 100-year return period is estimated between 180 and 210 mm (Internet 1). There are two annual precipitation peaks, with the main precipitation peak in autumn and the second in spring. An overview of past research An overview of all the implemented investiga- tions is listed in Table 1. The first research started within the Target Research Project (TRP): Debris flow risk assessment in Slovenia. Within the TRP project, geological mapping of the Koroška Bela hinterland, a study of alluvial fan deposits, and modelling of debris flows using a Flo-2D model were carried out. The project results showed that the alluvial fan of Koroška Bela consists of a se- quence of depositional layers related to several historic debris flows (Jež et al., 2008; Mikoš et al., 2008). Based on detailed engineering mapping of this area, the Urbas landslide was identified (for the first time), named and described as the Potoš- ka planina landslide (Jež et al., 2008). Further- more, the estimated magnitudes of debris flows for the Bela torrential watershed were calculated by Sodnik and Mikoš (2006), using various mor- phological parameters. By applying different empirical equations for the determination of the design discharge and flood volume with a 100- year return period, different debris flow magni- tudes were calculated, ranging from 19,687 m3 to 93,231 m3 (Sodnik & Mikoš, 2006). The Bela torrential fan was classified as a transitional fan, where debris flows are possible. For debris flow modelling, a Flo 2-dimensional model was used 133Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) with different numerical square grids generat- ed from digital elevation models (Sodnik et al., 2009, 2012, 2013; Sodnik & Mikoš, 2018; Bezak et al., 2020). The calculated results showed that the estimated inundated area ranges between 118 and 145 m² and the average maximum flow depth ranges from 0.6 to 1.2 m, depending on the square grid applied. The first monitoring at the Urbas landslide was established using a novel motion detection device that was developed within the EU funded project “Integrated Interferometry and GNSS for Pre- cision Survey (I2GPS)” founded by the Seventh Framework Programme (FP7-GALILEO-2008- GSA-1). The device integrated interferometric synthetic aperture radar (InSAR) and global navigation satellite system (GNSS) technologies. Two compact active transponder (CAT) units (In- SAR data) and a combined CAT and GNSS unit (providing 3D displacement assessments) were installed to monitor the surface displacements of the landslide and its vicinity (Komac et al., 2015). The InSAR and GNSS results showed rel- atively large (up to 32 mm horizontal and up to 15 mm vertical) displacements during a relative- ly short monitoring period (02/2011 – 08/2011), indicating a displacement of the central-upper and south-eastern parts of the landslide body (Komac et al., 2015, 2018). The Urbas landslide was further investigated and monitored within the framework of a PhD thesis (Peternel, 2017). To evaluate the kinematics of the Urbas land- slide, understand the characteristics of the slid- ing process and assess the surface displacement rates and changes in surface topography, period- ical monitoring was conducted using a variety of remote sensing and in-situ geodetic techniques (unmanned aerial vehicle (UAV) photogramme- try, terrestrial laser scanning (TLS), and tachym- etric surveys) (Peternel et al., 2015, 2017a; Peter- nel, 2017; Peternel & Komac, 2017). The surveys revealed that the Urbas landslide is a composite landslide (Cruden & Varnes, 1996) consisting of rock falls (upper part), deep-seated landslides (main body) and debris flow source areas (lower part) (Peternel, 2017; Peternel et al., 2017a). The long-term activity of the Urbas landslide over the past 138 years has also been reconstructed us- ing the dendrogeomorphological analysis of bent trees (Oven et al., 2019). Dendrogeomorphology has proven to be a highly useful method in the study of past slope mass movements and has re- cently been applied to the analysis of the debris flood magnitude in the Planica valley (Novak et al., 2020). The estimated risk and visible activi- ty of landslides have increasingly attracted the attention of the inhabitants of Koroška Bela and the Civil protection service. For this purpose, the socio-economic impact was assessed within the framework of the EU-funded project “RECALL Resilient European Communities Against Local Landslides”. In addition, a cooperative team of decision-makers, response authorities, technical experts and other stakeholders was formed to increase awareness and understanding of land- slide risk in the community (Jemec Auflič et al., 2017b, 2017c, 2019). In-depth research continued within the research project “Studying landslide movements from source areas to zone of deposi- tion using a deterministic approach (ARRS J1- 8153)”. As a part of the project, two additional research trenches, with depths between 3.00 and 3.95 m, were excavated on the alluvial fan where the Koroška Bela settlement has been developed; 11 samples of organic material were collected for radiocarbon dating. The main sedimentological units of the research trenches are layers of debris flow deposits interbedded with thick silty and sandy lenses, fluvial (fine-grained) deposits and flood/mudflow deposits (Jež et al., 2019b), suggest- ing several past depositional events. Age dating of the organic sediments revealed that most of the sediment was deposited during the Last Glacial Maximum (LGM), while two or three debris lay- ers were found in the upper part of the fluvial suc- cession, and these were deposited during the Hol- ocene. The youngest deposits were attributed to the debris flow which occurred in 1789 (Jež et al., 2019b). In addition, Sodnik et al. (2017) upgrad- ed their models by modelling debris flow source areas with a multi-model approach using field data, susceptibility and trigger modelling, im- plemented using LS-Rapid (Loi et al., 2020). This research project was granted as a strategically important project in the category of International Programme on Landslides (IPL) in 2017-2020. A very important milestone, which gave ad- ditional impetus to the study of landslide areas, occurred in April 2017, when part of the Čikla landslide collapsed and mobilised as a mass flow with a significant amount of talus material and vegetation. The debris flow had an estimated volume of 5,000 m³ and was triggered by heavy rainfall, with 200 mm of precipitation falling in 48 hours (Jež et al., 2019a; Peternel et al., 2022a). This event was one of the triggers for the inten- sification of detailed geological, geotechnical and hydrogeological investigations of landslides in the hinterland of Koroška Bela in 2017 (Ta- ble 1). For the spatial distribution of the applied 134 T. PETERNEL, E. ŠEGINA, J. JEŽ, M. JEMEC AUFLIČ, M. JANŽA, J. LOGAR, M. MIKOŠ & M. BAVEC methods and the description of the engineering geological and hydrogeological results of the Ur- bas and Čikla landslides, the reader is referred to previous studies (Peternel et al., 2017b, 2018; Janža et al., 2018). The initial investigations were limited and provided only a rough insight into the geological and hydrogeological conditions of the observed landslides. The main outcomes were (Peternel et al., 2017b, 2018, 2019; Janža et al., 2018): - In the hinterland of Koroška Bela there are more than 20 landslides, five of which have an area of more than 8,000 m2: the Ur- bas, Čikla, Potoška planina, Malnež and Obešnik landslides. Among these, Urbas and Čikla are considered the most active. - Landsliding mechanisms are defined by the complex lithological composition, intensive tectonic deformation and hydrogeological conditions. The rocks and sediments in the study area are heavily deformed and prone to weathering, which results in weak ge- omechanical properties of the bedrock. - In the Urbas and Čikla landslides, hydro- geological conditions are very heterogene- ous and cannot be characterised uniform- ly due to complex geological and tectonic conditions. The groundwater is recharging by a combination of infiltration of precipi- tation and subsurface inflow from the car- bonate hinterland. In the upper parts of the landslides, groundwater occurs at the con- tact between slope deposits and weathered clastic rocks. - Preliminary 3D reconstructions of the landslide body showed that the calculated volumes of the Urbas and Čikla landslides are 895,000 m3 and 141,000 m3, respectively. For landslide volume calculation, the GO- CAD-SKUA software was used. - The modelling results showed that the es- timated potential debris flows would have catastrophic consequences in the Koroška Bela settlement. In some densely populated parts, the simulated depth of the potential debris flow exceeds 5 m, which indicates that the application of mitigation measures is inevitable. - The results of the preliminary investiga- tions also revealed the need for continuous near-real and real-time monitoring and additional geological (hydrogeological, ge- ophysical), geodetic and geotechnical in- vestigations to obtain a basis for mitigation and remediation measures. Based on these results and the historical facts, the Municipality of Jesenice recognised the high risk and established a monitoring system for the Urbas and Čikla landslides. The monitoring net- work consists of extensometers, rain-gauges, pi- ezometers, inclinometers and motion detection cameras. All the electronic geotechnical sensors are wired and powered through the base station, which also serves as an automated data logger. To store and access all measured monitoring data, the eTeren platform (https://eteren.geo-zs. si/home) was developed in 2021. In 2019, additional geological, geotechnical and geodetic investigations were carried out in the frame of the project funded by the Ministry of the Environment and Spatial Planning. The main purposes of this project were the engineering, ge- ological and hydrogeological characterisation of landslides and the implementation of the stabil- ity analyses and feasibility studies of mitigation and remediation measures (Table 1). The Geolog- ical Survey of Slovenia (GeoZS), which led the project, also involved the Faculty of Civil and Geodetic Engineering (University of Ljubljana) and experts from the Italian Geological Survey (ISPRA) were called in as external consultants. Within the framework of the project, the follow- ing main outcomes were drawn (Bezak et al., 2020, 2021; Peternel et al., 2020a, 2020b, 2022a): - The preliminary results of real-time land- slide monitoring showed that the dynamics of the Urbas and Čikla landslides represent a combination of steadily sliding mass be- haviour as well as episodic, rapid displace- ments corresponding to increased rainfall. - Improved 3D reconstruction of the land- slides resulted in the following estimat- ed volumes: Urbas: 1,578,700 m3, Čikla: 330,500 m3, Malnež: 173,750 m3, Obešnik: 301,780 m3. For landslide volume calcu- lation, the GOCAD-SKUA software was used. - The study confirmed that the Urbas and Čikla landslides are a source area for de- bris flows and pose a direct risk to the set- tlement of Koroška Bela. - Observations of the Malnež, Obešnik and Potoška planina landslides, which were first identified in 2017, showed that their current state and dynamics pose a signif- icantly lower direct risk to the settlement. Nevertheless, monitoring should be contin- ued and improved. - The field investigation of the Bela stream and its tributaries revealed that more than 135Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) Du ra tio n Ty pe o f p ro je ct Pr oj ec t Ty pe o f r es ea rc h Lo ca tio n Re se ar ch p ap er s 1. 01 /0 6/ 20 06 30 /1 1/ 20 08 AR RS De br is flo w ri sk a ss es sm en t i n Sl ov en ia - t ra nc h (K or oš ka B el a) : 2 - g eo lo gi ca l m ap o f t he h in te rla nd o f K B - d eb ris fl ow m od el lin g us in g th e Fl o- 2D Hi nt er la nd o f Ko ro šk a Be la ; al lu vi al fa n So dn ik & M ik oš , 2 00 6; M ik oš e t a l., 2 00 8; Je ž e t a l., 2 00 8 2. 01 /0 1/ 20 10 31 /0 3/ 20 11 EU : F P7 I2 GP S – In te gr at ed In te rfe ro m et ry a nd G N SS fo r P re ci sio n Su rv ey - 1 st m on ito rin g of su rf ac e di sp al ce m en ts u sin g In SA R an d GN SS U rb as Ko m ac e t a l., 2 01 2a ; 20 12 b; 2 01 5; 2 01 8 3. 01 /1 2/ 20 12 31 /0 5/ 20 17 AR RS Dy na m ic s o f t he sl op e m as s m ov em en ts in th e Po to šk a pl an in a w ith an al ys es o f r es ul ts o f r em ot e se ns in g an d te rr es tr ic su rv ey s t ec hn iq ue s an d in -s itu m ea su re m en ts Lo w er p ar t: - T ac hy m et ric m .: 7 - U AV p ho to gr am m et ry : 7 U pp er p ar t: - T LS : 2 - U AV p ho to gr am m et ry : 2 U rb as Pe te rn el e t a l., 2 01 5; 20 17 a; P et er ne l & Ko m ac , 2 01 7; P et er - ne l, 20 17 4. 04 /0 5/ 20 15 04 /0 4/ 20 17 EU : D G EC HO RE CA LL R es ili en t E ur op ea n Co m m un iti es A ga in st L oc al L an ds lid es Co op er ati ve te am M on ito rin g us in g cr ac km et er m et ho d U rb as Je m ec A ufl ič e t a l., 20 17 b; 2 01 7c ; 2 01 9 5. 01 /0 5/ 20 17 30 /0 4/ 20 20 AR RS St ud yi ng la nd sli de m ov em en ts fr om so ur ce a re as to zo ne o f d ep os iti on us in g a de te rm in isti c ap pr oa ch . tr an ch (K or oš ka B el a) : 2 Ge ph ys ic al m .: ve rti ca l e le ct ric al so un di ng (V ES ) U rb as , Č ik la , Ko ro šk a Be la So dn ik e t a l., 2 01 7, Je ž e t a l., 2 01 9a ; 20 19 b 6. 21 /0 8/ 20 17 30 /1 1/ 20 17 M O P2 01 7 Im pl em en ta tio n of u rg en t e ng in ee rin g ge ol og ic al , h yd ro ge ol og ic al , ge op hy sic al , g eo m ec ha ni ca l a nd g eo de tic su rv ey s to d et er m in e th e ob je cti ve d eg re e of ri sk to th e po pu la tio n fr om sl op e m as s m ov em en ts in th e Po to sk a Pl an in a ar ea a nd p re pa re e xp er t d oc um en ta tio n pr po sin g m iti ga tio n m ea su re s. - g eo lo gi ca l m ap pi ng o f t he h in te rla nd o f K or oš ka B el a; - e ng in ee rin g- ge ol og ic al (E G) m ap pi ng o f l an ds lid es U rb as a nd Č ik la ; - h yd ro ge ol og ic al (H G) m ap pi ng + in -s itu in ve sti ga tio ns ; - 9 b or eh ol es (2 in cl in om et er s; 2 p ie zo m et er s) ; - 2 tr an ch es (1 U rb as , 1 Č ik la ); - s ei sm ic R ef ra cti on To m og ra ph y (S RT ): 4 cr os s- se cti on s (3 U rb as , 1 Č ik la ); - e le ct ric al re sis tiv ity to m og ra ph y (E RT ): 4 cr os s- se cti on s (3 U rb as , 1 Č ik la ); - l ig ht d et ec tio n an d ra ng in g (L iD AR ) U rb as , Č ik la Pe te rn el e t a l., 2 01 7b ; Pe te rn el e t a l., 2 01 8; Ja nž a et a l., 2 01 8 7. 04 /0 7/ 20 19 20 /1 1/ 20 20 M O P2 01 9 De ta ile d ge ol og ic al -g eo te ch ni ca l a nd h yd ro ge ol og ic al c ha ra ct er iza tio n of la rg e la nd sli de s i n th e hi nt er la nd o f K or oš ka B el a se tt le m en t f or th e st ab ili ty a na ly se s a nd fo r a fe as ib ili ty st ud y of m iti ga tio n m ea su re s - u pd at e EG m ap ; - E G m ap pi ng : M al ne ž, O be šn ik ; - m ap pi ng o f B el a st re am ; -1 7 bo re ho le s ( 9 in cl in om et er s; 8 p ie zo m et er s) ; - i n- sit u ge ot eh ni ca l i nv es tig ati on s ( pr es io m et er . 2 6; S PT : 4 ) - i n- sit u HG in ve sti ga tio ns (s lu g t.: 1 ; p um pi ng t. : 1 1; tr ac ki ng t. : 2 ) - g eo ph ys ic al m . ( SR T: ~1 .2 00 m ; E RT :~ 2. 40 0 m ; G PR : 2 70 m ); - i nc lin om te ric m .: 0 + 4/ 6 m es ur em en ts - t ac hy m et ric m .: 4 (d ec /1 9; a pr /2 0; se pt /2 0; ju n/ 21 ) - U AV p ho to gr am m et ry (U rb as ): 2 (s ep t/ 19 ; a vg /2 0) - t ra nc he s ( Ko ro šk a Be la ): 2 U rb as , P P, Či kl a, M al ne ž, O be šn ik : Pe te rn el e t a l., 2 02 0a ; 20 20 b; 2 02 2 Be za k et a l., 2 02 0; 20 21 ; K or en e t a l., 20 22 8. 01 /0 9/ 20 20 31 /0 8/ 20 22 AR RS De ep -s ea te d la nd sli de p re di cti on m od el lin g ba se d on a c om bi na tio n of ph ys ic al m od el lin g an d a da ta -d riv en a pp ro ac h - c on tin ue s m on ito rin g; - l an ds lid e dy na m ic m od el lin g; - l an ds lid e pr ed ic tio n m od el lin g. U rb as , Č ik la Pe te rn el e t a l., 2 02 0; 20 22 a; 2 02 2b 9. 01 /1 1/ 20 17 31 /0 1/ 20 21 EU : H 20 20 GI M S: G eo de tic In te gr at ed M on ito rin g Sy st em GN SS a nt en na : 7 U rb as Še gi na e t a l., 2 02 0 10 . 02 /0 6/ 20 19 on go in g m un ic ip al ity Je se ni ce Es ta bl ish m en t a nd m ai nt en an ce o f m on ito rin g sy st em a t U rb as a nd Č ik la la nd sli de s - r ai ng au ge s - e ks te nz io m et er s - w eb c am er a - m oti on d et ec tio n ca m er a U rb as , Č ik la re po rt s ( Ge oZ S ar - ch iv e) T ab le 1 . A n o ve rv ie w o f p a st l a n d sl id e ri sk r es ea rc h p ro je ct s in t h e K or o šk a B el a a re a. T h e co lo u re d b ac k g ro u n d i s d efi n ed b y th e ty p e of p ro je ct ( g re en – r es ea rc h p ro je ct s fu n d ed b y th e S lo ve n ia n R es ea rc h A ge n cy ; b lu e – p ro je ct s co fu n d ed b y E U P ro g ra m m es ; or a n ge – t ec h n ic a l p ro je ct s fu n d ed b y th e M in is tr y of t h e E n v ir on m en t a n d S p at ia l P la n n in g /M u n ic ip a li ty o f Je se n ic e) . 136 T. PETERNEL, E. ŠEGINA, J. JEŽ, M. JEMEC AUFLIČ, M. JANŽA, J. LOGAR, M. MIKOŠ & M. BAVEC 15,000 m3 of material was deposited in the Bela and Čikla watercourses as a result of previous debris flow events. Potential tor- rential floods could mobilise the debris ma- terial deposited along the Čikla and Bela streams. - The existing check dams do not have suffi- cient capacity to fulfil the sediment and de- bris flow management needed in the area. - It is essential to implement holistic reme- diation measures to protect the population and infrastructure. Primarily, remediation measures must be taken on the Čikla and Urbas landslides and the torrents below. - In the future, periodic and continuous monitoring of all landslides is essential in order to observe landslide dynamics and to verify the effectiveness of potential reme- diation measures. - Future impacts of climate change should also be considered when designing mitiga- tion measures. The analysis of the impact of future climate change shows that the negative impact of total and effective rain- fall, air temperature, evapotranspiration and runoff from the Bela stream catchment is expected to increase, compared to the previous period. In parallel, near real-time monitoring of the Urbas landslide was improved with the GNSS system developed in the frame of EU project GIMS: Geodetic Integrated Monitoring System (No. 776335). The system provides a continuous, simultaneous and accurate monitoring of sur- face displacement at multiple locations across the Urbas landslide (Šegina et al., 2020; Peter- nel et al., 2022b). The availability of the remote data and the ease of installation of the GNSS units proved that the system is very suitable for monitoring landslide areas that are difficult to access. For this reason, the Čikla landslide was also equipped with the GNSS unit to monitor the displacement of a large boulder located in the ac- tive part of the Čikla landslide. Currently, the main ongoing activities are carried out within the Postdoctoral Research Project (ARRS, Z1-2638). The main objective of the project is to investigate landslide triggering parameters (rainfall, groundwater level, etc.) and to develop a landslide prediction model based on quantitative methods using data collected through continuous monitoring of landslide dis- placements. The results of the study, carried out on the Urbas landslide, showed that the dynamics of a landslide vary, depending on local geologi- cal and hydrogeological conditions. Consequent- ly, certain parts of the landslide are at different evolutionary states and respond differently to the same external triggers (Peternel et al., 2022b). Long term evaluation of landslide activity using orthophotos and LiDAR-derived DEMs Since the in-situ landslide monitoring pro- vides spatially limited information, remote sens- ing data, such as digital orthophotography and laser scanning data, have been used to provide an overview of landslide activity across the en- tire area. The landslide activity was estimated through analysis/review of orthorectified aerial photography (digital orthophotos or DOFs) with a resolution 0.50 m, taken between 1994 and 2020, and digital elevation models (DEMs) with a resolution of 1 m, taken in 2014 and 2017. An overview of the spatial data used is presented in Table 2. The analysis provided the first informa- tion on landslide activity in the entire Koroška Bela hinterland, which encompasses a total area Table 2. An overview of orthophotos used and DEMs derived from LiDAR. Type of spatial data Number of the acquisition Date of acquisition Resolution Availability DOF 1 27. 7. 1994 (C26) 12. 9. 1999 (D26) 0.5 Public 2 22. 7. 2006 (C26) 20. 7. 2006 (D26) 0.5 Public 3 11. 8. 2011 0.5 Public 4 5. 7. 2015 0.5 Public 5 24. 8. 2017 0.5 Public 6 28. 7. 2020 0.5 Public DEMs 1 2014 1.0 Public 2 14. 11. 2017 1.0 Upon request 137Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) of 7 km². It also enabled the evaluation of the kinematics of landslides for the period when the monitoring system was not yet in place. First, a visual interpretation of six DOFs was used to obtain information on visible landslide changes over time. Particular attention was paid to changes that might indicate landslide activi- ty, such as disturbed or absent vegetation cover, deformation of the local roads and a growing ero- sion surface. The observed features were manual- ly digitised and compared to all available ortho- photos. Based on this comparison, the intensity of change was classified into three classes: low, medium and high change intensity (Fig. 3). Then, all observed changes were characterised by the type of phenomenon (landslide or deforestation), and verified by field investigations. The field in- vestigations revealed that some large areas were the result of deforestation and were excluded from further analysis. Based on the results of the change intensity as- sessment and field investigations, we identified three landslide active areas that have been ana- lysed in more detail (Fig. 3): - Active area 1: The lower part of the Urbas landslide, which has the potential to mobi- lise into a debris flow (see section 2.1). - Active area 2: Local road crossing the main body of the Urbas landslide (see section 2.2). - Active area 3: Pre and post-Čikla debris flow (see section 2.3). In addition to DOF analysis, elevation and volumetric changes resulting from landslide ac- tivity have also been estimated by comparing two successive DEMs. Elevations from the earlier DEM were subtracted from the latter on a cell- by-cell basis, at 1 m resolution. Decreases in ele- vation represent erosion zones (red colour), while increases in elevation indicate accumulation zones (Fig. 4). It should be noted that this method only indicates changes in surface elevation and volume and does not provide any insight into the overall mass balance of the area. The elevation difference calculated for the en- tire Koroška Bela hinterland between 2014 and 2017 shows that 43,616 m³ of material accumu- lated and 107,283 m³ eroded during this period, indicating that the Koroška Bela hinterland is a predominantly erosive area. The erosion is most- ly limited to the carbonate slopes, while accumu- lation occurs in the form of scree deposits under steep slopes. Carbonates on the Belščica slope are prone to extensive planar erosion, which is main- ly concentrated in gullies, indicating most of the torrential transport and removal of the availa- ble, mechanically weathered top layer of rock. Similar processes are anticipated on carbonate slopes of the Alničje ridge. The most intensive erosion and accumulation of material occurred in the Čikla landslide, where a typical sequence of downslope erosion, followed by accumulation (at the foot of the landslide) can be observed. Fig. 3. Assessment of change intensity by visual interpretation and comparison of multitemporal orthophotos. The highest change intensity due to landslides was observed in the areas crossed by the Bela stream and its tributaries. 138 T. PETERNEL, E. ŠEGINA, J. JEŽ, M. JEMEC AUFLIČ, M. JANŽA, J. LOGAR, M. MIKOŠ & M. BAVEC Active area 1: Čikla landslide Čikla landslide activity can already be seen by comparing the 2011 and 2015 DOFs (Fig. 5A and 5B), which shows that part of the Čikla land- slide collapsed before 2017. Although the exact date and trigger mechanism of this event are un- known, the erosion of about 1,200 m2 can be seen from the 2015 DOF. Another event occurred on the night of April 28-29, 2017, when part of the Čikla landslide body collapsed and mobilised a large amount of debris and vegetation into a mass flow. The sliding ma- terial flowed several hundred metres along the Čikla stream. The triggering of this event was attributed to heavy rainfall. The nearby mete- orological station (Javorniški Rovt) measured 204 mm of precipitation in 48 hours. DOF analysis indicates that about 5,500 m² of the area was affected by debris flow, representing 21 % of the total Čikla landslide area, determined by detailed engineering geological mapping (Pe- ternel et al., 2020). DEMs analysis indicates that approximately 1,214 m2 was affected by signifi- cant erosion. The volume of eroded surface mate- rial was approximately 4,700 m3 and the surface had subsided an average of 2 m (up to 6 m in peak areas) (Fig. 6, area 1). The lower part of the Čikla landslide is characterised by an accumulation of material, with an average increase of 2.0 m and a maximum increase of 2.5 m. The estimated vol- ume of accumulated material is about 1,700 m3 (Fig. 6, area 2). The deficit of material was either deposited further down the slope outside the ob- served area or it was removed by the torrent. Fig. 4. Spatial extent and temporal change of elevation changes of active areas based on the series of DEM analyses. 139Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) Active area 2: The lower part of the Urbas landslide Based on previous research (Peternel, 2017; Peternel et al., 2017a; Šegina et al., 2020) and field investigations, the lower part of the Urbas landslide is considered the most active landslide in the area of concern. It is crossed by the Bela stream, which has formed gully-type morpholo- gy. The sliding mass consists of tectonically de- formed and weathered clastic rocks, covered by a relatively thick cover of carbonate gravel and boulders. The Bela stream causes significant ero- sion and increases the possibility of downstream mobilisation of the sliding mass. The area is characterised by bare, rugged ground with fallen trees, strong gully erosion and flank ridges. The DOF analysis clearly indicates that ero- sion progressively increases during the year. The estimated extent of this area was determined by manual digitalisation of the available DOFs and is represented in Table 3 and Figure 7. No erosion was observed in the 1999 DOF, while the 2020 DOF was overexposed in this area, so digitisa- tion was not possible. Most activity was observed between 2015 and 2017, when the extent of the erosion area in- creased by about 1,200 m2 (or 400 m2 per year) which is twice as large as in previous years. The DOF analysis also shows that, after this event, the Bela torrent channel became enlarged due to the active erosion of the Bela stream. The wid- er area was also subjected to deforestation of its slopes, which could also increase erosion. Fig. 5. Digital orthophotos of the Čikla landslide from 2011 (A), 2015 (B) and 2017 (C). Red line represents the manually digi- talised boundary of the Čikla debris flow. Fig. 6. Surface elevation difference (z-axis) between two LiDAR-derived DEMs from 2014 and 2017. Cross-section shows ele- vation differences along the Čikla landslide. The yellow line indicates Čikla debris flow (Fig. 5C). 140 T. PETERNEL, E. ŠEGINA, J. JEŽ, M. JEMEC AUFLIČ, M. JANŽA, J. LOGAR, M. MIKOŠ & M. BAVEC DEMs analysis shows that approximately 87 m2 of the lower part of the Urbas landslide has been affected by erosion, while 12 m2 was character- ised by accumulation. The volume of eroded sur- face material was approximately 273 m3 and the surface had subsided between 2 and 5 m (Fig. 8, area 1 and area 2). The Bela torrent channel is characterised by an accumulation of material, with a maximum increase of 2 m. The estimated volume of accumulated material is about 24 m3 (Fig. 8, area 3). The deficit of material was either deposited further down the slope, outside the an- alysed area, or had been transported away by the torrent. Active area 3: Subsidence of the local road The strong landslide activity also affects the local road that crosses the main body of the Urbas landslide, which consists of decomposed siltstone and claystone. Above the road, near the Urbas spring, a minor scarp was formed (Fig. 9). Manu- al measurements of the minor scarp indicate that it has been opening at a rate of 0.5 to 0.8 m per year. This area consists of clastic rocks with very low permeability, which is reflected in the occur- rence of springs charged from the carbonate hin- terland. The Urbas spring is partly captured and used to supply water to nearby mountain huts but the rest of the surface water flows uncontrolled Fig. 7. Digital orthophotos of the lower part of the Urbas landslide from 2006 (A), 2015 (B) and 2017 (C). Red line represents the manually digitalised boundary of the Čikla debris flow. Fig. 8. Surface elevation difference (z-axis) between two LiDAR-derived DEMs from 2014 and 2017. The cross-section shows elevation differences along the Urbas landslide. The yellow line indicates the lower part of Urbas (Fig. 7C). Table 3. Estimated extent of the lower part of the Urbas landslide. Orthophoto 1999 2006 2011 2015 2017 2020 Area (m2) no evidenced activity 90 320 1,060 2,300 no data – overexposed image 141Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) along the landslide and across the road. Local geological and hydrogeological conditions are re- flected in continuous subsidence and occasional road collapses. The increased water flow causes intense road erosion and often makes it impassa- ble, particularly during periods of intense or pro- longed rainfall. For this reason, the road has been reconstruct- ed several times by adjusting the road level to the terrain, which can be observed on cross sections in Figure 10. Cross section 3 also indicates strong subsidence of the road level, ranging from 0.7 m (for the period from 1999 to 2017) to 11.0 m (for the period from 1999 to 2017) (Fig. 10). Road re- construction was carried out by backfilling with gravel material and log cribs. Backfilling mate- rial represents an additional weight that accel- erates sliding. Due to subsidence, the road level was later relocated by cutting into the slope. No appropriate drainage system has ever been im- plemented. Fig. 10. Position of local road that crosses the Urbas landslide. Cross-sections represent the position of the road’s left shoulder, which has been manually digitali- sed in orthophotos. Fig. 9. Longitudinal tension crack above the local road representing a minor scarp. 142 T. PETERNEL, E. ŠEGINA, J. JEŽ, M. JEMEC AUFLIČ, M. JANŽA, J. LOGAR, M. MIKOŠ & M. BAVEC Discussion Archival DOFs and LiDAR-derived DEMs were used to show the long-term evolution of landslide activity. This approach has been suc- cessfully applied in landslide mapping and char- acterisation by several researchers (Prokešova et al., 2010; Jaboyedoff et al., 2012; Bühler et al., 2012; Kenner et al., 2014; Popit et al., 2014; Đom- lija et al., 2019; Görüm, 2019). Our case review of the available archival data enabled the observation of landslide activity, even for the period when no monitoring system had been established. This approach turned out to be very useful when observing landslide ac- tivity for the entire hinterland of Koroška Bela, especially for the remote areas that have not yet been investigated in detail. The results were fur- ther validated using the data from the existing monitoring system. Analysis of the changes of the Čikla land- slide surface provides us with useful information about the extent and volume assessment of the debris flow that occurred in April 2017. At that time, no detailed investigations had been con- ducted and there was no monitoring system for the Čikla landslide. The detailed geological and hydrogeological investigations conducted after April 2017 showed that the Čikla landslide covers an area of 26,000 m2 and its volume was estimat- ed to be about 330,500 m3 (Peternel et al., 2017b, 2018, 2020a, 2022a). To date, the Čikla landslide has also been investigated by drilling five bore- holes. Two were installed with piezometers and three with inclinometers. The depth of the bore- holes was about 40 m. Boreholes and inclinome- ters indicate that the maximum shear surface oc- curs at a depth of 28 m, while the average sliding surface is about 23 m deep (Peternel et al. 2018, 2020a). Groundwater level appears at the con- tact between the debris deposits and the weath- ered bedrock. The 2017 debris flow occurred in the central part of the Čikla landslide, beneath a large carbonate block that is tectonically de- formed and structurally lies within the clastic Carboniferous rocks (Fig. 6, zone 2). In contrast to Čikla, the Urbas landslide has been the subject of several studies in the past (Jež et al., 2008; Komac et al., 2015; Auflič et al., 2017; Peternel, 2017; Peternel et al., 2017a). For example, the lower part of the Urbas landslide was investigated in detail between December 2012 and April 2016, using UAV photogramme- try and tachymetric measurements. Comparisons of the national orthophotos allows the calcula- tion of the extent of erosion over a long period of time, while the high-resolution DOFs and the DEMs derived from UAV photogrammetry pro- vide accurate displacement vectors and eleva- tion changes. Surface displacement patterns for the entire monitoring period (December 2012 to April 2016) were analysed based on the sum of displacement vectors for all observation periods. The determined displacements ranged from 0.9 to 19.0 m, with a clear SW directional orientation (Peternel 2017, Peternel et al. 2017a). No geotech- nical in-situ investigations (inclinometers or pie- zometers) were conducted in the lower part of the Urbas landslide due to difficult access and rug- ged morphology. Although these results only in- dicate changes at the surface and do not provide information on the depth of sliding surface, they contribute to a better understanding of landslide behaviour and kinematics. Reviewing the available archival orthopho- tos, we also observed the deformation of the lo- cal road that crosses the main body of the Urbas landslide. Digitised road shoulders provided in- formation about the continuous subsidence and collapse of the road. Nearby boreholes along the road indicated that the road crosses the landslide area with a depth of sliding surface ranging be- tween 8 and 15 m (Peternel et al., 2018). Moni- toring of the surface displacements using GNSS also indicates the constant displacements of the landslide main body. The GNSS antenna located below the road, and measuring the surface dis- placements in real-time, showed displacements of 37 mm over a period of one year (Šegina et al., 2020). Conclusions In this paper we summarise all conducted and on-going research and the main findings related to the landslide prone areas above the settlement of Koroška Bela. For the first time, we have also revealed the evolution of landslide activity over the last 25 years. According to investigations so far, the following main outcomes are defined: - The main landslides detected in the Ko- roška Bela hinterland are the Čikla, Ur- bas, Potoška Planina, Malnež and Obešnik landslides. All landslides are character- ised by high activity due to intense rain- fall events, long-term precipitation and/or groundwater table change. In addition, the moderate-high seismicity of the area could be another predisposing factor contribut- ing to the occurrence of threatening land- slides. 143Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) - Under the above conditions, the potential risk induced by the mass movements on the town of Koroška Bela can be generally as- sessed as follows: • very high risk due to the occurrence of fast-moving landslides (i.e. debris flows) that have already affected the urban area in the past and might happen in the future; • moderate to high risk derived by the slow-moving landslides detected in the slopes up-hill of Koroška Bela, some of them (i.e. the Malnež landslide) potentially threatening the eastern part of the town. - Even though a lot of knowledge and infor- mation has been gained, there are still un- knowns that need to be resolved and im- provements that have to be implemented. There is still a need for further geological and geotechnical investigations for: i) the improvement of the geological model of the study area, ii) integration with the equip- ment already installed and, iii) set-up of an integrated monitoring system for the control of slope deformations acting in the landslide areas in the Karavanke moun- tains. - Implementation of direct and indirect fea- sible, effective and sustainable landslide mitigation strategies aimed at reducing the landslide risk in the Koroška Bela settle- ment. - Due to the estimated risk, there was a need to set up a continuous and flexible monitor- ing system that could serve as a basis for a local landslide warning system. With the aid of landslide monitoring, early landslide activity can be detected and landslide im- pacts can be reduced. In this stage we set up customised dashboards that allowed access to all real-time monitoring sensors. In this way, GeoZS emergency services and stakeholders can access daily updated data, presented on a webpage, at any time. In fu- ture, we plan to upgrade the local warning system with email alerts sent to registered users when determined threshold values are exceeded. - This study is a step forward in landslide management in Slovenia. Several activ- ities that have been implemented within the framework of different projects have enabled the establishment of connections between national and local authorities, scientific and technical experts, and Civil Protection and residents of Koroška Bela. Acknowledgements This research was funded by the Slovenian Research Agency through grants Z1-2638, J1-8153, P1-0419, P1-0011, P1-0020 and P2-0180. Additional financial support was provided by the Ministry of the Environment and Spatial Planning, and the Municipality of Jesenice. The authors would also like to thank the revi- ewers and the editor for their constructive comments, which helped to improve the manuscript. References Bezak, N., Peternel, T., Medved, A. & Mikoš, M. 2021: Climate Change Impact Evaluation on the Water Balance of the Koroška Bela Area, NW Slovenia. In: Vilímek, V., Wang, F., Strom, A., Sassa, K., Bobrowsky, P.T. & Takara, K. (eds.): Understanding and Reducing Landslide Disaster Risk (WLF 2020), 221–228. https:// doi.org/10.1007/978-3-030-60319-9_25 Bezak, N., Sodnik, J., Maček, M., Jurček, T., Jež, J., Peternel, T. & Mikoš, M. 2021: Investigation of potential debris flows above the Koroška Bela settlement, NW Slovenia, from hydro- -technical and conceptual design perspecti- ves. Landslides, 18: 3891–3906. https://doi. org/10.1007/s10346-021-01774-7 Brenčič, M. & Poltnig, W. 2008: Podzemne vode Karavank: skrito bogastvo = Grundwasser der Karawanken: versteckter Schatz. Ljubljana, Graz, Geološki zavod Slovenije, Joanneum Research Forschungsgesellschaft m.b.H.: 143 p. Buser, S. 1978: Osnovna geološka karta SFRJ. List Celovec (Klagenfurt) 1:100.000. Zvezni geološki zavod Beograd. Buser, S. 1980: Osnovna geološka karta SFRJ 1:100.000. Tolmač lista Celovec (Klagenfurt). Zvezni geološki zavod, Beograd: 62 p. Bühler, Y., Marty, M. & Ginzler, C. 2012: High resolution DEM generation in high-alpine terrain using airborne remote sensing tech- niques. Trans. GIS 16/5: 635–647. https://doi. org/10.1111/j.1467-9671.2012.01331.x Cruden, D. M. & Varnes, D. J. 1996: Landslide Types and Processes. Landslides: investigati- on and mitigation (Special Report). National Research Council, Transportation Research Board, 247: 36–75. Đomlija, P., Bernat Gazibara, S., Arbanas, Ž. & Mihalić Arbanas, S. 2019: Identification and Mapping of Soil Erosion Processes Using the Visual Interpretation of LiDAR Imagery. 144 T. PETERNEL, E. ŠEGINA, J. JEŽ, M. JEMEC AUFLIČ, M. JANŽA, J. LOGAR, M. MIKOŠ & M. BAVEC ISPRS Int. J. Geo-Inf., 8: 438. https://doi. org/10.3390/ijgi8100438 Fifer Bizjak, K. & Zupančič Valant, A. 2009: Site and laboratory investigation of the Slano blato landslide. Eng. Geol., 105/3–4: 171–185. https://doi.org/10.1016/j.enggeo.2009.01.006 Gariano, S. L. & Guzzetti, F. 2016: Landslides in a changing climate. Earth. Sci. Rev., 162: 227–252. https://doi.org/10.1016/j. earscirev.2016.08.011 Görüm, T. 2019: Landslide recognition and map- ping in a mixed forest environment from air- borne LiDAR data. Eng. Geol., 258. https:// doi.org/10.1016/j.enggeo.2019.105155 Jaboyedoff, M., Oppikofer, T., Abellán, A., Derron M.H., Loye, A., Metzger, R. & Pedrazzini, A. 2012: Use of LIDAR in landslide investigati- ons: a review. Nat Hazards 61: 5–28. https:// doi.org/10.1007/s11069-010-9634-2 Jakša, J. & Kolšek, M. 2009: Naravne ujme v slovenskih gozdovih = Natural disasters in Slovenian forests. Ujma, 23: 72–81. Janža, M., Serianz, L., Šram, D. & Klasinc, M. 2018: Hydrogeological investigation of lands- lides Urbas and Čikla above the settlement of Koroška Bela (NW Slovenia). Geologija, 61/2: 191–203. https://doi.org/10.5474/ geologija.2018.013 Jemec Auflič, M., Jež, J., Popit, T., Košir, A., Maček, M., Logar, J., Petkovšek, A., Mikoš, M., Calligaris, C. & Boccali, C. 2017a: The variety of landslide forms in Slovenia and its immediate NW surroundings. Landslides 14: 1537-1546. https://doi.org/10.1007/ s10346-017-0848-1 Jemec Auflič, M., Kumelj, Š., Peternel, T., Milanič, B. & Jež, J. 2017b: Projekt RECALL: odpornost evropskih skupnosti ob lokalnih zemeljskih plazovih = Project RECALL: re- silient European communities against local landslides. Ujma, 31: 181–186. Jemec Auflič, M., Peternel, T., Kumelj, Š., Jež, J., Milanič, B., Dolžan, E. & Brunelli, G. 2017c: RECALL project: toward resilent European communities against local landslides. In: Mikoš, M., Arbanas, Ž., Yin, Y. & Sassa, K. (eds.): Advancing Culture of Living with Landslides. WLF 2017. Springer, Cham: 405–412. https:// doi.org/10.1007/978-3-319-53487-9_47 Jemec Auflič, M., Kumelj, Š., Peternel, T. & Jež, J. 2019: Understanding of landslide risk throu- gh learning by doing: case study of Koroška Bela community, Slovenia. Landslides, 16/9: 1681–1690. https://doi.org/10.1007/ s10346-018-1110-1 Jež, J., Mikoš, M., Trajanova, M., Kumelj, Š., Budkovič, T. & Bavec, M. 2008: Vršaj Koroška Bela - Rezultat katastrofičnih pobočnih do- godkov = Koroška Bela alluvial fan - The result of the catastrophic slope events; Karavanke Mountains, NW Slovenia. Geologija, 51/2: 219– 227. https://doi.org/10.5474/geologija.2008.022 Jež, J., Peternel, T., Milanič, B., Markelj, A., Novak, M., Celarc, B., Janža, M. & Jemec Auflič, M. 2019a: Čikla landslide in Karavanke Mts. (NW Slovenia). In: Uljarević, M., Zekan, S., Salković, S. & Ibrahimović, D. (eds.): Proceedings of the 4th Regional Symposium on Landslides in the Adriatic - Balkan Region. Geotechnical Society of Bosnia and Herzegovina, Sarajevo: 239–242. Jež, J., Markelj, A., Milanič, B., Mencin Gale, E., Atanackov, J., Mechernich, S., Peternel, T. & Jemec Auflič, M. 2019b: Preliminarni rezultati o starosti vršaja Bele v Koroški Beli. In: Rožič, B. (ed.): 24. posvetovanje slovenskih geolo- gov = 24th Meeting of Slovenian Geologists. Univerza v Ljubljani, Naravoslovnotehniška fakulteta, Oddelek za geologijo, Ljubljana: 49–51. Kenner, R., Bühler, Y., Delaloye, R., Ginzler, C. & Phillips, M. 2014: Monitoring of high alpine mass movements combining laser scanning with digital airborne photogrammetry. Geomorphology, 206: 492–504. https://doi. org/10.1016/j.geomorph.2013.10.020 Komac, M. 2005: Napoved verjetnosti pojavljanja plazov z analizo satelitskih in drugih pro- storskih podatkov. Geološki zavod Slovenije, Ljubljana: 284 p. Komac, M., Milanič, B., Jež, J., Bavec, M., Holly, R., Mahapatra, P., Hanssen, R., Van der Marel, H., & Fromberg, A. 2012a: Opazovanje plaze- nja s kombinacijo metod radarske interfero- metrije in GPS. Ujma, 26: 175–182. Komac, M., Jež, J., Celarc, B., Milanič, B. & Bavec, M. 2012b: Prvi rezultati merjenja pre- mikov površja na območju Jesenic in Potoške planine s kombinacijo InSAR in GPS meri- tev. In: Ciglič, R., Perko, D., Zorn, M. (eds.): Geografski informacijski sistemi v Sloveniji 2011–2012. Založba ZRC, Ljubljana: 25–31. Komac, M., Holley, R., Mahapatra, P., Van Der Marel, H. & Bavec, M. 2015: Coupling of GPS/ GNSS and radar interferometric data for a 3D surface displacement monitoring of lan- dslides. Landslides, 12/2: 241–257. https://doi. org/10.1007/s10346-014-0482-0 Komac, M., Peternel, T. & Auflič, M. J. 2018: TXT-tool 2.386-2.1: SAR Interferometry as 145Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) a Tool for Detection of Landslides in Early Phases. In: Landslide dynamics: ISDR- ICL landslide interactive teaching to- ols. Springer, Cham: 275–285. https://doi. org/10.1007/978-3-319-57774-6_19 Koren, K., Serianz, L. & Janža, M. 2022: Characterizing the Groundwater Flow Regime in a Landslide Recharge Area Using Stable Isotopes: A Case Study of the Urbas Landslide Area in NW Slovenia. Water, 14: 912. https://doi.org/10.3390/w14060912 Lacroix, P., Handwerger, A. L. & Bièvre, G. 2020: Life and death of slow-moving lands- lides. Nat. Rev. Earth Environ., 1/8: 404–419. https://doi.org/10.1038/s43017-020-0072-8 Lavtižar, J. 1897: Zgodovina župnij in zvonovi v dekaniji Radolica. 148 p. Loi, D. H., Sassa, K., Dang, K. & Le Luong, H. 2020: Landslide Hazard Zoning Based on the Integrated Simulation Model (LS-Rapid). In: Workshop on World Landslide Forum. Kyoto, Springer. Maček, M., Majes, B. & Petkovšek, A. 2016: Lessons learned from 6 years of suction mo- nitoring of the Slano blato landslide. Riv. Ital. di Geotec., 1: 21–31. Mikoš, M., Fazarinc, R. & Ribičič, M. 2006: Sediment production and delivery from re- cent large landslides and earthquake-indu- ced rock falls in the Upper Soča River Valley, Slovenia. Eng. Geol., 86/2-3: 198–2010. https:// doi.org/10.1016/j.enggeo.2006.02.015 Mikoš, M., Bavec, M., Budkovič, T., Durjava, D., Hribernik, K., Jež, J., Klabus, A., Komac, M., Krivic, M., Kumelj, Š., Maček, M., Mahne, M., Novak, M., Otrin, J., Petje, U., Petkovšek, A., Ribičič, M., Sodnik, J., Šinigoj, J. & Trajanova, M. 2008: Ocena ogroženosti zaradi delovanja drobirskih tokov. Ciljni raziskovalni projekt: Znanje za varnost in mir 2006–2010. Ljubljana, Univerza v Ljubljani, Fakulteta za gradbeništvo in geodezijo, Geološki zavod Slovenije: 224 p. Mikoš, M., Petkovšek, A. & Majes, B. 2009: Mechanism of landslides in over-consolida- ted clays and flysch. Landslides, 6: 367–371. https://doi.org/10.1007/s10346-009-0171-6 Mikoš, M. 2020: After 2000 Stože Landslide: Part I–Development in landslide research in Slovenia. Acta Hydrotech., 33/59: 129–153. https://doi.org/10.15292/acta.hydro.2020.09 Mikoš, M. 2021: After 2000 Stože Landslide: Part II-Development of landslide disaster risk re- duction policy in Slovenia. Acta Hydrotech., 34/60: 39–59. https://doi.org/10.15292/acta. hydro.2021.04 Ministrstvo za kmetijstvo, gozdarstvo in pre- hrano Republike Slovenije, 2016: Javno do- stopni podatki. Grafični podatki GERK za celo Slovenijo. http://rkg.gov.si/GERK/ (Pridobljeno 30. 4. 2022). Novak, A., Popit, T., Levanič, T., Šmuc, A. & Kaczka, R. J. 2020: Debris flooding magni- tude estimation based on relation between dendrogeomorphological and meteorological records. Geomorphology, 367: 107303. https:// doi.org/10.1016/j.geomorph.2020.107303 Oven, D., Levanič, T., Jež, J. & Kobal, M. 2019: Reconstruction of landslide activity using dendrogeomorphological analysis in the Karavanke mountains in NW Slovenia. Forests, 10/11: 1009. https://doi.org/10.3390/ f10111009 Peternel, T., Komac, M. & Oštir, K. 2015: Monitoring of Potoška planina landslide (Karavanke Mountains, NW Slovenia). In: Abolmasov, B., Marjanović, M. & Đurić, U. (eds.): Proceedings of the 2nd Regional Symposium on Landslides in the Adriatic-Balkan Region. University of Belgrade, Faculty of Mining and Geology, Belgrade: 49–54. Peternel, T. 2017: Dinamika pobočnih masnih premikov na območju Potoške planine z upo- rabo rezultatov daljinskih in terestričnih geodetskih opazovanj ter in-situ meritev: doktorska disertacija. Ljubljana, Univerza v Ljubljani, Naravoslovnotehniška fakulteta: 183 p. Peternel, T., Kumelj, Š., Oštir, K. & Komac, M. 2017a: Monitoring the Potoška planina landslide (NW Slovenia) using UAV pho- togrammetry and tachymetric measure- ments. Landslides, 14/1: 395–406. https://doi. org/10.1007/s10346-016-0759-6 Peternel, T., Jež, J., Milanič, B., Markelj, A., Jemec Auflič, M., Kumelj, Š., Celarc, B., Novak, M., Janža, M., Šram, D., Serianz, L., Bole, Z., Demšar, M., Klasinc, M. & Sodnik, J. 2017b: Izvedba najnujnejših inženirskogeoloških, hidrogeoloških, geofizikalnih in geomehan- skih ter geodetskih raziskav za ugotovitev objektivne stopnje tveganja za prebivalstvo zaradi masnih premikov na območju Potoške planine in izdelava strokovnih podlag s pre- dlogi zaščitnih ukrepov: izdelava celovitih geoloških strokovnih podlag in izdelava mo- dela monitoringa za oceno ogroženosti nase- lja Koroška Bela s pojavi pobočnega masne- ga premikanja. Geološki zavod Slovenije, Ljubljana: 129 p. 146 T. PETERNEL, E. ŠEGINA, J. JEŽ, M. JEMEC AUFLIČ, M. JANŽA, J. LOGAR, M. MIKOŠ & M. BAVEC Peternel, T. & Komac, M. 2017: Observing surfa- ce movement patterns of the Potoška planina landslide using geodetic techniques. In: Jemec Auflič, M., Mikoš, M. & Verbovšek, T. (eds.): Advances in landslide research: proceedings of the 3rd Regional Symposium on Landslides in the Adriatic Balkan Region. Geological Survey of Slovenia, Ljubljana: 77–81. Peternel, T., Jež, J., Milanič, B., Markelj, A. & Jemec Auflič, M. 2018: Engineering-geological conditions of landslides above the settlement of Koroška Bela (NW Slovenia). Geologija, 61/2: 177–189. https://doi.org/10.5474/ geologija.2018.012 Peternel, T., Jež, J., Milanič, B., Markelj, A., Sodnik, J., Maček, M. & Jemec Auflič, M. 2019: Implementation of multidisciplinary approa- ch for determination of landslide hazard. In: Mikoš, M. & Bezak, N. (eds.): Buildings and infrastructure resilience: WCF2019 book of abstracts with programme. Univerza v Ljubljani, Fakulteta za gradbeništvo in geo- dezijo, Ljubljana :124 p. Peternel, T., Jež, J., Janža, M., Zupan, M., Markelj, A., Maček, M., Logar, J., Smolar, J., Mikoš, M., Bezak, N., Sodnik, J., Serianz, L., Klančič, K., Šram, D., Jemec Auflič, M., Novak, A., Šegina, E., Oblak, A., Adrinek, S., Koren, K., Milanič, B., Zajc, M., Hrovat, M., Jurček, T., Kuder, S., Petrovič, D., Grigillo, D., Urbančič, T. & Kozmus Trajkovski, K. 2020a: Podrobna geološka-geotehnična in hidrogeološka ka- rakterizacija velikih plazov v zaledju naselja Koroška Bela za potrebe izdelave stabilno- stnih analiz in študijo izvedljivosti sanacij- skih rešitev: krovno poročilo. Geološki zavod Slovenije, Fakulteta za gradbeništvo in geo- dezijo, Ljubljana: 218 p. Peternel, T., Šegina, E., Zupan, M., Auflič, M. J. & Jež, J. 2020b: Preliminary Result of Real- Time Landslide Monitoring in the Case of the Hinterland of Koroška Bela, NW Slovenia. In: Tiwari, B., Sassa, K., Bobrowsky, P.T. & Takara, K. (eds.): Understanding and Reducing Landslide Disaster Risk. WLF 2020. Springer, Cham: 459–464. https://doi. org/10.1007/978-3-030-60706-7_49 Peternel, T., Jež, J., Janža, M., Šegina, E., Zupan, M., Markelj, A., Novak, A., Jemec Auflič, M., Logar, J., Maček, M., Bezak, N., Sodnik, J. & Mikoš, M. 2022a: Mountain slopes above Koroška Bela (NW Slovenia) – a landslide prone area. In: Peranić, J., Vivoda Prodan, M., Bernat Gazibara, S., Krkač, M., Mihalić Arbanas, S., Arbanas, Ž. (eds.): Landslide Modelling & Applications: Proceedings of the 5th Regional Symposium on Landslides in the Adriatic-Balkan Region. University of Rijeka, Faculty of Civil Engineering, Faculty of Mining; University of Zagreb, Geology and Petroleum Engineering, Rijeka: 13–18. Peternel, T., Janža, M., Šegina, E., Bezak, N. & Maček, M. 2022b: Recognition of Landslide Triggering Mechanisms and Dynamics Using GNSS, UAV Photogrammetry and In Situ Monitoring Data. Remote Sensing, 14/14: 3277. https://doi.org/10.3390/rs14143277 Placer, L. 2008: Principles of the tectonic subdi- vision of Slovenia. Geologija, 51/2: 205–217. https://doi.org/10.5474/geologija.2008.021 Popit, T., Rožič, B., Šmuc, A., Kokalj, Ž., Verbovšek, T. & Košir, A. 2014: A LIDAR, GIS and basic spatial statistic application for the study of ravine and palaeo-ravine evoluti- on in the upper Vipava valley, SW Slovenia. Geomorphology, 204: 638–645. https://doi. org/10.1016/j.geomorph.2013.09.010 Popit, T. 2017: Origin of planation surfaces in the hinterland of Šumljak sedimentary bo- dies in Rebrnice (Upper Vipava Valley, SW Slovenia). Geologija, 60/2: 297–307. https:// doi.org/10.5474/geologija.2017.021 Popit, T., Rožič, B., Šmuc, A., Novak, A. & Verbovšek, T. 2022: Using a Lidar-Based Height Variability Method for Recognizing and Analyzing Fault Displacement and Related Fossil Mass Movement in the Vipava Valley, SW Slovenia. Remote Sens., 14/9: 2016. https://doi.org/10.3390/rs14092016 Prokešová, R., Kardoš, M. & Medveďová, A. 2010: Landslide dynamics from high-re- solution aerial photographs: A case study from the Western Carpathians, Slovakia. Geomorphology, 115: 90–101. https://doi. org/10.1016/j.geomorph.2009.09.033 Rakovec, J., Vrhovec, T. & Gregorič, G. 2000: Osnove meteorologije za naravoslovce in teh- nike. Društvo matematikov, fizikov in astro- nomov Slovenije, Ljubljana: 239 p. Romer, C. & Ferentinou, M. 2016: Shallow land- slide susceptibility assessment in a semi- arid environment – A Quaternary catch- ment of KwaZulu-Natal, South Africa. Eng. Geol., 201: 29–44. https://doi.org/10.1016/j. enggeo.2015.12.013 Šegina, E., Peternel, T., Urbančič, T., Realini, E., Zupan, M., Jež, J., Caldera, S., Gatti, A., Tagliaferro, G. & Consoli, A. 2020: Monitoring Surface Displacement of a Deep-Seated Landslide by a Low-Cost and near Real-Time 147Review of the research and evolution of landslides in the hinterland of Koroška Bela settlement (NW Slovenia) GNSS System. Remote Sensing, 12/20: 3375. https://doi.org/10.3390/rs12203375 Sodnik, J. & Mikoš, M. 2006: Estimation of ma- gnitudes of debris flows in selected torrenti- al watersheds in Slovenia. Acta Geogr. Slov., 46/1: 93–123. https://doi.org/10.3986/AGS46104 Sodnik, J., Kumelj, Š., Peternel, T., Jež, J. & Maček, M. 2017: Identification of Landslides as Debris Flow Sources Using a Multi-model Approach Based on a Field Survey–Koroška Bela, Slovenia. Workshop on World Landslide Forum, Springer. Sodnik, J., Petje, U. & Mikoš, M. 2009: Terrain topography and debris flow modelling. Geod. Vestn., 53/2: 305–318. Sodnik, J., Vrečko, A., Podobnikar, T. & Mikoš, M. 2012: Digital terrain models and mathe- matical modelling of debris flows. Geod. Vestn., 56/4: 826–837. Sodnik, J., Podobnikar, T., Petje, U. & Mikoš, M. 2013: Topographic data and numerical de- bris flow modeling. Landslide Science and Practice. Claudio, M., Stefano, C. & Sassa, K. Berlin, Springer 1: 573-578. https://doi. org/573-578. 10.1007/978-3-642-31325-7_75 Sodnik, J., Kumelj, Š., Peternel, T., Jež, J. & Maček, M. 2017: Identification of landslides as debris flow sources using a multi-model approach based on a field survey - Koroška Bela, Slovenia. In: Mikos, M., Tiwari, B., Yin, Y. & Sassa, K. (eds.): Advancing Culture of Living with Landslides. WLF 2017. Springer, Cham: 1119–1126. https://doi. org/10.1007/978-3-319-53498-5_127 Sodnik, J. & Mikoš, M. 2018: Two-dimensional debris flow modelling and topograp- hic data: TXT-tool 3.386-1.1. Landslide dynamics: ISDR-ICL landslide interacti- ve teaching tools, 2: 235–250. https://doi. org/10.1007/978-3-319-57777-7_11 Verbovšek, T., Košir, A., Teran, M., Zajc, M. & Popit, T. 2017: Volume determination of the Selo landslide complex (SW Slovenia): in- tegrating field mapping, ground penetra- ting radar and GIS approaches. Landslides 14, 1265–1274. https://link.springer.com/ article/10.1007/s10346-017-0815-x Wheaton, J.M., Brasington, J., Darby, S.E. & Sear, D.A. 2010: Accounting for uncertainty in DEMs from repeat topographic surveys: imp- roved sediment budgets. Earth Surf. Process. Landf., 35: 136–156. https://doi.org/10.1002/ esp.1886 Williams, R.D. 2012: DEMs of difference. In: Geomorphological Techniques. British Society for Geomorphology: 17 p. Zupan, G. 1937: Krajevni leksikon Dravske bano- vine. Ljubljana, Uprava Krajevnega leksiko- na dravske banovine Ljubljana. Internet sources: Internet 1: http://meteo.arso.gov.si/uploads/pro- base/www/climate/image/sl/by_variable/ precipitation/mean-annual-measured-preci- pitation_81-10.png (30. 4. 2022)