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This paper presents a result of landslide inventory mapping at the Bednja Municipality and Lepoglava City study area in Hrvatsko Zagorje region, NW Croatia. The landslides were interpreted from the high resolution (30 cm) digital elevation model (DEM) and its derivatives (slope and contour map, hillshade). The DEM was interpolated from the point cloud obtained by airborne laser scanning undertaken in spring 2020. In the study area of 20.22 km2, the total number of interpreted landslides is 912, making the average density of 45.1 ls/km2. The average size of the recorded landslides is 448 m2. According to the spatial plans, most of the studied area is covered by forests, agricultural areas, pastures, and artificial areas. The highest density of landslides is also in the forest areas, while the lowest is in the artificial areas. Furthermore, almost 64% of the mapped landslides are located within 50 m of the roads, and more than 39% of the mapped landslides are located within 100 m of the buildings and residential houses. Due to the level of detail provided and its completeness, the presented landside inventory map is an important tool for risk management at the local level because it gives detailed information necessary for risk evaluation as well as to decide about feasible options for risk mitigation, e.g., stabilisation measures vs relocation of the development to a more favourable location.

The objective of this study is a presentation of the landslide susceptibility assessment on a national scale for the Republic of Croatia using a heuristic approach. The effects of controlling factors on landslide susceptibility were estimated using the Fuzzy logic approach based on a multiclass overlay of landslide predictor maps. The predictor set relates to topographic variables, geomorphological factors, geology factors, and land cover. The selection of relevant landslide factors and the final landslide susceptibility assessment depends on subjective factors, such as researcher knowledge of the study area, respectively knowledge of different landslides types and processes in the study area, etc. For these reasons, it was necessary to verify the final landslide susceptibility map with data on known landslides. During the last few years, the scientists from the Faculty of Mining, Geology and Petroleum Engineering in Zagreb were systematically collected data on landslides, and the database of 2, 186 landslides with the exact location of the occurrence was conducted. The Area Under the Receiver Operating Characteristic Curve (AUROC) was used to validate derived landslide susceptibility maps and select the final one for further classification into three susceptibility zones. With over 90% of mapped landslides falling in high and very high susceptibility zones, the results are considered satisfactory for national scale landslide modeling. The landslide susceptibility map of Croatia was created to give a general overview of problem areas for an entire country, and it can be used to inform national policymakers and the general public. The analysis showed that approximately 20% of the area of Croatia is potentially prone to sliding. Particularly landslide-prone areas in Croatia are lowlands and hills in the Pannonian Basin, the hills of the Istrian Peninsula, and isolated narrow valleys in the Dinarides, such as Rječina River Valley and Vinodol Valley in Primorje.

This paper presents the application of Light Detection and Ranging (LiDAR) data for landslide identification and mapping in the pilot area at the Istria Penninsula (Croatia) and the analyses on the influence of expert knowledge on the quality of landslide inventory. Visual interpretation of landslides was carried out on high-resolution airborne laser scanning (ALS) LiDAR dataset. Scanning was taken in March 2020 for the pilot area in the City of Buzet. Based on the characteristics of the acquired LiDAR Point Cloud, a bare-earth digital elevation model (DEM) with 30 cm resolution was created. Different topographic derivative datasets such as slope, hillshade, contour lines, and roughness maps were created to interpret the LiDAR data. Eight experts with different levels of expert knowledge on LiDAR interpretation were given one week to carry out visual identification and mapping of potential landslides in the pilot area (0.3 km2) at a large scale (1:200) to provide detailed landslide mapping. Statistical analyses were performed based on the collected data to determine differences in the mapping accuracy and the number of recognized landslides by the experts. Results show that the experts familiar with the geology of the study area and potential landslide mechanics obtained better results than the experts who mapped landslides based on only ordinary topographic and geomorphological features specific for landslides.

The scientific research project Methodology development for landslide susceptibility assessment for land-use planning based on LiDAR technology (LandSlidePlan, HRZZ IP-2019-04-9900 ), funded by the Croatian Science Foundation, deals with new and under-investigated subjects in respect of inventory mapping of small and shallow landslides and presents innovative approaches to scientific research of landslide susceptibility assessment using cutting-edge LiDAR technology for collection of input data. The project has three main scientific goals. The first goal is to create the best optimal digital model of the bare-earth terrain that shows realistic landslide footprints and differences between disturbed and undisturbed land that may influence land use. The second goal is to create the most reliable large-scale landslide susceptibility map with the best differentiation of landslide-prone and non- susceptible areas using scientific methods customised to specific engineering geological conditions of Croatian environments with sliding threats. And the third goal is to create maps depicting information about landslides tailored according to the needs of the system of physical planning in Croatia (particularly land-use planning), encompassing local and regional levels under, harmonised at the national level. Due to different natural conditions and land uses in different parts of Croatia, the methodology will be developed for pilot areas in the City of Zagreb, Hrvatsko Zagorje and Istria, selected based on characteristic geological settings and degree of urbanisation.

Landslide maps produced by LiDAR (Light Detection and Ranging) are very clear and detailed representation of the phenomena and in many cases reveal evidence of past landslides that are virtually invisible by other detection techniques due to vegetation cover. Over the last decade, airborne laser scanning (ALS) has been made available and has been used to identify and map landslide. LiDAR elevation data prove particularly effective for mapping of small and shallow landslides in areas that are partially or completely covered by dense vegetation that are difficult or impossible to identify using conventional identification techniques. The first topic covered in the talk is landslide detection and mapping using very high-resolution LiDAR DTM to obtain complete historical inventories of shallow soil slides. Few examples of landslide inventory maps from different geomorphological environment of the Republic of Croatia will be presented to show typical landslide distribution. For landslide spatial prediction, the talk focuses on the results of landslide susceptibility modelling and zonation performed for a range of coverages, starting with the largest encompassing whole territory of Croatia. Landslide susceptibility zonation was also performed for areas of two counties (Primorsko-Goranska County and Karlovac County) and few smaller pilot areas of cities and municipalities in the Pannonian Basin and in External Dinarides. The main objective of presented susceptibility zoning in different scales (national, regional and local scale) is to enable analysis of usefulness and reliability of map information (i.e., spatial distributions and rating of the terrain units according to their landslide propensity) for application in physical and urban planning. The needs of the decision-makers, planners and other stakeholders involved in landslide risk prevention are analyzed through the series of round-table discussions organized in Croatia in the framework of the project of applied research PRI- MJER (KK.05.1.1.02.0020). The ultimate goal is to create maps depicting information about landslides tailored according to needs of the system of physical planning in Croatia (particularly land use planning), encompassing local and regional level, harmonized at the national level. The research of the mapping methodologies is part of the project “Methodology development for landslide susceptibility assessment for land-use planning based on LiDAR technology, LandSlidePlan” (HRZZ IP-2019-04-9900) fully supported by Croatian Science Foundation.

An innovative technique for detailed landslide inventory mapping is airborne laser scanning and LiDAR-derived DTMs in high resolution. LiDAR data used in this study was obtained in the framework of the “Methodology development for landslide susceptibility assessment for land use planning based on LiDAR technology (LandSlidePlan IP-2019- 04-9900)” project fully supported by the Croatian Science Foundation. To select the optimal digital terrain model (DTM) for landslide delineation, quantitative and qualitative assessments were done individually for three landslides. The quantitative assessment included a comparison of minimum, maximum, mean, and standard deviation values of DTMs derived by using four interpolation methods (Kriging, IDW, Natural Neighbor, and ANUDEM) in six raster resolutions (0.15, 0.3, 0.5, 1, 2, and 5 m). Furthermore, by comparing point cloud LiDAR data and interpolated DTMs elevation values, the mean-absolute-error difference (MAE) and root-mean-square-error (RMSE) were calculated. Hillshade, roughness, and curvature morphometric maps were derived for 24 DTMs per landslide, resulting in the qualitative assessment of 216 different morphometric maps. The quantitative assessment showed minimum and negligible differences between DTMs for landslide areas ; therefore, the qualitative assessment prioritised determining the optimal DTM for deriving morphometric maps needed for landslide delineation. Based on visual interpretability of landslide parts (i.e. crown, ridges, and toe) and the terrain quality (i.e. expressed details, irregularities, and blurriness) on the derived morphometric maps, the LiDAR DTM derived using the Kriging method in 0.3 m resolution was selected for landslide inventory mapping in further studies.

A landslide inventory presents a detailed register of the distribution and characteristics of past landslides in a specific area. Landslide inventory maps and other maps such as landslide susceptibility, hazard and risk maps present an essential tool in landslide risk management, supporting authorities, practitioners and decision-makers in the more appropriate and sustainable land planning and risk mitigation strategy development. In recent years, Light Detection and Ranging (LiDAR) data have been commonly used to map landslide morphology and estimate landslide activity. LiDAR is a consolidated remote sensing technique used to obtain digital representations of the topographic surface for areas ranging from a few hectares to thousands of square kilometers. From elevation point clouds obtained by laser scanning, a detailed digital elevation model (DEM) and different DEM derivatives, such as slope, hillshade or contour maps, can be produced. In this study, a historical landslide inventory map of the Hrvatsko Zagorje area (NW Croatia), interpreted from LiDAR high-resolution DEM (HRDEM) derivates, is presented and analyzed regarding geomorphological characteristics. The study area comprises 20.22 km2 of the hilly terrain (88% of the area has slope angles >5°), mostly covered by forests (52%). The area is composed of Triassic carbonate rocks, Miocene clastic sedimentary rocks and soils and Quaternary alluvial soils. LiDAR data for the study area was acquired in the framework of the project „Methodology development for landslide susceptibility assessment for land- use planning based on LiDAR technology (LandSlidePlan)“ financed by the Croatian Science Foundation. The topographic derivative datasets used to interpret the landslide morphology were hillshade maps, slope maps and contour lines. Landslide identification on the LiDAR HRDEM derivatives (0.3 m resolution) was manual and GIS- assisted, based on recognizing landslide features (e.g., concave main scarps, hummocky landslide bodies, and convex landslide toes). The mapping was performed at a large scale (1:100–1:500) to ensure the correct delineation of the landslide boundaries. Totally 912 landslides were mapped. The total area of mapped landslides is 0.408 km2 or 2.02% of the study area, and the mean landslide density is 45.1 slope failures per square kilometer. The average landslide area is 448 m2 (median = 173 m2). The small size of the landslides is probably the result of geological conditions (mainly Miocene marls covered with residual soils) and geomorphological conditions, where the differences between the valley bottoms and the top of the hills are rarely higher than 100 meters. Geomorphological characteristics of mapped landslides were compared with characteristics of the stable terrain using different DEM derivatives, such as roughness and curvature. According to the analyses, roughness and curvature values are distributed differently in landslides and stable terrain. Knowledge of the difference between the geomorphological characteristics of landladies and stable terrain provides valuable information for the automatic mapping of landslides and potently unstable slopes.