Mapping landslide susceptibility using geographical information systems and its evaluation for forest roads in the Yigilca Forest Directorate

The Yığılca Forest Directorate is located in the Western Black Sea Region of Turkey, which has the highest occurrence of landslides in the country. The most important settlements in the 499-square-km study area are in Yığılca County of Düzce Province. Landslides are especially prevalent in areas near settlements and agricultural land. In addition, roads in the area trigger landslides and are generally affected by their occurrence. Thus, a landslide susceptibility map covering the study area has been generated based on the Geographical Information Systems (GIS) and evaluated in terms of forest roads. A landslide inventory based on susceptibility mapping was produced as a result of intensive fieldwork. In addition, a landslide inventory of the area created in 2005 by the MTA (Mineral Research and Exploration), scaled at 1/500000, was used for comparison. This inventory, generated through fieldwork, included 288 landslides.Eight parameters that have been observed in fieldwork as being effective for landslide occurrence were used for mapping landslide susceptibility. These are: land use, lithology, elevation, slope, aspect, distance to streams, distance to roads and plan curvature. Using these eight parameters, a landslide susceptibility map was created, as well as a digitized inventory map using the Logistic Regression (LR) method. For validation, the AUC (Area Under the Curve), which is the area under the ROC (Relative Operating Curve), was used to create the susceptibility map. The AUC value of the model was 0.905. The resulting map included five susceptibility classifications: very low susceptibility (0-0.2), low susceptibility (0.2-0.4), moderate susceptibility (0.4-0.6), high susceptibility (0.6-0.8) and very high susceptibility (0.8-1). According to these criteria, an area of 8439.9 ha was located in the very low-susceptibility class, an area of 20191.7 ha in the low-susceptibility class, an area of 14211.6 ha in the moderatesusceptibility class, an area of 6659.0 ha in the high-susceptibility class, and an area of 371.8 ha in the very high-susceptibility class. According to the road network plans, 931.9 km of roads, including forest roads, village roads and motorways, are to be found in the study area, 866.3 km of which have been constructed to date. An overlay analysis was made for evaluating these roads in terms of landslides. According to the results, of the roads in the network plan, 84.1 km were located over the very low-susceptibility areas, 413.5 km over the low-susceptibilityareas, 232.1 km over the moderate-susceptibility areas, 174.6 km over the highsusceptibility areas, and 22.7 km over the high-susceptibility areas. Of those roads which have not yet been constructed, 25.2 km are located over very low-susceptibility4class areas, 29.5 km over low-susceptibility areas, 9.9 km over moderate-susceptibility areas, and 1 km over high-susceptibility class areas. There are no planned roads over very high-susceptibility class areas. In the study area, real landslide frequency and general landslide frequency on the roads are 0.42 and 0.18, respectively. The real roadlandslide index and the general road-landslide index in the area are 0.10 and 0.04, respectively.Keywords: Geographical Information Systems, Landslide Susceptibility Map, Logistic Regression, Forest Roads, Yığılca