Extended TOBIA model for the assessment of deep-seated landslides

In order to assess landslide susceptibility, the selection of the controlling factors (i.e., the predictor variables) is crucial. The most important factors for deep-seated landslides are geological settings such as the bedding conditions of rock formations and the distance to faults. We developed a GIS-based semi-automatic method to extract information on the orientation of bedding planes. This method uses information captured by the interpretation of high-resolution digital terrain models (DTMs). In order to calculate dip and dip direction of the bedding planes we have developed the Morpho-Line concept, which uses geometrical information captured by a detailed interpretation of DTMs. To increase the number of data points, additional field measurements were added to the morpho-line data. We have implemented the "accumulated cost" tool, which is similar to thiessen polygons, to interpolate between the data points. This method takes valleys and faults as break lines into account when interpolating bedding orientation values. Dip and dip direction data has been used, in combination with the slope and aspect, to calculate an extended TOBIA model. TOBIA classifies slopes into anaclinal, cataclinal and orthoclinal classes. To obtain a more accurate picture of orthoclinal bedding conditions and their connection to landslides in these areas, we have subdivided the orthoclinal classes. The angle difference between topography and bedding dip has been calculated and divided into classes. According to that model, the highest abundance of landslides is found in slopes classified as cataclinal and orthoclinal. This means that landslides preferably occur where the geological layers are inclined with the slope (cataclinal) or the dip direction is perpendicular to the slope direction (orthoclinal).