The effectiveness of hillshade maps and expert knowledge in mapping old deep-seated landslides

Abstract

Large deep-seated landslides with a shear surface deeper than 3 m and a mean affected area of 4.2 ha are common features in the Flemish Ardennes. None of these deep-seated landslides are dated, but they are assumed to be rather old (>100 years). Because most of these landslides are located under forest, aerial photo interpretation commonly used for the creation of landslide inventories is not a suitable tool to map the landslides in the Flemish Ardennes. Therefore, an intensive 100-day field survey was carried out by two geomorphologists in a 430-km 2 study area. This resulted in a landslide inventory map, indicating the location of 135 large deep-seated landslides. But field surveys are time consuming and, thus, very expensive. Therefore, a cheaper and faster mapping technique was tested. A hillshade map was constructed for the study area in a GIS (IDRISI32) from a 5-m resolution digital elevation model (DEM). Seven experts were given 1 h to indicate all the hillslope sections, which they suspected to be possible landslides on a copy of the aforementioned map (scale 1:100,000). In total, this exercise took only 1 day (i.e., 7 person hours). Large differences in the number of presumed landslides and the extent of the hillslopes thought to be affected by landslides were reported among the seven experts. The polygon and pixel efficiency were introduced to estimate the quality of the landslide maps based on hillshade maps and expert knowledge. Compared to the field survey-based landslide inventory, the quality of the landslide inventories based on the hillshade maps and expert knowledge was relatively low. Experts familiar with the study area obtained somewhat better results than experts who visited the study area only once. A combination of the seven expert maps did not result in a good inventory map because too many unaffected hillslopes were incorrectly indicated as affected by landslides. The results obtained in this study are comparable to an investigation carried out by (Wills, C.J., McCrink, T.P., 2002. Comparing landslide inventories, the map depends on the method. Environmental and Engineering Geoscience 8, 279–293). Although the tested method can never replace a detailed field survey, taking into account the proposed improvements, it can be used for the creation of a regional inventory map of old landslides in a densely forested area where light detection and ranging (LIDAR) data are unavailable.