Volume estimation and evaluation of rotational landslides using multi-temporal aerial photographs in Çağlayan dam reservoir area, Turkey

Four landslide scarps in Neogene sedimentary rocks were observed in the Çağlayan dam reservoir area. This study aims to (1) estimate the volume of landslide material that slid into the dam reservoir area within 59-year period and (2) evaluate the reactivations with respect to triggering and predisposing factors such as slope gradient, precipitation, fire, and seismic activity. In this context, field survey was combined with innovative use of multi-temporal historical aerial photographs. Various methods of photogrammetry were employed on four sets of stereo aerial photographs (1953, 1970, 1995, 2012) to identify the boundaries of landslides, to generate digital elevation models (DEMs) from which cross-sectional data and slope maps were extracted. Volumes of landslides were estimated based on height differences between successive DEMs. Spatial mismatch and height errors between successive DEMs were minimized by image to image coregistration and height adjustment, respectively. The height adjustment technique was reevaluated and it was suggested to choose 11 sub-areas independent from the size and location. It is also recommended that the accuracy of the landslide boundaries interpreted from aerial photographs should be cross-checked by overlapped cross-sections of successive years to estimate the volume more precisely. A total volume of 121.78 × 10 4 m 3 material collapsed whereas the largest volume moved in the period between 1953 and 1970 which was linked to a seismic event. Unconsolidated debris and weak rock layers are expected to fail in relation to water level fluctuations in the dam lake. The volume of sliding material will contribute to siltation and shorten the operational life of Çağlayan dam.