Inventory of landslides and susceptibility mapping in the Dessie area, northern Ethiopia

A comprehensive study of landslide processes was carried out in the city of Dessie and its environs in northern Ethiopia. The study area consists of a graben bounded by steep fault scarps and characterized by highly weathered Tertiary basaltic rocks covered with Quaternary deposits. A method of landslide susceptibility mapping that adapts to the area of the hillside (about 16 km2) is presented. First, a preliminary zoning was made by dividing the area into a discrete number of smaller units having similar susceptibility to sliding, followed by semi-quantitative field investigation of the geological, geotechnical, geomorphological, hydrogeological, and anthropogenic factors that contribute to the susceptibility of the units to landslide activity. The final landslide susceptibility map was accomplished by using classical overlay procedures of established maps representing the different influencing factors. The validity of the method was checked by means of a landslide-hazard map that showed the actual distribution of landslide episodes that have occurred in the past. Finally, four broad landslide susceptibility zones and 22 specific active landslide sites were identified. The most important landslide types were complex earth and debris slides and flows in silty clay soils associated with alluvial and colluvial deposits overlying highly weathered basalts. Intact basaltic rocks experienced rock fall and toppling in steep slope areas. Large-scale landslides have been initiated mostly by surface water and groundwater during the wet season.