Landslide characterization and distribution in northern Ethiopia: a case study from the Adishu area

Landslides have resulted in large numbers of casualties and huge economic losses in the hilly and mountainous areas of the Ethiopian highlands. Adishu (Adi-Quchi and Quilet villages in particular) is one of the areas most highly affected by rainfall-driven landslides. Field investigations of this area have revealed displacement of 204 people, destruction of 56 rural houses and 5 km of rural roads, and effects on 58 ha of agricultural land due to landslides. In addition, a significant proportion of the population still lives in a terrain potentially endangered by landslides. The geology of the study area consists of Antalo limestone, Agula shale, Amba-Aradom sandstone, basalt flows, and unconsolidated sediments. Thirteen major landslides with clear downthrown head scarps and downspread toes were identified. Inventory of landslides was based on the interpretation of aerial photographs, topographic maps, satellite images, and field identifications, with the aim to assess the cause and type of landslides in the study area. The different methods applied to soils and rocks included the direct shear test, Schmidt hammer, and inverse auger hole methods. The results revealed that soil materials in the area have a low permeability (1.22 × 10−6–3.13 × 10−6 m/s), a high plasticity index (17.2–29.4), and cohesion values ranging between 10 and 26 kN/m2. Soil samples from the stable in situ layers showed relatively high cohesion and relatively small internal angle of friction compared to the residual layer. The common types of landslides in the study area were rock falls, rock topples, debris/earth flows, and debris/earth slides accompanied by translational and rotational sliding mechanisms. These landslides could be classified into three zones based on location, type of failure, mode of failure, and direction of failure. The upper layers of basalt and limestone rocks showed significant percolation and recharge to the downslope layer (marl-shale). We therefore concluded that the marl-shale layer is mainly responsible for sliding, whereby percolated and recharged water cause saturation and the development of pore water pressure in this layer due to its imperious nature. The Qulate (landslide zone one and zone two) and Adi-Quchi (landslide zone three) localities are highly affected areas where there are unconsolidated soil deposits underlain by marl-shale along the streams due to removal of underlying and lateral support by gully erosion.