Spatial patterns, causes and consequences of landslides in the Gilgel Gibe catchment, SW Ethiopia

The Gilgel Gibe catchment in SW Ethiopia is one of the areas in East Africa affected by landslides. To better understand the patterns and the causal factors of these landslides, all landslides in a small study area (14 km²) in the hilly parts of the Gilgel Gibe catchment were mapped and analyzed. In total, 60 landslides were mapped. These landslides caused a displacement of 1 million m³ slope material, which corresponds to a mean displaced volume of 50 ton ha−1 y−1 in the last 20 years. Moreover many landslides deliver directly sediment to the rivers and hence increase the sediment load in the rivers. This soil loss to the rivers was estimated at 11 ton ha−1 y−1 during the same period. High annual rainfall (ca. 2000 mm y−1), lithological and pedological properties and to a lesser extent steep (>16°) slopes turn the area into an inherent unstable situation and can be indicated as preconditions for the landslides in the study area. Distance to rivers is significantly the most important precondition, as slopes near rivers are less stable than slopes further away from the rivers. This is mainly caused by river incision and bank erosion which often occur in the area and which can be attributed to increased runoff due to deforestation over the past 20 years. Therefore recent deforestation caused more shallow landslides but also indirectly more deep-seated landslides close to the rivers. Heavy rainfall is indicated as the main triggering factor for almost all landslides.