Historical aerial and terrestrial photographs for the investigation of mass movement dynamics in the Ethiopian Highlands

The edges of the Ethiopian Plateau are affected by frequent landslides predisposed by pronounced seasonality in precipitation, thick weathering mantle of volcanic material, and rough relief. We analyzed the three-dimensional dynamics of three large landslides in Dessie using digital elevation models and ortho-images derived from a time-series of aerial photographs reaching back to 1936. Furthermore, we utilized repeated photography based on terrestrial photographs from the 1930s and 1940s to analyze landscape changes. It was revealed that the large sliding zone (25.4 ha) in lacustrine sediments at Kerra locality existed approximately in the present extent already before 1936. The volume of depleted material of the 1986-1994 rock slide at Doro Mezleya locality was assessed as 1.82 10(6) m(3) with mean vertical thickness of 48 m. Additionally, we described the nowadays inactive Hot Spring landslide (12.9 ha). We documented a large scale reforestation of the area carried out as a remediation measure and rapid changes in land cover and settlement structure. We conclude that two out of three studied large landslides existed before 1936 and thus are not induced by the increased human pressure on the landscape of the last decades. Additionally, we provide an overview about collections of historical aerial photographs of Ethiopia, and we discuss their potential and drawbacks for mass wasting studies