Slope failures in the Blue Nile basin, as seen from landscape evolution perspective

The Blue Nile basin is severely affected by slope failures, and the characteristics of its deep gorges and rugged valley walls called for a study on the relationships between topography and the process of landsliding and rock falling. Work was commenced with the conception of nine types of landforms on the basis of a one-to-one combination of lateral and vertical slope profiles and thence the determination of the effect of these landforms on the occurrence of slope failures. Observations showed that topographic surfaces with concave lateral profiles shelter mudflows and some retrogressive rotational slumps while slopes characterized by planar lateral profiles are sites mainly for translational slides. Landslides are rare in convex-shaped slopes but when they occur, they are big and deep-seated. As an effort to understand the significant contributions of landslides and rock falls to landscape development, direct and indirect methods are employed. Direct methods are based on quantitative relationships between the volume of material that had been removed from the area and the amount that could, in principle, be taken away based on available erosion rates. Indirect methods used the nature of river incision and the effect of the present-day landslides on the landscape. In general, discrepancy in calculated figures in the first, and the overall drop and form of the Abay River gorge coupled with the observed landslide-caused landform changes in the second, led us to deduce that slope failures were part of the mega-forces that shaped the entire Blue Nile basin, and in fact, played the dominant role in landscape evolution.