Landslide Susceptibility Mapping of Blue Nile and Tekeze River Basins using Oblique Rainfall-Aspect Rasters

This research was carried out to prepare the regional level landslide susceptibility maps by incorporating the oblique rainfall raster in the upper Blue Nile and Tekeze River basins. The oblique rainfall is the amount that actually falls on sloping surfaces, and varies considerably with slope inclination and aspect with respect to the prevailing trend of the wind direction. The monthly averaged precipitation data for the Kermit (July-September) and the Belg (March-April) rainfall seasons for the study area were acquired for the period of 1950 to 2000, and utilized to compute the oblique rainfall vectors at 40°, 45°, 50°, 55°, and 60° angles (representing "wind-driven" rainfall vectors). The weighted overlay index method using ArcGIS software was applied for this regional landslide susceptibility mapping (scales >1:100,000) by incorporating vertical rainfall intensity maps and aspect separately and as a combination (rainfall raster coupled with the slope aspect raster). The resulting landslide susceptibility maps were compared which reveals that the results obtained from using integrated rainfall/aspect raster's (combined) were found to be more reasonable towards computing high to very high hazards than using aspect and rainfall rasters as separate layers. The susceptibility maps were validated with landslide inventory maps as well as documented rockslides, scattered throughout the study area. This reconnaissance level study could serve as guide maps in identifying those areas where more detailed landslide hazard mapping might, or should be, undertaken in the future for detailed investigations.