Landslides in a densely populated county at the footslopes of Mount Elgon (Uganda): Characteristics and causal factors

Manjiya County on the Ugandan slopes of Mount Elgon is a densely populated mountainous area where landslides have been reported since the beginning of the twentieth century. The numerous fatalities and the damage done during the extreme rainfall events of 1997 to 1999 drew attention to this phenomenon. In order to better understand the causal factors of these landslides, 98 recent landslides in the study area, mostly debris slumps, were mapped and investigated. Together, they displaced 11 millions m³ of slope material. Statistical analysis shows that landslides dominate on steep concave slope segments that are oriented to the dominant rainfall direction (northeast) and at a relatively large distance from the water divide. Based on landslide occurrence and impact, four different zones can be distinguished within the study area. Causal factors as well as landslide characteristics differ greatly between the four zones.Besides the fact that steep slopes, high rainfall and typical soil properties and stratification turn Manjiya into an inherently unstable area, human interference cannot be neglected. Whereas deforestation has reduced the stability of the shallow soils on the eastern slopes of the study area, the excavation of slopes, mainly for house building, is an important destabilizing factor for the western slopes. The growing population density not only increases the risk on damage, but hampers the search for solutions for the landslide problem as well.