Landslide occurrences in the hilly areas of Bududa District in Eastern Uganda and their causes

Abstract

Although on a global scale Uganda is not known for landslide disasters, in the recent past especially in the years with above normal rainfall the loss of life and property from these disasters is increasing. This study assesses the factors that influence landslide occurrences in Bududa District and their impacts on to the livelihood of the people. Between 1997 and 2004, heavy rains left 48 people dead and ten thousand displaced and landless. The volume of debris from ninty eight landslides was 11 million m3 and this was deposited into rivers and streams. Twenty nine of these landslides dammed rivers resulting in destruction of bridges and roads when the dams broke. The main landslide types are debris slumps which occur on concave slopes where water concentrates. These landslides occur on steep slopes that are plano concave and between slope angles of 140 to 410. Slopes facing north-east are most prone to landslides which coincide with the dominant rainfall direction. The soil types in this area are those conditioned by topography and tropical climate namely Nitisols, Cambisols, Lixisols, Ferralsols, Leptosols, Gleysols, and Acrisols. The texture of the soil in the horizons was significant to the landslide occurrences especially in the western zone. In the eastern zone, soil profile horizon is significant in some of the landslides but in the shallow landslides the slope and the shallow depth which creates a discontinuity between the saprolite and the rock causing water stagnation is the main influence. The knowledge from farmers’ is almost similar to scientific observations. Farmers mentioned steep slopes, areas with concavities and those with flow of water from underground as areas prone to landslides. Although their observations have a limitation in that they cannot determine the threshold. The soils contain medium to high plasticity clays and according to the Atterberg limits they approximately fall in the categories of kaolinite and illite. The top soils also have a high infiltration rate which allows fast flow of water into the deeper clay rich horizons promoting water stagnation causing slope failure. The main triggering factor is rainfall and rainfall events of low intensity but prolonged for days are thought to be more disastrous however, this is an area that needs further investigation. Terraces are not popular among farmers in some of the areas because they believe that terraces promote water infiltration which triggers landslides. Using the LAPSUS-LS landslide model the slopes in Bududa District are identified as inherently unstable and the volumes of soil redistribution can yield four times higher than what was observed in 1997 (44,000,000 m3). These will end up in stream channels possibly damming rivers and causing damage to infrastructure or siltation and pollution of streams.