Modelling the effect of wind forces on landslide occurrence in Bududa district, Uganda

Abstract

The hilly area of Bududa District, in Eastern Uganda has suffered heavily from landslide occurrence, making the local populace unsafe. Although the area is well covered by vegetation, a catastrophic landslide occurred on the 26th June, 2012 and killed more than 18 people and wiped out the nearby community. The fallen trees due wind effect prior to the landslide coupled with intense rainfall are believed to trigger that landslide. Therefore, the effect of commonly known causative factor of rainfall and an uncommon landslide causal factor of wind forces on landslide occurrence were investigated in this study. The geometry of the modelled slope consists of 111 m height, 480 m length, and an inclination of 21°. Finite element seepage software, Seep/W and limit equilibrium software, Slope/W were used for modelling the landslide occurrence using five (5) slope models with and without additional shear strength in form of tree root cohesion. Each of the slope models was subjected to four failure criteria (FC). The first two failure criteria (FC1 and FC2) are assigned for group of tree along the slope (FC1) and a single tree at the middle (FC2) of the modelled slope. The other two failure criteria (FC3 and FC4) are assigned for the modelled slope with rainfall infiltration alone and rainfall plus several tree weights, respectively. The wind speed during which the landslide occurred was depicted using Beaufort wind scale 11 and was applied to the appropriate models. The results obtained show that with eucalyptus trees of Diameters at Breast Heights (DBH) of 25 cm, a minimum factor of safety (FOS) of 1.012 and 1.273 were recorded for FC2 in Slope model 1 and slope model 4 with and without increased cohesion, respectively. This shows an increase in the FOS of 23.81%. However, when the DBH was increased to a maximum of 60 cm (i.e. in slope model 3), the safety of the modelled slope diminishes and the FOS reduces to 0.601 for FC2 on the 25th day, without cohesion, just some hours before the fateful day of the 26th June 2012. This FOS remained below 1.0 (0.800 to 0.601) for FC2 with roots cohesion, although a gain in the FOS of 33.11% was realized. Hence, it can be concluded that the uncommon natural causative factors of wind have a great influence on landslide occurrence in the hilly areas of Bududa district.