Numerical Groundwater Flow Modelling for Slope Stability Analysis: Case of Blue Nile Gorge, Central Ethiopia

Abstract

Despite the vast researches on the landslide problem in the Blue Nile Basin, little is known about the perspective of groundwater level fluctuation in the region. The overall image that emerges from literatures is as it upswings in the rainy season from high infiltration into the ground and cause landslide problem. This assumption sustained more than nearly a century in all researches conducted towards understanding the landslide hazard, which causes life-threatening impact besides the negative economic effects to repair structures. Even though different mitigation measures are taken from these studies output and recommendation, the region is still unstable. Accordingly, there is a need for investigation of groundwater flow coupled with slope stability study. In this study, we determined the overall condition of groundwater in different seasons and checked the contribution of the level rise to instability of a slope in a specific site. After an exhaustive site visit and data study, the area is conceptualized based on a geographical, hydrological, and hydrogeological feature of the region, to fit the mathematical model in hand. The groundwater flow study is conducted with identified boundary conditions using saturated finite-difference grid model MODFLOW. Steady and transient states of flow are modelled with different hydrogeological parameters. The models are calibrated to satisfy the observed field conditions and expected results from the scientific point of view. Results show that in limited areas the fluctuation of groundwater is significant while in others the level remains stable in all seasons of the year. Following that, the result of groundwater flow model is exported to GeoStudio to simulate the slope stability of the selected slope. The factor of safety was calculated using slope/W. The effect of pore-water on the factor of safety is crosschecked by remodelling the slope without water. The results and sensitivity analysis of slope stability confirm that the rise of groundwater level decreases the factor of safety significantly only on critical slope section. From this paper, it is noticeable that in landslide study, coupling groundwater model with slope stability is not only effective but also economical in shallow groundwater sites before infrastructure development.