Hydro-geological and meteorological behaviors of typical landslide prone hillslopes in North-western Rwanda

Abstract

Landslide hazard prevention measures that include slope stabilization or an early warning system require an understanding of the hydro- geological and meteorological behaviors of the hillslopes prone to failures. This research aimed to understand the hydro-geological and  meteorological processes and the relationship thereof using two typical hillslopes (Karago and Rwaza) that experienced slow moving  rotational deformation. For each case study, geotechnical characterization and hydrological field and laboratory information was  collected, i.e., saturated permeability measurements, soil moisture and groundwater monitoring. The surface displacements were also  monitored and their linkage with hydrological processes was assessed. The geotechnical characterization indicated instability conditions  (Fs<1) at the Karago hillslope and marginally stable conditions (1<Fs<2) at the Rwaza hillslope. The slope deformation and landslides  occurred during the wettest conditions (i.e. soil moisture close to saturation and groundwater rises up to near surface). The surface  displacements control points revealed the toe and head units to move faster than the intermediate units. The highest acceleration at the  toe was attributed to the external incision agents like stream erosionwhile cracks and steeper failure plane were responsible for  acceleration at the head units. The regression analysis indicated a strong correlation (R²=79%) between surface displacement and depth  to groundwater and thus impactful for slope deformation and landslide initiation. The role of rainfall was also significant with long lasting  low intensity rainfall being more important than short and high intensity rainfall.