Geological and geomorphological factors influencing mass wasting susceptibility in the Nyabage Watershed, Kabezi Commune, Western Burundi

Abstract

Landslides are among the most destructive natural hazards in Burundi, particularly within the Nyabage watershed of Kabezi Commune, where they pose a serious threat to infrastructure, human lives, and land use sustainability. This study assessed the geological and geomorphological factors influencing mass wasting susceptibility in this region by integrating field investigations, laboratory testing, petrographic analysis, and spatial modeling. Twelve rock samples and over forty soil samples were collected and analyzed to evaluate key physical and mechanical properties, including grain size distribution, Atterberg limits, porosity, permeability, shear strength, and uniaxial compressive strength. Petrographic analysis of thin sections revealed that the watershed is underlain by biotite- and muscovite-rich lithologies, including granite gneiss, granite, and micaceous sandstone, which are structurally weak due to schistosity, foliation, and pervasive microfracturing. These lithologies significantly reduce shear strength and promote water infiltration. ArcGIS 10.8 spatial analysis combined with kriging interpolation reveals that landslides in the Nyabage watershed mainly occur on steep, concave slopes (20°–40°), eastern aspects, and areas with high moisture content. These zones feature fine-grained, cohesive soils with moderate to high plasticity (PI 8–19%), a liquid limit of 39.3%, a plastic limit of 27.1%, high porosity (32%), low permeability (8.8 × 10⁻⁶ m/s), and low shear strength (friction angle 18.64°; cohesion 44.3 kPa). When paired with structurally weak rocks and intensified land use or deforestation, these geotechnical and geomorphic factors primarily control landslide occurrences in the watershed. This study demonstrates that the susceptibility to mass wasting in the Nyabage watershed arises from a combination of lithological weakness, geotechnical vulnerability, slope configuration, and anthropogenic pressure. The integrated methodological approach offers a scientifically robust framework for hazard mapping and risk assessment. These findings contribute to regional disaster risk reduction strategies and support informed land-use planning and infrastructure development in geologically sensitive areas.