Contribution à l’étude des mouvements de masse dans le bassin versant de la rivière Gikoma

Abstract

Ground instability is one of the most spectacular and frequent geodynamic phenomena worldwide. Its occurrence causes significant morphological changes to the terrain and affects exposed infrastructure, human activities, and ecosystems. These phenomena result from several factors that contribute to their occurrence. The Gikoma River watershed is located partly in the Imbo plain and partly in the Mirwa region, in the province of Bujumbura. Due to its natural characteristics, this watershed is particularly prone to the development of numerous types of ground instability. These are influenced by several factors, including lithology, pedology, rock fracturing, land use, the hydrographic network, slope and exposure of the slopes. The highly rugged topography, torrential rains and reduced vegetation cover are the main factors contributing to the instability of the terrain in this watershed. During the rainy season, the clay soils become saturated with water and more susceptible to landslides. During the rainy season, clay soils become saturated with water and more prone to slipping. An inventory of ground instability was carried out by analysing aerial images and photographs and based on in situ observations. A total of 78 mass movements were identified: rockfalls (37.20%), translational slides (26.92%), rotational slides (7.69%), complex movements (6.41%), collapses (8.97%), fissures (3.85%), water erosion (3.85%), mudflows (2.56%), scouring (1.28%) and subsidence (1.28%). Slope and torrential rainfall are the main factors playing a major role in the process of land instability in the study area. A diachronic study of land use based on satellite images from 2002, 2014 and 2024 reveals that changes in land use play a major role in triggering ground movements within the Gikoma River watershed. The recent intensification of these phenomena is thought to be mainly linked to the gradual deterioration of the environment in the catchment area. After subjecting the factors responsible for ground instability to hierarchical multi-criteria analysis using an AHP method, the susceptibility map produced showed the stable areas (Buhayira, Giterama and Tenga) and unstable areas (Kindobwe, Kagoma, Kwigere, Masake and along the watercourses: Kigezi, Nkuri and Karunga) in our study area.