Distribution des glissements de terrain dans un environnement en mutation : focus sur la gorge de la Ruzizi située entre la RD Congo et le Rwanda

Abstract

The conversion of natural ecosystems into agricultural or urban areas can be accompanied by changes in geomorphological processes. This is common in areas where environmental conditions naturally predispose landscapes to landslides. Here we focus on the Ruzizi gorge, a region of the western branch of the East African Rift known for being affected by landslides and where the development of the city of Bukavu (DR Congo) have disturbed the landscape over the last decades. The aim of this research is to understand the regional spatio-temporal distribution of landslides in this context where the scarcity of information on geomorphological processes is commonplace. Data from field observations, historical aerial photographs, satellite imagery and archive analysis are combined to produce a multi-temporal inventory of 149 landslides spread in three morpho-lithological zones over an area of 94 km². The largest (up to 2 km²) and oldest landslides precede clearly human induced-changes of the last 60 years. There are mostly made of slides (planer and rotational) and topography, lithology (through weathering of volcanic formations) and river incision linked to Lake Kivu formation (~10 kyr BP) explain their distribution. The other landslides (58% of the inventory) have occurred in the last six decades. Most are flow-like landslides including flow slides, earthflows, debris avalanches and debris flows. These most recent and shallow landslides are mostly controlled by topography and land cover / land use changes. They are triggered by rainfall. For large landslides whose age is unknown, a hypothesis of seismicity can’t be ruled out in their triggering. But this requires a much more detailed study where the incision of the river and the progressive evolution of the slopes towards conditions of instability must be taken into account.