Interactions between landslides, land use and management : a case study on the rift flanks west of Lake Kivu (DR Congo)
(2023)
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- Tropical mountainous regions are often identified as landslide hotspots because of the growing population pressure. However, very little information is available for understanding landslide processes in these environments. This thesis investigates the interactions between landslides and land use and management in a rural environment located on the Rift flanks west of Lake Kivu (DR Congo). Multi-method approaches, combining remote sensing and field work, allowed us: (1) to analyze, at the regional scale, the distribution of the different categories of landslides and to identify the factors that favour their occurrence; (2) to investigate, at the local scale, the factors that drive land use on landslides after their occurrence; and (3) to quantify the rate of vegetation cover change in landslide-affected zones and analyze the environmental factors that drive revegetation after an intense convective rainfall event triggered landslides and flash floods. At the regional scale (5700 km²), a unique inventory for this type of environment was built, with more than 2730 landslides mapped. We identified old and recent (post 1950’s) landslides, making a distinction between deep-seated and shallow landslides, road landslides and mining landslides. Analyses of frequency-area statistics and predisposing factors by susceptibility models show that: (1) susceptibility patterns and area distributions are different between old and recent deep-seated landslides, which shows that natural factors contributing to their occurrence were either different or changed over time; (2) shallow landslides are more frequent, but of smaller size, in areas where deforestation has occurred since the 1950’s as compared to shallow landslides in forest areas, i.e. in natural environments ; and (3) mining-related landslides and road-related landslides are larger than shallow landslides but smaller than recent deep-seated landslides; and they are controlled by environmental factors different from the natural conditions of occurrence. At the local scale, field surveys and spatial analysis showed that farming is more common on landslides than on adjacent flanks, reflecting the strong human demand for agricultural land. Three quarters of the surveyed farmers were aware that they operate on a landslide and 89% of them currently assess the risk of reactivation as high to very high. Farmer satisfaction and the value of land inside landslides can be higher than adjacent land when they are characterized by higher fertility, weaker slopes, less stony and sandy terrain, deeper soil and greater moisture. These favourable natural conditions are generally more prevalent on deep-seated, old, large, "landslides" than on their adjacent flanks. Using the combined landslide-flash flood event of October 2014 in the vicinity of Kalehe as a case study, it was found that the source zones for the shallow landslides, on steeper slopes, revegetate more slowly than the transport-deposit zones (propagation sector). For the latter zones, the rate of vegetation recovery is affected by slope steepness and by the exploitation of earth and debris mobilized during the event for construction purposes. The doctoral research showed that, beyond natural predispositions, human activity plays an important role as a predisposing factor and, in the post-event period, in land use and vegetation cover change. For a better risk reduction strategy in tropical environments, there is a need to undertake further studies and actions to minimize the human impact on landslide occurrences, especially in the preservation and restoration of forest cover and land use planning.
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Maki Mateso, J.-C. (2023). Interactions between landslides, land use and management : a case study on the rift flanks west of Lake Kivu (DR Congo). https://hdl.handle.net/2078.5/233636