Characteristics and causes of natural and human-induced landslides in a tropical mountainous region: the rift flank west of Lake Kivu (Democratic Republic of the Congo)

Tropical mountainous regions are often identified as landslide hotspots with growing population pressure. Anthropogenic factors are assumed to play a role in the occurrence of landslides in these densely populated regions, yet the relative importance of these human-induced factors remains poorly documented. In this work, we aim to explore the impact of forest cover dynamics, roads and mining activities on the characteristics and causes of landslides in the rift flank west of Lake Kivu in the Democratic Republic of the Congo (DR Congo). To do so, we compile a comprehensive multi-temporal inventory of 2730 landslides. The landslides are of different types and are grouped into five categories that are adapted to study the impact of human activities on slope stability: old (pre-1950s) and recent (post-1950s) deep-seated landslides, shallow landslides, landslides associated with mining and landslides associated with road construction. We analyse the landslides according to this classification protocol via frequency–area statistics, frequency ratio distribution and logistic regression susceptibility assessment. We find that natural factors contributing to the cause of recent and old deep-seated landslides were either different or changed over time. Under similar topographic conditions, shallow landslides are more frequent, but of a smaller size, in areas where deforestation has occurred since the 1950s. We attribute this size reduction to the decrease in regolith cohesion due to forest loss, which allows for a smaller minimum critical area for landsliding. In areas that were already deforested in the 1950s, shallow landslides are less frequent, larger and occur on less steep slopes. This suggests a combined role between regolith availability and soil management practices that influence erosion and water infiltration. Mining activities increase the odds of landsliding. Landslides associated with mining and roads are larger than shallow landslides but smaller than the recent deep-seated instabilities, and they are controlled by environmental factors that are not present under natural conditions. Our analysis demonstrates the role of human activities on the occurrence of landslides in the Lake Kivu region. Overall, it highlights the need to consider this context when studying hillslope instability characteristics and distribution patterns in regions under anthropogenic pressure. Our work also highlights the importance of using landslide classification criteria adapted to the context of the Anthropocene.</p

Interactions between landslides, land use and management : a case study on the rift flanks west of Lake Kivu (DR Congo)

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.(AGRO - Sciences agronomiques et ingénierie biologique) -- UCL, 202

Landslides, land use and the anthropization of a rural environment: focus on the Rift flanks west of Lake Kivu (DR Congo)

The Rift flanks west of Lake Kivu are one of the regions in DR Congo naturally most affected by landslides. This mountain region, located between the sprawling cities of Goma and Bukavu, is also one of the most densely populated areas of DR Congo, with a density of up to 200 inhabitants per km2, and its population is expected to continue to grow in the future. Besides population growth, the region has been facing important land use/land cover changes. Despite the large impact landslides have in this area, little is known regarding the interactions between this land degradation process and land use/land cover changes. Here we present the research strategy and the methodology that was specifically designed for this data-scarce region which will allow us to investigate these intertwined issues so that better land management strategies can be proposed. First results at regional scale showing the distribution of more than 1,500 landslides with regard to land cover and anthropic activities (such as mining and quarrying) will also be shown

Characteristics, distribution and susceptibility of natural and human-induced landslides in a tropical mountainous region: the Rift flank west of Lake Kivu (DR Congo)

Tropical mountainous regions are often identified as landslide hotspots with particularly vulnerable populations. Whereas both natural (e.g., rainfall, lithology) and anthropogenic (e.g., deforestation, mining) factors are expected to be involved in landslide occurrence, the relative importance of such factors remains poorly documented. The objective of this study is to understand the role played by natural and anthropogenic factors in the occurrence of landslides in the Rift Flank west of Lake Kivu in DR Congo. First, we inventoried 2856 landslides of various types using Google Earth imagery, high resolution topographic data, historical aerial photographs from1950’sand intense field surveys. We made a distinction between old and recent deep-seated landslides, shallow landslides and mining-related landslides. The difference in susceptibility patterns and in size distributions between old and recent deep-seated landslides indicates that natural factors contributing to their occurrence were either different or changed over time. The shallow landslides are all recent. The susceptibility analysis shows that their regional pattern is mainly controlled by forest dynamics and the presence of roads. The occurrence of shallow landslides in forest areas, i.e. in natural environments, strongly increases with slope gradient. In areas with similar topographic conditions where deforestation has occurred since the 1950’s, shallow landslides are more frequent, but of smaller size. This size reduction is attributed to the decrease of regolith cohesion due to forest loss, which allows a smaller minimum critical area for landsliding. In areas that were already deforested in 1950’s, shallow landslides are less frequent, larger, and occur on less steep slopes. This suggests a combined role between regolith availability and soil management practices that influence erosion and water infiltration. Landslides associated with mining activities are larger than shallow landslides but smaller than the recent deep-seated instabilities. They are not well predicted by the susceptibility models, showing that they respond to environmental factors that are not present under natural conditions. This research highlights the importance of human activities on the occurrence of landslides and the need to consider this context when studying hillslope instability patterns in regions under anthropic pressure

Landslide inventory approach for regional-scale hazard assessment in a data-poor context in tropical Africa

peer reviewe

Living on landslides: land use on unstable slopes in a rural environment of the Rift flanks west of Lake Kivu (DR Congo)

Whereas landslides have been widely studied in terms of dangerous phenomena and land degradation processes, they may also be a source of opportunities for local communities in developing countries. Indeed, landslides modify topography, soil characteristics or local hydrology, which may lead to favorable conditions for human settlements, agriculture, or mining activities. However, little is known about the factors that influence landslide valorization. The aim of this study was to assess, based on the characteristics of the landslides and land user’s perception of risk, the extent, modes of valorization and degree of satisfaction in exploiting landslides in the Kalehe region on the rift flanks west of Lake Kivu. Seventy landslides were selected out of 785 inventoried landslides by stratified sampling according to three criteria (type of movement, size, and position of landslide on the rift flanks). In addition, the landslides were characterized in terms of the local lithology, their age and depth. A survey was carried out with farmers exploiting these landslides. In addition, a comparative study of land uses in and outside the landslides were performed. We collected opinions from 82 farmers on 57 landslides of the 70 initially selected due to accessibility or safety constraints. All visited landslides were exploited except for three. All respondents knew about landslides, and more than 75% could report signs of landslide activity on their parcel. Almost 90% of these farmers consider the landslide risk to be high to very high, especially in the case of recent landslides and those with a flow movement. Compared to adjacent flanks, land values are higher and farmer satisfaction greater inside landslides characterized by less steep slopes, higher fertility, deeper soils, wetter soil, and lower stoniness, which is preferentially associated with old, deep-seated, and large landslides with a slide movement. On the contrary, land in recent landslides is cheaper. Farmer satisfaction is lower inside these landslides that are less wet, less fertile, shallower, or richer in sand content than the adjacent flanks. Spatial analysis revealed a differentiation in land uses in the landslides compared to the surrounding flanks. Subsistence crops occupy a larger proportion inside the landslides, while eucalyptus plantations and pastures have higher proportion outside. Statistical tests revealed that landslide characteristics significantly explain the differences in land use proportions for subsistence crop, forest, and banana land uses. Landslides less favorable for valorization are generally small, with very steep topography. This study shows that almost all landslides are being exploited by farmers and that farmers adapt their land use to the type of landslide. Some types of landslides further offer more favorable conditions for cropping that land outside landslides. Despite the high perceived risk of landsliding, human settlement on unstable slopes would be justified because any future hazards are outweighed by the immediacy of the benefits. Better understanding the reasons for the settlement of rural populations on unstable landslides may help devise better risk mitigation strategies

Landslides in a Changing Tropical Environment: North Tanganyika - Kivu Rift Zones

status: publishe