Landslides, river incision and environmental change : the Ruzizi gorge in the Kivu Rift

The understanding of the interplay between natural and human induced factors in the occurrence of landslides remains poorly constrained in many regions, especially in tropical Africa where data-scarcity is high. In these regions where population growth is significant and causes changes in land use/cover, the need for a sustainable management of the land is on the rise. Here, we aim to unravel the occurrence of landslides in the 40 km-long Ruzizi gorge, a rapidly incising bedrock river in the Kivu Rift in Africa that has seen its landscape disturbed over the last decades by the development of the city of Bukavu (DR Congo). Careful field observations, historical aerial photographs, satellite imagery and archive analysis are combined to produce a multi-temporal inventory of 264 landslides. We show that the lithological context of the gorge and its extremely high incision rate (> 20 mm year-1) during the Holocene explains the presence of a concentration of large landslides (up to 2 km²) of undetermined age (well before the first observations of 1959) whose occurrence is purely natural. They are mostly of the slide type and do not show morphologic patterns of recent activity. The landslides that occurred during the last 60 years are flow-like shallower slope failures of smaller size (up to 0.12 km²) and tend to disappear rather quickly (sometimes within a few years) from the landscape as a result of rapid vegetation growth, land reclamation and (human-induced) soil erosion. They are primarily related to threshold slopes and precipitation plays a frequent role in their onset. However, land use/cover changes also affect their occurrence. This study provides useful information for a more accurate evaluation of the landslide hazard in the area, particularly with respect to the growth of the city of Bukavu that has developed without the consideration of naturally instable slopes. It also stresses the need and added value of building accurate landslide inventories in data-scarce regions

Glissements de terrain et éléments à risque dans le bassin versant de la Wesha (Bukavu, RD Congo)

The Eastern part of DR Congo is a region naturally prone to landslides. Their impact can be important in populated areas. The objective of this research is to make a preliminary local assessment of landslide elements at risk on a recent settlement area of Bukavu (Wesha watershed). We mapped 11 landslides by combining a Pléiades image, aerial photographs and Google Earth imagery with field survey. Most landslides are deep-seated rotational movements. Shallow translational features as well as mud flows are also observed. The largest landslides were already present in 1959, but their origin is certainly far much older. Land use has dramatically changed since then. In 1959, the watershed is rural, whereas in 2013 a large part is urbanized and intensely populated. This population influx has started since 2000. Many infrastructures are damaged or under threat (electric lines, water pipes, roads, schools). Several fatalities due to landslides were reported over the last years. This study is a first step towards the understanding of the processes at play in this environment subject to anthropisation.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Weathering, rock type, bedrock incision and landslides in a tropical environment : the Ruzizi gorge in the Kivu Rift, Africa

Tropical environments favour chemical weathering and regolith development. Weathering induces textural, mineralogical and chemical changes in rocks, modifying their strength and thus affecting slope stability. Degree of weathering is, however, not only a function of climatic conditions, but is also influenced by e.g. bedrock composition and structure, exposure length and intensity, and slope angle. To investigate the role of weathering and rock type on landslide occurrence, we focus on the Ruzizi Gorge in the Kivu Rift segment of the western branch of the East African Rift System. Stretching along the border between the DR Congo and Rwanda, development of this 40-km long bedrock river began about 10,000 years ago, rejuvenating the landscape at a very high rate, with rather invariant slope angles outside of the landslides. The gorge stretches across a region where two main types of rocks constitute the geological substrate, i.e. late Miocene to Pleistocene volcanic rocks and Mesoproterozoic metasedimentary rocks. The gorge is a hotspot of deep-seated landsides in the region, with slope failures of up to 2 km². For the present study, we sampled weathering profiles developed on both mentioned rock types, in each case with sampling points within and outside the landslides as well as within and outside the rejuvenated landscape. The chemical composition of rock and regolith samples was determined by Inductively Coupled Plasma–Optical Emission Spectroscopy (ICP–OES) analysis, and their mineralogical composition by X-Ray Diffraction (XRD) analysis and thin section observations. Geotechnical tests were used to determine mechanical properties. Overall, we observe that lithological aspects alone control regolith characteristics, and that slope angle and exposure to landscape rejuvenation hence play no significant role. In areas with volcanic rock substrate, where the largest, mostly slide-type, landslides develop, stratified weathering profiles are observed. These profiles show a greater weathering depth than those over metasedimentary rocks, where flow- and avalanche-type landslides are more common. The regolith derived from volcanic rocks has higher clay content, greater plasticity and stronger cohesion than the sandy to silty weathering material that overlies the metasedimentary rocks. These preliminary results show that weathering and rock type are more important than landscape rejuvenation in controlling the type of deep-seated landslides

Multi-Temporal DInSAR to Characterise Landslide Ground Deformations in a Tropical Urban Environment: Focus on Bukavu (DR Congo)

Landslides can lead to high impacts in less developed countries, particularly in tropical environments where a combination of intense rainfall, active tectonics, steep topography, and high population density can be found. However, the processes controlling landslide initiation and their evolution through time remains poorly understood. Here we show the relevance of the use of the multi-temporal differential radar interferometric (DInSAR) technique to characterise ground deformations associated with landslides in the rapidly-expanding city of Bukavu (DR Congo). We use 70 COSMO-SkyMed synthetic aperture radar images acquired between March 2015 and April 2016 with a mean revisiting time of eight days to produce ground deformation rate maps and displacement time series using the small baseline subset approach. We find that various landslide processes of different ages, mechanisms, and states of activity can be identified. Ground deformations revealed by DInSAR are found consistent with field observations and differential GPS measurements. Our analysis highlights the ability of DInSAR to grasp landslide deformation patterns affecting the complex tropical-urban environment of the city of Bukavu. However, longer time series will be needed to infer landside responses to climate, seismic, and anthropogenic drivers

Operational assessment of landslide risks in the sprawling city of Bukavu (DR Congo)

The expansion of informal and uncontrolled urban landscapes commonly overlooks the natural constraints from the environment. This is particularly true for urban environments affected by landslides. Landslide risk assessment relevant for urban planning and disaster risk reduction (DRR) strategy requires highly spatially-resolved datasets and approaches. It also requires that both physical and social local aspects of risk are studied in an interdisciplinary manner. Such assessment of hazard risk remains challenging and under-researched in many regions, especially in low- and lower-middle-income countries in the tropics, as it usually requires large and diverse datasets that are frequently unavailable or unreliable. In addition, specifically in urban contexts, human-induced environmental change impacts slope stability. Under these conditions of data-scarcity and land transformation, reliable and detailed landslide risk assessment encompassing the physical and societal aspects in an operational approach strongly relies on expert knowledge. In this research, we assess the risks associated with landslides in Bukavu, a city located in the eastern DR Congo where urban sprawling is high and the problem of landsliding is particularly acute. Firstly, we compiled a comprehensive multi-temporal landslide inventory covering several decades using remote sensing, archives, field survey and interviews with key informants. From this inventory, we derived three hazard zonations with multiple scenarios that allow to consider the interactions between various landslide processes and the role of human activities. Secondly, we obtained detailed socio-economic data from a sample population survey in morphological areas determined by remote sensing. Within two months, 10 specifically-trained local interviewers counted and located nearly 44,000 inhabitants living in about 6,580 households, and collected socio-economic baseline data over 10,880 people from 1,614 households. These demographic data were used to determine the variations in population density (exposure) in the city. These data were also key for the vulnerability assessment. For this, we designed a contextualised vulnerability index capturing the various dimensions of vulnerability with a set of selected indicators aimed at facilitating understanding, replicability and updating of the data collection. By combining hazard, exposure and vulnerability, we produced three risk zonation maps at a very high spatial resolution with the potential to be used operationally: one for shallow landslides, another for deformation within landslides and one for reactivation of deep-seated landslides. The development of these maps, as well as the collection of field-based information were carried out in close interaction with the city authorities and various stakeholders (e.g. civil protection, local community leaders) involved in DRR. A specific effort of awareness raising was also made through the organisation of dedicated workshops and radio programmes, and the implementation of a disaster risk information centre in Bukavu

Landslides, bedrock incision and human-induced environmental changes in an extremely rapidly formed tropical river gorge

Landslides are hillslope processes controlled by natural changing topographic conditions. Landslides are also influenced by human activities. Yet, understanding the space-time occurrence of landslides and their interactions with these typically long-term natural and short-term human-induced controls remains a key challenge in many regions, especially in tropical environments where data scarcity is commonplace. Here we decipher these dynamic processes in the Ruzizi Gorge located in the Kivu Rift (Central Africa), that is an exceptional geomorphological landmark whose origin is associated with the rerouting of >7000 km(2) of drainage area from Lake Kivu during the Holocene. This bedrock river has also seen its landscape disturbed over the past decades by the development of the city of Bukavu (DR Congo). In this study, we combine detailed field observations, historical aerial photographs, archive analysis and satellite imagery to compile a multi-temporal inventory of 385 landslides and constrain their dynamics. We show that extremely high incision rates during the early stage of the formation of the gorge explain the space-time clustering of thousand-year-old large (up to similar to 2 km(2)) landslides, independently from the lithological context. These landslides are currently non-active and poorly eroded. Their deposit areas partly cover the riverbed with boulders, armouring the channel and inhibiting further incision. The landslides that occurred over the last 60 years are shallower slope failures of smaller size and higher mobility. They tend to disappear rather quickly from the landscape, sometimes within a few years. Their distribution is primarily controlled by threshold slopes, lithology, and the past large landslides, the influence of the land use being less pronounced. Overall, the sediment mobilization rates associated with these high frequency landslides significantly outpace the extreme landslide erosion pulse associated with the gorge formation. Our results provide insight on interactions between channel-hillslope coupling and feedbacks among landslide processes and river gorge formation in a unique environment