Geochemistry and preliminary Sr-Nd isotopic data on the Neoproterozoic granitoids from the Bantoum area, west Cameroon: evidence for a derivation from a Paleoproterozoic to Archaean crust

The Bantoum area in west Cameroon is composed of migmatitic gneisses associated with parallel strips of amphibolites,quartz-monzonites,biotite-granites, two-mica leucogranites and granitic dikes.Quartz-monzonites are metaluminous (A/CNK=0.8-0.9)I-type,biotite-granites are peraluminous (A/CNK=1.0-1.10)I-type, leucogranites are peraluminous (A/CNK=1.14)S-type granitoids.All are hyper-potassic rocks defining a calc-alkaline trend.Quartz-monzonites gave an Rb-Sr isochron age of 720+-61 Ma assumed to be a mixing age.The thermometry estimated from major elements and zircon saturation indicate that the biotite-granites crystallized from high temperature melts (812-866゜C) whereas leucogranites crystallized from low temperature melts (719-745゜C). The trace element distribution diagrams are characterized by an enrichment in LILE and LREE (5<La_N/Sm_N<17),with negative Nb,Ta,Sr and Ti anomalies. Model initial ^87Sr/^86 Sr ratios (620 Ma)are 0.707614-0.708363 for quartz-monzonites,0.711242-0.713784 for biotite-granites,and 0.715835 for leucogranites.They have highly negative ε_Nd (620 Ma)(-19~-11) and T_DM model ages ranging from 1.9 to 2.9 Ga. These geochemical and isotopic features imply that the granites are generated at different temperatures and from different crustal materials;they are the witnesses of the recycling of a Paleoproterozoic to Archean crust with minor inputs of juvenile magmas during the Pan-African orogeny. Chemical similarities between gneisses and some biotite-granites suggest that the partial melting of these gneisses may have contributed to the formation of granites

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

Quantifying the direct impacts and risks of large urban gullies in the Democratic Republic of Congo

peer reviewedLarge urban gullies (UGs) cause major infrastructural damages and often claim casualties in many tropical cities of the Global South. Nonetheless, our insight into this new type of geo-hydrological hazard remains limited to some case studies and the overall impacts remain poorly quantified. Here, we aim to bridge this gap by making a first assessment of the number of persons affected by urban gullies at the scale of the Democratic Republic of Congo (DRC). We used Google Earth imagery in combination with local news sources and earlier research to identify 25 cities in DRC where UG occur at a significant scale (at least ten UGs). This list is likely exhaustive. Next, for each of these cities, we used Google Earth imagery and other high resolution satellite images to map all visible UG, evaluate their expansion rate and inventorize detectable damages to houses and roads. In total, >2,000 UGs were mapped across the 25 affected cities. Overall, the problem of UGs in DRC is especially acute in the cities of Kinshasa, Mbujimayi, Kikwit, Tshikapa and Kananga. Over 90% of these gullies were active during the observation period (typically from 2002 to 2020). Next, we assessed the total number of persons that are directly affected, as well as the number of persons currently at risk. Using available high resolution population density data and taking into account the current position of urban gullies, we estimate that around 68,700 people were directly displaced due the formation and expansion of UGs over the last 15 years. This corresponds to an average of ca. 4,300 persons per year. By considering the population that lives in the direct vicinity (<100 m) of an UG, we estimate that around 1.3 million people in D.R. Congo are currently at risk and/or experience significant impacts because of UGs (e.g. reduced land value, problems with trafficability, stress). This number has doubled over the past 10 years (2010-2020) and will likely continue to increase as a result of urban expansion and climate change. Overall, this research shows that urban gullying is a very serious problem in the Democratic Republic of Congo, but likely also in many other countries of the Global South. More research is needed to better understand this processes and, ultimately, to prevent and mitigate its impacts. The results and the database of this study provide an important step towards this

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

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