Depicker, Arthur

Dewitte, Olivier

Govers, Gerard

Havenith, Hans-Balder

Maki-Mateso, Jean-Claude

Van Rompaey, Anton

Landslides are one of the most important natural hazards in mountainous environments. They cause thousands of fatalities every year, especially in the Global South where the social and economic vulnerability is high. Since the beginning of the second half of the twentieth century, a period referred to as the Anthropocene, human activity has increasingly led to changes in the environment, exacerbating both the occurrence and impacts of landslides.
The North Tanganyika-Kivu Rift Region (NTK Rift) is a landslide-prone mountainous environment embedded in the western branch of the East-African Rift. The area encompasses parts of the eastern Democratic Republic of the Congo (DRC), Rwanda, and Burundi. In the past six decades, the population of the NTK Rift quintupled and a large portion of the forests was lost, especially in the DRC. Although landslide casualties are frequent in the NTK Rift, the region has hitherto received little attention in landslide literature, partly due to the scarcity of available data.
In this work, we investigate the spatial patterns of landsliding by providing a first regional landslide susceptibility (LSS) assessment for the NTK Rift. We show that such a regional assessment is much more accurate than the existing global or continental susceptibility maps that are openly available. Most of the added value of such a regional assessment is generated through the compilation of a regional landslide inventory, rather than the model choice or the availability of detailed regional variables. Besides the fact that the vast majority of landslides in the past few decades were triggered by extreme rainfall, we find that the most important driving factors of LSS in the NTK Rift are the slope gradient, moderate seismic activity that weakens and fractures the hillslope material, a concave slope morphology, and land cover (LC) (e.g. the presence of forest cover).
With regard to forests, we observe that deforestation increases shallow landslide activity by a factor of 2 to 8. This landslide wave persists for roughly 15 years. The landslide response to deforestation depends on the geomorphic context of the landscape. Rejuvenated landscapes, for example, exert a weaker response to deforestation compared to relict landscapes. This weaker response can be linked to three factors at play in the rejuvenated landscapes: (i) the occurrence of high-magnitude earthquakes in the past that depleted the most unstable regolith, (ii) a thinner regolith mantle due to the younger age of the landscape, and (iii) fewer extreme rainfall events that can trigger landslides.
We use these new insights to reconstruct how population pressure and forest cover changes have affected shallow landslide risk in the NTK Rift over the past six decades. To do so, we develop a holistic landslide risk model which evaluates 58 years of population and forest cover trends. To reconstruct the forest cover trends, we rely on the historical aerial photographs of 1958 that are conserved at the Royal Museum for Central Africa (RMCA) in Tervuren, Belgium. We show that the current landslide risk in the eastern DRC is twice as high as in neighboring Rwanda and Burundi. Congolese households, on average, populate more hazardous terrain, likely as a result of conflicts and economic pull-factors such as
mining. Moreover, the recent large-scale deforestation of primary rainforest in the DRC has increased the landslide risk by ca. 40 %. This analysis demonstrates how the legacy of deforestation, conflicts, and population dynamics resonates in the shallow landslide risk in the NTK Rift.
Finally, we explore how human disturbances in the landscape have exacerbated shallow and deep-seated landslide mobilization rates (a proxy for landslide erosion). For this purpose, we use the historical aerial photographs of the RMCA to differentiate between 'old' landslides (triggered before 1958) and recent landslides (triggered after 1958) that contributed to the overall mobilization rates in the Anthropocene. We find that humans have a limited impact on these rates (5-18 %). Within rejuvenated landscapes, the construction of roads has the most adverse impact, while, in relict landscapes, mining exerts the largest influence, especially in the eastern DRC. The average landslide mobilization rate in the rejuvenated landscapes of the NTK Rift is ca. 32 m3 km–2 year–1, which is one order of magnitude lower than what is commonly observed in other mountainous areas around the world. This is explained by our long observation period (61 years) and the extent of our study area which is at least twice as large compared to studies in other regions. Hence, we avoid that extreme landslide events skew our observed mobilization rates upward. Moreover, we did not observe any high-magnitude earthquakes which could considerably increase the mobilization rates and are expected to occur sporadically in the NTK Rift, as demonstrated by the dozens of old large (>1 km2) landslides in the vicinity of active faults.
In conclusion, landslides are frequent in the NTK Rift. Most instances are triggered by intense rainfall and controlled by factors such as slope gradient, moderate seismic activity, slope concavity, and LC. Deforestation, for example, leads to a two- to eight-fold increase in shallow landsliding for a period of 15 years. As such, we estimate that, during the Anthropocene, the growing human pressure on forests has led to an increase of 40 % in landslide disaster risk. While the human impact on landslide hazard and risk is large, the effect on landslide mobilization rates is more limited (5-18 %) and associated mainly with road construction and mining activity. In the long term, sporadic high-magnitude earthquakes are expected to considerably increase the observed landslide disaster risk and landslide mobilization rates. This work helps to better understand the landslide processes in a changing tropical environment and the manner in which these landslide processes collide with growing societies.status: publishe

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

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