Characteristics and Distribution of Landslides in the Populated Hillslopes of Bujumbura, Burundi

Abstract

Accurate and detailed multitemporal inventories of landslides and their process characterization are crucial for the evaluation of landslide hazards and the implementation of disaster risk reduction strategies in densely-populated mountainous regions. Such investigations are, however, rare in many regions of the tropical African highlands, where landslide research is often in its infancy and not adapted to the local needs. Here, we have produced a comprehensive multitemporal investigation of the landslide processes in the hillslopes of Bujumbura, situated in the landslide-prone East African Rift. We inventoried more than 1200 landslides by combining careful field investigation and visual analysis of satellite images, very-high-resolution topographic data, and historical aerial photographs. More than 20% of the hillslopes of the city are affected by landslides. Recent landslides (post-1950s) are mostly shallow, triggered by rainfall, and located on the steepest slopes. The presence of roads and river quarrying can also control their occurrence. Deep-seated landslides typically concentrate in landscapes that have been rejuvenated through knickpoint retreat. The difference in size distributions between old and recent deep-seated landslides suggests the long-term influence of potentially changing slope-failure drivers. Of the deep-seated landslides, 66% are currently active, those being mostly earthflows connected to the river system. Gully systems causing landslides are commonly associated with the urbanization of the hillslopes. Our results provide a much more accurate record of landslide processes and their impacts in the region than was previously available. These insights will be useful for land management and disaster risk reduction strategies.

Assessment of Landslide susceptibility and risk to road network in Mt Elgon, Uganda

“…The landslide road exposure risk varied spatially in this study area as also reported in other east African highland region (e.g. Kubwimana et al, 2021). The riskiest road exposure segment was the topographically complex Bulucheke-Bukalasi area in Bududa district.…”

Landslides and Gullies Interact as Sources of Lake Sediments in a Rifting Context: Insights from a Highly Degraded Mountain Environment

“…Our research is one of the very few that clearly demonstrate the role of landslides as processes favoring the occurrence of gullies [20,27,76], especially over such a large region. Of course, we cannot ignore the role of landscape rejuvenation within the scheme of landslide-gully interaction.…”

“…With respect to landslide activity, we identified two populations: active landslides that are relatively small and shallow, and large deep-seated landslides that are inactive. Small active landslides are slope-related processes whose signature in the landscape can disappear quickly, sometimes over a few years due to vegetation regeneration and land reclamation [75,76]. The oldest image in Google Earth used for the mapping being from 2016, we thus argue that the inventory of these small active failures provides information on the state of the landscape over a period of ~10 years.…”

“…In some cases, gully extensions of more than one kilometre in a few decades have been reported by [80]. When a gully forms, it can remain active over several decades [76,81]. For active gullies, we therefore interpret most of them to correspond with current natural environmental conditions, although the timing of their initiation and rate of expansion remains unconstrained.…”

“…Here, we further consider the seismotectonic control on the occurrence of old bedrock landslides through the analysis of the rejuvenated landscapes created by the upstream migration of fault-related knickpoints. Faults, fractures, knickpoint migration and landscape rejuvenation are parameters that act as environmental controls on bedrock landslides in other regions [31,34,83], including in the context of a rifting system such as the central portion of the western branch of the East African Rift [24,76].…”