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In urban areas, networks and life-support systems, such as roads, power lines and drinking water pipes, are key infrastructures which contribute to the functioning of the city. Their damaging or destruction can lead to harmful consequences for the population. Such situation has already occurred in several African cities following the impact of a hazard of natural or human-induced origin. Bujumbura, the economic capital of Burundi, is no exception to such scenario, particularly as a result of geo-hydrological hazards which regularly affect various neighborhoods of the city: floods, flash-floods, river bank collapse, large urban gullies. For instance, the February 2014 flash flood damaged several key infrastructures in the north of Bujumbura, including the National Road number 1, a medium-voltage line and drinking water pipes.The goal of this study is to show how, in an urban system like the city of Bujumbura, the vulnerability of these key infrastructures to geo-hydrological hazards can lead to the fragility of a large territory. This vulnerability is influenced by weaknesses related to their wear or age, their construction technique, and their dependence on external systems for their effective functioning. In addition to their own weaknesses, these key infrastructures are exposed to several natural hazards that can lead to their damage or destruction. In Bujumbura, the geo-hydrological hazards that regularly impact the city are accentuated by the lack or outdated rainwater drainage facilities and the inadequacy of the urban development plan.After listing, locating and characterizing all the infrastructures that play a role in the life of the population in the urban environment, a classification, based on a participatory approach, was developed to select those on which the vulnerability assessment focused. Six forms of vulnerability (intrinsic, dependency, hazard exposure, control capacity, alternative operation, and crisis management) were assessed for each key infrastructure. To assess each form of vulnerability, interviews were conducted with facility managers (28) to supplement the data from the literature and field collection. Geometric weighting was performed to highlight the most vulnerable infrastructure that could disrupt the functioning of others and, by extension, that of the city.The results of this study show that several road sections in the center and at the exit points of Bujumbura, as well as the drinking water treatment plant and the pumping stations, stand out among the most vulnerable infrastructures. Continued efforts are required to maintain, protect, and strengthen these networks and life-support systems in the face of geo-hydrological hazards, as their damage or destruction due to cascading effects caused by dependence on, or wear and tear of, infrastructure can affect a much wider area than just the hazard impact zone.

Disasters related to hydrological hazards are frequent, occur worldwide, and regularly devastate many African cities. The victims are commonly among the population in precarious situations, without solid infrastructure and with incomes too low to recover from disasters.Located in the western branch of the East African Rift, and squeezed between the shore of Lake Tanganyika and steep hillslopes, the city of Bujumbura hosts approximately 800,000 people. It is regularly affected by disasters related to lake and river floods, flash floods, riverbank collapses, and gullies. This research aims to assess people's vulnerability to these hydrological processes through the analysis of the territorial vulnerability of the city. To achieve this objective, we present here the first step which focuses on the evaluation of the exposure to past hydrological disasters.We built a comprehensive dataset using information from different sources, such the Civil Protection, that we combined and complemented with field data collection obtained from qualitative and quantitative surveys. Given the limited information on the susceptibility to the various processes studied, the distribution of past disasters is used as an indicator of the exposure of different parts of the city to hydrological hazards. This phenomenological approach is a key step to map and understand risk due to hydrological hazards.Between 1990 and 2021, the city recorded more than 210 catastrophic events. Flooding was the most frequent hazard. For example, with a rise of more than 2m above its normal level, the Lake Tanganyika flooding in April 2021 has induced the displacement of many people and the abandonment of many houses and recreation centers. Flash flooding and gullying have been reported mostly at the foot of the hillslopes. The collapse of river banks has also caused severe damages to infrastructures, along the major rivers crossing the city.Like many African cities, Bujumbura is characterized by the non-compliance with the law, an inefficient drainage system, the anthropization of the hillslopes overlooking the city, the increasing demand for housing and a lack of structure in its urban expansion. The older neighborhoods of central Bujumbura seem adapted to cope with these types of events, while the peri-urban areas are not. The combination of these vulnerability factors makes Bujumbura more exposed to disasters of hydrological origin.