Exposure to past disasters related to hydrological hazards: the case of Bujumbura city, Burundi

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.