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Article

Geo-Hydrological Hazard Impacts, Vulnerability and Perception in Bujumbura (Burundi): A High-Resolution Field-Based Assessment in a Sprawling City

by
Jean Nsabimana
1,2,3,*,
Sabine Henry
2,
Aloys Ndayisenga
3,
Désiré Kubwimana
4,
Olivier Dewitte
1,
François Kervyn
1 and
Caroline Michellier
1
1
Department of Earth Sciences, Royal Museum for Central Africa, Leuvensesteenweg 13, 3080 Tervuren, Belgium
2
ILEE (Institute of Life-Earth-Environment), Department of Geography, University of Namur, Rue de Bruxelles 61, 5000 Namur, Belgium
3
CREDSR (Centre de Recherche et d’Etudes sur le Développement des Sociétés en Reconstruction), Department of Geography, Environment, and Population, University of Burundi, Avenue de l’Unesco N° 2, Bujumbura 1550, Burundi
4
Department of Earth Sciences, University of Burundi, Avenue de l’Unesco N° 2, Bujumbura 1550, Burundi
*
Author to whom correspondence should be addressed.
Land 2023, 12(10), 1876; https://doi.org/10.3390/land12101876
Submission received: 4 September 2023 / Revised: 24 September 2023 / Accepted: 3 October 2023 / Published: 6 October 2023
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Abstract

:
Rapid urbanization, demographic pressure, and sprawl of cities are key factors in the vulnerability and damage related to geo-hydrological hazards. Dysfunctional urban services that favor informal settlements are at the forefront of elements that increase vulnerability. Cases of cities that suffer from geo-hydrological hazards are increasingly reported in many regions, especially in tropical countries in the Global South. Yet, studies on such examples are rare and commonly overlook the human and societal components of hazard risks. Here, we focus on Bujumbura, a city in Africa that has experienced rapid unplanned growth and sprawl into unserviced areas because of the non-application or a lack of a valid urban planning law. After filling in the gap in data collected using high-resolution field surveys and focus group discussions, this study highlights various factors of vulnerability to geo-hydrological hazards in the urban area. Indeed, 108 events of flood and flash floods and 81 gullies were inventoried in Bujumbura between 1997 and 2021. These geo-hydrological hazards have had a significant impact, particularly on housing, and have caused increasing displacement of the population. This vulnerability is exacerbated by the inefficiency of the rainwater drainage system in the urban space. Our result demonstrates how the failure of the institutions responsible for urban management is at the top of all the causes of the vulnerability of the sprawling city. We anticipate that our empirical approach is an effective way to obtain concrete information to develop practical strategies to prevent and mitigate vulnerability to geo-hydrological hazards in urban sprawling contexts.

1. Introduction

In 2018, around 55% of the world’s population lived in cities [1], and projections show that it will increase to 68% by 2050 [2]. The largest increase is projected to occur in developing countries in the tropics, especially in Africa [3]. In cities in the tropics, it is common for many households to be poor and live in informal settlements [4,5,6,7,8,9]. As a result of the unplanned rapid urban growth, cities are expanding towards marginal areas, without servicing, accessible to low-income people [8,10]. Urban sprawl and the development of informal settlements impact the quality of life of the population: poor building conditions, lack of basic infrastructure for proper sanitation, inadequacy of rainwater drainage systems, etc. [11,12].
The rapid pace of urbanization that follows population growth increases the exposure to and, therefore, the impacts of geo-hydrological hazards such as (flash-) floods and gully erosion [13,14,15,16]. Furthermore, urban growth impacts the hydrological system through soil sealing and road construction, for example, which in turn affects urbanized areas by flooding, flash flooding, and gullying [17,18,19,20,21,22,23,24].
Many cities around the world have already fallen victim to natural hazards, particularly geo-hydrological hazards [14,20,25,26,27]. American cities such as Boston, Chicago, New Orleans, Oklahoma City, Philadelphia, and Washington, DC, have already been severely affected by (flash-) floods [23,25,28,29,30,31]. In New Orleans, Masozera et al. [31] documented damage caused by Hurricane Katrina in 2005. In Latin America, Mexico city and Quito are both threatened by hydrological, volcanic, and earthquake hazards [25,32]. Asian cities have also suffered severe losses of lives and infrastructures because of flooding in Delhi, Mumbai, and Chennai in India, rainstorms in Shenzhen in China, and Kuantan in Malaysia, to name but a few [33,34,35,36,37]. Moreover, as the studies by Koks et al. [38], Paliaga et al. [39], and Marchi et al. [40] show, European cities are not spared from geo-hydrological hazards either.
Examples of cities that regularly suffer from geo-hydrological hazards are increasingly reported in tropical countries, and Africa is no exception, with significant examples in Nigeria [9,41], DRC [42,43], Mozambique [44], and Uganda [22,45]. However, these examples are rare, and they are usually oriented towards characterizing the risk equation’s hazard component, i.e., the natural process involved. In addition, these studies usually focus on a single type of hazard that is investigated in isolation. Yet, in reality, in a city often plagued by multiple hazards, solutions for isolated hazards are not able to solve the whole problem. As such, they clearly overlook issues related to the impacts and vulnerability associated with geo-hydrological hazards, and the population’s perception of the causes of their vulnerability is commonly absent [46,47,48].
Vulnerability is a concept that originates in the functioning of a city itself [49]. It is directly related to the exposed elements (assets and people) but also to the way a society organizes itself and its territory. According to Metzger and D’Ercole [50], vulnerability is the set of weaknesses or failures that can degrade a major asset, interrupt its functioning, or even destroy it. Vulnerability is dynamic, complex, multidimensional, and of essential interest in research on hazards and associated risks [51,52,53,54,55,56].
Knowledge of what and how people perceive as causes of their vulnerability to hazards is necessary to offer effective mitigation strategies for their impact [57,58,59]. Perception is an intuitive judgment based on the process of collecting, selecting, and interpreting signals concerning the uncertain impacts of events and choosing how to respond [60,61]. In other words, perception is an opinion of individuals, a construction based on the acquisition of knowledge about hazards or disasters. By living in an area with a high frequency of geo-hydrological events, the population acquires a level of experience and behavior that allows them to live and cope with these hazards [57,62,63]. Two main approaches are used to assess perception of disasters associated with natural hazards. First, the psychometric paradigm [64,65,66] focuses on individual cognitive processes and assumes that the existence of a threat leads an individual to make an assessment [66], and second, qualitative understanding of natural hazard disaster perception [67]. In this latter, which we adopted in this study, the perception of natural hazard disasters is seen as socially constructed and closely based on social and cultural interpretation [66,68].
As mentioned above, research carried out on this issue is limited and often disregards the anthropogenic aspects. They are rarely based on comprehensive, in-depth, field-based investigations, therefore missing the goal of delivering high-resolution outputs that could be relevant for operational decision support and disaster mitigation strategies [46,69,70,71]. The goal of our research is to contribute to filling this gap in hazard risk research by assessing the impacts, vulnerability, and perception of geo-hydrological hazards in the context of sprawling cities. Our study will contribute to improving the understanding of the driving forces behind vulnerability to these hazards and provide city authorities in charge of urban planning and disaster risk management with the knowledge they need to take thoughtful and effective measures to reduce vulnerability and impacts of these often-interacting hazards. We focus on the city of Bujumbura (Burundi), a place in tropical Africa recurrently affected by multiple hazards. Bujumbura is indeed known for being impacted by river and lake flooding, flash floods, and gully erosion, i.e., processes that often occur in interactions [72,73,74,75]. Here, we look at these four types of processes. Taking into account the evolution of the city over several decades, we first analyze the spatio-temporal distribution of the hazard impacts. Then, we determine the vulnerability factors in the urban area of these phenomena. Lastly, we focus on understanding how the population in frequently flooded or gullied areas perceive the causes of their vulnerability to such hazards. Field-based investigations and interviews are at the core of our study.

2. Study Area

Located in the western branch of the East African Rift, the choice of Bujumbura as a settlement was linked to the presence of Lake Tanganyika (Figure 1; [76]). Founded by the Germans in 1897 and developed by the Belgians from 1916 to 1962, Bujumbura has expanded considerably in space after Burundi’s independence [77]. Its extension, long dictated by natural constraints (north-south direction), now involves the steep slopes to the east and the shoreline of Tanganyika Lake to the west, originally reserved for woodlands and recreation.
The city of Bujumbura has long been the political and economic capital of Burundi until 2019 when it remained with the sole status of economic capital. As a result, housing all the country’s political and economic structures, the major hospitals, and other public and private institutions has attracted the population of the whole country. However, this strong attractiveness has not been accompanied by a sufficient supply of housing, whose last development dates back to the 2000s. Under these conditions, all the unoccupied spaces were invaded without any supervision of the urban planning services [10,78].
Bujumbura is a macrocephalic city undergoing major changes because of its high population density (almost 7500 inhabitants/km2) (Table 1, [79]). Currently, the urban area occupies approximately 114 km2, divided into 97 neighborhoods grouped into 14 municipalities and 3 communes, namely, from north to south, Ntahangwa, Mukaza, and Muha (Figure A1 and Table A1). The city expansion was accompanied by strong population growth, from about 23,000 inhabitants in 1955 to more than 770,000 in 2021 (Table 1, [79,80]).
In recent decades, Bujumbura has become the scene of impactful events associated with the occurrence of river floods, flash floods, and large gullies [72,73,74,75]. These hazards occur mainly during the rainy season, from October to mid-December (short wet season) and from mid-February to June (long wet season). To document the amount of precipitation, only the Bujumbura airport weather station provides data, which become publicly available when they concern extreme events, as they are then published in disaster reports. On the event of 9 February 2014, according to Nkunzimana et al. [75] and the Burundian government [84], 80 mm of rain fell in the Bujumbura region between 8 p.m. and 11.30 p.m. Nkukimana et al. [75] calculated that it represents 1/10 of the annual total recorded at the Bujumbura airport weather station, for the event of 11 November 2009, 142 mm of rain were recorded between 6 a.m. and 12 p.m. by the airport weather station [75]. Smaller, localized rainfall events are regularly recorded, but as they fortunately do not always turn into large-scale catastrophic events, rainfall data are less readily available. A particular threat to the western area of the city is the fluctuating height of Lake Tanganyika (up to a few meters), which regularly leads to year-long flooding in certain neighborhoods; the variability of this process is associated with rather complex climate patterns [85]. In the face of these events, many houses, power lines, roads, water supply systems, economic and social infrastructures (schools, health centers, markets), rainwater drainage systems, and crop fields (rice growing, for example) are at high risk [75,84].

3. Materials and Methods

A mixed-method approach, combining qualitative and quantitative data collection, was used to analyze the spatio-temporal distribution and impacts of these geo-hydrological hazards, determine the factors of vulnerability, and analyze how people usually affected by those hazards perceive their vulnerability.

3.1. Urban Growth Patterns

To analyze the evolution of Bujumbura’s spatial extension, we used direct observations, previous publications, and visual analysis of documents, such as the three Master Plans for Urban Development and Planning (SDAU) of Bujumbura [82,86] and historical maps, complemented and enhanced with recent information (2016 Pléiades image, OpenStreetMap and 2021 ©Google Earth imagery). These data allowed us to map the sprawl of Bujumbura and the phenomenon of peri-urbanization. The information used was taken mainly from Cazenave-Piarrot et al. [78], Groupe Huit/SHER [82], Mboga [87], Mboga et al. [77] and Ndayirukiye [76]. In addition, through interviews with engineers from the Agence Burundaise des Travaux Publics (ABUTIP) and the sanitation department of the Office Burundais de l’Urbanisme, de l’Habitat et de la Construction (OBUHA), supplemented by field observations and grey literature [88], we updated the database on the rainwater drainage system, revealing its failures and their implications for flooding.

3.2. Existing Quantitative Data of Geo-Hydrological Hazard Events

To determine the spatial distribution of geo-hydrological hazard events and the subsequent damage, we collected documentary information from an exhaustive panel of sources, mostly public institutions, UN agencies, NGOs, and Ph.D. theses available at the University of Burundi’s library. Key quantitative data come mainly from four sources: monthly reports on geo-hydrological events collected by the municipal Civil Protection Police coordination, the Displacement Tracking Matrix (DTM) published by IOM-Burundi, data published on ReliefWeb by OCHA-Burundi, and the Red Cross. Moreover, this was complemented by data available on social media (Table 2).
Due to the temporal bias in data collection, we used information from a complete period (2018–2020) to analyze the damage incurred to the (flash-) flood hazards and supplemented this inventory with a field survey conducted among the population (see Section 3.4 for further explanation).

3.3. Field Campaign

An intensive 42-day field campaign was conducted between 19 October and 23 December 2020 and between 7 and 28 January 2021 to inventory and delineate areas affected by past flooding and gullies, respectively. Regarding gullies, we analyzed their activity status (active, stable) and, when present, the nature of their biological or structural stabilization. We also identified damage to infrastructure (houses, roads, water pipes, electricity poles) and property, as well as the distance between the infrastructure and the gully, in order to determine the level of exposure of these infrastructures. The survey also aimed to identify the causes of these hazards. Google Earth imagery was also used to constrain the period of occurrence in these gullies.
For the floods and flash floods, the field survey consisted of recording GPS coordinates in order to delimit the affected areas. Flood maps also combine field inventory and Google Earth imagery.

3.4. Population Survey

3.4.1. Quantitative Data through Inhabitant Interviews

We used a non-probability (snowball) sampling method [89,90,91] to interview 46 inhabitants in eight municipalities, comprising 11 neighborhoods (Figure A1 and Table A1), that had experienced one or more hazards in the past years (Figure A1 and Table A1). The selection of these municipalities and neighborhoods was motivated by the information in Section 3.1, and the reports from the National Platform for Disaster Risk Prevention and Management obtained through the General Directorate of Police and Civil Protection. Neighborhood leaders assisted us in precisely targeting areas affected by past disasters. The interviewees, who were all heads of household, were recruited exclusively within these areas.
Individual interviews were conducted in Kirundi, based on a questionnaire first developed in French and using the Kobocollect application installed on a tablet. The questionnaire covered several topics, including (1) the type of natural hazard disaster recorded in the area, (2) the damage caused by the past disaster(s), and (3) the type of building materials used to construct houses in order to understand the level of vulnerability to flooding. Next, several questions related to people’s awareness and risk perception were asked, such as (4) whether they were aware of living in an area prone to geo-hydrological disasters and, if so, why they did not move, (5) whether they were informed of the area’s exposure, (6) whether there was an emergency service that could help in the event of disaster, (7) the frequency and violence of geo-hydrological events, and (8) their perception of what might amplify water-related disasters. Based on people’s testimonies and field observations, we also estimated the distance between the infrastructure and (1) the element causing the flood or the flash flood (river or undersized drains) or (2) the gullying area. In addition, this data collection process enabled us to assess the extent of damage and identify the infrastructures affected by (flash-)flooding and gullying. A final question focused on the apparent causes of the geo-hydrological hazards, highlighting the torrential runoff, overflowing of rivers and streams running through the city, and a lack of rainwater drainage.

3.4.2. Qualitative Data through Focus Group Discussion

To understand the population’s perception of the causes of their vulnerability to floods, flash floods, and gullies, a qualitative approach was developed. Focus groups reveal collective perceptions, highlighting the point of view widely shared by the participants [92]. Twenty focus groups were organized between 23 November 2020 and 27 January 2021: 16 for floods and flash floods and 4 for gullying (Table 3) in 13 neighborhoods out of a hundred, targeted because they experienced a geo-hydrological hazard (Figure A1 and Table A1).
According to Kitchin and Tate [93], each group consisted of three to five people who knew each other. These people were randomly selected in the neighborhoods where they live or work—close to the area of a past disaster. Data recording began after a presentation of the purpose and process of the study and obtaining the participant’s consent. A total of 79 people (~60% male) with an average age of 37 years participated. Forty-three percent of the participants were employed in agriculture (rice farming) and small trade; the rest were manual workers, masons, and students. Each focus group lasted between 45 and 55 min.
Participants were asked to share their perceptions on the occurrence of fluvial, pluvial, and lake flooding, flash floods, and gullies, including their causes and impacts. During each focus group, participants discussed until they reached a consensus. The longer the discussions went on, the more the participants came up with new ideas that reinforced what they had already discussed. Our interventions were limited to reframing the discussion when the group was straying from the topic.
First, we transcribed the discussions from Kirundi (the local language) into French before applying an open coding using the inductive generalization approach to develop the discourse (verbatim) based analysis grid [94]. An analysis grid was constructed and filled in based on isolated keyword segments sorted from the verbatim. In total, twenty-five categories/keywords for flooding and flash flooding and fourteen categories for gullying were formed; as some categories were broad and unwieldy, further refinement was necessary to be usable [93]. Categories were merged or combined to find new categories. The final stage of analysis involved linking the different categories according to similarity or meaning (synonym) to see how they fit into the original transcriptions. After this stage, nine categories were retained, representing the perceived causes.

4. Results

4.1. Bujumbura Urban Sprawl

Since its independence, the area covered by Bujumbura has expanded from 19 km2 to 37 km2 in 1983 [82], 46 km2 in 2000 and 109 km2 in 2020 [87]. In 2021, it was at 114 km2 (Figure 2). The sprawl has been accompanied by an increase in the population, which has more than doubled in the last two decades, from around 353,000 inhabitants in 2002 to over 790,000 in 2022 (Figure 3).
The city sprawls mainly to the north and south and a little less to the northwest in areas that are generally flat (Figure 2 and Figure A2). To the east, the growth occurs on much steeper terrain with slopes less favorable to built environments.

4.2. Distribution of Geo-Hydrological Hazards

Since 1997, 242 occurrences of floods and flash floods have been recorded in Bujumbura. Some of these occurrences occurred on the same day, leading to an inventory of 108 different events (Figure 3). Between 1998 and 2000, as well as in 2005, no observation was recorded despite similar annual rainfall. The number of recorded events increased from 2009 onwards. Flood and flash flood data recorded by IOM-Burundi between January 2018 and 2020 are the most recent and up-to-date. They provide an accurate insight into the impact of floods, such as that of 11 November 2009 and 9–10 February 2014, which caused enormous damage to the population and infrastructure, as noted by Nkunzimana et al. [75]. Other less significant events are poorly documented, and damage is poorly and often less accurately reported.
Over the past 10 years, several events have severely impacted the city and its functioning (Figure 3 and Figure 4a,b). Such events appear to be more frequent in the north of the city. Overall, these events generating the most damage are more likely to be recorded by different institutions. Flooding and flash flooding seasonal recurrence are reflected in the magnitude of damage caused by the events. Our results show that 43% of floods and 39% of flash floods are identified by the population as particularly violent. Moreover, 81 gullies are identified on the foothills to the east of the city (Figure 4a). Among these gullies, 32% originated before 2007, including very old gullies from the 1950s, 43% of them between 2019 and 2021, and more than 50% are still active (Figure 4b), i.e., with ‘little to no natural vegetation on the gully walls, evidence of recent channel incisions’ [69].

4.3. Impacts of Geo-Hydrological Disasters

Our quantitative survey shows that floods and flash floods affected about 4200 people and destroyed 1400 houses between 1997 and 2021. Damage to housing is mentioned to a greater extent than for other infrastructures, farms, and livestock losses (Figure 5a).
According to data recorded between 2018 and 2020 by IOM and Civil Protection, more than 20,000 houses were affected by the geo-hydrological events, especially flooding and flash floods, including almost 4000 completely destroyed houses, 8300 partially destroyed houses, and about 7900 flooded houses (Figure 5b). More than 67,000 people were affected by floods and flash floods (Figure 5c); 31,200 could stay in their houses after the event, while 36,270 were temporarily displaced in the city, sometimes for a few years before their houses were repaired. Moreover, at least 55 died and 98 were injured. According to these institutional data, the Ntahangwa commune (north and northwest) alone accounts for about 80% of the recorded damage, while those of Muha (south) and Mukaza (center) communes account for 20% and less than 1%, respectively. In terms of material damage, Buterere is the most impacted municipality, located in the Ntahangwa commune, followed by the Kanyosha municipality of the Muha commune. The more devastating events occurred in 2019 and 2020, affecting and destroying many houses.
Regarding gullies, almost 640 ha of land were affected over the past 25 years. Our field data show that it led to the destruction of more than 200 houses. In 2022, about 3700 houses were highly exposed to gully erosion, these houses being located in an area just a few meters (maximum distance of 20 m) away from the gullies. The houses and agricultural fields destroyed by the gullies are located in the eastern municipalities where slopes become steeper. Gullies also threaten and impact infrastructures such as schools, health centers, water supply pipes, and roads (Figure 5d).

4.4. Identified Factors of Vulnerability to Geo-Hydrological Hazards

Data from the quantitative survey reveal what the population living in the previously affected areas perceive as aggravating factors of vulnerability. In order of importance, the path of rainwater drainage systems in new neighborhoods and the lack of urban planning were mentioned by 33% of respondents, while 20% mentioned the lack of rainwater drainage and 17% the spontaneous construction in marginal areas. Indeed, the absence or inadequacy of rainwater drainage infrastructure in Bujumbura contributes to the city’s increasing vulnerability to geo-hydrological hazards (Figure 6). The level of development of rainwater drainage infrastructure varies between municipalities (Figure 6a; [95]). The northwest of the city has no rainwater drainage infrastructure (Figure 6a,b,d), while older municipalities in the city center are relatively well-equipped. These latter benefit from the highest density of rainwater drainage (27 km/km2), with drains aligned with the road network. In the other municipalities, such infrastructure remains insufficient, with a density varying between 0 and 7 km/km2, regardless of the population density (Figure 6a). Furthermore, where it exists, it is commonly undersized and unable to handle the flow of runoff water after rainfall (Figure 6b,f). Indeed, the neighborhoods most affected by flooding, recurrent flash floods, and gullies are often the most recent, which have not benefited from an appropriate development of servicing. These neighborhoods are located in the north, east, and south. They correspond to areas affected by urban sprawl. The most emblematic is Buterere municipality, created in 1994 as a site for the displaced population, without any form of service since then (Figure 2 and Figure 6a).
Our field survey shows that most of the rainwater drains are several decades old, and damage is common. In addition, in many areas of informal settlements, these rainwater drains are filled with sand and other materials from the heavily anthropized upstream areas. In the city center, drains are buried, and some are clogged with rubbish and plastic because of a lack of long-term maintenance. This leads to floods, even after a small amount of rain (Figure 6c). As an example from our field survey, Figure 6e shows a rainwater drain that has developed into a gully. Similarly, the construction of the Kidumbugwe and Gasenyi river canals has not been completed and has been abandoned in the rice fields of Kinyankonge, upstream of a densely populated area. These canals are nowadays responsible for flooding in several neighborhoods of the northwestern municipalities.

4.5. Perceived Causes of Vulnerability to Geo-Hydrological Hazards

The analysis of the qualitative survey data based on the 20 focus groups allows us to distinguish common perceived causes of flooding, flash flooding, and gullying. Overall, the focus group participants identified (1) pressure from the often-poor population associated with the densification of the built-up area, (2) urban expansion on marginal slopes and lack of control in land-use planning and plot sale, and (3) lack of, undersized, unfinished or aging of rainwater drainage systems, in adequation with the quantitative survey conducted among the previously impacted population (Table 4).
The level of perception of the causes varies depending on the municipality (Figure 7). For the northwestern municipalities (Buterere and Kinama), the population’s perception is more oriented toward the inappropriate rainwater drainage system and the undersized infrastructure. For example, as emphasized by the participants in the focus group N°4: ‘the problem is that they are building pipes upstream that discharge the water into the Kinyankonge river, forgetting that the downstream part is inhabited’. Similarly, ‘… the branch of the Kinyankonge that passes next to the Islamic High School overflows and floods the Mutakura-Buterere road because of the section of the pipes that cannot evacuate the entire quantity of water…’. This issue was also mentioned by other focus groups from the north of the city. Another case is given by the participants in the focus group N°3, who reveal that ‘[…] in 2015, the unfinished construction of the Gasenyi river canal downstream of the Carama neighborhood and the Kidumbugwe river canal were for us and the inhabitants of Buterere synonymous with ordeal, while it was a solution for the inhabitants of the parts where the canals are built and completed. Often, we are flooded when it hasn’t even rained in our neighborhood’.
The perception of the population in the southern area (Kanyosha and Kinindo) further links the problems of the rainwater drainage systems with the overflow of the Kanyosha river (Figure 6f) and the Mugoyi canal to demographic pressure. The focus group N°12 reveals that ‘the exploding urban population growth is the only reason that generates the other factors. Before, the hills were forested. The expansion of the city is the cause of the torrential nature of the Mugoyi canal’. The focus group N°6, which gathers people living in an area dominated by rice fields in the northern part of the city, mentions urbanization and uncontrolled occupation as causes. For this group, ‘the population is partly responsible for the causes of the flooding, especially the owners of the rice plots who, in order to enlarge their plots, gradually move the dikes, thus narrowing the Kinyankonge riverbed, which hence divides into several branches that cross the Buterere municipality’.
Our quantitative and qualitative surveys were complemented with regular field visits. These allowed us to document the rising water level of Lake Tanganyika, which was mentioned in focus group N°16. A rise of over two meters above the normal of 774 m was recorded, affecting more than 2560 people in three municipalities. As a result of the flooding, approximately 560 people were forced to leave their homes (partially or permanently). According to the focus group, ‘[o]ver the years, the water in Lake Tanganyika has gradually decreased and people have followed the decreasing water level and built up to the edge of the lake. Now, Lake Tanganyika is reclaiming its former bed and flooding houses. Many recreational areas such as the Lacosta bar, Saga Vodo have been completely flooded and abandoned’. The area along Lake Tanganyika was not prepared to face such flooding issues.
Finally, the risk reduction strategies highlighted by interviewees emphasized the need for better urban management and planning to avoid sprawl into hostile areas, as well as the updating and enforcement of all existing urban laws. To these measures, they added the development of an efficient drainage and rainwater evacuation system throughout the city.

5. Discussion

5.1. An Unprecedented Geo-Hydrological Hazard Event Inventory—A Reliable Data Source?

In the context of data scarcity, we have built an inventory of geo-hydrological hazard events. The inventory is based on information available in institutions or published by various local and international media associated with population surveys and fieldwork. However, although this inventory is unique and brings an unprecedented level of information for the city of Bujumbura, we must keep in mind that these data come from multiple sources of information and present some biases [53,73,96,97].
First, difficulties are related to the incompleteness of these reported data [2,96]. As noted by Michellier et al. [53] and Monsieurs et al. [73], the lack of a systematic recording of all events is one of the main causes of these data’s incompleteness. For example, in the 2020 monthly reports of the Municipal Police Coordination, only the months of January and December recorded cases of floods and flash floods; the other months were considered without disasters, while the IOM recorded several occurrences grouped into eight events in the same year. Such incompleteness leads to a misrepresentation of the spatial distribution and the frequency of events related to these hazards and, thus, to a minimization of the assets’ exposure. Moreover, the apparent recent increase in geo-hydrological events (Figure 3) cannot only be explained by the unplanned growth of Bujumbura, which has increased exposure to geo-hydrological hazards, but also by improved recording by the various institutions.
Moreover, this study supports previous observations [53,73,96,97,98] that the lack of geospatial coordinates of each event and its impacts is also a weakness. For an effective response, well-localized geospatial data are required to better understand and assess population and infrastructure exposure and vulnerability. In several sources of information, the location was only textually expressed, either by the smallest administrative unit (neighborhood) or by a more precise place name. Despite their imprecision, the information collected in the institutions enabled us to create an inventory to design maps and analyze the spatio-temporal distribution of floods and flash floods (Figure 3 and Figure 4a) and to assess their impact (Figure 5).
Another bias is due to the inaccuracy of the extent of the impact and damage reported. Indeed, the results confirm that the quality and accuracy of these data also depend on the magnitude of the event [96,97,98,99]. As Monsieurs et al. [73] have already pointed out, this observation does not apply to very large-scale events, which have always been particularly well documented. This was the case for old events, such as the flash flood of 9–10 February 2014, which, over our study period, was the deadliest and most destructive disaster in terms of the number of houses, socio-economic infrastructure, and homelessness it caused. More recently, the rise of Lake Tanganyika was also very accurately recorded. As data on the level of Lake Tanganyika were collected daily by IGEBU for the period from 1 January 2018 to 30 April 2021, it made it possible to monitor the evolution of the submergence zone (Figure 4a). Referring to other episodes of lake rise [83], the peak level reached on 27–28 April 2021 (776.5 m a.s.l.) ranks third, behind the episode of 1962–1964 (777.10 m a.s.l) and that of 1878 (783.6 m a.s.l). In addition to these hazard-related data, impacts due to such an exceptional event were also detailly reported, which makes it a benchmark to follow for reporting future events, even of lesser intensity.

5.2. Vulnerability Factors—Insights from People vs. Reports and Field Observations

In many cities in the Global South, population growth has led to rapid expansion of urban areas, resulting in (1) occupation of marginal areas, (2) unplanned urban development, and (3) inefficiency of rainwater drainage system [8,12,100,101].
In 2008, 497,000 inhabitants lived in Bujumbura, while in 2020, this figure exceeded 759,900 inhabitants [80]. Since the political crisis of 1993, migrants fleeing the war have settled in the city of Bujumbura, accelerating its spatial expansion and peri-urbanization towards the north and the escarpment to the east [76,78]. As a consequence, new buildings have encroached on agricultural land, transforming hundreds of hectares into residential areas in the eastern, northern, and southern parts of the city [102]. The anthropization of the Mirwa foothills, whose steep slopes overlook the city, has increased yearly. In addition, the buffer zones left by the lake and along the rivers and streams that flow through the city of Bujumbura are also occupied. As mentioned in the focus group discussion and reported in the literature [8,103], this situation is exacerbated, on the one hand, by the high cost of a plot of land in some neighborhoods, combined with the long waiting period for land and building permits and, on the other hand, by cheaper and more easily accessible land in marginal areas. As explained by Pelling [104], household poverty, a housing crisis, and marginalized communities, such as displaced people, are elements that contribute to vulnerability. People choose between the cost of living and the hazard, which they consider as a minor one-off phenomenon, and settle in the areas that are clearly at risk. In other words, such rapid and uncontrolled expansion of urban areas is a driving force for not only increased exposure but also vulnerability to geo-hydrological hazards [2,101]. As noted by Douglas [5], this intense upstream urban development conditions the level of vulnerability of the entire urban area to geo-hydrological hazards. Indeed, the conversion of agricultural or natural areas into built-up areas, including marginal ones, contributes to the sealing of the urban space and thus increases the susceptibility to gullies and flash floods [19,43,105,106].
In addition, consistent with the literature [8,107], this research emphasizes that the non-application or the lack of urban planning law further multiplies the factors of vulnerability. If Bujumbura already had three master plans for urban development and planning (SDAU) developed in 1956, 1983, and 2014, due to a lack of legal framework for its implementation, ‘the 2014 one has already become obsolete’ said Burundi’s Minister of Infrastructure at the Council of Ministers meeting on 24 November 2021. Despite the development of a new SDAU for the period 2016–2045 (neither yet approved nor implemented), the city’s development is hampered by the absence of a valid, up-to-date urban planning law, which prevents it from countering acute urban sprawl, with informal settlements proliferating on the northern and southern outskirts. In addition, the non-intervention of the urban planning authorities in the sale of vacant plots leaves owners to sell their plots themselves without necessarily leaving space for services, water pipes, and electricity lines. According to Groupe Huit/SHER [82], more than 39% of the urbanized area is occupied lawlessly without benefiting from adequate prior servicing development. In other words, the lack of land use planning has led to the densification of houses, which contrasts with a lack of servicing. Another important perceived cause of vulnerability is the non-application of urban planning norms and other environmental and sanitary regulations. For example, the 1983 SDAU reserved the shores of Lake Tanganyika and the mountains above the city of Bujumbura for recreation and afforestation, respectively, but this is no more respected. This non-application of the urban planning law has left the population free to occupy all open spaces.
Furthermore, the inefficiency of the rainwater drainage system, exacerbating the overflow and rainwater runoff after heavy rainfall, is another major cause of vulnerability to geo-hydrological hazards [8,21,108]. In the context of Bujumbura, it takes the form of the absence, inadequacy, undersized, and aging of the rainwater drainage system, associated with a lack of an efficient water management system (Figure 6a–d). Moreover, our field observations and the focus group discussions also pointed to the incomplete construction of rainwater drains as a vulnerability factor, causing gullying and/or flooding, as also observed in other contexts by, for example, Makanzu Imwangana et al. [43] and Godswill et al. [109]. Thus, the lack of maintenance or proper development of the rainwater drainage system results in its clogging, as it is not adapted to rapid urban development and the absence of roof water management. Indeed, this additional perceived cause of vulnerability, highlighted in our study, combines with the lack of waste management, which invades the drains and reinforces the problem [110,111,112]. Finally, it emerges that, to varying degrees, uncontrolled urban growth due to rapid population increase is the underlying factor in all the identified vulnerability factors identified in our study.

5.3. A Vulnerable City? The Responsibility of the Urban Planning Institutions

Bujumbura and its surroundings are regularly affected by floods, flash floods, and gullies. As shown above, the vulnerability of the sprawling city depends on several factors, both internal and external [8]. For Salami et al. [9], the city’s functioning requires a proper vulnerability assessment. In Burundi, the problem of the operational performance of the OBUHA, in charge of Urban Planning, Housing, and Construction, can be highlighted. This institution, created in 2019, is the result of six institutions merging, including the Public Real Estate Company (SIP), the Social Construction Framing and Land use planning (ECOSAT), the National Laboratory of Building and Public Works (LNBTP), the Municipal Technical Services (SETEMU), the General Directorate of the Building (DGB) and the General Directorate of Urban Planning and Habitat (DGUH). It is now responsible for sanitation, housing, and urban planning and development. This merger has resulted in the transfer of former staff to other institutions and the recruitment of brand-new personnel without any handover. This problem affects urban management and, consequently, risk prevention and management measures.
Other gaps have been identified in the institutions directly responsible for disaster risk prevention and management in Burundi. Several experiences have already shown that Civil Protection while striving to assist the population, fails to effectively mitigate and manage disasters. Despite its efforts to rescue disaster victims, it faces many challenges in terms of budget, adequate equipment, and qualified personnel [113]. The level of data collection and archiving of all disaster events by the Civil Protection remains unsatisfactory.
Reducing vulnerability, therefore, requires not only effective urban planning and management institutions and an appropriate policy framework (see Section 5.2), as suggested by UNISDR [114] and Ramiaramanana and Teller [8], but also strengthening the effectiveness of risk management institutions by promoting integrated geo-hydrological risk management systems [115]. These institutions could be empowered with the laws and resources needed to counter any non-compliant actions that do not contribute to disaster risk reduction and enforce appropriate regulations, some of which already exist [116]. This would enable concrete action on the ground, as advised by other studies [8,10].

5.4. The Added Value of an Operational Assessment

In the context of this research on the impacts, vulnerability, and perceptions of geo-hydrological hazards in Bujumbura, we chose to use high-resolution field-based assessment in an urban sprawl context. This approach made it possible to identify the components of vulnerability to multiple geo-hydrological hazards. In conditions of lack of databases on the different occurrences/events and damages incurred, the use of high-resolution field-based collection represents an effective way to obtain crucial information. The combination of different sources of information, including data collection, interviews, and focus groups to fill the gaps in institutional data collection, is an insight without which concrete prevention and mitigation strategies for vulnerability to geo-hydrological hazards are difficult to implement [46,59,70]. Indeed, it is hardly impossible to plan effective geo-hydrological risk mitigation strategies in a city without clearly identifying the area most vulnerable to these hazards [71,117]. Our approach makes it easy to identify such areas that have already been highly affected and where the hazards are frequently recurrent, increasing the vulnerability of infrastructures and the population. The various actions aimed at reducing risk should tackle the different causes of vulnerability identified.

6. Conclusions

This study assesses the impacts, vulnerability, and perception of geo-hydrological hazards in a sprawling city. In the Global South, cities are progressively expanding into marginal areas because of rapid population growth and the subsequent high demand for housing. As analyzed through the Bujumbura case study, this results in urban sprawl and the occupation of unserviced urban areas, including marginal land. After filling gaps in data collected by the institutions in charge of disaster risk prevention and management through high-resolution field surveys and focus group discussions, this study highlights various factors of vulnerability to geo-hydrological hazards in urban areas. This study shows that vulnerability is exacerbated above all by institutional failures. It strengthens the idea that the non-application or lack of urban planning law is upstream of the other causes of urban vulnerability to geo-hydrological hazards.
In order to mitigate further disasters linked to geo-hydrological hazards, urban planning authorities should promote controlled occupation of urban and peri-urban areas. In addition, the redevelopment of anarchically-occupied spaces, through the installation and maintenance of appropriate servicing infrastructures, could also help to reduce the risk of disaster. Moreover, the natural stabilization of gullies by planting soil-binding plant species, such as bamboo, banana, and vetiver grass, could be carried out in all unstable areas in order to slow down or even halt the process.
This study was constrained by a lack of detailed data on the impact of past disasters on urban infrastructures, as well as very limited information on the areas susceptible to the various hazards studied. Further research should then not only deepen studies of hazards occurring in urban contexts but also focus on an integrated approach to vulnerability, taking into account both population and infrastructure. This holistic approach could make a key contribution to the restructuration of urban space, and the development of new urban sites should count.
This study shows the importance of complementary field-based studies to assess factors of vulnerability to geo-hydrological hazards in cities of the Global South that are commonly not well documented. It also proposes an empirical approach at the nexus of natural hazards and vulnerability that offers potential for transferability, especially to the cities located in the Great Lake region. This could provide a better understanding of vulnerability factors at a more regional scale.

Author Contributions

Conceptualization, J.N., S.H., A.N. and C.M.; methodology, J.N., S.H. and C.M.; software, J.N.; validation, S.H., O.D. and C.M.; formal analysis, J.N., S.H. and C.M.; investigation, J.N., S.H., C.M., A.N. and D.K.; resources, J.N., S.H., A.N., D.K., O.D. and C.M.; data curation, J.N. and O.D.; writing—original draft preparation, J.N., S.H., A.N. and C.M.; writing—review and editing, J.N., S.H., O.D. and C.M.; visualization, J.N., S.H., O.D. and C.M.; supervision, S.H., A.N. and C.M.; project administration, S.H. and C.M.; funding acquisition, F.K. and C.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Development Cooperation program of the Royal Museum for Central Africa with support of the Directorate-General Development Cooperation and Humanitarian Aid of Belgium (RMCA-DGD) and HARISSA (RMCA-DGD 2019–2024; https://georiska.africamuseum.be/) projects.

Data Availability Statement

Data are available on request from the corresponding author.

Acknowledgments

We thank institutions and individuals who contributed to the completion of this work. We thank IOM Burundi and the General Directorate Civil Protection Police of Burundi for providing us with information on the occurrence of floods and flash floods. We thank Nicholus Mboga for sharing the spatial layers of the urban footprints for the 1940 to 2020 period. Finally, we thank Carolin Mayer for English proofreading.

Conflicts of Interest

The authors declare no conflict of interest.

Appendix A

Land 12 01876 g0a1
Figure A1. Spatial distribution of the qualitative focus group survey and the quantitative individual interview survey. Names indicated in black correspond to the five communes, and those indicated in red to the 14 municipalities. The administrative names of neighborhoods (listed below) were used to identify areas affected by geo-hydrological hazards.
Figure A1. Spatial distribution of the qualitative focus group survey and the quantitative individual interview survey. Names indicated in black correspond to the five communes, and those indicated in red to the 14 municipalities. The administrative names of neighborhoods (listed below) were used to identify areas affected by geo-hydrological hazards.
Land 12 01876 g0a1
Table A1. Legend of Figure A1.
Table A1. Legend of Figure A1.
MunicipalityCode in MapNeighborhoodMunicipalityCode in MapNeighborhood
Buyenzi32INgagara22I
31II16II
30III17III
29IV24IV
28V23V
27VI25VI
39VII18VII
Bwiza42IKamenge21Gituro
40II20Heha
43III19Kavumu
41IV15Gikize
33Jabe I14Songa
26Jabe II9Teza
34Jabe III8Twinyoni
Cibitoke7INyakabiga36I
13II37II
6III38III
10IV35Kigwati
12VKinama1Muramvya
21VI2Gitega
5VII3Ngozi
4Muyinga
Land 12 01876 g0a2
Figure A2. Sprawl in marginal areas and gully erosion in the south–eastern slope of Bujumbura. ©Google Earth imagery from 22 October 2022.
Figure A2. Sprawl in marginal areas and gully erosion in the south–eastern slope of Bujumbura. ©Google Earth imagery from 22 October 2022.
Land 12 01876 g0a2

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Figure 1. Overview and key characteristics of the study area. (a) Location of the city of Bujumbura in the western depression of Burundi, along the shore of Lake Tanganyika; (b) Urban footprint of the city. Most of the city is located in a lowland area, between 774 m and 1000 m a.s.l. The eastern part of the city is bordered by escarpments (Mirwa foothills) and the Congo–Nile ridge, which rises to over 2650 m and is the source of the rivers that flow across the city. The background image is from ©Google Earth (9 February 2020); (c); Lake flooding in 2021; (d) Destruction of a bridge along the Kinyankonge river; (e) Flash flood north of Bujumbura (d and e: pictures taken by the Civil Protection, after the disaster of 9–10 February 2014); (f) Example of a large active gully in the urbanized foothills in 2020.
Figure 1. Overview and key characteristics of the study area. (a) Location of the city of Bujumbura in the western depression of Burundi, along the shore of Lake Tanganyika; (b) Urban footprint of the city. Most of the city is located in a lowland area, between 774 m and 1000 m a.s.l. The eastern part of the city is bordered by escarpments (Mirwa foothills) and the Congo–Nile ridge, which rises to over 2650 m and is the source of the rivers that flow across the city. The background image is from ©Google Earth (9 February 2020); (c); Lake flooding in 2021; (d) Destruction of a bridge along the Kinyankonge river; (e) Flash flood north of Bujumbura (d and e: pictures taken by the Civil Protection, after the disaster of 9–10 February 2014); (f) Example of a large active gully in the urbanized foothills in 2020.
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Figure 2. Sprawl of Bujumbura between 1907 and 2021 with the location of the 14 municipalities. Hillshade derived from the 10-m resolution digital elevation model (DEM) of Central Geomatics Office (BCG), 2012.
Figure 2. Sprawl of Bujumbura between 1907 and 2021 with the location of the 14 municipalities. Hillshade derived from the 10-m resolution digital elevation model (DEM) of Central Geomatics Office (BCG), 2012.
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Figure 3. Flood and flash flood events temporal distribution was obtained by compiling data from the sources presented in Table 2. Rainfall data are from a rain gauge station located in Bujumbura (data provided by IGEBU—Institut Géographique du Burundi). Population data were extracted from statistical yearbooks and other reports from INSBU (Institut National de la Statistique du Burundi).
Figure 3. Flood and flash flood events temporal distribution was obtained by compiling data from the sources presented in Table 2. Rainfall data are from a rain gauge station located in Bujumbura (data provided by IGEBU—Institut Géographique du Burundi). Population data were extracted from statistical yearbooks and other reports from INSBU (Institut National de la Statistique du Burundi).
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Figure 4. (a) Flood, flash flood, and gully spatial distribution in Bujumbura (Table 2 and field campaign); (b) Gully activity status. Hillshade derived from the 10-m resolution DEM of BCG, 2012.
Figure 4. (a) Flood, flash flood, and gully spatial distribution in Bujumbura (Table 2 and field campaign); (b) Gully activity status. Hillshade derived from the 10-m resolution DEM of BCG, 2012.
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Figure 5. Impacts of geo-hydrological hazards in Bujumbura s. (a) Types of infrastructure affected during disaster events according to the quantitative survey of the 46 inhabitants. For example, 4% of the respondents mention the destruction of a bridge; (b) Impacts of events on houses and (c) on people based on data recorded between 2018 and 2020 by IOM and Civil Protection.; (d) Examples of the destruction of houses and other infrastructure (a school, a church) by a gully that formed in 2019 (photo taken on 27 March 2019). The gully also threatened a road of great importance for the Musaga municipality.
Figure 5. Impacts of geo-hydrological hazards in Bujumbura s. (a) Types of infrastructure affected during disaster events according to the quantitative survey of the 46 inhabitants. For example, 4% of the respondents mention the destruction of a bridge; (b) Impacts of events on houses and (c) on people based on data recorded between 2018 and 2020 by IOM and Civil Protection.; (d) Examples of the destruction of houses and other infrastructure (a school, a church) by a gully that formed in 2019 (photo taken on 27 March 2019). The gully also threatened a road of great importance for the Musaga municipality.
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Figure 6. Characteristics of the rainwater drainage components in the city of Bujumbura. (a) Drainage density map at the scale for the city. These initial data [88] were updated with information collected in the field. In the map the names indicated in black correspond to the 14 municipalities; (b) Under-sizing of a drainage collector; (c) Absence of a rainwater drainage system ((b,c): pictures taken by the Civil Protection, after the disaster of 9–10 February 2014); (d) Flooding due to clogged collector (picture taken in 12 November 2021); (e) Active gully that originated from a poorly maintained rainwater drain (© Radio Culture, 13 November 2019). Not the presence of sand back to try to stabilize the gully; (f) Kanyosha river fluvial flooding (picture taken on 26 December 2022).
Figure 6. Characteristics of the rainwater drainage components in the city of Bujumbura. (a) Drainage density map at the scale for the city. These initial data [88] were updated with information collected in the field. In the map the names indicated in black correspond to the 14 municipalities; (b) Under-sizing of a drainage collector; (c) Absence of a rainwater drainage system ((b,c): pictures taken by the Civil Protection, after the disaster of 9–10 February 2014); (d) Flooding due to clogged collector (picture taken in 12 November 2021); (e) Active gully that originated from a poorly maintained rainwater drain (© Radio Culture, 13 November 2019). Not the presence of sand back to try to stabilize the gully; (f) Kanyosha river fluvial flooding (picture taken on 26 December 2022).
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Figure 7. Perceived causes of vulnerability by the municipality of Bujumbura based on the focus group analysis. The number in the grey bar shows the number of focus groups that did not perceive the cause factor, and the numbers in other colors show the focus groups that perceived the causes.
Figure 7. Perceived causes of vulnerability by the municipality of Bujumbura based on the focus group analysis. The number in the grey bar shows the number of focus groups that did not perceive the cause factor, and the numbers in other colors show the focus groups that perceived the causes.
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Table 1. Evolution of the city of Bujumbura since the beginning of the 20th century.
Table 1. Evolution of the city of Bujumbura since the beginning of the 20th century.
Year1907 *1920 *1941 *1955 *1983 *1994 **2002 **20142020 ***2021
Area (km2)0.326123743.849.860109113.9
Population25003000 10,000 23,427162,201-353,236588,336759,901776,258
(*) Evolution according to the Master Plan for Urban Development and Planning (SDAU) of 1983; (**) Evolution according to Sindayihebura [81]; 2014 area according to [82]; (***) 2020 area [77]; 2021 area estimated with ©Google Earth image of 18 May 2021. Population data from [76,79], except for 1920 and 1941 [83], and for 2021 [79].
Table 2. Overview of available reports and inventories containing data on flash floods, floods, and other hazards.
Table 2. Overview of available reports and inventories containing data on flash floods, floods, and other hazards.
Name InstitutionIncluded ProcessesStart (AD)End (AD)
Civile Protection PolicePFN/PGRC: Plateforme Nationale pour la Prévention et la Gestion de Risques de Catastrophes (Burundi)Lake, pluvial and fluvial flood, landslide, fire, road accident, heavy rainfall20092020
DTM-IOMIOM-BurundiPeople displacement, flood, landslide, heavy rainfall20152021
ReliefWebOCHA—BurundiFlood, flash flood, landslide-2020
Red-CrossRed-Cross—BurundiLake, fluvial flood and flash flood, landslide, fire, epidemic-
FloodListCopernicus, the European Union (EU)Flood-related issues: warning system, mitigation and control, flood recovery, flood damage repair and restoration, flood insurance1995
Thesis available University of Burundi’s libraryFlood, riverbank dynamics, gully erosion, landslide2000
Social media Isanganiro, Mashariki-TV, Yaga-BurundiFlood, flash flood, heavy rainfall, landslide-
Table 3. Composition of the focus groups (FG) at the different collection sites.
Table 3. Composition of the focus groups (FG) at the different collection sites.
N°
FG		Date of Data CollectionCommuneMunicipalityNeighborhoodHazardParticipants
Male (46)Female (33)Average Age (37)
1.23 November 2020NtahangwaKinamaBuhinyuzaFlash flood0340
2.23 November 2020NtahangwaKinamaBuhinyuza (Rice field)Flash flood3140
3.23 November 2020NtahangwaKinamaBukirasazi I- cell 2Fluvial flooding1342
4.23 November 2020NtahangwaButerereMugaruroFluvial flooding1232
5.24 November 2020NtahangwaKinamaBukirasazi IIFluvial flooding3031
6.24 November 2020NtahangwaButerereButerere IIB (Kinyankonge Rice field)Fluvial flooding3143
7.24 November 2020NtahangwaKinamaBukirasazi II- cell 5Pluvial flooding2447
8.25 November 2020MuhaKanyoshaGisyoLake and fluvial flooding3033
9.8 December 2020NtahangwaNgagaraSabePluvial and fluvial flooding4042
10.9 December 2020MuhaKanyoshaKajijiFlash flood4038
11.9 December 2020MuhaKanyoshaGisyo IVGully0427
12.19 January 2021MuhaKanyoshaNkenga Busoro (Mugoyi Canal)Flash flood2267
13.19 January 2021MuhaKanyoshaBusoroGully0336
14.19 January 2021MuhaKanyoshaNkenga-BusoroGully4229
15.20 January 2021MuhaKinindoMisabiroLake flooding3330
16.20 January 2021MuhaKinindoKibenga-RuralLake flooding1226
17.26 January 2021MutimbuziRubiriziGatunguruGully3041
18.27 January 2021MutimbuziRubiriziGatunguruFlash flood2231
19.27 January 2021NtahangwaKinamaCaramaPluvial and fluvial flooding4021
20.27 January 2021MutimbuziRubiriziNyakabondo 1Flash flood3135
Table 4. Perceived causes of vulnerability to flooding, flash flooding, and gullying based on the focus group analysis.
Table 4. Perceived causes of vulnerability to flooding, flash flooding, and gullying based on the focus group analysis.
HazardComponentCause of Vulnerability
Flood, flash floodPlanning problemAge of drainage infrastructure, insufficient or undersized channels
InstitutionalNon-compliance with the law
NaturalOverflow and destruction of riverbanks
AnthropicRiverbed narrowing and/or detour; dumping of garbage and filling of channels or rivers
GullyPlanning problemAgricultural exploitation of marginal areas
AnthropicIncrease in the amount of water that households send to the drainage upstream of the gully; increase in roof water
NaturalVery fragile sandy soils; heavy rainfall increases erosion
Flood, flash flood, gullyPlanning problemLack of control in urban development and sale of plots; unfinished construction of collector
UrbanizationUrban expansion on marginal slopes; densification of the built environment
AnthropicStrong increase in the population; household poverty
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Nsabimana, J.; Henry, S.; Ndayisenga, A.; Kubwimana, D.; Dewitte, O.; Kervyn, F.; Michellier, C. Geo-Hydrological Hazard Impacts, Vulnerability and Perception in Bujumbura (Burundi): A High-Resolution Field-Based Assessment in a Sprawling City. Land 2023, 12, 1876. https://doi.org/10.3390/land12101876

AMA Style

Nsabimana J, Henry S, Ndayisenga A, Kubwimana D, Dewitte O, Kervyn F, Michellier C. Geo-Hydrological Hazard Impacts, Vulnerability and Perception in Bujumbura (Burundi): A High-Resolution Field-Based Assessment in a Sprawling City. Land. 2023; 12(10):1876. https://doi.org/10.3390/land12101876

Chicago/Turabian Style

Nsabimana, Jean, Sabine Henry, Aloys Ndayisenga, Désiré Kubwimana, Olivier Dewitte, François Kervyn, and Caroline Michellier. 2023. "Geo-Hydrological Hazard Impacts, Vulnerability and Perception in Bujumbura (Burundi): A High-Resolution Field-Based Assessment in a Sprawling City" Land 12, no. 10: 1876. https://doi.org/10.3390/land12101876

APA Style

Nsabimana, J., Henry, S., Ndayisenga, A., Kubwimana, D., Dewitte, O., Kervyn, F., & Michellier, C. (2023). Geo-Hydrological Hazard Impacts, Vulnerability and Perception in Bujumbura (Burundi): A High-Resolution Field-Based Assessment in a Sprawling City. Land, 12(10), 1876. https://doi.org/10.3390/land12101876

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