Quantifying the direct impacts and risks of large urban gullies in the Democratic Republic of Congo

peer reviewedLarge urban gullies (UGs) cause major infrastructural damages and often claim casualties in many tropical cities of the Global South. Nonetheless, our insight into this new type of geo-hydrological hazard remains limited to some case studies and the overall impacts remain poorly quantified. Here, we aim to bridge this gap by making a first assessment of the number of persons affected by urban gullies at the scale of the Democratic Republic of Congo (DRC). We used Google Earth imagery in combination with local news sources and earlier research to identify 25 cities in DRC where UG occur at a significant scale (at least ten UGs). This list is likely exhaustive. Next, for each of these cities, we used Google Earth imagery and other high resolution satellite images to map all visible UG, evaluate their expansion rate and inventorize detectable damages to houses and roads. In total, >2,000 UGs were mapped across the 25 affected cities. Overall, the problem of UGs in DRC is especially acute in the cities of Kinshasa, Mbujimayi, Kikwit, Tshikapa and Kananga. Over 90% of these gullies were active during the observation period (typically from 2002 to 2020). Next, we assessed the total number of persons that are directly affected, as well as the number of persons currently at risk. Using available high resolution population density data and taking into account the current position of urban gullies, we estimate that around 68,700 people were directly displaced due the formation and expansion of UGs over the last 15 years. This corresponds to an average of ca. 4,300 persons per year. By considering the population that lives in the direct vicinity (<100 m) of an UG, we estimate that around 1.3 million people in D.R. Congo are currently at risk and/or experience significant impacts because of UGs (e.g. reduced land value, problems with trafficability, stress). This number has doubled over the past 10 years (2010-2020) and will likely continue to increase as a result of urban expansion and climate change. Overall, this research shows that urban gullying is a very serious problem in the Democratic Republic of Congo, but likely also in many other countries of the Global South. More research is needed to better understand this processes and, ultimately, to prevent and mitigate its impacts. The results and the database of this study provide an important step towards this

How fast do gully headcuts retreat?

© 2016 Elsevier B.V. Gully erosion has important on and off site effects. Therefore, several studies have been conducted over the past decades to quantify gully headcut retreat (GHR) in different environments. Although these led to important site-specific and regional insights, the overall importance of this erosion process or the factors that control it at a global scale remain poorly understood. This study aims to bridge this gap by reviewing research on GHR and conducting a meta-analysis of measured GHR rates worldwide. Through an extensive literature review, GHR rates for 933 individual and actively retreating gullies have been compiled from more than 70 study areas worldwide (comprising a total measuring period of >19 600 years). Each GHR rate was measured through repeated field surveys and/or analyses of aerial photographs over a period of at least one year (maximum: 97 years, median: 17 years). The data show a very large variability, both in terms of gully dimensions (cross-sectional areas ranging between 0.11 and 816 m2 with a median of 4 m2) and volumetric GHR rates (ranging between 0.002 and 47 430 m3 year- 1 with a median of 2.2 m3 year- 1). Linear GHR rates vary between 0.01 and 135 m year- 1 (median: 0.89 m year- 1), while areal GHR rates vary between 0.01 and 3628 m2 year- 1 (median: 3.12 m2 year- 1). An empirical relationship allows estimating volumetric retreat rates from areal retreat rates with acceptable uncertainties. By means of statistical analyses for a subset of 724 gullies with a known contributing area, we explored the factors most relevant in explaining the observed 7 orders of magnitudes of variation in volumetric GHR rates. Results show that measured GHR rates are significantly correlated to the runoff contributing area of the gully (r2 = 0.15) and the rainy day normal (RDN; i.e. the long-term average annual rainfall depth divided by the average number of rainy days; r2 = 0.47). Other factors (e.g. land use or soil type) showed no significant correlation with the observed GHR rates. This may be attributed to the uncertainties associated with accurately quantifying these factors. In addition, available time series data demonstrate that GHR rates are subject to very large year-to-year variations. As a result, average GHR rates measured over short (100%) uncertainties. We integrated our findings into a weighted regression model that simulates the volumetric retreat rate of a gully headcut as a function of upstream drainage area and RDN. When weighing each GHR observation proportional to its measuring period, this model explains 68% of the observed variance in GHR rates at a global scale. For 76% of the monitored gullies, the simulated GHR values deviate less than one order of magnitude from their corresponding observed value. Our model clearly indicates that GHR rates are very sensitive to rainfall intensity. Since these intensities are expected to increase in most areas as a result of climate change, our results suggest that gully erosion worldwide will become more intense and widespread in the following decades. Finally, we discuss research topics that will help to address these challenges

The origin and control of mega-gullies in Kinshasa (D.R. Congo)

This study aims to investigate the relation between mega-gully (> 5 m width) distribution and urbanization in Kinshasa (D.R. Congo), to establish what governs mega-gully location and plan form and to illustrate the concepts behind mega-gully treatment. For this purpose, the diachronic distribution of mega-gullies has been mapped in Kinshasa. All mega-gullies have been reported in ArcGis 9.3 on the orthorectified SPOT 2007 image. A newly elaborated DEM enables the mega-gullies to be placed in their natural topographical context. The GIS inventory on the SPOT 2006/2007 anaglyph indicates the mega-gully situation in the high town of Kinshasa 5 years ago: 308 mega-gullies with a cumulated length of 94.7 km, a mean drainage density of 0.4 km km− 2 and an average width and depth of 17 m and 6 m respectively. On the WorldView 1 (WV1) coverage, the number of mega-gullies has more than doubled between 2007 and 2010 from 160 to 334. The study shows that mega-gullies only develop within the urbanized perimeter of the high town of Kinshasa and only 5 to 10 years after incipient urbanization. The study also indicates that neither the location, the plan form or the downslope course of mega-gullies in Kinshasa are controlled by the natural topography. Forty-three point eight percent of the mega-gullies in Kinshasa are ‘axial’, occupying urban structures which function as artificial runoff drainage lines: roads, tarred or not, with or without side-road trenches, gutters in all forms and materials from concrete to sand, also foot paths and further all artificial runoff drainage lines. The study reveals that every mega-gully is directly or indirectly induced by human activities, but that every gully also finally ends to grow after an initial phase of sudden development. Mega-gully treatment follows two principles, often combined. The first is to stop the alimentation of the mega-gully head with water. The second includes a complete stabilization of the channel and walls inside the mega-gully. This study emphasizes that gully prevention can basically be achieved by control of the runoff discharges in the artificial stream network, as well as beside the roads

Strategies Des Resiliences Urbaines Face A La Production Des Services De Base En Eau Potable, Electricite Et Assainissement Dans Les Quartiers Batumona A Kimbanseke Et Talangay A N’sele/Rd.Congo

La réflexion sur la problématique des résiliences urbaines liées à l’approvisionnement en eau potable, électricité et assainissement dans les quartiers BATUMONA à Kimbanseke et TALANGAY à N’sele est basée sur le fait que la périurbanisation rapide des pays en voie de développement est liée à une consommation excessive de la ressource foncière pour l’habitat BEAUGARD & HALLEUX, (2015) ; ANGEL & ali,(2016).Le concept de résilience urbaine paraît savant et, est  très mal connu par les sujets enquêtés. Cette situation se justifie probablement par le niveau très bas d’instruction des sujets enquêtés dû à l’insuffisance de moyens financiers, au faible revenu (salaires toxiques), dont le dénominateur commun est la précarité de conditions de vie de la population et leurs conséquences néfastes TREFON, (2O11), MABIN, BUTCHER & BLOCH, (2013) ; ANGEL & ali, (2016). Il s’agit là, d’une conséquence de l’urbanisation rapide et de la répulsion du centre-ville, en raison de la forte pression foncière, de coûts élevés de la vie pour les ménages à faible revenu à Kinshasa en général et en particulier dans l’aire d’étude à Kinshasa-Est. Il y a lieu de faire également référence à l’inexistence de politique de planification urbaine WATSON, (2009), BINZANGI & FALANKA, (2014).Globalement, l’aire d’étude fait face à une croissance extraordinaire de son contenant et doit être gérée rationnellement, pour assurer sa durabilité environnementale. Malheureusement, l’état actuel des sites étudiés est confronté à des précarités à outrance des conditions de vie urbaine. Cette précarité a engendré la pauvreté urbaine de masse qui a ouvert la voie aux vulnérabilités des conditions de vie urbaine et celles des habitants. Face à la crise économique persistante, les sujets enquêtés ont développé d’autres stratégies de survies appelées «  résiliences urbaines ». Les sujets enquêtés éprouvent de difficultés énormes de tous ordres qui constituent des conditions majeures de viabilité et vivabilité environnementales urbaines. Ces difficultés constituent des facteurs limitant de possibilités de la plupart des ménages enquêtés dans l’aire d’étude, à prévenir les risques environnementaux et leurs aléas sur les vies humaines. A ce titre, les difficultés précitées soustraient l’aire d’étude des exigences de durabilité environnementale urbaine

Démarrage et longueur de la saison de pluies dans la région de Kinshasa et risques induits

peer reviewedLa détermination et la prévision du démarrage de la saison de pluies sont des questions cruciales à travers la RDC en général et en particulier dans l’hinterland de la ville de Kinshasa étant donné que l’agriculture y est pluviale. Elles permettent de déterminer le calendrier agricole, particulièrement le semis, et influence le rendement. La question qui se pose toujours est celle de savoir s’il y a un changement climatique dans la région de Kinshasa qui se traduirait par la variation de la saison des pluies de 1961 à 2010. Et si c’est le cas, quels en sont les impacts sur l’activité agricole et autres ? Cette étude mène une analyse de la variabilité interannuelle et décennale de la saison des pluies à partir de deux séries chronologiques dans la région de Kinshasa. [...

Contribution of Remote Sensing in the Estimation of the Populations Living in Areas with Risk of Gully Erosion in Kinshasa (DR Congo). Case of Selembao Township

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