Landslide Susceptibility Assessment Using Frequency Ratio: A Case Study of Kiliba (Sud-Kivu/DR Congo)

The conversion of natural ecosystems into agricultural or urban areas can alter geomorphological processes, particularly in landslide-prone regions. Landslides in such areas can be triggered by natural events like heavy rainfall or earthquakes, as well as human activities such as deforestation and unplanned urbanization. Their impacts can be severe, resulting in significant socio-economic damage. Uvira Territory, in the western part of the East African Rift Valley, frequently experiences these events. It is located between the Ruzizi Plain to the east and the Mitumba Mountains to the west, with diverse geology comprising precambrian formations and quaternary sediments. The topography has a stepped relief with altitudes ranging from 770 to 3250 meters. The climate is tropical and humid, with a rainy season from September to May and a dry season from June to August. The area features coastal plains and mountain slopes, with many waterways flowing into Lake Tanganyika or the Ruzizi River. Detailed studies on landslide susceptibility mapping in this area are limited. This study aimed to map landslide susceptibility in the Kiliba River catchment to assist policymakers in land management. It used Google Earth images, GPS surveys, and field observations, applying a Frequency Ratio (FR) model that considered seven geo-environmental factors: slope, aspect, elevation, distance to watercourses, topographic wetness index, vegetation cover, and land use/landcover. The inventory identified 106 landslides in the study area, with densities of up to 11.25 landslides per km². Key factors in predicting landslide susceptibility were slope, elevation, and vegetation cover. The prediction model had an accuracy rate of 72.2%. The study shows that regions at medium elevation with steep slopes and low vegetation cover are mostly at risk for landslides. These findings are key for land management and disaster prevention. Future studies should consider more factors and a broader geographic range to enhance risk managemen

EVOLUTION RECENTE DU BASSIN VERSANT DE LA LUBUDI ET SON IMPACT A KINSHASA ENTRE 1979 ET 2021

Notre bassin versant de la rivière Lubudi a un indice de compacité de Gravelus de 2,023, elle a une forme allongée, sa densité de drainage est de 0,9509 km/Km², et ce bassin versant est dans un état de maturité, avec un périmètre de 37,81km et d’une superficie de 27,37 Km². Sur base des enquêtes observés sur terrain, nous confirmons nettement que le bassin versant de la Lubudi est une région sure du point de vue de certains risques naturels notamment les inondations. Ce phénomène s’observe lorsqu’il y a la présence d’une pluie diluvienne. Les résultats de l’occupation des sols dans le bassin versant de la Lubudi montrent qu’il y a une forte croissance d’activité urbaine en défaveur de la végétation. Ceci implique de nombreux problèmes environnementaux entre autres l'érosion, les inondations et la sédimentation. Sur base des résultats de notre étude, nous pouvons déduire la vitesse moyenne de la disparition de la végétation entre 1979 à 2021 (soit 42 ans) serait égale à 0,245683 Km²/an. Cet inventaire nous a permis de connaitre la perte du couvert végétal et de réaliser aussi les cartes d’occupations des sols à différentes période (1979, 2001, 2017 et 2021). Nous demanderons aux décideurs politico-administratif et experts de notre pays de biens vouloirs prendre en comptent de résolutions de notre recherchent en utilisant de nouvelle approche (SIG et Télédétection) lors des travaux d’aménagement durable de territoire en tenant comptent de risques naturelles (inondation) auxquelles les populations sont soumises afin d’intégrer dans le planning de leurs activités

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

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

Mapping and spatial-temporal assessment of gully density in the Middle Volga region, Russia

© 2018 John Wiley & Sons, Ltd. A large-scale mapping of gully density was carried out for the Middle Volga region of the Russian Plain (188 000 km2) based on the interpretation of aerial photographs (scale 1:17 000; surveys undertaken during 1956–1970). In addition, spatial-temporal dynamic of gully density were assessed for some parts of the study area (the Udmurt Republic and the Mesha and Ulema River basins of Tatarstan), based on the interpretation of aerial photographs (survey 1986–1991) and high resolution satellite images (2012–2015). Information on factors potentially controlling gully formation and development were collected and a geographic information system (GIS) analysis was conducted. Results show the strong development of gullies in the study area over the 1956–1970 period with an average gully density of 0.21 km km−2. For the Udmurt region, we found that gully densities varied little in the period 1956–1986, during which the total active gully length reduced with only 2%. This period was characterized by low variable climatic conditions and a stable fraction of arable land with a relatively continuous crop rotation system. However, gully dynamics seems to have changed more strongly during recent decades. We found a strong (order of magnitude) reduction in active gully density for the period 2010–2015 as compared to 1986–1991. The main reason for this is likely the increasing winter air temperatures. This leads to a significant reduction in surface runoff during spring as a result of snowmelt. Nonetheless, in some regions (i.e. the Udmurt Republic in the taiga zone), the abandonment of arable land after 1991 likely plays a significant role. Likewise, a decline in the frequency of extreme rainfall events (> 50 mm) may have played a role. All of these factors contribute to a reduction of surface runoff to the gullies and their subsequent stabilization. © 2018 John Wiley & Sons, Ltd.status: publishe

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