Abstract :
[en] Urban gully erosion is an escalating problem in many tropical cities of the global south, particularly in the Democratic Republic of Congo (D.R. Congo), where thousands of urban gullies cause severe damage in cities like Bukavu, Kikwit, and especially Kinshasa. To mitigate the impacts of these rapidly expanding gullies, various gully prevention and stabilization initiatives are undertaken. These range from large structural interventions by the state and NGOs to numerous bottom-up efforts by local populations at risk. Despite the significant investments in these measures, many seem to fail, and the reasons for this remain unclear, largely due to a lack of systematic documentation and assessment. This work aims to address several of the research gaps surrounding urban gully erosion, its impacts, and effective control strategies. As such, the general objective of this PhD thesis is to better understand the effectiveness of measures aimed at stabilizing urban gullies in tropical cities by drawing lessons from both failures and successes of already implemented initiatives.
To achieve this, we first documented and assessed the impacts of urban gullies and their associated costs at the scale of Kinshasa. A total of 802 household surveys were conducted with families living within 100 meters of an urban gully, complemented by interviews with 12 real estate experts. The findings revealed that around 60% of the surveyed households had already experienced material damage, with the most frequently reported impacts being parcel size reductions, cracks, and significant damage to homes. The average reported cost for these damages was approximately $4,320 per affected family. We also documented the involvement of these households in local bottom-up initiatives aimed at stabilizing urban gullies. Despite widespread participation in such initiatives, the financial constraints faced by many households likely limit the effectiveness of these efforts. Finally, we show that urban gullies have a significant impact on real estate values. Properties near gullies (< 100 m) can easily lose over half their value on the official housing market. At the scale of Kinshasa, urban gullies have contributed to an estimated real estate value reduction of $979 million USD.
Next, we investigated the variety of gully control measures currently implemented and their overall effectiveness. To do this, we conducted extensive field campaigns in Kinshasa, Kikwit, and Bukavu, documenting 398 urban gullies, including 69 where the impact of specific stabilization measures could be evaluated. Our results show that most gullies have been addressed through ad-hoc measures by the affected populations, while more comprehensive engineering solutions were implemented in only 20-30% of cases. Among the various strategies, only runoff diversion measures showed significant effectiveness in reducing gully expansion rates.
Building on these results, Chapter 4 analyzes in greater detail why upstream water retention structures, which aim to limit runoff to gullies, appear to have limited success. We conducted detailed field surveys in two representative catchments in Kinshasa and used the Soil Water Management Model (SWMM) to evaluate the structures' effects. Despite a high prevalence of runoff retention structures (77%-88% of parcels), their impact on catchment runoff was minimal (typically < 20% runoff reduction), primarily due to their small size, poor maintenance, and unequal distribution. However, coordinated implementation of such structures with capacities proportional to parcel sizes offers great potential for better controlling urban gully erosion.
Chapter 5 further examines gully stabilization measures based on vegetation. Using high-resolution imagery and a dataset of 1,335 gully sidewall sections, we analyzed the relationship between vegetation and gully stability. While stable gully sections generally exhibited more vegetation, there was no statistically significant link between vegetation and gully stability. Instead, factors such as sidewall steepness and proximity to the outlet were stronger predictors of gully activity, offering new insights into identifying risk zones and targeting future stabilization efforts.
In the concluding chapter, we provide concrete answers to our specific research questions and offer recommendations for policymakers and stakeholders involved in preventing and controlling urban gully erosion. Furthermore, we outline areas for further research.
