The potential of runoff retention structures as a strategy to control urban gullying in tropical cities

Rapid and chaotic urbanization leads to the formation of urban gullies in many tropical cities of the Global South. To prevent and limit the destructive impacts of these gullies, runoff retention structures are often constructed. Yet, earlier research indicates that these measures are largely ineffective in reducing urban gully expansion. This study aims to understand why. We conducted detailed field surveys in two representative catchments affected by urban gullies in Kinshasa (Democratic Republic of the Congo) and characterized all existing runoff retention structures. We then used the Soil Water Management Model (SWMM) to evaluate the likely influence of these structures on runoff volumes and peak discharges. Although most parcels (77%–88%) in the catchments have at least one runoff retention structure, their overall effect catchment runoff is limited (e.g., only 25–30% reduction of the total runoff volume for an event with a return period of 2 years). One key reason is that many structures are too small and poorly maintained, reducing their already limited storage capacities. Additionally, they are typically unequally distributed within the catchments (with fewer measures upstream) and not proportional to the parcel size, leading to some oversized and many undersized structures. Overall, we demonstrate that, while current efforts are largely ineffective, coordinated implementations of runoff retention structures proportional to parcel size offer promising perspectives for better controlling urban gully erosion.