Mapping urban gully erosion in the Democratic Republic of the Congo

Democratic Republic of the Congo (DRC), quantifying the extent of the phenomenon, its recent dynamics, and its human impacts at the scale of an entire country. This approach, which can be replicated in other cities and countries, offers a tool to address a rapidly growing problem with severe human and financial costs.

Geomorphological and hydrogeological landform, a gully is a large and deep incision carved by rainwater as it concentrates and flows with high erosive power through susceptible terrain. In urban areas, gullies can extend for several hundreds of meters in length and tens of meters in width, cutting across neighbourhoods and severing infrastructure. Once formed, they tend to expand with each intense rainfall event. The consequences are devastating. “Urban gullies are silent disasters,” explains Dr Guy Ilombe Mawe of the Université Officielle de Bukavu, first author of the article. “They rarely make headlines, yet each year they displace hundreds or even thousands of people in Kinshasa alone.”

Although gully formation is initially a natural process, the scale and rate of their development in Africa have accelerated in recent years, largely due to human activities and landscape modification associated with urban expansion. “While natural factors such as intense rainfall play an important role in triggering gullies, the predisposing factors are almost always anthropogenic,” explains Aurélia Hubert, geologist at University of Liège. “Unplanned urbanization on steep slopes, poorly drained roads, and insufficient water collection and retention infrastructure are key drivers that exacerbate the phenomenon.”

By combining very high-resolution satellite imagery with demographic datasets, the Congolese–Belgian research team quantified the extent, drivers and human impacts of this erosion process across urban areas of the Democratic Republic of the Congo. “We identified 2,922 gullies in 26 cities, with an estimated 118,600 people displaced between 2004 and 2023, with a clear acceleration after 2020,” explains Matthias Vanmaercke, professor at KU Leuven and specialist in gully erosion. “Comparisons with aerial photographs from the 1950s show that almost all of these gullies formed in connection with recent urbanization and road networks, as only 46 gullies existed prior to major built-up expansion.”

Beyond material destruction, these dynamics represent a level of risk comparable, in terms of human impact, to better-known hazards such as landslides or earthquakes, yet they remain largely underestimated in urban policies. In this context, the study proposes concrete avenues for action, with a particular emphasis on prevention. “Prevention is preferable to remediation,” stresses Aurélia Hubert. “Stabilizing a single gully can cost over one million US dollars. Rethinking drainage, road design and land use planning is far less costly than repairing the damage afterward.”

The study notably calls for:

• integrating gully erosion risk into urban planning regulations and road design standards;
• prioritizing exposed areas to protect residents and redesign drainage systems accordingly;
• developing predictive models to anticipate where and when gullies may form, and to intervene at an early stage, since the initial gully length strongly conditions the future extent of damage.

A transferable method

A key strength of this work lies in its methodology: a countrywide gully inventory based on satellite images with spatial resolution ≤ 1 m, validated through field surveys (434 gullies) and combined with demographic datasets to assess population exposure and displacement.

Originally designed to analyze large areas of the DRC, this approach can be replicated in other cities of the Global South experiencing rapid urban growth. “At a time when urban Africa is expanding at high speed, urban gullies represent a new type of geo-hydrological hazard with major consequences, whose occurrence and associated risks can be managed,” concludes Olivier Dewitte, geographer at the Royal Museum for Central Africa (AfricaMuseum). “By revealing its magnitude, drivers and human costs, this study provides an operational basis for action-supporting earlier planning and intervention to prevent streets from turning into canyons.”