Source: ForeignAffairs4
Source: The Conversation – Africa – By Matthias Vanmaercke, Associate professor BOF Faculty of Science, KU Leuven
In fast-growing cities like some in the Democratic Republic of Congo (DRC), heavy rains are carving huge scars into the land. Known as urban gullies, these deep erosion channels can swallow homes, destroy roads and displace entire communities.
They can grow to hundreds of metres long and dozens of metres wide, splitting neighbourhoods in two. Once established, they keep expanding with each major downpour.
The consequences are devastating. In Kinshasa, the DRC’s capital, heavy rainfall in December 2022 triggered rapid gully expansion, destroying homes and claiming dozens of lives.
Urban gullies form when rainwater runoff cuts deep channels into fragile soils. The erosive force of concentrated water exceeds the strength of these soils. The gullies usually form after intense rain on steep slopes. Urbanisation makes the situation worse as vegetation is removed to build houses, greatly increasing the likelihood that heavy rainfalls will simply run off the top soil. Roads also play a critical part as they can change how water flows across the landscape, forming direct pathways along which runoff can accumulate.
Our new study reveals the staggering scale of the problem in the DRC. Our research team of Congolese and Belgian earth scientists and geographers identified 2,922 urban gullies in 26 DRC cities.
We used satellite imagery and population data to identify the gullies. Our detailed, nationwide mapping effort – the first to map gully erosion across an entire country – shows that this is not a series of isolated incidents but a widespread and fast-growing hazard.
But urban gullies can be avoided by adequate urban planning and infrastructure. This includes adapted zoning plans and measures such as better road drainage, rainwater retention and infiltration systems, increased vegetation cover and targeted engineering works to divert runoff safely.
The crisis in numbers
Many of the urban gullies in the DRC are huge. A typical example is easily 250 metres long and 30 metres wide. Together, they stretch nearly 740 kilometres.
Kinshasa alone has 868 mapped gullies (221km in total). With about 17 million inhabitants, it is the DRC’s largest city and one of Africa’s megacities, where rapid, unplanned growth (around 6.6% per year) makes gully erosion a major urban hazard. Kinshasa is also tropical with annual rainfall typically above 1,000 millimetres.
By reconstructing how these features expanded between 2004 and 2023, we calculated that 118,600 people in the DRC were forced from their homes. Displacement has accelerated sharply: before 2020, about 4,600 people were displaced annually; today, the figure is more than 12,000.
The study also looked ahead. In 2023, some 3.2 million Congolese lived in areas considered at risk of future gully expansion. Of these, more than half a million are in zones where the chance of losing their homes within a decade is very high.
Several factors make Congo’s cities especially prone to gully erosion. Many are built on steep slopes with sandy soils that are highly erodible. Rapid, unplanned urban growth strips vegetation and increases impermeable surfaces such as rooftops and roads, which funnel runoff into concentrated flows.
The link with roads is particularly striking: 98% of all mapped gullies were connected to the road network, either forming along unpaved streets or fed by runoff from poorly drained roads.
The problem is set to worsen. Congo’s urban population is booming, driven by both natural growth and migration. Informal neighbourhoods often lack basic infrastructure, leaving rainfall to carve its own destructive paths.
Climate change adds another layer of risk. Rainfall intensity in tropical Africa is projected to rise by 10%-15% in the coming decades. Since heavy downpours are a trigger for gully formation, expansion rates could double if no action is taken.
Prevention over cure
Once formed, gullies are extremely hard and costly to stabilise. Local communities often try to slow their advance, but without proper engineering solutions, most efforts fail. Stabilising a single large gully can cost the DRC more than US$1 million, an impossible burden for most municipalities.
The study shows that prevention is the only viable long-term strategy. That means paying careful attention to how cities are planned and built. Measures such as better road drainage, rainwater retention systems and strategic vegetation cover can reduce the risks.
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Above all, improved spatial planning is crucial to stop new neighbourhoods from being built in vulnerable areas. The effectiveness of specific urban gully control measures remains largely unknown and poorly documented, apart from an earlier case study in the DRC that showed that many measures fail. But such measures should not be confused with better spatial planning. This means avoid constructing houses and roads in areas that are sensitive to urban gully formation, or at least making sure that rainwater is safely stored or evacuated.
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We argue that the best strategy for limiting the impacts of urban gullies is preventing them.
Above all, urban gullies must be recognised as a disaster risk on par with floods and landslides. Only then can policies and investments be developed that are needed to protect vulnerable populations.
A problem in the rest of Africa too
Although the DRC is at the epicentre of the crisis, similar problems are emerging elsewhere in Africa, including Nigeria, Uganda, Burundi and Madagascar.
Read more:
Flooding in Nigeria is on the rise – good forecasts, drains and risk maps are urgently needed
With urban populations across the global south expected to nearly triple by 2050, gully erosion could become one of the defining urban hazards of the century.
The deep scars running through Congo’s cities are not just features of the landscape, they are reminders of the urgent need to rethink how urban growth is managed in vulnerable regions.
Matthias Vanmaercke receives funding from the University of Leuven. The research behind this article was funded through the Belgian ARES research collaboration project PREMITURG (Prevention and Mitigation of Urban Gullies: lessons learned from failures and successes, D.R. Congo)
– ref. Soil erosion is tearing DRC cities apart: what’s causing urban gullies, and how to prevent them – https://theconversation.com/soil-erosion-is-tearing-drc-cities-apart-whats-causing-urban-gullies-and-how-to-prevent-them-264497