Deep gashes are tearing through cities, swallowing houses and displacing vast numbers of people

Urban gullies are rapidly expanding in cities built on sandy soils with inadequate drainage, where heavy rains channel water into unprotected ground and carve deep trenches that swallow houses, businesses and infrastructure and sometimes cause deaths. Researchers identified 2,922 urban gullies across 26 of 47 cities from 2021–2023 satellite imagery, totaling nearly 740 kilometres, and found only 46 gullies in 1950s aerial photographs, linking the phenomenon to urbanization. Between 2004 and 2023, 99% of gullies expanded by at least 10 square metres and the average gully was 253 metres long, displacing large populations, notably an estimated average of 118,600 people in the DRC over that period, with projections of hundreds of thousands more displaced without urgent action.

Gigantic trenches known as gullies are opening up in cities in Africa, swallowing up homes and businesses, sometimes in an instant, a study has found. About 118,600 people, on average, in the Democratic Republic of the Congo (DRC) alone were displaced between 2004 and 2023, according to researchers reporting their findings in Nature. Without urgent action, researchers estimate that hundreds of thousands of people across Africa are likely to be displaced within the next 10 years, including more than one-quarter of the 770,000 or so people in the DRC living in the expected expansion zone of these gullies.

"It's an underestimated and severely under-researched hazard," says study co-author Matthias Vanmaercke, a geographer at the Catholic University of Leuven (KU Leuven) in Belgium. It is caused by "a combination of natural and human factors", he says, but this is "not at all unavoidable".

Vanmaercke and his colleagues used satellite images taken between 2021 and 2023 to identify 2,922 urban gullies in 26 of 47 cities, covering a cumulative distance of nearly 740 kilometres. The team cross-checked these images with historical aerial photographs stored at the Royal Museum for Central Africa in Belgium and found that only 46 of the gullies were present in the 1950s. This "gave the first clear indication that this is indeed attributable to the ongoing urbanization", Vanmaercke says.

Read at Nature