In late October this year, tragedy struck in the Philippines. Typhoon Nalgae, which made landfall five times, killed 123 people across the Philippines, including at least 61 who died in floods and landslides. The country’s president visited the affected areas, noting that deforestation and climate change increase the risk of a landslide and exacerbate the damage these landslides can do. Those are fair observations, but there’s another parameter impacting landslides: urbanization.

The Philippines is far from the only country dealing with landslides. In fact, landslides are some of the most common geological events, and they seem to be happening more and more. A part of that is indeed because of climate change, but according to a new study, urbanization — particularly unplanned urban sprawl that affects surface and subsurface water flows — also makes landslides more likely.

Landslides are natural phenomena. They occur when large masses of soil or rocks (or both) move down a slope en masse, and are generally caused by abrupt disturbances in the slope, which can be from an earthquake, a heavy rainstorm, or a sudden change in water table level. But human activity can also affect the risk of landslides.

The removal of trees and other vegetation, for instance, has been known for a long time to destabilize soil and favor the development of landslides, as it reduces the amount of water the soil can absorb. Land management practices can also affect water flow, so it makes sense that they would also increase landslide risk. However, not much is known about the influence of urbanization on landslides, note the authors of the new study.

The current rate and scale of urban growth are unprecedented in human history, particularly in densely populated areas in Asia and Africa. This urban growth often occurs in an informal and sprawling way, often overlooking natural constraints and hazards, exposing the population to these hazards.

To make matters even worse, climate change is indeed, through extreme rain, erosion, and more frequent wildfires, is making landslides more common. To see how all of this is impacting landslide risk, a team led by Antoine Dille  from the Royal Museum for Central Africa, Tervuren in Belgium studied a site in Bukavu, Democratic Republic of the Congo. Bukavu lies at the very eastern edge of the country, and it’s been growing steadily year after year. Just 50 years ago, Bukavu had barely over 100,000 people; in 2000, it had 384,000 people; now, it’s at around 1,200,000 million people, and shows no signs of stopping its urban growth. Parts of Buvaku are also sloping, making it a good area to study this effect.

Dille and colleagues looked in particular at deep-seated landslides. Shallow landslides are violent and fast, but deep landslides are more pervasive threats — they move at a slow but steady pace year after year. The fastest sections in Bukavu move at about 0.7 meters per year (2.3 feet).

The researchers analyzed satellite radar data from 2015 to 2019, along with the past 70 years’ worth of aerial photos, exploring the link between urbanization and landslides.

They found that the more heavily a slope area is urbanized, the more likely it is to be destabilized and the more prone to landslides. The destabilization was most severe in areas where the re-routing of surface runoff and leaky underground infrastructure have combined to increase soil water saturation. Also, the researchers looked at seismic and meteorological weather and found that earthquakes and heavy rains alone can’t account for this change.

The landslide woes of Bukavu are likely common for numerous cities, particularly in rapidly-expanding urbanized areas in tropical areas, the researchers note. To address these concerns, community-based solutions should be used to eliminate or reduce the hazard — especially for managing water levels, drainage, and water courses (both over and under the ground).

“While landsliding is not the primary concern of the urban population of Bukavu — primary concerns include access to potable water, sanitation, health or education services and (food) security — community-based approaches should be promoted to prevent loss of life and infrastructure due to landsliding,” the study authors note. “As hillslopes of the world’s cities are being urbanized at accelerating paces, we believe that more studies are needed to improve our understanding of how anthropogenic activity influences surface processes and landscape evolution. This would ensure the valid evaluation of landslide hazard and optimization of mitigation strategies.”

Urbanization can contribute to landslides in other ways, as well. Building heavy structures on sloped areas can increase the load of the soil, potentially destabilizing the slope. In addition, urbanizing areas often use soil from the hill itself for various construction programs. Deforestation is also a problem linked to urbanization.

A separate study from 2022 found that climate change will undoubtedly aggravate the conditions causing landslides, and unregulated urbanization will compound on this problem, affecting more and more settlements in underdeveloped regions of the world in the future — particularly in the tropics, where heavy rainfall and informal urbanization are expected to destabilize slopes.

The study was published in Nature.