Slow-moving landslides increasing risk to expanding mountain communities

Slow-moving landslides increasing risk to expanding mountain communities
by Robert Schreiber
Berlin, Germany (SPX) Sep 19, 2024

As populations grow in mountainous regions, more people are building on unstable slopes, which are prone to slow-moving landslides, according to a recent study. Although these types of landslides are often overlooked in risk estimates, they pose a potential danger to hundreds of thousands of people worldwide, the researchers warned.

These landslides, which can move between one millimeter and three meters per year, are particularly concerning because the movement can be imperceptible. This leads people to believe the areas are safe for settlement. However, over time, the landslide movement can damage homes and infrastructure. Furthermore, slow-moving landslides can suddenly accelerate, especially during periods of heavy rainfall, increasing the risk of significant damage and even fatalities in extreme cases.

As urban development pressures increase, particularly when lower elevations are threatened by flooding, people are pushed to resettle on these risky slopes. According to the Intergovernmental Panel on Climate Change (IPCC), about 1.3 billion people live in mountainous regions, a number that is continuing to rise.

"As people migrate uphill and establish settlements on unstable slopes, a rapidly rising population is facing an unknown degree of exposure to slow landslides - having the ground move underneath their houses," said Joaquin Vicente Ferrer, a natural hazards researcher at the University of Potsdam and the study's lead author.

This research marks the first global study focused on the risk of slow-moving landslides, which are not typically included in most assessments of landslide threats to human populations. The study was published in 'Earth's Future', a journal focused on research about the planet's past, present, and future.

Tracking Slow-Moving Landslides
The authors created a new global database of 7,764 slow-moving landslides, each covering at least 0.1 square kilometers (about 25 acres), in areas designated as "mountain risk" by the IPCC. Using statistical models, they investigated global and regional factors that increase exposure to these landslides.

Of the landslides cataloged, 563 (approximately 7%) were found to be inhabited by hundreds of thousands of people. The highest concentrations of these slow-moving landslides were identified in northwestern South America and southeastern Africa. Significant populations were also found on landslides in Central Asia, the Tibetan Plateau, and northeast Africa. The Alai Range in Kyrgyzstan had one of the largest clusters of inhabited slow-moving landslides.

The study focused exclusively on permanent settlements, excluding refugee and nomadic communities.

The researchers observed that urban sprawl in all regions of the study area contributed to an increase in the risk of slow-moving landslides. As cities expand, especially in poorer regions, development often takes place on unstable land, including areas already known to be susceptible to slow-moving landslides.

Flooding and Climate Change
The study also explored the link between climate conditions and the activation of slow-moving landslides. While the exact triggers are still not well understood, scientists believe that intense rainfall and fluctuations between dry and wet conditions can cause these landslides to speed up. These same climatic changes can also increase the likelihood of flooding, which often forces people to relocate to higher, and potentially less stable, ground.

Regions affected by flooding tended to have more settlements on slow-moving landslides. Western North America and southeast Africa had the strongest correlation between flood exposure and landslide settlement.

The study also noted that certain areas with known landslide risks, such as the Hindu-Kush Himalayas, lack sufficient landslide mapping and monitoring data, underscoring the need for better detection efforts in these vulnerable regions.

"We highlight a need to ramp-up mapping and monitoring efforts for slow-moving landslides in the East African Rift, Hindu-Kush-Himalayas, and South American Andes to better understand what drives exposure," said Ferrer. "Despite a limited number of landslide inventories from Africa and South America, we found communities in cities are densely inhabiting slow-moving landslides there."

Even in regions with advanced landslide mapping, such as northern North America (Canada and Alaska) and New Zealand, communities remain at risk from slow-moving landslides, even though these areas were not covered by the study.

"Our study offers findings from a new global database of large slow-moving landslides to provide the first global estimation of slow-moving landslide exposure," Ferrer added. "With our methods, we quantify the underlying uncertainties amid disparate levels of monitoring and accessible landslide knowledge."

Research Report:"Human settlement pressure drives slow-moving landslide exposure"