[Reading Science] "'A city of 80,000 people' faces total collapse risk"

NASA's Study on Africa's Congo 'Land Sliding' Case
"Groundwater Movement is the Main Cause"

"The ground of an African metropolis with a population of 500,000 is entirely at risk."

As global warming has increased the frequency of heavy rainfall worldwide, a study has found that the movement of groundwater is the biggest cause of the slow but massive landslide phenomenon known as 'land creep.'

On the 3rd (local time), NASA's Jet Propulsion Laboratory (JPL) announced that it reached this conclusion after investigating satellite and aerial imagery of Bukavu, a city of about 500,000 people located on a plateau in the Democratic Republic of the Congo. Bukavu is currently a large city with a population of approximately 500,000. It is a rapidly growing city expected to double to 1 million by 2030. The problem is that a significant portion of the city has been rapidly constructed on unstable sloped ground without proper planning. Originally built along the flat shoreline of a lake, many buildings, roads, and other infrastructure were established on slopes during the expansion process.

Bukawushi, Democratic Republic of the Congo, Africa. Part of the city, dangerously built on a slope, is at risk. Photo by NASA.

During this process, Bukavu has suffered from slow but continuous destruction of infrastructure due to ground instability. In particular, the Funu area, home to about 80,000 people living in poor housing conditions, experiences land creep of up to 3 meters annually, causing fear among residents.

The research team utilized satellites operated by the European Space Agency (ESA) and the Italian Space Agency (ISA) to conduct radar measurements. They also examined factors that could influence ground movement, such as recent urban development, earthquakes, and rainfall. They referred to 70 years of aerial photographs taken from 1947 to 2018, preserved at the Royal Museum for Central Africa in Belgium. Notably, ground movement was measured weekly over a period of four and a half years.

The results showed that rainfall, ground activity, and urban development all influenced land creep, but the greatest risk factor was the flow of groundwater. Groundwater seeps into cracks in underground bedrock, weakening it, and the roads, stormwater drains, and broken water pipes installed during urban construction alter the flow of water, turning the slope soil into mud and destabilizing it.

Alexander Handwerger, a JPL researcher, explained, "Through this, we were able to confirm a clear correlation between urban growth and land creep. We found that land creep is caused not by the weight of additional housing on the upper layers, but by the flow of water."

Land creep is generally less threatening than fast-moving landslides, which cause thousands of casualties and billions of dollars in economic damage annually. However, it occurs gradually each year, causing mountains to collapse and sometimes triggering massive landslides that are difficult to repair. A representative case is the landslide at Mud Creek in Big Sur, California, USA, in 2017. A massive 5 million cubic meters of rock and soil covered the highway, causing damage that took a considerable time to repair. Rapid urbanization is occurring at an unprecedented pace worldwide. Abnormal weather such as heavy rainfall, which causes land creep, is also becoming more frequent due to global warming. The likelihood of a massive land creep landslide like Mud Creek occurring anywhere at any time is increasing.

Meanwhile, NASA plans to launch the SAR satellite (NISAR) in 2024 in collaboration with the Indian Space Research Organisation (ISRO), capable of day and night radar observation of surface movements, to accurately measure ground movement worldwide. NASA explained, "Monitoring subtle surface movements related to natural disasters such as landslides, earthquakes, and volcanic eruptions will help protect lives and property."