Slow-moving landslides creep just a few centimeters or meters per year, but they persistently alter landscapes via erosion.
Scientists have now analyzed hundreds of slow-moving landslides in Northern California to trace how their activity is linked to rainfall. The team found a pronounced uptick in the number of landslides in 2017, California’s second-wettest year on record. The slow-moving landslides active in 2017 also tended to be smaller, shallower, and faster than their brethren in drier conditions, the researchers showed.
Finding so many active landslides has implications for both hazard analyses and studies of how sediments are transported, the scientists concluded.
So Slow They’re Missed
The plodding pace of slow-moving landslides means they’re often missed.
“Sometimes people don’t even know they’re there and build on top of them,” said Alexander Handwerger, a geoscientist at NASA’s Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and the lead author of the new study.
But slow-moving landslides are important because they afford researchers a long-term look at how landslides, even the classical ones that rush down hillsides in a matter of seconds, unfold. “They’re essentially governed by the same physics as fast landslides,” said Handwerger.
Handwerger and his team studied slow-moving landslides in the Eel River catchment in Northern California. This catchment’s clay-rich, mechanically weak soil—paired with the area’s seasonal precipitation (mostly between October and May) and active plate tectonics—makes this region a hotbed for landslides, said Handwerger.
A Jet for Science
The researchers collected radar observations of roughly 4,700 square kilometers of the grassland-dominated landscape using NASA JPL’s Uninhabited Aerial Vehicle Synthetic Aperture Radar mounted on a modified Grumman Gulfstream III business jet. By comparing radio observations collected eight times between April 2016 and February 2018 using a technique called synthetic aperture radar interferometry, Handwerger and his team pinpointed areas where the ground had moved over timescales ranging from roughly a month to nearly 2 years.
The scientists focused on a time period that included a range of precipitation levels: 2016 was the final year of a historic drought, 2017 had above-average rainfall, and 2018 was again a drier year. “It was a really interesting time to study landslides because it went from extremely dry to extremely wet in a very short time period,” said Handwerger.
Smaller, Shallower, Faster
The researchers measured a significant jump in the number of slow-moving landslides in 2017. They found 312 landslides in water year 2017 (the time period between 1 October 2016 and 30 September 2017), more than double the number of slow-moving landslides in water years 2016 and 2018.
“That so many landslides were active in that short period was surprising,” said Handwerger.
Furthermore, the scientists showed that the landslides active in 2017 tended to be smaller, shallower, and faster than their counterparts active in drier conditions.
“The extreme rainfall of 2017 triggered a widespread but short-lived increase in the activity and velocity of the landslides,” the researchers reported in June in the Journal of Geophysical Research: Earth Surface. That makes sense, Handwerger said, because smaller landslides, which tend to be shallower, are more sensitive to individual rain storms. Rainfall boosts the water pressure within soil, increasing its density and making it more likely to shear down a slope.
Slow-moving landslides will likely remain a fixture of the Northern California landscape, said Handwerger. That’s because annual precipitation and transitions between dry and wet conditions are both predicted to increase in California.
These slides will alter the environment by changing the shapes of hills and rerouting drainage networks, the researchers concluded. “That could have big implications for the sediment flux and hazard potential,” said Handwerger.
This “interesting and novel” study presents a largely complete and unbiased sample of rainfall-triggered landslides, said George Hilley, a tectonic geomorphologist at Stanford University in California not involved in the research. Despite the known link between rainfall and landslide activity, said Hilley, “seeing how many and how extensive these features are is enlightening.”
Handwerger and his team are looking forward to collecting more observations of slow-moving landslides, particularly in years with abnormal levels of precipitation. The researchers recently secured three more years of funding to use NASA JPL’s Uninhabited Aerial Vehicle Synthetic Aperture Radar.
“We want to keep extending the time series,” said Handwerger.
—Katherine Kornei (@katherinekornei), Freelance Science Journalist