Decreased slope stability from logging combined with increased rainfall from climate change will mean more landslides in the future, according to a Washington State University study.
The study, published in Engineering Geology, was modeled after clearcuts on the Olympic Peninsula and based off of the predicted climate of the year 2045. It forecasted that there is likely to be an increase between 7 and 11 percent of areas that are highly vulnerable to landslide events. Researchers noted that those findings are applicable to the Cascade Mountain Range as well.
A press release from WSU noted that the study is the first to look closely at the future of landslides in the Pacific Northwest as they relate to climate change.
“Logged landscapes become more susceptible to landslide activity under climate change,” said Jennifer Adam, an associate professor of civil and environmental engineering and associate director of the State of Washington Water Research Center, in the release.
Adam says that the study can be used as both a warning to the general public and a tool by land managers as they work to identify key identifying features of vulnerable slopes that can help to inform harvesting decisions. The WSU researchers found that vulnerable slopes tend to be comprised of talus or sandy soils at elevations above 1,600 feet with a slope degree of at least 40 degrees.
“This study allowed us to look at exactly what characteristics of the landscape became more susceptible, allowing us to identify win-win situations in which both environmental and economic outcomes can be improved by targeted logging locations,” Adam said in the release. “It’s not that we’re saying, ‘Don’t log because you’re going to have more landslides.’”
According to the study’s lead author, Muhammad Barik, the study “is telling people, if you are cutting trees on this slope, it might be OK today. But in the future, it might not be, so plan according to that. If you do logging in this area without considering future projections, it might become susceptible to landslides.”
The deadliest landslide in U.S. history occurred outside of Oso, Washington, in 2014 when 270 million cubic feet of mud covered a neighborhood. That event killed 43 people and destroyed 49 homes. An engineering report conducted after the landslide found that recent logging activity was likely to have increased the amount of water on the slope but stopped short of pinpointing a single cause.
Landslides can cause billions of dollars of economic loss as well as damage to sensitive aquatic habitats, like those of the region’s endangered salmon. The study noted that clear-cutting timber reduces the amount of rainfall that is caught by leaves and decreases the ability of tree roots to reinforce the hillside.
“Wet soil is not cohesive, so it becomes very unstable,” said Adam in the release. “If you don’t have a lot of vegetation and deep roots holding that soil in place, then it becomes susceptible to landslides.”
The western edge of the Olympic Peninsula was chosen by the researchers because of its varied geography and prolific rainfall. The Queets watershed in the Olympic Experimental State Forest receives between 96 and 236 inches of rain per year. However, the researchers noted that the terrain they studied is typical of rainforest areas from Oregon to southeast Alaska.
Climate change is forecast to bring warmer and wetter winters to the region in addition to more frequent bouts of extreme rainfall that are known to cause slope failures.
“The combination of warming, precipitation and less snow means more liquid precipitation, which will then sit in the soil and keep it wet and unstable,” said Adam in the release.
One model that used the lowest anticipated greenhouse-gas emissions between now and 2045 predicted a 7.1 percent increase in areas susceptible to landslides. Another scenario, which predicted the highest level of emissions, forecast a 10.7 percent increase in landslide prone areas.
“We’re giving you a tool to see into the future,” explained Barik. “Most of the landslide studies are based on historical data. Here, along with historical data, we also used climate models so you can look at future projections.”
The full study can be viewed online at www.sciencedirect.com/science/article/pii/S0013795216307682.