As a rainy winter plods on, scientists, resource managers and property owners from coast to coast cast worried eyes on America’s hillsides for signs of this common natural geologic hazard – a landslide.
Landslides can happen anyplacethat has unstable hillslopes and a contributing factor such as an earthquake, a wildfire severe enough to alter soil properties, human modification of the landscape or, most commonly, rain. The West Coast, particularly California, is vulnerable to landslides because of its steeplands, its seasonal rainfall patterns, and the way its ongoing tectonic deformation exposes vulnerable sediments,not to mention the large population that lives in areas that could be affected by landslides, said Lynn Highland, a geographer with the USGS National Landslide Information Center in Golden, Colo.
The USGS is working on several fronts to better understand landslide hazards both in the United States and internationally and to provide information that will help resource managers and the public make safe choices in the face of landslide hazards. USGS scientists have mapped landslides, studied why landslides can repeat in the same places, created near-real-time monitoring systems, and calculated the economic costs of landslides. A “Did You See It” feature recently added to the USGS Landslide Hazards Program’s website allows people to report landslides anywhere in the United States and to see where landslides have been reported in the past.
In California, most people think of landslides as following prolonged heavy rain. They reference such tragedies as the San Francisco Bay Area’s Love Creek landslide, which claimed 10 lives in January 1982, or the Scenic Drive landslide in nearby La Honda, which rendered uninhabitable more than $40 million worth of homes in February 1998. Intense West Coast storms that hit from mid-December to April are much more likely to trigger landslides than storms earlier in the season, because they add water to steepland soil that has already accumulated significant moisture in its fissures and pores, said USGS research geomorphologist Jonathan Stock. In late November 2012, a heavy storm brought more intensive rain to some parts of the Bay Area than they received during the 1982 disaster. “But there was no widespread landsliding, because the soil pores were not yet fully saturated,” Stock said.
Stock and USGS research civil engineer Brian Collins have created a near-real-time landslide monitoring system in the San Francisco Bay Area. Sensors at four stations monitor pore water pressure and soil moisture conditions that could precede widespread landsliding. Similar USGS monitoring systems exist in parts of Oregon, southern California and Colorado. Years of observation using such systems are typically required to understand what combinations of soil moisture and precipitation lead to landslides.
It’s already known that many California landslides are associated with the airborne masses of water vapor called atmospheric rivers, which, because they originate in the semitropics of the Pacific Ocean and travel west, can be tracked several days before they reach the West Coast. Now, Stock, Collins and others are working on ways to integrate precipitation and soil moisture data to forecast susceptibility to widespread landsliding.
Hillsides that have been swept by wildfires also pose risk of post-fire debris flows. These post-fire debris flow warnings are issued by the National Weather Service to local emergency managers and the public. Here, the triggering process is different. Not only is vegetation destroyed that might intercept rainfall, but soil particles themselves can become coated with volatilized organic matter that makes them water-repellent, explained USGS emeritus research geologist Sue Cannon. More rainwater cascades off hillslopes than soaks in.
As it moves downslope, the rainwater collects increasing amounts of soil and rocks. The mix becomes denser and heavier as it travels, and in as little as 15 minutes can become a debris flow powerful enough to threaten structures or block highways, as happened after the 2009 Station fire in southern California and the June 2012 High Park fire west of Fort Collins, Colo.
Because this process does not depend on soil saturation, post-fire debris flows can be triggered by the first big rain of the year, Cannon said. USGS scientists created debris-flow hazard maps of the High Park burn area and found that some drainage basins had up to an 84 percent chance of producing debris flows if they received 25mm (just under an inch) of rain in an hour. USGS has also installed monitoring sensors in areas with high potential for post-fire debris flows, including basins burned by the 2009 Station Fire that threatened several cities at the base of the San Gabriel Mountains in southern California. The potential for post-fire debris flows generally lessens after two to three years, however, as burned areas become revegetated and sediment supplies are depleted.
USGS’ Landslide Hazards Program has developed methods to characterize debris-flow hazards from recently burned areas, including empirical models to predict for a given point along a drainage network 1) the volume of debris flow; 2) the probability of debris flow; and 3) the downstream path that the debris flow would take . “A set of models were originally developed for burned areas in the Intermountain West,” Cannon said, “and we are now in the process of developing models specific to conditions within Southern California because of differences in the region’s fire behavior and the large number of people potentially impacted there.”
An important part of mitigating landslide hazards is the understanding that landslides can recur at the same sites, Stock said. Water can seep off the toe of an old slide and cause failures there, he said. Other slope failures calve off earlier failures, as happened in the seaside community of La Conchita, Calif., in 1995 and again in January 2005. No one was injured in the 1995 event, but a debris flow a decade later killed 10 people. Both landslides were within the footprint of an ancient landslide on the slope behind them, as aerial photography and LIDAR imagery reveal. La Honda’s Scenic Drive homes sat atop an ancient landslide as well.
Landslide hazard maps of many areas are available from USGS as well as the California Geological Survey and other states’ geological offices, along with resources and tips on how to help avoid landslide hazards. These landslide maps identify areas where the greatest threat to property exists from the movement of deep-seated landslides.
Other USGS science investigates how landslides fit into larger ecological processes and economic assessments. Highland studies direct economic losses from landslides in addition to indirect losses. Landslide losses in many cases are ascribed to earthquakes or floods, but actually are due to landslides that the earthquakes or floods have triggered. In the case of the 1964 Alaska earthquake, much of the economic damage was actually done by landslides, Highland said. Other USGS scientists are studying large-scale landslide hazards threatening people and property in coastal Oregon and Washington, Appalachia, China, Micronesia, and Haiti, as well as rockfall associated with the 2001 earthquake in Nisqually, Wash., and the 2011 earthquake in Mineral, Va.
More USGS landslide resources:
FAQs on landslides
Other USGS publications on landslides
Images of landslides around the world
Pilot project toward a national landslide inventory
USGS geologists working on landslide research
How to find landslide information in your state
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