Mark E. Reid, and Richard G. LaHusen, 1998, Real-time Monitoring of Active Landslides Along Highway 50, El Dorado County: adapted from: California Geology, v.51, n.3, p.17-20
Late in the rainy evening of January 24, 1997, tons of earth gave way down a steep Sierra Nevada canyon slope and slid onto a major northern California highway (Photo 1). The since-named Mill Creek landslide closed U.S. Highway 50 and briefly dammed (5 hours) the nearby South Fork of the American River, about 25 miles east of Placerville (Sydnor, 1997). The slide damaged or destroyed three cabins (Photo 2), and waters dammed by the landslide flooded two vehicles on the highway. Fortunately, there were no fatalities and the waters subsided after the river cut through the dam later that night. However, before Highway 50 could be reopened, an estimated 350,000 cubic yards of slide material (35,000 truck loads) had to be removed over a 4-week period, at a cost of $4.5 million (California Department of Transportation, 1997). Indirect economic costs due to highway closure were estimated at more than $1 million per day.
Following the Mill Creek landslide, the U.S. Geological Survey (USGS), in cooperation with the Eldorado National Forest, acted quickly to install monitoring equipment that would measure landslide conditions and provide the results in real-time. The system was installed on the nearby active Cleveland Corral landslide which has the potential of blocking Highway 50 and possibly damming the American River if the entire slide moved rapidly. This landslide, with its downslope edge about 150 feet above the highway, had moved during the wet winter of 1996 and continued to move slowly downslope during the winter of 1997 (Photo 3).
Many other large landslides along this corridor of the South Fork of the American River have moved in the geologic past, and some may impact Highway 50 in the future (Wagner and Spittler, 1997). Although most slides in this canyon are dormant during dry times, they typically become active during or following extended periods of rain or snow melt due to increased ground-water pressures. These elevated pressures, in turn, reduce the overall strength of the slide and induce downslope movement. Many landslides along the corridor move slowly, traveling perhaps only a few inches over many days. Occasionally, however, a landslide will move rapidly, traveling hundreds of feet in a matter of minutes, as did the Mill Creek landslide in January 1997. Another occurrence upriver in 1983 closed the highway for 75 days (Kuehn and Bedrossian, 1987).
Prior to the installation of monitors, landslide movement patterns and associated hydrologic conditions along Highway 50 were not systematically measured. During the wet sinter of 1996, U.S. Forest Service geologists observed ground cracking in the hillslope that would later become the Mill Creek landslide. These field observations, however, were not sufficient to indicate that sudden and rapid movement would occur the following year. Elsewhere in the world, studies of landslides have shown that rapid slope failure may be preceded by gradually accelerating movement (Terzaghi, 1950; Varnes, 1983; Voight, 1989; Fukuzono, 1990). In order to detect these kinds of possible precursor movement for active landslides along Highway 50, continuous real-time monitoring was needed.
Soon after the Mill Creek landslide, the USGS installed a real-time monitoring system at the nearby active Cleveland Corral Landslide. A real-time monitoring system provides near-continuous measurements on the hydrologic conditions and ground movement of the landslide. This system is borrowed from USGS data acquisition and telemetry systems developed for remote monitoring of active volcanoes. Sensors for the system are installed in or on the landslide and the data are transmitted via radio telemetry to USGS computers (Photo 4). Data collected at such a continuous rate and in real-time will greatly increase the understanding of dynamic landslide activity and behavior in the Highway 50 corridor. The data will enable geologists to detect changes in landslide movement, monitor the rainfall and ground-water conditions, and hopefully anticipate possible catastrophic movement at the Cleveland Corral landslide.
Landslide movement and hydrologic conditions are being monitored using a variety of sensors. The amount of downslope movement is recorded by extensometers, anchored to the ground surface at the edge of the landslide (Photo 5). Ground vibrations associated with slide movement are monitored by geophones buried within the landslide. (These geophones measure a wider dynamic response than standard earthquake seismometers, and have successfully detected large debris flows from volcanoes [Hadley and LaHusen, 1995; LaHusen, 1996]). Ground-water conditions within the slide are monitored by pore-water pressure sensors, and on-site rain gages record rainfall. Data are sampled from these sensors every second and transmitted to the USGS every 10 minutes. However, data are transmitted immediately in the event of strong ground vibrations associated with massive landslide movement.
The USGS also has a cooperative project with the California Department of Transportation (Caltrans) to monitor five active landslide sites (including the Mill Creek landslide) along the Highway 50 corridor. Real-time data from the overall monitoring system, which involves 11 stations and 58 surface and subsurface instruments (Photo 6), are relayed through USGS computers to Caltrans engineers and geologists. These data may provide Caltrans with early notification of landslide activity and may also aid Caltrans engineers in the design of remedial measures to slow or halt these active landslides.
We thank Steve Ellen and Richard Iverson (USGS), and Roy Bibbens (Caltrans) for their helpful reviews of this manuscript.
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