Geologists eye shifts in canyon walls
This Friday’s reopening of U.S. Highway 50 following cleanup of the Mill Creek Slide will be accompanied by close monitoring of six additional slide risk areas in the American River Canyon.
The Jan. 24 Mill Creek Slide occurred in one of seven potential slide areas identified last year by geologists for the U.S. Forest Service.
No additional fallout is expected from the slide that closed the highway for four weeks, said Rod Prysock, a geotechnical engineer for the California Department of Transportation.
However, the potential for additional slides in other sections along a nearly 10-mile section of the canyon between Kyburz and Riverton will have Caltrans officials monitoring the highway 24 hours a day.
Forest Service geologists last February noticed movement in portions of the canyon walls near Riverton. This caught the attention of these scientists and their counterparts with Caltrans and the U.S. Geological Survey.
Ironically, their attention was focused on the Cleveland Corral Slide near Riverton when the Mill Creek Slide occurred about five miles to the east, said Chuck Mitchell, watershed group leader for the Forest Service.
“(The Mill Creek Slide) had shown a less movement than some of the other areas we had been looking at,” Mitchell said.
“The slide at Mill Creek certainly surprised folks,” Prysock added. “I don’t think that could have been predicted in advance.”
Prysock said mudslides typically do not occur as rapidly as the Mill Creek event, which trapped four vehicles traveling on the highway.
He believes that Caltrans officials would notice movement on the slopes prior to a major slide at the other problem areas.
If any slide does give way, it will likely occur during a rainstorm.
Rainfall totals calculated at more than twice the average for both December and January are suspected to be the major cause of the Mill Creek Slide, according to a report by Forest Service Geologist Anne Boyd.
Future rainfall will likely be met with anxiety.
“With 24-hour road patrolling, we will revue the situation if it starts raining and decide if we should close the road,” Prysock said.
Boyd’s report, dated Feb. 5, noted that the Cleveland Corral Slide showed “at least 10 feet of downslope movement occurred between early March and April of last year.”
Since then, the Forest Service and USGS have installed electronic monitoring devices in the slide area and are establishing a method to monitor the slide on the Internet, Mitchell said.
Boyd’s report noted that during a survey on Jan. 29, geologists determined that the slide had progressed upslope “so that it now reached Ice House Road.”
The report does not address how quickly the earth could give way in a major rainstorm or assess the potential magnitude of damage that could occur if this slide was to imitate the Mill Creek Slide.
Even with monitoring, Mitchell acknowledged that it is difficult to predict if and when a slide will give way.
“It’s not an exact science of predicting ‘if we get this much moisture level, (the slide) is going to fail,'” he said.
To minimize the risks, Caltrans crews and the agency’s contractors have taken steps to ensure maximum drainage of the canyon’s walls. In some areas, horizontal drains have been installed. In other areas, crews have dug channels to prevent water from forming pools on the hillside.
Prysock is confident that these steps combined with monitoring will ensure public safety on the highway.
“We can’t give a 100 percent guarantee, of course, that some location might not come down,” he said. “But we feel it’s safe to reopen the road. We feel we are taking prudent precautions while providing service to the public.”
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