Aerial Reconnaissance, Central Sierra Nevada Mountains
Aerial Reconnaissance of Potential Landslide Activity in the Central Sierra Nevada Mountains
Here is a brief summary of the aerial reconnaissance of potential landslide activity in the central Sierra Nevada, flown by three USGS geologists, Mark Reid, Steve Ellen (emeritus) and Ray Wilson, on Wednesday, January 8, 1997. The purpose of the flight was to ascertain the extent of landslide activity from the heavy storms of the previous week (New Year's 1997 Storm). News accounts had suggested that many of the highways through the Sierras were blocked and extensively damaged by mudslides and other landslide activity.
An unusual aspect of the New Year's storm of particular technical interest was the precipitation of rain from a subtropical air mass onto a heavy, pre-existing snow pack, a "rain on snow" event. Based on our aerial reconnaissance of landslides in Washington and Oregon from a similar storm in February 1996, we (Reid and Wilson) expected a concentration of debris-flow activity in elevations between the pre-storm and post-storm snow levels. For the New Year's storm in the central Sierras, this corresponds roughly to 4000 to 8000 feet. The focus of our reconnaissance was this band of elevations on the western slope of the Sierras.
After departing from Palo Alto airport, we flew to the southwestern corner of Yosemite National Park where news accounts had described a massive rock fall onto State Route (SR) 140. We located this failure 6 miles east of El Portal, on the north side of the Merced River. We have since learned that this feature actually occurred in 1982. The road was reconstructed on a fill placed over the rock debris, and the New Year's (1997) storm washed away this fill, uncovering the old rock fall. In the air photos published by the news media, the road surface appeared to lie below the rubble, but actually lies above the old debris fan.
Approximately 30 miles to the northwest, a massive debris flow was observed crossing SR-4 just east of the hamlet of Dorrington. This debris-flow is approximately 300 feet wide and flowed a distance of some three miles, losing 2000 feet of vertical elevation, from the ridge crest down into and across the Stanislaus River. SR-4 has been cleared and re-opened, but the debris flow also destroyed the bridge across the Stanislaus at Board's Crossing.
Several large debris flows dropped tons of mud and woody debris into Salt Springs Reservoir on the Mokelumne River. A number of smaller debris flows, distributed between several small (<5 sq. mi.), widely scattered areas, were observed between the Stanislaus and Mokelumne Rivers.
We then flew north to US Highway 50, where near Kyburz, we observed a concentration of smaller, shallow debris flows in a large area burned in the summer of 1995. Further east, about 2 miles west of Strawberry, we observed a large debris flow that had covered US 50 with mud and granite boulders for a distance of several hundred feet. Several large debris flows crossed the highway in this vicinity.
North of US 50, we observed a several large debris flows in the "Royal Gorge" canyon along the American River, south of Snow Peak. These debris flows fell from the top of the canyon over a thousand feet down into the river.
We continued north to Interstate 80, then flew eastward along I-80 from Soda Springs to the Nevada border, seeing nothing unusual, despite news accounts of mudslides partially blocking the roadway.
We then turned around and flew westward back over the summit and followed the Yuba River from Lake Spaulding to Englebright Reservoir. We saw several small debris flows, very widely scattered, on the slopes on either side of the Yuba, but no significant concentrations of landslide activity. From Englebright Reservoir, we flew back to Palo Alto Airport, concluding our reconnaissance.
Summary
We saw much less landslide activity than we expected, given the magnitude of the storm and the evidence for high runoff and moisture levels we could observe from the air (standing water, heavy flow in waterfalls, scouring along creek beds, and so forth). Certainly, there was much less activity than we had observed in the Cascade Range after the February, 1996 storms. On the other hand, a few of the Sierra landslides we did observe (e.g. Dorrington) are spectacular. Steve Ellen kept a running tally of landslide features observed during our flight--one large rock fall (SR 140), one significant debris slide (near Beardsley Reservoir), 62 debris flows (11 of these large features with long runouts), and one slump (along the Yuba River). Other than those landslides that impacted roadways (US 140, SR 4, and US 50), we saw no damage or potential hazard to public safety from landslides from the New Year's Storm in the central Sierra. However, it is a well known characteristic of the largest, deepest landslides to initially deform by very slow creep, exhibiting little or no evidence of movement for periods of up to several weeks after a major storm, then suddenly accelerate their movements, with potentially catastrophic effects, long after the storm has passed. Finally, the winter is only half over, with large storms and the spring snowmelt season yet to come.
Figure 1
Figure 1 Large debris flow near Dorrington on State Route 4. Total runout distance is about three miles. Photograph by Mark Reid—USGS (336K)
Figure 2
Figure 2 Toe of Dorrington debris flow. The debris flow actually crossed the Stanislaus River and deposited material on the opposite bank, indicating that the slide moved at a high velocity. Photograph by Mark Reid—USGS (255K)
Figure 3
Figure 3 Large debris flow crossing US Highway 50 between Kyburz and Strawberry, closing the main road to South Lake Tahoe. Photograph by Steve Ellen—USGS emeritus (270K)
Figure 4
Figure 4 Another debris flow that crossed US 50, blocking the highway about a mile east of the larger debris flow in the previous figure. Photograph by Steve Ellen—USGS emeritus, (279K)
Contact Information
| Raymond C. Wilson Earthquake Hazards Team, Eng. Geology and Geophysics Section U.S. Geological Survey (MS-998) 345 Middlefield Rd. Menlo Park, CA 94025 |
