Ferguson Rock Slide Buries California State Highway Near Yosemite

Recent Landslides Landslides Edwin L. Harp . Mark E. Reid . Jonathan W. Godt . Jerome V. DeGraff . Alan J. Gallegos DOI 10.1007/s10346-008-0120-9 Received: 14 June 2007 Accepted: 18 January 2008 Ferguson rock slide buries California State Highway near © Springer-Verlag 2008 Yosemite National Park Abstract During spring 2006, talus from the toe area of a rock- about future movement of the approximately 800,000-m3 slide mass. block slide of about 800,000 m3 buried California State Highway Normally, the highway accommodates about 800,000 vehicles per 140, one of the main routes into heavily-visited Yosemite National year. During the 92 days in 2006 when the highway was completely Park, USA. Closure of the highway for 92 days caused business closed, the nearby city of Mariposa sustained losses in tax and losses of about 4.8 million USD. The rock slide, composed of slate business community revenue of about 4.8 million USD (Rick Benson, and phyllite, moved slowly downslope from April to June 2006, Mariposa County Administrator, personal communication, 2007), creating a fresh head scarp with 9–12 m of displacement. traffic congestion increased on other routes to Yosemite National Movement of the main rock slide, a re-activation of an older Park, and local residents and school children endured greatly slide, was triggered by an exceptionally wet spring 2006, following lengthened commutes. Moreover, future downslope movement of a a very wet spring 2005. As of autumn 2006, most of the main slide large part of the slide mass could potentially impact the highway appeared to be at rest, although rocks occasionally continued to realignment and dam or alter the Merced River, a designated fall from steep, fractured rock masses at the toe area of the slide. National Wild and Scenic River. A potential rock-slide dam could Future behavior of the slide is difficult to predict, but possible pose additional hazards from inundation of upstream infrastructure, scenarios range from continued scattered rock fall to complete homes, and businesses and downstream flooding of campgrounds, rapid failure of the entire mass. Although unlikely except under very roads, and other facilities should such a dam breach rapidly. Here, we destabilizing circumstances, a worst-case, rapid failure of the entire review documented nearby large slides in the Sierra Nevada rock slide could extend across the Merced River, damming the river Mountains, describe the Ferguson Rock Slide reactivated in the and creating a reservoir. As a temporary measure, traffic has been spring of 2006, discuss the continuing hazards posed by this slide, reroutedtotheoppositesideoftheMercedRiverataboutthesame and present a brief overview of mitigation and monitoring efforts. elevation as the buried section of Highway 140. A state-of-the-art monitoring system has been installed to detect movement in the steep Nearby large slides in the Sierra Nevada Mountains talus slope, movement of the main slide mass, local strong ground Compared to some mountain ranges elsewhere in the world, such motion from regional earthquakes, and sudden changes in stream as the European Alps or the Himalayas, the Sierra Nevada levels, possibly indicating damming of the river by slide material. Mountains generate relatively infrequent massive rock slides. Nevertheless, there have been a number of recent large slides in Keywords Rock slide . Yosemite National Park . Monitoring . and near Yosemite (Fig. 1), similar in size to the Ferguson Rock GPS data . Hazard Slide. These slides have had severe impacts on people, commu- nities, and infrastructure (Table 1). Some large failures, both Introduction historic and prehistoric, have created landslide dams in adjoining A wide range of rock falls and rock slides, originating in both steep river canyons. Some of these dams failed rapidly and plutonic and metamorphic rocks, sculpt the Sierra Nevada catastrophically, others created long-lived impoundments. Mountains of California, USA. Massive slope movements have been triggered by rainfall, increased groundwater levels, and Ferguson Rock Slide earthquake shaking (Harp et al. 1984; Wieczorek 2002). Yosemite During the wetter-than-average spring of 2006, talus from the toe National Park, a UNESCO (United Nations Educational Scientific of the Ferguson Rock Slide, located in the central Sierra Nevada and Cultural Organization) World Heritage Site of immense foothills, completely blocked California State Highway 140 (Figs. 1 granitic cliffs visited by about 4 million people each year, has and 2). The activity of the slide was first noticed on April 25, 2006, been the location of numerous rock falls and slides. Some of these as a few scattered rocks rolled onto Highway 140. At that time, have resulted in fatalities, damage to property, and road closures surficial displacement of soil and rock produced a distinct track (Wieczorek et. al. 1992, 2000). During the wet spring of 2006, a large extending from the road to its origin 60 to 70 m upslope. This rock slide reactivated the upslope of California Highway 140, about initial surficial sliding announced the eventual reactivation, over 10 km west of Yosemite National Park (Fig. 1). Talus from the toe of the next few months, of the entire rock-slide complex, having a the slide, locally known as the Ferguson Rock Slide, completely volume of about 800,000 m3. Rock-fall activity from the toe of the buried the highway, necessitating temporary rerouting of the slide accelerated in late May 2006, destroying recently emplaced roadway to the opposite side of the steep-walled Merced River road barriers and rock-fall protection fencing, burying the entire Canyon. The talus also encroached about 10 m into the river channel. roadway, and extending about 10 m into the Merced River. Rock- Because Highway 140 is one of three year-round routes into fall activity was greatest during late May and early June; it then Yosemite National Park and the only one without tunnels that is not decreased substantially over the summer. The main slide mass significantly affected by winter snow, there is widespread concern appears to be a reactivation of an older rock slide; the older Landslides Recent Landslides Fig. 1 Location maps showing the 119°54' position of the Ferguson Rock Slide at regional and local scales 37°40' Ferguson Rock Slidee 2 km 120°0' 119°50' 119°40' YOSEMITE N.P. S140 El Portal 37°40' Briceburg Ferguson YOSEMITE N.P. Rock Slide Midpines Wawona C 37°30 ALIFORNIA S140 Mariposa S49 20 km headscarp varies from 45 to 75 m in height. Review of available (Fig. 2). Immediately downslope of this area, the surface steepens aerial photography from 1944 to the present shows no evidence of from approximately 35°–40° to greater than 50°. This area is the significant pre-2006 movement on this feature. source of most of the rock fall that forms the talus slope that has The main slide complex emanates from a steep slope (38°–45°) buried the highway and extended into the river. This steep rock-fall in metamorphic slate, phyllite, and chert of the Phyllite and Chert source area may reflect a local steepening of the basal failure of Hite Cove (Bateman and Krauskopf 1987). The overall surface. The distal portion of the landslide is a steep (∼42°) talus dimensions are approximately 244 m in length at its midsection, slope of about 100 m length (Fig. 2). Talus fragments are coarse and 187 m wide at its widest extent, and 30–40 m deep (Gallegos and range between gravel and pebble sizes to boulders larger than cars. DeGraff 2006), although the depth to the failure surface is not well- Fragment shape is typically bladed prismatic and highly angular. known and may be highly variable. The main mass is a rock-block The basal shear surface of the rock slide appears to toe out below slide with numerous internal slumps below the headscarp. the steep, disrupted rocks at the head of the talus slope but also Movement of the main body of the slide mass appears to be appears to extend farther downslope to near the bottom of the slope primarily translational. Recent motion on the older main head- as if its dip steepens in the direction of the toe (see Fig. 2). The right scarp (Fig. 3) is about 9–12 m. Approximately 50 m downslope from side (looking downslope) of the slide toe has shed enough rock debris the reactivated headscarp is an area of multiple internal scarps recently to exhume what appears to be a shear surface in this area. Landslides Table 1 Other large slides in the Sierra Nevada Range Landslide Location and date Size and material Impact Sourgrass Debris Flow Stanislaus National Forest North 190,000 m3, glacial Destroyed electric transmission line, bridge, and telephone (DeGraff 2001) Fork Stanislaus River; till overlying volcanic cable; dammed Stanislaus River for several hours; deposit Jan. 1, 1997 mudflow breccia reduced capacity of McKay reservoir Glacier Point Rock Fall Yosemite National Park, Glacier 23,000–38,000 m3, Air blast destroyed 1,000 trees, one fatality, several injuries (Morrisey et al. 1999) Point, July 10, 1996 granitic intrusive Mill Creek Landslide Village of White Hall in El Dorado 1.5×106 m3; Destroyed three cabins and dammed the South Fork (Sydnor 1997) County., California, near US colluvium overlying American river for 17 h; closed US Highway 50 Highway 50; Jan. 24, 1997 mafic plutonic rocks for 27 days. US Highway 50 Landslide White Hall in El Dorado County, 765,000 m3; granitic Dammed the South Fork American River for 6 h and closed (Kuehn and Bedrossian California, near US Highway 50; rocks Highway 50 for 75 days; canal for irrigation and electric 1987) April 9, 1983 power disrupted; several houses submerged by lake Slide Mountain Rock Slide Yosemite National Park, 1.9×106 m3; quartz Dammed Piute Creek in northern Yosemite National Park, (Bronson and Watters 1987; Piute Creek; prehistoric monzonite marshy meadow remains Huber et al.

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