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APPENDIX A Case Studies A Widespread Problem The following examples are presented to give the reader an aware- ness of the characteristics and variety of landslides how they are triggered, their size and speed and various community, institu- tional, or technological responses to these hazards. Coastal Erosion, California. Much of the coastline of California consists of bluffs composed of relatively soft and poorly consolidated sediments (Figure A.1) that do not readily withstand erosion and undercutting by ocean waves and currents. The crest of the bluffs is an old, relatively level marine erosion surface that constitutes a highly desirable residential location, with easy access from inland and attractive ocean views. Many residents do not recognize the hazard of landslides and bluff retreat. Extensive damage results when winter storms and heavy rainfall combine to cause the coastal bluffs to fail. Recently, a small landslide on a coastal bluff received local notice; the following report was printed in the North County Times (San Diego County, California) on Saturday, May 25, 2002: DEL MAR Another chunk of the city's bluffs crashed onto the beach Friday morning, bringing the unstable sandstone much closer to the railroad tracks and creating a sheer precipice along a popular beach access point. The 20-foot-long, 5-foot-deep section slidfrom beneath afoot path at 11th Street at about 6:30 a.m. A pile of sandstone and sandstone boulders spread across the narrow beach below, nearly to the water's edge. Atop the 60-foot bluff, a number of deepfissures near the collapse suggest that more of the cliffmay slide. The access trail at 11th Street is afavoritefor many surfers and residents, although the city does not maintain the path, nor does it authorize its use. 117
118 APPENDIX A FIGURE A.1 Overview of the sea cliffs near Del Mar, California. The upper two- thirds of the cliff are formed of relatively soft, young (Pleistocene) Bay Point Formation, while the lower steep cliff is composed of more durable, older (Eocene) Torrey Sandstone. Both units retreat under wave attack and require costly retain- ing walls to provide some erosion protection. SOURCE: Photo by Chris Metzler, Aura Costa College (http://www. m~racosta.cc.ca.us/ home / cmetzler / field_trip / top .html). This landslide was modest in size; it injured not a soul and caused no property damage. Yet it occurred in a well-populated area and was wit- nessed by early morning beachgoers. The landslide hazard at the Del Mar bluffs persists, and the risk to the nearby railroad has increased. Fatalities in Oregon. On November 18, 1996, heavy rains on the Oregon Coast Range dropped more than 18 cm of rain in a 24-hour period, lead- ing to widespread shallow landsliding and debris flow generation. Along Rock Creek, near the town of Roseburg, people had built homes on the sloping surfaces of debris fans formed at the base of steep rocky hillslopes. When they built their houses, these hillslopes were covered with dense old forest. However, in the 1980s the slopes were clear-cut, despite con- cerns raised by Rock Creek residents, and waste wood was tossed into downslope steep gullies (Squier and Harvey, 2000~. Removal of the trees caused the elaborate root systems that were laced through the stony, loose
APPENDIX A 119 soil to fade away, as first the fine root hairs and then the larger roots died. With this loss of root strength, the soils became highly vulnerable to slope instability. In the late afternoon of November 18, the heavy rains of the day had progressively increased the soil water content until patches of soil in two small valleys failed and flowed downslope. The first failure tumbled down a canyon toward a house, but slowed and came to rest in wooded areas of the broad lower valley. The second tumbled down a steep canyon, where it picked up debris as it traveled at 5 to 7 m/s. By the time the debris flow had reached the low, gentler slopes on which houses were built, the mass had increased by 40 times. More than 4,000 m3 of material, traveling upward of 9 m/s, smashed through one house, instantly killing the parents of a child who managed to run out the front door and escape as it swept by. The flow tore down the valley, sweeping away two more people and headed straight for another house. Fortunately it banked and turned to travel downstream, eventually coming to rest as it entered the mainstem Hubbard Creek. This entire event, from initial landslide, to the crushing death of four individuals, to the halting of the debris flow at Hubbard Creek, took only a few minutes. Subsequently, material disturbed by the slide, including wood and many household items (e.g., family pictures and clothing), continued down Hubbard Creek, depositing all the way out to the confluence with the Umpqua River (12 km from the headscarp) and beyond. This tragedy, and others in the storms of 1996 and 1997, led to the development of a warning system, compila- tion of hazard maps, and new legislation regarding forest practices. Rapid Debris Flow, New York. At approximately midday on April 27, 1993, a large landslide occurred along the foot of Bare Mountain in LaFayette, Onondaga County, New York, about 12 miles south of Syra- cuse. The landslide flowed rapidly toward the middle of the Tully Valley, involving approximately 50 acres of land. It destroyed three homes and caused the evacuation of four others. Luckily, most residents were away from their homes at the time, so no fatalities or serious injuries resulted from the landslide (Wieczorek et al., 1998b). The New York State Geologi- cal Survey reported that this was the largest landslide to have occurred in the state in more than 75 years. However, several parts of New York State, including the Finger Lakes region and the Hudson and Mohawk valleys, and other limited areas in the northeastern United States, such as Boston, and in the Puget Sound region of the Pacific Northwest are covered with similar glacial clays, originally deposited in lakes or in marine environ- ments. Such clays are characterized by unstable internal structures and ~http: / /www.oregongeology.com/Landslide/Landslidehome.htm.
20 APPENDIX A are prone to sudden landslide failures and rapid flowage of their debris, even over very gentle or level terrain. As a consequence, this type of land- slide hazard became the subject of collaborative studies by the U.S. Geo- logical Survey (USGS) and the State of New York (Wieczorek et al., 1996a). Onondaga County has begun to identify areas of landslide hazard and to zone them accordingly (eager and Wieczorek, 1994) Huge Coastal Slide, Michigan. When local resident George Weeks walked his dog along the shore in Sleeping Bear Dunes National Lakeshore, Michigan, on an unusually warm February morning in 1995, he was shocked to find that where there had only recently been a beautiful beach was now a steep 100-foot drop into Lake Michigan (USGS, 1998; Figure A.2~. The millions of cubic feet of sand that made up the beach and part of the high bluff above it had disappeared beneath the waters of the lake in a huge coastal landslide. Luckily, no one was on this popular beach when it slid off into the lake. The USGS was asked to investigate and determine the causes of the landslide. In 1997, USGS scientists studied the under- water part of the 1995 slide. Near the shore they found a deep hole where formerly there had been a gently sloping lake bottom. They also found that a thick blanket of slide debris extended more than 2 miles offshore FIGURE A.2 Warning sign put up by National Park Service rangers immediately after the 1995 slide. SOURCE: USGS (1998).
APPENDIX A 121 into water depths greater than 250 feet, much further and deeper than expected. Underwater video of the deeper part of the slide showed trees, which had been growing on the bluff and had been swept into deep water by the slide, protruding from the sand. The most probable cause was in- creased pore pressures within the bluff, resulting from entrapment of water from snowmelt behind the frozen bluff face or within confined sand layers. This mechanism is supported by history. Two similar landslides at Sleeping Bear Point in December 1914 and March 1971 also occurred in unseasonably warm weather during winter months. Sequential Coastal Slides, Massachusetts. Landslides are common along the Atlantic and Pacific coasts. An example from coastal southern Califor- nia has been presented already. Cape Cod is formed of unconsolidated glacial deposits, and the onslaught of Atlantic waves and surges has caused the coastal bluffs on its outer arm to retreat in a series of land- slides. The Highland Light lighthouse was built in 1797 and replaced in 1857, set back from the 183-foot bluff a distance of 500 feet. By 1990, a sequence of landslides caused the bluff to retreat to within 100 feet of the lighthouse. In 1996, the lighthouse was moved inland a distance of 450 feet. This sequence of events is an example of mitigation by "strategic retreat." Madison County Debris Flows, Virginia. The foothills of the Blue Ridge in central Virginia are dotted with working farms along meandering rivers. The setting is peaceful to the casual observer, but closer inspection reveals massive boulders at the base of hills. These are signs of past vio- lent geologic events catastrophic large landslides and debris flows that have sculpted the local landscape. An intense storm on tune 27th, 1995, produced 30 inches of rain in 16 hours over sections of the foothills of the Blue Ridge in Madison County, Virginia. Hundreds of debris flows were triggered on steep slopes and moved rapidly down mountain channels (Wieczorek et al., 1995~. Small flows joined to form larger flows that, upon entering lowland valleys, spread mud, boulders, and other debris and inundated homes and farms (Figure Aid. One debris flow traveled nearly 2 miles, and an eyewitness estimated that it moved at a speed approach- ing 20 miles per hour. Because of the severity of the storm's effects, rural communities were isolated when bridges, roads, and power and telephone lines failed (Burton, 1996~. The full extent of the damage was not recog- nized until aerial surveys were made several days later, and the county was declared a federal disaster area. Scientists have documented 51 his- torical debris flow events between 1844 and 1985 in parts of the Appala- chians most of them in the Blue Ridge area. Radiocarbon dating of plant remains from debris flow deposits indicate that these processes have occurred repeatedly over the last 34,000 years (USGS, 1996) and that
122 APPENDIX A FIGURE A.3 Aerial view of Madison County, Virginia, debris flaws; note destroyed house in upper right. SOURCE: Morgan et al. (1999~. recurrence intervals for individual river basins are not more than 2,000 to 4,000 years (Eaton et al., 2003~. Multiple Landslide Types, Pacific Northwest. The Pacific Northwest coastal mountains are frequently subjected to episodes of numerous relatively small landslides following winter storms. A typical episode occurred after two separate regional storms in November and December 1998. The November storm triggered several small landslides in south King County, Washington, that blocked a few roads, including north- bound Interstate 5 near the Seattle airport, but caused no serious damage.
APPENDIX A 123 FIGURE A.4 Damage from the head scarp of a small earth slide closed Oregon State Highway 229, 13 miles north of Siletz, Lincoln County, Oregon. The land- slide is typical of the numerous landslides caused by heavy winter rains in No- vember and December 1998 in the Pacific Northwest. SOURCE: Baum and Chleborad (1999~. The December storm was more serious. It followed a cold spell with con- siderable snow accumulation, and the runoff from snowmelt combined with rainfall caused flooding and triggered landslides throughout western Washington and, especially, western Oregon. A few of the landslides caused significant damage, and many temporarily blocked roads and highways (Figure Am. USGS scientists conducted reconnaissance surveys to assess the landslide triggering mechanisms (Baum and Chleborad, 1999), and reported that there were a variety of landslide types, including earth slides, rock slides, rock falls, rapid earth flows, and debris flows. Yosemite Valley Rock Falls, California. Rock falls and other types of mass movements are an important element of landscape development in Yosemite Valley. More than 400 rock falls have occurred in historic times; nine people have been killed and many others injured. The largest rock fall in memory occurred in fuly 1996, when two large rock blocks, with a combined weight of nearly 70,000 tons, fell more than 2,000 feet from the cliff face at Glacier Point to the valley floor near Happy Isles, a popular
24 APPENDIX A FIGURE A.5 Rock fall damage to E1 Portal Road, Yosemite National Park, February 12, 2001. SOURCE: National Park Service photo. trailhead and concession stand (Wieczorek et al., 1992~. The rock fall created an air blast that flattened about 2,000 trees in the vicinity. One person was killed at the concession stand, and 14 people were seriously injured. The National Park Service and USGS geologists conducted assess- ments and ultimately developed maps showing hazard zones and areas of rock fall potential in Yosemite Valley (Wieczorek et al., 1998a; Wieczorek and Snyder, 1999; Wieczorek et al., 1999~. However these maps do not predict when or how frequently a rock fall will occur; consequently, neither the probability of a rock fall at any specific location nor the specific risk to people or facilities can be assessed. Rock fall hazard is a continuing problem at Yosemite. On February 12, 2001, a rock fall closed the E1 Portal Road (Highway 140) approximately one-half mile east of the park boundary for about 24 hours. A slab of granite of unknown size was released approximately 1,000 feet above the road. On impact, the slab broke into many smaller pieces ranging in size from 2 to 12 feet in diameter, causing damage to the roadway (Figure Add. Earthquake-Induced Landslides. Earthquakes trigger landslides (NRC, 2003~. There are many examples from Alaska, California, Montana, and
APPENDIX A 125 other states prone to earthquakes. As well as the massive 1964 "Good Friday" Alaska earthquake, the 1994 magnitude 6.7 Northridge earth- quake in southern California triggered more than 11,000 landslides the vast majority were highly disrupted, shallow falls and slides of rock and debris that occurred over a wide area (Harp and libson, 1995~. Landslide damage from the Northridge earthquake was only moderate because the area was neither heavily developed nor populated. However landslides did block roads, damage and destroy homes, disrupt transportation links and lifelines, and damage oil and gas production facilities. Volcanic and Submarine Landslides. The slopes of volcanoes frequently experience landslides (e.g., Zimbelman et al., 2003~. The cone of a volcano is built of material that falls or flows to the angle of repose, so that moder- ate shaking or subsequent eruptions can cause the volcano flanks to slide. The landslide hazard for volcanoes in the United States is geographically restricted and is confined to Alaska, California, Hawaii, Oregon, and Washington. In some cases, volcanic landslides can occur underwater, with the potential to cause destructive tsunamis (NRC, 2000~. Giant slides have been identified surrounding the Hawaiian Islands (e.g., Moore and Clague, 2002) and on many of her continental margins (e.g., Lewis and Collot, 2001~. These landslides are among the largest known on earth, and most have occurred within the past 4 million years. Understanding giant sub- marine landslides is critically important because although they occur infrequently, they can produce destructive tsunamis and accordingly have the potential to cause enormous loss of life, property, and resources throughout surrounding coastal regions (e.g., the 1998 Papua New Guinea tsunami; Bardet et al., 2003; Dengler and Preuss, 2003; Liam Finn, 2003; Okal, 2003; Wright and Rattle, 2003~. REFERENCES Bardet, J.P., C.E. Synolakis, H.L. Davies, F. Imamura, and E.A. Okal, 2003. Landslide Tsunamis: Recent Findings and Research Directions. Pure and Applied Geophysics, 160~10- 11~: 1793-1809. Baum, R.L., and A.F. Chleborad, 1999. Landslides triggered by Pacific Northwest Storms, November and December 1998. U.S. Geological Survey web page, online; available at http://landslides.usgs.gov/Wash-Or/PNW98.html; accessed June 2003. Burton, W.C., 1996. When the earth moved in Madison County. Washington Post, June 12. Dengler, L., and J. Preuss, 2003. Mitigation Lessons from the July 17, 1998 Papua New Guinea Tsunami. Pure and Applied Geophysics, 160~10-11~: 2001-2031. Eaton, L.S., B.A. Morgan, R.C. Kochel, and A.D. Howard, 2003. Role of Debris Flows in Long-Term Landscape Denudation in the Central Appalachians of Virginia. Geology, 31: 339-342. Harp, E.L., and R.W. Jibson, 1995. Inventory of Landslides Triggered by the 1994 Northridge, California Earthquake. U. S. Geological Survey Open-File Report 95-213.
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