Map of major landslides in the Western U.S. Click map to enlarge.

GENERAL IDEA…

 

Landslides are the downward and outward movement of earth materials on a slope, which generally move by the falling, sliding, or flowing of rock and or soil, or by a combination of these and other less common types of movement. Also, its causes include earthquakes, reservoir drawdowns, heavy precipitation, and floods. Hence, costly landslide events are those where there was loss of human life or damage to public or private property.

 

Landslides are widespread which represent a major geologic hazard in particular the United States. Indeed, it occurs in all 50 states and territories, which causes $1-2 billion in damage and more than 25 victims on average each year. Furthermore, landslides particularly threatens highways and structures that support fisheries, tourism, timber harvesting, mining , and energy production as well as general transportation. Thus, landslide activities usually occur in connection with earthquakes, volcanoes, wildfires, and floods.

Thistle landslide in 1983

 

A destructive landslide was active for a two-week period in April, 1983, near the town of Thistle, about 90 km south-southeast of Salt-Lake City. Indeed, together with the direct and indirect costs from the Thistle landslide exceeded about $400 million, making it the most expensive single landslide in U.S. history. Also, the resulting lake, nearly 5 km long and 60 m deep, inundated Thistle, three major highways, and an important route of the Denver & Rio Grande Western Railway, and it posed a flooding hazard to downstream communities. Thus, this disaster occurred when central Utah experienced unusual heavy precipitation during the late 1982 to Spring, 1983, which saturated slopes in the Wasatch Range and nearby mountain ranges triggering numerous landslides and debris flows.

 

WHERE AND WHEN…

 

Landslides are common throughout the Appalachian region and New England in the East, the Ohio, Missouri, and Mississippi valleys in the Midwest, and all mountainous area of the West. In addition, landslides are also common in Alaska and Hawaii, as well as Puerto Rico and other U.S. properties. However, major hazards in the eastern U.S. are from debris flows and from sliding of soils. For example, debris flows resulting from heavy rainfall in 1969 and 1995 have taken many lives in the Blue Ridge of Virginia and landslide damages in urban areas such as Pittsburgh, PA, and Cincinnati, OH, are among the greatest per capita in the U.S. Moreover, landslides also cause serious property damage along the shores of the Great Lakes and on bluffs of major rivers throughout the Midwest. Nonetheless, rapidly melting snow late in the spring of 1983 reactivated hundreds of large landslide deposits and triggered thousands of debris flows throughout central and northern Utah and caused hundreds of millions of dollars in damage.

The 1983 Thistle, Utah, landslide buried parts of two State highways and tracks of the Denver and Rio Grande railroad.

(Air photograph provided by the USGS National Landslide Information Center)

Video of Sultan River Landslide in the US. (youtube.com) 

LANDSLIDE-DEBRIS FLOW…

 

Debris flows occur wherever lithology and weathering patterns produce ample loose material on steep slopes; periodic heavy rains or rapid snow melt trigger debris flows in these areas. On the other hand, loss of vegetation and ground cover that occurs during wildfires further enhances debris-flow susceptibility. In addition, sixteen people lost their lives in debris flows on December 25, 2003, when heavy rain triggered flows in recently burned areas near San Bernardino, California. Indeed, major storms have caused widespread flooding and landslide events along the Pacific coast.