Читать книгу Disaster Response and Recovery - David A. McEntire - Страница 30

Geologic hazards are those hazard agents associated with the earth’s soil and rock surfaces. Landslides are the most damaging kind of geological hazards. This movement of earth occurs due to a number of variables such as slope angle, moisture content of the soil, and physiology of rock. The absence of

vegetation may also be a reason why landslides occur. A lack of root systems from trees and plants may make the soil more unstable. Landslides may move swiftly and occur without warning or creep at a slow and perhaps unnoticeable pace. Such events are possible in any hilly or mountainous area but are probably most common along the Rocky Mountain region and the Pacific Coast. In 1983, a major landslide blocked a major highway in Thistle, Utah. The sediment and rock created an earthen dam that backed up a river and flooded a city. In 2005, a sizable portion of the mountain separated in La Conchita, California, and fell to the valley floor below. It buried 15 homes, damaged 16 others, and killed several individuals. And, on March 22, 2014, the Oso mudslide in Washington killed 43 people and destroyed or damaged 49 homes and other structures. Sadly, there had been many warnings about slope instability in this area for many decades. The risk was not taken seriously, and lives and property were lost as a result.

Besides landslides, there are also geologic hazards related to subsidence and expansive soils. Subsidence occurs when the water table or underground rivers erode the soil around them and the earth collapses. This type of sinkhole is common in Florida (see Figure 1‐4). In August 2013, a sinkhole on Florida’s surface swallowed parts of the Summer Bay Resort near Walt Disney World. The sinkhole was about 60 feet wide and 15 feet deep. Fortunately, those vacationing in the area were able to evacuate quickly as the building started to crumble and fall into the sinkhole. Another cause of subsidence is the mining for coal and ore, dredging canals or the pumping of groundwater out of a certain geographic area. New Orleans and Mexico City are both sinking due to these latter activities. In contrast to subsidence, expansive soils may actually rise due to the presence of moisture in ground. This hazard is especially prevalent in locations that have clay soils. Although expansive soils are found most often in the south and west, they can be present in many parts of the United States. Expansive soils do not necessarily kill people, but they can create a large amount of property damage (especially to the foundations of buildings).

Figure 1‐4 This 45-foot deep sinkhole formed in Monticello, FL, after Tropical Storm Debby produced excessive rains and flooding. David Fine/FEMA.