Liquefaction and levees potential double disaster in the american midwest

Liquefaction is a process during which sudden shaking of certain types of water-saturated sands and muds turns these once-solid sediments into a slurry having a liquidlike consistency. Liquefaction occurs when shaking causes individual grains in the soil to move apart, then water moves between the grains, making the whole water/sediment mixture behave like a fluid. Earthquakes often cause liquefaction of sands and muds, and any structures built on soils or sediments that liquefy may suddenly sink into them as if they were resting on a thick fluid. Liquefaction causes sand to bubble to the surface during earthquakes, forming mounds up to several tens of feet (~ 10 m) high, known as sand volcanoes, and ridges of sand to squeeze into cracks in the Earth. Liquefaction is responsible for the sinking of sidewalks, telephone poles, building foundations, and other structures during earthquakes. Famous examples of liquefaction occurred in the 1964 Alaskan earthquake, when entire neighborhoods slid toward the sea on liquefied sand layers, and in the 1964 and 1995 Japan earthquakes, when entire rows of apartment buildings and shipping piers rolled onto their sides but were not severely damaged internally.

Recent earthquakes and floods in the Midwest region of the United States passed with locally huge amounts of damage, but were not catastrophic events for the entire region. But one must consider what could have happened if the earthquakes were slightly larger (which is possible) and occurred while the rivers were at high stages (which happens for several months each year). One real threat is that many apparently stable levees may experience mass failure by liquefaction during an earthquake, potentially causing catastrophic flooding of regions behind these levees.

The U.S. Geological Survey reports that there is a significantly high threat of many of the soils on the floodplains of the Mississippi, Missouri, Ohio, and Illinois Rivers—in a report to the U.S. House of

Representatives in 2006, Eugene Schweig (of the U.S. Geological Survey) stated, "If the earthquakes were to occur when the Ohio and Mississippi Rivers were high, loss of levees is likely along with flooding of low-lying communities." When the soils in the levees are saturated with water, such as during flood or high-water events, the potential for liquefaction is much higher. Geological analysis of the banks of many rivers in the region has revealed that a magnitude 6 or 7 earthquake struck about 40 miles (65 km) east of St. Louis some 6,500 years ago, and another event struck about 4,000 years ago. Both events caused massive liquefaction of the thick river sediments on the floodplains. Large earthquakes have continued to hit the region in historic times, as shown by the 1811-12 sequence of magnitude 7-8 events, which also caused liquefaction of huge areas in southern Missouri and surrounding states. The April 18, 2008, earthquake in Illinois and aftershocks remind and warn us that the potential consequences of earthquakes on levees during high water must be considered.

The two earthquake swarms in the Midwest in 2008, although minor, occurred when the local rivers were high. What would have happened if the earthquakes were slightly larger, say a magnitude 6 or 7? Would the levees have experienced mass failure and collapse by liquefaction? Levees fail for several reasons. They may be overtopped by high water, scoured at their bases and sides, or have water seep through the pores between the sand and mud, weakening the structure until the pressure from the high water causes it to collapse. The most catastrophic type of failure can occur when the soils of the levee liquefy from the pressure of the high water, by shaking from earthquakes, or both. In these types of failures, hundreds of linear yards of levee may suddenly collapse, sending torrents of water into the "protected" areas behind the levees. Studies by the U.S. Army Corps of Engineers suggest that many of the levees in the region are not strong enough to withstand shaking during an earthquake and may fail by liquefaction. The problem is especially critical in communities such as those surrounding East St. Louis, Illinois, where the entire levee system is in the process of being decertified, as the levees do not meet modern standards for safety in earthquakes and from other stresses, such as floods. If these levees fail during high water, the force of the Mississippi will surge into East St. Louis with the force of Niagara Falls, pushing into and covering the floodplain with many feet (several m) of water. Approximately 130,000 people live on the floodplain in the Metro East area near East St. Louis and rely on the levees for protection. If an earthquake causes massive liquefaction and failure of the levees, there will be little time to react, and many people will be stranded on rooftops, creating scenes reminiscent of New Orleans and Hurricane Katrina.

What can be done? The U.S. Army Corps of Engineers estimated that it will take $180 million to upgrade the Metro East levees to modern standards, and much of that money would need to come from local communities. If that is too expensive, residents on the floodplain behind the levees need to be aware of the possibility of liquefaction and mass failure of the levee system. Flood-hazard and earthquake-hazard risk maps need to be compared to determine which areas have the greatest threat for liquefaction, and emergency management plans need to be established for the contingency of such a catastrophe. This could save many lives. Homeowners, businesses, local governments, and insurance issuers need to understand these risks and plan accordingly.

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