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<br /> <br />FLOODING DUE TO GROUND FAILURE: SUBSIDENCE AND LIQUEFACTION <br /> <br />Subsidence occurs in parts of all fifty states. The problem is particularly serious in <br /> <br />southern California, the Houston-Galveston area and southern Louisiana. Lowering of the <br /> <br /> <br />land surface - caused by extraction of subsurface fluids and other materials, soil com- <br /> <br /> <br />paction or other processes - can increase flood depths and duration. <br /> <br /> <br />Liquefaction occurs when unconsolidated sands and silts temporarily flow like <br /> <br />thick fluids, usually triggered by earthquakes. Liquefaction can be a major cause of <br /> <br /> <br />flooding if dams or levees are damaged or if structures on filled wetlands sink below <br /> <br /> <br />water level. <br /> <br />FLUCTUATING LAKE LEVELS <br /> <br />Thousands of inland lakes, inCluding the Great Lakes and the Great Salt Lake, are <br /> <br /> <br />subject to long-term fluctuations in water levels. Recent fluctuations of the Great Salt <br /> <br /> <br />Lake have caused over one hundred million dollars in losses in a single year. Lake levels <br /> <br /> <br />can rise and remain high for years, with damage compounded by ice and shore erosion <br /> <br />and detrimental impacts on water quality. <br /> <br />ICE JAMS <br /> <br />Thirty-five northern and midwestern states suffer high risk flooding due to ice <br /> <br /> <br />jams. Dams of ice increase flood levels upstream, then unleash ice floes and deep, high <br /> <br /> <br />velocity floodwaters downstream. The combination of ice, debris and water can cause <br /> <br /> <br />tremendous physical damage to structures. <br /> <br />MUDSLIDES <br /> <br />Heavy rains can trigger mud floods or mudflows in areas of steep slope, limited <br /> <br /> <br />vegetation and unconsolidated soils. Flows can be sudden and deep, but the greatest dam- <br /> <br /> <br />age is often done by the debris. Mudflows and mudfloods are a particular problem in <br /> <br /> <br />Southern California, the San Francisco Bay area and the communities surrounding the <br /> <br /> <br />Great Salt Lake. <br /> <br />A-3 <br />