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<br />Ground Failure <br /> <br />Areas subject to ground failure often suffer from mud flows and mud floods, <br />two forms of landslides. Urban development alters hillslope configurations <br />and upsets established equilibrium, triggering the natural instability of many <br />slopes and sometimes reactivating old landslides. Mud and debris may fill <br />drainage channels and sediment basins, causing flood waters to suddenly <br />inundate areas 'outside the floodplain. Mud flows and mud floods may cause <br />more severe damage than other flooding because of the force of the debris- <br />filled water and the combination of debris and sediment. <br />Both. natural and human. induced subsidence can increase flood damage in <br />areas of high groundwater, tides, storm surges, or overbank stream flow. It <br />can also block or othenvise alter drainage patterns, leading to deeper or unex- <br />pected flooding. Subsidence occurs in at least 38 states. <br />Liquefaction is a type of ground failure triggered by seismic waves passing <br />through unconsolidated and saturated soil. Depending on the character of the <br />soil, the amount of water, and the drainage potential, the soils may sink or <br />become liquid. This can result in serious flooding of structures built on fill or <br />saturated soils-as in parts of San Francisco and Anchorage. <br /> <br /> <br />Mud /lows and mud floodr are two 1]'Pes if landrlidrr tho.t can be aggravated b;' hwrum dneloflmRnl. Addi- <br />tionally, thq can remlt from otha natural hazards, such as earth.zWlirLs and vokanu: rruptimu <br /> <br />1iJutle Rh'fr. WashineUm Slnle, (oUou;inf!, t~ m~fJticn I:!f Mt. St. Helens, :\.fay 1980. <br /> <br /> <br /> <br />,'/atural coastal erosion can be greatly aaelerated by wave <br />acticn during storms and hurmartfs. Combined with inap- <br />propriate construdion in coastal areas, this natural prouss <br />wn resul1 in disaster. <br /> <br />Big Rock Beach, Malibu, California, jolf<Jwing Ruijic <br />win1l7 swrm, 1983. <br /> <br />15 <br />