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Hazard Concept . . The Hazard Area Concept. specifies the area with a water depth of 18 Inches or greater and a water velocity of 3 Ips or greater as the floodway which must be kept free of development. Low Hazard Area High Hazard Area Low Hazard I Area 18" Deep 3 FPS 18" Deep The Hazard Area Concept IS based on depth and velocity enterla. Its application. simple to understand. does not require as much detailed engineering as the rise concept. This concept IS most applicable to the con. fined. narrow, and steep gradient streams found In the mountain regions of the state. The criteria relate to potential damages produced by static and dynamiC flood forces caused by the depth and velocity of the water. The hazard area concept does not work well in wide floodplainS where develop' ment pressure IS great. It IS. however. often applicable In wide areas with shallow flooding and low velocities. . Mapping Dam Failure Flood Zones The engineering Information that you have just read deals with determining flood elevations and floodplain boundaries for floods caused by preCIpitation. One type of flooding which is not considered in most engmeenng studies is dam failure flooding. Techniques do exist for preparing maps Showing dam failure flood lones. As was discussed In Chapter 2 of this manual. as of July 1. 1983. maps shOWing the extent of flooding that would result from failure of state regulated high hazard dams are to be prepared by the State Engineer. There Will stili be many areas downstream of moderate or low hazard dams. and of unregulated dams. where such information will not eXIst. Locai governments may want to consider contacting the National Weather Service (NWS). the U.S. Army Corps of Engineers (Corps). the Soil Conservation Service (SCS). the State Engineer. and the U.S. Bureau of Reclamation IUSBR) for assls, tance in preparing such maps to better delineate potential hazards. NOTE: Most detailed tloodpfain studies. including the majonty of those conduc- ted by Federal and State agencies. do not conSider the POSSibility of dam failure. Regulation of floodplalns.n the United States IS based on the 1 CO-year flood which IS a preCIpItation-caused flood. As eVidenced by the flood on Fall River near and through Estes Pal1<. In July, 1982, where the flood was, on the average, 2 1/2 times as deep as the estimated 500- year flood stage and where flood waters greatly surpassed the SOO-year flood boundaries Indicated in the City'S Flood Insurance Study, dam failure floods may exceed the 1 Oo-year and SOO-year floods calculated In most detailed studies. .