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1 DO-Year Floodplain fS. B Rise Concept Flood Fringe Rise Floodway High Hazard Concept Low Hazard Area High Hazard Area Low Hazard I Area A 3 FPS 18" Deep 1 8" Deep Another limitation of the rise concept is that the specified but arbitrary rise criterion of 0.5 foot or 1.0 foot may actually increase damage potential in some areas, When portions of the flood fringe are filled, and protected by that fill, other areas may suffer as unconfined flows are forced into previously dry areas due to the increase in the water surface elevations. There is another limitation which can be misleading. On steep gradient streams, encroachment onthe floodplain can often appear to lower water surface pro- files as the computer solution of flow passes from the subcritical into the supercritical regime of flow with shallow depths and high velocities, The Hazard Area Concept is based on depth and velocity criteria. 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, Floodway computations must use the H EC-II Computer Program or an acceptable backwater-step procedure. If a floodway has been computed it must be shown on the community's administrative map, 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 engineering studies is dam failure flooding. 'Techniques do exist for preparing maps showing dam failure flood zones, 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 still be many areas downstream of moderate or low hazard dams, and of unregulated dams. where such information will not exist. Local 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 (USBR) for assis- tance in preparing such maps to better delineate potential hazards.@ NOTE: Most detailed floodplain studies, incl.uding the majority of those conduc- ted by Federaland State agencies, do not consider the possibility of dam failure. Regulation of floodplains in the United States is based on the 1 OO~year flood which is a precipitation-caused flood. As evidenced by the floQd on Fall River near and through Estes Park 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 Hood waters greatly surpassed the SOa-year flood bou'ndaries-indicated in the city's, Flood Insurance Study, dam failure flo'ods may exceed the ,qD,-year and SOD-year floods calculated in most detailed studies~ . , .._ 22