1997-05-28_REVISION - M1981302 (4)

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0 Memo to File M-81-302 -2- May 25 , 1997 foundation stability, settlement, and interior drainage . Riverine levees must provide a minimum freeboard of three feet above the water surface level of the one percent annual chance flood. Areas behind levees and floodwalls are often subject to severe internal drainage problems . The exclusion of floodwater also contributes to retention of storm water runoff . The area delineated as the regulatory floodway is the area that will carry the waters of a particular magnitude flood without increasing that water surface elevation more than a designated level . Areas outside the regulatory floodway but still within the designated one percent floodplain are referred to as the "flood fringe . " In delineating a floodway, it is assumed that the flood fringe areas will be fully developed and completely obstruct floodwater. The regulatory floodway will then serve to convey the floodwater. FIA minimum regulations require that the regulatory floodway be delineated so that it can pass the one percent annual chance flood without increasing the water surface elevation within the regulatory floodway by more than one foot at any point . Several states and communities have adopted more stringent requirements for the regulatory floodway by limiting the increase in water surface elevation to less than one foot, generally resulting in a wider floodway. Floodway boundaries are determined by application of hydraulic modelling techniques . A hydraulic model is developed to reflect existing conditions, and the model is manipulated to reduce the area in the flood fringe until the water surface within the floodway rises a foot or less . FIA minimum regulations also prohibit encroachments (fill, new construction, substantial improvements) within the adopted regulatory floodway that would result in any increase in flood levels (above the one-foot rise already allowed) during a one percent chance annual flood. Levees and floodwalls can be designed to protect relatively small areas, but they may create a false sense of security since the level of protection is limited. These structures may actually increase flood elevations on adjacent upstream and downstream properties by obstructing or accelerating flood flow and/or increasing flood peaks . For purposes of making special flood hazard area determinations for the National Flood Insurance Program (NFIP) , FEMA has established