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1 OO-Year Floodplain . Floodways Two basic concepts for dividing the 1 OO'year floodplain Into a floodway and a flood fringe are the Rise Concept and the Hazard Area Concept. Floodway computations must use the HEC'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. Rise Concept The Rise Concept. specifies a maximum allowable rise (.5' or 1,0') in water surface elevations due to encroachment Into the floodplain while st,lI allow- ing passage of the 1 OO'year flood (provided hazardous velocities. which are those in excess of 3 feet per second Ifps), are not produced). The Rise Concept is based on an "acceptable rise" criterion. This rise criterion assumes that encroachment occurs on the edges of the floodplain on both sides until the 1 OO.year water surface is increased by.5 foot or 1.0 foot. It is suited to wide. flat'sloped. and well'defined floodplains where con. veyance capacity must be preserved. By assuming encroachment into the floodplain from both sides. the floodway line may be calculated. Often this is conservative. since development may not take place at every point. B . Flood Fnnge Rise Floodway The nse floodway determination has some drawbacks. It IS an expensive process and it may allow for inconSistent development. FEMA's floodway gUidelines reQuire eQual reduction In stream conveynace capacity from both sides of the floodplain, This eQual reduction is an attempt to treat all land' owners on both sides of the stream eQuitably and give all an opportunity to develop some of this capacity by placement of fill. There IS. however. some provISion for vanance from the reQuirement. Use of thiS variance provIsion can mean that some property owners may have more developable land In the floodplain and some property owners may have iess. Another limitation of the nse concept IS that the specified but arbitrary rise cntenon of 0,5 foot or , ,0 foot may actually increase damage potential in some areas. When portions of the flood fnnge 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 on the floodplain can often appear to lower water surface pro- files as the computer solution of flow passes from the subcrillcal into the supercrillcai reg,me of flow with shallow depths and high velocities. .