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4. O. FLOOD PLAIN MANAGEMENT APPLICATIONS A prime purpose of the National Flood Insurance Program is to encourage State and local governments to adopt sound flood plain management pro- grams. Each Flood Insurance Study, therefore, includes a flood boundary map designed to assist cOllllllunities in developing sound flood plain management measur~s. 4.1 Flood Boundaries In order to provide a national standard without tegional discrimina- tion, the lOa-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of flood plain manage- ment measures. The SO a-year flood is employed to indicate 'additional areas of flood risk in the cOllllllunity. For each stream studied in detail, the bowHlaries of the 100- and SOO-year floods have been' delineated using the flood elevations determined at each cross section; ~encr05S sectio,*,'.'l:bebbUll&l'ies were Interpdlated using topogr.phic maps at-a scale of 1:1200, with a contour interval of 1 foot (Reference 8). In cases where the 100- and SOO-year flood boundaries are close together, only the laO-year flood boUndaTY ha.. been 'shown. Approximate flood boundarie.. in some portions of the. study area were taken from the Federal IIllOurance Adm,,-ni..t:ration' s Flood Ha:o:ard Boundary Map for Weld County, Colorado (Reference 10), Small areas within the flood boundaries may lie above the flood elevations and, therefore, not be subject to flOOding, tiwil'lg to limita~ions of the map scale, such areas are not shewn. 4.2 Floodways Encroachment on flood plains, such as artificial fill, redUces the flood-carrying capaoi.t)o.and incr....ses flood. lHIighes,; thUS' increasing floodllazar<is in .._ beyond.tme..encr_ch.3nt.il;self. ;9neaspect of flood plain managenent involves balancing the economic gain from flood plain development against!.1zhe resulting increase in: flood hazard. For purposes of the National Flood Insurance Program, the concept of a floodway is used as a tool to assist local conim\inities in this aspect of flood plain mal'laqement. Underi:his eofteept, the area of the lOa-year flood is divided into afloodway arid a floodway fringe. The floodway is the channel of a stream, plus any adjacent flood plain area.., t:hat must he"kept. free of encroachtitent: in- order that the laO-year flood be carried without; -substantial increases in flood heights. As minimum standards, the Federal Insurance lldminis- tration lim! ts such increases in. flood heights '100 1.0 foot, provided that hazardous velocities are not produced. In locations where the Tri-Area Drainageway abuts the Union Pacific Railroad, flood plain encroachment was considered only on the east side of the dra:inageway. The floodway in this study was computed on the basis of equal convey- ance reduction from each side of the flood plain, The results of 6