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V. FLOOD PLAIN INFORMATION Extent of F100dinq Areas inundated by the 100.year Pawnee Overflow and Sand Creek flood events are shown in Plates 2 through 19. ~aximum flooding south of Highway 14 results from Pawnee overflows. ~orth of Highway 14, the highest lOO-year flood depths and inundated areas are produced by Sand Creek. The 5-, 10-, 25., 50-, 100-, and 500+year flood events '~ere the frequencies selected for this analys1s. Accord1ngly, the information contained in this report will be useful not only for regulation purposes and H.B. 1041 designations, but also for Federal Emergency Management Agency flood insurance rate studies. , . ~ ... .. , ,'.... ." ."" ,~~ ,..,~ ~.^" 'i~t~cn 3ca,~. to,a u,~u .~~J~C' .0 ,,,...n,,,~",cn "y .."" ",""-y"~' "VVY CuI< ~'" Floodway 1/ Encroachment on floodplains, such as artifical fil1,reducesthe flood-carrying capacity and increases flood heights, thus increasing flood hazards in areas beyond the encroachment itself. One aspect of flOOdplain management involves balancing the economic gain from flood- plain development against the resulting increase in flood hazard. For purposes of the ~ationa1 Flood Insurance Progrlm. the concept of a floodway is used as a tool to assist local comunities in this aspect of floodplain management. Und~r this concept, the ~rea of t~e 100-year flood is divided into a floodway and a floodw~y fringe. The floodway is a channel of a stream. plus any adjacent floodplain areas. that ~st be kept free of encroachment in order that the 100~year floo~ be carried without substantial increases in flood heights. As minimum standards, the ~at1onal Flood Insurance Program limits such increases in flood heights to 1.0 foot. provided t~at h~zar~uus velocities are not produced. The area between the floodway and the boundary of the 100-year flood is termed the floodway fringe. The floodway fringe thus encompasses the portion of the flOOdplain that could be completely Obstructed ~ithout increaSing the water-surface elevation of the 100-year flood more than 1.0 foot at any point. Typical relationships between thefloodwayand the floodway fringe and their significance to floodplain development are ,hown in tile figure below. A floodway sMwn as the shaded area in ?lates 2 thrn'lgh 10 ;<; b~<;pri on a depth of flooding of 2 feet or more dcwns~ream from Sidney Avenue. and a combination 2-foot depth an~ 1.foot rise criteria upstrea'1l from Sidney Avenue. The floodway is a narrow strip of land along the ~ioneer Ditch south of Highway 14. A compl,.te dhcu5sion 0" the flocday Jnd flOOd fringe ~eterr:1i nations is contained in the TeChnical Addenrlu"'(ledf, 1983). 5~. 10-. 25-, and 50-year Events A kncwledgeoffloods of these magnitudes is important for future engineering studies and land use planning related to drainage, sub- divisions. highway construction, channel improvements. parks and recreation, and agricultural lands. Design and planning based on floods of this magnitude should be made only when the hazard to life is non-existent. lOO-year ~vent Aside from the design ofclvil works projects. the most important ,-,se of the lCJO~year nood event 1 ies in designation and regulation of the flood plain as set forth in state and fe<leral statutes. lnColoradO, flood plain regulations (seeH.B. 1041) are based on the 100-yea revent. ParticipationintheNationalF1oodlnsuranceProgramalsorequires that the juriSdictional authority designate and manage the 100-yearflood plain. lJponofficial aooroval of this report by the Colorado '.ater Conser- designated a flood plain and regulated accordingly by Logan County dnd the City of Ster1fng. 7hefloodpr<lfiles(FigureP-l through P-7} maybe used on occasions where there is uncertainty as to flooding depths. Since the profiles show flood elevations arK! distances from a ~nown point. high water ~ark5 can be located on site to correct a~y discrepancies on the base;r>aps which follow. J6 lJ, This section provide<l by the Coiorado Hater Conservation aoard. ,. .'