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~~-1~ 09°37 FRDP1 ~'~t REGIOhI 8 NTFm 70 ~9130349302i3 P. 08 <br />The V.S. Army Corps of Engineers, Q~aha District, provided the <br />data for the l0-, 50, and 100-year discharges for the Sig <br />Thompson River through 4oveland (Reference.2). The 500-year <br />discharge was estimated by a straight-sine plot an log probability <br />paper. The 50D-year discharge was approximately the same as that <br />of the Standard Project Flood computed by the U.S. Artily Corps of <br />Engineers; therefore, the Standard Protect Flood discharge was <br />used as the SOD-year discharge. <br />Resource Consultants, Inc. reviewed the discharges for the Big <br />Thompson Rivets to verify the consistency of discharges used in <br />other flood-plain studies conducted for this reach and the <br />adjacent reaches of the Big Thompson River (References 2, 9, 10 <br />and 11). The discharges used in this study are consistent with <br />those used in previous studies of the Big Thompson River. <br />' A summery of drainage area-peak discharge relationships for <br />the Big Thompson River is shown in Table 1. <br />TAOLE 1 - SUMMARY OF DISCHARGES <br />FLOpDING SOURCE DRAINAGE AREA PEAK DISCHARGES (cfs <br />AND LOCATION (Sq. Miles) 10-Year~6-Year 00-Year 500-Year <br />i <br />Big Thompson River <br />Rlailroad Avenue 515 4,700 12,0 19,000 44,000 <br />3.2 Hydraulic Analyses <br />Analyses of the hydraulic characteristics of the flooding <br />sources studied in detail in the community were carried out <br />t0 provide estimates of the elevations of floods of the <br />selected recurrence intervals along each of these flood <br />sources, <br />Cross sections for the Big Thompson River were initially <br />obtained by photogratmtetric and field survey methods in August <br />1976. Tfiis survey information and base maps were then updated <br />by the same methods to reflect May 1981 conditions (Reference <br />12 ). All bridges and culverts were surveyed to obtain elevation <br />data and structural geometry. <br />Locations of selected cross sections used in the hydraulic <br />analyses are shown on the Flood Profiles. For stream segments <br />for which a floodway was computed (Section 4.2), selected <br />cross-section locations are also shown nn the Flood Boundary <br />and Floodway Map, <br />hater-surface profiles were developed using a NEC-2 step-backwater <br />