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<br />I <br /> <br />I <br /> <br />HYDRAULIC ANALYSES <br /> <br />The topographic maps were field checked by Greiner Engineering to assure na- <br />tional mapping accuracy standards. <br /> <br />I <br /> <br />I <br />I <br /> <br />For Big Dry Creek and its principal tributaries, flood elevations for the fu- <br />ture condition 100-year flood event were computed using the U.S. Army Corps of <br />Engineers HEC-2 Step Backwater Computer Program (Reference 15). Cross sec- <br />tions used in the computations were obtained using aerial photogrametric pro- <br />cedures based on recent topographic mapping (Reference 16). <br /> <br />SUMMARY <br /> <br />I <br /> <br />All major bridges, culverts and drop structures within the study reach were <br />field inspected and measured. All stream crossings modeled using HEC-2 were <br />based on unobstructed flow and were assumed to stay in place during the 100- <br />year storm event. Flow separation was assumed for all irrigation ditches and <br />no flow diversions were considered. <br /> <br />This report includes the hydrologic analysis of the Big Dry Creek basin from <br />its headwa ters to Interstate 25 and the de linea tion of the 100-year <br />floodplains for Big Dry Creek and its principal tributaries from Standley Lake <br />to 1-25 assuming ultimate developed basin conditions. <br /> <br />I <br /> <br />l1annings Roughness values ranged from 0.03 to 0.08 in channel areas and 0.04 <br />to 0.15 in overbank areas (Reference 17). <br /> <br />This report was prepared by Greiner Engineering Sciences, Inc. (GES) under an <br />agreement with the Urban Drainage and Flood Control District. All existing <br />condition hydrology and technical data used in this report was compiled by <br />(GES) under contract EMW-83-C-1173 to the Federal Emergency Management Agency <br />for the City of Westminster Flood Insurance Study. <br /> <br />I <br /> <br />The 100-year starting water surface elevation on Big Dry Creek was obtained <br />from the Adams County Flood Insurance Study just downstream of 1-25. (Refer- <br />ence 18). On tributaries to major streams affected by the backwater from the <br />major stream, the starting water surface elevations were based on the back- <br />water flood elevation at the time to peak on the tributary stream. In the <br />event that the tributary was independent of the major stream, the slope area <br />method was used to estimate the starting water surface elevation. <br /> <br />This report supercedes the previous report "Mas ter Plan - Major Drainageway <br />Planning Big Dry Creek", prepared by the UD&FCD in 1973 (Reference 2) and will <br />be used as the basis for future major drainageway planning studies on Big Dry <br />Creek and its tributaries. <br /> <br />I <br /> <br />I <br /> <br />FLOODPLAIN AND FLOODWAY DATA <br /> <br />I <br /> <br />The floodway developed for this study was based on equal conveyance reduction <br />for each side of the floodplain with a maximum surcharge of 0.5' for the ener- <br />gy grade line. The floodway is that portion of the lOa-year floodplain which <br />is required to pass the lOa-year flood event without raising the flood profile <br />more than an acceptable amount (i.e. 0.5 feet) and also represents the portion <br />of the floodplain that is most hazardous to personal safety and welfare. <br /> <br />I <br /> <br />I <br /> <br />The laO-year floodplain and floodway data developed for this report are shown <br />on the Floodplain and Floodway Reference Data (Tables 8 to 22). The floodway <br />boundaries are shown on unpublished work maps on file at the Urban Drainage <br />and Flood Control District. <br /> <br />I <br /> <br />The locations of selected cross sections used in this report are shown on the <br />plan and profile drawings located in the Appendix. The velocities associated <br />with the cross sections are channel velocities. All cross sections were taken <br />left to right facing downstream with a starting left station of 200. In the <br />event that a starting station shown on the map is different than 200, the sta- <br />tion has been so noted. <br /> <br />I <br /> <br />I <br /> <br />FLOODPLAIN DELINEATIONS <br /> <br />I <br /> <br />The 100-year floodplains developed for this study have been delineated using <br />the flood elevations determined at the cross section locations. Between cross <br />sections, the boundaries have been interpolated using the topographic maps at <br />a scale of 1"=200' with a contour interval of 2 feet (Reference 16). The to- <br />pographic maps used for this study were prepared by Delta Aerial Surveys, <br />based on 5-3-76, 6-11-83 and 11-17-83 flight dates. <br /> <br />10 <br /> <br />I <br /> <br />I <br /> <br />