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<br />0.100. Cross-sectional dala were obtained by field survey. All bridges were surveyed <br />to oblain elevation data and structural geometry. <br /> <br />For Spring Creek, water-surface profiles were developed using the HEC-2 hydraulic <br />model. Cross-sectional data were taken from topographic maps, field-surveyed sections, <br />and information obtained from certified construction drawings. The majority of the <br />mapping is from 1: 1,200 topography, with 2-foot contour intervals, provided by the City <br />of Fort Collins (References 31,32, and 33). Other mapping included 1990 topographic <br />aerials at a scale of 1:480, with a I-foot contour interval, in the area upstream of Lemay <br />A venue to upstream of Stuart Street, and 1991 topographic field-surveyed mapping at a <br />scale of 1:600. with a I-foot contour interval, in the area downstream of Shields Street. <br />Bridge and culvert openings were field measured and taken from information provided <br />in the City of Fort Collins' Master Plan Study (Reference 16). <br /> <br />Starting water-surface elevations for Spring Creek were taken from the hydraulic analyses <br />prepared for the Letter of Map Revision (LOMR) issued on October 30, 1989, for the <br />City of Fort Collins (Reference 34). For the 10-, 50-, and Ioo-year storms, the critical- <br />depth elevations determined at the first cross section in the hydraulic analyses for the <br />LMMP project exceeded the actual starting water-surface elevations taken from the 1989 <br />LOMR. Therefore, the starting water-surface elevations for these flood frequencies are <br />at critical depth. <br /> <br />Manning's "n" values for the channel <br />overbanks between 0.025 and 0.10. <br />inspection. <br /> <br />range between 0.015 and 0.075, and for the <br />These values were estimated based on field <br /> <br />A LOMR was issued on January 4, 1995, to remove a portion of Lot 4, Fort Collins Club <br />Office Park PUD, from the floodplain along Spring Creek based on the placement of fill. <br />The lot is located along the north side of Spring Creek, west of Welch Street. This <br />property is now located in Zone X (shaded), an area between the limits of the 100- and <br />5oo-year floods. <br /> <br />Water-surface profiles for Cooper Slough were developed using the HEC-2 model. Cross <br />sections from the Simons, Li & Associates, Inc., Boxelder Creek study were realigned <br />and extended and new cross sections were developed from 1: 1 ,2oo-scale maps provided <br />by the City of Fort Collins and Larimer County (Reference 5). These maps were <br />compiled by photogrammetric methods from aerial photography exposed between 1984 <br />and 1988. Stream crossing structures were surveyed by the Larimer County Surveyor's <br />office. <br /> <br />The Manning's "n" roughness coefficient used for the Cooper Slough channel and <br />overbanks was 0.045. The Manning's "n" value used for the overflow south of SH-14 <br />ranged from 0.025 to 0.045, and the "n" value for the flow along Sherry Drive ranged <br />from 0.045 to 0.1. Based on field observations, the following debris-blockage <br />percenlages for the culverts were determined: 100 percent for the 36-inch corrugated <br />metal pipe (CMP) at Vine Drive due to size, 100 percent for the 48-inch CMP at the <br />C&SRR due to the amount of sediment present. 25 percent for the double 48-inch CMPs <br />at SH-14, and 100 percent for the box culvert along Lake Canal at Sherry Drive. <br /> <br />16 <br />