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<br />sections were set up such that f10w in the right <br />overbank represents the quantity of flow overtopping <br />either north or southbound 1-25 respectively. <br /> <br />Cross-section topographic data were obtained from the <br />following sources: <br /> <br />o 1"=100' topographic mapping with 2-foot contours <br />used in the draft flood hazar.d study (Reference 2) <br /> <br />o Field Survey data prepared by URS Company for the <br />portion of Larimer County Road 50 east of 1-25 which <br />were adjusted to U.S. GeOlogical Survey datum by <br />subtracting 2.8 feet. <br /> <br />o Roadway elevations for both 1--2,5 and the existing <br />frontage road were obt.ained from r.cladway <br />construction plans adjusted to U.S. Geological <br />Survey datum by subtracting 2.8 feet. The typical <br />section of 1-25 used in the BEC.2 program is shown <br />in Figure 5. <br /> <br />c). Cross-section Numbering <br /> <br />For ease of refE~rence, mainline Interstate Highway 25 <br />stations were used to number HEC.2 cross-sections. <br /> <br />d). Manning's Roughness Coefficients <br />field n = 0.06 <br />roadside ditch and median n = 0.045 <br />pavement n = 0.016 <br /> <br />c\c <br /> <br />el. Expansion and construction coefficients <br /> <br />expansion Ce = 0.3 <br />contraction Ce = 0.2 <br /> <br />f). Starting Water Surface Elevation <br /> <br />The HEC.2 program was run for each pr.ofile usng the <br />slope-area option with a starting energy gradient of <br />0.005 feet per foot. <br /> <br />g). Rating Curves - Culverts under north bound 1-25 <br /> <br />o assume energy gradeline elevation of culvert entrance <br />coincides with water surface elevation east of 1-25 <br /> <br />o assume entrance loss of half a velocity head of flow <br />through culvert, Ke = 0.5. <br /> <br />10 <br />