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<br />and possible overtopping of the northbound lanes of <br />Interstate Highway 25 to the median ditch. Profiles were <br />run and the flows readjusted based on the resultant water <br />surface profile elevations and continuity. Rating curves <br />developed for each of the existng cross culverts under the <br />northbound lanes of 1-25 are presented in Tables 1 and 2. <br />The starting water surface elevation for each profile was <br />determined using the slope-area option with a starting <br />energy gradient of 0.005 feet per foot. Approximate flow <br />quantities associated with each flow path alnd mechanism are <br />shown schematically in Figure 4. Supporting HEC.2 computer <br />runs associated with Figure 4 are contained in the <br />Appendix. <br /> <br />Under existing conditions the Larimer County Road 50 <br />overpass embankment at 1-25 causes the upstream water <br />surface elevation to rise until the sum of runoff conveyed <br />under the Larimer County Road 50 overpass alnd over Larimer <br />County Road 50 east of 1-25 equals the total inflow runoff <br />to the northeast quadrant of 1-25 and Larimer County Road <br />50. <br /> <br />Under existing conditions, during the 100-year spill from <br />Boxelder Creek of 3320 CFS, approximately 1261 CFS would <br />flow to the south via the 1-25 median ditch of which <br />approximately 131 CFS would be from overtoI~ing of <br />northbound 1-25 immediately north of Larime!r County Road <br />Approximately 1303 CFS would flow to the south under the <br />Larimer County Road 50 overpass via the frclntage road and <br />northbound 1-251 and approximately 756 CFS would overtop <br />Larimer County Road 50 to the south on the east side of <br />1-25. <br /> <br />\ ' <br /> <br />50. <br /> <br />~~~ - <br /> <br />Pertinent data and assumptions used in running the HEC.2 <br />program are presented as follows: <br /> <br />a). Datum <br /> <br />U.S. Geological Survey datum was used. U.S. Geological <br />Survey datum is approximately 2.8 feet lower than <br />Colorado Department of Hi.ghways Datum. <br /> <br />b). Cross-Sections <br /> <br />Cross-sections were spaced at approximately 200 feet <br />intervals starting at the south side of the Larimer- <br />Weld irrigation canal as shown in Figure 3. Cross- <br />sections were coded from east to west and thence <br />northward along either the Wl=st edge of pavement of <br />northbound 1-25 or the east edge of palvement of <br />southbound 1-25 to the next cross-'section. Cross- <br /> <br />6 <br />