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I I I I I The cross section location, channel bottom elevation, bridge and road locations, and profiles of the 10-, 50-, 100-, and 500-year floods for Boxelder Creek and Cooper Slough are presented in the water surface profiles, plates 1 to 12. This information is also tabulated for each cross section in Tables 12 and 13 for Cooper Slough existing and developed basin conditions, and in Table 14 for Boxelder Creek. Hydraulics of bridges and culverts were calculated separately as a check of the HEC-2 calculations. Bridge and culvert capacities are listed in Table 15. Debris blockage was not considered to be significant because an evalua- tion of the floodplain revealed no major sources of large debris. Only the State Highway 14 Bridge over Boxelder Creek has adequate capacity to pass the 100-year flood peak. Flood flow from Cooper Slough will overtop Vine Drive, the Colorado and southern Railroad, State Highway 14, the Lake Canal, and the Cache la Poudre Reservoir Inlet. Flood flow from Boxelder Creek,'within the study reach, will overtop Vine Drive, the Larimer-Weld Canal, 1-25, the Lake Canal, and Prospect Street. I I Table 11. Flow Split Data Cooper Slough I I Location Flood Recurrence Interval'i' Total Flow Overland Flow Channel Flow I I Existing Conditions I Reference Point 3* Colorado Highway 14 I Reference Points 8.5 - 9.5 I Reference Point 10 Colorado and Southern Railroad I Developed Conditions I Reference Point 3 Colorado Highway 14 I I Reference Points 8.5-9.5 I Reference Point 10 Colorado and Southern Railroad I I 25 year 50 year 100 year 500 year 430 cfs 720 cfs 1060 cfs 1680 cfs I 131) cfs 420 cfs 760 cfs 1380 cfs 300 cfs 300 cfs 300 cfs 300 cfs I 100 year 500 year 920 cfs 1015 cfs 225 cfs 285 cfs I 25 year 50 year 100 year 500 year 430 cfs 720 cfs 1060 cfs 1680 cfs 695 cfs 730 cfs 70 cfs 90 cfs 140 cfs 665 cfs I 10 year 25 year 50 year 100 year 500 year 500 cfs BBO cfs 1240 cfs 1680 cfs 2310 cfs 360 cfs 630 cfs 920 cfs 1015 cfs I Floodway A 100-year floodway was delineated using the Larimer County guideline of allowing encroachment to the point where the water surface elevation is not more than one-half foot above the base flood water surface elevation. Because flow splits occur at several locations, a floodway that carries the full discharge could not meet this one-half foot rise criteria. The Boxelder Creek flow is reduced from 3840 cfs to 1500 cfs at 1-25 due to the bridge capacity and a floodway downstream of 1-25 could not carry the full discharge with a rise in water surface elevation of less than one-half foot. The floodway for this reach is defined to be the same as the floodplain. On Cooper Slough, separate flood channels exist downstream of Vine Drive, downstream of the Colorado and Southern Railroad, and downstream of State Highway 14. At these locations, the water surface will rise more than one- half foot because of the flow increase in the main channel. For these loca- tions, the f100dway was set at the limits of the floodplain. In no case was the rise in water surface elevation more than one foot for the full discharge f100dway in Cooper Slough except downstream of State Highway 14. The floodway widths and water surface elevations are given in Table 16 for Cooper Slough and Table 17 for Boxelder Creek. F100dway limits for Cooper Slough and Boxelder Creek were also delineated on the topographic mapping at a scale of 25 year 50 year 100 year 500 year 765 cfs 1075 cfs 1470 cfs 1810 cfs 200 cfs 580 cfs 940 cfs 1380 cf s 2010 cfs I 10 year 25 year 50 year 100 year 500 year 500 cfs 880 cfs 1240 cfs 1680 cfs 2310 cEs 300 cfs 300 cEs 300 cEs 300 cfs 300 cEs I 100 cEs 315 cfs 630 cEs 900 cfs 665 cEs 760 cEs 840 cEs 910 cEs I I 60 cfs 115 cfs 165 cfs 210 cfs 500 cfs 440 cfs 765 cfs 1075 cfs 1470 cfs lBlO cfs I I *Al1 Cooper Slough flow will reach Colorado Highway 14 through two separate channels. I *F10w split data listed only for recurrence intervals that flow split occurs. I I I 34 I 35 I