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Collins has developed a plan for controlling flows from the Canal Importation Basin into Spring Creek. The basin north of Spring Creek has been designed as the Canal Importation Basin and is separated into north and south portions. Historically, the canals in the north portion have been able to carry approximately a 100year flow from the basin area north of Prospect Road. Flows exceeding this are either detained to the 10-year rate or flow past the canals to outfalls north of the Spring Creek basin and. therefore, are non- contributory. The effects of the importation of flows from the north portion of the basin are accounted for in the hydrologic model by including JO-year input hydrographs for the area north of Prospect Road for all storm frequencies, Flows from the south portion of the Canal Importation Basin area also are intercepted by the canals, However, these flows remain in the Spring Creek basin and contribute runoff to Spring Creek for all storm events. Therefore, the south portion of the Canal Importation Basin is modeled directly into the hydrologic program and is included as part of the Spring Creek basin. The peak discharges for Cooper Slough were established using the SWMM computer program as developed by the EPA and later modified by the USACE, Missouri River Division (Reference 22). The original SWMM model for the Cooper Slough watershed was developed by Simons, Li & Associates, Inc, (Reference 23). This model was revised to include the overflows from the Boxelder Creek basin. The peak discharges flowing into Cooper Slough from Boxelder Creek were obtained from the USBR at the following points: Larimer-Weld Canal, Vine Drive, a low point on 1-25 south of Vine Drive, C&SRR, and SH-14 (Mulberry Street) (Reference 24). The following table lists the peak discharges (in cfs) leaving Boxelder Creek across 1-25 into Cooper Slough, provided by the USBR: Location ~ 50-Year ] 00- Year 5oo-Year At Larimer-Weld Canal 57 258 258 258 At Vine Drive 191 930 1,450 2,430 South of Vine Drive 18 68 104 ]58 At C&SRR' 47 74 94 132 AtSH-14' 275 530 726 1,136 'The canal and railroad flows go through underpasses at 1-25. 'The flow to the 1-25 median was not subtracted from these values. These discharges represent the discharges from the Boxelder Creek watershed that flow into the Cooper Slough basin, except at SH-14. To determine the amount of flow that travels down the 1-25 median and the discharge that overtops the highway into the Cooper Slough basin, the area of overtopping was surveyed. A normal-depth equation was used to determine the amount of discharge that flows down the highway median and a weir equation was used to determine the amount of discharge that overtops 1-25, The following table lists the amount of flow at SH-14 as a result of overtopping 1-2'5: Recurrence Interval Tota] Flow from Boxelder Creek (cfs) Flow to Cooper Slough (cfs) Flow to 1-25 Median (cfs) 10- Year 50-Year loo-Year 500- Year 275 530 726 1,136 29 173 301 590 246 357 425 546 9