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Rex Bums, PeE, and Bob Smith, PeE, Final Report - Hydrologic Analysis of the Cooper Slough Within Larimer County, Colorado Page 3 Creek basin for each storm event, but did not distingLlish what portion of the di.scharges flow down the highway median and what portion flows owr the southbound lanes into the Cooper Slough basin. Therefore, the area of overtopping was surveyed by the Larimer County Survey Department to detemtine the distribution of th(~ overflow, 111'" su!v('yed section is shown in Appendix A. ," ," 'v! ",) ,I". \ lVI' C-_I A normal depth equation was used to detennine the amount of disc~ge that flows down the highway median to return to Boxe1d'er Creek; and the weir equation was used to determine the amount of di~charge that overtops the southbound lane and flows itltO the Cooper Slough basin, A rating curve table of water surface d,evation versus discharge can be found in Appendix A. 111e following table lists the updated overtopping flows at State Highway 14, TABLE.2 1-25 OVERTOPPING DISCIMJ:;[GJE:S AT STATE HWY 14 FLOW STOR.1\,.1 RECIJRRENCE INTERVAL 10-year 50-year 1 DO-year SOD-year TOTAL FLOW FROM BOXELDER (ers) 275 530 726 1136 FLOW TO , COOPER SLOUGH __JcfsJ! 29 l73 301 '590 FLOW TO 1-25 l'vIEDIAI\' --.frill 246 357 425 546 The values listed in the "Flow to Cooper Slough" COlUllUl above, as well .as the di<;charges listed in Table 1 (except for State Highway 14) are used to (;ompute the inflow hydrographs thaI will be added to the updated Cooper Slough S\VlVllvl model, INFLOW HYDROGRAPHS To incorporate the timing of the 1-25 overtopping discharges into the Cooper Slough S\VJ\-IM, inflow hydrographs were created for;! locations into the Cooper Slough Basin (see Appendix B), The three locations are at Vine Street, the C &: S Railroad, and at Highway 14, Dischargl)s attributed to the Vine Street location include the, discharge fi-om the Larimer-Weld Canal. Discharges from the C & S Railroad include the dis,~harge downs!tream of Vine Stn~et. Because the fIve 1-25 overtopping locations are located in the same hydrologic sub-basin (sub-,basin 42), of the Boxelder SWMJ\1 model (see Figure 2, Reference 4), the inflow hydrographs were determined by multiplying the hydrograph discharge of sub-b..sin 42 from the Boxelder S\\'J'v!JvI by the ratio ofI-25 overtopping peak flow to Cooper Slough at a designated location to the peak flow for sub- basin 42. Copies of the inflow hydrographs developed for the 10-, 50-, 100-, and 500-year stonTtB are presented in Appendix B. Value,s from the inflow hydrographs were incorporated into the Cooper Slough S\VMM, however sine;e the model can input only 20 discharge values, the 20 most representative discharge values for each hydrograph were input. ~I ,