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Rex Burns, PeE, and Bob Smith, PeE, Final Report - Hydrologic Analysis of the Cooper Slough Wi~lin Larimer COUllty, Colorado Pa"oe 4 SWMM MODEL The Simons and Li 1987 Cooper Slough SWMIv[ model used a 3..hour rainfall event. The USBR Boxelder Creek SWMM model used a I-hour rainfall event Since the inflow hydrographs an: developed from the I-hour Boxelder S\\'1-.1M mode~ the Cooper Slough SWMM model was converted to a I-hour storm to be consistent This was agreed. upon in the December 10, 1992 Cooper Slough L!\iTh.1P progress meeting (Reference 5), It was also agreed. upon at the meeting to assume that the storm evenl occurs over both basins at the same time. Figure 3 shows the: locations of sub-basin numbers, channel numb(:rs and storage numbers used in developing the: S'\'1-.1M model. The inflow hydrographs at Vine Drive and at State Highway 14 were combined in the Cooper Slough SWMlvf model without routing because, the travel time from the overflows to the Cooper Slough floodplain would be minimal during a 100-year storm. The inflow at the railroad was analyzed to determine the overland route that flow would take to Cooper Slough. Three pos~-ible routes were HNiewed: south along the ditch between the access road and the highway; west along the north side of the railroad tracks; and southwesterly across the field. The inflow at the railroad most likely flows in all tlu'ee of these directions, howevcr, the majority of the flow travels southerly to State Highway 14, Therefore, the entire railroad inflow hydrograph was routed to State Highway 14, Hydrollogically it does not mattcr which route the inflow takes because the combined flows at State Highway 14 changes by less than 0,5% with anv of the possible routes, Routing the flow due west to enter Cooper Slough at the railroad lrack crossing changes the flow at the State' Highway 14 by less than 0.:5% and changed the flow at the railroad crossing by less than 5%, RESULTS OF HYDROLOGICAL ANALYSIS The table below lists the discharges for the variousc storm recurrence intcrvals for the Cooper Slough watershed, TABLE j COOPER SLOUGH DISCBARGES (ds) Location (S\VMM ID#) At Vine Street (13) At C&S Railroad (8) At State Hwy 14 (4) At confluence with Boxelder Creek (42) Cooper Slough /) Drainage Area:- ~ (SQ. mi.) , lO-vear 0.25 250 0,60 246 1.06 337 _,__50-vear 1,260 1,260 1.592 lOO-vear 1,868 1,864 2,407 500-vear 3,050 3,033 3,977 2.00 328 1,569 2,370 3,783 "'The contributing drainage are:a is only that portion of the basin that is south of the Larimer-Weld Canal. All flow north of the canal is intercepted by pond~ owned by the Anheuser- Busch Company and is released at a controlled rale to the canal.