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The starting water-surface elevations for Cooper Slough upstream of SII-14 wcrc determined by assuming the centerline of the westbound lane is at critical depth and acts as a weir in the HEC-2 model. Flow through the two 48-inch CMPs was determined by hand calculations, and the starting water-surface elevations for the Sherry Drive Overflow and the State Highway 14 Overflow were taken from the 1981 Simons, Li & Associates, Inc., study (Reference 2). A separate analysis was done for the divided flow (referenced as the East Island Divide) around an island of high ground north of the C&SRR between Vine Drive and SH-I4. This area is subject to shallow flooding during the I ()().. and 5oo-year floods. Cooper Slough overtops its eastern bank between Sections E and G, creating a split flow. Flows travel due south and rejoin Cooper Slough at Cross Section C. At Cross Section P, BoxeIder Creek Overflows from the Larimer-Weld Canal and Vine Drive were added to Cooper Slough. The Boxelder Overflow south of Vine Drive flows south to the C&SRR, west along the north side of the railroad embankment, and overtops the railroad approximately 900 feet west of 1-25, joining the overflow from Cross Section G of the main channel of Cooper Slough. The Boxelder Overflow under 1-25 at the C&SRR travels south and spreads out to join the Boxelder Overflow resulting from overtopping the railroad. This results in an area of shallow flooding from the boundary of the Cooper Slough Ioo-year floodplain to 1-25 and from the C&SRR to SH-14. This hydraulic analysis was field verified. At Cross Section B, Boxelder Overflows at SH-14 were added to the analyses. The Sherry Drive Overflow is the result of only a small portion of the ponded flow passing under SH-14 through the Cooper Slough culvert. This flow joins with Lake Canal approximately 700 feet downstream of SH-14. It then overtops the canal and flows south along Sherry Drive. At a low point east of Sherry Drive, it overflows to Prospect Road where it merges with backwater from Boxelder Creek. The analyses of the State Highway 14 Overflow to its confluence with Boxelder Creek indicated that flows leave the Overflow floodplain at Lake Canal and travel southeast between Lake Canal and the Cache La Poudre Reservoir inlet, eventually joining the Boxelder Creek floodplain. The majority of the flow ponded at SH-I4 overtops the highway at a low point approximately 1,225 feet east of the Cooper Slough channel and flows south to its confluence with the Boxelder Creek floodplain. This overflow was analyzed separately because it is not hydraulically connected to the Cooper Slough channel flow. For the Boxelder Creek Overflow Channel, the USACE HEC-2 model was used to develop the JOO-year water-surface profile, using a cross-section orienlation of left to right, looking upstream. This orienlation was selected to be in agreement with the Simons, Li & Associates, Inc., study (Reference 2). Cross sections were laid out in the overflow channel with consideration given to the canal, road embankments, and low places on 1-25. The roughness coefficients for the overflow channel ranged from 0.035 to 0.065. The overflow channel does not contain a defined ditch where most of the discharge will flow; rather, it is wide with a nearly flat bottom. 17