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Alternative No.2: This alternative will restrict 100-Y":1f flood flow from overflowing into the left overbank floodplain and entering the agricultural lands and the City of Sterling. Thiis alternative was the recommended alternative from the "City of Sterling Drainage and Flood Control Implementation Plan" dated June 1997. This alternative consists of a system of flood control levees. In general, the levees would follow along the north side of Pawnee Creek beginning upstream of CR. 29 and continuing east to HWY 6. Since the levees would be d"signed for the I OO-year storm, they would be required by FEMA to have 3 feet of freeboard. The existing bridges at HWY 6 would be improved to convey the 100-year discharge urlder the bridges. For the southbound lanes, the existing concrete bridge would be expanded by adding 4 cells of 20' wide X 8' tall RCBs to the south of the existing bridge. The existing timber bridge along the northbound lanes would be replaced. The Colorado Department of Transportation (CDOT) currently has plans to replace tile timber bridge within the next 3 to 5 years. In the summer of 1996, the Burlington Northern Railroad (BNRR) replaced their bridge located just downstream of HWY <5 for structural reasons. Design of the new railroad bridge was such that the new bridge will ultimately convey the 100-year peak discharge. Since the bridge improvements would allow more flow downstremn of HWY 6, a second system of levees and a floodwall are proposed to confine ~:tormwater downstreanl of the highway to the South Platte River. These proposed improvements would eliminate th,: Pawnee Creek overflows from draining into the City. The overflows are estimated to be about 5,000 cfs during the 100-year storm. Eliminating these overflows removes a substantial anlOunt ofland and many structures from the floodplain in both Logan County and the City of Sterling. Alternative No.3: The proposl:d alternative will temporarily store the floodwaters in detention structures in the upper watershed and will allow controlled, attenuated releases. This alternative is a series of upstream flood control reservoirs as presented in the report entitled "Flood Plain Management Study, Pawnee Creek and Logan Creek, Colorado" dated April 1992, and its supplement, along with supplemental analysis completed by ICON to update construction costs. The reservoirs are located in the upper portions of the basin, with only one being in Logan County and the remainder being in Weld County. In general, these dams m'e designed for peak flow reduction and are not intended to provide permanent ponds. All of the structures would be large fadlities that would fall under the jurisdiction of the Colorado State Engineer's office, and accordingly would require emergency spillways designed to prevent overtopping from the design flood. Alternative No.4: This alternative will collect the Pawnee Creek overflow and divert these flows directly to the South Platte River in the general vicinity of the Cemetery just south of Sterling. This alternative consists of construction of a berm having an approximate height of 5 feet to the north and west of the Riverside Cemetery. This berm would collect and prevent Hows from the Pawnee overflow from entering the City along th,: HWY 6IRailroad embankments. Included in the alternative is lowering of CR. 24, construction of a channel with a concrete floodwall, a closure structure for HWY 6, construction of box culverts under the railroad, a siphon for the Sterling No. 1 Ditch, and construction of a trapezoidal channel to the South Platte River. Alternative No.5: The proposed alternative will provide a conveyance structure at Town of Atwood, increasing the bridge capacities at the mainstem of Pawnee Creek, chmmel cleaning, (debris removal) and stabilization measures upstremn ofHWY 6 to CR 29, and ail the project features of Alternative No. 4. This alternative consists of construction of Alternative No.. 4, along with modifications and new HWY 6 bridges, channel and levee improvements downstream of tile UPRR at Town of Atwood. 1-9