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was formulated by first reviewing the existing Logan County FIRM map showing the <br /> Pawnee Creek Overflow limits. Based on the flood width shown in the area of CR 22.5, <br /> it appeared that if this water could be collected and directed to the South Platte River that <br /> Sterling's and a portion of Logan County's flooding problems would be eliminated. <br /> Some of the first questions that needed to be addressed included how would adding 5000 <br /> cfs from the Pawnee Creek Overflow on the upstream (south) side of Sterling affect the <br /> South Platte River floodplain and flooding along the South Platte banks through Sterling? <br /> CWCB and the COE are currently sponsoring a project to study reservoir release impacts <br /> to the South Platte River. The existing HEC-RAS hydraulic model of the South Platte <br /> River was obtained and used to determine impacts to Sterling due to the change in <br /> location since the Pawnee Overflow will enter the South Platte River. The results of this <br /> evaluation determined that the South Platte River 100-year floodplain would not be <br /> affected by the change in the Pawnee Creek Overflow outfall location. The more <br /> frequent events on the South Platte River may be impacted by the new outfall location, <br /> however, the probability that the Pawnee Creek Overflow would be carrying water at the <br /> same time the South Platte River is experiencing a significant event to cause any damage <br /> is greater than approximately an equivalent 1000-year event on the South Platte by itself. <br /> Another area of concern included what tailwater elevation should be used to size and <br /> analyze the new channel? According to FEMA, the ratio of drainage basin sizes needs to <br /> be within a specific range to require the use of coincidental flows between the Pawnee <br /> Creek Overflow and the South Platte River. In this case,the Pawnee Creek basin is much <br /> smaller than the South Platte's basin and is below the FEMA defined range. The new <br /> channel was sized and analyzed using a normal depth boundary condition to start the <br /> hydraulic analysis. <br /> Meetings were held with local landowners to present the project and to understand their <br /> issues. The channel was originally planned to be located at the south edge of the <br /> cemetery. Landowners pointed out that erosion would work its way into their fields after <br /> any flood event that required the use of the channel. The erosion would cause upstream <br /> fields to become un-irrigatable and would require leveling. Because of this, the channel <br /> location was moved to the south and positioned immediately north of CR 22.5. If erosion <br /> occurs with the new alignment, it will impact the county road instead of an irrigatable <br /> field. The channel alignment is positioned along property lines to eliminate dividing <br /> farm property. <br /> Plan Description <br /> The selected plan consists of improvements near CR 22.5, where flood protection will <br /> prevent Pawnee Creek Overflow flows from being trapped west of the HWY 6 and <br /> railroad embankments forcing flows into the City of Sterling. <br /> This alternative will collect the Pawnee Creek overflow and divert these flows directly to <br /> the South Platte River in the general vicinity of County Road 22.5 just south of Sterling. <br /> This alternative consists of excavation of an interceptor channel designed to carry the <br /> 5000 cfs, 100-year flow rates. The channel will extend west past the point where water <br /> 31 <br />