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<br />Study Area <br /> <br />The project area is located in northeastern Colorado. The City of Sterling and the project area are <br />within Logan County, which is approximately 120 miles northeast of Denver. A general location map <br />of the Logan County/Sterling area is shown on Figure l. The watershed of interest is the Pawnee <br />Creek Basin, which is within the South Platte River Basin. The basin is a left bank tributary with its <br />confluence with the South Platte River located upstreanl of the City limits of Sterling, Colorado. <br />Major tributaries to Pawnee Creek include: Raymer Creek; South Pawnee Creek with its tributary <br />Wildhorse Creek; North Pawnee Creek with its tributaJ~1 19o Creek; Cottonwood Creek; Horsetail <br />Creek; and Spring Creek. These creeks are basically intermittent with rurming water in the lower <br />reaches. <br /> <br />Predominate topographic features of the basin include the: Pawnee Buttes in the uppe:r reaches of the <br />basin. The basin's drainage area is approximately 645 square miles. Elevations in the basin range <br />from approximately 5,400 feet to 3,980 feet. Th.: wate:rshed is mainly native grass with localized <br />regions of cultivated agriculture. Soils are predominately loams and clay loams, with sandstone, shale <br />and siltstone outcroppings. Hydrologic curve numbers are in the range of 77 to 79 with the primary <br />vegetation being native range with some winter wheat and a significant amount of irrigated cropland in <br />the lower part of the basin. <br /> <br />The Pawnee Creek/South Platte River confluence is located approximately 4 miles southwest of the <br />City of Sterling along U.S. Highway 6 (HWY 6) and the Union Pacific Railroad (UPRR). During large <br />storm events, stormwater overtops the banks of Pawnee Creek and flows into the City. These <br />overflows are estimated to be 5,000 cfs during the 100-year storm. This study focuses primarily on the <br />reach along Pawnee Creek from its confluence with the South Platte River upstream to County Road <br />29 (CR. 29), a distance of about 7.5 miles. The Vicinity Map (Figure 2) presents the lower portion of <br />the Pawnee Creek Basin with respe,~t to the City of Sterling. The Pawnee Creek Basin Map (Figure 3) <br />shows the overall Pawnee Creek Drainage Basin, along with a tabulation of pertinent basin <br />characteristics. <br /> <br />Previous Reports <br /> <br />Several studies have been completed to investigate the existing flooding conditions in the City of <br />Sterling and surrounding Logan County. Some of these ~~udies address alternatives for mitigation to <br />the flooding problems. The City of Sterling is subject to flooding firom four major sources which <br />include the South Platte River, Pawnee Creek, Sand Creek, and the Pioneer Drainage D itch. One <br />recent study, "City of Sterling Drainage and Flood Control Implementation Plan" dated June 1997, by <br />ICON Engineering, Inc. concluded that addressing the flooding problems associated with the Pawnee <br />Creek should be given the City's highest priority, and would result in the most feasible and economical <br />plan of improvements for the area. As a result of flooding events that occurred in 1997 and discussed <br />in detail within this report, the City and County have determined that resolution of the Pawnee Creek <br />drainage problems must be appropriately implementt:d. The following reports were reviewed as a part <br />of this study: <br /> <br />1-2 <br />