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I I I I I I I I I I I I I I I I I I I City of Sterling, Colorado Pawnee Creek Flood Hazard Mitigation Feasibility Study I. INTRODUCTION The City of Sterling, Colorado, is bordered by Sand Creek and Pioneer Ditch to the north and west, the South Platte River to the east, and Pawnee Creek to the south. Each of these drainage ways provides a source of flooding for the City during large storm event. Three major floods and at least three other minor floods have impacted the City in the last 65 years. Through previous studies, improvements to the Pawnee Creek flood plain would create the greatest benefit to the City and portions of the county. The objective of this project is to develop a Flood Hazard Mitigation Feasibility Study for Pawnee Creek and to provide a report to be included with the Colorado Water Conservation Board (CWCB) loan application. This report is formatted to meet the criteria outlined by the CWCB for their Water Project Construction Loan Program. II. PROJECT SPONSOR The City of Sterling is the county seat for Logan County. The City is located approximately 125 miles northeast of Denver, and 126 miles southeast of Cheyenne, Wyoming. A general location map of the Logan County/Sterling area is shown on Figure 1. Sterling is the largest community in Logan County, having a population of about 11 ,000 (1995 estimate). The total population within Logan County is about 19,000 (1995 estimate). The City operates with sources of revenue from taxes, licenses and permitting, intergovernmental, charges for services, and fines. Approximately 75% of the City's total revenue comes from taxes of which approximately 70% of the tax revenue comes from sales tax. The climate of Sterling is primarily affected by northwestern winds. The average annual precipitation is 19 inches and the average annual snowfall is 39.4 inches. The mean annual temperature is 48.60 F., and the average annual number of clear days is 299. The coldest month is January with a mean temperature of 38.80 F. and the warmest month is July with an average temperature of 89.00 F. The topography of the City consists of high rolling plains, with the predominate feature being the South Platte River which flows through the City and County from the south to the north. The South Platte River floodplain is broad and relatively flat. Sterling is bisected from southwest to northeast by State Highway 6 and a parallel railroad track operated jointly by BNRRlUPRR. Pawnee Creek is a left bank tributary to the South Platte River and drains an area of about 645 square miles. The upper portions of the basin are located in Weld County, and the lower portion, along with the confluence with the South Platte River are located in Logan County. The mouth of the creek is located about 5 miles southwest of Sterling and about 1 mile northeast of Town of Atwood in Logan County. The watershed elevation ranges from about 3,980 feet to 5,400 feet. The soils in the basin are mostly loams and clay loams, with sandstone, shale, and siltstone outcroppings. Land use consists of open range land and a significant amount of irrigated cropland in the lower part of the basin. Several major and many minor irrigation canals and ditches cross the study area, generally flowing parallel to the South Platte River. Significant among the canals are the Pawnee Ditch, Springdale Ditch, and the Sterling No.1 Ditch.