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<br />. <br /> <br />. <br /> <br />are characterized by moderate peaks, large volume and long <br />duration. This type of flood is usually not as damaging as those <br />resulting from rainstorms. Nevertheless, severe flooding may <br />occur when rainfall accompanies the snowmelt. <br /> <br />~LQcglgg!~ 6021L2!2 <br /> <br />The U.S. Army Corps of Engineers, Omaha District, has completed <br />a hydrologic investigation for the entire Saint Vrain Creek <br />basin which includes the study reach. Discharges for the 5-, <br />10-, 50-, 100-, and 500-year floods were developed by the Corps <br />for Saint Vrain Creek from just below the confluence of the <br />North and South Saint Vrain Creeks in Lyons, Colorado to the <br />mouth of the Saint Vrain Creek. <br /> <br />For the South Saint Vrain Creek, floodflow frequency data were <br />based on regional relationships for statistical parameters of <br />log-Pearson Type III distribution (reference II. The regional <br />relationships were developed through statistical analyses of <br />streamflow records at ten USGS gauging stations located in the <br />upper Saint Vrain Creeks and Big Thompson River basins <br />(references 2, ~, 4, 5, 6, 71. Table 1 lists these gauging <br />stations. Rainfall and snowmelt floods were separated in the <br />frequency analyses and then statistically combined to obtain the <br />composite floodflow frequency curves. <br /> <br /> <br />Flows on North Saint Vrain Creek directly above the confluence <br />with South Saint Vrain Creek at Lyons, Colorado were established <br />by determining the runoff volume for the various frequencies per <br />drainage basin unit area above Buttonrock Reservoir multiplying <br />those values by the drainage basin area below Buttonrock <br />Reservoir and adding the reservoir spillway outflow values. <br />These values correlated very well with the frequency flows <br />determined at the confluence. Figure 1 is a schematic <br />representation of flows along the South, North and St. Vrain <br />Creeks to the Boulder-Weld County line. Table 2 gives a <br />tabulation of flows along the entire St. Vrain Drainageway. <br /> <br />!:;l22!O t:!g!'i!~liOg <br /> <br />Rainfall-runoff characteristics for the St. Vrain basin were <br />modeled by the Corps utilizing the EPA Storm Water Management <br />Model (SWMMI and the Missouri River Division version of Harder's <br />diffusion routing model. The routing model extended from Lyons <br />to the mouth of the St. Vrain Creek. Calibration of the models <br />to conditions before Buttonrock Dam was. operational was <br />accomplished from a discharge probability analysis for Saint <br />Vrain Creek at the mouth near Platteville, Colorado. The <br />analytical methods presented in Bulletin No. 17 published by the <br />Water Resources Council were used for each analysis. The gauge <br />at Lyons is located below the confluence of North and South St. <br />Vrain Creeks and has records available from 1888 to the current <br />year. <br /> <br />2 <br />