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<br />floods were gener.lted by the Corps of EngLleers at various design points <br /> <br />lvt thin the basin. <br /> <br />The Middle and South St. Vrain Creeks 000-:1 frequency data we!"e b,eLsed on <br /> <br />regiona.l relationships for statistical pa ra.meters of log-Pearson TYPE~ [J r <br /> <br />distributions (Reference 6). <br /> <br />The regioTlnJ relationships were developed <br /> <br />through statistical analyses of strE~amElo1;\' records at ten rSGS gaugJng <br /> <br />stations located in the upper St. Vrain (;reek and Big Thompson basins. <br /> <br />Rainfall and snowmelt floods were separatel~ in the frequency <::Hl<3.1ysl?:s, <-tnc~ <br /> <br />t.hen. statistically combined to obtafn t'f-l€~ composite flood flovl frequ.ency <br /> <br />curves. <br /> <br />Discharges at the mouth of North St. \iIain Creek were derJ~ved by <br /> <br />determining the peak rate of runoff per clICalnage basin unit are~, above <br /> <br />Buttonrock Reservoi r for the 10-, 50.." ]In.." and 500-ye8r flo'ws. <br /> <br />Tbese <br /> <br />peak flows per unit area were then multiplied hy the drai.nagE~ basi.n area <br /> <br />between Buttonrock Reservoir and the mouth :)f the North St. Vrain Creek, <br /> <br />and added to the spillway discharges for var:~ous recurrence intervaJ s from <br /> <br />Buttonrock Reservoir. <br /> <br />These values correl.ated we],l with the peak flc>od <br /> <br />flows from the South St. Vrain, and the flows calculated by the Corps or <br /> <br />Engineers innnediately downstream of the eo]~.f] uenee of the >Jorth and South <br /> <br />St. Vrain Creeks. <br /> <br />In view of the fact that the statistical ,c.nalysis of streamflow record~; <br /> <br />did not accurately address tbe effcects 011 peak flood nc>ws through <br /> <br />Buttonrock Reservoir, the Corps of Enginee'C.3 developerl a modl:~] based on <br /> <br />the Environmental Protection AgenCY's (EPII 'j ~)torrn Wa.ter Management Model <br /> <br />(S,\\TMH) (Reference 7), and the Missouri R:!Y':~r Divis; on ~ s version of thE' <br /> <br />J q <br />