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<br /> TABLE 1 <br /> PIONEER DITCH <br /> 10- YEAR FUTURE CONDmONS <br /> LOCAL DRAINAGE <br /> PEAK DlSCHARGEPROFlLE <br /> 10- YEAR PEAK <br /> HEC-2 CROSS DISCHARGE <br />NODE SECTION LOCATION (cfs) <br />1 1180 Burlington Northern Railroad 365 <br />2 1130 Main Street 560 <br />3 1090 Franklin Street and North 7th Avenue 875 <br />4 1060 North Division Avenue and Pioneer Road 1037 <br />5 980 Burlington Northern Railroad, Union Pacific Railroad, and 1212 <br /> U.S. Hi.hwav 138 Crossin.s <br /> <br />3.2.4 A channel hydrograph was needed to make detention pond routing calculations. <br />Examining the 1988 Leaf document and preceding documents analyzing several different <br />sources of flooding in Sterling, it was determined that the Soil Conservation Service (SCS) <br />runoff method was used to calculate the flows shown in Table 1. Therefore, a hydrograph <br />representing local drainage flows in Pioneer Ditch was npt available. Table 7 in the 1988 Leaf <br />document shows lO-year, 6-hour inflow hydrographs for existing and future conditions for <br />inflow into the detention pond sized for that study. Calculations or narratives describing the <br />source of the hydrographs could not be found, so it was assumed that the future conditions <br />inflow hydrograph was the most accurate information available. This hydrograph was used to <br />calculate a Pioneer Ditch hydrograph. Table 5 of the 1988 Leaf study showed that the total 10- <br />year future runoff volume at node 4 was 301.2 acre-feet and that the lO-year peak discharge <br />was 1040 cubic feet per second (cfs). The future conditions inflow hydrograph from Leaf <br />(1988) was modified to peak at 1040 cfs and to have a total volume close to 301.2 acre-feet. <br />The original detention pond inflow hydrograph from the 1988 report and the calculated channel <br />hydrograph are shown in Table 2 and on Plate 2 of this report. <br /> <br />4 <br />