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<br />2.6 ESTIMATION OF 100-YEAR PEAK FLOWS <br /> <br />Using plausible combinations of lag times, curve numbers, and durations of storm events, several <br />estimates of the 100- and 25-yr peak flows of Frenchman Creek at Holoyoke have been <br />developed. Simulations are made to estimate the 25-yr peak flow at Holoyoke because previous <br />estimates of the 25-yr peak flow are available for comparison, The output of the HEC-l model <br />indicated that, for the case with lag times computed by the stream hydraulics method using an <br />average velocity of 2 ft/sec, the 100-yr peak flow at Holoyoke occurs at 24.0 hours after the start <br />of the storm. But the 25-yr peak flow occurs 25.0 hours after the start of the storm, This <br />suggests that, for this case, peak flows for 48-hr duration storms should also be evaluated, Thus, <br />the following sets of lag times and CNs are used for rainfall-runoff simulations: <br /> <br />(i) Lag times given by the USBR method with CN = 60 and 64 and 24-hour (24-hr) duration <br />storm, <br /> <br />(ii) Lag times given by the Stream Hydraulics method with V = 2 ft/sec and CN = 60 and 64 <br />and 24-hour duration storm. <br /> <br />(iii) Lag times given by the Stream Hydraulics method with V = 2 ft/sec and CN = 60 and 64 <br />and 48-hour duration storm. <br /> <br />(iv) Lag times given by the Stream Hydraulics method with V = 1 ft/sec and CN = 64 and 68 <br />and 48-hour duration storm, <br /> <br />The estimated peak flows for each case are shown in Table 6. <br /> <br />Table 6 <br />Estimated Peak Flows Using Rainfall-Runoff Modeling <br /> <br />Case Description Modeling Results <br /> Return Period and Estimated peak Flows (cfs) <br /> Duration of Storm <br />1 USBR Method; 1 OO-yr 24-hr 13,314 (occurs at 23.25 hrs) <br /> CN = 60 and 64 25-yr 24-hr 6,808 (occurs at 24 hrs) <br />2 Stream Hydraulics Method I OO-yr 24-hr 12,145 (occurs at 24 hrs) <br /> V = 2 ft/sec; 25-yr 24-hr 6,260 (occurs at 25 hrs) <br /> CN = 60 and 64 <br />3 Stream Hydraulics Method 1 OO-yr 48-hr 12,250 (occurs at 24.5 hrs) <br /> V = 2 ft/sec; 25-yr 48-hr 6,288 (occurs at 25 hrs) <br /> CN = 60 and 64 <br />4 Stream Hydraulics Method 1 OO-yr 48-hr 11,587 (occurs at 42.75 hrs) <br /> V = 1 ft/sec; 25-yr 48-hr 6,312 (occurs at 43.75 hrs) <br /> CN = 64 and 68 <br /> <br />As expected, peak flows for the 48-hr duration storm (Case 3) are slightly higher than those for <br />the 24-hr duration storm (Case 2). <br /> <br />Draft Letter Report <br />Independent Review of Hydrologic Analysis for Frenchman Creek <br />Contract No. DACW41-00-D-0026-0001 <br /> <br />Flood Plain Management Services Special Study <br />Holyoke, Colorado <br />June 18,2001 <br /> <br />Page 9 <br />