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<br />3.0 OTHER PEAK FLOW ESTIMATES <br /> <br />3.1 PREVIOUS ESTIMATES OF PEAK FLOWS <br /> <br />In an earlier hydrologic/hydraulic study for the Frenchman Creek watershed (USACE, 1980), 10- <br />, 50-, 100-, and 500-yr peak flows of Frenchman Creek for a drainage area (D.A,) of 227 sq <br />miles were estimated to be as shown in Table 7, These peak flows fit the lognormal distribution <br />reasonably well (Figure 1), The 25-yr peak flow is estimated by interpolation using the <br />lognormal distribution, The corresponding peak flows for a drainage area of 247.5 sq miles.are <br />estimated using the drainage area ratio. These peak flows are also shown in Table 7. <br /> <br />Table 7 <br />Previously Estimated and Extrapolated Peak Flows of Frenchmen Creek <br /> <br />Return period Previously Estimated Peak Extrapolated Peak Flows (cfs) <br /> Flows (cfs) for D.A. = 227 sq for D,A. = 247.5 <br /> miles sq miles <br />10-yr 2,750 2,998 <br />25_yra 4,506 4,913 <br />50-yr 6,200 6,760 <br />100-yr , 8,200 8,940 <br />500-yr 14,900 16,246 <br /> <br />a Interpolated using lognormal distribution. <br /> <br />3.2 INDIRECT PEAK FLOW ESTIMATES <br /> <br />Using high water marks at certain locations along the cross sections of Frenchman Creek in the <br />vicinity of Holoyoke, the peak flow at the golf course in Holoyoke was estimated to be 3,600 to <br />4,000 cfs (CWCB, 1997). This flow resulted from a 3-hr storm event, which occurred on June 14 <br />and 15, 1997. The estimate is based on hydraulic computations using Manning's equation with <br />channel slope estimated from USGS Quadrangle maps. The cross sections of the creek were <br />prepared from field surveys and Manning's roughness coefficients were selected by judgment <br />based on observations in the field. Using the information given in Floodplain Information <br />Report, Frenchman Creek, Holoyoke, Colorado (USACE, 1980; Table 7) and regional regression <br />equations for the Republican River Basin, a 25-yr return period was assigned to this estimate. <br />Because of the methods of estimation, this estimate of 25-yr peak flow is judged to be <br />approximate and indicative of a ballpark value. If the D,A. at the golf course is 247.5 sq miles, <br />then the return period interpolated from the values in Table 7 may be about 13 years for 3,600 <br />cfs and about 17 years for 4,000 cfs (see Figure 1), <br /> <br />Draft Letter Report <br />Independent Review of Hydrologic Analysis for Frenchman Creek <br />Contract No. DACW4I-00-D-0026-0001 <br /> <br />Flood Plain Management Services Special Study <br />Holyoke. Colorado <br />June 18,200 I <br /> <br />Page 10 <br />