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<br />length to width ratio, precipitation, and discharge-frequency was used to estimate discharge- <br />frequency for Willow Creek. The study included only the 1949 historical flood event in Willow <br />Creek, as the event occurred during the construction of the masonry flume and may be a more <br />reliable estimate. Table 2.2.2 compares flood flows estimates at Willow Creek gage from the <br />various studies as well a log-Pearson analysis of the gage data with various historic flood <br />estimates. Gage data are from 1951 to 1982 and do not include gage data available from 1999. <br /> <br />Table 2.2.2. Comparison of flood flow estimates in cfs (adapted from Yochum and Hyde 2002) <br /> <br />Discharge NRCS, CWCB, Army Army Gage Gage Gage Gage <br />Frequency 2002 1986 Corps, Corps, Data, no Data, + Data, + Data, + <br />1989 1999 est. '49 est. '49, 48' all est. <br />500-yr - 1680 2810 - - - - - <br />200-yr - - 2000 6000 - - - - <br />100-yr 1073 1120 1530 2300 674 982 1244 2591 <br />50-yr 915 910 1160 1500 583 803 988 1914 <br />25-yr 769 - 867 1000 494 643 769 1377 <br />10-yr 586 510 568 600 382 460 527 843 <br />5-yr 458 - 392 400 300 339 375 542 <br />2-yr 288 - 203 210 187 193 202 244 <br />1.25-yr 185 - 111 - 115 113 113 117 <br /> <br />The NRCS 2002 discharge-frequency estimates are similar to the CWCB 1986 estimates, but are <br />significantly smaller than the Army Corps estimates. The discrepancy arises in the confidence <br />placed in the historic flood estimates versus the confidence that the Willow Creek watershed has <br />a discharge-frequency relationship comparable to other similar watersheds in the area. <br />Determining which of the discharge-frequency estimates is the most appropriate is beyond the <br />scope of the current study. Although the NRCS used a non-traditional methodology, it used a <br />comprehensive and defensible approach, and the NRCS discharge-frequency relationships were <br />used in the current study. Table 2.2.3 details flood-flow levels at various points within the <br />Willow Creek watershed (rather than just at the gage) provided by the NRCS study. <br /> <br />Table 2.2.3. Flood discharge-frequency values (cfs) recommended by Yochum and Hyde 2002 <br /> <br />Location Discharge- Frequencies <br /> 100yr 50yr 25yr 10yr 5yr 2yr 1.25yr <br />Willow Creek at confluence with Rio Grande 1213 1046 888 689 546 353 232 <br />Willow Creek at Railroad Crossing 1131 969 817 627 493 313 203 <br />Willow Creek at Creede gage station 1073 915 769 586 458 288 185 <br />West Willow Ck. at confluence with East Willow Ck. 532 457 386 296 232 144 90 <br />West Willow Creek at Nelson Creek (inclusive) 382 325 273 206 159 96 58 <br />East Willow Ck. at confluence with west Willow Ck. 942 807 681 523 412 264 172 <br />East Willow Ck. at road crossing near Phoenix Mine 733 623 522 398 311 196 127 <br /> <br />2-10 <br />