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<br />3.1 Hydrologic Analyses <br /> <br />Hydrologic analyses were carried out to establish the peak dischargc- <br />frequency relationships for floods of the selected recurrence inter- <br />vals for each stream studied in detail in the community. <br /> <br />Two streamflow gaging stations are located in the vicinities of the <br />study area: one on the Animas River, having 39 years of record, and <br />the other on Mineral Creek, having 14 years of record. However, a <br />regional relationship relating the basin area to the peak discharge <br />was the principal method used. Gaging stations located within the <br />Animas River Basin and used to establish the regional relationship, <br />are shown in Table 1. <br /> <br />The method developed by the U.S. Water Resources Council was used to <br />determine the flood flow frequency relationship based on log-Pearson <br />Type III distribution for each selected gaging station (Reference 4). <br />Gaging stations used for developing the regional relationship were <br />selected according to the criteria of (1) stations having 10 or more <br />years of record and (2) stations where peak flows are not affected <br />by regulation and diversion. <br /> <br />Because the annual peak discharges published in the U.s. Geological <br />Survey's Water Supply Papers (References 5, 6, and 7) and the u.s. <br />Geological Survey's Surface Water Records (Reference 8) were a mix- <br />ture of floodflows caused by snowmelt and rainfall events, the <br />regional relationship established by using these unnual peak discharges <br />does not reflect intense rainfall-produced flooding. A more detailed <br />method was used for each gaging station to develop regional relation- <br />ships for this area. The following procedures were used (1) separating <br />the rain and snowmelt data at each gaging station, developing statistics <br />for each type of event, and statistically COmbining the two frequency <br />curves (streamflow records were obtained from the u.s. Geological <br />Survey, Denver Federal Center, and District Office in Albuquerque, <br />and the Colorado State Engineers' Office); (2) adjusting short- <br />period statistics to reflect long-period values if they are closely <br />correlated; (3) considering outliers and historical data during the <br />development of floodflow-frequency relationships for each gaging <br />station; and (4) developing regional relationships relating basin <br />areas to peak discharges for the 10-, 50-, 100-, and SOO-year floods. <br />Peak discharges used for hydraulic analyses at various locations for <br />each of the 10-, 50-, 100-, and SOO-year floods were determined by <br />the regional relationships. <br /> <br />Peak discharge-drainage area relationships for the Animas River, <br />Mineral Creek, and Cement Creek are shown in Table 2. <br /> <br />7 <br />