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During the same period, the mean discharge from the Trout Creek watershed above <br />• TR-a was 38 cfs representing approximately I.0 feet of runoff. Because the Trout <br />Creek runoff during the 67 week period is essentially equally to the value believed <br />to be The annual mean it is expected That the 0.62 feet of runoff measured at O K- <br />a is also equal to the annual mean. By simple ratios, The mean annual runoff from <br />the Oak Creek watershed above OK-a is estimated to be 0.62 feet, equal to that <br />measured for the 67 week period of this study. The mean low flow in Oak Creek, <br />as estimated from data collected in the fall of 1979, is about 5 cfs. As in the case <br />on Trout Creek, the low flows occur during the fall and winter. <br />Flood Estimates -Flood discharges and depths were estimated at five <br />locations along Oak Creek for storms of 6 and 24 hour duration and recurrence <br />intervals of 2, 5, 10, 25, 50 and 100 years. Rainfall for each of the Twelve events <br />was estimated using methods presented by Miller, et al., (1973). Point values of <br />precipitation were obtained from isofluvial maps given by Miller, et al., and adjusted <br />by the appropriate areal reduction factor. Table 2.5-4 presents the depth of <br />precipitation for the Oak Creek drainage basin for each of the twelve events. The <br />location of the cross-sections used for flood routing are presented in Table 2.5-5 and <br />shown on Exhibit 2.5-10 and 2.5-2. Cross-Sections I, 3 and 4 are surveyed cross- <br />. sections on Oak Creek while Cross-Section 2 was taken from the 1:4800 scale map <br />of the area and data obtained during discharge measurement at The Oak Creek <br />Gaging Station. Cross-Section 5 was taken from a 1:24000 scale map. <br />Runoff hydrographs for each storm were prepared using the method developed <br />by the Soil Convervation Service (SCS). The composite curve number for the Oak <br />Creek Drainage Basin was estimated to be 69 (Antecedent Moisture CondiTion II) <br />based on land use and soil type. For this curve number at least 0.90 inches of <br />rainfall are required to produce runoff; therefore, the 6 hour, 2 year storm does not <br />provide runoff and runoff hydrographs were not prepared for this storm. The SCS <br />method of developing runoff hydrographs requires development of the Unit Hydro- <br />graph for the area. The SCS (1972) presents the following equations for developing <br />the Unit Hydrograph based on basin characteristics of oreo in square miles (A) and <br />lag time in hours (t~): <br />tp = T~/1.2 <br />484A <br />qp = <br />tp <br />Revised 7-81 2.5-IS <br />