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<br />~ <br /> <br />Synthetic Hydrograph Analysis <br /> <br />(McCabe Creek Segment) <br /> <br />Soil Conservation Service Technical Release No:-20 was used to develop <br /> <br />and route hydrographs from the upper reaches of McCabe Creek to the San <br /> <br />Juan River. This procedure involves the use of rainfall frequency data, <br /> <br />soils and ground cover data, and time of concentration in developing <br /> <br />hydrographs. Soils information was obtained from the Forest Service for <br /> <br />that portion within the San Juan National Forest and from local soil <br /> <br />surveys for the balance of the watershed. Cover conditions in the <br /> <br />~Iatershed were obtained by on-site inspection. Time of concentration <br /> <br />was obtained by using surveyed channel cross sections at selected locations <br /> <br />throughout the ,"atershed in conjunction with mannings equation to <br /> <br />compute velocities and travel times. Channel roughness coefficients <br /> <br />were obtained by on-site inspection. Photographs of the channel were <br /> <br />taken as supporting data. Hydrographs and corresponding elevations of <br /> <br />the peaks were obtained from the SCS TR-20 computer program. Cross <br /> <br />section rating tables were included as input to this program. These <br /> <br />rating tables were developed from previous water surface profile computations <br /> <br />using SCS WSP-2 Computer Program. This program is discussed further <br /> <br />under the hydraulics section of this report. <br /> <br />Data from the TR-20 program were used to plot boundaries of the 500 year <br /> <br />and 100 year frequency floods as well as profiles of the necessary flood <br /> <br />frequencies. <br /> <br />laav <br /> <br />~ <br /> <br />I <br />l. <br />, -....~ <br /> <br />- <br /> <br />" <br />t;; <br />, <br /> <br /> <br />,.'- <br /> <br /> <br />,",' . <br /> <br />1 ,,' <br /> <br />"'.1 <br /> <br />~ <br />~~ <br /> <br />. <br />