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<br />ce <br /> <br />~- <br />,; . <br />" " <br /> <br />l4e <br /> <br />6.0 TRIBUTARY WATERSHED ANALYSIS <br /> <br />A small tributary watershed is located on the northeast slope of the <br /> <br />ridge immediately to the southwest of the proposed Water Treatment Plant as <br />shown on Figure 2. The steep watershed contains approximately 94 acres of <br />forested land and grassland. lOO-year flows from this watershed pose a <br />hazard to the facilities due to possible innundation and damage from high <br /> <br />velocities. <br /> <br />6.1 Watershed Characteristics <br /> <br />The watershed area of 94 acres is very steep; the crest elevation of <br /> <br />7.560 drops to elevation 6,600 at the mouth of the watershed in only 4.400 <br /> <br />feet. Due to the steep slope. averaging 22 percent. runoff from the watershed <br />has a very high velocity and a short time of concentration. Approximately <br />one-ha1 f of the watershed is covered by a ponderosa pine forest and the <br />other one-half by grassland and brush. The watershed appears to be covered <br />by a thin layer of soils overlying bedrock. Smooth 'bedrock is exposed in <br /> <br />the thalweg of the stream as shown in the photo in Figure 6c. <br /> <br />6.2 Peak Runoff <br /> <br />The Rational Method and the Soil Conservation Service method were used <br /> <br />to calculate runoff resulting from the 100 year precipitation on this water- <br /> <br />shed. <br /> <br />The Rational Method is appl icable to small watersheds having short <br /> <br />times of concentration. The rational formula is: <br /> <br />Q = Ci A. <br /> <br />6-1 <br />