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<br />. ' <br /> <br />c. Manninq's "n" and Infiltration Losses <br />Due to the variable geography throughout the basin, <br />surface permeability, channel and overbank roughness, and <br />overland roughness varied substantially. The Manning's "n" <br />coefficient used for channels ranged from 0.03 in the <br />downstream region of the basin to 0.05 in the mountains. A <br />value of 0.012 was used in the residential areas where <br />concrete and asphalt streets served as the main channels. <br />Overbank Manning's "n" varied from 0.03 to 0.08. In the upper <br />region of Ralston Creek, where the Snyder Unit Graph method <br />was used for runoff calculation, Snyder's standard lag time <br />varied from 0.87 to 1.32 hours based on the Cherry Creek <br />constants (ct = 0.51 and 640 Cp = 537). In the lower region, <br />where the kinematic wave technique was used to calculate <br />runoff, the resistance factors used for overland flow were <br />0.15 for impervious areas and 0.4 for pervious areas and were <br />weighted according to the percent of specific surface <br />conditions found in each subbasin. An initial infiltration <br />rate of 0.5 inches and uniform infiltration rate of 0.75 <br />inches/hour ("Soil Survey of Golden Area, Colorado", US Soil <br />Conservation Service, 1980) were kept constant throughout the <br />basin. <br />A study was performed to ascertain the sensitivity of <br />model peak flows to initial losses, uniform losses, and <br />overland flow resistance. Peak flows at the mouth of Ralston <br />Creek were evaluated using a range of values for each <br />parameter. Initial loss was varied from 0.0 to 1.0 inches <br />with a control value of 0.5 inches, uniform losses varied from <br />0.3 to 1.0 inches with a control value of 0.6 inches, and <br />overland flow resistance varied from 0.2 to 0.6 with a control <br />value of 0.4. Table 5 presents the sensitivity of these <br />parameters to the model. <br /> <br />7 <br />