2011-09-08_REVISION - M1977342 (35)

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rainfall excess to the time at which 50 percent of the runoff has passed the concentration point. The following equation was used to determine the lag time: where, Infiltration Lg = G (L /S° 5`0.33 L = lag time, in hours; = a constant equal to 26K; L = length of the longest watercourse, in miles; L� = length along the longest watercourse from the point of interest to a point opposite the centroid of the basin, in miles; S = overall slope of the longest watercourse from the point of concentration to the drainage basin divide, in feet/mile. The G value is approximated as 26 times K,,, where K„ is estimated to be equivalent to the average Manning's roughness value for the principal watercourses in the drainage basin. The K values used in our evaluation were consistent with typical values used in the Rocky Mountains region. The USBR Flood Hydrology Manual provides unit hydrograph lag relationships for both the general storm and local storm. For the Rocky Mountain Region, K ranges from 0.130 to 0.260 for general storms, and from 0.050 to 0.073 for local storm events. In addition, K„ was also approximated according to information provided by Sabol. Sabol determined that for mountain watersheds K, ranges from 0.045 to 0.055 with an average value of 0.050. Based on the USBR's and Sabol's suggested ranges for K,, a value of 0.050 was selected to be used for all the subbasins in the Henderson watershed. A time interval of 15 minutes was used in the generation of the hydrograph for all subbasins. The unit hydrograph calculation was based on the recommendations for a local storm, consistent with the use of the Opal precipitation event. The uniform infiltration method was used in the HEC -1 model to estimate infiltration losses during the PMF. Infiltration losses were estimated by calculating the weighted average of the soils identified in the SCS Soil Survey of Grand County Area Colorado in each 7