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(iv) USBR Method (USBR, 1987) Based on observed flood hydro graphs for a number of watersheds, the US Bureau of Reclamation developed the following equation for lag time: tL (hrs) = 26 Kn x ( L . Lea / ."j S)0.33 in which Kn = Manning's roughness coefficient; L = sub-watershed length (miles); Lea = length from the centroid to the outlet of the sub-watershed (miles); and s = average slope of the sub- watershed (ft/mi) , This equation is similar to the Snyder's equation except that it includes watershed slope as an independent variable in addition to L and Lea and predicts an average value of lag time for the sub-watershed: For watersheds in the Great Plains west of the Mississippi River and east of the Rocky Mountains, the values of Kn were found to vary from 0.03 to 0.070. The higher value, 0.070, reflects basins with considerable overland flow before reaching moderately well-defined water courses. Using a value of 0,069 for the relatively flat sub-watersheds in Frenchman Creek basin with considerable lengths of overland flow, the following equation is adopted, tL(hrS) = 1.8 x (L, Lea / ."j S)0.33 (v) Stream Hydraulics Methbd (USDA, 1972) This involves estimation of travel time from the divide to the outlet of the sub-watershed using average velocities for overland and channel flows and estimating the lag time as 0.60 times the total travel time, Weighted average velocities of 1 and 2 ft/sec are adopted for the sub- watersheds of Frenchman Creek. The estimated lag times for each sub-watershed are shown in Table 4, A review of the estimated lag times included in Table 4 suggests that the estimates obtained by the Kirpich method may be too low and those obtained by the SCS method and Stream Hydraulics method with V = 1 ft/sec may be too large. In fact, the estimates given in the last two columns of Table 4 suggest that a storm duration of 48 hours or more may be required to ensure surface runoff contribution from the entire drainage area at the basin outlet. The estimates based on the Stream Hydraulics method with V = 1 ft/sec may be used for a 48-hour duration storm. Estimates of lag times given by the Kerby and USBR methods are not significantly different from one another and suggest that a storm duration of 24 hours may be adequate. The estimates based on the Stream Hydraulics method with V = 2 ft/sec may be used as the upper range of lag times for which a 24-hour duration storm may be adequate, For this case, computations have also been made using a storm duration of 48 hours, Draft Letter Report Independent Review of Hydrologic Analysis for Frenchman Creek Contract No. DACW41-00-D-0026-0001 Flood Plain Management Services Special Study Holyoke. Colorado June 18,2001 Page 6