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<br />consideration channel storage along the study reach. As a result, <br />some peak discharge values for portions of the river show an <br />increasing velocity with a decreasing drainage area. <br /> <br />Peak discharges for the 10- and 100-year floods on Sheep Draw were <br />computed using the SCS Technical Release No. 20 computer program <br />(Reference 6). Basin characteristics needed for the program were <br />taken from the USGS topographic maps, at a scale of 1:24,000, with <br />a contour interval of 10 feet (Reference 7). Precipitation <br />distributions were developed from information provided in the <br />National Oceanic and Atmospheric Administration Atlas <br />(Reference 8). The SO- and SOO-year discharges were obtained by <br />straight line interpolation and extrapolation of the 10- and 100- <br />year values plotted on log-probability paper. <br /> <br />Discharge hydrographs for the 10- and 100-year floods on Ashcroft <br />and Eaton Draws were developed using methods suggested in Chapter <br />16 of the National Engineering Handbook, published by the SCS <br />(Reference 9). Drainage areas and stream lengths necessary to <br />construct the hydrographs were taken from USGS topographic maps at <br />a scale of 1:24,000, with a contour interval of 10 feet (References <br />10 and 11). Precipitation distributions were developed from <br />information provided in the National Oceanic and Atmospheric <br />Administration Atlas (Reference 8). The hydrographs for Ashcroft <br />Draw were routed through Arrowhead Reservoir using the Storage- <br />Indication Method outline in Chapter 17 of the National Engineering <br />Handbook (Reference 9). The SO- and SOD-year discharges were <br />obtained by straight line interpolation and extrapolation of the <br />10- and 100-year values plotted on log-probability paper. <br /> <br />.' <br /> <br />Discharges for the Tri-Area Drainageway were computed using the SCS <br />Technical Release No. 20 (Reference 6). Rainfall data used in the <br />hydrologic analyses were obtained from the National Oceanic and <br />Atmospheric Administration Atlas for Colorado (Reference 8), and <br />soil and land use information were taken from maps prepared by the <br />SCS (Reference 3). <br /> <br />Discharge magnitudes for floods on the Big Thompson River were <br />based upon an analysis of stream gaging data at the USGS stream <br />gages located near Drake and La Salle. Because there are no <br />streamflow gaging records between the La Salle gage and the <br />upstream limit of study, discharge-probability relationships were <br />developed for the intervening drainage area using unit hydrographs <br />and flood routing techniques and rainfall probabilities from U.S. <br />Weather Bureau Technical Publication No. 40. <br /> <br />" <br /> <br />.' <br /> <br />The peak discharges for the 10-, SO-, 100-, and SOO-year floods for <br />Coal Creek were developed by the SCS in a report entitled Flood <br />Hazard Analyses. Coal Creek and Rock Creek. Boulder and Weld <br />Counties. Colorado (Reference 12). The SCS used synthetic <br />rainfall-runoff procedures, as described in the SCS's National <br />Engineering Handbook (Reference 13), and the TR-20 computer program <br />for flood routing (Reference 14) in order to establish the selected <br />discharges along the stream. <br /> <br />10 <br />