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The area and rainfall discrepancies appear to warrant a reevaluation of South <br /> Boulder Creek hydrology and perhaps other watershed hydrology by the Corps that <br /> is still being used, such as Boulder Creek . <br /> CORPS' MODELING CALIBRATION AND SENSITIVITY TESTS <br /> From review of the information provided, it is apparent that the Corps' work was <br /> strongly oriented toward calibration to a Boulder Creek gaging station two miles <br /> ' upstream from its confluence with the St. Vrain. The tributary area to the <br /> confluence was reported as 440 square miles. The gaging record encompassed 28 <br /> ' years, from 1927 to 1949 and from 1951 through 1955. The Corps conducted a <br /> statistical analysis of this data using procedures outlined Bulletin No. 17 of the <br /> Federal Water Resources Council and corrected for expected probability. The <br /> ' watershed SWMM model and lower basin stream routing modeled was apparently <br /> "calibrated" to result in a discharge frequency curve similar to the statistical data, as <br /> evidenced by a comparative plot of the model results. <br /> Some of the key parameters settled upon by the Corps in comparison with values <br /> typically used in UDFCD and Boulder manuals are given in Table III-1 . As stated <br /> before, the rainfall used by the Corps is low for the 1 hour and shorter incremental <br /> peak rainfall, leading to an under prediction of peak flows. <br /> The use of steady state infiltrations of 0.5 inches per hour in urban areas and 1 .0 <br /> inches per hour in the mountains, the significant increase in urban percent <br /> impervious that has occurred, and the rather unusual parameters used for <br /> depression storage, may result in somewhat compensating effects. <br /> 1 <br /> ' III - 5 <br />