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' Table III-2 <br /> REVIEW OF CORPS' SENSITIVITY STUDY OF PARAMETERS <br /> CHANGE RESULT IN FLOW AT BOULDER CREEK AT MOUTH <br /> ' At West Edge of Boulder Above South Boulder Creek <br /> Base Case 10,365 cfs 13,200 cfs <br /> ' Increase 6 hour rainfall from 12,031 (+16%) 14,855 (+13%) <br /> 2.72 to 2.84 inches <br /> Decrease 6 hour rainfall from 4,224 (-59%) 7,592 (-42%) <br /> ' 2.72 to 2.15 inches <br /> Increase channel roughness 9,026 (-13%) 11,357 (-14%) <br /> 25% (appears this was used: <br /> base channel 0.035 to 0.05, <br /> overbank 0.045 to 0.06) <br /> ' Increase channel roughness 7,193 (-31 %) 8,995 (-32%) <br /> 75% <br /> ' Pervious depression storage 14,538 (+40%) 16,682 (+26.3%) <br /> from Base 0.184 to 0.05 <br /> (0.05 was used) <br /> ' Pervious overland roughness 8,835 (-15%) 11,670 (-12%) <br /> (from n = .12ton = .2) <br /> ' Very limited qualitative conclusions can be drawn by Corps' sensitivity tests <br /> regarding potential problems identified in our Phase I and II effort. For example, we <br /> are interested in the effects of changing the 1 hour rainfall. At best, the 6 hour <br /> rainfall sensitivity test indicates peaks may increase due to a rise in the peak 1 hour <br /> incremental rainfall. Channel roughness sensitivity tests qualitatively indicate that <br /> ' flows will decrease with effective channel routing and roughness parameters <br /> commonly associated with mountainous streams. Variation in depression storage <br /> probably changed model results by 26 to 40%, which indicates major creek flow is <br /> ' sensitive to effective volume of runoff and thus sensitive to infiltration and <br /> depression losses. Results appear sensitive to overland runoff, but this may have <br /> resulted from selection of "W" parameters that equate to unrealistically long <br /> ' overland flow parameters. <br /> We believe that the Corps' SWMM model has numerous parameters which do not <br /> depict physical features accurately. Some of these and their effects include <br /> parameters tabulated on Table III-3. Adjustment of many of these parameters may <br /> have offsetting effects. The missing basin areas and rainfall will likely lead to <br /> ' increased flow predictions. <br /> III - 7 <br /> 1 <br />