Laserfiche WebLink
<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />Table 111-2 <br /> <br />REVIEW OF CORPS' SENSITIVITY STUDY OF PARAMETERS <br />CHANGE RESULT IN FLOW AT BOULDER CREEK AT MOUTH <br /> <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,B55 (+13%) <br />2.72 to 2.B4 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%) B,995 (-32%) <br />75% <br />Pervious depression storage 14,53B (+40%) 16,6B2 (+ 26.3%) <br />from Base 0.1 B4 to 0.05 <br />(0.05 was used) <br />Pervious overland roughness B,B35 (-15%) 11,670 (-12%) <br />(from n = .12 to n = .2) <br /> <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 /> <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 111-3. 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 /> <br />III - 7 <br />