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MODELING <br /> The Corps' model was adapted to refine hydrology modeling of the West Valley of <br /> South Boulder Creek. Apparently, the purpose of the Corps' model was to attain <br /> general flood discharge quantities along the main streams. The purpose of the <br /> ' refinements herein was to begin the process of estimating specific flow paths and <br /> discharge amounts in the West Valley area resulting from South Boulder Creek spills <br /> and local drainage. <br /> The initial model simulates local subbasin runoff and key drainage conveyance <br /> ' components within the West Valley from the Viele Channel to the Wellman Ditch. <br /> The West Valley is part of the alluvial fan of South Boulder Creek. As with most <br /> alluvial fans, flood waters spread into many flow paths across the entire valley. <br /> The works of man have caused this water to follow set paths controlled by streets, <br /> culverts, and grading. A complicated drainage network then exists which <br /> effectively splits the flood along many routes, which often recombine and then <br /> resplit. This report specifically addresses many of the key flow splits that occur in <br /> the West Valley from Highway 36 to the area of the Wellman Ditch. <br /> The modeling and hydrology effort revealed some shortcomings of the Corps' <br /> hydrology model. Review of the Corps' model and comparison with physical data <br /> and parameters established by other Federal agencies and local criteria suggests <br /> ' significant discrepancies and a likely under prediction of South Boulder Creek flood <br /> flows by the Corps' model. The rainfall data does not conform to either the <br /> National Oceanic and Atmospheric Administration (NOAA) Atlas procedures or <br /> current local standards. We estimate that the Corps' rainfall is from 0.67 to 0.72 <br /> inches short for the peak one hour duration. In addition, the Corps' peak rainfall <br /> intensity was 2.7 inches/hour for 30 minutes in comparison to 8.0 inches/hour for <br /> five minutes used in local criteria. At least two square miles of the tributary area in <br /> the South Boulder Creek watershed is missing from the Corps' model. Correcting <br /> these issues would cause the projected runoff to increase. On the other hand, the <br /> Corps does not adequately model stream routing. Correcting this issue may <br /> somewhat mitigate the aforementioned increase. Resolutions of other issues such <br /> as infiltration, depression storage, and overland runoff characteristic could also <br /> affect the hydrology, probably to a lesser degree. An increase in peak flow can <br /> likely be expected as part of future efforts. <br /> FINDINGS <br /> The following findings can be more easily understood by examining Drawing No. 3 <br /> in the back of this report. (Drawings 1 and 2 are with the Phase I report.) <br /> Understanding the many flow splits is necessary to understanding the hydrology. <br /> Some of the more significant flow splits during the 100 year flood include the <br /> following: <br /> ' 1 - 3 <br />