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I I I I I I I I I I I I I I I I I I I I MODELING The Corps' model was adapted to refine hydrology modeling of the West Valley of South Boulder Creek. Apparently, the purpose of the Corps' model was to attain general flood discharge quantities along the main streams. The purpose of the refinements herein was to begin the process of estimating specific flow paths and discharge amounts in the West Valley area resulting from South Boulder Creek spills and local drainage. The initial model simulates local subbasin runoff and key drainage conveyance components within the West Valley from the Viele Channel to the Wellman Ditch, The West Valley is part of the alluvial fan of South Boulder Creek. As with most alluvial fans, flood waters spread into many flow paths across the entire valley. The works of man have caused this water to follow set paths controlled by streets, culverts, and grading, A complicated drainage network then exists which effectively splits the flood along many routes, which often recombine and then resplit, This report specifically addresses many of the key flow splits that occur in the West Valley from Highway 36 to the area of the Wellman Ditch. The modeling and hydrology effort revealed some shortcomings of the Corps' hydrology model. Review of the Corps' model and comparison with physical data and parameters established by other Federal agencies and local criteria suggests significant discrepancies and a likely under prediction of South Boulder Creek flood flows by the Corps' model. The rainfall data does not conform to either the National Oceanic and Atmospheric Administration (NOAA) Atlas procedures or current local standards, We estimate that the Corps' rainfall is from 0.67 to 0.72 inches short for the peak one hour duration, In addition, the Corps' peak rainfall intensity was 2,7 inches/hour for 30 minutes in comparison to 8.0 inches/hour for five minutes used in local criteria, At least two square miles of the tributary area in the South Boulder Creek watershed is missing from the Corps' model. Correcting these issues would cause the projected runoff to increase. On the other hand, the Corps does not adequately model stream routing, Correcting this issue may somewhat mitigate the aforementioned increase, Resolutions of other issues such as infiltration, depression storage, and overland runoff characteristic could also affect the hydrology, probably to a lesser degree. An increase in peak flow can likely be expected as part of future efforts. FINDINGS The following findings can be more easily understood by examining Drawing No.3 in the back of this report. (Drawings 1 and 2 are with the Phase I report.) Understanding the many flow splits is necessary to understanding the hydrology. Some of the more significant flow splits during the 100 year flood include the following: 1- 3