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<br />Routt County Commissioners <br />June 4, 2001 <br />Page 4 of 8 <br />gaging data for the Yampa River(Gage 09244410), Trout Creek (Gage 09244100) and the Elk River <br />(Gage 09242500). <br />Another questionable error in the hydrological assumptions appears in the gage flow values used by <br />the Applicant as input data to develop the 100-year flood peak using the Log Pearson III distribution. <br />On Table 1 of Appendix A in the Applicant's report, the gage flow values shown for the Yampa River <br />(Gage 09244410) aze 93 to 95 percent of the recorded USGS gage values. And for Fish Creek, Table <br />l of Appendix B in the Applicant's report, the gage flow values shown for the Fish Creek (Gage <br />0924410) are 53 to 68 percent of the recorded USGS gage values. Although the Apphcant's engineer <br />may be trying to adjust for downstream contributions in the gage, this is never stated in regazds to <br />the gage data, and it would be entirely incorrect to reduce the Fish Creek gage by any amount. <br />Neither Fish Creek nor Trout Creek flows should ever be reduced when evaluating the effects of <br />flooding on the Applicant's property, as the subject property is located at furthest downstream end <br />of Trout Creek. <br />Furthermore it was incorrectly assumed in the Applicant's report that the flood peak on Trout Creek <br />needs to be removed from the Yampa River gage or peak flood flow. The proper methodology for <br />modeling the confluence of two streams in a subcritical flow regime (modeling stream junctions, <br />HEC-RA5 Manual) is to begin downstream of the confluence (junction) where the flows aze <br />combined then step upstream of the confluence on the mainstream adjusting the peak flow values <br />accordinely. <br />FEMA's published study of the Routt County Floodplain (1989), ended approximately 2 miles <br />upstream of the project site at the confluence of the Yampa and Elk Rivers. Albeit the FEMA study <br />did not include the combined flow, the study participants were apparently not interested in analyzing <br />the effects that a backwater curve would have immediately upstream of the confluence. Unlike the <br />FEMA study, the focal point of interest in the Applicant's application is located at the confluence and <br />immediately upstream of it. Since the Apphcant's property is contiguous and surrounded on both <br />sides by the Yampa River and Trout Creek, performing a combined flow analysis may show that tfie <br />Applicant has considerably more property within the 100-year floodplain than stated in the <br />Applicant's Flood Delineation Report. <br />Hydraulics <br />1. Yampa River <br />Review of the Applicant's HEC-RAS water surface x-section results, raises a few concerns. Already <br />discussed is the survey data of the river sections. Review of the Applicant's model results shows the <br />