FLOOD07152

• County: Delta
• Community: Orchard City
• Stream Name: Sruface Creek, Cedar Run, Tongue Creek
• Basin: Gunnison
• Title: Orchard City, Colorado 100-Year Floodplain Delineation
• Date: 2/20/2003
• Prepared For: CWCB
• Prepared By: Gared Grube, Thomas Gilman
• Floodplain - Doc Type: Floodplain Report/Masterplan

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5.0 Results

5.1 Hydrologic Analysis

For Tongue Creek, the CWCB Regional Regression equation for the Lower Gunnison River
Subregion gave a resulting 100-year peak discharge of3832 cfs. The USGS Regional Regression
equation for the Northwest region of Colorado gave 2829 cfs for the 100-year peak discharge.
HEC-FFA indicated that the 100-year peak flow would be 2190 cfs (the program output is
attached in Appendix E) and the Weibull Method resulted in a peak discharge of 2544 cfs. The
2544 cfs peak discharge found using the Wiebull method was determined to be the best
candidate for the floodplain study since it resulted in the highest flow value for detailed analysis.

For Cedar Run, approximate 100-year flows were found using the same CWCB and USGS
regional regression equations as for Tongue Creek. However, since the Cedar Run drainage basin
area increases from 5 square miles to over 6 square miles, multiple flow locations were found.
The drainage area for each cross section was found using the HEC-GeoHMS ArcView extension
identify contributing area tool. The HEC-HMS configuration described above resulted in runoff
from this basin of 3 .11 ems, or 110 cfs. The output for HEC-HMS can be found on the data CD.
The result from HEC-HMS was increased by the same amount that the USGS regional
regressions equation indicated would be increased for each cross section. This method for
determining the flow at multiple flow locations was necessary since the HEC-GeoHMS
extension would not successfully generate an HMS project for any point below the upstream
study limit.

The table below summarizes the results of the calculations for the regional regression analysis
for Tongue Creek and the multiple flow locations for Cedar Run. The cross-section (XS)
identifier that is given is the one that was selected in the field data collection. The order they are
presented in the table is from upstream to downstream. Those that are lumped together are the
upstream and downstream cross-sections of a bridge, culvert, or dam. The Hydrologic
Engineering Center's Hydrologic Modeling System results for Cedar Run and the data from the
1981 study are also presented.

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