FLOOD10363 ‎(2)‎

• County: Gunnison
• Community: Gunnison County
• Stream Name: East River, Cement Creek, Alkali Creek, Ohio Creek
• Title: Gunnison County Flood Hydrology, Contract # W99-D-0018 Final Report
• Date: 11/1/2004
• Prepared For: US Army Corps of Engineers, CWCB and Gunnison County
• Prepared By: Tetra Tech, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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5.7 REACH 8 - TOMICHI CREEK FROM THE GUNNISON RIVER TO THE GUNNISON AIRPORT

5.7.1 Description of Reach 8

. Reach 8 extends along T omichi Creek from the confluence with the Gunnison River approximately 2
miles upstream, past the Gunnison Airport. The contributing drainage area is 1,061 square miles at the
downstream end of the reach and the entire reach length is unregulated. The reach is located just south
of the City of Gunnison and the Gunnison Airport. The watershed is primarily located in a high mountain
valley environment with elevations lower than the other reaches in this study. While annual peak
discharge values are primarily due to snowmelt runoff, the amount of snow pack in this watershed is
significantly less than the typical snow pack of the watersheds of the other reaches involved in this study.

This reach is located well below the significant snowmelt runoff sources; however, peak flows are still
primarily due to snowmelt runoff. The USGS gage Tomichi Creek at Gunnison (ID# 09119000) is located
along this reach and has a complete record of peak flows since 1938.

5.7.2 Flood Records for Reach 8

The USGS gage Tomichi Creek at Gunnison includes peak flow data from 1938 through 2000. The
T omichi Creek at Gunnison gaged flows are representative of the flows along the entire Reach 8. The
reported family of flows for the USGS gage Tomichi Creek at Gunnison is presented in Appendix A-
Table A.7.

5.7.3 Results for Reach 8

Before any analysis was completed, the data record for the Tomichi Creek gage was filtered for rain on
snow events, using the 0.5 inch criteria discussed in Section 4.1.1. Eight rain on snow events were found
in the record of the Tomichi Creek gage.

The REGFREQ and HEC-FFA programs were used to perform the analysis on the flow record in
Appendix A - Table A.7. Prior to using the REGFREQ program, HEC-FFA was used on Tomichi Creek to
test for low and high outliers based on a methodology described in Bulletin 17B. Any outliers were
manually removed from the REGFREQ data set. The 1984 was determined to be a high outlier. The
results from the analysis are presented in Figure 5.7.3,1. Additionally, the adopted statistics and the 10, 2,
1, and 0.2 percent chance exceedence flows are presented in Table 5.7.3.1.

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