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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5.8 REACH 9 - GUNNISON RIVER FROM OHIO CREEK TO TOMICHI CREEK

5.8.1 Description of Reach 9

Reach 9 encompasses the Gunnison River between Ohio Creek and the confluence with T omichi Creek.
Originally, this reach was divided into two reaches: Reach 6b - Gunnison River from Ohio Creek to the
north side of the City of Gunnison and Reach 9 - Gunnison River from the north side of the City of
Gunnison to Tomichi Creek. This separation in reaches was based on city-county boundaries, but is not
required for the hydrologic analysis. Therefore, these two reaches have been combined into one reach,
Reach 9 - Gunnison River from Ohio Creek to Tomichi Creek.

The approximate combined reach length is 4 miles with a total drainage area of 1,012 square miles at the
gage located near the middle of the reach. The reach generally has steep banks, and a steep bluff is
located just past the right bank (looking downstream). The portion of the reach located within the city
limits was studied for the City of Gunnison in 1977 (ECI) and 1982 (FEMA). There have been some
above-average runoff events since that study, which affect the hydrology for this reach, warranting a new
study. In addition, after the previous study, the city built a levee on the left side of the river (looking
downstream) that directs flood water back to the river.

5.8.2 Flood Records for Reach 9

The USGS gage Gunnison River near Gunnison is located in this reach. This gage provides a record of
reported peak flows for 1912 through 1914, 1916 through 1928, and 1945 through 2000. However, since
this gage is affected by the operations of Taylor Park Reservoir, the regulated data is transformed to
unregulated flows by accounting for the daily change in storage (see Section 4.3). Unfortunately, daily
storage information is not available for the Taylor Park Reservoir (construction began in 1935) until 1958.
Therefore, there is a gap in unregulated flows from 1935 to 1958, as unregulated flow data could not be
simulated.

The travel time from Taylor Park Reservoir to the Gunnison River near Gunnison gage is estimated to be
less than 12 hours; therefore, no lag is applied in the conversion to an unregulated data set.

An estimate of the instantaneous peak flows to corresponding daily values is determined for this gage
using values prior to 1935. The ratio of 1.0687 peak-to-daily flows is determined by plotting a straight
trend line for the data. The R2 value for the line is 0.9313 as shown below in Figure 5.8.2.1. This value is
significantly less than the peak-to-daily ratio that is applied for the Taylor River at Almont peak flow
record.

This ratio is applied to the maximum daily value for each year of the unregulated data set to estimate
peak values. The peak flows for the USGS gage Gunnison River near Gunnison are presented in Table
A.8.

The REGFREQ program was used to determine the correlation of the available flow data to nearby gages
on the Gunnison River system. A high degree of correlation (>0.9) between the deregulated Gunnison
River near Gunnison gage and the following gages was found.

· Taylor River at Almont (09110000)
· Taylor River below Taylor Park Reservoir (09109000)
· East River at Almont (09112500)

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