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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Figure 4.2.1

Comparison of Regulated vs. Unregulated Instantaneous Peaks at Taylor River
below Reservoir

3000

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.

2500

o 2000
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af
~ 1500
.t:
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o
is
1000

500

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~~~~~~~~~~~~~~~~~~~~~
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Year

In addition to the Taylor River reach immediately below the reservoir, the Taylor River at A1mont and the
Gunnison River at Gunnison gages are affected by the reservoir, though the level of influence decreases
as additional unregulated drainage area contributes flow.

The regulated flow data sets at both of these stream gages are adjusted to simulate unregulated
conditions. The adjustments are made by first adding or subtracting the change in Taylor Reservoir
storage values (converted to daily flow) to the regulated flow data sets. The second step is to identify the
annual maximum daily flow. Lastly, the peak to daily ratio is applied to estimate the instantaneous peak
flow. For the Taylor River at A1mont, the peak to daily maximum ratio was determined for water years
1913, 1916, and 1920-1935 to be 1.1164. This ratio is applied directly to both the regulated and change in
storage portions of the flow to create the unregulated peak flow record for that location.

At the Gunnison River near Gunnison, the data are treated differently. First, the daily regulated flow is
added to the daily change in storage for all values from 1959 through 2000. Next, the annual maximum
daily value is identified based on the sum of the values. The instantaneous peak is estimated by applying
two different factors: one for the change in storage flow and one for the regulated portion of flow. The
change in storage is multiplied by the ratio (1.2481) determined for the Taylor River at Taylor Park gage.
The regulated flows are multiplied by the peak to daily ratio of 1.0687 derived for this gage from water
years prior to the reservoir construction.

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