FLOOD06449

• County: Delta
• County: Montrose
• Community: Collbran, Cedaredge, Paonia
• Stream Name: Buzzard, Plateau, Grove Creeks
• Basin: Gunnison
• Title: Hydrology Report Unincorporated Areas of Delta and Montrose Counties
• Date: 4/1/1979
• Prepared For: FIA
• Prepared By: Meuer Serafini & Meuer, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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HYDROLOGY:
Montrose, Delta and Mesa Counties, Colorado

discharge index points on the study length were used. If not, the
average of the two endpoint flows was used throughout the study
length. Table 6 summarizes the 10-, 50-, 100- and 500-year flows
used.

Special consideration was given to Plateau Creek at Collbran
because of the presence of Vega Dam about 10 miles east of Collbran.
The dam has been regulating flow since 1958 when construction began
on it. Because of the lack of hydrologic data about the watershed
and hydraulic data about the reservoir and dam, a routing procedure
was not practical. Instead the gage station data before and after
the building of the dam was analysed to estimate the effects of its
regulation on the discharges at Col1bran.

The U.S.G.S. stream gage No. 09096500 was used in the analysis. The
gage is located approximately 6 miles east of Collbran and 2 miles
downstream of the Dam. The drainage area at the gage is 80.4 sq.
miles and the drainage area behind the dam is 24.4 sq. miles. The
gage data was divided into two groups. The first group includes
peak flows up to water year 1958 (unregulated flow) and the second
includes flows after 1958 (regulated).

The combined flood flow frequency curves for snowmelt and rainfall
were computed for both the unregulated and regulated conditions
(See Figure 45).

To find the discharges at Collbran (drainage area 113 sq. miles)
the regional flood-frequency study was used to compute the
unregulated discharges produced by the drainage basin between the
gage and Co11bran, and these flows were added to the regulated
discharges at the gage. To account for the time difference between
the peaks, the unregulated discharges produced by the 33 square
mile basin below the gage were not taken directly off the regional
frequency curves for a drainage area of 33 sq. miles. Instead the
differences in discharges computed for the entire basin (113 square
miles) and the gage (80.4 square miles) were used. Figure 47
summarizes the calculations.

For Plateau Creek below the confluence with Buzzard Creek no flood
reduction due to Vega Dam regulation was considered because most of
that total watershed is unregulated (270 out of 295 sq. mile).

For comparison purposes flow discharges for key points were
computed using two published regional methods. Colorado Water
Conservation Board's Technical Manual #1 presents regression
equations for natural streams in four general areas of Colorado. A
summary of the method and calculations used for this report appears
in Figures 42-44. A second method from U.S.G.S. Water Supply Paper
1683 Magnitude and Frequency of Floods in the United States was

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