FLOOD08334

• County: Mesa
• Community: Grand Junction
• Stream Name: Colorado River
• Basin: Colorado Mainstem
• Title: Colorado River Flood Analysis
• Date: 1/1/1984
• Prepared For: Mesa County
• Prepared By: Greenhorne & O'Mara, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

An estimate of the flood frequency for the 1983 flood on the Colorado River at
Grand Junction, Colorado 'Nas made by the Colorado Water Conservation Board using
gaging information obtained through the Grand Junction office of the U.S.
Geological Survey. Table 2 shows the estimated peak discharge and frequency for
the 1983 flood in Grand ~Iunction. For dischar~le comparison purposes, Table 2
also shows the correspondin9 100-year discharge at each location.

TABLE 2

PEAK DISCHARGE AND FLOOD FREQUENCY *

Location

Peak
Discharge (cfs)

Flood
Freque'l!.9!.

100-Year
Discharge (cfs)

Colorado River downstream
of Gunnison River.

62,200*

20- Y,~ar*

82,000

Colorado River upstream
of Gunnison River.

41,000*

ll-Year*

63,000

Gunnison River at
conf 1 uence with
Colorado River.

21,200*

150-Year*

20,000

* These values are estimates made by the Colorado Irlatelr Conservation Board.
Final values will be published by the USGS.

HYDRAULIC ANALYSIS

A hydraulic analysis was completed along the five and one-half mile study reach
to determine water surface profiles for the 2-year, 5-year, 10-year, 50-year,
100-year and 500-year flo,Jd events for exist'ing stream conditions after the 1983
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flood. The water surfac~ elevations were comp leted using the HEC-II computer-'
model for backwater anal:rsis (Reference 4).

Stream cross section information was generally taken from the U.S. Army Corps of
Engineers 1976 Report comleted on the Colorado River through Grand ~Iunction.
(Refl~rence 3). This information is the same used for the 1978 Flood Insurance
Study for Mesa County and was supp lemented as follows (Reference 1). Stream
cross-sections and other hydraulic information near the new Redlands Bridge was
taken from the study completed for Armstrong Engineers, which was used for their
design of the bridge in Januar/otT98T.~'TR;:f~~~~~~e 2). Finally, surveyed

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