FLOOD10330

• County: Mesa
• Stream Name: Colorado River, Big Salt Wash, Little Salt Wash
• Basin: Colorado Mainstem
• Title: Hydrology Report - Flood Insurance Studies in Mesa County, Colorado
• Date: 5/1/1989
• Prepared For: FEMA
• Prepared By: J. F. Sato & Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Having estimated the magnitude of flood peaks on the Gunnison
River, a comparison of the flood hydrographs for the Colorado and
Gunnison Rivers was made to determine when the peaks occurred.

Figures 4, 5, and 6 show hydrographs during the high .flow years
of 1957, 1962, and 1984. It can be assumed that flood peaks on
the Colorado and Gunnison Rivers occur at the same time since
these hydrographs are nearly parallel.

Thus, for study reaches downstream of the mouth of the Gunnison,
the peak discharges on the Colorado are added to the flood peaks
computed for the Gunnison.

Thus, the proposed de sign discharges on the Colorado River study
reach are as follows:
10-yr 50-yr 100-yr 500-yr
study area above mouth 32,900 44,400 49,300 61,000
of Gunnison
study area below mouth 50,600 73,100 83,700 111,400
of Gunnison

The previous FISs used the peak discharges listed in the
following table.

Previous FISs 10-Yr 50-yr 100-yr 500-yr
study area above mouth 40,000 56,400 63,000 82,000
of Gunnison
Study area below mouth 55,000 73,000 82,000 107,000
of Gunnison

B. Big Salt Wash and Little Salt Wash

The Flood Hazard Information Report for Fruita, 1976 (3),
discusses the hydrologic studies conducted by the Sacramento
District of the Corps of Engineers on Big Salt Wash and Little
Sal t Wash. Because streamflow gauging information was not
available for these two streams, the Corps derived unit
hydrographs by the S-graph method, based on an S-graph developed
for Rifle Creek at Rifle. A standard project storm was developed
for the washes from a compilation of precipitation records.
Using frequency curves developed for other regional drainage
basins similar in size to the study area drainages, ratios of
100-year and 500-year floods to the standard project flood were
determined. These peak discharges developed by the Corps will be
used in the on-going studies also. The 1976 hydrologic studies
also developed information on 10-year and 50-year flood events
but the design discharges are not listed in the report.

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