FLOOD00880

• County: Mesa
• Community: Rifle
• Stream Name: Colorado River
• Title: Floodplain Information Report - Colorado River Mainstem From Rifle to Utah-Colorado State line
• Date: 4/1/1996
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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3.0 HYDROLOGIC ANALYSIS

3.1 Existing Data

For locations where existing detailed hydrologic analyses were available, those analyses were used
to obtain peak 50-year and 100-year flows. For other locations, statistical analyses of stream
gages and existing regional hydrologic analyses were examined. As is explained in the next
paragraph, those analyses were found to be inconsistent with the existing adopted analyses and
were not used for this report.

A draft floodplain study being jointly prepared by the u.s. Army Corps of Engineers and the
CWCB and published FEMA studies provided 50-year and 100-year flows for much of the study
reach. Corps of Engineers flows were available for the Colorado River from Rifle, Colorado, at
the upstream limit of this study, downstream to DeBeque Canyon. FEMA flows were available
for the Colorado River from Palisade, Colorado downstream to Fruita. The USGS performed
frequency analyses on gage locations near DeBeque, near Cameo and near the stateline. The
USGS gage analysis val~es were lower than the Corps values by 7 to 10 percent. In order to
maintain consistency, the Corps values were selected for this study.

3.2 New Hydrologic Information

The remaining hydrologic points can be divided into two groups: Group 1) Three hydrologic
points were located within reaches that had already been studied by detailed methods, but there
weren't any published flow values for those specific points; Group 2) Two points were in
reaches downstream of the Sato Hydrology Report, so they had not previously been analyzed.
Hydrologic analyses of the 50-year and 100-year flood flows for the hydrologic points in Group
1 were performed through interpolation of known flow values. Since detailed information already
existed, the flows were not used to compute flood depths and no changes were made to the
detailed floodplain maps. (The flow values are provided in this report simply for information
purposes. )

The known values for points near the Group 1 points were entered into a curve fitting computer
program which selects an equation that provides a close description of the plotted curve. Flow
values for the three Group 1 locations on the Colorado River (river miles 80-75, 42-45 and 25-
28) were calculated. A similar procedure involving extrapolation instead of interpolation was
used for the hydrologic points in Group 2 (river miles 0-10 and 10-12). The curve fitting
program computed the flows at these locations by using the corresponding drainage areas at each
location. Table 3 summarizes the data at the selected hydrologic locations. The equation for the
Colorado River below Rifle is:

where:

Q=k* A C
Q = 50-year or 100-year flow;
A = drainage area;
k = a constant; and
c = an exponent

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