FLOOD09681

• County: Mesa
• Community: Grand Junction
• Basin: Colorado Mainstem
• Title: FIS - Grand Junction
• Date: 7/15/1992
• Prepared For: Grand Junction
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

watershed to determine the flood discharges. This was accomplished
by using the COE HEC-l rainfall-runoff computer program (Reference
13). For this analysis, basin characteristics which define the
size, shape, and runoff characteristics of the watershed, as well
as rainfall amounts based on the selected recurrence intervals, are
used to compute flood hydrographs for various design points in the
basin.

The flows for Indian Wash were computed in Hvdrologic Analysis.
Indian Wash. Grand Junction. Colorado, prepared by MSM Consultants,
Inc., Denver, Colorado (Reference 14).

Peak. discharge-frequency relationships for Leach Creek.
obtained from the Mesa County, Colorado Flood Insurance
(Reference 14).

were
Study

The City of Grand Junction conducted a hydrologic study of Horizon
Drive Channel in 1988 (Reference 15) in order to determine the
effects on the lOa-year flood peak of a newly constructed detention
pond upstream. The city used the COE HEC-l rainfall-runoff
computer program (Reference 13) to compute the 100-year discharge.
The same model was used in this Flood Insurance Study with
appropriate preci pi tat ion values for the 10-, 50-, and 500-year
storms.

The HEC-l computer program utilizes as input the following data:
rainfall, loss rates, unit hydrographs, base flow, and routing
criteria. Rainfall computations for the 10- and 50-year storms
were based on precipitation-frequency data and areal reduction
factors derived from the National Oceanic and Atmospheric
Administration Atlas. To obtain the precipitation value for the
SOO-year flood, a frequency-precipitation curve for the stream was
plotted using the 10-, 50-, and laO-year value, and the curve was
extrapolated to a 500-year value. Unit hydrographs were derived by
the Snyder Method and loss rates were established by hydrologists
familiar with the area. A base flow of 60 cfs was used.

Peak discharge-drainage area relationships for the Colorado River,
Indian Wash, Leach Creek., and Horizon Drive Channel are shown in
Table 2.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding
sources studied were carried out to provide estimates
elevations of floods of the selected recurrence intervals.

from
of

the
the

Locations of selected cross sections used in the hydraulic analyses
are shown on the Flood Profiles (Exhibit 1). For stream segments
for which a floodway was computed (Section 4.2), selected cross
section locations are also shawn on the Flood Insurance Rate Map
(Exhibit 2).

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