FLOOD01227

• Designation Number: 208
• County: Mesa
• Community: Grand Junction
• Title: FIS - Grand Junction, Mesa County, Colorado
• Date: 7/1/1982
• Designation Date: 1/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

The flows for Indian Wash were computed in Hydrologic Analysis,
Indian Wash, Grand Junction, Colorado, prepared by MSM Consultants,
Inc., Denver, Colorado (Reference 8).

Peak discharge-frequency relationships for Leach Creek and Horizon
Drive Channel were obtained from the Mesa County, Colorado, Flood
Insurance Study (Reference 1).

Peak discharge-drainage area relationships for 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 the flooding sources
studied in the community were carried out to provide estimates
of the elevations of floods of the selected recurrence intervals
along each of these flooding sources.

For Colorado River, water-surface elevations of the 100-year flood
were computed by the U.S. Army Corps of Engineers for the Mesa
County, Colorado, Flood Insurance Study in November 1976 (Refer-
ence 2) and revised in May 1980. The water-surface elevations
for the 10-, 50-, and 500-year floods were computed in May 1980
using the revised HEC-2 hydraulic model (Reference 9). For Indian
Wash, the water-surface elevations of floods of the selected recur-
rence intervals were computed through use of the U.S. Army Corps
of Engineers HEC-2 Step-Backwater Computer progran (Reference 9).
Water-surface elevations for Leach Creek and Horizon Drive Channel
were obtained from the Mesa County, Colorado, Flood Insurance
Study (Reference 1).

Cross sections for the backwater analyses of Indian Wash were
obtained by field survey and by use of 2-foot contour orthophoto
maps (Reference 10). Cross sections for the channel improvements
were obtained from as-built plans from the city. All bridges,
dams, and culverts were field surveyed to obtain elevation data
and structural geometry.

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 is computed (Section 4.2), selected cross
section locations are also shown on the Flood Boundary and Floodway
Map (Exhibit 2).

Roughness factors (Manning's "n") used in the hydraulic computations
for the study streams were chosen by engineering judgment and
based on field observations of the flooding sources and flood
plain areas. Roughness values for the main channel of Colorado
River ranged from 0.017 to 0.04, and flood plain roughness values
ranged from 0.035 to 0.07. Channel roughness coefficients in

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