FLOOD01186

• County: Garfield
• Community: Rifle
• Title: FIS - Rifle
• Date: 1/3/1986
• Prepared For: Rifle
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

Peak flows along Rifle Creek were based on the regional regression
relationship and the routing of the inflow hydrograph through Rifle
Gap Reservoir. The hydrograph was developed from U.S. Soil Conser-
vation Service (SCS) methodology. The inflow peaks corresponded
to the peak as determined from the regression relationship, and
the volume was checked to correspond with the excess volume as
determined from the SCS analyses.

Peak discharge-drainage area relationships for the flooding sources
of the City of Rifle are shown in Table 1.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding from the
sources studied were carried out to provide estimates of the eleva-
tions of floods of the selected recurrence intervals.

Water-surface elevations of floods of the selected recurrence inter-
vals were computed using the COE HEC-2 step-backwater program
(Reference 6).

Cross section data for the Colorado River were developed from
topographic maps furnished by the Colorado Department of Highways
(References 7 and 8). Cross sections for Rifle and Government
Creeks, and Hubbard Gulch were digitized from aerial photographs
flown in May 1980 (Reference 9). All bridges 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 flcodway was computed (Section 4.2), selected cross
section locations are also shown on the Flood Boundary and Floodway
Map (Exhibit 2).

Roughness factors (Manning's On") used in the hydraulic computations
were computed from stream gage data or determined by field obser-
vation using engineering judgment. Roughness values for the main
channel of the Colorado River ranged from 0.035 to 0.040, whereas
overbank roughness values ranged from 0.060 to 0.070. Roughness
values for the main channel of Rifle Creek ranged from 0.040 to
0.050, whereas overbank roughness values ranged from 0.050 to 0.080.
Roughness values for the main channel of Government Creek ranged
from 0.035 to 0.040, whereas overbank roughness values ranged from
0.035 to 0.045. Roughness values for the main channel of Hubbard
Gulch ranged from 0.035 to 0.045, whereas overbank roughness values
ranged from 0.035 to 0.050.

The starting water-surface elevations for the Colorado River,
Government Creek, and Hubbard Gulch were determined by the slope-
area method. The starting water-surface elevation for Rifle Creek
was taken as the normal depth for the given discharge.

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