FLOOD09509

• Designation Number: 330
• County: Mesa
• Community: Unincorporated Mesa County
• Basin: Colorado Mainstem
• Title: Flood Insurance Study - Mesa County, Colorado, Unincorporated Areas
• Date: 7/1/1992
• Designation Date: 9/1/1992
• Floodplain - Doc Type: Floodplain Report/Masterplan

Cross section data from Horizon Drive Channel were obtained from
previous studies conducted by the City of Grand Junction using
orthophoto maps prepared by the USBR in 1974 (Reference 20), and
supplemented by field surveys conducted by the city at the request
of the study contractor.

"

Cross sections for the backwater analyses of Plateau and Grove
Creeks were obtained by aerial photogrammetry (Reference 21). The
below-water sections were obtained by field measurement. All
bridges, dams, and culverts were field surveyed to obtain elevation
data and structural geometry. On Grove Creek, several small
private driveway bridges were not considered sturdy enough to
withstand peak floodflows and were therefore considered to be
washed out in the computations.

,.

Cross section data and reach length information for the backwater
analyses for Roan Creek were obtained from photogrammetric maps
prepared especially for the study of flooding on the Parachute and
Roan Creeks with contour intervals of 2 feet and a scale of 1:4,800
(Reference 22).

Cross sections for West Creek were obtained by field survey.
Supplementary data were obtained from field observations and from
USGS 7.5-minute series topographic quadrangle sheets (Reference
23).

Water-surface elevations of
intervals were computed using
program (Reference 24).

floods
the COE

of the selected recurrence
HEC-2 step-backwater computer

Channel and overbank roughness factors (Manning's "n") used in the
hydraulic computations were chosen by engineering judgment and were
based on field observations of the stream and floodplain areas.

The nn" values assigned on the streams studied are shown as
follows:
Roughness Coe f fi c i en t s (Manning's "nn)
Stream Channel Overbanks
Colorado River 0.035 0.060 to 0.073
Big Salt Wash 0.020 to 0.035 0.045 to 0.060
. Little Salt Wash 0.020 to 0.045 0.030 0.060
to
Leach Creek 0.035 to 0.070 0.050 to 0.180
Horizon Drive Channel 0.016 to 0.035 0.024 to 0.060
Plateau and Grove Creeks 0.035 to 0.080 0.050 to 0.100
Roan Creek 0.040 0.050 to 0.055
West Creek 0.035 to 0.070 0.050 to 0.180

Starting water-surface elevations for both the Colorado River and
Horizon Drive channel were determined using critical depth. Known
elevations based on the previous COE study (Reference 15) in 1976

23