FLOOD08776

• County: Moffat
• Community: Craig
• Basin: Yampa/White
• Title: FIS - Craig
• Date: 3/28/1984
• Prepared For: Moffat County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

Peak discharge-drainage area relationships for Fortification Creek,
Brotherton Gulch, Cedar Mountain Gulch, Pine Ridge Gulch, and
Tributaries 1 and 2 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.

For Fortification Creek, Brotherton Gulch, Cedar Mountain Gulch,
and Pine Ridge Gulch, the water-surface elevations of floods of
the selected recurrence intervals were computed through use of
the COE HEC-2 step-backwater computer program (Reference 6).

Cross sections used in the backwater analyses for Fortification
Creek and Brotherton Gulch were obtained from the Colorado Water
Conservation Board and the city. The below-water sections were
obtained by field measurement. Cross sections for Cedar Mountain
Gulch and pine Ridge Gulch were field surveyed. 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 was computed (Section 4.2), selected cross
section locations are also shown on the Flood Boundary and Floodway
Map (Exhibit 2).

Roughess factors (Manning I s "nll) used in the hydraulic computations
for the detailed 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 Fortification
Creek ranged from 0.040 to 0.045, and flood plain roughness values
ranged from 0.045 to 0.065. Roughness values for the main channel
of Brotherton Gulch ranged from 0.030 to 0.045, and flood plain
roughness values ranged from 0.045 to 0.060. Roughness values
for the main channel of Cedar Mountain Gulch and Pine Ridge Gulch
ranged from 0.030 to 0.050, and flood plain roughness values ranged
from 0.035 to 0.080.

Flood profiles were drawn Showing computed water-surface elevations
to an accuracy of 0.5 foot for floods of the selected recurrence
intervals (Exhibit 1). The results obtained from the HEC-2 computer
program were adjusted in the vicinity of bridges to more accurately
represent actual flooding conditions (Reference 7). Starting
water-surface elevations for Fortification Creek and Brotherton
Gulch were determined by the Slope-area method. For Pine Ridge
Gulch and Cedar Mountain Gulch, starting water-surface elevations
were determined from hand calculations taken at culverts at State
Highway 13 and the First Street Bypass.

10