FLOOD00232

• Designation Number: 318
• County: Weld
• Community: Unincorporated Weld County
• Title: Flood Insurance Study - Weld County, Colorado, Unincorporated Areas and Eaton, Colorado
• Date: 9/1/1991
• Designation Date: 11/1/1991
• Floodplain - Doc Type: Floodplain Report/Masterplan

Peak discharge-drainage ar~a relationships for South Platte River,
Cache La poudre River, Sheep, Ashcroft, and Eaton Draws, Tri-Area
Drainageway, Coal Creek, and the Big Thompson River are shown in
Summary of Discharges (Table 1).

Discharges for U.S. Highway 34 Levee Overflow Channel were obtained
using a weir flow equation,

Q ~ CLH3/2

where Q =
C =
L =
H ~

Total flow over the weir
Coefficient of discharge
Effective length of weir controlling flow
Difference between the energy grade line elevation and
the roadway crest elevation

.

Values of H were determined by using the COE HEC-2 computer program
(Reference 15) where the flow was confined to the channel. The
100- and SOO-year discharges ranged from 550 to 600 cfs and from
7,000 to 12,500 cfs, respectively.

The 10-, 50-, 100-,
Drainageway Overflow
respectively.

and 500-year discharges for the Jri-Area
Channel are 240, 780, 850, and 1,100 cfs,

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of streams in the county
were carried out to provide estimates of the elevations of floods
of the selected recurrence intervals along each stream studied in
the county.

,~

Cross section data for the South Platte River were obtained from
field surveys done by the COE for the Special Flood Hazard
Information Report (Reference 5) and by field surveys performed by
the study contractor. All bridges were surveyed by the COE to
obtain elevation data and structural geometry. Roughness
coefficients (Manning I s "n") were estimated by field inspection.
Channel "n" values ranged from 0.035 to 0.100. Water-surface
profiles were computed using the COE HEC-2 computer program
(Reference 12). Starting water-surface elevations for the South
Platte River were determined using the slope area method 2,000 feet
downstream of the study reach.

c.

Cross sections for the Cache La Poudre River were taken from
topographic maps by COE for their Flood Plain Information Report
(Reference 4). All bridges were field surveyed to obtain elevation
data and structural geometry. Roughness coefficients were selected
by the COE. The channel "n" value was 0.040, and the overbank "n"
values ranged from 0.060 to 0.100. The 100-year water-surface
profile was computed by the COE (Reference 15). Profiles for the
10-, 50-, and SOO-year floods were hand calculated using the step-

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