FLOOD00802

• Designation Number: 355
• County: Weld
• Community: Greeley
• Stream Name: Cache La Poudre
• Title: Floodplain and Floodway Analysis
• Date: 1/1/1994
• Designation Date: 3/1/1994
• Floodplain - Doc Type: Floodplain Report/Masterplan

2

The flow rates in the Poudre River as cited in the Frs studies are shown in Table 1 for
the 10-, 50-, 100-, and 500-year events.

Table 1 Flows in the Cache La Poudre River

-
Frequency of stonn Q (cfs)
(year) - --
10 3,500
- --
50 8,000
-.-
100 10,700
- --
500 19,000
-

Hydraulic Models

Water surface profIles (floodplains) were prepared for the 10-, 50-, 100-, and 500-year
rainfall events utilizing the COE HEC-2 Water Surface ProfIle computer program which is the
accepted FEMA methodology. Additionally, floodway analysis was prepared for the 100-year
rainfall event. The HEC-2 input and output for the models are induded as Appendices F
through J.

The HEC-2 program calculates water surface profiles for steady, gradually varied flow
in natural or man-made channels. The effects of various obstructions such as bridges, weirs,
and structures in the floodplain are considered in the computations. The computational
procedure is based on the solution of the one-dimensional energy equation with energy loss due
to friction evaluated with Manning's equation.

BC's model achieves the FEMA requirements which dictate that all hydraulic models
submitted for a LOMR start at a downstream FEMA 100-year water surface elevation and
converge back to the upstream FEMA 100-year elevation. For the Greeley model, BC began
with the 1973 FEMA data downstream of Ash Avenue. BC then develop('.d a new baseline
model from Ash Avenue upstream to 5th Street.

BC prepared topographic mapping of the study area to develop the baseline model
condition and for documentation and presentation of the model output. BC used the services of
Arnold Analytical Services as subconsultant for the mapping. Mapping was prepared from the
City's 1987 aerial photography at a scale of 1" = 100' with a 2-foot contour interval.

BC perfonned field surveys to supplement the aerial mapping and provide the basis of
an accurate hydraulic model. Bridge configurations were included in the model based on field
surveys and as-built data obtained from the Colorado Department of Highways and the City.
Above 5th Street, the original FEMA model was again us('.d until convergence was attained with
the 1973 model.

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