FLOOD06682

• County: Weld
• Community: Firestone
• Basin: South Platte
• Title: FIS - Firestone
• Date: 6/1/1979
• Prepared For: Firestone
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

Peak discharge-drainage area relationships for the Tri-Area Drainage-
way are shown in Table 1.

Table 1. Summary of Discharges

Flooding Source
and Location

Drainage Area
(Square Miles)

Peak Discharges
(Cubic Feet per Second)
10-Year 50-Year 100-Year 500-Year

Tri-Area Drainageway
At Downstream Study
L.i,Jqi. t
Upstream of Union
Pacific Railroad

5.2

690

2030

2270

3200

4.0

540

1720

1910

2550

3.2 Hydraulic Analyses

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

Cross section data for the Town of Firestone were obtained from
topographic mapping at a scale of 1:1200, with a contour interval of
1 foot, produced as a.patt of a 1975 drainage study for Weld County
(Ref.r~ce 8). Bridges were measured to obtain 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 tloodway is computed (Section 4.2), selected cross
section locations are also shown on the Flood Boundary and Floodway
Map (Exhibit 2).

The water-surface profiles for. the 10-, 50-, 100-, and 500-year
flooqs were cOmputed using the U.S. Army Corps of Engineers HEC-2
step-backw<lter oomputer program (Reference 9). Starting water-
surface.elevations were obtained by estimating water-surface elevat'ons
at F~est9ne Lake and calculating a backwater profile 800 feet
upstream to the corporate limits.

Flood profiles .were drawn -Showing computed water-surface elevation.
to an accuracy or 0.5 foot for floods of the selected recurrence
intervals (~hibit 1).

Roughness coefficients (Manning' s "n") were estimated by field
~nspection. The roughness coefficients for the Tri-Area Drainageway
and overbank areas generally ranged between 0.045 and 0.060. In
some overbank areas, "nil values up to 0.090 were appropriate.

All elevations are referenced to the National Geodetic Vertical
Datum of 1929 (NGVD). Elevation reference marks used in the study
are shown on the maps.

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