FLOOD06065

• County: Weld
• Community: Frederick
• Stream Name: Tri-Area Drainageway
• Basin: South Platte
• Title: LOMR - Affected Panel 080244 0001 B - Case No. 04-08-0410P
• Date: 1/12/2005
• Prepared For: Frederick
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

3.1 Hydrologic Analyses
Hydrologic analyses were carried out to establish the peak discharge-
frequency relationships for floods of the selected recurrence inter-
vals for each stream studied in detail in the community.

No streamflow gaging information is available for the major drainage-
way flowing through Frederick. Gaging information on small watersheds
in Colorado in currently being collected by the State Highway Depart-
ment, but records to date are sporadic, and the per~od of record is
short. The U.S. Soil Conservation Service method for developing
synthetic hydrographs was, therefore, used to determine frequency
discharges in this basin (Reference 6). This method is particularly
suited to small agricultural watersheds. Rainfall data used in the
hydrologic analyses were obtained from the National Oceanic and
Atmospheric Administration Atlas for Colorado (Reference 7), and
soil and land use information was taken from maps prepared by the
U.S. Soil Conservation Service (Reference 3).

Frequency discharges for the 50-, 100-, and SOO-year floods in the
Town of Frederick reflect overtopping of the levee at the south edge
of town. Discharges resulting from the levee failure were estimated
using the California Breach Method and criteria used by the Colorado
State Engineers' office. Discharges were routed downstream through
procedure (Reference 8). Failure of the levee increased the 100-
year flood discharge by approximately 2S percent through Frederick.

Peak discharge-drainage area relationships for the Tri-Area Orainage-
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 confluence of
Tri-Area East Tributary

1.2

250

693

896

1,269

3.2 Hydraulic Analyses

REVISED DATA

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

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