FLOOD11444 ‎(2)‎

• County: Fremont
• Community: Fremont County and Incorporated Areas
• Basin: Arkansas
• Title: FIS - Fremont County and Incorporated Areas
• Date: 9/19/2007
• Prepared For: Fremont County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

115 determined (drainage area 3,546 square miles) by an area proportion. The
discharges obtained in the previous calculations fell within the 90 percent
confidence limits of the USACE Canon City gage-station-analyses discharges;
therefore, the hydrologic analyses performed by the USACE are considered
valid. A summary of drainage area-peak discharge relationships for each stream
studied in detail is shown in Table 2.

There are no stream flow records for the Northeast Canon Drainage Basin. Peak
discharges for the I-percent -annual-chance frequency flood were therefore
developed using the synthetic rainfall-runoff evaluation procedures (Reference
12). Synthetic runoff hydrographs were developed from the U.S. SCS Triangular
Hydrograph Equation using incremental hydro graphs. All runoff computations
were based on existing land use and cover conditions in the watershed as of
March 1980. Precipitation data were taken from National Oceanic and
Atmospheric Administration (NOAA) Atlas #2, Precipitation-Frequency Atlas of
the Western United States, Volume III, Colorado (Reference 13).

Peak discharges of various recurrence intervals for Coal Creek and Oak Creek
were determined by Camp Dresser and McKee, Inc. To predict the peak 50-, 4-,
and 1- percent -annual-chance flow rates for these creeks a synthetic unit
hydro graph was developed, using the basin characteristics presented in the
Arkansas River Basin, Florence, Colorado, Design Memorandum No.1, prepared
by the USACE (Reference 14). A SCS Type II-A storm distribution was applied
to each basin developed from rainfall data from NOAA, to calculate the peak
discharges. The peak discharges for 50-,4-, and 1-percent-annual-chance events
were then plotted on logarithmic - probability graph paper and peak discharges
for the 10-, 2-, and 0.2-percent-annual-chance frequencies were estimated.

Peak discharges for the Coal Creek East and West Overflows on Coal Creek
below the Denver and Rio Grande Western Railroad were determined based upon
the hydraulic analysis of Coal Creek (as described in Section 3.2, below).

The USACE undertook a hydrological analysis of Coal Creek Basin in 2001
specifically to determine peak discharges and frequency relationships for Coal
Creek and Oak Creek and associated tributaries at various locations within the
watershed (Reference 15).

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding from the sources studied
were carried out to provide estimates of the elevations of floods of the selected
recurrence intervals. Users should be aware that flood elevations shown on the
FIRM represent rounded whole foot elevations and may not exactly reflect the
elevations shown on the Flood Profiles or in the Floodway Data tables in the FIS
report. Flood elevations shown on the FIRM are primarily intended for flood
insurance rating purposes. For construction and/or floodplain management
purposes, users are cautioned to use the flood elevation data presented in this FIS
in conjunction with the data shown on the FIRM.

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