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

of flow separation, rating curves were constructed using backwater results and
added to obtain a composite rating. Flows for each frequency were proportioned
for each flow path by reference to the rating curves. From the computations, the
probable flow direction was determined and equal potential water-surface
contours were projected into adjacent ponding or low velocity areas. The
constrictions that produce the divided flow also limit the quantities of flow in the
channels downstream. The flow regulation imposed by these structures limit the
discharge in the downstream channel to one maximum value for all frequency
floods, as reflected in the profiles for these areas.

In the City of Florence, flow overtops the channel banks of Coal Creek at the
Denver and Rio Grand Western Railroad Bridge and the East Main Street Bridge
and moves east and west from the main channel. A divided flow analysis was
used on the railroad bridge and other bridges within the City of Florence to
proportion the flow between the channel and the over banks of Coal Creek.
Additional cross sections were obtained and hydraulic computations made to
define the water-surface elevations along the east and west over bank flow paths.

The flooded areas determined for the East and West Overflows of Coal Creek in
the City of Florence can be characterized as shallow flooding, with depths
ranging from 1 to 3 feet. This flooding occurs with flows in excess of the 10-
year frequency on Coal Creek, due primarily because of the constriction of Coal
Creek at the Denver and Rio Grande Western Railroad. Overflows moving east
and west from the main Coal Creek channel were determined upstream of the
Denver and Rio Grande Western Railroad and analyzed using a uniform depth
analysis (Manning's equation) up to the point where flow was determined to
move northward through the City of Florence, via the east and west overflow
paths.

3.3 Vertical Datum

All FIS reports and FIRMs are referenced to a specific vertical datum. The
vertical datum provides a starting point against which flood, ground, and
structure elevations can be referenced and compared. Until recently, the standard
vertical datum used for newly created or revised FIS reports and FIRMs was the
National Geodetic Vertical Datum of 1929 (NGVD29). With the completion of
the North American Vertical Datum of 1988 (N A VD88), many FIS reports and
FIRMs are now prepared using NA VD88 as the referenced vertical datum.

Flood elevations shown in this FIS report and on the FIRM have been converted
to NA VD88. Table 4 shows Individual datum conversion elevations for each
stream studied in this community.

Table 4, "NGVD29 to NA VD88 Conversion Elevations"

Conversion from NGVD
Stream Name to NA VD (feet)
Arkansas River 3.32
Chandler Creek 3.30
Coal Creek 3.26

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