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

the confluence with each tributary. The Arkansas River is the regulatory
flooding source in Canon City. Flood profiles were drawn showing computed
water-surface elevations to an accuracy of 0.5 foot for floods of the selected
recurrence intervals.

Starting water-surface elevations for Coal Creek East Overflow, Coal Creek
West Overflow, and Oak Creek were determined from critical depth elevations at
the confluence with the Arkansas River. Starting water-surface elevations for
Coal Creek were obtained from the water-surface profile elevations for the
Arkansas River.

Starting water-surface elevations for the tributary creeks of Oak, Coal, and
Chandler Creeks, and the Northeast Canon Drainage Area, were determined from
a water -surface profile calculation on the Arkansas River using a coincidental
flow relationship for the discharge in the Arkansas River (References 11 and 19).

Roughness Factors

Channel roughness coefficients (Manning's "n" values) for these computations
were assigned on the basis of field inspection of the floodplain area with ground
level photography used as an aid in the selection of the roughness factors and
aerial photography used to determine the limits of each roughness zone. The
channel and over bank "n" values for the streams studied by detailed methods are
shown in Table 3.

The hydraulic analyses for this study were based on unobstructed flow. The
flood elevations are thus considered valid only if hydraulic structures remain
unobstructed, operate properly, and do not fail. For Coal Creek in the City of
Florence, blockage of flow from buildings and other unnatural obstructions were
simulated in the over bank calculations to predict the impacts these features have
on flood elevations.

Table 3, "Manning's "n" Values"

Stream Name
Arkansas River
Arkansas Tributaries
Northeast Canon Drainage Area
Oak Creek
Coal Creek
East and West Coal Creek Overflows
* = Data not available

Roughness Coefficients
Channel Over banks
0.030 - 0.320 0.025 - 0.065
0.016 - 0.060 0.030 - 0.060
0.035 - 0.065 *
0.032 - 0.045 0.030 - 0.080
0.032 - 0.047 0.040 - 0.090
0.024 - 0.030 0.040 - 0.100

For the Arkansas River and Chandler, Coal, and Oak Creek, divided flow
analysis was used on the railroad bridges and other bridges to proportion the flow
between the channel and the overbanks of the streams. Additional cross sections
were obtained and computations made to define the separated flow characteristics
of the overbank flow paths. Backwater computations, using HEC-2, were made
for each independent flow path using a random range of discharges. At the point

11