FLOOD11732

• Designation Number: 377
• County: Boulder
• Community: Boulder
• Stream Name: Bear Creek, Fourmile Canyon Creek
• Basin: South Platte
• Title: FIS - Boulder
• Date: 8/4/1988
• Prepared For: Boulder
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

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3.2

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any 50-year period is approximately 40 percent (4 in 10), and, for
any 90-year period, the risk increases to approximately 60 percent
(6 in 10). The ~nalyses reported herein reflect flooding
potentials based on c:onditions existing in the community at the
time of completion of this study. Maps and flood elevations will
be amended periodically to reflect future changes.

Hydrologic Analyses

Hydrologic analyses were carried out to establish the peak
discharge-frequency relationships for each flooding source studied
by detailed methods affecting the community.

Because no stream gage data are available for the study streams, a
rainfall-runoff analysis was conducted on the watersheds to
determine the flood discharges. This was accomplished by using
the UDFCD CUHP-B rainfall-runoff computer program to develop the
storm hydrographs (Reference 9) and the COE HEC-1 computer program
for the stream and reservoir routings (Reference 10). For this
analysis, basin characteristics of the watershed as well as
rainfall amounts based on the selected recurrence intervals are
used to compute flood hydrographs for various design points in the
basins. All stream and reservoir routings were accomplished using
the Modified Puls Method.

The SOO-year storm runoff values for various locations along each
stream were extrapolated from the discharge-frequency curves.

A more detailed description of the input variables
and the HEC-l rainfall-runoff analysis, as well
computer output and the summary of the final
output, are located in a technical addendum
Insurance Study (Reference 11).

for the CUHP-B
as the CUHP-B
HEC-1 computer
to this Flood

Peak discharges for Gregory Creek, Bluebell Canyon Creek, Kings
Gulch, Viele Channel, Anderson Channel, and Davids Draw were taken
from reports prepared by Wright-McLaughlin Engineers (References 4
and 16) as part of a plan to manage the city's storm drainage.

Peak discharge-drainage area relationships for the streams studied
by detailed methods are shown in Table 2.

Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding
sources studied were carried out to provide estimates
elevations of floods of the selected recurrence intervals.

from the
of the

For all streams studied, the water-surface elevations of floods of
the selected recurrence intervals were computed using the COE
HEC-2 step-backwater computer program (Reference 12).

11