FLOOD01759

• County: Broomfield
• Community: Broomfield
• Title: Flood Insurance Study - City and County of Broomfield, CO
• Date: 11/25/2003
• Prepared For: Broomfield
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

3.1 Hydrologic Analyses

Hydrologic analyses were carried out
discharge-frequency relationships for each
by detailed methods affecting the community.

to establish the peak
flooding source studied

Peak discharges for the 10- and 100-year floods for the City Park,
Nissen Reservoir, Gay Reservoir, and West Lake basins were obtained
from a 1979 UDFCD report (Reference 2). In the UDFCD report, the
10- and IOO-year flood discharges were computed using the Storm
Water Management computer model {Reference 3). The rainfall figure
used was based on Volume III of the Precipitation-Frequency Atlas of
the Western United States (Reference 4).

The SC determined the 50- and SOD-year flood discharges by plotting
the 10- and 100-year flood discharges on log-probability paper and
interpolating or extrapolating as appropriate. The 50-year flood
discharge was determined by interpolation; the SOa-year flood
discharge was determined from extrapolation.

Peak discharge-drainage area relationships for all flooding sources
are shown in Table 2.

3.2 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
of

the
the

For all streams studied by detailed methods, the water-surface
elevations (WSELs) for floods of the selected recurrence intervals
were computed using the U.S. Army Corps of Engineers (USACE) HEC-2
step-backwater computer program (Reference 5).

Cross sections used in the backwater analyses for all streams were
obtained by aerial photogrammetry (Reference 6). The below-water
sections of all cross sections were obtained by field measurement.
All bridges, darns, and culverts were field surveyed to obtain
elevation data and structural geometry.

Locations of selected cross sections used in
are shown on the Flood Profiles (Exhibit 1).
for which a floodway was computed (Section
section locations are also shown on the Flood
Map (FBFM) (Exhibit 2).

the hydraulic analyses
For stream segments
4.2), selected cross
Boundary and Floodway

Roughness factors (Manning's "nil) used in the hydraulic computations
for the detailed study streams were chosen by engineering judgment
and based on field observations. Roughness values for the main
channels of the streams studied ranged from 0.02 to 0.065; overbank
roughness values ranged from 0.02 to 0.10.

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