FLOOD00653

• Designation Number: 253
• County: Boulder
• Community: Jamestown
• Title: Flood Insurance Study - Jamestown, Boulder County, Colorado
• Date: 1/1/1983
• Designation Date: 3/1/1986
• Floodplain - Doc Type: Floodplain Report/Masterplan

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3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish the peak discharge
frequency relationships for floods of the selected recurrence
intervals for each flooding source studied in detail affecting
the community.

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Discharges for James Creek and the downstream portion of Little
James Creek were taken from a U.S. Army Corps of Engineers report
(Reference 2). Technical Manual No.1, developed by the U.S.
Geological Survey (Reference 3), was used to obtain peak discharges
on the upstream portion of Little James Creek and Unnamed Tributary
to Little James Creek.

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Peak discharge-drainage area relationships for James Creek, Little
James Creek, and Unnamed Tributary to Little James Creek are
shown in Table 1.

3.2 Hydraulic Analyses

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

Water-surface elevations for the 10-, SO-, 100-, and SOO-year
floods were computed using the U,S. Army Corps of Engineers HEC-2
step-backwater computer program (Reference 4). The results obtained
from the HEC-2 computer model were verified by comparing them
to ground photographs of the 1969 flood through Jamestown.

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Cross sections were obtained by field measurements. Bridges
in this study were analyzed using a blockage criteria dependent
upon bridge construction and water depth. Concrete and steel
bridges were assumed unobstructed until the upstream water-surface
elevation reached the bridge "low steel" elevation, at which
time the bridge was assumed fully obstructed. Wooden bridge
decks were assumed destroyed due to debris. This type of bridge
was assumed unobstructed at all discharges with wingwalls and
abutments in place but the deck removed. Head losses at fully
obstructed bridges were determined by weir computations. Unobstructed
bridge losses were computed by using the normal bridge routine
in HEC-2.

Locations of selected cross sections used in the hydraulic analyses
are shown on the Flood Profiles (Exhibit 1). For stream segments
for which a floodway is computed (Section 4.2), selected cross
section locations are also shown on the Flood Boundary and Floodway
Map (Exhibit 2).

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