FLOOD01279

• Designation Number: 215
• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Title: Flood Hazard Area Delineation - Boulder Creek
• Date: 1/1/1983
• Designation Date: 5/1/1983
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Areas of shallow flooding are designated to show areas where the flood profile
is not high enough to cause inundation, but the energy and momentum relation-
ships from upstream flooding and road grade overtopping result in shallow
inundation of areas immediately downstream.

Study Reach No. 4 (Broadway to 9th Street)

Study Reach No.4 begins immediately upstream of Broadway (Cross Section No. 54)
and ends upstream of 9th Street (Cross Section No. 61). Floodplain limits for
this study reach are shown on Sheets 2 and 3, and the water surface profile is
shown on Sheet 16.

A typical cross section for this reach, as shown on Sheet 15, represents the
separate hydraulic profiles.

The 9th Street bridge is the major stream crossing in this reach. Other cross-
ings include the City Hall footbridge and the Boulder Public Library (Sheet 19).
The library crossing is well above estimated flood levels.

Study Reach No.3 (Folsom to Broadway)

Reach No.3 begins immediately upstream of Folsom (Cross Section No. 37) and
ends immediately upstream of Broadway (Cross Section No. 54).

Overbank flooding in this reach affects properties north and south of Boulder
Creek. The limited capacity of Broadway combined with the wide road grade
produces little backwater effect during the 100-year event. Upstream of Broadway
overbank flooding affects a number of public properties, including City Hall and
the Boulder Public Library. Numerous private properties will also be affected
by flooding north of Canyon Boulevard and south of Boulder Creek. The new 9th
Street bridge has high flow capacity, but the low road grade to the north allows
significant flow around the bridge during the 100-year event.

Floodplain limits for this study reach are shown on Sheets 3 to 5 and the water
surface profile on Sheets 15 and 16. Major stream crossings in the reach include
17th Street, Arapahoe Avenue and Broadway. Minor stream crossings include Folsom
Field Bridge, Stadium Footbridge, 19th Street Footbridge, and the High School
Footbridge (Sheets 18 and 19). A diversion dam located immediately downstream of
the Broadway crossing directs flows to the North Boulder Farmers, Boulder and
Left Hand Ditch.

As with Reach No.2, significant property damages would likely result during an
occurrence of the 100-year flood. Bridge superstructures are also inundated
and such a flood would likely wash out or cause severe damage to all stream
cross i ngs.

Study Reach No. 5 (9th Street to the Canyon Mouth)

Study Reach No. 5 begins at Cross Section No. 61 and ends 100 feet upstream of
the Arapahoe Avenue crossing (Cross Section No. 80). Floodplain limits for
this reach are shown on Sheets 1 and 2, and the water surface profiles are shown
on Sheets 16 and 17.

Areas of shallow flooding are designated to show areas where the ground profile
exceeds the flood profile, but upstream energy and momentum of flood waters is
sufficient to cause inundation. The backwater model indicates that the channel
downstream of 17th Street has the capacity to convey more than 90 percent of
the total 100-year discharge, while upstream energy relationships would not
permit that degree of flow contraction. This hydraulic condition further
supports the designation of the shallow flooding areas. A typical cross section
shows this relationship downstream of 17th Street (Sheet 16).

6th Street and Arapahoe Avenue are the only stream crossings within this study
reach.

Flooding in this reach is characterized by higher velocities and a more confined
floodplain as compared to the lower reaches of Boulder Creek. Between 9th and
6th Streets, 100-year flooding is confined by berming along the north side of

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