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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The following paragraphs contain a general description of flooding for each
study reach.

Typical cross sections showing flow rates, channel velocities and flood levels
are included on the flood profile sheets.

Study Reach No.1 (Valley View Road to Arapahoe Avenue)

Study Reach No. 1 begins at Cross Section No. 2 immediately downstream of
Valley View Road and ends at Cross Section No. 20 upstream of the lower
Arapahoe Avenue crossing. Floodplain limits for this reach are shown on
Sheets 8 to 12, and the water surface profile on Sheets 13 and 14.

Wide overbank flooding is typical for this reach of stream. Major stream
crossings in this reach include Valley View Road, Colorado and Southern
Railroad, 47th Street (Foothills Parkway), and Arapahoe Avenue (Sheet 18).
In the right overbank area between Valley View and the Colorado and Southern
Railroad, flooding is confined by a levee running along the north and west
boundary of an existing, commercially-developed area.

Study Reach No. 2 (Arapahoe Avenue to 24th Street)

Study Reach No.2 represents main stem flooding beginning at Cross Section
No. 20 and ending upstream of 24th Street (Folsom) at Cross Section No. 38.
Floodplain limits for this study reach are shown on Sheets 5 to 8, and the
water surface profile on Sheets 14 and 15. This main stem reach parallels
the Arapahoe Avenue Overflow floodplain with similar information being shown
on the same drawings. The cross sections, stationing and flood profile for
the overflow reach have been identified separately on the drawings to simplify
user interpretation.

For the floodplain area downstream of the C & S Railroad, a special analysis
was conducted to determine separate flood profiles for the right overbank (ROB)
conveyance channel and the main channel of Boulder Creek between Cross Sections
7 and 9. (See: Sheet 11 of 19). Flow contributing to the ROB channel was
determined by an iterative process involving a study of weir flow over the
railroad embankment and flow passing through the bridge. The weir section was
defined along the C & S Railroad grade beginning at the bridge center, running
1300 feet to the east and tying back into the northwest corner of the large
building located east of Cross Section 11. The separate profiles are shown on
Sheet 14, and the split flow discharges representing bridge flow and weir flow
respectively are listed in Table II for Cross Section 9. The floodplain map
also indicates that flow will depart from the main floodplain to the east along
the C & S Railroad grade.

Major stream crossings within this study reach include 30th Street, 28th Street
and 24th Street (Folsom). The Hilton Harvest House footbridge is also included
in this study reach (Sheet 18). This hydraulic effect of the footbridge cross-
ings were considered negligible as the crossings only present a minor obstruc-
tion in the vicinity of the channel and would likely be destroyed during a major
flood event such as the 100-year flood.

Development through this area is subject to significant damages during the
occurrence of the 100-year flood. Limited bridge capacities in this reach produce
significant backwater effects and contribute to the split flow discharges used
for the Arapahoe Avenue Overflow.

The flood prone areas upstream of 47th Street result from a combination of
main stem and Arapahoe Avenue Overflow flooding. The Arapahoe Avenue Overflow
flooding is described on page 12 of this report. Within the reach between
47th Street and Arapahoe Avenue, the two separate flood profiles become
hydraulically connected at about Cross Section No. 16.1.

Special consideration was given to the area surrounding the Gold Run Condominiums.
The right bank profile east of the Wellman Canal diversion dam exceeds the
100-year profile, thereby confining flood flows to the channel and left (north)
overbank. Some floodwaters for the 100-year event will spill into the Wellman
Canal at the diversion dam, however, the relatively small amount of flow entering
the canal is not considered a major flood problem at this time. Considering the
uncertain nature of flooding along the Wellman Canal, it is recommended that the
low-lying area south of the canal be elevated above the 100-year flood level as
part of any future development plan.

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