FLOOD09722

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Request for LOMR - City of Boulder
• Date: 1/1/1989
• Prepared For: Boulder County
• Prepared By: Love & Associates, Inc.
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

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east, gradually spilling back into
Boulder Creek along the study reach,
forms the upstream limit of this study

the left overbank of
Section 22 of the FRAD
(Fig, 5a),

A detailed discussion of the Arapahoe Avenue Overflow as
defined in the FRAD is beyond the scope of this study, The
Arapahoe split was assumed to be valid and flows returning
south to the left overbank and the main channel of Boulder
Creek were considered in this study as noted later. These
flows as published in the FHAD are reproduced in Table 2 and
shown in Fig, 5b,

TABLE 2

FLOWS RETURNING TO BOULDER CREEK FROM ARAPAHOE AVENUE

SA~t:ion

23
22
21
20.1
19
17
16.1

2500
2300
2300

RAt. 1'1 ow BouldAr CreAk
7300
1600 8900
8900
150 9050
600 9650
9650
1500 11150

Ara'D AVA _

2100

The FHAD model considers Boulder Creek and the adjacent right
overbank to be a hydraulically connected body of water during
the 100 Year Flood (Fig 5b). Actually, the right bank of the
creek is considerably higher than the adjacent right overbank
(Fig, 6). A 10-foot wide concrete bikepath running the length
of the creek in effect forms a weir along the right bank of
the channel through the UCBRP. Flows which spillover this
path become hydraulically disconnected from the main channel
and flow eastward to Foothills Parkway. This results in a
rather complex situation in which three separate bodies of
flood flows exist along a single reach of the creek: the
Arapahoe Avenue Overflow, Boulder Creek Main Channel,
and the Right Overbank or tJCBRP (Fig. 5b) (note that
sheets 7 to 10 of the FHAD are reproduced in Appendix la, and
detailed hydraulic information on a section by section basis
can be found in Appendix lb and lc).

Certain control points exist throughout the City where
spatially varied flows are forced back into the main channel
by geographical constraints. The bridge spanning Boulder
Creek at Foothills Parkway and its embankment is one of these
control points. Foothills Parkway is elevated in this area
and floodwaters are generally restricted to flowing through
the bridge. For this reason, Section 13 downstream of the

Love & Associates, Inc.

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