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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downstream limit of the Arapahoe Avenue Overflow, Boulder
Creek and UCBRP grading modifications,

3.2 .3 BOil] der Creek IBCSC-Appendix 2a I

Upstream of section 16.1, 100 Year flood flows from the
Arapahoe split (1500 cfs) were considered separately from the
Boulder Creek model, Thus out of a total of 11,150 cfs from
the combined flows, 9,650 cfs comes from Boulder Creek and
the UCBRP (Fig. 5b),

As mentioned earlier, part of this 9,650 cfs spills over the
bikepath running along the south bank of Boulder Creek
upstream of the Arapahoe Avenue bridge, The Split Flow option
of the HEC-2 program was used to define these flows (Ref, 6).
A weir was defined along the right bank of the creek along
the concrete bikepath extending from the Arapahoe bridge
upstream to section 21 (Fig, 5c). This resulted in a
separate model for the main channel flows (BCSC, Appendix 2a
- see Fig. 2a and b for water surface profiles along this
reach) ,

Two sections were added between the base section (16,1) and
the Lower Arapahoe bridge to reflect conditions below the
bridge:

Section 16.2 represents an extension of the section used in
the Arapahoe Avenue Split. It was encroached to the left bank
of the creek channel to confirm the assumed spatial variance
between the Boulder Creek model and the Arapahoe Avenue
Overflows to the north (Appendix 1). Inspection of the
results confirms that these are hydraulically disconnected
flows at this point,

Section 16.3 was added to represent recent modifications to
the creek below the bridge. The section was surveyed as a
part of this study.

Sections 17 and 18 represent the downstream and upstream
faces of the Lower Arapahoe Bridge, and were as-built
surveyed as a part of this study. This bridge was originally
modeled in the FHAD using the normal bridge method of HEC-2
under the assumption that the right overbank above Arapahoe,
i.e" the C.U. Research Park, was hydraulically connected to
the main channel, By careful examination of the physical
constraints above Arapahoe Avenue, it can be shown that
rising flows in the creek will be confined to the main
channel until the low chord of the bridge becomes submerged
and eventually the deck is overtopped. This will create a
pressure flow condition, which is more readily modeled using
the Special Bridge method (Ref. 5). It is the backwater from

LOVI! &. Associates, Inc.

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