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

, I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

bridge was used in this study as the downstream limit (Sheet
10 of 19, Appendix la),

3.2 2

Rasf> Sf>ction

In order to model the split flow upstream of Foothills
Parkway, a Base Section had to be developed which adequately
described the backwater situation as the spatially varied
flows returned to the main channel.

Section 13, downstream of Foothills Parkway, was used
unchanged from the FHAD. All backwater models for this study
were started with water surface elevations taken directly
from the FHAD.

Sections 14 and 16 represent the downstream and upstream
faces of the Foothills parkway Bridge. Above the bridge,
water from the Arapahoe Avenue split, Boulder Creek, and the
right overbank all come together to form a large pool
upstream of Foothills Parkway (Fig. 5a). Part of this water
will flow on across the intersection of Foothills Parkway and
Arapahoe Avenue, which is modeled as a weir (BT cards,
Section 16). The rest will flow through the bridge.

The bridge opening was originally described as having a very
high left levee (X3.B), but the right levee (X3.9) was left
too low to restrict flows to the bridge opening in the FHAD
model (Appendix 1). This results in the cross sectional area
in the right overbank being used for flow computations when
actually this area is a large pool of water behind Foothills
Parkway, Large losses incurred as a majority of the flows are
forced to pass north over Arapahoe Avenue and through the
bridge are not reflected in backwater computations upstream.
For this reason, the right levee (X3.9) of sections 14 and 16
was raised as part of this study to restrict flows to the
bridge, The remaining data which physically describes the
bridge was used unchanged from the FHAD,

Section 16.1, just upstream of the Foothills Parkway bridge,
represents the confluence of the three spatially varied
flows, and as such, forms the Base Section. At this point,
the flows from the Arapahoe Avenue Overflow, the main channel
of Boulder Creek, and right overbank flows from the UCBRP are
combined (Fig. 5c),

Data to the left of GR station 715 (Arapahoe Avenue north
across Boulder Creek) was used unchanged from the FHAD, and
data to the right of GR station 715 (UCBRP) was digitized
from the as-built drawings. The section was encroached from
the right to GR station 1250 to limit effective flows to pond
3 (see construction drawings), Section 16.1 represents the

Love & Associates, Inc.

Page 6