FLOOD04661

• County: Boulder
• Community: Boulder
• Stream Name: South Boulder Creek
• Basin: South Platte
• Title: Floodplain Analyses of South Boulder Creek at Flatiron Property
• Date: 8/12/1996
• Prepared For: University of Colorado at Boulder
• Prepared By: Love & Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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was found to spill to the north along the right bank at three points. These three spill
points included a right bank spill discharging towards Manhattan Circle between Cross
Section 140 and 150 and a right bank spill discharging at the RTD Park-N-Ride facility
just downstream of Cross Section 130 and the main spill location at Cross Section 90 at
Apache Road. The delineation and flow spills for this condition are depicted on Figure
6.

The right bank spill discharging toward Manhattan Circle was calculated using the split
flow option in HEC-2. The discharge from the RTD Park-N-Ride facility was calculated
only for the opening located on the east side of the facility. The discharge through the
opening was calculated using normal depth calculations.

The split flow analysis in the main channel model at U.S. 36 for Condition 2, 100-Year
FEMA Certified Flood Protection Levee, indicates approximately 2,620 cfs of the 100-
year flood discharge of 6,160 cfs in South Boulder Creek leaves the main channel and
flows to the west along U.S. 36. The spill discharge of 2,620 cfs is approximately 1,000
cfs greater than the spill discharge of 1,660 cfs reported in the regulatory (0&0) model
at this point. The remaining 3,540 cfs flows to the north within the main channel and
through the U.S. Highway 36 bridge structure to the north. Of the 2,620 cfs loo-year
spill discharge, 2,020 cfs of this flow enters the U.S. 36 Overflow area and flows along
the highway embankment in a westerly direction, while 600 cfs enters the U.S. 36 South
Overflow.

The 2,020 cfs discharging along the U.S. 36 Overflow flows westwardly along the east-
bound lanes until it overtops the median upstream of Cross Section 140. The water
surface elevations equalize between the east and west bound lanes at this point. The
HEC-2 analysis indicates that the spill towards Manhattan Circle is about 195 cfs. The
right bank spill at the RTD Park-N-Ride structure is estimated to be 400 cfs and spills
toward South Boulder Road. The remaining 1,425 cfs rejoins the 600 cfs flowing along
the U.S. 36 South Overflow route downstream of the Park-N-Ride. A total of 2,025 cfs
leaves U.S. 36 at Cross Section 90, discharging towards Apache Road.

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