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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Spill discharges from the main channel of South Boulder Creek were identified using the
split flow option in the HEC-2 model. The split flow control line was located at the
highest ground point between the Flatiron Property and South Boulder Creek. To
establish the sensitivity of the magnitude of the spill discharge to the assumed 2.6 weir
coefficient, the coefficient was varied in the models to try and determine if more waters
remained in the South Boulder Creek main channel section or if more waters were

spilling between the main channel and the eastern edge of the Flatiron Property along the
berm. It was determined that the magnitude of the spills from South Boulder Creek
would decrease by about 15 percent with the coefficient of 2,0 and increase less than 2
percent with a coefficient of 3.0, Thus, the weir coefficient of 2.6 appears reasonable for
these models, This weir coefficient is a function of the HEC-2 split flow model and is
required in order to complete the HEC-2 modeL

The starting water surface elevation was obtained from the regulatory model at Cross
Section 20180 upstream of South Boulder Road. The discharge at this Cross Section in
the regulatory model equals the total regulatory lOO-year discharge of 6,160 cfs_
Because the main channel spills upstream of Cross Section 20180, it is likely that a
discharge lower than 6,160 cfs would be realized at the Cross Section during a l00-year
event. However, identifying the discharge would require identifying the direction and
path of the spills north of U.S, 36, which exceed the scope of this study.

An estimate of the sensitivity of the two models to starting water surface elevation was
made. As a result of this analysis, it was decided to use Cross Section 20180 as the
starting section but recognize spill discharges could change with additional revisions to
the regulatory HEC-2 downstream of U.S. 36.

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Since it was determined that the berm surrounding the Flatiron Property was not a FEMA
certified levee, Love & Associates, Inc. developed l00-year floodplain models for the
Flatiron Property based on two conditions, Condition 1 assumed that the site perimeter
berm was did not exist and a model was set up entitled "loo-Year Floodplain - No Levee
with 200-foot Wide Channel". The 200-foot wide interior interceptor channel was

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