FLOOD01787

• County: Boulder
• Stream Name: South Boulder Creek
• Title: South Boulder Creek/ Flatirons
• Date: 12/5/1997
• Prepared For: Boulder
• Prepared By: Consultants
• Floodplain - Doc Type: Community File

.
.
11.
.
.
.
.
! .
.
.
.
.
.
.
.
.
.
I .

,

I

~TAGGART
~, EflGlNEEAING ASSOCtATES, INC.
2480 West 26th Avenue. Suite 340-8 . Denver, Colorado 80211 . [phone] . FAX [phone]

December 18, 1996

Mr. Bill DeGroot
Chief, Floodplain Management
Urban Drainage and Flood Control District
2480 West 26th Avenue, Suite 156B
Denver, CO 80211

RE: Evaluation of Certain Engineering Studies of South Boulder Creek, From Upstream of
Highway 93 to Downstream of South Boulder Road, Phase I

Dear Mr. DeGroot:

We have reviewed the Flood Hazard Delineation (FHADl - South Boulder Creek, Greenhorne
and O'Mara (G&O), July 1986 for the Urban Drainage and Flood Control District (UDFCDl,
and Boulder County (County); and the FloodDlain Analvsis of South Boulder Creek at the
Flatirons ProDertv in Boulder Countv, Love & Associates, Inc. (Love), 1996. Our evaluation
was to determine which of these two studies was more appropriate to use, with or without
modification, for South Boulder Creek floodplain management efforts in the reach from
upstream of State Highway 93 to just below South Boulder Road.

For the purposes of our analysis, the 100-year storm hydrology, originally prepared by the
U.S. Army Corps of Engineers (Corps) and used in the G&O FHAD was assumed. We
obtained a copy of the Corps' model and were able to determine a more detailed distribution
of the flow from the tributaries, such as Viele and along the floodplain. The hydraulic
models used in our analysis were largely based on current topographic conditions including
the Flatirons Pit area and levees.

It is important to note that the G&O and Love studies were done for different purposes. The
G&O study was a general floodplain delineation study of South Boulder Creek from the C&S
Railroad upstream into the canyon. The Love study was intended to identify conservative
flood hazards associated with the Flatirons gravel mining property, as a part of due diligence
investigations initiated by the University of Colorado (CU). Most of Love's effort focused
on the short reach from the upstream side of the Flatiron's Levee to Highway 36.

The Love study identified a spill from South Boulder Creek that could overflow U.S. 36 into
existing neighborhoods on the west side of the South Boulder Valley. Previous studies
(G&O, R.W. Beck, Corps of Engineers) reflected the potential for this, but did not include any
details on flows, probabilities, areas of flooding, and the potential hazards.

BACKGROUND

Prior studies don't adequately explain the hazards of the changing alluvial floodplain. South
Boulder Creek historically created an alluvial fan forming a wide valley bottom over a mile

Hydrology. Hydraulics. Sediment Transport .
CIVil Engineering for Flood Control. Waterways, Water Supply and Parks.
Environmental Mitigation . ForenSIC Services .