FLOOD03253

• County: Boulder
• Community: Boulder
• Stream Name: South Boulder Creek
• Basin: South Platte
• Title: South Boulder Creek Draft Hydrology Study Phase III
• Date: 2/10/2000
• Prepared For: UDFCD
• Prepared By: Taggart Engineering Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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PRIOR HYDROLOGY STUDIESG ~ 0 \

In 1977 the U.S. Army Corps of Engineers (Corps) c Clucted a hydrology study
that included the South Boulder Creek Basin. Tha tudy provided the hydrology for
the Urban Drainage and Flood Control District ( FCD) Flood Hazard Area
Delineation (FHAD) by Greenhorne and O'Mar South Boulder Creek Flood Hazard
Area Delineation}, and the Federal Emergency Management Agency (FEMA) Flood
Insurance Study (FIS) and Flood Insurance Rate Maps (FIRMs) for South Boulder
Creek.

Although Gross Reservoir is not a flood control facility, it does affect flood runoff in
two ways. First, if the reservoir level is lower than the spillway, it will trap, for
later release for water supply purposes (up to 43,060 AcFt.), flood runoff. Second,
even if the reservoir is full, dynamic flood storage will occur above the spillway
level, which lowers the peak flow rate )( downstream and delays the hydrograph
volume from upstream of the reservoir in joining the runoff occurring downstream.
The earlier R.W. Beck study determined that a release of 300 cfs would occur
during the 100-year flood based upon examination of Denver Water Board records
and policy. The G&O FHAD was based on Corps of Engineers modeling which
assumed that the reservoir was full and that the 100-year rainfall flood from above
the reservoir would route through the storage volume above the spillway. The peak . %~cK ~
flow out of the reservoir was modeled as 3900 cfs, which was several hours . .-,'
behind the downstream subbasin times of peak flow. Th~'- roeably
more realistic. The second is conservative, because it follow' nventional
UDFCD, CWCB, and FEMA practice to assume the storage below the spillway crest
is not available for flood routing purposes because it is not owned or controlled by
UDFCD and the Boulder communities and therefore unavailable for flood control.

In 1996, Love and Associates (Flood Plain Analvses of South Boulder Creek at the
Flat Iron Prooertv) identified that South Boulder Creek could spill greater flows than
identified in the FHAD and that these spills could travel to the west side of the
valley and overtop Highway 36, largely west of Foothills and thus posed a hazard
to the neighborhood"north of Highway 36.
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The TEA Phase I effort reviewed the FHAD and Love's investigation, and identified
1 OO-year spills on the order of 2700 cfs from South Boulder Creek above Highway
36. The spill was shown to commingle with local drainage from the Dry Creek
Ditch No.2 and the Viele watershed. The Phase I effort obtained a copy of the
Corps' hydrology model to identify the timing of the Viele inflows and allow
simplified simulation of its drainage network. Some general problems were noted in
the Corps' model, but the effort focused on the determination of how much of the
South Boulder Creek spill would return via Viele and how much would continue to
the West Valley spill route. Phase I determined that about 1800 cfs would
continue to the West Valley north of Highway 36.

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