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

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2480 West 26th Avenue. Suite 340-8 . Denver, Colorado 80211 . [phone] . FAX [phone]

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March 19, 1997

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

RE:

Initial Reconnaissance of Drainage Network and South Boulder Creek's
Hydrology, Highway 36 to Approximately the Wellman Ditch, Phase II.

Dear Mr. DeGroot:

Our Phase I effort identified that the Flood Hazard Area Delineation Report
(FHAD) underestimated the flow leaving South Boulder Creek at Highway 36 and
did not portray spillage paths to the west side of the valley, particularly along
Foothills Parkway. This Phase II report provides an initial reconnaissance of the
overflow routes and approximate flows of the West Valley Overflow north of
Highway 36 to the area of the Wellman Ditch. Drawing No.3 depicts the results
of this overflow analysis (Drawings 1 and 2 are with the Phase I report).

We obtained and reviewed additional hydrology information by the U.S. Army
Corps of Engineers (Corps). In Phase I we modified the Corps' Storm Water
Management Model (SWMM) model to better reflect the Viele watershed and its
outfall channel to South Boulder Creek. In this Phase II we have greatly refined
the SWMM model detail downstream of Highway 36 in order to identify
significant flow paths and approximate discharges.

The 100 year spill from South Boulder Creek just upstream of Highway 36 would
flow to the northwest along the south side of the freeway, with an initial portion
returning to South Boulder Creek through the Viele outfall, spillage from the Dry
Creek Ditch No.2, and the Anderson Ditch. The majority of the spill would
overtop Highway 36 west of Foothills and flow northerly to the intersection of
Baseline and Foothills. The intersection topography would largely control the
hydraulics of the spill distribution and would result in six surface flow paths and a
seventh path through the culverts under Foothills between the small detention
ponds that are south of Baseline on either side of Foothills. Three of the surface
paths would follow Foothills to the north (two travel lanes and the westerly
borrow ditch). Two would follow Baseline to the east, and one would overtop
Foothills slightly south of the intersection. The last three routes would
commingle and split again, following the streets to the north and the Crossover

Hydrology' Hydraulics' Sediment Transport .
Civil Engineering for Flood Control, Waterways, Water Supply and Parks'
Environmental Mitigation' Forensic Services'