FLOOD00385

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Title: South Boulder Creek Interim Hydrology Study
• Date: 7/1/2000
• Prepared For: Boulder County
• Prepared By: Taggart Engineering Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Creek just upstream of Highway 36 overflow to the West Valley, and potentially
flood the urbanized area downstream.

South Boulder Creek downstream of Highway 36 to Arapahoe Road is largely
natural with adjacent housing downstream of Baseline (Drawings 6, 7, and 8). The
floodplain downstream of the Wellman Ditch is developed and is the location of
another split across the golf course to the West Valley (Drawing 7). While the
stream is natural here, it shows signs of aggregation from sedimentation,
overgrowth, and resultant loss of capacity. Downstream of Arapahoe, South
Boulder Creek is largely channelized.

The West Valley Overflow route downstream of Highway 36 is urbanized. The area
is largely residential, with some commercial and office space near Foothills and
Baseline and along and downstream of Arapahoe. Flood waters take many routes
as illustrated on the drawings. The main overflow path starts downstream of
Highway 36 on the west side of Foothills and travels northerly to the intersection of
Baseline and Foothills.

At the intersection of Baseline and Foothills the flows split in seven directions.
Three paths follow Foothills to the Wellman Canal. Some of this flood water enters
the Wellman, but significant portions of the two roadway flows will largely leave
the basin and enter Bear Canyon Creek. At the same time, Wellman Ditch flows
from the west will enter as a transbasin flow, but the model, per contract and
standard UDFCD practice, assumes none. There are probably some offsetting
effects. That is, the Wellman brings flow in from Bear Creek while the flows on
Foothills go to Bear Canyon.

Two of the flows from the Foothills and Baseline intersection follow Baseline. The
last two are conveyed over and under Baseline, and join the Baseline flows after
going through a minor storage, except for a small portion that crosses under
Baseline to the drainage ditch on the north side, referred herein as the Crossover
Ditch.

Spills from Baseline travel northerly through the Country Club Park, Centennial
Trails, Arapahoe Ridge, and commercial areas to the north. The Wellman then
receives most of these flows. While it has a significant cross section, its slope is
flat, resulting in limited capacity which cannot convey significant flood waters.
Spills would begin on the east side of the golf course, but as the flood discharge
increases, sizable spills would occur to the west of 55th. Much of the flow on the
west side of 55th will spill northerly to Boulder Creek through the commercial area.

The overflows on the golf course drain to Dry C~eek Ditch No.2, which is a large
channel to the north of Arapahoe, then join South Boulder Creek flows and
discharge to Boulder Creek.

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