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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The 1969 Corps study "Flood Plain Information, Boulder and South Boulder Creek"
used these data (through 1 969) as its basis for hydrology using a Log-Pearson Type
III statistical methodology. A 100-year flood' of 5000 cfs was recommended for
South Boulder Creek.

The 1973 R.W. Beck study "Major Drainageway Planning, South Boulder Creek"
used the 1 969 flood event data, including rainfall and the Eldorado gage data to
determine unit hydrograph coefficients. Then they used NOAA rainfall statistics,
assumed a design rainfall event below Gross Reservoir, and routed subbasin
hydrographs down the stream. This technique allowed prediction of discharges
along the stream as various streams joined in. Their 100-year flood estimates for
South Boulder Creek varied from 4300 cfs at Eldorado Springs, 5300 cfs at
Highway 36, and 5000 cfs above the confluence with Boulder Creek. Based on
assumed 1990 development conditions the flows increased to 5600 cfs at
Highway 36 and 5400 cfs above the confluence, with peak discharges from
Marshall Gulch of 2300 cfs and Viele Gulch of 900 cfs.

Thus, the studies support a 1 OO-year discharge at Eldorado Springs in the 4300 to
5000 cfs range. However the Beck study assumed a constant discharge from
Gross Reservoir of 300 cfs, rather than analyzing a rainfall runoff flood above
Gross.

The latter Corps hydrology, which was used in the 1986 G&O study, was based on
the SWMM computer program. In that model, they assumed rainfall flooding in the
entire watershed and routed the flow through Gross Reservoir, assuming the
reservoir was full to the spillway. G&O reports a 1 OO-year flood at Eldorado
Springs of 4800 cfs, which is similar to the 5000 cfs 1969 Corps statistical
determination.

EXAMPLE FLOOD EVENTS

Two events are of particular interest, August 31 to September 4, 1938, because it
is the largest historical event on South Boulder Creek and gives graphic evidence of
the stream's capability to erode, scour, and move sediment and debris; and May 4
to 8, 1969, because it is a recent event, is of frequent recurrence interval, and
actually flooded Highway 36.

31 AUQust - 4 Seotember 1938

The following excerpts from the Corps' 1 969 report described the event:

"This storm produced general rains over all of eastern Colorado. The
largest amounts of precipitation occurred in the mountains where over 6
inches was reported west of Eldorado Springs. Boulder reported 3.62

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