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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SECTION II

SOUTH BOULDER CREEK WATERSHED

GENERAL FEATURES

There are numerous features which influence South Boulder Creek flows arriving at
the Highway 36 Bridge.

There are approximately 93.2 square miles of mountainous watershed above Gross
Reservoir, ranging in elevation from about 7300 feet to the Continental Divide
(11,900 to 13,294 feet M.S.L.). Flooding at these altitudes is often dominated by
, snowmelt runoff. Rainfall data is sparse and the NOAA Atlas statistics are less
reliable at these elevations.

Gross Reservoir captures much of the basin runoff and stores it with diversions
from the west slope. The west slope diversions are reduced when the reservoir is
nearly filled. Practically, most flood runoff events would probably be trapped for
later release. Even with a conservative assumption that the reservoir was full at
the beginning of the flood, as made by the Corps and herein, the spillway and
reservoir characteristics would greatly attenuate inflows and delay the upstream
peak flows from joining downstream peak flows.

Downstream of Gross Reservoir to Eldorado Springs there are approximately 19.4
square miles of steeply sloping mountainous watershed ranging in elevation from
5720 to 8900 feet M.S.L. The Eldorado Stream Gage is about one mile upstream
of Eldorado Springs. There, the tributary area below Gross is 18.2 square miles.
This area can experience significant extreme rainfall with resultant large peak flows.

South Boulder Creek emerges from the mountains at Eldorado Springs and flows to
Highway 93, gaining another 7.9 square miles of tributary area (27.3 square miles
below Gross) of steep watershed ranging from 5400 to 8760 feet.

South Boulder Creek begins to have alluvial fan characteristics above Highway 93.
Modest flood flows can spill from the main channel, which is generally on the east
side of the valley, and travel along a west overflow path with two irrigation ditches
(hereafter referred to as the West Valley Overflow). One of the ditches is the Dry
Creek Ditch No.2, which travels a separate 6 mile route before rejoining South
Boulder Creek. Ground elevations in the valley are often lower west of Dry Creek
Ditch No.2, therefore, flood flows can be carried west of this Ditch.

From Highway 93 to Highway 36 another nine square miles are potentially tributary
to South Boulder Creek at Highway 36 (36.3 square miles below Gross), ranging in
elevation from 5350 M.S.L. to 8461 M.S.L. at Bear Peak. The character of the

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