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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SECTION II

FEATURES AFFECTING LOCAL DRAINAGE
AND SOUTH BOULDER CREEK OVERFLOWS
FROM HIGHWAY 36 TO THE AREA OF THE WELLMAN DITCH

GENERAL FEATURES

There are numerous features which influence South Boulder Creek flows arriving at
the area of Highway 36 Bridge over the creek. Some of these are indicated on
Drawing 3 in the back of this report and the watershed figure in Section III.

There are approximately 93.7 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.l. Flooding at these altitudes is usually dominated
by snowmelt runoff. Rainfall data is sparse and the NOAA atlas statistics the least
reliable.

Gross Reservoir captures much of the basin runoff and stores it with diversions
from the west slope. 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, 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 Gauge is about one mile upstream
of Eldorado Springs. There the tributary area below Gross is 18.2 square miles.
Such areas 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 7.6 square miles of tributary area (27.0 square miles
below GrossI of steep watershed ranging from 5400 to 8760 feet.

As documented in the Phase I report, 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 is the Dry Creek Ditch No.2, which travels 6 miles to very near
the South Boulder Creek confluence with Boulder Creek before rejoining South

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