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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3. The Leggett Canal to Val mont, which can receive headgate overflows and
convey flood waters to Boulder Creek

KEY FLOOD CONTROL FEATURES

There are numerous features which presently control and/or potentially can change
and affect the distribution of South Boulder Creek floodplain flows. For purposes of
simplification and organization these can be grouped as follows.

The UDDer or Hiahwav 93 SDlit (See elements 1423 and 1413 and Drawings 4, 5,
and 9.)

Two splits redistribute a major portion of the South Boulder Creek flow to the west
side of the valley at Highway 93. There is a complex of several facilities and
topographic characteristics, including the South Boulder Ditch and Dry Creek Ditch
No.2 diversions, low main channel capacity and flow characteristics, Highway 93
bridge capacity and the downstream Marshallville Ditch diversion, and the profile of
the local access road which controls much of the overflow hydraulics and
interaction with the spills just above Highway 93.

It is important, for the purposes of floodplain management, to realize that changes
of anyone component are unlikely to effectively reduce the spill. For example, if
one enlarged the Highway 93 bridge over the main channel in order to decrease the
spill, the upper portion of the net spill at the ditches would probably increase to
somewhat negate the improvement.

The capacity of the double box culvert on the west side of the valley will be
significantly exceeded during the 100-year event. The 93 roadway will be
overtopped to the east of the box culvert because of the narrowing valley section.
This, coupled with the cross slope of the super elevated road, should direct most
of, if not all, of the flood waters to the west valley floodplain below, and not to the
north along Highway 93, except for the local irrigation lateral and drainage ditch
culvert on the west side of Highway 93.

FloodDlain Characteristics between Hiahwav 93 and the Flatirons Levee (Drawing 5)

For the most part, significant South Boulder Creek flood events are carried in two
paths, one on the east side of the valley in the main channel, and the second,
carrying the majority of extreme floods, in the left or West Valley Overflow route
along the South Boulder and Dry Creek No.2 ditches. Generally the west side of
the valley is lower, except where the flow approaches the Flatirons Levee and is
diverted east to the main channel. Because of the elevation differential, and
locations where pits have been created in the floodplain, there is some risk that