PROJ02027

• Contract/PO #: FS0044FX
• Contractor Name: Larimer County, Board of County Commissioners
• Contract Type: Miscellaneous
• Water District: 0
• Loan Projects - Doc Type: Feasibility Study

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SECTIIIFIVE

Flood 1III0UoO lod Ilvenloo

The purpose for the flood detention and diversion scheme is to reduce the flood flows within Dry
Creek to a rate of flow that will allow more options for diversion. The section describes the
alternatives considered for the flood detention and diversion scheme.

5.1 DOUGLAS DAM

Douglas Dam and Reservoir are located on Dry Creek near the middle of the watershed. As
such, any flood waters generated in the upstream watershed must pass through the emergency
spillway structure assuming the reservoir is full. Douglas Dam and Reservoir was designed for
storage of irrigation water. The existing structure attenuates flood flows to some degree,
however, additional storage in the reservoir could further attenuate peak flood flows and
decrease flood discharge downstream.

5.1.1 Conceptual Design of Douglas Dam Fuseplug

The presence of the dam and reservoir on Dry Creek allows for existing attenuation and storage
of upstream floodwaters. A zoned earthfill fuseplug at the Douglas Dam spillway would allow
for additional storage of floodwaters in the reservoir during extreme runoff events. The addition
of a fuseplug structure would further increase the ability of the structure to attenuate Dry Creek
flood flows without compromising the safety of the dam.

A fuseplug structure is typically an earthen or mechanical structure which adds additional
freeboard to the crest of an emergency spillway but is designed to fail in case of extreme flows.
The current emergency spillway of Douglas Dam has been designed to pass 75 percent of the
Probable Maximum Flood (pMF') or 39,750 cfs. As such, in order to attenuate flows up to the
lOO-year frequency event, a fuseplug approximately-3 foot high would be required. This height
is based on a hydrologic model incorporating the routing of the upstream hydrograph through
reservoir and spillway. '

The preliminary conceptual design of a segmented earthen fuseplug structure for Douglas Dam is
provided on Figure 5-1. In general, the design consists of dividing the fuseplug spillway into
three (3) sections by constructing two (2) cast-in-place concrete piers. These piers will be
designed to be stable under the design flood and to minimize hydraulic impacts on the ultimate
capacity of the spillway. The first section will remain at the existing grade of the spillway crest
and will therefore not affect the normal operating water surface elevation within the reservoir.
Also, the more frequent flood flows occurring upstream of the reservoir (up to and including the
100-year frequency inflow design flood) will pass directly through this opening. The second and
third sections will have earthen fuseplug structures constructed to different heights at the existing
spillway crest. The second quadrant will be constructed to a maximum height of 3-1/2 feet
above the existing spillway crest. An approximately 10 foot wide cut in the fuseplug section is
provided so that erosion of the structure will begin for depths greater than 3 feet. A similar
fuseplug will be constructed in the third quadrant to a depth of 4 feet. This staggered fuseplug
effect will minimize the magnitude of the release rate of the reservoir should one of the fuseplugs
be overtopped, while ensuring that the evacuation time of the reservoir is not affected. Access
will be accommodated along the upstream side of the fuseplug structure.

The design of each fuseplug section includes four (4) earthfill zones. The four zones will be
designed to minimize seepage through the structure while providing for controlled erosion of the

T:\Projects\68246600ry CrkDiversionLarimerCounly\Sub_00\12,DWord ProcIFIood Control Pfan-Rptdoc 04/24100(11:14 AM) 5-1