FLOOD10417

• County: Mineral
• Community: Mineral County
• Stream Name: Willow Creek
• Title: Willow Creek Channel Improvement, Mineral County, Colorado - Channel Replacement Feasibility Report
• Date: 8/13/2003
• Prepared For: Willow Creek Reclaimation Committee
• Prepared By: Chou Cha, P.E., Civil Engineer
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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which indicated the original channel in dashed lines. This width is approximately 16 feet
wide. '

The disadvantage of this option is that fencing would need to be erected along both sides
for safety protection. The length of such fencing would be from the beginning of the
channel to the end of the channel. Only at locations of pedestrian bridge crossings lmd
vehicular bridge crossings would the safety fence be interrupted. The costs of additional
earthwork to excav~te, backfill, and compact the backfill for this channel shape is y,et
another disadvantage. This design has a lower capacity of passing debris that may now
in the channel versus Alternative 1, the trapezoid shape. The 16 feet top width will
accommodate most debris but is less width than the trapezoidal shaped channel that
varies in width between 24 feet and 28 feet. This could pose as a maintenance issue and
would require inspection during any flood event to make sure no debris flows build at th~:
bridge crossings.

The cost ofthis altemative is estimated at $3,040,000 that includes the fencing mate:rial,
but excludes appurtenance materials. The bypass construction cost is less due to room to
design and construc~ a water bypass system within the existing width ofthe channel thus
eliminating the need for a pumped bypass system.
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6.3 ALTERNATIVE 3 - PARTIALLY FULL FLOWING PIPE

This alternative incl\ldes large diameter pipes that will accommodate the design
discharge. The most appropriate pipe material that is considered for this alternative is a
precast concrete pipe. Other pipe materials have been considered but were determined to
offer less resistance to corrosion and erosion and are not readily available in the larger
diameters. As preli~inary sized, a ten feet diameter pipe will accommodate the design
flood discharge. The advantages and disadvantages of the design are as presented in the
following paragraph~. Preliminary design drawings can be referenced in Figure A4 in
Appendix A.

The advantage of this covered alternative is the elimination of the pedestrian bridges,
safety fences, and safety cables. Some safety fencing and equipment may be needed in
the upstream intake of this alternative to prevent any illegal access and possible upstream
rescue before enteriqg the pipe. The efficient cross section of this alternative is smaller
than Alternative 2, the rectangular channel section. This is an added advantage, as this
will provide additional room for the construction of the water bypass system.

The major disadvant~ge to this design is the high potential for clogging with debris. The
design pipe diameter will accommodate the floodwater, however, the efficient pipe
section may not allow for large debris to safely pass. This is a factor that could result in
the flooding ofthe town of Cree de if the debris were to plug at the upstream end. Once a
debris plug occurs t~e water will not be able to return to this conveyance system and. it
could result in the flooding ofthe town. Another disadvantage is construction access to
install such large diaineter pipes due to the weight ofthe pipe sections. Large cranes will
be required to install.the pipes. Because of the tight access room around the channell

NRCS Northern Plains Engineering Team 8