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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4. REPLACEMENT OPTION

Before covering the different alternatives, the advantages and disadvantages are
discussed as to why a complete replacement may be necessary. A complete replacement
ofthe channel will include the demolition and disposal ofthe existing channel and new
construction. Complete replacement will include all appurtenances that will improve the
existing channel both in conveyance capacity and safety. It should also provide other
structures that will help future restoration projects. This replacement option will start
from the channel upstream intake to the downstream outlet structure. The removal ofthe:
rail road bridge would be recommended. New construction ofthe channel will utilize
today's construction materials and practices.

The advantage to a complete replacement is that it would provide the conveyance
capacity for the design flood. It ensures that any foundation concerns are addressed and
lining material would be maintainable. A replacement would include the use of
reinforcing steel, concrete air entraining additives and high compressive strength
concrete. Steel reinforcement allows the concrete to adequately stand temperature
variations and helps keep cracks to minimal widths. Air entrained concrete is the use of
additives in the concrete mix to form microscopic voids which aid by absorbing the
thermal expansion and contraction ofthe concrete in a freeze-thaw cycle. The high
compressive strength mix design of concrete will resist abrasion from sediment. As
noted by the past inspection reports and their conclusions, these problems are the
contributing factors that cause concerns about the existing channel potentially failing
during the design flood flow.

A disadvantage of a complete replacement is the initial capital cost of the demolition and
construction. Demolition cost would include the labor, equipment time, hauling, and
disposal of the material. Because ofthis particular site, another replacement
disadvantage is the limited construction access due to encroached buildings, other
structures, and utilities such as overhead power and communication lines and propane gas
tanks. Construction access is necessary for the construction of a temporary water bypass
system for Willow Creek. Access room must also allow for the positioning of equipment
and stockpiling of materials. Construction easement and access were probably an issue
during the initial construction ofthe existing channel, but growth and development of the
town since then has challenged any complete replacement.

The areas not covered in this report as part ofthe complete replacement option are the
upstream diversion embankment and the concrete cutoff sill structure. However, a new
transition would be designed and constructed between the concrete sill and the
appropriate design channel alternative. Other areas where replacement construction
would not impact would be the vehicular bridges located at Wall Street, Third Street,
Fifth Street and Colorado State Highway 149. The bridges would remain in place during
construction because the abutments for the bridges sit on separate foundations. Caution
must be exercised during construction excavation in these areas so that the abutment's
structural integrity is not compromised.

NRCS Northern Plains Engineering Team 4