C150009 Feasibility Study

• Contract/PO #: C150009
• Contractor Name: Boulder & Left Hand Irrigation Company, The
• Contract Type: Loan
• Water District: 5
• County: Boulder
• Bill Number: SPL
• Loan Projects - Doc Type: Feasibility Study

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Boulder and lefl Hand Repair of Outlet Works
CWCBFeasibilityStudy
NovemberS, 1999

3. Replace the Outiet Works and Construct a New Spillway:

Replacing the existing outlet works will address the requirement of not losing discharge release
capacity, and constructing a new spillway will address the requirement of passing the design
inflow flood without the dam overtopping,

Various options for replacing the outlet works and constructing a new spillway were
considered, These included three outlet pipe options, two spillway options, and two options
for the stabilization of the dam embankment at the outlet structure where the Ditch Company
plans to construct a vehicle turnaround area,

The three outlet pipe alternatives examined included steel pipe, reinforced concrete pipe, and
high density polyethylene pipe, A 30-inch diameter pipe was investigated for each of these
alternatives,

The spillway options included the construction of a knee-wall and concrete spillway over the
embankment. This alternative is technically feasible, but the associated costs make this option
unacceptable to the Ditch Company, The other option evaluated was construction of a drop-
inlet spillway tied directly to the new outlet works, This option is technically feasible and the
costs are considered acceptable, The sizing of the drop-inlet spillway and associated flood
routing were analyzed in the May, 1999 Hydrology Study performed by RMC for the Ditch
Company and approved by the SEO on June 22, 1999,

A reinforced earth wall and a concrete cantilever retaining wall were investigated as the two
options for stabilizing the embankment at the vehicle turnaround area,

Cost Estimates of Feasible Alternatives

Costs were estimated for the replacement of the outlet works and the construction of a pipe spillway,
Estimated outlet works costs for steel, reinforced concrete pipe (RCP), and high density polyethylene
pipe (HDPE) are presented on Table 4, Construction cost estimates for the installation of a drop-inlet
spillway and new outlet works are presented on Table 5, Also included on Table 5 are the costs to
raise the dam crest as recommended in the Hydrology Study performed by RMC. Construction cost
estimates for the installation of a vehicle turnaround area at the outlet structure are presented on Table
6, Despite the slightly higher initial costs, the drop-inlet spillway with a steel outlet pipe is the
preferred option over the RCP and HDPE options, because of its durability and ease of maintenance,
Thus, the steel outlet pipe may, in the long-term, provide a cost savings to the Ditch Company,

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