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 Left Hand Repair of Outlet Works
CWC8FeasibilitySludy
NovemberS, 1999

Alternatives Evaluated

A range of alternatives were considered, including: 1) don't repair or replace the existing outlet works
and spillway, 2) repair the existing outlet works and enlarge the existing spillway, and 3) replace the
existing outlet works and construct a new spillway, Several options within the third alternative were
examined, including: evaluation of different outlet pipe materials, evaluation of two different spillways,
and evaluation of two options to stabilize the dam embankment at the outlet structure where the Ditch
Company plans to construct a vehicle turn-around area,

A subjective evaluation of the alternatives follows:

1. Don't build the project:

By not repairing or replacing the outlet works and enlarging or building a new spillway, the
SEO would require the Ditch Company to abandon Twin Lakes, Shareholders would need a
new storage facility for equalizing the ditch flow and replace lost irrigation water. The
associated cost of building a new 218 acre-foot storage facility, which is the deceed capacity of
Davis No, 1 and 2, is valued between $2,3503 per acre-foot and $3,5004 per acre-foot, and
would result in costs to shareholders ranging from approximately $512,000 to $763,000, The
costs to replace storage would be substantially more than repairing or replacing the outlet
works and spillway, and are not considered economically feasible,

2. Repair the Existing Outlet Works and Enlarge the Existing Spillway:

The existing 30-inch diameter CMP is corroded beyond repair, The extent of the corrosion
also puts into question whether the pipe could be slip-lined, Further, if the pipe were slip-
lined the loss in conveyance would not be acceptable to the Ditch Company for current ditch
operations,

The SEO has determined the existing spillway is not adequate to pass the design inflow storm
through Twin Lakes (SEO, June 17, 1998), The existing spillway is located at the western of
the two Twin Lakes, The eastern of the two Twin Lakes is the jurisdictional structure, No
enlargement of the existing spillway structure is possible that would allow passage of the
design inflow storm without water overtopping at the eastern embankment.

Repair of the existing 30-inch diameter CMP and enlargement of the existing spillway are
therefore considered technically not feasible, Thus, this alternative was eliminated from further
consideration,

3

Approximate value per acre-foot that Denver and South Adams County are paying LaFarge Corporation
to develop gravel pit storage on the South Platte River at the Howe-Haller Pit.

4

Approximate value per acre-foot the City of Westminster paid for storage at the Jim Baker Reservoir.

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