C153804 Feasibility Study

• Contract/PO #: C153804
• Contractor Name: Left Hand Ditch Company
• Contract Type: Loan
• Water District: 5
• County: Boulder
• Bill Number: HB 95-1155
• Loan Projects - Doc Type: Feasibility Study

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to elevation 5,359.2. All flood flows up to the one in 100 year flood would be carried by the

existing spillway exclusively. The north dam/spillway would be overtopped only for storms with

recurrence intervals greater than one in one hundred years.

Providing the same protection, without raising the south dam, is accomplished in Plan D-2
which will have the RCC overtopping section on the north dam constructed to an elevation of
5,353.2 feet. A fuse plug with a crest elevation of 5,354.6 will be constructed on the RCC spillway
floor. The RCC section will be low enough so that the head available between the RCC floor and
the top of the south dam at elevation 5,358 will be sufficient to pass the design flood through the
reservoir without overtopping the south dam.

The RCC overlay for both plans will be constructed to sound rock at the abutments and along

the toe of the north dam, thus protecting the overlay from undercutting. The cutoff to the bedrock

at the toe of the dam can be constructed from RCC unless the excavation is too deep in which case

a concrete curtain wall will be constructed underneath the first RCC layer. Because of the protection

afforded by the RCC contact with the bedrock around the perimeter of the dam there will be no need

for a traditional stilling basin. A certain amount of riprap protection will be provided at the toe to

reduce nuisance erosion from small overflow events and rain falling on the face of the dam.

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