WSP11761

• File Number: 8441.400
• Description: Colorado Big Thompson Project - Reports
• State: CO
• Basin: South Platte
• Water Division: 1
• Date: 4/1/1987
• Title: Joint-Use Reservoir and Green Mountain Exchange Projects - Summary Report
• Water Supply Pro - Doc Type: Report/Study

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en UNA DAM AND RESERVOIR: The Una Dam Site is located on the Colorado River at the.
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tv Mesa-Garfield county line. Itlies between the towns of Parachute and DeBeque, 35 miles northeast
of Grand Junction. Both dam abutments rise steeply from the 1100-foot wide valley floor. The
reservoir would extend 9.7 miles up the Colorado River valley to the town of Parachute. The
reservoir was limited to 150,000 af to keep the water surface below elevation 5060 feet and minimize
reservoir encroachment in Parachute.

The dam would be founded on a thick $equence of sandstone and claystone lenses. Both an

earth embankment dam and an RCC gravity dam were considered for the Una Site. The controlling

design parameter was the large spillway capacity required to pass the estimated PMF of 340,000 cfs.

The RCC gravity dam design allows most of the crest length to be used as a spillway. Approximately

9 miles of railroad and 7 miles of highway would have to be realigned along the reservoir.

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Hydropower Plant: The substantial river flows and the drop in elevation over the dam

indicate a maximum power generation potential of 17,600 kW. Two Francis turbines operating on

flows bypassed or released to meet downstream requirements and flows which would otherwise be

spilled would produce an average of 88.5 million kWh per year. The relatively low estimated cost of

power production, presented in Table S.3, indicates the potential for an attractive financial rate of

return.

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CONVEYANCE SYSTEM

The conveyance $ystem is the pumpback element of the Green Mountain Exchange Project.

It Is a pump and pipeline system that would pump water from Green Mountain Reservoir to Dillon

Reservoir. Both reservoirs are located on the Blue River, a tributary to the Colorado River. Green

Mountain Reservoir is located 26 miles downstream of Dillon Reservoir. The difference in' water

surface elevation between the two reservoirs Is 1070 feet.

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The selected pipeline route follows the west shore of Green Mountain Reservoir and west

side of Colorado Highway 9 to the town of Silverthorne. It would cross under Interstate 70 alongside

the Blue River and proceed to a below-ground terminal near the east abutment of the dam at Dillon

Reservoir. Using this route would allow the installation of the pipeline in the highway right-of-way

where practical and would requIre only one crossing of the Blue River. A map of the route is

presented In Figure S.2.

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