PLACERVILLE, CALIFORNIA

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Tunnel option: Flumes 42-46 under study

By From page A1 | May 28, 2014

Flumes 42/43 through 46 are being studied for replacement by Carlton Engineering.

The $98,000 contract was approved unanimously by the El Dorado Irrigation District Board of Directors April 14.

The five-year Capital Improvement Plan designates $190,000 total for this study.

One of the options under consideration will be replacing these flume sections with a tunnel. If so, it would make the fourth tunnel section on the 22-mile El Dorado Canal that starts at a diversion dam on the South Fork of the American River and moves up to 85 cubic feet of water per second through canal, flumes and tunnels, eventually terminating at Forebay Reservoir in Pollock Pines.

Called Project 184 by the Federal Energy Regulatory Commission, the canal and diversion dam capture not only some in-stream flow from the South Fork, but also releases sent downstream from four alpine reservoirs. The water not only serves EID customers but also runs a 21-megawatt power plant that generates $8 million of revenue on the average.

In 2001, Carlton Engineering was tasked with conducting a comprehensive assessment of the flumes after EID acquired Project 184 from PG&E in 1999. That assessment was updated by Carlton in 2007. EID staff have annually updated the flume assessment since 2008.

“The majority of the degraded flumes have been replaced with wood, precast concrete, or bypassed with completion of the Mill-Bull tunnel (2002), reducing the total number of flumes down to 24, nine of which remain a high priority of replacement,” the staff report said.

The estimated cost of replacing Flume 42-43 by traditional method is $3.2 million, according to the five-year CIP. Flume 44 is budgeted for $150,000, Flume 45A for $500,000. “With the exception of Flume 46, these high-priority replacement flumes are next on the list to be replaced…” wrote Daryl Noel, water/hydro engineer.

The concern with these flume sections is that a failure of one section would threaten Highway 50 at the same time it would interrupt delivery of one-third of the district’s water supply and halt hydro generation. Flume 42/43 was breached in 2009, causing a landslide that closed Highway 50 and left the district short of water for an extended period of time.

“Flumes 42/43-46 are aligned closely in an area where it may be feasible to construct a tunnel bypass,” Noel wrote.

The five flumes total 3,130 linear feet and are all wood, according to a March 10 summary. Various sections were replaced in 1948, 1956, 1966, 1948/1991 and 1948-2007.

Carlton Engineering will do a geotechnical analysis —  its specialty — and do a cost analysis weighing flume replacement against tunneling.

On May 12, the board approved a consent calendar item to purchase $64,000 worth of custom cut, milled and treated lumber to restock its supply. It will take four to six months for the order to be completed and delivered. It will be used as an emergency reserve and to meet scheduled replacement when the canal is dewatered in the October. This type of construction is done by in-house construction crews.

Planned during the fall outage is replacement of 10 flume boxes and marine plywood to cover 400 feet of degraded cedar lining at Flume 46, according to Noel.

It was last replaced in 1966 and has no year listed for relining that 128-foot-long flume.

Michael Raffety

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