Construction workers blasted a hefty notch out of the Glines Canyon Dam on the Elwha River on Saturday afternoon, resuming work on the teardown of the dam after a year’s hiatus.
Work to remove the dam on the Olympic Peninsula was suspended in October 2012 because heavy sediment from the increased river flow was clogging downstream water-treatment facilities.
Those facilities were upgraded earlier this year with heavy-duty gravel pumps capable of handling the sediment. So, following the end of the strong summer chinook salmon run in September, the effort to take down the dam piece by piece has resumed.
Explosives placed in about 70 holes drilled into the 25-foot-thick concrete blasted a hole 28 feet high and 40 feet wide on the right side of the dam, dropping the dam’s level 10 vertical feet below an earlier notch on the left side.
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“It went off exactly as was designed,” said Aaron Jenkins, dam-removal project superintendent for Bozeman, Mont.-based Barnard Construction. “The blast pushed most of the rubble downstream, where we wanted it.”
To remove the dam in stages, workers are taking out successive notches on either side so the flow is diverted alternately to the left and right sides.
With 150 vertical feet of the 210-foot dam already removed, the lake behind the dam has been drained for the past year, leaving only the river flow through the dam and a great mass of sediment behind the dam wall.
Barb Maynes, spokeswoman for Olympic National Park, said the lowering of the dam Saturday will not increase the water flow, but it will boost the amount of sediment carried in it.
The work on the Glines Canyon Dam and the 108-foot Elwha Dam farther downstream is the largest dam-removal project in U.S. history and is intended to restore salmon runs.
Crews can do the blasting only during certain time windows free of fish runs. This month is one window. The next will be January through April.
Water-treatment facilities, completed in 2011, were designed to divert and clarify water clouded with high levels of sediment by the dam removal.
The water supplies the city of Port Angeles, as well as a paper mill in the city, a salmon-rearing channel used by the Department of Fish and Wildlife and a fish hatchery run by the Lower Elwha Klallam Tribe.
But last fall, the water-intake part of the facility, intended to filter out larger debris, was constantly clogged with rocks, gravel and sand, requiring manual removal that slowed and reduced the output of clean water.
Only about half of the sediment projected to eventually flow downstream has been released so far.
To solve the problem at the water-treatment facilities, the National Park Service paid to install two gravel pumps of a robust design more typically used to sift gravel for gold mining in Alaska.
Maynes said the pumps functioned well during the recent rainstorms, which produced much higher water flows in a matter of hours. She said engineers believe the intake facility is now ready to cope with further lowering of the Glines Canyon Dam.
“We feel very confident it’s ready to handle the increased sediment,” Maynes said.
She said the scientists monitoring the movement of the sediment recommended taking the dam down another notch now, in advance of heavy fall and winter rains, to minimize the release of large-scale sediment and ease the load on the water-treatment facilities.
The Elwha Dam was fully removed in March 2012. After Saturday’s blasting, about 50 vertical feet remains of the Glines Canyon Dam.
Jenkins said Saturday’s blast will, over the next few days, dry out the left side of the dam, and his crew will blast there in January.
He said three more blasts early next year will be needed to complete the demolition of the dam wall. Afterward, his crew will clear rubble from the bottom with an excavator.
Dam removal is scheduled to be finished by next September.
Dominic Gates: 206-464-2963 or firstname.lastname@example.org