Huge Weir Arrives at Dam
by Andy Porter
Walla Walla Union-Bulletin, October 24, 2007
LOWER MONUMENTAL DAM - After some 280 miles of smooth sailing, a 1,000-ton aid to salmon migration reached its new home here Tuesday.
Watched by a small crowd of U.S. Army Corps of Engineers officers and employees, a 10-story high removable spillway weir was slipped through a navigation lock and into the lake above with only a minor, although loud, scrape on a concrete guide wall.
During the next three days divers working 120-feet below the lake's surface will fasten the massive structure into place behind one of the dam's eight spillways, said Cary Rahn, project manager.
Once in place, the weir will allow migrating juvenile salmon and steelhead to reach the spillway and slide over it at fairly shallow depths. As originally built, the spillways forced juvenile fish to dive to 50-60 feet to find a passage route, subjecting them to large pressure changes and rapid accelerations.
While aiding juvenile fish passage, the weir also allows dam operators to continue normal operations, including hydropower generation, navigation and irrigation.
"This is where we've taken a structure that wasn't designed to take water over the top and designed something that allow fish to go over it," Rahn said.
The weir is "removable" because, if necessary, it can be lowered to the reservoir bottom so the spillway can be opened to allow maximum flow during a major flood event.
The $15-million structure began its journey Friday as it was floated into the Columbia River in the Portland area where it was built.
Pulled by the tugboat Noydena and pushed by the towboat Clarkston, both owned by Foss Maritime Co., the weir traveled up the Columbia and into the Lower Snake River, arriving at Lower Monumental Dam ahead of schedule Tuesday.
The Lower Monumental Dam weir is the third one installed on Snake River dams operated by the Corps. The first was placed at Lower Granite Dam in 2001 and the second in 2005 at Ice Harbor Dam.
A fourth removable spillway weir is scheduled to be installed at Little Goose Dam in 2009, according to the Corps.
Although the massive structure drew sightseers as it floated up the Columbia and Snake rivers on its journey, when it goes to work, there won't be much to see, Rahn said.
"Once this is in place, it's like an iceberg. Eighty percent of it is out of sight," he said.
Prime contractor - Advanced American Construction, Inc., fabricated by Oregon Iron Works.
Cost - About $15 million.
Dimensions - 120 feet high (10 stories), 80 feet wide, 70 feet deep and weighing 2 million pounds (1,000 tons).
Attached to dam by - 150 bolts, each 1 inches in diameter, inserted into flanges 120 feet below the surface.
WHAT'S A `WEIR'?
It's a very old word that originally described a low dam built to back up water or a fence in a stream intended to catch fish. A third definition is "an obstruction placed in a stream, diverting the water through a prepared aperture for measuring the rate of flow." Etymologists have traced the term's roots back through Middle and Old English, Old High German, Old Norse, Gothic, Latin, Greek and Sanskrit.
(Sources: Webster's Third New International Dictionary and Webster's New World Dictionary)
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