Corps awards removable weir for 810-MW Lower Monumental

The Corps of Engineers’ Walla Walla District has awarded a $15 million contract to Advanced American Construction Inc. to build a surface-bypass structure for 810-MW Lower Monumental Dam.

Oregon Iron Works, a subcontractor to Advanced American, is manufacturing the structure, a removable spillway weir. (HNN 2/24/06) It is to be delivered to the dam site and installed by Advanced American, of Oregon City, Ore., in February 2007.

It will be the district’s third removable spillway weir, and is intended to improve passage conditions for migrating juvenile salmon and steelhead in the lower Snake River.

A prototype removable weir was installed in 2001 at 810-MW Lower Granite Dam. The second weir was installed in 2005 at 603-MW Ice Harbor Dam.

The weir passes juvenile salmon and steelhead over a raised spillway crest, similar to a waterslide, and uses less water than traditional spill methods, the Corps said. The fish pass the dam near the water’s surface under lower accelerations and lower pressures than if they passed the dam below raised spillway gates.

The weir is designed to be �removable� by controlled descent to the bottom of the dam forebay. The structure is hinged to allow rotation to the river bottom for removal, making it possible for the Corps to return the spillway to original flow capacity during major flood events.

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