July 17, 2003
A salmon-friendly solution on the Snake
By M. REECE VOSKUILEN
Often described as the Academy Awards of engineering, the ACEC competition is a showcase of the nation’s engineering accomplishments. A few of the previous winners include the I-205 Columbia River Bridge, the Vandenburg Space Shuttle Complex, the Fort McHenry Tunnel and the St. Louis Metrolink project.
The top project this year was not a mega sports complex nor a major bridge structure — it was a fish passage device at Lower Granite Lock and Dam in Eastern Washington designed by Jacobs Civil. It’s a project that few people will ever see.
The challenges encountered for the Lower Granite Lock and Dam removable spillway weir (RSW) project were significant. We had operational constraints imposed by possible flood conditions. We had an unusual structural shape dictated by a combination of biology, hydraulics and structural limitations. We had a massive and unusually shaped structure to build and then transport hundreds of miles from its fabrication site to the project.
All of this had to be done on an aggressive schedule.
The massive RSW structure is constructed from nearly 2 million pounds of steel plate, cut and formed to create the desired geometry and to mesh smoothly with the existing dam structure. It is over 100 feet tall, 83 feet wide and nearly 60 feet from upstream to downstream.
The RSW was built in Vancouver at Thompson Metal Fabricators. Dix Corp. of Spokane was the general contractor.
Despite weighing more than two fully loaded 747-400’s, the structure was floated to the project site using two tugs. The journey was completed in about four days.
Because the new weir partially blocks the spillway and reduces the flood handling capacity of the dam, it was necessary to design a way for it to be removed.
The solution was a unique but simple method for removing the structure from the spillway ahead of flood flows. The massive structure attaches to the dam with large hinges connected to supports deep in the reservoir. Using a complex system of internal air ballasting in watertight internal compartments, the structure was designed to rotate back into the reservoir to a fully submerged position to restore the full spillway capacity.
In the lowered position, the RSW rests on a landing pad constructed and precisely placed in water over 100 feet deep.
Testing of the completed RSW during the 2002 spring migration documented the effectiveness of the structure’s unique design with better than expected fish passage results. Test data for 2003 has been collected and is currently being processed.
The success of the RSW at Lower Granite has led the Army Corps to consider other potential sites where RSWs might simultaneously improve fish passage and water usage.
The goal of restoring salmon populations in the Northwest, while balancing human needs has been, and continues to be, a difficult task. A unique and creative engineering community is required to be able to adapt designs to the biological requirements of fish. In the process, engineers have become an instrumental part of fish restoration efforts. This has resulted in enhancement of not only fish populations, but also the stature of the engineering profession as stewards of the environment.
The Lower Granite RSW has served as an example of the creative application of engineering to a unique project and gives us some optimism that the challenges that face us are not insurmountable.
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