Boise, Idaho – Contractors for the Bureau of Reclamation continue to make progress replacing the 108-year-old Minidoka Dam Spillway on the Snake River, located 10 miles northeast of Rupert, Idaho. The improvement project, begun in 2011, will be a vastly more efficient and safer system.
As of now, the southern of the two gated headworks is complete and operational, feeding water to the Minidoka Irrigation District. Three thousand cubic feet per second was exiting the south gate on this visit. The north gated headworks, which feeds the Burley Irrigation District, is nearing completion.
Much of the construction work this summer is focused on construction of a an overflow spillway between the two gated headworks. This involves the placement of concrete into wooden box frames called forms. It’s a process involving a lot of concrete and labor.
”They’re putting in 10-plus hours working,” says Eduardo Lopez-Owsley, resident engineer for the U.S. Bureau of Reclamation, referring to the RSCI contractors. “Sometimes to get a placement ready, they’ll work until late, or during concrete placements until all of the concrete is placed. We’ve been here sometimes up to midnight placing concrete.”
This effort to connect the headwork gates is slowly moving south from the north gate. Much of the work done on this day was building the second row of blocks. Work is progressing in leap frog motion, with one form placed and then the next two forms down.
The concrete must meet stringent specifications, both coming from the batching plant and in the actual placement itself. It contains larger aggregate and less water than a typical mix. Use of this drier concrete mix reduces the heat generated during cure and therefore diminishes cracking. Third party contactors on site test samples for air and water content, slump factor, and temperature to ensure quality throughout construction.
The original structure consists of 150 gates, each blocked with a dozen wooden 6-inch stop-logs measuring 5 to 6 feet wide. Dam workers had to go out along a walkway to manually lift individual boards to adjust flows and to serve maintenance.
When completed in 2015, flows will be adjusted with the push of a button.
“One of the great benefits of the project is it reduces the labor intensive process of removing and placing stop-logs,” says Brandt Demars, Regional Construction Officer. “In order to control the elevation of Lake Walcott (above the dam), workers would actually have to go out and using a long steel tool, reach in and pull out these heavy pieces of #1 Douglas Fir lumber. It is labor intensive, it's inefficient, and this new spillway structure is really a modern, 21st century design.”
Overall there is a good sense that work is moving well along and Demars feels his is a great project for Reclamation, our stakeholders, and to work with a local Idaho contractor.
“I just see a win‑win situation all around,” said Demars.