A dam upgrade in Bend, Ore., will provide three distinct river channels, all engineered to match differing—and often competing—needs.
As part of an upgrade to the Deschutes River at the Colorado Dam, the Bend Park & Recreation District hired engineer Otak and contractor Hamilton Construction to turn some 500 ft of in-city river into three channels, serving the needs of wildlife, recreational boaters and white-water paddlers and maintaining the irrigation capacity provided by the roughly 100-year-old, 10-ft-tall sheet-pile dam.
Combining so many elements into one project is the first effort of its kind on the West Coast, says Kevin Timmins, Otak project engineer. The resulting design "is a reflection of the different river stewards at this location coming together to create something unique and interesting for the community," he says.
The Colorado Dam will remain in place as part of the $9.68-million, one-year project that kicked off on Oct. 1, but the river near the dam will look completely different. Starting about 100 ft upstream from the dam and ranging about 400 ft below it, one side of the river—about 20 ft to 30 ft wide—will turn into a safe-passage channel. The 80-ft-wide middle channel will morph into a challenging white-water recreation path. The third, nearly 90 ft wide, will become a habitat channel.
The project keeps the dam height the same but raises the riverbed between 6 ft and 10 ft using existing ¼-in. to 3-in. river rock, "nearly accomplishing the same things as if we removed the structures," says Evan Stuart, Hamilton's project manager. The dam remains critical to water-right owner needs.
"When all is said and done, a navigable channel will go up and over the dam [and be] the same size as the original river," Stuart says.
Hamilton will build the safe-passage and white-water portions as one unit, isolating the river by driving sheet panels between the habitat and the white-water portions. Once those sections are built, Hamilton will switch water containment and build the habitat. Temporary holes cut into the dam will help to regulate water flows during construction, the trickiest part of the project, Stuart says. To control water levels during potential flood events, the gates will be programmed to lower using sensors installed above the dam.
The habitat channel, spearheaded by environmental interests and permitting agencies, will have a significantly reduced flow. Waterfalls will aerate water to increase oxygen content. The habitat for the endangered spotted frog will be expanded via small pools, islands, habitat logs and boulders.
Newly created peninsulas and the island will be graded to match the elevation of the existing wetlands to create shallow pools and deep still water, Timmins says.
The safe-passage channel reestablishes a connection for boaters to the upper and lower reaches of the river by creating a set of 8-in. ripples that start above an existing pedestrian bridge, which Hamilton will replace toward the end of the project, to gradually bring boaters through the drops. During the summer, the steady current will keep the water smooth.
The white-water sections will provide Bend Paddle Trail Alliance, a prominent river paddling club that contributed $1.1-million to the project, with a "playground in the middle of this," Stuart says. The channel will include pneumatic-controlled gates and bladders that have small locks and gates to raise and lower the water elevation. Water features will include three drops into sets of pools, rubber rocks and 20 inflatable bladders that manage adjustable waves. Submerged concrete will create three fixed waves.