Culvert technology may help young salmon muscle their way upstream

Tens of thousands of culverts lie beneath roads in the Pacific Northwest, successfully moving water under the roadbed to preserve pavement and prevent flooding. At the same time, many are blocking juvenile salmon from migrating upstream to the habitat they need to survive and grow.

Pacific Northwest National Laboratory is participating in a consortium led by the Washington State Department of Transportation (WSDOT) that includes the transportation departments of other West Coast states and Alaska. The consortium funds a program to find viable retrofits for the thousands of culverts in the Pacific Northwest that may be preventing juvenile salmon from completing their life cycles.

The culvert test bed program promises not only to evaluate current and future retrofits, but to do so in a comprehensive way. "This project is a true interdisciplinary project because we're blending the expertise of hydraulics engineers, mechanical engineers, statisticians, fish biologists and fish behavior specialists to find a solution to a problem that faces the entire Northwest and has implications for culverts throughout the country," said Walter Pearson, who manages the project for PNNL. "We're doing this in a systematic, scientific way, using well-designed experiments in a well-engineered test bed."

Located at the Washington Department of Fish and Wildlife Skookumchuck Hatchery near Tenino, Wash., the test bed is a physical device, into which scientists can place a culvert. By changing the culvert's water flow and slope settings, scientists can measure hydraulic conditions, or how water flow interacts with the culvert system to influence water velocity or create turbulence.

Scientists are ready to begin trials with juvenile fish to determine how well fish pass through various retrofit designs. The test bed will enable scientists to set the hydraulics and observe fish behavior and then adjust the hydraulics and observe behavioral changes. "There are hundreds of possibilities for bed configurations. A particular design will stop passing fish at some flow or some slope and that's what we'll be looking for," Pearson said.

Recent research has shown that upstream movement of juvenile salmon is important in their freshwater phase and preliminary tests of the PNNL system confirm this.

"Evidence of juvenile salmon moving upstream in our reference culvert was the last piece we needed to confirm the system will work," Pearson said. "We've proved the fish will move upstream in the culvert system. How many fish move upstream will determine the success of the various retrofit designs."

Attempting to retrofit culverts is not a new endeavor. Baffles, weirs, ladders and other physical structures have been added to culverts to enhance fish passage over the years, but little monitoring has been done to test the effectiveness of such additions.

Installation of the test bed is complete and researchers have tested the mechanics of the device. The science team has begun to test fish in the reference culvert system and will begin to test retrofit designs soon. WSDOT and its consortium funded PNNL through the U.S. Department of Energy to conduct the culvert test bed program for evaluating retrofit culverts.