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October 12, 1999

Build it well and salmon will swim through it

  • WSU comes up with recipe for better culvert design
    Journal Environmental editor

    In the town of Pullman, out in the rolling Palouse wheat fields, a team of Washington State University researchers is focused on fish.

    Using a grant from the state Department of Transportation and WSU's Albrook Hydraulic Laboratory, the team has spent a year figuring out how to build culverts that are passable to salmon, and which would remain stable under various conditions.

    Now the first phase of the research is finished, and a report has been submitted to the DOT for comment and review. Among other things, it contains what is essentially a recipe for good culvert design.

    Thanos Papanicolaou, of the university's civil engineering department, said he and three other researchers -- Mike Barber, Ken Campbell and Rollin Hotchkiss -- found that the best culvert is the one that most closely matches the streambed above and below it.

    However, being engineers, they arrived at this conclusion very methodically. The team used a 75-foot recirculating flume to examine slopes of 2 degrees to 15 degrees, with water flows up to 40 cubic feet per second and gravel beds with 2-inch to 8-inch rocks.

    Culvert cut away
    WSU engineers have found that these design features can help ensure stable fish-passage conditions in culverts.
    Data from the flume yielded a graph that predicts, for any given flow, the depth at which gravel is eroded. This "scour depth" data could be used to properly size culverts to avoid erosion.

    The highest flows represent an extreme flood, a "200-year" event.

    "We tried to find stable bed configurations, which means we have marginal erosion," Papanicolaou said. "The natural condition is that you have sediment passing through the culvert, but you are not going to see the whole material totally eroded."

    Other essential culvert features, the team discovered, are things that nature creates very well: pools, riffles and steps.

    In round numbers, a good culvert recipe is as follows.

    • The "gravel" in the culvert should be made up of rocks with a diameter no greater than 25 percent of the culvert's diameter. So, a 12-foot culvert should contain rocks smaller than 3 feet across.
    • About 35 percent of the culvert pipe must be submerged in the gravel.
    • Instream structures called sills must be built to stabilize the gravel, especially on steep slopes.

    According to Papanicolaou, if gravel larger than 25 percent of the pipe diameter is placed inside it, the gravel's irregular surface will trap trash and debris. This will degrade the culvert's fish-passage characteristics and will contribute to blockages, flooding and washouts.

    The research done so far has not involved live fish. That comes in the next phase of the study, when the team will measure the effects of velocity on salmon.

    Washed out road
    Poor culvert design isn't just bad for fish. A clogged culvert caused this road to wash out, stranding an entire community in western Washington.
    Jim Schafer, director of the environmental research program at the DOT, said the WSU data will have immediate practical value to the department. And it will be applied to the next phase of the research, which includes not only live fish testing at WSU but also fish-behavior studies at the University of Washington.

    It is hoped that this last element will reveal what motivates juvenile fish to move upstream against strong currents, and when this trigger occurs. All new information will be utilized as quickly as possible by the DOT and the state Department of Fish and Wildlife, who will disseminate it to builders and contractors.

    Then, assuming money for salmon restoration finds its way into culverts, it could mean plenty of work for contractors. The state DOT has found that, of 1,585 culverts that are important to fish, about 32 percent are partially or completely impassable to them.

    And those are just the culverts on the state's own 7,000 miles of roads. When you add in 42,000 miles of county roads and 11,000 miles of city roads, the number of bad culverts could approach 19,000.

    In addition, state and federal forest roads, which total another 21,000 miles in Washington, very likely have thousands more impassable culverts.

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