Technology▼Solutions Taming rivers with engineered logjams
WASHINGTON STATE DEPARTMENT OF TRANSPORTATION
that logjams benefit the region’s multimillion-dollar fishing industry. Roger Peters, a fisheries biologist with the U.S. Fish and Wildlife Serrock, 60 feet below the riverbed. This When the Hoh River threatened vice, says studies have shown that fish core is draped with spruce trees 36– to wash out U.S. Highway 101 for do better around structures made of 48 inches in diameter, with the root the fourth time in 20 years, John woody debris. “Rock is just not that ball still intact and most of the large Hart gave up. The river, which runs good because it does not allow the debranches cut off. “We have a stanthrough Washington’s Olympic Navelopment of diverse habitats,” he says. dard of practice, going through ‘spec’ tional Park, had already gobbled “It’s very simple,” says Hart. “Riprap sheets for each of the cumulative up $2.2 million of the rock riprap kills fish.” However, riprap construcparts and for the forces of the entire shielding the road, and the project tion is a familiar technology whose structure,” says Abbe. engineer with the state’s Departperformance is well ment of Transportaknown. It is also quite tion was willing to try expensive; in a North a “new” technology Dakota state report, ofthat was actually quite ficials estimated that old—logjams. it would be cheaper to Today, the Hoh River buy private property project is the largest along a section of the engineered logjam in Missouri River than the world. Completed to restore banks with in the summer of 2004, rock riprap. the project has a price The Hoh River tag of $7 million, well project has yet to below the $40 million withstand a large rain it would have taken event, but Abbe says to move the highway. other engineered But these are no ordiTo protect this highway from the Hoh River, engineers replaced rock riplogjams have shown nary woodpiles. “We rap with logjams along the road’s edge, pushing the river back into its bed. favorable results. coined the term ‘engiAnother set of logjams helps to dissipate the river’s energy and break up the Ten years ago, he deneered logjam’ because flow by creating side channels. signed a logjam to we did the math,” exprotect a stretch of private property Once the large pieces are placed, plains Tim Abbe, a geomorphologist along the Cowlitz River, which flows the structure is then backfilled with with Herrera Environmental Consuloff the slopes of Washington state’s river alluvium, as would occur with a tants. This novel approach could proMt. Rainier. Riprapping to protect 427 natural logjam. With seasonal floods, vide communities with a cheaper and meters of shoreline was estimated to the logs should anchor themselves greener alternative to flood control cost $900,000, while the logjam design fur ther into the riverbed, and pieces and river repair, predict experts. came in at $10,000. The structure was of floating wood will wedge into the Computer modeling determined built from nearby trees, pushed over trees, creating a natural-looking logthe size and placement of each logto save their root balls. The trees were jam with greater structural stability. jam. Eight structures protect the edge then dug into the riverbed so that they Although he doesn’t have the data of the highway and act like bumpers would not be swept away. As the curyet, Abbe suspects that engineered to bounce the river back, while four rent pushes against each tree, the root logjams create more natural flow patother jams sit directly in the river’s ball acts like a plow, digging the tree terns through the river’s hyporheic path, chopping up the main channel even deeper into the soil sediment. zone, and this encourages normal and diverting it into smaller streams. Abbe expects these logjams to water chemistry, temperature, and This region of the United States regulast just as long as any conventional pH. The hyporheic zone is the soil relarly receives 10–12 feet of rain anstructure such as a bridge, or maybe gion beneath and adjacent to a river nually, and the project is designed to longer. “When the wood is waterwhere surface and groundwater mix. withstand a 100-year flood—when logged, it can last a long time,” he Bacteria in the hyporheic zone can the Hoh River rushes down the valley says. “There are logjams that are convert nitrogen to harmless nitrate, floor at 73,000 cubic feet per second. hundreds of years old.” and clay sediments can trap organic Each structure has a steel H-pil—PAUL D. THACKER pollutants. Even better, research finds ing core driven into the underlying © 2005 American Chemical Society
MAY 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY ■ 199A
