Mimicking Mother Nature

Public Roads, Jan-Feb, 2006 by Megan Hall, Steve Moler

Data from the analysis suggested that the area's relatively unstable glacial geology may have caused large quantities of sediment to be deposited in the water. Over thousands of years, these sediments accumulated in large bars that, over time, caused the river to migrate back and forth across the valley. During the past 50 years, erosion from extensive logging in the valley led to increased sediment deposits. Also, during major floods, the river's accelerated side-to-side movements increased erosion and destructive forces at MP 174 and other problem areas.

The reach analysis found that doing nothing would leave the highway vulnerable to continued flood damage and, possibly, catastrophic failure. Options for action included moving the highway away from the river, which would have involved carving a massive roadcut into a landslide hazard zone, at an estimated cost of $40 million, or constructing a bridge structure at an estimated cost of up to $20 million. Alternatively, the construction of ELJs provided a potentially long-term solution that incorporated habitat-forming elements, at an estimated cost of about $7 million. The study concluded that ELJs were the best option.

After the 2003 flood, during which the river chewed up most of a 214-meter (700-foot)-long riprap revetment and required $500,000 of additional riprap for emergency temporary stabilization, WSDOT asked FHWA for help. After reviewing the reach analysis, design, past history, and cost data, FHWA determined that the site qualified for funding under the Emergency Relief (ER) program and approved $7 million to implement the ELJ alternative. The logjam approach was approved as a "betterment involving added protective features" under the provisions of the ER program.

"Band-aid recurring repairs were no longer considered cost beneficial, and a long-term solution was in order," says Gary Hughes, operations team leader in FHWA's Washington Division. Other regulatory agencies agreed. The U.S. Army Corps of Engineers and the U.S. Fish and Wildlife Service strongly endorsed the project and issued the appropriate emergency approvals.

"When we reviewed WSDOT's proposal, we really liked what we saw," says Dan Mathis, FHWA's division administrator for the State of Washington. "We were impressed with how their solutions dealt so well with balancing both engineering and environmental issues. Instead of the same old solution of rock revetments, they came to us with an innovative plan."

The novel approach to solving chronic flooding problems excited engineers at WSDOT as well. "Everyone involved in this project was ready for something new," says WSDOT hydrologist Jim Park. "This project showed that we can think 'outside the box,' that we're willing and able to develop innovative new approaches and get out of situations where we keep doing the same old things over and over again."

Engineering the Right Logjam

The Hoh River ELJ project near MP 174 included construction of four logjam structures in the main channel to divert the river into side channels and help dissipate and redirect the destructive energy. The project also included six bank ELJs extending 15 meters (50 feet) into the channel along the roadbank, plus two smaller bank ELJs, to prevent the river from eroding the highway embankment.