Transportation Industry

Mimicking Mother Nature

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

For more information, contact Steve Moler at 415-744-3103 or steve.moler@fhwa.dot.gov.

RELATED ARTICLE: The Inspiration Behind the Engineered Logjam

Before European settlers came to North America, logjams littered rivers and streams in heavily forested areas, helping tame the rivers and creating vast and complex networks of wetlands rich in fish and wildlife. But early pioneers considered logjams and other woody debris a safety hazard and an impediment to river transportation and hence to agricultural and industrial development. Pioneers cleared many rivers and streams of snags, jams, and old-growth trees, and transformed the waterways into simple channels, thereby reducing the wood supplies needed to create natural logjams.

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Geomorphologist Tim Abbe conceived the idea of engineering logjams to protect human infrastructure during a hike in 1991. "I came upon a natural logjam and noticed that the logjam was directly influencing the hydraulics and morphology of the stream," Abbe recalls. "Here was this deposit of wood affecting where the stream went and how it behaved."

This observation inspired Abbe to begin considering the idea of building manmade logjams to return rivers to their natural state, to the extent possible for any given situation. In 1995, he supervised construction of what is believed to be the first ELJ project in the United States-and possibly the world-to provide emergency bank protection for private land along the Upper Cowlitz River near Packwood, WA. For this project, Abbe designed three logjam structures using trees found onsite, already fallen over and still retaining their root-balls.

He found that the root-ball is a critical factor in the stability of a river snag, as it acts as both a plow and an anchor. As flow hits the root-ball, it scours the riverbed, and the root-ball sinks further into the substrate to create a snag that can resist even the most formidable floods. This self-strengthening process is what gives natural logjams some of their structural integrity and durability.

When conducting additional research, Abbe found that, in at least one Washington river, natural logjams had lasted for more than 1,000 years. The ELJs constructed on the Cowlitz River withstood a 25-year flood only 6 weeks after being constructed, and the landowner has actually regained some of the previously eroded land, as a result of sedimentation behind the logjams.

Since 1995, more than 50 ELJ projects have been constructed in the Pacific Northwest and 15 in Australia. All are functioning today as intended.

"Reintroducing wood into streams and rivers is not a passing fad," says Abbe. "It's a scientifically recognized component of restoring Pacific Northwest river ecosystems and is also essential to salmon recovery."