Mimicking Mother Nature

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

Washington State constructed an engineered logjam to help safeguard a vital roadway from chronic flooding and at the same time improved fish habitat.

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For the better part of two decades, a remote two-lane stretch of U.S. Highway 101 in western Washington State took a recurring beating from the floodwaters of the Hoh River, which flows to the Pacific Ocean from the glaciers and rainforest of the Olympic Mountains. Major floods in 1981, 1997, 2001, and 2003 inflicted heavy damage on a particularly vulnerable road section where the Hoh takes a 90-degree turn and slams head-on into the highway embankment and then flows along it for about 366 meters (1,200 feet) before turning away. The vulnerable segment, at the highway's Milepost (MP) 174, is about 24 kilometers (15 miles) south of Forks, WA, in Jefferson County.

The Washington State Department of Transportation (WSDOT) spent $2.2 million over a 12-year period on emergency repairs, mostly placing rock groins and riprap revetments extending from the roadway shoulder to the river below. However, significant erosion of the riverbank and shoulder continued. Highway 101 remained in danger of severe undercutting and possible loss of the entire roadway section from catastrophic failure of the unstable slope.

The battered highway, in addition to being one of the State's most scenic routes, is a critical economic lifeline to Washington's northwestern coast, because it is the only route on the Olympic Peninsula capable of carrying commercial truck traffic. A lengthy closure was not feasible.

Innovation Needed

The WSDOT engineers urgently needed to find a long-term solution that would not only protect the highway infrastructure, but also would minimize environmental impacts. With the help of the Federal Highway Administration (FHWA), WSDOT was able to implement an innovative strategy consisting of an emerging technology called "engineered logjams" (ELJs).

These manmade logjams mimic those found in nature. In a natural river system, logjams typically form when a large tree falls into the water and becomes embedded in the river bottom, creating a snag that captures additional logs and debris moving downstream. Such logjams are capable of redirecting the channel and slowing the water's destructive forces. As an additional benefit, the logs and debris can create or enhance fish habitat.

Similarly, in the right situations, the construction of ELJs can have beneficial effects on the natural environment by improving fish habitat, while protecting critical transportation infrastructure by stabilizing and taming river channels during flooding. "Despite seeming counterintuitive, placing woody debris in rivers as stable logjams can provide an effective means of managing unstable woody debris that poses a risk to infrastructure," says geomorphologist and leading ELJ expert Tim Abbe of Herrera Environmental Consultants, Inc. WSDOT hired Abbe as a consultant on the Hoh River project.

The project is believed to be the first nationally significant application of ELJs for infrastructure protection, and experts at WSDOT and elsewhere are keeping a watchful, hopeful eye.

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"Many methods have been used routinely in the past to prevent erosion--rocks, car bodies, dikes, walls--all of which have had temporary success," says John Hart, the lead WSDOT engineer on the Hoh logjam project. "But ELJs may give us the long-term, environmentally friendly method we've been looking for to protect property and infrastructure."

Why Use Logjams?

"Expanding the use of ELJs for infrastructure protection is what separates this project from other logjam projects," says Hart. Prior to the Hoh River solution, most ELJ projects in the past were constructed for habitat enhancement and relatively small-scale bank protection.

Enhancing Fish Habitat With Logjams

According to geomorphologist Tim Abbe, woody debris in rivers is a positive and necessary natural process in many situations. Woody material helps create diverse fish habitat, protect endangered species, enhance water quality, and sustain complex ecosystems. Specifically, currents that flow through the logs scour deep pools where fish, particularly salmon, can rest, find refuge from predators, and reproduce. Logjams also shade the water so that it remains at a cool temperature suitable for spawning, and the decaying logs serve as a source of nutrients for plants and fish.

Studies have shown that fish living around structures of woody debris are healthier than those living around rock structures, according to Roger Peters, a fisheries biologist with the U.S. Fish and Wildlife Service. In an April 6, 2005, article in Environmental Science & Technology Online News regarding the Hoh River logjams, Peters wrote: "Rock is just not that good because it does not allow the development of diverse habitats."

So what prompted WSDOT to try ELJs, something so new and different?

After the 2001 flood inflicted heavy damage on MP 174, WSDOT conducted a reach analysis (a detailed site investigation) of the river alonga 13-kilometer (8-mile) section upstream and downstream from MP 174. The reach analysis provided WSDOT with a full understanding of the problem, based on sound science and engineering, and a thorough assessment of long-term solutions for protecting Highway 101 in an environmentally friendly way.