Engineered logjams designed to control riverbank erosion near highway
ROCKPORT – State Route 20 is back on its original course east of Rockport thanks to the completion of a project to keep the Skagit River in its current channel.
A contractor crew for the Washington State Department of Transportation finished installing engineered logjams along the north bank of the river four months early. The logjams are designed to eliminate erosion close to the highway.
Three times in the past decade the river eroded the roadway embankment, threatening SR 20. Installing large rocks as an emergency measure proved to be only a temporary fix, as the Skagit continued eroding farther upstream and downstream of the repairs. A more permanent fix was needed.
Since winter, contractor crews from Trimaxx Construction installed five engineered logjams and four protective log-based barriers on approximately 1,000 feet of the Skagit River’s north bank a few miles east of Rockport. The logjams and barriers are held in place by large concrete blocks called dolosse (plural, pronounced “doh-LOH-suh”), which resemble jacks from the children’s game. But these jacks are about 8 feet on a side and weigh 8 tons each, large enough to keep the logs in place.
The logjams are designed to deflect the river flow away from the shoreline, stopping erosion in that area. The barriers are designed to protect the highway embankment from erosion in areas between the logjams. They also provide habitat for young salmon to grow before heading out to sea.
The original project schedule included a break from May to July to avoid delays for drivers on SR 20 during most of the North Cascades Highway summer travel season and heightened river levels during the spring snow melt. The break also was designed to preserve fish passage during a vulnerable salmon spawning period. Because the contractor finished early, crews from Trimaxx will not need to return to finish the project in August.
“The contractor built the logjams much faster than we anticipated,” said WSDOT project engineer Shane Spahr. “This is only the second time a project like this has been done in Washington. It’s much larger than the other one along the Puyallup River, and the water here is a lot swifter, so that made estimating how long it would take a challenge. But it’s great to have the work done early, which will help traffic through the area and reduce the effects on wildlife.”
The early completion also means SR 20 returns to its normal route ahead of the opening of the North Cascades Highway. During construction, the highway was detoured onto a 1,900-foot temporary bypass and the speed limit was reduced from 50 to 30 mph.
For the next couple of weeks, drivers may experience reduced speeds and single-lane closures near the work zone, as contractor crews remove the bypass pavement. This fall, drivers may once again experience flagger-controlled single-lane closures while crews do landscaping along the riverbank.
The total project cost is $10.2 million.