SEATTLE - The Washington State Department of Transportation lists 270 bridges as "scour-critical," meaning their supports could be undermined by the intense pressure of swollen creeks and rivers. Two hundred and seventy out of almost 3,100 bridge structures in the state's inventory that are 20 feet or more in length.
We learned of a similar term in May, when an oversized load brought down a span of the Skagit River bridge over I-5 after a corner of the load clipped an overhead beam. That term was "fracture-critical," referring to certain bridges that if hit in just the right place can collapse.
But the state has seen more bridges fail due to having the earth under piers and supports erode than having trusses and other piece of overhead structure impacted by vehicles.
"When you're in a flood event when the water is bubbling and boiling and churning, it can erode a lot of material in a short amount of time," said DeWayne Wilson, a WSDOT bridge engineer and expert on the vulnerabilities of bridges due to water.
Scour failures can happen anywhere. They can turn fatal if the bridge fails under or ahead of cars and trucks. As a precaution, WSDOT will close bridges in flood events if engineers feel the structure is vulnerable. Sometimes rafts of logs and other floating debris that hangs up on bridge supports can make the problem worse by concentrating the energy from rushing water along the bottom of the river speeding up erosion under bridge supports.
Some bridges are more vulnerable than others, and currently the state is ranking the top 20 to prioritize state funding to armor or protect those bridges. Costs can vary widely, from a few hundred thousand dollars to add boulders around supports, to multi-million dollar projects involving diverting water in the stream bed and environmental mitigation for bigger and more complex jobs.
Right now, the state lists its top five as the U.S. 101 Bridge over the Chehalis River in Aberdeen, the SR 529 bridge over Union Slough between Everett and Marysville, the U.S. 2 bridge over the south fork of the Skykomish River, the SR 108 bridge over Wildcat Creek in McCleary and the U.S. 12 bridge over Cedar Creek north of Oakville.