Cause of mudslide that led to 10-hour closure on I-5 in Bellingham revealed

The city of Bellingham revealed in a statement Monday what caused the mudslide that led to a 10-hour closure of I-5 north Sunday.

A spokesperson for the city reported that, according to a preliminary investigation, a clogged culvert, blocked by a roll of carpet padding from an unknown location, prevented water from flowing through the storm drainage system. That blockage led to a buildup of water, which then “saturated the soil and ultimately destabilized the slope.”

Stormwater Solutions, a publication that delivers “up-to-date information on technology, products and trends in the stormwater and erosion control industry” defines a culvert as “a short tunnel structure that primarily conveys water beneath an obstruction.” The publication adds they are “particularly important for the effective management of surface runoff across roads and highways.”

Engineers from the city reviewed the surrounding structures’ safety, including houses on Maryland Street, and “there is no indication of immediate risk,” the statement from the city reads. In addition, the city is working with the Washington State Department of Transportation (WSDOT) to address the impact the slide. WSDOT geotechnical engineers assessed the slope and determined it to be stable at the time of the release.

The city’s release added WSDOT and staff from the city of Bellingham will continue to monitor the embankment.

Mudslide in Bellingham stops traffic on I-5

The mudslide shut down the northbound lanes of the freeway just north of Bellingham’s Iowa Street, WSDOT reported on X. The agency also reported the I-5 south lanes were also affected by the slide.

WSDOT reported “about 2,000 cubic yards of debris spilled across lanes of I-5.”

That 2,000 cubic yards is about 54,000 cubic feet of debris that had to be removed from the freeway. The debris was primarily made up of mud but also included hillside vegetation.

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The agency didn’t track the weight of the debris that was cleared, and it’s difficult to calculate since the density of debris varies depending on materials in it. But, as The Bellingham Herald explains, in its coverage it’s still possible to get a general idea of how much debris was cleared.

Clay soil typically weighs around 1.5 grams per cubic centimeter.

“That translates to roughly 2,500 pounds for each cubic yard — and that’s before taking into account the amount of water absorbed by the soil in a landslide — meaning the debris that the department cleared likely weighed in the thousands of tons,” the outlet in Bellingham explained.

A mudslide covers the freeway in Bellingham, shutting down all northbound lanes. (Photo: Washington State Department of Transportation/@wsdot_north on X)

During the closure, officials asked drivers to avoid the area. Thousands of vehicles trying to head north through Bellingham had to resort to alternate routes to get around the freeway.

The agency confirmed on X just after 7:30 a.m. a mudslide caused the freeway closure. At that time, transportation officials reported on X that Whatcom County was getting soaked with rain. Close to two inches of rain fell in the Bellingham area Sunday morning.

WSDOT also reported there is also a lot of flooding on local roads in Bellingham and Whatcom County.

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WSDOT, via its @wsdot_north X account, first alerted travelers of an “incident” in a post at 4:46 a.m. 

Editors’ note: This story originally was published in the morning of Sunday, Oct. 27. It has been updated and republished multiple times since then.

Tom Brock is a reporter, editor and anchor at KIRO Newsradio. You can read more of Tom’s stories here.