Pierce County’s mudslides haven’t been as large as Oso’s

But it lacks good data on slide history and soil makeup, laser scans of ground

Staff writerMarch 30, 2014 

If anything good came out of the disastrous mudslide in Snohomish County, it might be this: Other counties throughout Washington began taking closer looks at their own mudslide risk assessments, wanting to make sure they’re not surprised in similar ways.

Emergency management officials in Snohomish County initially said the catastrophic mudslide March 22 took them by surprise, even though previous mudslides at the Oso site — one twice as big — had been well documented by geologists.

Is a lack of communication between scientists and public agencies concealing other disasters waiting to happen? Are other communities in danger?

On Thursday, Pierce County Executive Pat McCarthy called department heads together for a post-mudslide summit to answer those questions. She says she came away reassured.

“Existing records in Pierce County don’t reveal mudslides that come anywhere close to the magnitude of the Oso slide, which comprised an estimated 6 to 7 million cubic yards of earth,” McCarthy said. “We regularly handle small slides — 10 to 100 cubic yards — that involve material slipping off of wet slopes, but not wholesale collapses of hillsides, especially in populated areas.”

And yet, several potentially disturbing facts have been revealed in the days since the Oso slide.

Emergency management and planning officials in Pierce County have no comprehensive inventory of historic mudslides, which geologists say are the best indicators of future slides.

The county’s official study identifying potential mudslides and assessing risks, a document prepared in 2010 to be eligible for federal grants, relies almost entirely on the steepness of slopes — which geologists say is a good indicator of mudslide potential but by no means the only one.

The county’s risk assessment does not include mudslide data compiled by the United States Geological Survey, the state Department of Natural Resources or the state Department of Ecology, and for most of the county, it does not consider soil types.

It makes no use of new technology, including LIDAR, which uses laser scanning to see beneath vegetation and give a clear picture of underlying landforms.

County planners also say they have never completely rejected a building project based on steepness of slope or mudslide potential.

The tension in that regard is the same here as in Snohomish County: public safety versus the right of people to do what they please with their property.

“Our role as a local government is to balance the need to protect the public from harm and to protect private property rights,” McCarthy said.

The questions and answers that follow are intended to further clarify the situation:

Question: Why are there so many mudslides in Western Washington?

Answer: According to geologists, it’s a combination of soil type — mostly loose sand and gravel and clay left over from past ice ages — steep slopes and a heavy concentration of rain during just a few months of the year. University of Washington climatologist Cliff Mass refers to Washington as “The Mudslide State,” making the case that Washington experiences more catastrophic mudslides than any state in the United States.

Q: Why do mudslides suddenly decide to let loose?

A: Gravity is the main and omnipresent cause, but triggers include heavy rainfall, which reduces the strength of soil and lubricates junctions between different soil layers or builds pressure. Earthquakes, even small ones, can be triggers, as well as volcanic activity.

Human modifications to the landscape, including logging, grading and road building can trigger mudslides by changing water flow.

Q: Where are the most vulnerable areas in Tacoma and Pierce County?

A: Geologists say past mudslides are the best indicators of where future mudslides will occur.

Areas most often affected in the Tacoma area are the bluffs along the Tacoma Narrows, specifically the Salmon Beach area, where in 1996 mudslides pushed two houses into Puget Sound. Mudslides are common at the northwestern tip and along the western side of Fox Island and along bluffs south of Gig Harbor, especially those wrapping around Point Evans and Point Fosdick.

Mudslides on the eastern side of the Narrows have frequently disrupted rail traffic. Mudslides near DuPont and between Brown and Dash points also are common.

In Tacoma, there have been numerous mudslides below Stadium High School and above Schuster Parkway and across the Tideflats along Marine View Drive, which runs below the Hylebos Bluffs.

Elsewhere in the county, areas south of Carbonado along state Route 165, along state Route 410 east of Enumclaw, and the Jovita Canyon near Edgewood and Milton are frequently hit. In 1991, a mudslide near Fort Lewis blocked the Nisqually River.

Numerous mudslides occur in the mountains of East Pierce County, including the Carbon River drainage, but they draw less attention because of their distance from developed areas. Public agencies generally concentrate on places that are threats to life or property.

An exception is the Department of Natural Resources, which manages state forest land and has done extensive monitoring and predictive work to manage timber harvests.

Q: Is the steepness of a hillside a good indicator of mudslide danger?

A: Yes, but not always. Steep slopes can be stable, depending on the underlying composition of soil and rock, according to James McDougall, a University of Washington Tacoma geologist who teaches classes in natural hazards and disasters. Large, deep-seated mudslides can occur on slopes as low as 15 percent. Other factors, such as liquefaction, or the tendency of soil to lose strength when saturated with water, also are important.

Q: What has the city of Tacoma done with regard to planning and emergency response to mudslides?

A: The city has mainly deferred to the county’s Department of Emergency Management with regard to emergency response. As far as land-use planning goes, the city has mapped mudslide hazard areas in its Comprehensive Plan and has code requirements associated with activities near them.

Not all development is prohibited. Generally, developers need to do geologic analyses and propose acceptable engineering solutions to be allowed to build in hazard areas.

Last year, the city applied for a Rockefeller Foundation grant to finance planning for disasters, but it was not among the cities selected.

Q: Could mudslides as big as the one in Snohomish County happen here?

A: Definitely. And much bigger if you include mudslides from Mount Rainier.

Because Pierce County’s populated areas lie downstream from the mountain, mudslides on the mountain have and — according to geologists, almost certainly will again — cause mudflows that make the Oso slide seem insignificant by comparison. Past mudflows, or lahars, from Mount Rainier have made it to Commencement Bay, burying everything along the way.

Similar events now, geologists say, could be comparable to the 1985 mudflow from Nevado del Ruiz in Colombia, which killed about 23,000 people.

“Pierce County is perhaps one of the best known locations for destructive mud or debris flows in the country, or the world,” the UWT’s McDougall said.

Otherwise, such large catastrophic failures have, at least recently, been unusual here. Land movement generally is slower in the South Sound, triggering periodic small slides that damage property and close roads but rarely kill people.

Q: Emergency management and planning officials in Snohomish County apparently were unaware of studies that warned the Oso site was dangerous and that mudslides had happened there before. Is it possible a similar situation exists in Pierce County?

A: County officials say that’s possible but highly unlikely.

“The history of mudslides in Pierce County indicates there’s never been an event that even approaches anything of that magnitude,” said Harold Smelt, a manager in the county’s Department of Public Works and Utilities.

Q: Does Pierce County have a comprehensive database of past mudslides?

A: No. The inventory of mudslide databases is sparse in most of Washington. DNR is working to consolidate and digitize historical mudslide data. Generally, mudslides have been understudied compared with other geologic hazards such as earthquakes, volcanoes and tsunamis.

Q: What is the process for assessing the mudslide risk with regard to permitting new development in Pierce County?

A: The county tries to prevent danger to people by requiring developers in steep areas to submit an evaluation of slide danger before deciding on building permit requests. County codes also call for setbacks in certain areas, both from the top and bottom of hillsides.

The burden of proof is on geo-technical experts hired by developers, whose professional reputations are at stake, said Dennis Hanberg, director of the county’s Planning and Land Service Department.

“We set the standards,” he said, “and it’s up to them to convince us, ‘This will work.’ ”

While the county has never rejected a proposed project outright because of landscape risk, Hanberg said, it routinely places conditions on development.

Q: Have attempts been made in the county to remove homes or developments from areas known to be at risk from mudslides?

A: No. Such efforts are taking place for areas known to be at risk from flooding, but not mudslides.

Q: Pierce County did a mudslides risk assessment in 2010. Doesn’t it do a good job?

A: It could be better.

Lowell Porter, director of the county’s Department of Emergency Management, says his faith in the document has not been shaken since the Oso slide.

“We’re where we should be,” Porter said Friday.

But he admits the study is incomplete. For most of the county, it’s based only on slope gradients and doesn’t include soil analysis or a thorough inventory of past slides.

Pierce County recently gained access to LIDAR technology, which will enhance its ability to identify danger areas. That technology will be used in the next version of the mudslide-risk assessment, which Porter said is in the works. The revision is not because of Oso, Porter said. It was planned anyway.

Rob Carson: 253-597-8693
Staff writer Adam Lynn contributed to this report.

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