Pretty Deadly

>> Monday, March 30, 2009

In light of all the news Mount Redoubt is making with its ash plumes, I'd like to post a reminder about Mt. Rainier, which is far more dangerous than Mt. Redoubt. Although, the 6 million barrels of oil sitting at the base of Redoubt is another story. With earthquakes in California (a 4.8 on the 24th and a 4.3 today) and Redoubt erupting until who knows when, it seems like the US portion of the Ring of Fire is heating up. Now, if the "Big One" hits California, we'll have a national disaster the size of Katrina or bigger. Now, if Rainier goes as it is expected to some day, we could/will have a disaster on the magnitude of the 2004 tsunami.

The Mount St. Helens lahar and even the Nevado del Ruiz lahar are minuscule compared to most of the 60 major lahars that scientists now know have surged over the millennia from Mount Rainier. In studying them, scientists found conclusive evidence that many lahars—including Rainier's second largest but still one of the greatest ever recorded, the Electron Mudflow—didn't coincide with any eruptions involving magma. Mount Rainier was a virtual factory of landslides. Lahars could come at any time, even, according to the USGS, after a landslide caused by a regional earthquake 60 miles away on the Seattle fault.
There is more snow and ice in the glaciers of Mount Rainier than all the other 12 Cascade Mountain volcanoes put together. "It's almost like a positive feedback system," says Pat Pringle, a geology teacher at Centralia College and a geologist formerly with the USGS and the state Department of Natural Resources. Pringle, who has spent most of his career studying Rainier's lahars, is considered by many geologists to be one of the top experts in his field. The glaciers, Pringle says, are carving away at the mountain's flanks, creating unstable steep slopes while contributing to groundwater that comes in contact with heat and sulfur. The acidic liquid that results helps weaken rock, resulting in landslides of claylike material that has a high water content. This porous material becomes a lahar that behaves and looks like wet concrete. Of course, lahars could also result from volcanic activity.

In 1995, four of the top lahar scientists in the country, including Pringle, wrote a USGS paper on the Osceola Mudflow. "The inundation area of a modern cohesive lahar of the same size could extend to Puget Sound, through Tacoma along the Puyallup River and through Seattle by way of the Green River system and the Duwamish Waterway," they wrote. "The lower resistance to flow of the modern unforested river valleys would allow a recurrence of this flow to go farther and faster than the original. . . . " Scientists now understood that giant lahars could come without warning and destroy much of the Puget Sound region "within a matter of hours," according to Don Swanson of the USGS, considered to be one of the world's elite volcanologists.

Because of the enormous size and complexity of the Osceola Mudflow, scientists are reluctant to estimate how much larger a modern-day flood on the same scale would be. There are differences that suggest it could be bigger. Besides fewer forests, an Osceola-size flood today would not encounter the vast arm of water that extended 30 miles inland from where the Port of Seattle is now. It would be much easier for a lahar to travel over this land, and that means Seattle could feel the brunt of a same-size event today. The experts who penned the USGS paper and many other scientists agree that a lahar disaster on the scale of the Osceola, with the potential of almost unimaginable destruction, should be included in government hazard plans.

The extent of a lahar threat to Pierce County and Tacoma has been known for a while. But the peril lahars hold for Seattle is a relatively recent discovery. It wasn't until 1996 that geologists digging at the Port of Seattle's Terminal 107 on the Duwamish Waterway made a startling discovery. They found a layer of sand they recognized could only have come from Mount Rainier. They had already found similar sand near the Emerald Downs racetrack in Auburn and beneath the Puyallup Fairgrounds. "We were amazed, just amazed," Pringle says.

The sand came from a relatively small lahar compared to the Osceola but one big enough that its deposits apparently filled the lower Duwamish Valley from wall to wall, all the way to Elliott Bay. This flood of mud and debris occurred only about 1,200 years ago.

This discovery and others like it since have dire implications for Seattle and the region. Even a lahar much smaller than the Osceola could have catastrophic effects on Seattle, Tacoma, and the suburbs. Geoff Clayton, a geologist with one of the pre-eminent engineering firms in the state, RH2, was asked by Seattle Weekly to evaluate the potential impact of a lahar on Seattle. Using a software program that analyzed present topography, he concluded that "a mudflow from Mount Rainier is the most catastrophic natural disaster that could happen to this area." Before approaching Seattle, a lahar, he says, would have "wiped out Enumclaw, Kent, Auburn, and most of Renton, if not all of it."
If a lahar comes off Mt. Rainier and only kills as many people as the Nevado del Ruiz volcano in Colombia killed (21,000), we'll be lucky. I know that sounds cold, but it's the truth. A major lahar comes off Mt. Rainier about every 500 years. The Electron Mudflow came off the mountain about 530 to 550 years ago. Yeah, we're due.


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