Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Pe Ell Landslide, Pe Ell, Lewis County

The Pe Ell Landslide failed during the December 3rd Storm of 2007, closing State Route 6 just west of Pe Ell.

Pe Ell Landslide - Photo by WSDOT

Pe Ell Landslide - Photo by WSDOT

The debris avalanche/slide flowed across the highway and pushed a truck into the living room of the house across the way. Remarkably, most of this was caught on tape by the residences of the house.

Pe Ell landslide impact of house - WSDOT Photo

Pe Ell landslide impact of house - WSDOT Photo

On December 11, Kelsay and I arrived at the landslide. The drive through the Chehalis valley was spooky to me, a lingering stench filled the air and misery could be seen all around. Home after home, farm after farm all showed damage from the floods. By time we arrived, WSDOT had already arranged for an emergency contract with Scarsella (on December 9th) to begin work on clearing State Route 6. Unfortunately, with all of the heavy equipment working on the site, we decided to stay on the periphery of the landslide and investigate the damage to the structures.

The damage was localized to the western lobe of the landslide. It impacted the houses at a low speed, warping and pushing them.

Pe Ell landslide impact to a house - DNR/DGER Photo

Pe Ell landslide impact to a house - DNR/DGER Photo

Pe Ell Landslide impact to second house - DNR/DGER Photo

Pe Ell Landslide impact to second house - DNR/DGER Photo

Meanwhile, WSDOT was working hard on figuring out the landslide. The WSDOT Geotechnical Division has access to many really neat tools to help with their investigations. Here is a 3-D representation of the landslide mass created by their division:

Pe Ell Landslide 3D Model - WSDOT Geotechnical Division

Pe Ell Landslide 3D Model - WSDOT Geotechnical Division

They also compiled a small scale geologic map of the landslide mass (with an amazing aerial photo of the landslide):

Pe Ell Landslide Geologic Map - WSDOT Geotechnical Division

Pe Ell Landslide Geologic Map - WSDOT Geotechnical Division

In the end, WSDOT removed over 47,000 cubic yards of material to stabilize the landslide mass at a cost of around $4 million dollars. The project was completed on March 13th, 2008, over three months after the storm.

The landslide prompted a debate on logging, landslides, and highway safety. The landslide itself was logged weeks before the storm. The interesting part, this landslide wasn’t caused by root strength loss, it was probably too deep anyway to have much impact. The lack of canopy, however, might have played a roll in the landslide initiation. Canopy plays a role in reducing the rate rainfall from reaching the ground (to a certain point) or slow melting of snow on the ground by reducing rain rates and buffering changing temperature. It is difficult to say in an intense storm how much it might have slowed the rainfall, or reduced snow melt (by reducing the warm rain and temperature from reaching the snow), but the lack of trees, even with this intense rainfall, probably did increase the likelihood for its initiation.

Cause aside, the cost of repairing these landslides is expensive. This is just one of probably hundreds of landslides to fall on our highway systems each year. Figuring out why these landslides fail and if we can either mitigate or possibly find better management practices to help reduce landslides would help save millions of dollars and reduce injury and death.

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Each week we will feature a new landslide in Washington State. Washington State is covered with dynamic and sometimes quirky landslides.

Mt. St. Helens Debris Avalanche, Skamania County

Today is May 18th and it seems fitting that the Mt St Helens Debris Avalanche would be the landslide of the week. The Mt St. Helens Debris Avalanche is perhaps one of the most well known landslides in the world.

Area: 23 sq.mi.
Volume: .67 cu. mi.
Age: 29 years

Mt St Helens Debris Avalanche

Mt St Helens Debris Avalanche

An Excerpt from: REPORT: Eruptions of Mount St. Helens: Past, Present, and Future

“May 18, a Sunday, dawned bright and clear. At 7 a.m. Pacific Daylight Time (PDT), USGS volcanologist David A. Johnston, who had Saturday-night duty at an observation post about 6 miles north of the volcano, radioed in the results of some laser-beam measurements he had made moments earlier that morning. Even considering these measurements, the status of Mount St. Helens’ activity that day showed no change from the pattern of the preceding month. Volcano-monitoring data-seismic, rate of bulge movement, sulfur-dioxide gas emission, and ground temperature-revealed no unusual changes that could be taken as warning signals for the catastrophe that would strike about an hour and a half later. About 20 seconds after 8:32 a.m. PDT, apparently in response to a magnitude-5.1 earthquake about 1 mile beneath the volcano, the bulged, unstable north flank of Mount St. Helens suddenly began to collapse, triggering a rapid and tragic train of events that resulted in widespread devastation and the loss of 57 people, including volcanologist Johnston.

Debris Avalanche
Although the triggering earthquake was of slightly greater magnitude than any of the shocks recorded earlier at the volcano, it was not unusual in any other way. What happened within the next few seconds was described by geologists Keith and Dorothy Stoffel, who at the time were in a small plane over the volcano’s summit. Among the events they witnessed, they

“noticed landsliding of rock and ice debris
in-ward into the crater… the south-facing wall
of the north side of the main crater was especially
active. Within a matter of seconds, perhaps 15 seconds,
the whole north side of the summit crater began to
move instantaneously. … The nature of movement
was eerie…. The entire mass began to ripple and
churn up, without moving laterally. Then the entire
north side of the summit began sliding to the north
along a deep-seated slide plane. I (Keith Stoffel)
was amazed and excited with the realization that
we were watching this landslide of unbelievable
proportions. … We took pictures of this slide
sequence occurring, but before we could snap off
more than a few pictures, a huge explosion blasted
out of the detachment plane. We neither felt nor
heard a thing, even though we were just east of the
summit at this time.”

Realizing their dangerous situation, the pilot put the plane into a steep dive to gain speed, and thus was able to outrun the rapidly mushrooming eruption cloud that threatened to engulf them. The Stoffels were fortunate to escape, and other scientists were fortunate to have their eyewitness account to help unscramble the sequence and timing of the quick succession of events that initiated the May 18 eruption.

The collapse of the north flank produced the largest landslide-debris avalanche recorded in historic time. Detailed analysis of photographs and other data shows that an estimated 7-20 seconds (about 10 seconds seems most reasonable) elapsed between the triggering earthquake and the onset of the flank collapse. During the next 15 seconds, first one large block slid away, then another large block began to move, only to be followed by still another block. The series of slide blocks merged downslope into a gigantic debris avalanche, which moved northward at speeds of 110 to 155 miles an hour. Part of the avalanche surged into and across Spirit Lake, but most of it flowed westward into the upper reaches of the North Fork of the Toutle River. At one location, about 4 miles north of the summit, the advancing front of the avalanche still had sufficient momentum to flow over a ridge more than 1,150 feet high. The resulting hummocky avalanche deposit consisted of intermixed volcanic debris, glacial ice, and, possibly, water displaced from Spirit Lake. Covering an area of about 24 square miles, the debris avalanche advanced more than 13 miles down the North Fork of the Toutle River and filled the valley to an average depth of about 150 feet; the total volume of the deposit was about 0.7 cubic mile. The dumping of avalanche debris into Spirit Lake raised its bottom by about 295 feet and its water level by about 200 feet.

1983 Photo North Fork Toutle River

1983 Photo North Fork Toutle River


Mudflows and Floods

Volcanic debris flows-mobile mixtures of volcanic debris and water popularly called mudflows -often accompany pyroclastic eruptions, if water is available to erode and transport the loose pyroclastic deposits on the steep slopes of stratovolcanoes. Destructive mudflows and debris flows began within minutes of the onset of the May 18 eruption, as the hot pyroclastic materials in the debris avalanche, lateral blast, and ash falls melted snow and glacial ice on the upper slopes of Mount St. Helens. Such flows are also called lahars, a term borrowed from Indonesia, where volcanic eruptions have produced many such deposits.

Mudflows were observed as early as 8:50 a.m. PDT in the upper reaches of the South Fork of the Toutle River. The largest and most destructive mudflows, however, were those that developed several hours later in the North Fork of the Toutle River, when the water-saturated parts of the massive debris avalanche deposits began to slump and flow. The mudflow in the Toutle River drainage area ultimately dumped more than 65 million cubic yards of sediment along the lower Cowlitz and Columbia Rivers. The water-carrying capacity of the Cowlitz River was reduced by 85 percent, and the depth of the Columbia River navigational channel was decreased from 39 feet to less than 13 feet, disrupting river traffic and choking off ocean shipping.

Mudflows also swept down the southeast flank of the volcano-along the Swift Creek, Pine Creek, and Muddy River drainages and emptied nearly 18 million cubic yards of water, mud, and debris into the Swift Reservoir. The water level of the reservoir had been purposely kept low as a precaution to minimize the possibility that the reservoir could be overtopped by the additional water-mud-debris load to cause flooding of the valley downstream. Fortunately, the volume of the additional load was insufficient to cause overtopping even if the reservoir had been full.

On the upper steep slopes of the volcano, the mudflows traveled as fast as 90 miles an hour; the velocity then progressively slowed to about 3 miles an hour as the flows encountered the flatter and wider parts of the Toutle River drainage. Even after traveling many tens of miles from the volcano and mixing with cold waters, the mudflows maintained temperatures in the range of about 84 to 91 degrees (F); they undoubtedly had higher temperatures closer to the eruption source. Shortly before 3 p.m., the mud- and debris-choked Toutle River crested about 21 feet above normal at a point just south of the confluence of the North and South Forks. Another stream gage at Castle Rock, about 3 miles downstream from where the Toutle joins the Cowlitz, indicated a high-water (and mud) mark also about 20 feet above normal at midnight of May 18. Locally the mudflows surged up the valley walls as much as 360 feet and over hills as high as 250 feet. From the evidence left by the “bathtub-ring” mudlines, the larger mudflows at their peak averaged from 33 to 66 feet deep. The actual deposits left behind after the passage of the mudflow crests, however, were considerably thinner, commonly less than 10 percent of their depth during peak flow. For example, the mudflow deposits along much of the Toutle River averaged less than 3 feet thick.”