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.

Hornby Landslide, Glenoma Area, Lewis County

This is one of the more fascinating landslides that occurred during the January 2009 storm. Numerous debris flows and avalanches dotted the slopes above Mark and Jon Hornby’s farm.

Map of landslide in Glenoma Area, Lewis County

Map of landslide in Glenoma Area, Lewis County

Hornby Landslides

Hornby Landslides

Series of Events

At about 9:00 am on January 8, a large debris flow moved into Mark Hornby’s farm pasture, plugging a culvert and covering it with mud and debris. About a half an hour later, another landslide came off the slope, nearly striking Mark and his brother, Jon. I think this landslide also struck a bull and carried it a ways across the pasture. There full story can be found here.

View of debris flow deposit near house; DGER/DNR Photo

View of debris flow deposit near house; DGER/DNR Photo

The debris avalanche/flow in the middle of the clear cut slope is very shallow, less than 2 feet of incision in many places. The landslide incised down to bedrock in most places, which was probably why the landslide was so shallow. When it reached the pasture, the landslide turned into a short debris flow and then transformed into a hyperconcentrated flow and made its way across the Hornby’s Farm pasture. The landslide ponded against Highway 12, flowing to the Hornby’s driveway and then onto Highway 12. One of the oddities discovered in Glenoma was that many of the hyperconcentrated flows that reached the valley floor were entrapped into roads by snow berms from plowed snow.

Hornby Landslides; DGER/DNR Photo

Hornby Landslides; DGER/DNR Photo

January 7-8 Storm Summary

In December of 2008 and into January of 2009, cold air from British Columbia created an ideal condition for snowfall across Washington State. Snow accumulations preceding the storm were low in the Puget Lowland, with at only inches on the ground in most places. On January 7, a stream of moisture originating from around Kauai (Hawaiian Islands) flowed into Washington State, bringing warm temperatures and high amounts of rain, rapidly melting what snow remained in the lowlands and eating away at the snowpacks in the mountains. By January 8, the largest evacuation in the state’s history was under way, forcing more than 30,000 people living in the Puyallup River area to flee. The town of Orting, with a population of more than 26,000, was almost completely flooded. For the second year in a row, flood waters closed Interstate 5 in Centralia/Chehalis. In the rest of the state, rivers were also flooding—the Stillaguamish, Snohomish, Chehalis, Naselle, Hoko, Cedar, and Cowlitz were the most significant, peaking above the 100-year flood level.

King and Snohomish County were least affected, as a rain shadow from the Olympic Mountains shielded their low-lying areas. In the Puget Lowland, rainfall totals ranged from 1.5 inches in Seattle to 5 to 7 inches in southwest Washington and 3 to 6 inches in the northwestern counties. As the storm progressed into the Cascades, the higher elevation forced the clouds to release water as they moved over the mountains, leaving more than 20 inches of rain in two days. The rainfall saturated slopes, many already wet from melting snow, triggering debris flows and debris avalanches throughout most of western Washington. Areas sensitive to high-intensity storms, such as Glenoma, Concrete, and Van Zandt, were the site of numerous large debris flows, blocking roads, limiting emergency response, and destroying homes. In the end, more than 1,500 landslides were reported or recorded from Washington Division of Geology and Earth Resources (DGER) field and aerial surveys.

Logging and Landslides

DGER and AEG hosted a field trip in the Glenoma Area (field trip guild). The purpose of this stop was to discuss logging and landslides. Unfortunately, the conversation never got going very well. It certainly caught my attention when I was looking over the photos coming in. One of the first things that caught my attention was the prominent deep-seated landslide on the west side of the clearcut. It is difficult to see if it is active from a photo, but when I first saw it from the aerial photo I thought it probably had some recent movement (within the last 100 years, maybe?). In the subsurface, there are places of thick, mostly unconsolidated pumice. So, is this logging related? It is possible. Was it illegal? Probably not. I didn’t do any detailed ground survey of this area, but just at a general glance, I cannot think of any forest practice rules that they might have broke. Maybe we need to look at if the FP rules are protecting our slopes, especially in Lewis County.

Harvest related landslides have been in the public eye for awhile. Hal Bernton and Justin Mayo of the Seattle Times (and photographer Steve Ringman) have really brought landslides and logging into the minds of people in Washington, if not many places in the nation. Science backs that logging can cause landslides with a lot of research focused on rooting strength and the 3-15 year (or so, see Loughlin and Ziemer, 1982) window of weakening roots (variable with climate, soil, tree species, among other things). Perhaps less studied are canopy coverage and its impacts to landslides. A study in Canada (Horel, 2006) has shown that in their study area, landslides failed in the years after harvest and reduced in landslides when rooting strength was at its lowest. So, perhaps we have a myriad of factors for how trees can influence landslides. That is just an introduction to the following area. I have been working on digitizing landslides from the January 7-8th storm with help from Trevor Contreras and Kelsay Davis-Stanton. A lot of the landslides I digitize in are in a mixture of timber ages, from clear cut areas (0-5 years), young stands (5-15 years), and submature timber (15-50 years). However, there are areas where we see a higher incident of landslides in a single age of trees or specific landuse.
This area is located along the White River (and Highway 410) at about Twin Creek, an area that was hit hard by landslides during the January storm. This was one of the areas where we had reports of landslides during the storm, but was blocked from investigating these landslides due to flooding and debris flows across the road. On Saturday morning, I was digitizing the landslides in from our oblique photos and ran across this:

Area map of landslides

Area map of landslides

Area map of historical landslides

Area map of historical landslides

Historically, this area has had many debris flows and is covered in deep-seated landslides. The red stars indicate where landslides have initiated from the January 7-8th, 2009 storm.

These maps are made with a 2006 NAIP Orthophoto and as you can see on the photos, harvest has occurred between 2006 and now:

Photo of area covering the western side of the map

Photo of area covering the western side of the map

Photo of area covering mid-western section of map

Photo of area covering mid-western section of map

Photo of area covering the mid-eastern side of map

Photo of area covering the mid-eastern side of map

Photo of area covering the eastern side of the map

Photo of area covering the eastern side of the map

So, is this clearly a case of clearcuts causing landslides? Lets look at some of the data of this area (this data is for example use only and is very much in draft form).

Graph of landslides verse landuse

Graph of landslides verse landuse

Eight landslides initiating in clearcuts, three in submature timber and eight in roads, is that clearly a clearcut answer? Not really, first we would need to normalize the data, that is quantify the size of different landuses and then divide that with the landslides. That way, if there is, say, 16 acres of clearcuts and 6 acres of submature timber, then the data would not suggest that landslides were more frequent in either landuse. In this area, roughly speaking, there are 11,200 acres of clearcut and 6,400 acres of submature timber. Normalized that would come out to about 7 landslides/1,000 acres for clearcuts, 5 landslides/1,000 acres for submature timber. Not that this sample is statistically large enough, but the difference between the two landuses is not all that significant.

This data doesn’t mean that clearcuts aren’t causing landslides, but the rate of landslides is similar to that of submature timber. Landslides are a naturally occurring event, but can sometimes be aggravated by various landuses. The real question is; is the rate of landslides greater than that of natural rates, that is, is our management of the forests creating a greater number of landslides. With that, we would have to look at historical records to determine a pre-harvest landslide rate and compare that to the various landuses today. Unfortunately, such data would be very difficult to create and the accuracy for a specific area would almost be impossible. Some data exists for site specific areas, but no statewide data exists. Feel like arguing in a circle for awhile? We are faced with a question that cannot be clearly answered with our given knowledge of landslides in Washington, but we can continue to collect data and make inferences as to how landuse might be impacting our land.