Precipitation is an important component into landslide movement. During the investigation into the Alderwood-Banyon and the Carlyon Beach-Hunters Point Landslides, long-term precipitation (over five years) had been above the mean average. This is thought to be the main driver of these landslides. In the same thought, maybe the Nile Landslide has experienced above average rainfall over a period of time, similar to the other two landslides. We asked Cliff Mass (click here for his Blog) at the University of Washington Atmospheric Sciences to help us figure out the the precipitation history of this area. The data, emailed from Mark, an colleague of Cliff Mass, isn’t a smoking gun. The email below:

I have looked at water year annual precipitation for 2 snotel sites situated on the east slope of the Cascades but somewhat north of the Niles Landslide. They are Blewett Pass and Grouse Camp snotel sites.
There are no snotel sites in the immediate vicinity of the Niles Landslide.

Over the past water year (Oct 2008-Sept2009) precipitation totalled 10% above the long term average (1983-2008) at a composite of the two snotel sites.
Over the past 2 years ==> 2% above the long term average (1983-2008).
Over the past 3 years ==> 6% above the long term average (1983-2008).
Over the past 4 years ==> 7% above the long term average (1983-2008).
Over the past 5 years ==> 0% above the long term average (1983-2008).
The 2005 water year was unusually dry bringing the 2005-2009 5-year average back to nearly the same as the long term average.

Hmm, well, looks like we are back to the drawing board.

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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.

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.

One of the more interesting landslides I have ran across was a debris flow triggered during the December 3rd, 2007 storm west of Pe Ell, Lewis County. Kelsay and I were conducting a reconnaissance of SW Washington to try and find out just how bad the landslides were. We pulled up through Pe Ell to find two debris flows that had come across State Route 7 and surrounded a house.

Pe Ell Debris Flow - Impact to House

Pe Ell Debris Flow - Impact to House

We parked and I decided to hike up the western debris flow as Kelsay went to look at the eastern debris flow. Unfortunately, I didn’t see the ditch that was covered in mud and quickly went up to my hips in mud (and since I do all my field work in a skirt, that was about as awful as it got!).

Western Pe Ell Debris Flow

Western Pe Ell Debris Flow

Past falling in the mud, the hike up was fairly easy and the scarp was amazing. A thin layer of soil and dirt has slid off of a hollow area (not to be confused with the DNR Forest Practices definition of a bedrock hollow), that is a volcanic tuff.

Western Pe Ell Debris Flow Scarp

Western Pe Ell Debris Flow Scarp

The soil is between 6 inches and two feet, depending on which side of the scarp you are at. The convergent topography with intense precipitation probably greatly contributed to the landslide moving. The rainfall in this area was probably between 16-20 inches during the storm (the majority falling on December 3rd). Intense rain + shallow soils + impermeable substrate = landslide. Actually, that is the formula we saw again and again for almost all landslides during the December 3rd storm.

The other interesting thing to point out, the area was recently harvested. The lack of canopy coverage can increase the rate that rainfall will reach the ground (from a timed delay to no delay). On weak storms and wet winters, this could increase landslide activity, but we haven’t seen it very many compelling cases around Washington State (but there is a nice study from Canada). However, during the December 3rd storm, the intense precipitation and lack of canopy might (and I will go out on a limb and say almost certainly) have increased landslide activity. To what extent and what increase, that remains to be seen.