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.

This landslide has brought together an surprising amount of scientists from various agencies around Washington State. One that certainly deserves mention is John Vidale, Director of the Pacific Northwest Seismic Network, and his crew, who has been of invaluable help to us in helping to unravel the timeline of this landslide. Here is an excerpt and some data that John had given us:

These are spectrograms, which plot frequency content of the seismogram the vertical axis against time on the horizontal axis. The number on the horizontal axis is hours after the start of Saturday, for example, 34 is 10am Sunday. I think you can see more detail on them by looking at them in a graphics program. This 1st plot runs from 25 to 35 hrs, the bright red spot is the landslide noise at 10am Sunday. The vertical axis is frequency – 0 at the top grading to 10Hz at the bottom. The 5Hz sound of the landslide grows from imperceptible on the left until the racket at 34, then fades slowly.The industrial source at 9Hz is visible as the pulsation on the bottom, and the pops are too short to see in this plot. The 5-10Hz smears in the lower right are probably unrelated cultural noise that starts at daybreak after a quiet night.

Close-up of the noisy part, spanning about 1.3 hrs or 80 minutes.

This is the noisiest part of Saturday, hours 5-21, on the same color scale. More cultural noise 5-10Hz starts about 7am, as appeared above for Sunday. There is not a signal similar to the 5Hz band above, which is apparently how the landslide appears on this station. Also, the patches of signals present do not match the timing of energy on the other nearby station ELL. So maybe some Saturday landslide noise could be invisible on this plot, but it would be less than the noise on Sunday.

Here is an example of the pops at their most frequent, 2 hours before the big noise. The plot spans about 15 minutes, and the pops appear on the upper half of the plot, 1-5Hz, and agree in timing with pops seen on station ELL.

This is the burst at 7:38 Sunday in a 15 minute window. Note the strong 1-2Hz energy, more so than during the rest of the landslide-related signals, and most of the action takes place within 1 minute.

This is the 4:55am Sunday burst in a 15-minute window, weak but with the same frequency range and gradual onset as the other slide related shaking.

This sort of data allows us for form a timeline to the landslide movement. Combined with eye witness reports, we can reconstruct the various parts of the landslides and when they moved. With that data, we can look at the places of initial movement and evaluate the pre-failure conditions to see if there is any likely event that might have triggered this landslide. Vary preliminary data, however, has been suggesting that landslide movement might have been prior to 2002 (we are still working on this), but this movement was quite slow, just about creeping. I am currently working on tying together a series of aerial photos to determine the amount of movement and hopefully to constrain the first start of movement.


Sampling the Ice

October 22, 2009

I just got back from GSA, so I apologize for the delay in the posts. We have had a lot of talks about this landslide at the conference and have a lot of heads thinking and working on it. Particularly, the ice. It actually might play a big part in the landslide failure. If the landslide formed deep cracks through previous slower movement, it could have allowed warm air into the area where the ice was. This could have resulting in melting of the ice, which would have added in water to the system or just made the perfect slide for a block of ice to flow on. The low angle of the landslide would suggest something like this could have occurred. Once we gain a sample of the ice, we will run some tests on it, try and determine its age and so forth. Hopefully it will shed some further clues on this landslide.

As I thought might have occurred, the places where we first found the ice had collapsed and it was impossible to safely gather a sample of ice (ever try digging on an unstable talus slope?). So, that clue looks like it might be lost unless we can recover something from drilling.

Quick Report

October 16, 2009

Okay, lack of sleep and long hours have been catching up to me. Here is the latest:

On October 15, 2009, Jack Powell (FP), and Isabelle Sarikhan (DGER) investigated the Nile landslide on Highway 410 and the older Sanford Pasture Landslide approximately 10 miles west of Naches. There is concern that the Sanford Pasture Landslide could reactivate as a result of the recent movement of the Nile Landslide. We also investigated potentially new activity within the Nile Landslide, continued risk to public safety and the safety of crews working on the ground, and collected additional data.

No immediate danger was found for continued progression of failures. No cracks or fissures were found above slope of the Nile Landslide and a traverse of the crest of the hill above the Nile Landslide also found no cracks or fissures. Further, evidence was found that the Nile Landslide occurred below the failure plane of the Sanford Pasture Landslide, greatly reducing the risk of movement by the release of pressure from the Nile Landslide. However, because of the size and wide scale impact that the movement of Sanford Pasture can cause, further surveys for recent cracks and fissures further uphill on the western edge of the Sanford Pasture Landslide should be conducted.

The Nile Landslide had moved several feet since Sunday. During our visit on Oct. 15, no rapid signs of movement were occurring, but the landslide is probably still moving at a speed of inches per day. The speed will gradually slow, but movement might not completely stop for months. Sudden catastrophic movement is unlikely and the risk of public safety and workers on the ground should be low if they stay a safe distance from the landslide. The most unstable parts of the landslide are the toe, which is oversteepened and can result in shallow failures and the scarps, both internal and the main scarp, which can ravel or result in shallow failures without warning.

Nile Landslide Field Visit

October 15, 2009

Tomorrow looks rainy, really rainy out by the landslide. In the dryer climate of Central Washington, dirt roads are not always built to drive on when wet. So, we might not be able to drive very far up on the Sanford Pasture Landslide tomorrow. We are still going to investigate on and around the Nile Landslide. We have heard that there is uplift of up to 8 feet on the old Naches River bed. We will be examining for cracks around the landslide to see if the landslide is propagating (expanding) and potentially going to continue to fail. If we can, we will also try and hike or drive up above the Nile Landslide to look for stress or tension cracks from the release of material below. I will be posting probably on the trip on Friday Morning.

Oct. 13 SR 20 Landslide

October 15, 2009

In all of the focus on the Nile Landslide, I just about missed this landslide up on SR 20.

DATE/TIME: October 13, 2009, 2:19 p.m.
DESCRIPTION: Both lanes of SR 20/North Cascades Highway are blocked by a rockslide at milepost 145.5, the Skagit/Whatcom County line.

LOCATION: SR 20 at the Skagit/Whatcom county line
START: October 13, 2009, 2:05 p.m.
Est. END: unknown
OTHER: WSDOT maintenance crews are en route to the slide and will provide an estimated reopening time once they assess the area.
CONTACT: Bronlea Mishler, 206-440-4455

Ice in the Nile Landslide?

October 14, 2009

One of the odd things that we discovered during our investigation on Sunday was chunks of ice in the Nile Landslide. Ice. I guess I should qualify this a bit better. The ice is between the coarse rocks and it doesn’t look much like ice when you are next to it. Here is what I mean:

Ice in the Nile Landslide

Ice in the Nile Landslide

To get it out of the way, the ice probably didn’t play much, if any role in the landslide failure. It is probably a product of the talus, which in Eastern Washington can sometimes form ice cores. These are well known to many in Eastern Washington and even were mined early on as natural cold storage. This isn’t something I know a whole lot about, but my geologist friend Jack from Eastern Washington explained it to me. Cold air from the surface is able to permeate into the talus, forming a barrier from the hot air. As water infiltrates down into the talus, either through precipitation, snow melt, or shallow groundwater (or other ways), the water hits that cold air and starts to freeze (as long as the air is colder than freezing). The cycle continues as cold air can continue to permeate down, keeping the ground cool and protecting it from the hot air above.

I will be heading into the field on Thursday, either with WSDOT on a helicopter flight or by vehicle. Either way I will be hiking on the west side of the Sanford Pasture Landslide, checking for stability. I’ll keep you posted.

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It is a strange thing to see how TV works from the production side. The interview I had was maybe 15 minutes or so, but it all boiled down to about 10 seconds of film on air. I am glad to see that this landslide is getting the attention it deserves. This is a major event in Washington’s history, at least, landslide history.

Features of the Nile Landslide

Features of the Nile Landslide

This is a generalized map of the different features present in the Nile Landslide. The main landslide mass is located in red and deformation at its toe is yellow. The salmon color block is an area where cracks were noticed along the hillside. The orange is a block of material that was pushed and cracked by the Nile Valley Landslide main landslide body. The diverted Naches River is blue in color. These offer a representation and a general boundary of the features. More detailed delineation will be conducted once the landslide is safe to work on.