This morning there were only a few reported landslides, one that occurred near Port Angeles that unfortunately resulted in the death of Neal Richards of WSDOT, who was struck by a branch while working on clearing the landslide. The only other reported landslides so far was by Jefferson County, who said they have received reports of mudslides. As the morning goes on, we should be able to gather more information on these landslides.

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Nile Failure Plane

November 19, 2009

One of the things we like to try and figure out is at what depth is the landslide moving and what is it sliding on. Usually, landslides like to slide on beds of weakness, in Eastern Washington, this can be interbedded silts and sands between basalt flows. In Western Washington, the Lawton Clay in the Seattle area perches water of the Esperance Sand, creating a perfect failure plane for landslides in that area. The Nile landslide is a little different, since we are talking about an area that has experienced at least three major sliding events. The subsurface could be torn apart and we would never really be able to figure out a good clear place where it is sliding. However, we may just be in luck. On the area known as the Woodshed Restraint a recent well log might have given us a giant leap forward on what the sliding plane might be:

Well Log on Woodshed Restraint

One word of caution, well logs can be misleading and sometimes downright falsified. However, this log does seem to fit quite well to what we know of the area. So, jump down to the Black and Brown Broken Clay. Although we could jump to the conclusion that is was broken due to previous sliding, there would be some other arguments coming into that as well as to what could have broken it up. This clay could very well be an old paleosol (old soil layer). The groundwater looks like it is right in that area as well, so clay + water = great sliding plane. What we think has happened in this area of the Woodshed Restraint is that this block was pushed over various sliding activities into the valley. I think this well log helps to support that claim, since below the soil is gravel and boulders, most likely fluvial deposits within the valley floor. Basically, we are thinking this is the historic sliding plane and the plane we are expecting it to continue to use. So, before we make any final conclusions, WSDOT has a lot of drill holes and we are studying the other well logs as well, so we need to try and correlate these findings with other data points in the area. Stay tuned…

Two bigs storms are hitting Washington State, one blowing in last night, another blowing in this afternoon. So far, no landslides have been reported over major roadways or have made it into the media (although where we had rainfall so far isn’t well covered by the media). However, this last storm added water into already soaked hillsides, setting up the stage for the potential for sliding this evening and into tomorrow.
We don’t have a forecasting system up yet (so far I have been swamped by other projects and haven’t been able to spend enough time getting it going). But, we can try and make some estimation of areas that will have a higher chance of sliding. I would put this akin to a back of the napkin calculation.

Landslide Risk Map Nov 19-20

Landslide Risk Map Nov 19-20

This is our forecasted rainfall for the next day or so (including some of the precipitation from yesterday). The things to note in all of this, much of the higher elevations where higher rainfall is shown is mostly snow, I never parsed that out in the file. Next, I just used forecasted inches of rainfall to determine where the difference between low and high should be. It is a little arbitrary, but I did look back at the other smaller storms with somewhat similar soil saturations to help determine when we started seeing landslides initiating. This is more of an experiment at this time to see if we can make a really simplified forecasting system that tries and predicts which counties will be at risk of landslides during a storm. A note of caution, even in the low areas we can expect landslides, especially in urban areas. In less urban areas, water usually knows where it wants to go, has been going there for a long time. In urban areas, we have a lot more control over that water, we channel it on roadways and usually discharge it into sewers or into creeks. The problem, if a road channeling water is blocked (either by leaves, debris or some other thing), that water can be diverted, saturating a nearby hillside and causing a landslide, even though rainfall was low. This can also occur with property owners concentrating water on their property. There are a lot of other factors involved of course, but you get the idea.

I talked with Gus Melonas of Burlington Northern Santa Fe. He said that the reported landslides are not quite landslides (well, one probably is a small slump or something). A landslide occurred 4 miles north of Carkeek Park on November 17th, around 6-7am. The landslide was about 30 feet in length, 4-6 feet deep (mostly brambles and blackberry bushes) and composed of mud (no rock). He said it didn’t flow across the track, but any landslide can close the Amtrak Service in that area, so sorry train folks, you have 48 hours to wait. The ‘landslide’ in Skykomish was only a tree that was blown over onto the tracks.
There is one other landslide, one reported up on the Hoko Drainage (Olympic Peninsula), reported by one of our Forest Practice Geologists who helps evaluate that area. That brings our total to 5 landslides from this last storm.

Here was the original post (now defunct):
Two landslides were reported on Burlington Northern Santa Fe railroad lines during this storm, but the news is slow to get out. A landslide in the Carkeek Park area north of Seattle closed Amtrak passenger service for the mandated 48 hours. Another landslide was reported to be on BNSF tracks near Skykomish (I suspected there were a couple of landslides up there). I am attempting to contact BNSF to get some more information on these landslides.

November 16-17, 2009 Storm

November 17, 2009

Here is a precipitation map of this last storm (with reported landslides):

I suspect we have a landslide or two up in the area north of Highway 90, but so far, nothing has been reported.

I should have given the chance of landslides a bit higher than low to moderate, but at the time, the rainfall might not have been intense enough.

We have a couple of landslides across Washington State this morning, most notably, a landslide out near Hoodsport on US 101 at milepost 324. The road was closed at 11/16/2009 9:39 PM, so the landslide occurred somewhere in the 9:00 PM hour. This is very near the same spot that a handful of landslides occurred on the December 3rd Storm in 2007 and this area is quite susceptible to intense precipitation (storm) events.

Another landslide occurred on the Olympic Peninsula on State Route 112 at milepost 2.00. Apparently this occurred on 11/16/2009 at 1:00 PM, but I didn’t catch wind of it until this morning.

If you see a landslide, please report it to us. You can contact me at: 260.902.1425 or by email: isabelle.sarikhan@dnr.wa.gov

A small landslide occurred in Gig Harbor today (Nov. 16), damaging a garage and two vehicles. The landslide occurred at 29th Street Court Northwest near Shawnee Drive around 1:30am.

Landslides in urban areas, especially during these first storm events, could be caused by misdirected water or water inadvertently being directed into an unstable area. Something like a drain blocked by leaves causing water to flow onto the hillside. Unfortunately, I have been dealing with my computer dieing this morning, so I haven’t been able to investigate this landslide on the scene. Hopefully I will make it out there tomorrow.

This ground is starting to get to a good saturation level. The chance is probably low to moderate for at least one or two more landslides around the Puget Sound Area from this storm.

It has been awhile since I completed a Landslide of the Week. I think the Sanford Pasture Landslide is a good candidate since it has gotten so much press lately and what we know about it is fairly limited (at least, in publications).

The formation of the Sanford Pasture Landslide started back in the late Miocene and early Pliocene epochs, where the eruptions covered much of Eastern Washington with basalt, known as the Columbia River Basalts. Between the eruptive cycles, sandstones, generally fluvial in origin, deposited on top of the flows, only to be covered by the next pulse of magma. These are known as interbeds and are suspected to be Ellensburg Formation. At the Sanford Pasture Landslide, the dominant flows of the Columbia River Basalts are the N2 and R2 flows of the Grand Ronde Basalts, some of the last recorded flows of the eruptive cycle. Much of the deposits were lain horizontally, but as we know them today, the geologic units are folded and faulted. This is accomplished by stress from the subductive oceanic plate pushing its way underneath the continental crust that we live on here in Washington State. The force of the collision compresses Washington State, forming wrinkles and faults as the stress is dissipated through the plate. In the Naches area, this folding resulted in the formation of Cleman Mountain as a steeply dipping anticline. The area was not able to just fold to reduce the stress on it, it faulted as well, forming the Nile Thrust Fault. The failure mechanism is something that we probably do understand. The oversteepened anticline combined with the weak interbed layers of sandstone created a perfect weak plane for the above rock to slide on. An earthquake, probably on the Nile Thrust, or perhaps something larger like a Cascadia Subduction Earthquake, probably reduced the restraining forces enough to start the material moving downhill, depositing where we see it today (more on that below). These events occurred after the Columbia River Basalts and interbeds were lain in place, giving us a limiting age on the landslide. Given the flow age, coupled with the folding and faulting of the area, the general estimation of the landslide is 2 million years old.

Determining the age of a landslide is often difficult. Dates can be acquired through a couple of different methods, most often coring into sag ponds, or lake bed deposits (on older landslides that have dammed rivers), or by coring old tree snags that have been drowned. The goal is to find datable material or stratigraphic reasoning to determine a specific of general age. For the Sanford Pasture, there are no found lake bed deposits up valley of the landslide initiation and the landslide is too old to support sag ponds that formed during its initial movement. The general thought is that the landslide occurred prior to glacial times.

The Sanford Pasture landslide moved across what is today the Naches Valley and deposited material almost a mile inward from the valley’s edge. During this time, the Naches Valley was less incised and contained much less water (remember, no lake beds deposits), so whatever damming of the paleo-fluvial system here, it was minor. During the age of glaciation in the Quaternary Period (predominantly alpine glaciation influences at the Sanford Pasture). Advances and retreating of the glaciers, combined with their constant run-off carved much of the valleys and fluvial systems we see today in the area. I should point out, I don’t think any glaciers have reached the Sanford Pasture Landslide area. The melt water flowing through what is now the Naches Valley would have eroded out the landslide and continued to incise into the valley, exposing in-place Columbia River Basalt Flows on the western side and eastern side of the valley. Unfortunately, all of this erosion created yet another unstable element into the system. The eroding river removed much of the lateral strength that the landslide had when its mass continued for another mile. It literally shortened the landslide by half. In response, the Sanford Pasture landslide didn’t fail as one large piece, but as smaller failures within the older landslide material.

This image of the Sanford Pasture Landslide is a quick drawing of the possible major landslide events. There are dozens of smaller events throughout the landslide. The most difficult part to figure out is the northwest section of the landslide, that appears to have gone through a series of deformations, probably more than I have drawn here. That is something we are going to try and unravel down the road. It is difficult to determine if the last major movement was on the eastern or western section of the landslide. The only sag pond that exists on the landslide is on the eastern side, known as Dog or Mud Lake. This makes me suspect that the last major movement has been on the eastern side. Other evidence also suggests that the morphology is younger, less stream development and incision on the eastern side. Regardless, the western side is the side where the Nile Landslide initiated off of and probably has a much more active, smaller landslide activity.

The area where the Nile Landslide has occurred has experienced several large landslide events. Looking at the history, the Nile Landslide is probably the 4th in a series of movements in the area (Sanford Pasture, Largest block in purple, smaller block in green, then Nile Landslide). That is the larger movements. Further evidence looks like smaller landslides have been recent in the same area as the Nile, maybe being able to form and move every couple of hundred years (not sure how far back this might go, but maybe a thousand or two years, depending on when the major movement of the largest block in purple and smaller green block occurred). Granted, that is a bit of speculation. In the 1940’s photo, there is clearly areas without vegetation that look hummocky that might indicate recent movement, like within the last 50 years. Comparing that 1940’s photo to today, areas that were once void of vegetation now are supporting sparse tall trees, indicating a possible regrowth period. Maybe we are looking at something that is geologically common here.

The last work, Sanford Pasture Reactivation. This has been pushed around in the media about State Geologists concerned about future movement of the Sanford Pasture Landslide. They are right, we are concerned, I being on of them. The removal of lateral support by the Nile Landslide could reactivate something larger uphill. Remember, this is really torn up landslide material, it has its strength reduced and it looks like it is sliding on something that is fine grained. Reactivation of the Sanford Pasture Landslide, worst case scenario, would completely block the Nile Valley, forming a massive lake (Lake Naches?) behind the debris. The threat would then continue into the competency of the material to hold the water, a race to safely dewater the lake and the possible major dam-burst flood into the Yakima Valley. The destruction of that last one would be unlikely, but something we have not seen the likes of in modern society.

Orthophoto and Fissures

November 9, 2009

WSDOT released an orthophoto of the landslide last Friday (at least, this is when I got it). The image is spectacular and helps give us some much needed data. This weekend I worked on mapping out all of the fissures in the “Woodshed Restraint”, as well as other places (that was much quicker, since the cracking was predominantly localized there).

A note of caution on this map, these fissures haven’t been field verified, so they could change, move, or disappear. Especially some of the cracks outside of the Woodshed Restraint area. We have some data on these as well as for the types of movement (uplift, down dropped or translational movement) and that will help us map the stresses and block movement within this mass of earth.

Last week DNR issued an order to suspend mining at the Simmons Quarry. There is a long story within that, but also one that may lead to some legal issues. The continued potential for danger and instability in the area gave us concern for public safety in the area, especially since we have residences living in houses right on the landslide. More on that later.