We received a tip yesterday that a landslide recently became active, probably in the storms that blew through the area in mid to late November. The landslide occurred in a quarry that has been inactive for some time. The highway south of the quarry is State Route 112.

The quarry is an old clay mine that use to supply clay to the cement mills in Seattle. It is actually still owned by a cement company that has a working plant in Seattle. The report we got was the landslide had rocks that were actively rolling town a scarp and some of the pictures we received are amazing.

Photo by Dave Parks, Department of Natural Resources

Photo by Dave Parks, Department of Natural Resources

The good thing, so far this landslide isn’t threatening any houses, pipelines, powerlines, or such. If it starts progressing uphill, it could eventually impact State Route 112, but at the moment it seems unlikely. The area of the site is part of the Miocene-Oligocene marine sediments of the Pysht formation of the upper Twin Rivers Group, which is locally interbedded carbonaceous and fossiliferous claystone, siltstone and some fine grained sandstone. A clay layer within the Pysht formation at the site was mined for cement manufacturing in Seattle. Also the Burke Museum has a whale skeleton (Miocene) from this quarry, probably by Jim Goedert. The Pysht formation also contains many near shore fossils, like bivalves and gastropods. So, we will keep our eyes out for another whale skeleton (although we might need a bigger truck). Before you think this would be a great place to head out, know that this area is extremely dangerous. Rocks are currently raveling down the scarps and toe of this landslide and sections of the landslide can move and give way at any moment. It isn’t worth your life to try and collect fossils here. The ground is torn apart and you could easily fall into a covered (or uncovered) crevasse and get trapped. Sudden movement of the landslide or the snapping of tree roots could knock over trees without warning. This area is also private property and no tolerance will be given to anyone caught in this area, especially given the extreme danger that this area faces. If you would like to collect fossils from the Pysht Formation (or anywhere like that), contact the DNR Geology and Earth Resources Division, they can help you find safe and legal locations to collect on.

Nile Landslide LiDAR

December 10, 2009

Sometimes you get a Christmas gift you weren’t expected. I guess in this case, it was more of a birthday gift. LiDAR for the Nile Landslide has finally preliminarily been released to various people to help in analysis on the landslide, but has not yet been released to the general public until the proper QA/QC has been established on the data. If you are working on the Nile Landslide (and I have talked with many of you already) and would like a copy of the LiDAR, contact me and I’ll see what we can arrange.

That being said…

I am going to analyze the lidar a bit more, but some interesting things. You can clearly see some of the uplifted areas, although it might not be as extensive as we previously thought. There are a few secondary landslides within the landslide mass, one major one by the quarry. Near the center of the landslide, there are some odd peaks (near the toe), not sure what those are, but I’ll try and find out today. Anyway, stay tuned, this is a key piece of data to help us understand the Nile Landslide. The Lidar also covers the Sanford Pasture Landslide as well and this will give us clues to the activity and morphology of this landslide as well.

Well, we almost got through Thanksgiving without a landslide. There was another landslide on Chuckanut Drive near Spokane Street. The first time I saw reported was 11:45am on 11/26/2009. That makes 11 reported landslides so far.

These little storms that roll through are really a wealth of information. Since we are embarking on trying to make a landslide forecasting system, we need to figure out the precipitation thresholds. The nice thing is, these small landslides are probably failing just at the border of that threshold. The problem is figuring out the precipitation variables. As far as I can figure, it doesn’t look like the USGS Seattle Landslide threshold model is holding up in the rest of the state (nor should we expect it to).

So, I threw together the precipitation data for a 3-day, 15-day USGS threshold models. We are assuming that the areas are probably at or very close to antecedent moisture threshold values. As you can see in the map, almost all of western Washington was above the threshold and guess what, we did have landslides. However, the amount of false positive that this map portrays is fairly concerning, unless there are a ton of landslides we haven’t seen. I guess the model is fairly simple in that it just says there is a likely chance that landslides could occur past the threshold. I think we might be able to improve upon this, maybe we can come up with a threshold model and as we pass the threshold, we can come up with a estimates landslides per acre, or something relevant that we could better plan or manage for an impending disaster. The biggest hurdle I am facing right now is, what sort of day ratio is the correct one. I don’t think the 3 day-15 day is going to work, or if it is, we need to drastically change the threshold value for various parts of Washington. Second, do we have indicator areas when the thresholds are starting to get reached, say, Chuckanut Drive or Eagle Cliffs down on SR 4. I have a dozen other questions, both those are foremost on my mind today.