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.

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.