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.

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.

We tried getting into the Pilchuck headwaters yesterday to no avail. Roads were washed out and gates were locked. We did see a number of landslides on the Sultan Basin Road heading up to Olney Pass. These landslides bare the mark of a large rainstorm event and almost certainly moved during the January 7-8th, 2009 storm event

Sultan Basin Road Landslides

Sultan Basin Road Landslides

The picture has a backdrop of 2003(? I think it was later than that) Snohomish County LiDAR.

The most interesting of these landslides is a debris avalanche at the bridge crossing at Olney Creek. It was probably dealt a one-two punch, the swollen Olney Creek was probably eating away at the bank (and probably has been for years) and the saturated ground allowed enough driving forces to overcome the resistive forces. It also moved a good amount of timber into the creek, which might cause a problem down the road by creating a debris dam behind the bridge.

Sultan Basin Rd Debris Avalanche.  Photo by Carol Serdar

Sultan Basin Rd Debris Avalanche. Photo by Carol Serdar