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.

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.

Thunderstorms over Chelan County have caused a small landslide that destroyed a driveway on the Stemilt Creek Road. According to the Wenatchee World:

” “I wouldn’t say our driveway is damaged. I’d say its nonexistent,” Tammy Liebert said of the 10-foot by 30-foot mud slide that swept across the gravel road to her home at 3550 Stemilt Creek Road. “My husband’s out there with a backhoe now.”

Liebert, her husband Terry Liebert and their 17-year-old daughter were home during the slide, which happened around 8:30 p.m.

“It rained huge drops about the size of chicken eggs, then it hailed about the size of quarters and then there was nothing, and then a huge downpour,” Liebert said. “We heard a noise like a semi skidding down pavement. It was the rumbling of rocks rolling together down the gully.”

Besides garbage can-sized boulders, tree trunks and mud, the Lieberts’ road is also covered in tires.
Liebert said the slide took with it part of a hill along Wenatchee Heights that is a popular teenage drinking spot.

The teens take tires up there and roll them over a guardrail to see how far they go, Liebert said.

“The kids strictly do it for entertainment,” she said. “Once we saw one bounce about 10 feet in the air.”
She said neither the home nor their vehicles were damaged in the slide, but neighbors also received mud damage to their driveways and an orchard.

The storm itself doesn’t appear to be too devastating with rain, so this probably barely reached a threshold limit on an area that is already very unstable. ”

Precipitation Map of Eastern Washington

Precipitation Map of Eastern Washington

The rainfall was probably under .5 inches of rain in the area. However, the area that the landslide initiated out of came is the lateral scarp of the Stemilt Landslide and is a fairly unstable area.

Stemilt Road Landslide

Stemilt Road Landslide

This photo shows a fairly steep scarp with older landslides and scree along the cliff side. A failure in one of these gullies or an area that ponded water could easily turn into a debris flow or debris avalanche and be able to do some damage downhill.