Thursday, April 3, 2014

Oso/Hazel/Steelhead: Foreseeable

Dave Petley asks a fundamental and very important question regarding the Oso/Hazel/Steelhead landslide Could it have been foreseen? I fully agree with his conclusion. And I will add the answer to his question is very important.

The LiDAR imagery makes that conclusion relatively easy to reach. However, even before LiDAR was available for this area, previous geologic mapping done without LiDAR had mapped out the large landslide deposits from the landslide area just to the west of the Hazel Slide.

Part of the Sauk 1:100,000 Geologic Map (2002)

The landslide hazard at this location was foreseen by Dan Miller dr-millers-oso-hazel-landslide-reports and Tracy Drury and I suspect a few other geologist that had worked in the area. This landslide and the hazard it posed was foreseen. The Hazel/Steelhead landslide had been actively moving on and off for many years. Analysis of the slope had indicated large run out distances. The risk that the slide would expand was raised. The LiDAR images as well as the previous published mapping indicated that landslide run outs from failures of the same units from very nearby had run outs that crossed the entire valley floor. That is the LiDAR that Dr. Petley used as an obvious indicator.

Rereading the Miller and Sias paper as well as other reports by Miller and a report by Drury is digging into the early stages of a disaster and watching it unfold. In a report on the slide Miller did in 1999 he provided an analysis of the then active landslide area and potential run out distance that was very close to the slide run out that took place in 2006. But he as well as Drury both stated concern about the slide area expanding and Miller noted that if the slide expanded to include a previous old slide block the results would be an order of magnitude greater. That is 10x greater in science geek lingo.

The papers and reports by Miller and Drury are the ones I know of. But I also have known a number of geologists and scientists that were familiar with this site and they all expressed grave concerns to me. Especially after the large slide in 2006. It is why that I knew as soon as I heard the news of a river blocking slide that it was this slide. It took me 5 minutes to put up the first post I did on the slide HERE.

The 2006 slide and the slide movements after lowered the support for the very high and very unstable slope above. It was just a matter of time and the threshold of the disaster would be crossed. Predicting when that threshold would be crossed is not something we can do, but this disaster was clearly close at hand. It was not some "oh way off in the future" event. To say otherwise would be irresponsible.

Understanding this from a hazard management perspective is very important. If the landslide could be foreseen and indeed was foreseen, this slide does have policy implications that we should learn from. Yes, geologic disasters do happen that may humble us because we can not predict accurately when or how severe, but in the case of this landslide geologists had a very good idea this was a bad landslide that posed a significant threat. The problem is not with the science, but with policy. 


Paul Johanson said...

Dan: I greatly enjoy your blog and have for quite some time. I have especially appreciated recent posts and links on the Oso tragedy. It seems clear that this event may alter the way we deal with environmental threats here in the PNW, and I'm interested in your thoughts on what we might do differently. It seems as though there are multiple levels of large-scale cognitive dissonance coming to light, e.g., multiple specialists seemingly aware of major imminent threat yet multitudes including even the highly curious (like me) comparatively oblivious to specific high-risk sites like this. Aside from several others you have addressed recently, at risk of stirring up fear, something we have way too much of already in today's world, how many comparably scary threats looming above human development in western Washington would you think there could be?

Eric Bilderback said...

Dan, thanks very much for this concise reiteration of what I suspect many geomophologist who have worked in the Puget Sound region believe. I personally was mildly dismayed by the 4 April Science news and analysis piece “Even for Slide-Prone Region, Landslide Was Off the Chart”. While there is some good information in the piece the headline and some of the content of the piece gives the impression the scale of the disaster could not have been foreseen. Richard Iverson, a landslide geologist who I have immense respect for based on past work, makes the argument that the Oso/Hazel/Steelhead slide was considerably more mobile than other slides of that size. This is presumably based on a global landslide database and at face value is a true statement. However as you state, there is clear evidence from the lidar that landslides in this section of the Stillaguamish River have run out across the entire valley in the past. Thus the comparison to the global data set is interesting but not relevant to the risk in that section of the Stillaguamish. I view the Science article and any other media that furthers the “we could not anticipate this” narrative surrounding the Oso slide as a real disservice to humanity and the science of geologic hazards.

Bob in the Pacific Northwest Wilderness said...


Since 1990 passage of the Growth Management Act counties have had the responsibility to designate Critical Areas that are of concern for development.

I gave up trying to find on line information for Snohomish County but it must exist.

Dan McShane said...

I have schemes to address all three of the above comments in future posts as they are important - just need some time. Thanks