The death toll from the Steelhead landslide near to Oso in Washington State is continuing to rise. Latest reports suggest that there are now 14 known fatalities, but 176 people are reported to be missing. It is quite normal in this sort of event for the number of reported missing people to exceed substantially the actual number of victims, so this maximum toll may reduce in the next few days. However, it is still likely to be the costliest landslide in terms of lives lost for many years in the USA.
Details are slowly emerging of the landslide history of this site. It is clear that major landslides have occurred here on many previous occasions; indeed so much so that the landslide is known as either the Hazel landslide or the Steelhead landslide; at this stage I am opting for the matter given that the inundated area is known as Steelhead Drive.
Better images of the Steelhead landslide
I very much appreciate the help that numerous people have given me over the last few days to put together this post – too many to name, but thanks to you all. The best graphic that gives an overview of the slide is in the Seattle Times:
The original version has a very impressive slider function that allows the user to flip from one image to the other. I can’t replicate that, but putting the two images side-by-side shows the extent of the devastation. The number of inundated houses is large, suggesting that the loss of life will be high, especially bearing in mind that the slide occurred on a Saturday.
The best set of aerial images of the slide are on the Flicker page of Governor Jay Inslee – there are some wonderful images there. This image shows the source of the landslide:
The landslide has been widely reported as a mudslide. In terms of the lower portion, which did the damage, this is correct, although in places it might have been more of a mudflow than a mudslide. However, the upper portion is a rotational landslide – the rotated block with the fallen trees is very clear. A working hypothesis would be that this block failed catastrophically, transferring load onto the block below, which in turn generated very high pore water pressures, causing fluidisation and a very rapid mudflow that struck the settlements across the river.
1949: A large landslide (1000 feet long and 2600 feet wide) affected the river bank
1951: Another large failure of the slope; the river was partially blocked
1967: Seattle Times published an article that referred to this site as “Slide Hill”
1997 report, by Daniel Miller, for the Washington Department of Ecology and the Tualialip Tribes
1999: US Army Corps of Engineers report by Daniel and Lynne Rodgers Miller that warned of “the potential for a large catastrophic failure”
25 January 2006: large movement of the Steelhead landslide blocked the river
There is a good presentation about the 2006 landslide available online (NB pdf). This includes the following (somewhat blurry) image of the source of the 2006 landslide:
The slope that formed the scarp of the 2008 slide was undoubtedly very over-steepened, and of course was formed from weak materials. This looked like an accident waiting to happen.
Unfortunately the toll from Saturday’s Oso landslip is rapidly mounting. Latest reports suggest that at least eight people have lost their lives and that up to 18 more might be missing in the debris. Unfortunately, the site remains very dangerous, such that substantial areas have yet to be entered. This will be the worst landslide in the USA for many years. The last event on a similar scale of which I am aware was the 25th December 2003 debris flow in San Bernadino County, California, which killed 16 people. It looks likely that this landslide will be worse.
..
Oso landslip resources
Whilst I am referring to this as the Oso landslip, in fact it is a reactivation of an existing landslide, known as the Hazel Landslide. This landslide is known to have moved 1988, and went through a second phase of movement in 2006. It is well described in a blog post from 2009 that can be found at: https://slidingthought.wordpress.com/tag/north-fork-of-stillaguamish/
The landslide did occur in glacial sediments, as I indicated might be the case yesterday. There are some excellent resources on the landslide at the following three blog posts:
A very large landslide occurred yesterday morning near to the town on Oso in Washington State, USA. Unfortunately, three people have been killed by the Oso landslide, and newspaper reports suggest that three survivors, including a six months old boy, are in a critical condition, and that two more people have been seriously injured. At present it is not clear as to whether there may be other victims in the landslide debris.
The best set of images of the slide is to be found on the Seattle Pi website, which includes this overview shot:
The landslide is complex, but appears to have occurred in weak sedimentary deposits; I would guess with a glacial origin. The landslide appears to have a large arcuate scar with a large, rotated and partly disaggregated central block:
The toe of the landslide appears to have fluidised and flowed laterally (i.e. up and down the valley), suggesting that the landslide would have been rapid and highly destructive, which accounts for the fatalities:
There can be little doubt that this is a rainfall triggered landslide, though given its size there might have been a substantial time gap between the triggering event and the slide itself whilst pore pressures built up. An interesting aspect of the landslide is that the valley is now blocked. National Weather Service Seattle tweeted the gauging station data for the north fork of the Stillaguamish River downstream from the landslide:
NWS Seattle
..
The very rapid decline in water depth is very clearly apparent, but note also the short but dramatic spike in water depth immediately after the landslide, presumably caused by a surge of water induced by the slip entering the river. A key management task over the next few days is likely to be the creation of a bypass channel to reopen the river.
Acknowledgements
Thanks to John Garver, Lee Allsion, Peter Weisinger and Bryan O’Sullivan for helping me to track down the material for this post.
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the landslide in Washington State
Submitted by John on Wed, 03/26/2014 - 09:55.25 March 2014
The Steelhead landslide in Oso, Washington State
The Steelhead landslide
The death toll from the Steelhead landslide near to Oso in Washington State is continuing to rise. Latest reports suggest that there are now 14 known fatalities, but 176 people are reported to be missing. It is quite normal in this sort of event for the number of reported missing people to exceed substantially the actual number of victims, so this maximum toll may reduce in the next few days. However, it is still likely to be the costliest landslide in terms of lives lost for many years in the USA.
Details are slowly emerging of the landslide history of this site. It is clear that major landslides have occurred here on many previous occasions; indeed so much so that the landslide is known as either the Hazel landslide or the Steelhead landslide; at this stage I am opting for the matter given that the inundated area is known as Steelhead Drive.
Better images of the Steelhead landslide
I very much appreciate the help that numerous people have given me over the last few days to put together this post – too many to name, but thanks to you all. The best graphic that gives an overview of the slide is in the Seattle Times:
Seattle Times
..
The original version has a very impressive slider function that allows the user to flip from one image to the other. I can’t replicate that, but putting the two images side-by-side shows the extent of the devastation. The number of inundated houses is large, suggesting that the loss of life will be high, especially bearing in mind that the slide occurred on a Saturday.
The best set of aerial images of the slide are on the Flicker page of Governor Jay Inslee – there are some wonderful images there. This image shows the source of the landslide:
Image from Jay Inslee
..
Whilst this image from the same source shows the entire landslide mass.
Image from Jay Inslee
..
The landslide has been widely reported as a mudslide. In terms of the lower portion, which did the damage, this is correct, although in places it might have been more of a mudflow than a mudslide. However, the upper portion is a rotational landslide – the rotated block with the fallen trees is very clear. A working hypothesis would be that this block failed catastrophically, transferring load onto the block below, which in turn generated very high pore water pressures, causing fluidisation and a very rapid mudflow that struck the settlements across the river.
The history of the Steelhead landslide
The Yakima Herald has a very nice article that details the chronology of events on the Steelhead landslide. This includes:
There is a good presentation about the 2006 landslide available online (NB pdf). This includes the following (somewhat blurry) image of the source of the 2006 landslide:
http://www.stillaguamish.nsn.us/Publish/Steelhead%20Landslide.pdf
..
The slope that formed the scarp of the 2008 slide was undoubtedly very over-steepened, and of course was formed from weak materials. This looked like an accident waiting to happen.
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24 March 2014
Oso landslip: useful resources and the rising human cost
Oso landslip costs
Unfortunately the toll from Saturday’s Oso landslip is rapidly mounting. Latest reports suggest that at least eight people have lost their lives and that up to 18 more might be missing in the debris. Unfortunately, the site remains very dangerous, such that substantial areas have yet to be entered. This will be the worst landslide in the USA for many years. The last event on a similar scale of which I am aware was the 25th December 2003 debris flow in San Bernadino County, California, which killed 16 people. It looks likely that this landslide will be worse.
Oso landslip resources
Whilst I am referring to this as the Oso landslip, in fact it is a reactivation of an existing landslide, known as the Hazel Landslide. This landslide is known to have moved 1988, and went through a second phase of movement in 2006. It is well described in a blog post from 2009 that can be found at: https://slidingthought.wordpress.com/tag/north-fork-of-stillaguamish/
You can find a geological map of the area here: http://www.dnr.wa.gov/Publications/ger_ofr2003-12_geol_map_mounthiggins_24k.pdf
The landslide did occur in glacial sediments, as I indicated might be the case yesterday. There are some excellent resources on the landslide at the following three blog posts:
http://washingtonlandscape.blogspot.com/2014/03/arm-waving-notes-on-stilliguamish.html
http://washingtonlandscape.blogspot.com/2014/03/geology-of-silliguamish-blocking-slide.html
http://washingtonlandscape.blogspot.com/2014/03/aerial-history-and-lidar-of.html
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23 March 2014
Oso landslide in Washington State: three people killed and the river is blocked
Oso landslide
A very large landslide occurred yesterday morning near to the town on Oso in Washington State, USA. Unfortunately, three people have been killed by the Oso landslide, and newspaper reports suggest that three survivors, including a six months old boy, are in a critical condition, and that two more people have been seriously injured. At present it is not clear as to whether there may be other victims in the landslide debris.
The best set of images of the slide is to be found on the Seattle Pi website, which includes this overview shot:
Image from Seattle Pi
..
The landslide is complex, but appears to have occurred in weak sedimentary deposits; I would guess with a glacial origin. The landslide appears to have a large arcuate scar with a large, rotated and partly disaggregated central block:
Image from Seattle Pi
..
The toe of the landslide appears to have fluidised and flowed laterally (i.e. up and down the valley), suggesting that the landslide would have been rapid and highly destructive, which accounts for the fatalities:
Image from Seattle Pi
There can be little doubt that this is a rainfall triggered landslide, though given its size there might have been a substantial time gap between the triggering event and the slide itself whilst pore pressures built up. An interesting aspect of the landslide is that the valley is now blocked. National Weather Service Seattle tweeted the gauging station data for the north fork of the Stillaguamish River downstream from the landslide:
NWS Seattle
..
The very rapid decline in water depth is very clearly apparent, but note also the short but dramatic spike in water depth immediately after the landslide, presumably caused by a surge of water induced by the slip entering the river. A key management task over the next few days is likely to be the creation of a bypass channel to reopen the river.
Acknowledgements
Thanks to John Garver, Lee Allsion, Peter Weisinger and Bryan O’Sullivan for helping me to track down the material for this post.
Comments/Trackbacks (1)>>
John
“Civilisation exists by geological consent, subject to change without notice.” - Will Durant