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1 The Washington Landslide Disaster of March 22, 2014: An Armchair Interpretation. 1.Introduction This tragic landslide occurred at 10:37am on Saturday, March 22, 2014 in the valley of the Stillaguamish River near the town of Oso, about 55kms northeast of Seattle, Washington. As with many such tragic events, this was a disaster in waiting. The signs of potential disaster were evident and reported upon long before the event. Indeed, the site has been variously referred to as “Slide Hill”, “Hazel Landslide” and “Steelhead Haven Landslide”. Unfortunately, the reports passed unseen, were seen and ignored, or were seen and misjudged and residences and infrastructure were built adjacent to the site. All too often it takes a tragedy to bring the picture into focus. If there is a positive side, it is that useful things can be learned after the fact from such events. This essay briefly outlines factors contributing to the disaster, albeit from an armchair position, using media and other reports and interpretations of photographs from the scene before and following the event. Any disaster has two components: a hazard or hazards, and a condition of vulnerability or exposure of people, property and/or infrastructure to the hazard. In this case, the hazard, namely a landslide or landslides, developed over many thousands of years involving geological, hydrological and other processes. The condition of vulnerability developed over a much shorter period of time, arising from the settlement of the valley, the construction of roads and other infrastructure, and, most recently, the residential development on the left-bank flood plain of the Stillaguamish River, opposite the slopes that produced the landslide (Photos 1a and b). Photos 1a and b: Situation of the landslide (Source: www.cbc.ca) Before After

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Page 1: The Washington Landslide Disaster of March 22, 2014: An … · 2014-04-01 · The Washington Landslide Disaster of March 22, 2014: An Armchair Interpretation. 1.Introduction This

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The Washington Landslide Disaster of March 22, 2014: An Armchair Interpretation.

1.Introduction

This tragic landslide occurred at 10:37am on Saturday, March 22, 2014 in the valley of the Stillaguamish

River near the town of Oso, about 55kms northeast of Seattle, Washington. As with many such tragic

events, this was a disaster in waiting. The signs of potential disaster were evident and reported upon

long before the event. Indeed, the site has been variously referred to as “Slide Hill”, “Hazel Landslide”

and “Steelhead Haven Landslide”. Unfortunately, the reports passed unseen, were seen and ignored, or

were seen and misjudged and residences and infrastructure were built adjacent to the site. All too often

it takes a tragedy to bring the picture into focus. If there is a positive side, it is that useful things can be

learned after the fact from such events. This essay briefly outlines factors contributing to the disaster,

albeit from an armchair position, using media and other reports and interpretations of photographs

from the scene before and following the event.

Any disaster has two components: a hazard or hazards, and a condition of vulnerability or exposure of

people, property and/or infrastructure to the hazard. In this case, the hazard, namely a landslide or

landslides, developed over many thousands of years involving geological, hydrological and other

processes. The condition of vulnerability developed over a much shorter period of time, arising from the

settlement of the valley, the construction of roads and other infrastructure, and, most recently, the

residential development on the left-bank flood plain of the Stillaguamish River, opposite the slopes that

produced the landslide (Photos 1a and b).

Photos 1a and b: Situation of the landslide (Source: www.cbc.ca)

Before After

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(Source: www.accuweather.com)

2. The Landslide Hazard

Reports from local people indicate that this has been troublesome site for some time and a 1999 report

filed with the US Army Corps of Engineers by Daniel and Lynne Miller warned of the landslide hazard on

the basis of detailed research at the site. The pictures above tell us that a landslide hazard has been

present for some time. This is evident from half bowl-shaped feature on the hillside on the right-bank of

the river before the landslide (Photo 1a). The feature is even more pronounced after the landslide

(Photo 1a). The shape is indicative of a progressive downward slumping and, following the landslide, a

massive slumping and outward flow of material. Another feature to notice is that the river cuts into the

base of the slope on the outside of its meander bend. This suggests that the toe or base of the slope is

being eroded, and weakened over time, by the river. It is a hazard because the slope has been unstable

and people, buildings, and roads on the opposite side of the river have been exposed. The March 22

landslide and disaster was an extreme expression of the hazard. It is useful to know what caused the

hazard and what likely triggered the specific event. Factors that must be considered include: the

topography, type of material making up the slope, erosion processes at the site, vegetation and other

ground cover conditions, weather conditions, and human use.

The Stillaguamish River meanders westward across a floodplain bounded on the sides by low hills rising

to mountains to the north and south. Of relevance are the river, its floodplain, and bounding hills which,

in places where the river has cut laterally into them, slope steeply to the floodplain. It is in such a

location that the landslide hazard developed. The scar left by the March 22 landslide exposed the types

of materials and their arrangement that were factors in the instability of the slope (Photo 2). Shown in

the exposure are fine-grained sediments, dominated by fine sands and layers or pockets of finer silts and

Slide Area

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clays. The clays have a typical blue-grey color as seen in the photo, and as evident in the very loose

slurry of mud seen in the landslide deposit (Photo 3). The origin of the material composing the hills and

slope was an ancestral Stillaguamish River that drained large quantities of water during the melting of

glaciers at the close of the last ice-age, more than 10,000 years ago.

Photo 2: The scar left by the March 22 landslide showing the fine-grained sand, silt and clay (blue-gray

color) that made up the landslide (Source: modified from www.wbir.com)

Photo 3: Part of the deposit showing the blue-grey, slurry-like, clay material involved in the landslide

(Source: www.globalnews.ca)

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The clay, silt and fine-sand were deposited by the river in a lake that was present in this location at the

time. As such, they are termed “glaciolacustrine” (i.e. glacial lake) deposits. The surficial geology maps

and reports provide a much more precise explanation of the materials and processes involved (see:

www.dnr.wa.gov/Publications/ger_ofr2003-12_geol_map_mounthiggins_24k.pdf). The point to be

made here is that a full understanding of the hazard and the March 22 landslide requires some

exploration of processes in the distant past. Clay deposits soak up water very slowly (i.e. have low

permeability) but they can hold very large quantities of water (i.e. have high porosity) with enough time.

When saturated with water, a clay deposit usually contains a greater volume of water than sediment. In

other words, it is structurally very weak and flows like water if disturbed, carrying other materials and

objects with it. The photos, and the comments of first responders and members of the rescue teams,

indicate that water-saturated clay was an important causal factor in the landslide and an impediment to

rescue and recovery efforts.

A consideration of recent erosion and deposition (i.e. geomorphological and hydrological) processes

adds to our understanding of the hazard. The location of the slope and landslide site on the outside of a

meander bend of the Stillaguamish River suggests that it has been cut into through lateral erosion by the

river, possibly steepening the slope and weakening its base. A review of images of the site over the past

25 years indicates that the slope has been unstable for at least that period of time. Earlier air

photographs from as early as the 1930s and reports, such as that by Daniel and Lynne Miller and filed

with the US Army Corps of Engineers, indicate an even longer period of instability and several landslide

events at the site. The photos since 1989 also reveal that there have been some attempts to limit the

basal erosion caused by the river, indicating that the instability has been recognized as a problem for

some time. Media comments indicate the same, and it is likely that a review of local knowledge would

provide further details. The following sequence of photos shows the progression of slope failure at the

site from 1989 to 2013.

Photo4a: Sept. 5, 1989. (Source: Google Earth) Photo 4b: July 21, 2003. (Source: Google Earth)

Slope failure line/scarp

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In Photos 4a and 4b it is clear that the landslide area (circled in yellow) has been active before and after

1989. Though partially covered in deciduous vegetation, there is a well-defined slope failure line or scarp

visible through the upslope vegetation, and the bare surface in the lower left of the landslide is evidence

of recent movement. By 2003 (Photo 4b), there has been further slippage at the failure line, disturbance

of a greater part of the slide area, and the river channel has cut further into the landslide area base. On

the floodplain across from the landslide area, State Highway 530 is present and, between 1989 and

2003, there have been land clearing, development of side roads, and some housing development.

Photo 5a: July 31, 2005 (Source: Google Earth) Photo 5b: March 31, 2006 (Source: Google Earth)

Between July 2005 and the end of March 2006, the landslide area underwent a major failure with

displacement of debris down slope and across the river, damming it temporarily and altering its course.

The timing of the event was January 25, 2006. The inhabited area was not directly impacted but the

number of dwellings in a potential landslide runout zone had increased substantially over the previous 2

years.

Photo 6a: April 30, 2009 (Source: Google Earth) Photo 6B: July 14, 2013 (Source: Google Earth)

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Following the 2006 landslide, much of the landslide release area appears to have stabilized through

2009 to 2013, as shown by the increased vegetation cover in Photos 6a and 6b. However, the slope

failure scarp at the head of the landslide area remains very evident in 2013. The river channel had

stabilized since 2006, in part because of 400m long wall of boom logs anchored by concrete blocks

constructed to control channel erosion at the base of the landslide.

Photo 7a: July 14, 2013 (Google Earth). Photo 7b: March 24, 2014 (National Post).

Photos 7a, 7b, and 7c convey the magnitude of the 2014 event. Estimates of the volume of material

involved in the landslide exceed 1 million m3. The active landslide area (scar) has expanded by one-third,

with most of the expansion occurring in a previously undisturbed forested area upslope from the failure

scarp shown in the 2013 image. The mode of failure in this area was a rotational slump. Most of the

slumped material (the slump block), with the trees fallen but still at the surface, is seen just above the

mid-point of the slide area shown in Photos 7b and 7c. The material in the landslide deposit on the

Stillaguamish floodplain was derived primarily from the pre-2014 landslide area shown in Photo 7a. The

2014 landslide deposit is made up of two major components: the lighter colored, hummocky sands and

silts with some surface vegetation; and, visible at the leading margin of the deposit (Photo 7b) and at

Photo 7c

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the upper margin of the slump block (photo 7c), the blue-grey clay (see also Photo 3). The 2014 landslide

is distinctive from previously known landslides at the site in at least two ways: the much larger volume

of material involved, and the distance of its runout. The composition of, and the arrangement of

materials in the deposit, indicate that the coarser material was rafted on the much more mobile, water-

saturated clays. The January 25, 2006 landslide also involved the blue-grey glaciolacustrine clays which

are visible in Photo 5b as slurry at the leading edge of the landslide deposit. The ability of the clays to

absorb and hold large quantities of water that causes them to liquefy, and their distribution in the

landslide deposit, indicate that they were one of the primary factors in causing the landslide as well as

its volume, rapid movement, and distance traveled.

Weather conditions in the weeks preceding the landslide provide a key to understanding how the clay

and other material became wet or saturated and how the slope weakened. Precipitation amounts

during the winter of 2013-14 up to mid-February in the Pacific Northwest and southeastern British

Columbia were well below normal. In mid-February, the skies opened and precipitation, especially rain,

has been much above normal (Figures 1 and 2). March to date has been among the wettest on record in

the region. The March 2014 precipitation recorded at SEC-TAC airport is shown in Figure 1 as being

much above normal. Also shown is the fact that much of the precipitation came in three distinct periods

with none in the week before the landslide. Figure 2 shows that precipitation amounts for the region

between February 21 and March 22 ranged between 150 and 300% of normal.

Figure 1: Precipitation recorded at SEC-TAC Feb 25 to Mar 23, 2014 (Source:

www.cpc.ncep.noaa.gov/products/global_monitoring/precipitation)

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Figure 2: The 2014 precipitation anomaly for the period Feb 21 to Mar 22.

There are no precipitation data for the landslide site but nearby stations at Arlington, Darrington, and

Gold Hill provide some data. Between March 1 and 22, Arlington received 186mm (7.33in) of rain, 233%

above normal. For the same period, Gold Hill received 417mm (16.41in) of rain, 250% above normal

(this figure must be treated with caution as the GH normal is based only on 9 years of record). Only

February data are available for Darrington which received 282mm (11.12in) of rain, mostly in the final

two weeks of the month. This amount was 130% above normal (see:

www.climate.washington.edu/events/2014landslide). The main point to be made from this is that the

landslide area received an exceptional amount of rain in the 6 weeks leading up to March 22. This was a

significant contributing factor in wetting and weakening the sensitive clay, adding weight to the slope,

and initiating the landslide.

There have been suggestions that land uses, such as logging, may have contributed to the landslide.

Clear cut logging along with its access roads has been a contributing factor in many landslides in the

Pacific Northwest and southwestern British Columbia. However, its connection to the event in question

is tenuous but can’t be ruled out. There has been recent clear cutting in the general area known as

Whitman Bench but most in the immediate area of the landslide occurred many years ago. The most

substantial clear cut within the immediate area (within 250m) of the landslide occurred pre-1989. In the

immediate surroundings of the landslide site a 7 acre parcel was cut in 2005. Most of the previously cut

area appears to be in second-growth timber. Nonetheless, we cannot discount the possibility that the

early clear cutting in the area did help to destabilize the site at the time, and that the 2005 cut

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contributed further to destabilization. There appear to have been few other human factors that could

have contributed to the landslide. Indeed, the efforts to control toe erosion at the site indicate

awareness of part of the hazard and an attempt to provide stability.

Finally, there are reports of low magnitude (Magnitude <2) seismic (i.e. earthquake) activity on March

10, prior to the landslide. High magnitude earthquakes (Magnitude > 5) are very significant landslide and

rock fall triggers in mountain areas. As well, they destabilize or weaken slopes making the slope more

prone to failure in the future. With the very sensitive seismic monitoring instruments now in use, the

action of landslides and rock falls themselves are recorded as seismic events. However, a causal

relationship between the March 10 tremor and the landslide has now been discounted.

3. Secondary Hazards

Secondary hazards arise from the process that created the initial hazard event. They are very common

during the unfolding of a disaster and may magnify and/or exceed the impact of the initial hazard event.

Landslides of all types often block rivers, causing a lake to form behind the dam and/or causing the river

to shift its channel. Both can be hazardous. A landslide-dammed lake may cause flood damage. A river

forced to take a new course may cause flood and erosion damage along that course. Furthermore, one

of the great hazards arising from a landslide dam is the potential for the dam to burst suddenly,

resulting in catastrophic downstream damage. Such landslide-dam burst floods have been very

destructive in mountain regions globally.

The March 22 landslide blocked, and continues to block, the normal course of the Stillaguamish River

creating a secondary hazard of upstream flooding and the potential for a downstream landslide-dam

burst flood. Upstream flooding caused damage to structures and infrastructure (roads), and disruption

to transportation and related processes. In recognition of downstream flooding, evacuation orders for

exposed areas were issued shortly after the landslide event. Fortunately, the slurry-like nature of some

of the landslide material enabled the river to quickly find a new course through the dam and thus relieve

some of the threat. However, additional heavy rain and increasing stream discharge has added further

uncertainty and flood watches remain in effect long after the landslide. The upstream flooding has been

making rescue and recovery efforts even more difficult.

Landslides sometimes beget landslides. In other words, a slope weakened by a landslide may fail again,

creating a secondary landslide. This was a danger in the aftermath of the March 22 event. Continued

movement in the slide debris and local collapses of material on the landslide scarp caused authorities to

remove rescue teams from the landslide area for a period, delaying rescue operations. Constant watch is

directed at the landslide as local collapses continue to occur in the scarp area and, with continuing rain,

there remains a chance of a further large-scale slope failure. Photos 7b and 7c illustrate the very large

volume of material (e.g. the slump block) poised on the hillside. It is only a matter of time and the right

circumstances for the slope to fail and create another large landslide.

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Other secondary hazards arise in the aftermath of destruction and damage. Of particular relevance to

the March 22 event is damage to containment for sewage, waste, fuels, chemicals, etc. that may lead to

the spread of pathogens, toxins, and other harmful materials over a wide area. These, the wreckage

from structures, vehicles, and machinery, and downed power lines pose a significant hazard for

survivors, first responders, and rescue/recovery and security personnel in the short term. Some of the

harmful effects can persist for long periods and hamper restoration and future use of the area. During

the rescue and recovery operations following the March 22 event, these hazards are proving to be a

major challenge and workers are being decontaminated when going off duty.

4. Vulnerability and Exposure

The people, objects and processes of value that are exposed to a hazard and vulnerable to harm from it

are the second component of a disaster. Here, lies the tragedy of disaster in general and most certainly

that of the March 22 landslide. Interpretation of Photos 4 to 7 indicates that roads, other infrastructure,

and some buildings were in place in the affected area in the early 1990s. It appears that lot subdivision

for residential properties began in 2004-2005 and structures were in place by the end of 2006. More

building has occurred since then. Even though there was local knowledge of the slope instability across

the river, a major landslide in January 2006, and attempts to stabilize the river erosion at the foot of the

landslide area, it is evident that the thinking did not extend to the possibility of an extreme landslide

runout into the area being developed. This development translates into elevated exposure and

vulnerability in disaster language. Current reports tell that at least 29 of the structures in the affected

area were permanently occupied. Permanent residence, as opposed to temporary or seasonal

residence, elevates the exposure and vulnerability component more.

The building of residences, and other large structures, raises vulnerability through the increased

economic value of property. Accompanying vehicles and other objects raises potential economic losses

more.

The specific timing of a hazard event, especially rapid onset hazards such as landslides, earthquakes, and

tornadoes for example, plays a role in shaping the impact by influencing exposure and vulnerability. The

March 22 landslide occurred at 10:37am on a Saturday morning, a day and time when many residents,

especially children are likely to be at home. Thus, the number of exposed and vulnerable people was

relatively high as compared with that at mid-morning on a school/work day, though less than would

have been the case if the landslide occurred at night. Time of year also influences the level of exposure

and vulnerability of people. It is evident that some of the residences in the affected area were used on a

seasonal basis and, thus, may not have been in use during the rainy late winter, thus reducing exposure

and vulnerability.

Another of the important costs in a disaster, are the losses of functions or processes. In fact, the

definition of disaster includes wording that describes the total or partial disruption of normal economic,

social, and civil society functions and processes. Among the more vulnerable functions is ground

transportation and communication. State Highway 530 was destroyed at the landslide site. It served as

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the most important transportation conduit in the region, serving many commercial, administrative,

security, educational, personal, and other purposes. Its loss of function imposes enormous economic

and other costs over an extended period of time. These are costs that come before, and apart from, the

substantial reconstruction costs. The mountain topography makes ground transport and utilities

corridors particularly vulnerable to landslides and other natural hazards. The current disaster illustrates

this. The growth of population, settlement, and economic and civil activity in the Stillaguamish River

valley over many years has raised the need for and reliance on Highway 530, raising the exposure and

vulnerability component of the disaster. Other disruptions and loss of function are enumerable and

some, such as the long-term grief, anxiety, stress, and sense of loss, are beyond our ability to truly

measure.

5. Conclusion

The March 22, 2014 landslide disaster is the result of a major landslide event that buried an area of

residential and infrastructural development. The event occurred at a known landslide hazard site

adjacent to which development had increased exposure and vulnerability of people and property to the

hazard. In other words, the risk of disaster had increased. Landslides of lesser magnitude had occurred

at the site, and geological reports and local knowledge had identified the landslide problem before.

Several factors intersected to elevate the hazard and risk, and cause the March 22 event and help to

explain its magnitude. Some factors date from the distant past, while others relate to conditions in the

weeks and days leading up to March 22. These factors are:

Relatively steep valley-side slopes abutting the Stillaguamish River floodplain.

Slopes composed of fine sands, silts and clays deposited in a lacustrine (lake) environment at the

end of the last ice age (10,000 to 12,000 years ago).

Among those materials, blue-grey clay that, under high moisture conditions, becomes water-

saturated, subject to liquefaction, and capable flowing and rafting other material over long

distances.

Periodic undercutting of the toe of the landslide area on the outside of a meander bend of the

Stillaguamish River over a long period.

Rainfall well in excess of normal amounts from mid-February to mid-March.

Risk of disaster at the site was raised through the development of exposed and vulnerable residences,

roads, and other infrastructure in the potential runout area of a landslide. The development of full-time

residences, magnified the level of vulnerability and risk. The mid-morning occurrence of the landslide on

a Saturday probably meant that more people were vulnerable than otherwise might have been the case,

other than during a night-time occurrence. The presence of Highway 530, which was destroyed in the

landslide area, and its importance to the regional population and economy, added to the level of

vulnerability and risk. Through a combination of natural and human factors, risk of disaster increased

over time. The landslide hazard remains at the site. In fact, through the large amount of material left on

the slope, the hazard probably is elevated. Control of the factors that could precipitate another major

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landslide will be very difficult. However, risk of future disasters can be lessened and controlled through

regulation of land uses in the exposed area by the appropriate authorities. To do so would be one

positive outcome of the tragedy.