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2014 Oso landslide

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2014 Oso landslide
File:Oso landslide (WSP).pngOso mudslide, looking northwest
DateMarch 22, 2014 (2014-03-22)
Time10:37 a.m.
LocationOso, Washington
Coordinates48°16′57″N 121°50′53″W / 48.28256°N 121.84800°W / 48.28256; -121.84800
CauseSuspected soil saturation from heavy rainfall.
Deaths36 confirmed
Non-fatal injuries4 serious
Missing10 reported
Property damage49 homes and other structures destroyed

On Saturday, March 22, 2014, at 10:37 a.m. local time, a major mudslide occurred 4 miles (6.4 km) east of Oso, Washington, United States, when a portion of an unstable hill collapsed, sending mud and debris across the North Fork of the Stillaguamish River, engulfing a rural neighborhood, and covering an area of approximately 1 square mile (2.6 km). As of April 8, 2014, the Snohomish County Medical Examiner's office confirmed that 36 people had died and 10 people remained missing or unaccounted for. Excluding landslides caused by volcanic eruptions, earthquakes or dam collapses, this is the deadliest single landslide event in United States history.

Overview

2009 view to the southwest overlooking the slide site (on the left) and the Steelhead Haven plat across the river. The unstable area is covered with a red alder, bigleaf maple and black cottonwood forest (light green, right of center and below the North Fork of the Stillaguamish River). The mudslide flowed to the upper left, across the river. Most, or all, of the houses visible in the image were destroyed.

Active earth movement at the Hazel Landslide dates back to at least 1937. The latest landslide event engulfed nearly fifty properties in an unincorporated neighborhood known as "Steelhead Haven" 4 mi (6.4 km) east of Oso, Washington. It also dammed the river, causing extensive flooding upstream, and blocked State Route 530, the main route to the town of Darrington (population 1,347). The geologic feature itself is called the Hazel Landslide, however the active slide events on the geologic feature are also called landslides, with the latest event being dubbed the "Oso mudslide" by the press.

The Hazel Landslide has a history of instability, and the area experienced up to 200 percent normal rainfall over the previous 45 days before the latest slide event. This event was described by witnesses as a "fast-moving wall of mud" containing trees and other debris cutting through homes directly beneath the hill. A firefighter stated, "When the slide hit the river, it was like a tsunami". A Washington state geologist stated the slide was one of the largest landslides he had personally seen. The mud, soil and rock debris left from the mudslide is 1,500 ft (460 m) long, 4,400 ft (1,300 m) wide and deposited debris 30 to 40 ft (9.1 to 12.2 m) deep.

Casualties and damage

More than 100 first responders from Snohomish and surrounding counties were dispatched to assist with medical aid and search-and-rescue efforts, including search-and-rescue helicopters from nearby Naval Air Station Whidbey Island. As of March 30, 2014 approximately 620 personnel, including 160 volunteers, were working on landslide recovery operations.

Late in the evening of March 22, 2014, Washington Lieutenant Governor Brad Owen declared a state of emergency in Snohomish County. Governor Jay Inslee toured the area by air the following day before joining county officials at a news conference.

As of April 9, 2014, the Snohomish County Medical Examiner's office had officially confirmed 35 fatalities, 31 of whom had been identified; 10 people are missing. Officials no longer expect to find any survivors. As of April 7, 2014, four survivors of the slide were still in Seattle: three at Harborview Medical Center (two in intensive care), and one released to a rehabilitation facility. Forty-nine homes and other structures were destroyed.

The slide blocked the North Fork of the Stillaguamish River, which backed up eastward. Because of concerns that the mud and debris dam could fail, causing downstream flooding, a flash flood watch was issued by the National Weather Service. On April 2, with the river flowing in a new channel at the north end of the debris dam, the flash flood watch was lifted. Flooding due to the partially obstructed river continued upstream of the debris dam. Following the slide, Highway 530 was closed indefinitely by the Washington State Department of Transportation with an alternative route around the slide opened when snow was cleared from the unpaved portion of Mountain Loop Highway south of Darrington.

Federal reactions

On April 3, the mudslide was declared a major disaster by President Barack Obama. The declaration was requested on April 1 by Governor Jay Inslee, who said that about 30 families needed help with housing and other needs, and who estimated that financial losses had reached $10 million. John Pennington emphasized the importance of registering with FEMA itself for access to the aid.

On April 7, 2014, the United States House of Representatives passed the United State Senate's bill the Green Mountain Lookout Heritage Protection Act. The bill would prevent the United States Forest Service from removing a building from the Glacier Peak Wilderness Area in Washington State unless the agency determines that the structure is unsafe for visitors. Senator Patty Murray (D-WA) argued that the bill should be passed in order to help the tourism industry in the area, but protecting the lookout point in question. Murray argued that the bill would be "a very small step in what will be a very long recovery" and that it would "provide a glimmer of hope for the long-term recovery of this area."

Initiation and possible seismic precursors

Aerial view of the damage

The seismic signals (ground vibrations) generated by the Oso landslide were recorded at several regional seismic stations, and analyzed by the Pacific Northwest Seismic Network (PNSN). The main event began 10:37:22 a.m. local time (PDT; 17:37:22 UTC) and lasted about 2.5 minutes. This is believed to the initial collapse of the material previously disturbed and weakened by the 2006 slide; this is the slide that impacted the neighborhood below. Another large slide occurred at 10:41:53 PDT; this may have been the collapse of the slope above the 2006 slide area that created the scarp now visible at the top of the slide area. Additional events, most likely smaller landslides breaking off the headscarp, continued for several hours. The last notable signal came 14:10:15.

Data collected by the PNSN shows a Magnitude 1.1 earthquake in the vicinity of the Oso landslide (about 2 ±0.8 km to the northeast), at a depth of 3.9 ±1.9 km, twelve days earlier on March 10. This is the quake that Snohomish County Emergency Management Director John Pennington speculated might have triggered the landslide.

Careful examination of the records from the nearest seismic station, about 7 mi (11 km) to the southwest, found many very small events that started around 8 a.m. and stopped in the late afternoon. However, they were not detected at the next nearest seismic station. They are also seen in the days before and after the slide, but only during daylight hours. They are believed to be related to some kind of human activity. No other indications of possible precursors have been found.

Geological context

Shaded-relief geomorphological map of Oso Landslide of 2014 and adjacent areas. Oso is two miles west of this map, Hazel (former mill site) one mile east. Colored areas are older landslides, "D" is oldest. Upper "A" is 2014 landslide, lower "A" Skaglund Hill. Topography shown is from 2006; red line is approximate location of current headscarp. Red cross-hatching is the runout area, now buried in mud and debris. Terrace in upper-left is Whitman Bench. Image from USGS & OFR 2014-1065.

The landslide occurred at the southeastern edge of Whitman Bench, a terrace about 800 ft (240 m) above the valley floor, consisting of gravel and sand deposited during the last glaciation. When the Puget Lobe of the Cordilleran Ice Sheet came south from British Columbia and filled the Puget Lowland it dammed the various mountain valleys, forming lakes. Sediments washed down from the higher mountains settled in the lake bottoms to form a layer of clay. As the glacial ice pressed higher against the western end of Mount Frailey the water flowing around the edge of the ice from the north was forced around Mount Frailey, eventually pouring in through the long valley extending to the northwest now occupied by Lake Cavanaugh. Where this flow entered the glacial lake the sand and gravel it carried dropped out to form a delta, the remnant of which is now Whitman Bench. After the glacier retreated and released the lake, the river carved out most of those deposits, leaving the former delta "hanging" approximately 650 ft (200 m) above the current valley floor.

The sand portion of this deposit has very little clay or "fines" to cement it together, so is very weak structurally. It is also sensitive to accumulations of water, as this increases the internal "pore" pressure, which contributes to failure. For the most part water infiltrating from the surface flows right through, except at the contact with the less permeable clay, where the water accumulates and forms a zone of weakness. Such variations in pore pressure and water flux are one of the primary factors leading to slope failure, the other primary factor being erosion of the base of the slope when the river impinges on it as it meanders across the valley. These conditions have created an extensive series of landslide complexes on both sides of the valley. Additionally, the history of multiple slides at this particular location, which lies across the inferred location of one of the strands of the Darrington—Devils Mountain Fault, and possibly the conjunction with the Larch Lake fault as well may be due to seismic fracturing and uplift of the underlying rock. Additional benches on the margin of Whitman Bench are due to deep-seated slumping of large blocks, which also creates planes of weakness for future slippage and channels for water infiltration.

History

According to a report from 1999:

The Hazel landslide has been active for over half a century. Thorsen (1996) noted a tight river bend impinging on the north bank with active landsliding visible in 1937 aerial photographs. The next 60 years involves two periods of relatively low landslide activity, and two periods of relatively high activity, the last of which extends to this day .

Known activity at this specific site includes the following:

  • 1937: aerial photographs show active landsliding.
  • 1951: mudflow from a side channel briefly blocked the river.
  • 1952: movement of large, intact blocks, leaving headscarps 70 ft (21 m) high. Later photographs show persistent activity through the next decade.
  • 1967 January: slump of a large block and accompanying mud flows push the river channel about 700 ft (210 m) south. This protects the toe from erosion, activity is minor for about two decades.
  • 1988 November: erosion of the toe leads to another slide, and the river is again moved south, but not as far as in 1967.
  • 2006 January 25: large slide blocks the river, new channel is cut to alleviate flooding.

Slope stability and effects of logging

Numerous parties have raised the question as to whether logging was a contributing factor to the 2014 Oso (aka Hazel) mudslide. In the past, logging on the Whitman Bench above the Hazel Landslide has been correlated with previous slide events, and this has led to concerns that recent logging may have been a contributing factor to this slide. Further, logging activities have long been known to trigger landslides; in the past decade, a series of spectacular landslides that followed timber harvest have heightened public awareness of the links between logging and landslides in Washington. Such events include the 2007 Stillman Creek landslides, the 2007 Highway 6 landslide, and the 2009 Whatcom County landslides.

Scientists have published several reports and papers that assess the potential impact of logging on the stability of the Hazel Landslide. Below is a synopsis of their findings.

In their 1998 peer-reviewed article on the Hazel Landslide, Miller and Sias describe the mechanisms through which timber harvest can affect the stability of the Hazel Landslide, as paraphrased below:

Timber harvest reduces the amount of water that trees remove from the soil by evapotranspiration (ET). When water accumulates in the soil, it adds mass to the soil and it reduces frictional resistance between soil (e.g. clay) layers, both of which increase the potential for slope failure. Forests reduce soil moisture both by withdrawing water from the soil through their roots, and intercepting rainfall with their canopies such that it never reaches the soil. In a warm, low elevation site such as the Hazel Landslide, conifers actively transpire throughout the year, including winter. So as rainstorms increase soil moisture levels, conifers reduce soil moisture levels, and the relative magnitude of these two dynamic opposing factors influences the potential for slope failure. Because most rainfall occurs in the late fall, winter and early spring, the ability of forests to remove water during this time is critical to reducing landslide risk. Average annual conifer forest ET at Hazel is between 45-75% of annual rainfall, and winter ET accounts for 50% or more of annual ET. Clearcuts (and hardwood forests) have almost no ability to remove water during the winter.

The underlying geology (and hence groundwater hydrology) of the glacial deposits on the Whitman Bench is complex, making it difficult to accurately differentiate between slide-prone areas, areas where forest removal would affect landslide hydrology (e.g. groundwater recharge areas) and areas where forest removal would have no effect on landslide hydrology. As an example, based on the complex hydrology-slope stability-groundwater recharge model that Miller and Sias constructed for the Hazel Landslide, the Department of Natural Resources (which regulates logging in Washington) delineated an area above as a “groundwater recharge” area. However, recent maps published in newspapers show that much of this recharge area failed during the 2014 event, indicating that it was part of the landslide itself rather than a recharge area to the slide. Miller and Sias noted the limitations of models in accurately delineating groundwater recharge areas and suggest that field measurements of water table elevations would be useful for testing the model.

There's a widely recognized link between logging and shallow debris flows, in which a muddy slurry fed by soil near the surface runs downhill in a destructive torrent, he said. Logging can remove trees whose roots help anchor the soil in place. But deep landslides like the one in Oso are more rare, and they haven't received the kind of attention that might illuminate what sets them off, said Josh Roering, a geomorphologist at the University of Oregon. However, it is not well understood as to the mechanisms whereby forest removal can affect deep-seated landslides such as the Hazel slide, nor the extent of the areas around such slides that should be protected from logging. As of March 31, 2014, a review of recent news articles suggests there is not yet consensus among experts as to whether the existing protective measures were sufficient to eliminate logging as a contributing factor to the 2014 landslide.

Controversy

Claim that slide was unforeseen

Controversy erupted on the third day of the slide (March 24) when John Pennington, the Director of Snohomish County's Department of Emergency Management, stated at a news conference: "This was a completely unforeseen slide. This came out of nowhere."

On the morning of the same day, The Seattle Times published an article that "Site has long history of slide problems". On Tuesday the paper followed up with a full page article on the various reports about previous slides at this location and the likelihood of more, along with the comments of geologists, engineers, and local residents, including a report that the area was known locally as "Slide Hill". Snohomish County Public Works Director Steve Thomsen was reported as saying: "A slide of this magnitude is very difficult to predict. There was no indication, no indication at all."

Another article the next day was about a 2010 study commissioned by the county (a follow-up of a 2005 report) that warned that the hillside above Steelhead Drive was one of the most dangerous in the county. According to one of the authors of the 2010 report: "For someone to say that this plan did not warn that this was a risk is a falsity." The criticism of county officials received national attention in a New York Times op-ed piece. The Seattle Times also reported that, in 2004, county officials became concerned about the possibility of a dangerous landslide in the Steelhead Haven area, and considered buying out the homes of people living in the area. However, they rejected the idea, and chose instead to build a new wall to attempt to stabilize the slope. Some disaster experts have criticized this decision as a serious mistake.

Aside from the question of whether this slide should have been foreseen is another question of what could have been done. A rock revetment installed in 1962 to protect the toe of the slide area from erosion was overrun by a slide two years later, and an effort in 2006 to move the river back 430 feet was made moot when another landslide moved the river back 730 feet. The simplest option, of not building in an area exposed to both flooding and landsliding, is unpopular, and attempts to limit it have been viewed as big government trampling on property rights. According environmental engineer and applied geomorphologist Tracy Drury, after the 2006 slide "They didn't even stop pounding nails." As to any kind of buy-out program, Drury said: "I think we did the best we could under the constraints that nobody wanted to sell their property and move elsewhere."

Logging

Grandy Lake Forest Associates of Mount Vernon, Washington proposed a 15-acre clearcut at the upper edge of the Oso landslide zone in 2004. Washington state forester Aaron Everett stated in an interview with KUOW that the application was rejected and "The one that was approved in the end eliminated the part of the harvest that would have been inside the groundwater recharge area." Everett further states the resulting 7-acre clearcut operation went to the edge of the groundwater danger zone. An investigation is being conducted to determine whether Grandy Lake crossed into the restricted area that could theoretically feed groundwater into the landslide zone, affecting it for up to 16–27 years.

See also

References

  1. Berman, Mark (March 24, 2014). "Everything you need to know about the Washington landslide". Washington Post. Retrieved March 24, 2014.
  2. ^ Snohomish County Medical Examiner’s Office (April 9, 2014). "Snohomish County Medical Examiner's Office Media Update". Retrieved April 9, 2014.
  3. ^ "Crews start work on berms to ease search for mudslide victims". The Seattle Times. April 7, 2014. Retrieved April 7, 2014.
  4. ^ Snohomish County Sheriff's Office (April 9, 2014). "SR 530 Slide Area Missing Person List". Retrieved April 9, 2014.
  5. ^ "14 dead; 108 reports of missing people". The Seattle Times. March 24, 2014. Retrieved March 24, 2014.
  6. "Worst Landslides in U.S. History". Wunderground. Retrieved March 31, 2014.
  7. ^
  8. "Mudslide witness: 'Everything was gone in 3 seconds'". Everett Herald. Retrieved March 23, 2014.
  9. Mark Leberfinger (March 24, 2014). "Death Toll From Washington Landslide Climbs to Eight". AccuWeather.com. Retrieved March 24, 2014.
  11. "Whidbey Island agencies assist in Oso mudslide response". Whidbey News Times. March 26, 2014. {{cite news}}: |first= missing |last= (help)
  12. Snohomish County Medical Examiner’s Office (April 1, 2014). "Sunday night 530 landslide update".
  13. "Landslide kills three, injures others in Washington state". Reuters. Retrieved March 23, 2014.
  14. Lornet Turnbull; Jennifer Sullivan (April 2, 2014). "Day 12: 'We are trying to be as honest as we can'". ]. Retrieved April 7, 2014.
  15. "Flash Flood Watch". National Weather Service. Retrieved March 27, 2014.
  16. "SR 530 Landslide". Washington State Department of Transportation. Retrieved March 26, 2014.
  17. "Obama declares major disaster for Oso landslide", KING 5 News and Associated Press via kvue.com, April 3, 2014. Retrieved 2014-04-03.
  18. "S. 404 - Summary". United States Congress. Retrieved April 5, 2014.
  19. ^ Cox, Ramsey (April 3, 2014). "Senate approves small bill to help Oso recovery". The Hill. Retrieved April 8, 2014.
  20. Allstadt, Kate (March 26, 2014), "Seismic signals generated by the March 22nd Oso Landslide", Seismo Blog: Updates and dispatches from the PNSN
  21. M1.1 - 18km WNW of Darrington, Washington (BETA)
  22. ^ Allstadt 2014 (PNSN).
  23. Doughton, Sandi (March 25, 2014), "Scientists say there's little chance tiny quake triggered slide", The Seattle Times; Lanela, Mike (March 25, 2014), "Washington state mudslide preceded by small earthquake", CBC News
  25. Tabor, R. W.; Booth, D. B.; Vance, J. A.; Ford, A. B. (2002), "Geologic Map of the Sauk River 30- by 60- minute quadrangle, Washington", U.S. Geological Survey, Miscellaneous Investigations map I-2592, 2 sheets and pamphlet, scale 1:100,000
  26. Details can be seen on the Mount Higgins (shows only the eastern part of the bench) and Darrington—Devils Mountain Fault geological maps. The areas of predominately gravel (which drops out first) and sand are shown as "Qgoge" and "Qgose", respectively. "Qgle" and "Qglv" mark exposures of the underlying clay and silt. "Qls" marks landslide complexes.
  27. ^ Miller 1999, p. 1 harvnb error: multiple targets (2×): CITEREFMiller1999 (help).
  28. Miller 1999, p. 4 harvnb error: multiple targets (2×): CITEREFMiller1999 (help).
  29. Mount Higgins geological map; John Vidale, quoted in Doughton, Sandi (March 24, 2014), "River likely undercut slope, experts say", The Seattle Times, p. A5
  30. Miller, Dan; Sias, Joan (1997), Environmental Factors Affecting the Hazel Landslide (PDF), Figure 1.3.
  31. Miller 1999, p. 2 harvnb error: multiple targets (2×): CITEREFMiller1999 (help).
  32. Miller 1999, pp. 2–3 harvnb error: multiple targets (2×): CITEREFMiller1999 (help).
  33. Alexander, Brian (January 27, 2006), "Slide diverts river; Oso homes at risk", The Seattle Times. See also Steelhead Landslide pictures.
  34. Baker, Mike; Armstrong, Ken; Bernton, Hal, State allowed logging on plateau above slope, The Seattle Times, retrieved March 30, 2014 {{citation}}: Unknown parameter |lastauthoramp= ignored (|name-list-style= suggested) (help)
  35. ^ Ryan, John, Oso: Clearcut Extended Into No-Logging Zone, Northwest Public Radio, retrieved March 30, 2014
  36. ^ Kiley, Brendan, Is There a Connection Between the Mudslide and Our State's Historical Mishmash of Logging Regulations?, SLOG, retrieved March 30, 2014
  37. ^ Ryan, John, Concern Over Landslide-Logging Connection Near Oso Is Decades Old, KUOW, retrieved March 30, 2014
  38. Benda, L. (1988). "Report of the ID Team Investigation of the Hazel Landslide on the North Fork of the Stillaguamish River". Unpublished. DNR NW Region, FPA: 19–09420. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  39. Sidle, Roy C.; Hirotaka Ochiai (2006). Landslides: processes, prediction, and land use. Vol. 18. American Geophysical Union. ISBN 0875903223.
  40. Bernton, Hal; Mayo, Justin, Logging and landslides: What went wrong?, The Seattle Times, retrieved March 30, 2014
  41. Bernton, Hal; Mayo, Justin, Slides putting our highways in danger, The Seattle Times, retrieved March 30, 2014
  42. Stark, John, Lands commissioner tours landslide areas in Whatcom County, The Bellingham Herald, retrieved March 30, 2014
  43. ^ Miller, Daniel J. (October 18, 1999), Hazel/Gold Basin Landslides: Geomorphic Review Draft Report (PDF), U.S. Army Corps of Engineers, retrieved March 30, 2014
  44. ^ Miller, Dan; Sias, Joan (1997), Environmental Factors Affecting the Hazel Landslide (PDF), TerrainWorks, retrieved March 30, 2014
  45. ^ Miller, Daniel J.; Joan Sias (1998). "Deciphering large landslides: linking hydrological, groundwater and slope stability models through GIS" (PDF). Hydrological Processes. 12 (6): 923–941.
  46. Benda, L. (1988). "Report of the ID Team Investigation of the Hazel Landslide on the North Fork of the Stillaguamish River". Unpublished. DNR NW Region, FPA: 19–09420. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  47. Cornwall, Warren, As Scientists Examine Landslide, Questions About Logging's Potential Role, National Geographic, retrieved April 8, 2014</
  48. Forest Practices Rules, Washington State Department of Natural Resources, retrieved March 31, 2014
  49. Knickerbocker, Brad, Washington mudslide: logging eyed as contributing cause, The Christian Science Monitor, retrieved March 30, 2014
  50. Armstrong, Carter & Baker 2014.
  51. Egan, Timothy, "A Mudslide, Foretold", New York Times, March 29, 2014. Retrieved 2014-03-30.
  52. Armstrong, Ken; Carter, Mike; Baker, Mike (March 25, 2014), "'Unforeseen' risk of slide? Warnings go back decades", The Seattle Times, pp. A1, A5.
  53. See External links for a partial list.
  54. Brunner, Jim; Berens, Michael J. (March 26, 2014), "2010 study ranked area among the most hazardous", The Seattle Times, pp. A1, A7.
  55. Rob Flaner, quoted in Brunner & Berens 2014.
  56. Egan, Timothy (March 29, 2014), "A Mudslide, Foretold", The New York Times, p. SR3.
  57. Knickerbocker, Brad (April 5, 2014). "Authorities knew of mudslide danger, but didn't tell residents". Christian Science Monitor. Retrieved April 5, 2014.
  58. Armstrong, Carter & Baker 2014.
  59. Schwartz, John (March 29, 2014), "No Easy Way to Restrict Construction in Risky Areas", The New York Times, p. A12.
  60. Armstrong, Carter & Baker 2014.
  61. Concern Over Landslide-Logging Connection Near Oso Is Decades Old
  62. "Oso: Clearcut Extended Into No-Logging Zone | Northwest Public Radio". Nwpr.org. Retrieved March 31, 2014.
  63. Mike Baker and Justin Mayo (March 26, 2014). "Logging OK'd in 2004 may have exceeded approved boundary". Seattle Times.

External links

Victims and aftermath

Scientific and engineering maps and studies

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