Earthquake, rain behind deadly mudslide in Oso, Washington State

Listen to this article

Aerial photograph of the Oso mudslide, taken by the Washington State Patrol on March 23. (Wikipedia)

On Saturday morning, March 22, just before 11 a.m., a massive mudslide wiped out roughly 50 homes along the Stillaguamish River in Snohomish County, Wash., between the towns of Oso and Darrington. Reports estimate that the sliding earthen mass reached speeds of up to 100 mph. Two mudslides occurred, one after the other, from the same hillside. Each mudslide lasted about two minutes and stretched nearly a mile out from where they calved from the hillside.

- 2014-mudslide-before1
A Google Maps view of affected area before the 2014 mudslide.

The death toll as of the morning of March 25, 2014, was 14. No one knows how many bodies are buried under all that mud, reportedly up to 50 feet deep in some areas. Travis Hots, chief of Arlington, Wash., rural fire district, expects more fatalities to be confirmed in the coming weeks as recovery efforts continue.

A distant view of the affected area on March 23, 2014. (Carl Niedermeyer)
A distant view of the affected area on March 23, 2014. (Carl Niedermeyer)

Search-and-rescue organizers have set up a telephone and email hotline for people to report missing persons. The hotline as of late Monday had received nearly 200 sources of input. Some media sources are listing this number as “missing persons,” but, in fact, the total is merely the number of separate sources of input the hotline has received. This means that, for example, several sources of input may be for any one person reported missing, such that the actual number of missing persons amount to far fewer than 200.

A direct view of the affected area on March 23, 2014. (Carl Niedermeyer)
A direct view of the affected area on March 23, 2014. (Carl Niedermeyer)

Further hampering rescue efforts is the concern that yet more earth may slough off of the original hillside. Worse, while weather has been dry since the weekend, more rain is forecast for much of the remaining week, with some flood warnings. Rescue workers describe attempting to walk on the mucky goo as like walking on quicksand.

Geologists have described the material that came loose in the mudslide as clay and sandy glacial soil piled onto bedrock. When that loose material becomes thoroughly saturated from a season of rain, conditions are prime for gravity-induced disaster. Recent reports suggest that a small earthquake in the area several days prior to the mudslide may have contributed to the event.

This same hillside shed some earth in 2006, but on a far smaller scale. Carl Niedermeyer, a pilot from nearby Everett, Wash., took aerial photographs then and now, several of which are featured in this article. The 2006 photos show how that slide blocked a bend in the Stillaguamish River. No casualties or injuries were reported from the 2006  mudslide.

This image shows the extent of the 2006 mudslide. Note the community to the left of the straight section of river, which re-joins the blocked sections of river. (Carl Niedermeyer)
This image shows the extent of the 2006 mudslide. Note the community to the left of the straight section of river, which re-joins the blocked sections of river. (Carl Niedermeyer)

Recent news-helicopter reports show countless homes on the upstream side of the Stillaguamish river flooding from the backup.  The river did start flowing downstream late Sunday, perhaps 18 hours after the blockage occurred, according to USGS river-flow graphs online.

- 2014-Stilly-USGS-3-22
USGS graph shows river flow at Arlington, Wash., downstream from the mudslide before and after the slide.

Some observers wonder if effects of global warming may have contributed to the mudslide. According to rainfall data, the rainfall in the mudslide area in the last seven months has totaled about 9 inches. Additional rain in the coming days could further endanger, and delay, recovery efforts.

3 thoughts on “Earthquake, rain behind deadly mudslide in Oso, Washington State

  • March 28, 2014 at 9:06 AM

    The horrible tragedy in Oso, Washington is very similar to much larger disaster that occurred in northern Italy in October 1963. That disaster was caused when the entire hillside above the east side of the lake created by the Vajont Dam gave way and collapsed into the lake in a matter of seconds causing much of the water behind the dam to be instantly displaced. The dead numbered between 1,900 and 2,500 and numerous villages on the west side of the lake behind the dam were wiped out by the gigantic displacement wave and when the wave continued over the dam, the entire valley below the dam, and all of the villages in that valley, was utterly destroyed. Many of the dead were never recovered.

    I’ve taken the liberty of copying a Wikipedia reference of this much larger but largely forgotten catastrophe:

    The Vajont Dam (or Vaiont Dam) is a disused dam, completed in 1959 in the valley of the Vajont River under Monte Toc, 100 km (60 miles) north of Venice, Italy.

    One of the tallest dams in the world, it is 262 m (860 ft) high, 27 m (89 ft) thick at the base and 3.4 m (11 ft) at the top. On 9 October 1963 a massive landslide caused a tsunami in the lake, the overtopping of the dam and around 2,000 deaths. This event occurred when the designers ignored the geological instability of Monte Toc on the southern side of the basin. Warning signs and negative appraisals during the early stages of filling were disregarded, and the attempt to safely control the landslide into the lake created a 250 metre (820 ft)[1] mega-tsunami (ten times higher than predicted) that brought massive flooding and destruction to the Piave Valley below, wiping out several villages completely.

    On 12 February 2008, while launching the International Year of Planet Earth, UNESCO cited the Vajont Dam tragedy as one of five “cautionary tales”, caused by “the failure of engineers and geologists”.

    The dam was built by SADE (Società Adriatica di Elettricità, English: Adriatic Energy Corporation), the electricity supply and distribution monopoly in northeastern Italy.

    The dam and basin were intended to be at the centre of a complex system of water management in which water would have been channeled from nearby valleys and artificial basins located at higher levels. Tens of kilometres of concrete pipes and pipe-bridges across valleys were planned.[citation needed]

    In the 1950s, SADE’s monopoly was confirmed by post-fascist governments and it purchased the land despite opposition by the communities of Erto and Casso in the valley, which was overcome with government and police support. SADE stated that the geology of the gorge had been studied, including analysis of ancient landslides, and that the mountain was believed to be sufficiently stable.[citation needed]

    Construction work started in 1957, but by 1959 shifts and fractures were noticed while building a new road on the side of Monte Toc. This led to new studies in which three different experts separately told SADE that the entire side of Monte Toc was unstable and would likely collapse into the basin if the filling were completed.[5] All three were ignored by SADE. Construction was completed in October 1959, and in February 1960, SADE was authorised to start filling the basin.[citation needed]

    Early signs of disaster
    Throughout the summer of 1960, minor landslides and earth movements were noticed. However, instead of heeding these warning signs, the Italian government chose to sue the handful of journalists reporting the problems for “undermining the social order”.[citation needed]

    On 4 November 1960, with the water level in the reservoir at about 190 metres (620 ft) of the planned 262 metres (860 ft), a landslide of about 800,000 cubic metres (1,000,000 cu yd) collapsed into the lake. SADE stopped the filling, lowered the water level by about 50 metres (160 ft), and started to build an artificial gallery in the basin in front of Monte Toc to keep the basin usable even if additional landslides (which were expected) divided it into two parts.

    In October 1961, after the completion of the gallery, SADE resumed filling the narrow reservoir under controlled monitoring. In April and May 1962, with the basin water level at 215 metres (705 ft), the people of Erto and Casso reported five “grade five” Mercalli scale earthquakes. SADE downplayed the importance of these quakes. SADE was then authorized to fill the reservoir to the maximum level.[citation needed]

    In July 1962, SADE’s own engineers reported the results of model-based experiments on the effects of further landslides from Monte Toc into the lake. The tests indicated that a wave generated by a landslide could top the crest of the dam if the water level was 20 metres (66 ft) or less from the dam crest. It was therefore decided that a level 25 metres (82 ft) below the crest would prevent any displacement wave from over-topping the dam. However, a decision was made to fill the basin beyond that, because the engineers thought they could control the rate of the landslide by controlling the level of water in the dam.[citation needed]

    In March 1963, the dam was transferred to the newly constituted government service for electricity, ENEL. During the following summer, with the basin almost completely filled, slides, shakes, and movements of the ground were continuously reported by the alarmed population. On 15 September, the entire side of the mountain slid down by 22 centimetres (8.7 in). On 26 September, ENEL decided to slowly empty the basin to 240 metres (790 ft), but in early October the collapse of the mountain’s south side looked unavoidable: one day it moved almost 1 metre (3.3 ft). There is no known record of any warning or evacuation order being issued to the populace.[citation needed]

    Landslide and wave
    On 9 October 1963, engineers observed trees falling and rocks rolling down into the lake where the predicted landslide would take place. Prior to this, the alarming rate of movement of the landslide had not slowed as a result of lowering the water, although the water had been lowered to what SADE believed was a safe level to contain the displacement wave should a catastrophic landslide occur. With a major landslide now imminent, engineers gathered on top of the dam that evening to witness the tsunami.

    At 10:39 P.M., a massive landslide of about 260,000,000 cubic metres (340,000,000 cu yd) of forest, earth, and rock fell into the reservoir at up to 110 kilometres per hour (68 mph), completely filling up the narrow reservoir behind the dam. The landslide was complete in just 45 seconds, much faster than predicted, and the resulting displacement of water caused 50,000,000 cubic metres (65,000,000 cu yd) of water to overtop the dam in a 250-metre (820 ft) high wave.

    The flooding from the huge wave in the Piave Valley destroyed the villages of Longarone, Pirago, Rivalta, Villanova and Faè, killing around 2,000 people and turning the land below the dam into a flat plain of mud with an impact crater 60 metres (200 ft) deep and 80 metres (260 ft) wide. Many small villages near the landslide along the lakefront also suffered damage from a giant displacement wave. Villages in the territory of Erto e Casso and the village of Codissago (it), near Castellavazzo, were largely wrecked.

    Estimates of the dead range from 1,900 to 2,500 people, and about 350 families lost all members. Most of the survivors had lost relatives and friends along with their homes and belongings.

    The dam itself was largely undamaged. The top 1 metre (3.3 ft) or so of masonry was washed away, but the basic structure remained intact and still exists today.

  • Mark Forseth
    March 26, 2014 at 7:25 PM

    In more related news, recent reports reveal that a geologist had written a paper on this area, suggesting that the area was ripe for landslide. Locals jokingly referred to that hill as Slide Hill because it has moved so much over the decades. This hillside had slid in the ’50s, ’60s, and ’80s as well. How the area across the river from that hillside was ever zoned for residential occupation may be a question the local zoning commission will have to answer.

  • March 26, 2014 at 6:32 AM

    This mudslide is just a total tragedy and an absolute disaster. I feel for the families of the people who died and are still lost. Our prayers are with you !

Comments are closed.