2014 OSO MUDSLIDEMatthew Mortimer

IntroductionOn Saturday March 22nd 2014, at 10:37 a.m. local time, a major landslide occurred 6.4 km east of Oso, Washington, United States. A portion of an unstable hill collapsed suddenly, mud and debris engulfed the North Fork of the Stillaguamish River, submerging a rural neighbourhood. The landslide covered a total area of approximately 2.6 km². 43 people were killed.

CausesThe area where the landslide occurred had a history of instability dating back to 1937. Before the March 2014 mudslide, the area experienced 200 percent of normal rainfall over the previous 45 days. The slide contained trees and other debris which cut through homes directly beneath the hill on the south side of the Stillaguamish River. Scientists questioned whether logging could have been a factor contributing to the hillside collapse. Grandy Lake Forest Associates, Washington proposed a 15-acre clear cut at the upper edge of the Oso landslide zone in 2004. The cutting of trees increases surface run off and means soil becomes saturated far quicker. Furthermore, the removal of the roots loosen the soil, making it far more prone to movement.

EffectsThe March 2014 landslide engulfed 49 homes and other structures in a neighbourhood known as Steelhead Haven, 6.4 km east of Oso, Washington. The slide also dammed the river which caused extensive flooding upstream as well as blocking State Route 530, the main access point to the settlement. The mud, soil and rock debris left from the mudslide covered an area 1,500 ft long, 4,400 ft wide and deposited debris 30 to 70 ft deep. There were 43 fatalities and 4 serious injuries.

ManagementFEMA (Federal Emergency Management Agency) have created a 4 step scheme to prevent a similar event happening again. These steps are: 1. Organize resources, 2. Assess risks, 3. Develop a mitigation plan, 4. Implement plan and monitor progress. Examples of mitigating steps that could be taken are planting low growing vegetation as ground cover. They advise that local people should meet regularly over time to implement, monitor, and evaluate the hazard, and also to highlight further hazards.