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Oso landslide report highlights need for risk assessment

Findings of a new report into the Oso landslide in Washington State, US has said that the incident highlights the importance of assessing and managing risk from natural slopes

The landslide, which killed 43 people, has been described as the US’s deadliest landslide disaster in the document published by the US-based Geotechnical Extreme Events Reconnaissance (GEER).

The landslide destroyed sections of Highway 530 and more than 50 homes and Washington Officials have estimated that the capital losses associated with the landslide are at least US$50M.

According to the GEER report, the landslide occurred in glacial sediments featuring interbedded layers of clay, silt, sand, gravel, cobbles and boulders. Investigations have shown that these deposits are susceptible to landslides, especially when they form steep slopes or have abundant groundwater.

Heavy rainfall in the three weeks before the Oso landslide is believed to have been the trigger.

Geer’s site investigation of the local geomorphology revealed a number of historic landslides in area around the Oso site over the last 6,000 years. Historical data shows that the ground above Oso had suffered stability issues since before 1950 with the most recent incident recorded in 2006.

GEER has said that hazard mapping was carried out and the risk of landslides had been identified at Oso but the impact of an event focused on the effects on the river rather than on local residents and infrastructure.

The report added that there are no national or state guidelines in the US concerning levels of risk due to natural landslides that warrant action. It its recommendations, GEER has suggested that risks to people and property should be better assessed and communicated, monitoring and warning systems should be considered where there is a high risk and regular re-assessment must be carried out.

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