When monitoring began, the landslide mass was moving at a rate of approximately 1.0 ft/week. Based on recent monitoring, we know that the landslide mass above the quarry is now moving downhill toward the south (~190-degree azimuth), at a rate of 1.5 ft/week based on the most recent measurements (5/20/2018). For several months, the landslide was moving at a steady rate of ~1.6 ft/week; however, the landslide is showing a very slow rate change over the past several weeks.
Significant surface cracks are visible across the body of the landslide and represent the slow movement downslope. Cracks have been widening and new cracks have developed since October.
It is estimated that the landslide volume is ~4 million cubic yards and covers an area of about 20 acres.
The honest answer is no one knows for certain. There are a number of possibilities. The most likely scenario is that the landslide will continue to slowly move to the south, where the landslide mass will fall into the quarry pit and accumulate. Monitoring data suggests most of the mass will remain in the pit and on the hillside. Some rocks are expected to fall around the pit and to the west in this scenario, likely reaching Thorp Rd. In this scenario the landslide is not expected to reach I-82 or the Yakima River.
In less likely scenarios, the landslide may move beyond the quarry and potentially damage the houses to the south of the quarry and Thorp Rd. and possibly reach I-82. A very unlikely scenario is that the landslide will run out beyond I-82 and reach the Yakima River. Based on the measured movement direction of the landslide, the slope of the sedimentary interbed, a landslide that moves west and blocks I-82 and the Yakima River is extremely unlikely. Plans are in place to deal with this unlikely scenario.
Based on the rate and behavior of the landslide, it is uncertain when the landslide will stop moving. It is very unlikely that a “large event”, such as sudden and rapid movement, will occur. Instead, the debris will continue to accumulate in the quarry to the south, eventually halting movement of the landslide.
What Are We Doing About It?
Multiple agencies have collaborated to deploy GPS monitoring stations, seismometers, total station receivers, aerial imagery, and terrestrial lidar. Geologists conduct a regular conference call to discuss observations and notable changes.
Large shipping containers (Conex boxes) filled with concrete blocks have been placed on the southwest corner of the landslide mass along Thorp Rd. to block rockfall from I-82. Yakima County has closed Thorp Rd.
A Level-3 evacuation was issued for residents south of the landslide. Yakima County lifted the evacuation and some residents have returned.
Preparing for the worst
Local, state, and federal agencies, the Yakama Nation, and mine operators are working together to monitor the landslide and adjust plans as more information is gathered. Detour routes have been planned for I-82, and the interstate will be closed if the landslide accelerates. Plans have been implemented for possible scenarios where the landslide could impact or dam the river to the south or west.
Who Is Involved in Monitoring the Situation?
Columbia Asphalt (quarry operators)
Yakima Valley Office of Emergency Management
Washington Geological Survey (Washington State Department of Natural Resources)
Washington State Department of Transportation
Yakima County and several local city officials
Washington State Patrol
Mine Safety and Health Administration
U.S. Bureau of Reclamation
Washington State Department of Ecology
Washington State Department of Fish and Wildlife
Pacific Northwest Seismic Network