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Prehistoric landslide complex, fault runs along Ledgewood
The massive landslide in the community of Ledgewood south of Coupeville displaced 5.3 million cubic feet of earth and one house in a matter of moments in the early morning hours of March 27, according to the Washington state Department of Natural Resources.
That’s about 40,000 dump truck loads.
Geologists from the department visited the scene of the landslide this week to take initial stock of the cause and risks.
Joe Dragovich, a geologist with DNR, said experts won’t be able to do in-depth work at the site until the area stabilizes, which could be a matter of weeks or months. Geologists on the scene said they heard a tree crack and fall while they were looking around.
The four geologists, however, released a “Quick Report for the Ledgewood-Bonair Landslide” on Thursday.
The report states that the landslide ran 1,100 along the shoreline and jutted 300 feet into Puget Sound.
The beach was lifted up 30 feet above the shore.
Dragovich explained that it was a deep-seated landslide, which basically means it originated from deep in the earth. The report states that the failure plane is likely below sea level, but the depth is currently unknown.
The reports states that the landslide area was a smaller portion of a much larger landslide complex. The entire landslide complex is 1.5 miles long and may date back as much as 11,000 years.
DNR geologist Michael Polenz identified the prehistoric landslide in 2005; it was published as part of a geologic map in 2009. “Landslide activity” demolished a beachfront residence from 1996 to 1997, approximately 800 feet north of this week’s landslide.
Polenz’ work suggests that the sediment at the top of the slope consists of Ice Age seafloor sediment, till deposited by glacial ice and outwash.
The mid-slope sediment may include sediment from non-glacial times.
Dragovich said the sediments tend to be poorly consolidated, which means instability.
“It’s basically just a pile of debris,” he said.
The report states that a portion of South Driftwood Way, the road taken out by the landslide, was moving from one to three feet each year from 2002 to 2009.
Dragovich said the cause of such deep-seated landslides isn’t completely clear. While the type of materials and steepness are obvious factors, it’s debatable whether groundwater or surface run-off can contribute.
Water can cause surface landslides, he said, but the evidence for deep-seated landslides is unclear.
Doug Kelly, an Island County hydrogeologist, said 2012 was the wettest year on record in Coupeville. The data for the Coupeville Station, just a few miles from the landslide, goes back to 1895.
Another intriguing piece of evidence, Dragovich said, is that a strand of the southern Whidbey Island fault runs across the landslide complex. He said the landslide wasn’t caused by an earthquake or movement of the fault, but there’s a possibility it could cut up through the sediments.
The report predicts that some sliding may continue. It states that “upland portions of the landslide body may remobilize to move the body into equilibrium” and that calving may occur as the near-vertical headscarf erodes.
The geologists conclude their report with advice for those who choose to return to their homes along the prehistoric landslide. The residents should watch for clues of an upcoming landslide, which may include any of the following:
• Newly developing cracks, mounds, or bulging on streets, sidewalks, or the ground.
• Sagging or taut utility lines; leaning telephone poles, fences, or trees.
• Sticking windows or doors; new or growing cracks in walls, ceilings, or foundations.
• Broken or leaking underground or surface utilities, such as water lines.
• Separation of foundation from sill plates; movement of soil away from foundation.
• Changes in water well levels.
• Increase or changes in spring or seep activity; ground becoming soggy or wet.