Slip Sliding Away
Geology Bats Last
Landslides have been part of the city "from a time to which the memory of man runneth not back."
City of Seattle Engineer Reginald Heber Thomson, Feb 27, 1897
“It is no small thing to re-engineer the basic geology of the region, which is what the Plaintiff’s position would lead to.”
King County Superior Court Judge Kathleen Learned, July 1999
Once again, our local geology is in the news. On January 7, Seattle Times reporters wrote of a man being rescued from his house on Perkins Lane, in the Magnolia neighborhood. Fortunately no humans were injured but sadly one of the family’s dogs died. The reporters wrote that the recent rain-saturated slope above the house had slid and moved it 15 to 20 feet off its foundation.
This was not the first time that stable Perkins Lane structures had been transformed into mobile homes. Twenty five years ago, the situation was far worse and far more damaging. In late December 1996, several inches of wet snow fell, followed by what we now call an “atmospheric river.” (This is another one of those phrases that is being bandied about more frequently because of climate change.) When the rains hit on January 1 and 2, they rapidly melted the snow, saturating the ground and triggering landslides that damaged scores of houses, several beyond repair. (Of course, this isn’t the lone landslide area around here. Eastside. Westside.)
In addition to precipitation, the other culprit in these landslides was the last ice age and the deposition of three layers of sediments: Vashon till atop Esperance Sand atop Lawton Clay. The problem is that the extremely fine-grained clay layer is impermeable to water so when it rains, water trickles through the upper layers, reaches the clay, and is perched, which creates a slippery surface susceptible to sliding.
As Mr. Thomson noted above, we have long known of the challenges of Seattle’s topography and geology. Attempting to combat the issue, 150 workers began a WPA project in 1936 to dig drainage trenches into the slopes above Perkins Lane. The plan was to provide conduits to collect water and get it out of harm’s way. It would require “men who are inexperienced, underfed and unwilling, [to work] in trenches 15 to 25 feet in depth where the saturated ground is moving and under unfavorable weather conditions,” according to a city council memorandum. Crews eventually dug and lined about 2,500 feet of trenches with one extending more than 700 feet into the hill, ultimately reaching 100 feet below the surface.
Despite the efforts of the WPA crews, the slopes of Perkins Lane still gush water. And, if you have walked the stairs between Magnolia Blvd and Perkins, you know the bluff isn’t stable because the stairs make you feel drunk, they are so uneven, tilted, and mismatched. I understand why people choose to live along the road—quiet and great views—but I am not alone in saying that you couldn’t pay me to live there. We have enough problems in the world without having to worry about intersection of the geologic time scale and the human time scale; it’s rare when humans come out on top when this happens.
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Reader Response: A reader in Portland wrote the following: "I like to keep track of sidewalk masons(?). I mean the imprinted name and year of concrete sidewalk installation. Here in Portland many are 100 years old (I'll have to double check that) and often one sees the same names.” I know that one can do the same in Seattle. Perhaps I’ll write about this in a subsequent newsletter.