WEST SEATTLE WEATHER: Saturated slopes slide

As our year gets off to a soggy start – more than twice the normal rainfall as of early today – saturated soil has slid in at least two West Seattle spots:

That slope between the 2000 block of Bonair and the 1700 block of Alki had a slide last Wednesday, witnessed and photographed by Kevin Freitas. He alerted the city, which has since “yellow-tagged” one house on Bonair. Department of Construction and Inspections spokesperson Bryan Stevens told WSB, “The slide was approximately 15 feet wide as it ran down the slope and was contained within the boundaries of this property.”

Stevens continued, “We didn’t observe damage to the home itself (built on concrete piles), but the existing block retaining wall was damaged. Upon inspection, we posted a yellow tag, which allows occupancy of the home but notes the need to repair the wall and hillside. The owner was notified to obtain a Geotechnical Engineer to evaluate and stabilize the hillside.”

We also asked him about another reader report, a slide in the Eddy Street Ravine area northeast of Lowman Beach, north of the 6400 block of dead-end 49th SW: “A surface slide occurred at the top of the slope and deposited debris at the bottom of the unopened street end. We have relayed this to SDOT, and Seattle Public Utilities is taking lead on followup, given the potential impact to their infrastructure.”

If you are on or near a slope, landslide awareness is vital, especially in times like right now when there’s little time for the ground to dry out between storms. City advice on prevention, and what to do if a slide happens, is here.

3 Replies to "WEST SEATTLE WEATHER: Saturated slopes slide"

  • Graciano January 12, 2021 (7:46 pm)

    My experience with a Geotechnical Engineer, call the city first and ask them for the preferred list of Geo tech’s..  This is because the city has absolutely no Idea what they are looking at. 

  • Julia January 13, 2021 (10:24 am)

    Landslides will always happen to a certain degree but I have to wonder how much of this is brought on by removing the trees and native vegetation. All I see in these pictures are invasive species like ivy, blackberry, and laurel. Anyone qualified able to comment on this?

    • Colin January 13, 2021 (5:25 pm)

      Trees, shrubs, and ground covers play a role in helping stabilize the upper layers of our soil. Erosion and soil saturation play a much bigger part, however.Interfaces can form in the soil column where two types of soil with different grain size meet. For example, sandy soil can sit on top of heavy clay soil. Water has trouble passing through these interfaces as it tries to percolate down (check out a visual of there in this video at the 8 minute 30 second mark: https://youtu.be/ego2FkuQwxc). Thus, if these interfaces exist, and are at an angle, heavy rains will saturate the top layer of soil to the point that it is heavy enough to begin sliding down on top of the layer below it.Alki has always been a peninsula that has been eroded from below, giving it its sheer cliffs that have sloughed off over time. Building on these slopes requires cutting into them and disturbing the soil columns. This disruption means that there isn’t as much soil downslope to help prop up the soil uphill. The amount the soil needs to be saturated in order to slide is lessened by this, so slides occur more frequently.Plants can help lessen the risk of slides in a number of ways. They are able to suck up water from the soil column, lessening the weight bearing down on the upper levels from over saturation. Unfortunately, it is winter, so the plants aren’t soaking up nearly as much as they would be if it were spring or summer. Their effect here is negligible. Plants can also help break up interfaces in the soil column by sending their roots down through them, helping aid the water in its travel downward. Trees are good for this. Their roots can also act as a stabilizing element to the soil, both through helping prop the soil uphill from them with their trunks and stems, but also with their fibrous roots that can hold clumps of soil together.So when it comes to invasives, it’s better to have them than to have nothing. They can soak up water from the soil column when they are photosynthesizing, and their roots can help act as a stabilizing presence in the soil. However, the presence of a hillside covered in invasives probably tells a larger story: one where development has taken place in that area, cutting into hillsides and removing established trees and shrubs that break up the soil column and help prop up the hillside.I hope this has been helpful. It’s a myriad of things: Alki’s natural formations, development, a loss of mature plants, and I barely touched on this but it’s significant: our increasing precipitation rates due to climate change.

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