Aerial History and LiDAR of the Stilliguamish Blocking Landslide

Aerial and LiDAR images explain a great deal about the landslide that blocked the NF Stilly (geology-of-silliguamish-blocking-slide and arm-waving-notes-on-stilliguamish). The LiDAR particularly shows the extent of past sliding and the scale of the past landslides. This leads to policy issues, but I will save that for a later day or two or week. 

 

This first image is from 1989. I marked in red the river route in 1989 as well as outlined the slide activity at that time. Obviously the river is eroding the base of the steep slope setting up an unstable situation and the ground is starting to unravel. Note the relatively new clear cut harvest. There have been and is ongoing debate regarding clear cut harvest and water infiltration into deep-seated glacial sediments in part triggered by this slide but also another set of slides in deep steep canyons elsewhere nearby. In this case, I will offer an opinion that the river erosion at the toe is the big driver of slope instability.

This image is from 2003. As can be seen the river and slide area has shifted a bit westward. This image is before the slide that took place in 2006 that blocked the river channel in a very similar manner to the Nooksack River blocking event of 2014.

This image is from 2006, post river channel blocking slide. The river has a new channel as it was deflected southeast by the mass of slide material. Note how much the slide has expanded westward. What can not be seen is that the west slope of the slide is much higher in elevation and thus contains a lot more volume. The new channel was partially excavated and rock lined by the Army Corps of Engineers to maintain flow away from the landslide toe area. Also note how much the river had been pushed towards Steelhead Drive.

 This image is from 2009 and shows the slide area is greening up except for the steep headwall scarp on the west limb of the slide. Note as well the straight line edge of the river channel on the slide side of the river. The channel has become wider than the 2006 channel due to erosion and the straight edge river bank is an engineering effort to contain the northwest directed erosion.

This image is also from 2006 and shows active stream channel work taking place. I am not entirely up on the project, but it was part of mix of salmon recovery and hazard management work conservationregistry.org/projects.

The 2013 image shows that the main body of the slide has greened up more. But note the headwall on the west side is still bare in part because it is so steep. However, part of the bare ground is the result of headwall collapses dumping material onto the unstable slide mass - not a good thing and it is what I observed when I took a look at this slide last summer. It was still active and clearly not done.

LiDAR is a great tool for assessing landslides. The bare earth images have made mapping large landslides and large landslide deposits an office exercise with focused field trips. The active slide area is easy to see, but there is so much more to be gleaned from the LiDAR in regards to scale.

This first image marks the active slide area at the time of the LiDAR in red. Blue arrows indicate the river flow. The orange line marks the boundary of an older headwall scarp. The area between the red line and orange line had not been actively sliding over the past 20 plus years, but that scarp indicates that some time in the past that area was likely part of a big landslide. Based on the Seattle Times images, I suspect that this area between the red and orange lines may have collapsed and the pre existing deep-seated slide may have expanded. This area between the red and orange lines and just where the new headwall scarp is located is my main interest as the pre failure and post failure geometry may provide some geology slope stability lessons. A hard and costly lesson.

This next LiDAR image pulls back away a bit to show the bigger picture of the slope stability of the area.

The smallish red marked area on the right is my estimate of the active slide area. I did not include the deposition area of the slide as I don't have a good image yet, but can say that the deposit extended clear across SR530 on the map.

The larger red outlined area in the central portion of the image is from a much larger slide. The deposit area from that slide was even bigger than what is indicated, but the southern end has been eroded and removed by the river. It would be very interesting to know how old this slide is. And also might be informative to look at the geometry of the headwall area.