Oso Landslide - Update

"To the geologic eye all the surface of the earth is a fluid form, and man moves upon it as insecurely as Peter walking on the waves to Christ." Will and Ariel Durant. Lessons of History (Simon & Schuster 1968).

There is much now that has been posted in other blogs by those who are more expert than I.  My initial observations founded in some limited online research were correct.  I had observed the scarp of a former landslide as marked by an arcuate band of whitish bare soil.  This actually was the location of a previous albeit somewhat smaller landslide in 2006 that ended up sliding into the river course and moving the river course southward with a little human engineered assistance.

For some additional insight on the Oso landslide, I refer you to these blog posts from Dan McShane's Reading the Washington Landscape:

Arm Waving Notes on the Stilliguamish Blocking Landslide

On the geology:

Geology of Stilliguamish Blocking Slide

Excellent historical imagery discussed:

Aerial History and LiDAR of the Stilliguamish Blocking Landslide

Cliff Mass' Weather Blog provides some insights into our wet March that was a factor in this landslide event.

The Meteorological Background for the Stillaguamish Landslide

The tragedy of all this is that conditions conducive to repeated occurrences of landslides were well known and documented.

Oso Landslide

I'll preface this blog post by acknowledging that I am not a landslide expert.  It's safe to say that I have an environmental science and geology background by education and profession.  With that, I can examine and research some of the factors that resulted in this catastrophic earth movement.  These factors are:

• Rainfall
• Geology
• Gravity

Rainfall has been of nearly record levels during March in our western Washington area.  Almost five inches of rainfall has been recorded during March in the Everett area and probably more than that fell in the landslide area.  Water is heavy and some of it soaks into the ground.  A wet sponge is heavier than a dry sponge.  Gravity then exerts more force on the heavier mass.  Combine the added mass with a steep slope and you have the potential for a landslide.

Here's a basic diagram of a typical slump block landslide, looking very much like the type of landslide that occurred here.

From the United States Geological Survey (USGS)

When I first looked at the area on Google Earth it became obvious that landslides had occurred in the past along both sides of the North Fork Stillaguamish River valley.  I could see some relict scarps, some fresher looking scarps, and hummocky surfaces where landslides had rolled out and came to rest.  Here's a Google Earth Image of the landslide area.  You can see the whitish area that could be a relict scarp from a previous landslide.  The actual landslide broke a little farther back from the assumed relict scarp marked by the whitish area and then flowed across the river and Highway 30 taking out everything in its path, including the houses seen in the foreground.

Pre-landslide view.  Note the whitish area at center which marks the area of the recent landslide.  (Google Earth Image used for a noncommercial purpose)

I was curious about the geology and so looked at some geologic maps available online.  On these maps, old landslides and landslide deposits are clearly mapped up and down the river valley on each side of the valley.  The stratigraphy of the valley reflects the recent glacial history and shows that the underlying geology was a significant contributing factor.

Let's look at the recent geologic history of the Puget Sound region.  During the last Ice Age, perhaps 16,000 years ago, the Puget Lobe of the great continental ice sheet moved into our area from the north, part of the vast ice sheet that covered much of North America.  The Puget Lobe essentially dammed up the river valleys along the front of the Cascade Range and even moved up these valleys.  This is somewhat counter-intuitive because one pictures glaciers moving down a valley, not up.  As these river valleys were dammed by the ice sheet, glacial ice margin lakes formed and in those lakes were deposited primarily clays and silts (Qglv - Advance glaciolacustrine deposits).  Then the ice sheet moved up and overrode these lacustrine deposits forming another layer of deposits called till (Qgtv), basically a mish-mash of silt, clay, sand, and gravel that is ground and packed under a glacier as it moves.  Both the till and advance glaciolacustrine deposits were densely packed under the hundreds of feet of glacial ice.

As the ice sheet receded some 13,000 years ago, it again left ice margin lakes with similar clay and silt deposits to the advance lacustribe deposits (Qgle - Recessional glaciolacustrine deposits).  The final top layer of the glacial sediment cake were the recessional outwash deposits (Qgoe) composed mainly of sand and gravel disgorged from the melting and receding glacier.  The recessional lacustrine and outwash deposits were never overridden and packed under glacial ice and are comparatively loose and unconsolidated.

As a refresher, the glacial deposits along the side of the river valley are from top to bottom (newest to oldest):

• Recessional outwash (sand and gravel; loose)
• Recessional glaciolacustrine deposit (silt and clay)
• Till (silt, sand, and gravel; dense)
• Advance glaciolacustrine deposits (silt and clay, dense)

The following map is a geologic map that shows the glacially-derived sediments discussed above and landslide deposits with arrows showing direction of historic landslide movement.

2003. Washington Division of Geology and Earth Resources. Geologic map of the Mount Higgins 7.5- minute quadrangle, Skagit and Snohomish Counties, Washington. Open File Report 2003-12. Dragovich, J.D., Stanton, B.W., Lingley, W.S., Jr., Griesel, G.A., and Polenz, Michael

The following photo shows what I interpret as the upper stratigraphic units discussed above.  At the top in light gray is the sand and gravel of the recessional outwash.  This is a relatively porous deposit that could hold a lot of water.  The darker gray band below the outwash is likely the recessional glaciolacustrine deposit composed of silt and clay.  With the clay and silt, the lacustrine deposit would be comparatively less permeable and would act as a perching layer holding up the rain-soaked recessional outwash and preventing water from infiltrating deeper.  The added mass of the rain-soaked recessional outwash at the top of the slope combined with the force of gravity have caused the slope to fail resulting in the massive landslide.  This is only an interpretation on my part.

Landslide main scarp. Note that the trees are probably 50 feet, if not 100 feet tall.  (Snohomish County)

As it stands now, the landslide has dammed the river and water is building up behind the dam.  The river will eventually overtop or work its way around the dam, either in a catastrophic flash flood event or perhaps a more gentle cutting of a new course.  Three people are confirmed dead and 18 are missing.  My hopes and prayers to all affected.

A Spring Walk in the Woods

Our backyard borders 120 acres of woods, in reality a typical western Washington second growth forest already 75 to 100 years in the making.  Western red cedar, western hemlock, and Douglas fir are in ascendancy while huge bigleaf maple and especially red alder are in decline.  It's spring time and the trillium and salmonberry are in bloom.

Trillium (Trillium ovatum)

  Salmonberry (Rubus spectabilis)

Fringecup (Tellima grandiflora)

Salish Crossing Redevelopment Project - Edmonds, WA

The Salish Crossing redevelopment project is underway here in my little waterfront town of Edmonds, Washington.  The site is owned by and the redevelopment project is being led by a group with local ties.  For all the 22 years that I've lived in Edmonds, this shopping center has always been anchored by the Waterfront Antique Mall, housed in the former Safeway store.  The former Safeway building is in the classic Marina-style, with the broad eyebrow arch and glass-fronted facade.  This building, according to plan, will fortunately be retained.  So far demolition of the southern building has occurred and the parking lot planters and trees have been removed.

Here's a couple links to articles:

New owners of Salish Crossing plan to enhance, not replace, Antique Mall structure

Demolition work begins on Waterfront Antique Mall/Salish Crossing site

I'll try and follow progress weekly via flickr: Salish Crossing Redevelopment Project - March 30, 2013

Spring

You dig a hole in the fall, drop in a dried-up looking little nugget, and then come spring it emerges as something of bright-colored delicate beauty.  Sort of the ugly duckling of the plant world.  Bulbs are so pathetically easy to grow.  It requires no skill whatsoever other than the ability to dig a shallow hole and drop in a little time capsule.  And you only have to plant them once for years of color every spring.  So here's the show from our garden, with a couple shots thrown in from elsewhere.

This was our best year for daffodils.

I salvaged this Hellebore years ago from a home garden slated for demolition.

These tulips are actually in front of the Edmonds Museum.

Early spring rotation on our front porch.

We have a huge camellia, could be more than 40 years old.

Not in our garden, but a flowering plum tree in the neighborhood.  One of the first flowering trees of spring in western Washington, now in full peak bloom across the region.

Birdbox Replacement

After over 20 years of use by generations of chestnut-backed chickadees, the birdbox on the big Douglas fir finally wore out.  Another bird, perhaps a woodpecker, had excavated a hole in the side and was in the process of stuffing it with shredded cedar bark.  I never saw the usurper.  The old birdbox had been occupied by chickadees every spring, except one year when I didn't clean it out, bumblebees took it over.

I went out to Wild Birds Unlimited and bought a brand new replacement birdbox, one constructed of fine-grained cedar with a metal-clad roof.  This one should last another 20 years.  The old birdbox was nailed to a young cedar out in front.  Maybe bumblebees will take it over again or maybe a family of wrens.

The old and the new.

A big hole in the side and smaller holes made by powder post beetles.

The new birdbox ready and waiting.

The Planet Hoth

Our winter sometimes brings days of clear, cold weather when a high pressure ridge plants itself over the eastern Pacific.  Our house is in constant shade this time of the year because of tall evergreens located behind us to the south.  As a result, hoarfrost develops and accumulates on everything.