Long Runout Landslide in Skagit Valley

Perkins, Reid and Schmidt (2017, Geology) compared landslides associated with glacial terraces in three valleys of the Cascade Range, the Cedar, North Fork Stillaguamish and Skagit valleys. Although the terraces have similar topographic appearances, the underlying glacial related sediments at each terrace differed and thus the mechanics and scale of the slides are differed. The terrace reviewed in the Skagit Valley is called Burpee Hill and is located west of Concrete.

Lidar derived figure from Perkins, Reid and Schmidt (2017) of the slides on the south side of Burpee Hill

Oates (2016) investigated the slides at Burpee Hill mapped the geology units and thicknesses of the units. She noted that the run out distances of these slides was truncated by the river. However, the deposit of one of the slides still remains on the opposite bank of the Skagit. 

The slide run out from the upper slopes of the slide area to the distal end of the deposit is over 6,000 feet. The slide deposit is a location of numerous homes in part because the slide deposit has created an elevated area above the flood levels of the Skagit River.

Regardless of the different mechanisms and style of slide initiation, all three terraces reviewed by Perkins, Reid and Schmidt (2017) are capable of generating long run out landslides. The hazard areas posed by these long run outs is a challenging policy issues at many government levels.  

Trade War Notes

The trade wars have begun. Washington State does a great deal of international trade. In part, due to our northern ports facing the Pacific. But the State also produces a lot of products that are shipped overseas. The US Chamber of Commerce put together a state by state highlight of the value of exports that will face tariffs. For Washington State the value of goods facing tariffs is more $6 billion (uschamber.com/tariff_data/one_pagers/wa.pdf).

Mexico has been especially strategic and that nations's retaliation will hit Washington fruit hard as well as potatoes. The Canadian strategy is to avoid supply chain issues for Canada.

I am not convinced about the Trump administration strategy, particularly with regards to Canada, Mexico and the EU. And I suspect that China situation is not well understood and will lead to long term harm.

Krugman knows a bit about trade and suggests that Trump will be well remembered for his statement that "trade wars are good and easy to win": Krugman: how-to-lose-a-trade-war

It appears the European thinkers will be strategic as well voxeu.org/new-cold-trade-war.

For Washington State - exports will remain important. It will be hard to consume all those apples.

Hole in the Clouds: Von Karman Vortex

Back in early February I observed spiral of low stratus clouds off the U.S. west coast. I put up a short note about it (swirling-hole-in-clouds).

Doing some random research on a favorite topic I came across this NASA image:

Holes in the stratus down wind of Tristan da Cunha

As a side note I have been fascinated by the South Atlantic island of Tristan da Cunha in part because I once had a map of the world completely covering on my wall and Tristan was right above the table I ate at - a remote island in the ocean demanding attention.

The hole in the clouds I observed is the same phenomenon and has a name - Von Karman vortex. And turns out that what I observed caught the attention of others and made the local news: https://weather.com/science/weather-explainers/news/2018-02-04-southern-california-von-karman-vortex

Possible Wave Cut Slope on Marrowstone

Much of the shoreline bluffs on Marrowstone Island are very abrupt, with a steep vertical drop due to the hard glacial till that can stand for decades as vertical cliffs. 

Stepping off the bluff edge on this bluff would hurt

Bluff is 65 feet and mostly silty glacial till

 
Many of the shoreline bluff I have inspected on Marrowstone have this sharp edged drop at the top edge of the bluff. The bluff above is exposed to fairly frequent wave energy such that any material that calves or ravels off the bluff face is washed away. That said, the bluff retreat at this particular bluff can not be discerned comparing historic aerial photographs dating  back 70 years. 

A recent bluff I was inspecting was even higher than the bluff shown above; however, it was not as steep on its upper part due to the higher amounts of cobbles and gravel within the upper bluff. About 20 feet down the slope it became vertical similar to the bluff above.

Test pits for septic systems often save me a bit of time for looking at the surface geology of sites.   

The above two pits were excavated into an area mapped as glacial till. At depth, the soil does become siltier and more typically of poorly sorted glacial drift observed elsewhere on the island bluffs as well as the steep cliff bluffs at Port Townsend. The round cobbles and slightly stratified nature of the upper units suggest some reworking of the sediments and forming a gravel/cobble lag. Glacial till can have some stratified layers due to water flowing at the base of the ice, and that was my working theory as to the high gravel and cobble  content in the test pits.

But on the drive over the mid section of the island, this slope is suggestive of a possible other explanation.

Maybe this higher portion of Marrowstone was reworked by wave action during seawater incursion during the late stages of the last glacial period.

The mass of glacial ice that filled the Puget lowlands caused the land surface to deform downward hundreds of feet. When the ice began to pull back, there was brief period when the sea invaded the lowlands and flooded the area at relatively higher levels than the current local sea level because the land surface was lower due to the ice loads. The water formed shorelines along the edges of areas inundated. After the land rebounded, theses former shorelines were uplifted to their current elevation.

These wave cut terraces are often rather subtle features. They are readily apparent in higher quality lidar (lidar-wave-cut-terraces-on-orcas-island).

Lidar imagery of southern San Juan Island

These wave cut terraces are pretty easy to see

The wave cut terraces on San Juan Island were recognized by geologists over 100 years ago (Bretz described them) as they are readily apparent. 

There is lidar coverage of Marrowstone; however, the quality is not as good as the more recent lidar coverage elsewhere and it is not clear if the slope above is a wave terrace.

But the slope is at the right elevation for having been a former shoreline. The hill slope is 170 feet. Across the water on Whidbey Island. Polenz, Schasse and Petersen (2006) mapping of the Freeland Quadrangle recognized former shoreline features at 125 to 130 feet at Double Bluff and 180 to 190 feet at Greenbank. Projecting across the water to Marrowstone the daisy covered slope shown above is midway between those two sites.

Deer Harbor Notes

I had a wet early morning stop on the upper end of Deer Harbor on Orcas Island. The view looking south from the upper bay down this protected inlet shows a few docks as well as the marina dock in the upper end of the bay.  

The upper part of Deer Harbor inlet

San Juan County used some grant money to replace the bridge across the upper part of the inlet a couple of years ago.

The first bridge across the inlet was built in 1915. This latest bridge replaced the span that had been built in 1970 (deer-harbor-bridge) and is a wider structure. The project was partially significantly supported by a Nearshore Ecosystem Restoration Project. By widening the estuary opening and removing a rocked bottom under the old bridge, the estuary is supposed to drain more freely and recover some of the original habitat features.

The harbor got its name from Hudson Bay Company hunters as the site was apparently a good deer hunting site. One of the early Hudson Bay hunters, Louis Cayou, stayed on and married a Lummi Indian. His name can be seen on an early survey and was later the name given to the estuary at the head of the inlet. 

San Juan Travels

I do feel very fortunate that my field work includes periodic trips out to the San Juan Islands. 

Before leaving the dock at Anacortes on the early morning boat

Mount Baker and the Twin Sisters and Obstruction Pass

Sunset off Lopez

Other passengers enjoying the trip and views

Willow Island

View and Notes on Racehorse Slide

I had a nice view of the Racehorse Slide (cool-oblique-lidar-image-of-racehorse) (first-description-of-racehorse-slide)on a recent trip up the Nooksack River valley. If the clouds and rain are not low, the slide area can readily be seen. 

2018 view of slide

2015 view of slide

I marked up the next picture with the arrow indicating the area of the block that detached and slid as down into the lower Racehorse Creek canyon in 2009. The headwall area of the older and much larger slide that spilled out across the Nooksack River valley is marked with a dashed line. Note that additional logging has taken place between 2015 and 2018 along the southwest side (right in the picture) of slide.

Logging had taken place on and adjacent to the 2009 slide shortly before the failure took place. As can be seen in the photograph above additional logging has been taking place along the older large slide headscarp and along a block that has remained attached on to the right of the slide. That block is located on the south side of the slide. 

lidar showing large slide area and deposit

The impacts of logging on large deep-seated landslides is uncertain. It certainly is not positive, but a direct connection between logging and reactivation of deep-seated bedrock landslides has not been firmly established. The 2009 slide was certainly coincidental with the logging that had taken place, but it also took place during a very intense rain on snow event that was likely on the order of a 25 to 50 year event. Estimating the water input difference between the slope harvested and not harvested  might be an informative exercise. I am not aware if anyone has tried this. The land in question is managed by the State DNR.