Van Zandt

The small town of Van Zandt lies about 10 miles away to the Southwest. It is home to a wonderful store and deli, a couple of churches, it has its own low-power FM radio station and that is about it.

It is also home to a potential large landslide and this has happened before. Here are two blog posts about the landslide and LIDAR

First - from The Landslide Blog:

The Van Zandt landslide: another great example of landslide mapping using LIDAR
I recently came across another great example of the use of LIDAR for mapping landslides in difficult terrain.  The Washington Geological Survey has a wonderful mapping tool online that gives the user the ability to look at a range of geological data.  The Bellingham Herald has a nice article about the LIDAR dataset, which can be accessed through the portal, in which they highlight the example of the Van Zandt landslide on the Nooksack River.  This landslide is located at 48.803, -122.151 if you want to take a look on Google Earth.  Imagery of the location suggests that there might be a landslide here – below is a Google Earth perspective of the site showing a huge scoop out of the ridge and a river that may have been pushed over to the opposite side of the valley.  However, the area between the ridge is densely forested in places, with vegetation of various types covering almost all of the landscape.  It is extremely difficult to map a landslide deposit in these circumstances:

20170507-vz01.jpg

The author - Dave Petley - talks about using topographic maps and direct visualization and then come out with the money shot:

But a LIDAR system fires many pulses of laser energy at the ground in the process of mapping.  Even in a forested area a few of these make it through the vegetation canopy to strike the ground before being reflected back to the instrument.  Whilst the proportion of laser pulses that make this journey is small, the high density of pulses sent by the laser means that these can be used to map the underlying terrain.  So the pulses that have reflected off the tree canopy are removed through processing, leaving just those that reached the ground.  These are then combined to produce a bare earth LIDAR hillshade.  The spatial resolution of this is less good, but the ability to capture the underlying relief is amazing.  Here is the bare earth hillshade for the Van Zandt landslide:

20170507-vz02.jpg

And suddenly the hummocky topography of the landslide deposit is plain to see, and is easily mapped. And what becomes evident from this is that this was another highly mobile landslide with a long runout. Interestingly, a back of the envelope calculation suggests that the H/L ratio for this landslide is about 0.132, close to the supposedly extraordinary mobility value of 0.105 for the Oso landslide.

The Oso landslide in 2014 killed 43 people.

Second, from local geologist Dan McShane's Reading the Washington Landscape:

The Van Zandt Slide Gets Some More Attention
Dave Petley calls attention to the use of LiDAR (light detecting and raging) and the Van Zandt landslide in Whatcom County (agu.org/landslideblog/2017/05/05/lidar/). His post is in part derived from a Bellingham Herald article that covers the recent work of the Washington State Department of Natural Resources Division of Geology and Earth Resources (DNRGER). He also gave my blog a nice compliment - I did a short write up on the Van Zandt Landslide (van-zandt-landslide-introduction) a while back after a trip to the slide with Geoff Malick. Goeff is getting near completion of his thesis on the slide.

The DNRGER has developed and recently expanded their geology information map portal https://fortress.wa.gov/dnr/protectiongis/geology/?Theme=wigm. The portal provides geologic maps but has also recently added LiDAR coverage.

I typically do not use the portal very often as I have most of the maps I need and their accompanying pamphlets/booklets. However, a separate portal of really good LiDAR imagery has recently been developed by the DNRGER (http://lidarportal.dnr.wa.gov). This portal does not include as much LiDAR derived imagery as the main geology portal as a lot of LiDAR areas have yet to be added. But the quality of the imagery is sharper and allows one to pick which LiDAR flight to view when there are multiple LiDAR flights available. It also allows turning on or off the bare earth imagery. The site can also be used to download the raw data.

The LIDAR portal is an excellent way to spend a few hours - looking for existing slides in the area. I know of several in one valley over and they show up loud and clear on the map.

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About this Entry

This page contains a single entry by DaveH published on May 7, 2017 4:31 PM.

Back home again was the previous entry in this blog.

Big book - The Klencke Atlas is the next entry in this blog.

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