This blog provides commentary on interesting geological events occurring around the world in the context of my own work. This work is, broadly, geological fluid dynamics. The events that I highlight here are those that resonate with my professional life and ideas, and my goal is to interpret them in the context of ideas I've developed in my research. The blog does not represent any particular research agenda. It is written on a personal basis and does not seek to represent the University of Illinois, where I am a professor of geology and physics. Enjoy Geology in Motion! I would be glad to be alerted to geologic events of interest to post here! I hope that this blog can provide current event materials that will make geology come alive.

Banner image is by Ludie Cochrane..

Susan Kieffer can be contacted at s1kieffer at gmail.com

Saturday, February 19, 2011

National Christmas tree toppled!

Photo of toppled tree behind the White House
Note the stump and the break about 2' off the ground
Photo from CNN.com, no photographer given
The news today is that winds gusting from 25 to 60 miles per hour toppled the White House Christmas tree, which had been left standing, though unlight, from the Christmas season. If you feel sorry for this one poor tree, then pity those destroyed by the lateral blast at Mount St. Helens in 1980. The blast devastated more than 500 square kilometers, and more than 4 billion board feet of timber.  (A board foot is a piece of timber 1 foot wide by 1 foot long by 1 inch thick.) It's not trivial to go out onto the WWW to try to find out how many board feet are in a typical tree! All sorts of definitions, because it's important to foresters to figure out how much useable wood is in a tree. But, I find that a red oak, 60' high with a diameter of 14" contains 189 board feet.

Natural tree blowdowns are well known.  In 1999, a severe storm called a "derecho" traveled from North Dakota through Quebec, 1300 miles in 22 hours, damaging homes, knocking down power lines, and blowing down an estimated 25,000,000 trees. A derecho is a straight-line windstorm common in here in the Midwest, and they can pack winds blowing up to 120 miles per hour.

So, is it difficult to blow down a tree?  The branches and canopies cause the drag on trees to be high, although trees change shape under wind conditions to minimize drag. Drag coefficients depend on a number of parameters, including the wind velocity itself.  They are highest for fir and spruce, and lowest for Western hemlock and Lodgepole pine.  Douglas fir and Scotts pine are intermediate.  Typical velocities that knock over trees are about 100 feet/second, or 68 mph.

Drag on a tree is proportional to both the wind velocity and the density of the wind itself (clear air).  At Mount St. Helens, the "wind" was full of rocks and ash (best described as an ash hurricane), so the density was higher perhaps by one to two orders of magnitude.  The velocity required to blow down the trees there was accordingly less, perhaps by one or two orders of magnitude as well.

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