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Merry Christmas and its very merry to
be with the living this Christmas.
The following incident occurred over the Thanksgiving weekend, must
have been about 4:00 pm Saturday, November 28, 2009 as it became dark pretty shortly
after. Earlier and for several hours that day moderate wind gusts
rustled through the crowns of the trees. Weather turned and there
was just a hint of snow an occasional very fine hail. At that
moment of the toppling it was a little bit sunny for a change, but with
the occasional wind gust.
I figure now is a good moment to reflect on the subject of what to do
in a forest should one encounters a massive tree falling in your
path. I have poked around the internet only to read one recent
and gruesome account of errant tree dodging. I never really
though about it but yes, a full size pine tree should just about
pulverize the head and shoulder of its victim. That's gravity for
you. When it takes only a moment to release the energy
collected in hundred and some odd years of growth, focusing it on a few
hundred square feet of ground. Just by comparison lightning,
though more concentrated energy may be relatively benign. Whereas
lightning may actively seek its victim out in the shortest (and
sometimes human aided path) from the negatively charged atmosphere to
the ground, a tree (and assuming it were a random event) falls where it
does. Even if one were standing right next to it you would have a
significant chance of coming out only worse for their nerves.
This is all strictly unaided tree falling and not felling of which
seems to be well practiced art by foresters. Generally well
practiced except for the the subject of the previously mention
account. The take away from the official report was that its very
important to quickly assess the danger your fellow tree harvesters
warning you of, and not to mistake their concern for some less
significant danger. When your fellow foresters bellow "watch
out", look up first or else you might avoid a minor danger only to jump
into the path of a falling tree.
Given that a forest of any significance is littered with fallen trees,
there would have to be at least a single significant tree fall each
year for every few acres. Certainly much more than say lightning
strikes per acre per year, yet so many more people get struck.
Seems at least to be many more accounts of lightning strike than tree
strikes. Maybe this is just another media phenomenon 'struck by
tree' might be less reported than 'struck by lightning'. At any
rate, what you might be curious about is what to do or what it is like
to have a large tree cross you path. First of all lets figure out
the dimensions, the perspective may be a little poor in my camera but
this tree pictured below spanned the distance of two power poles.
I paced it out to be an excess of 125 feet. The top 15 feet
entirely disintegrated by either the fall (the tip was pushed down by
the heavier base right). The diameter I would have to
guess is a relatively skinny for its height at approximately 3
ft. The breaking point was several feet in length not far up the
trunk. After reviewing various forestry publications I will just
estimate the weight as being a simple cone with density of 34 lbs./ft^3
1/3 h Pi R^2 D -> 1/3 (125) 3.14 (3/2)^2 (34) -> 10K lbs
-> 4.5 tons
I think that this might be a little light comparing it to some of the
charts for a tree taller than 100 feet but certainly in the ball park.
As hazardous as it is to extrapolate off just a couple of data points
(having witnessed the snow stressed breaking of 25 feet the top of an
approximately 75 foot tree) and seeing much the damaged aftermath of
many a snow, rain or wind storm you are pretty likely, if ever, to
encounter a falling tree while there is a strong wind or else with a
moderate to heavy snow or ice load. In my judgment the time
between the crack, and the moment where true direction of the fall
could be determined was about 3 seconds. Very fortunate for me,
there was no competing sounds as my perspective was not the best
(particularly initially looking directly up at it) to determine the
direction of the fall. Traveling by foot while reading paperback
novel (Catherine Wells, Mother Grim) I was quickly aware of the
specific danger by the distinctive and significant crack. In
moving from the edge of the one lane road to the center in order to
gain best advantage, I could see a moment later I was safely
positioned. The tree would land right where I was initially
walking on the downhill side of the road and I could very comfortably
achieved another 12 feet of safety before the tree hit the
ground. This second 3 seconds didn't compel me to scale the wall
of the hill to get further away. I was already decidedly so.
Seems my perception seems approximately correct as the time to fall
should be about 3 seconds:
SQ(2h/g) -> 2 (125 ft) /32 ft/s^2 -> 2.8 seconds
The rotational inertia of the tree as it falls could easily account for
the additional time. I distinctly recall being thankful that I
had not lost my page in the novel, but afterward I thought I should be
much more grateful that I had not been listening to loud music or
distracted by another noise, or that a car didn't happen to be driving
by at precisely that moment. I'm not sure my eyes, even directly
facing the tree, could have helped more than my ears had done.
Other witnesses in cabins within the radius of a hundred yards claimed
that there was a significant shaking both at the breaking and the
crashing to earth. I didn't feel much. The owner of the
house adjacent to the tree was pretty lucky too. The tree landed
all the way up his skinny driveway. Both his truck and his house
were within twenty feet of the fallen tree, but it did not look like he
suffered much property damage.
-
Chris
8 December 2009
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