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Re: Asian tsunami

Subject: Re: Asian tsunami
From: bobw
Date: Jan 11 2005 07:42:06
As a first mumbling, I'm mostly against equation dragging; seems to me,
it interferes greatly with intuition...
My thoughts involved a debris-filled flow of water including parts of
houses and a car or two. These pressing against a tree top are
dramatically different from a flow of ordinary "empty" water.
And then again, these conditions could "extract" a tree from the
ground--which is not uprooting as we might generally think of it. The
tsunami wave carries many items of debris at many levels in the surge,
not only in a lateral movement, but in a vertical movement as the wave
rises in height. A rise is very likely accelerated as a downstream
series of obstacles impede the flow. With little movement at that
moment, a rise is expectable, and provides a vertical force that need
not be too significant with a sandy soil.
Perhaps, the weight of water is irrelevant. Even if I extended the water
depth to the deck of the Titanic, and reached with a robotic arm for a
coat rack with a very wide solid base, I ought to be able to tip it or
lift it easily. There might be a small reluctance attributable to the
weight of water--in getting it out of the "way", but water is
incompressible and ought not to increase in "density" as a function of
depth... I seem to think that water at a Titanic depth would be about
the same reluctance or resistance as water at a depth of 33 ft.
It might be that we're caught up in the anecdotal observation in the
first post; "how many trees seemed to be left standing when everything
else in the path of the waves was swept away/ demolished?" It's hard to
tell if we don't have before and after information. Anything at all left
standing would have a notable effect on our senses, and maybe, they'd
tend to be "overcounted." That's not a criticism, but rather just a
reasonable puzzlement.
Thanks for a good reference article:
But, I'm not sure of the mitigation of coral reefs and mangrove stands.
Water at a height is stunningly insistent on finding its equalized
levels. It will get there with surprising speed. The main mitigation is
actually the retreat of the surge for exactly the reason I stated above.
A small perturbation can mean a shorter reach and lower velocity. A
surge that overtops the height of an island above sea level will
respect or be mitigated by very little.
We have initiated many butterfly effects on a huge complex energy
engine, and while I don't think that results in earthquakes, what do I
know? I feel sorry for my grandkids, who will inherit staggering changes
and profound local disasters, and be just as ignorant about what to do. 

xxx

Bob Wulkowicz
ps: Palms are fascinating, aren't they. Maybe supple is better than
rooted. (?) 

-----------------------------------

Scott Cullen wrote:
It would be straight forward enough to run drag equations for both
water and air against trunks. Take drag coefficient of a cylidrical
trunk as 1.0 and it is same for water and air. Substitute density of
waater for density of air. I would guess that the enormously greater
density would more than offset the shorter lever arm... i.e. a greater
bending moment at the base.
But if a tree uproots, it is becuase the bending moment exceeds the
resitance moment. The resistance moment includes the weight of the
above ground parts (net of off-center gravity load as the system
deflects) plus weight of roots and attached soil plus the "pull out"
resistance of roots beyond the "anchoring root plate-ball" (limited by
the tensile strength of those roots). When there is air above that
subsurface mass it exerts negligible downward pressure. Water by
contrast exerts great downward pressure... ~8lbs/gal, maybe a little
less in salt water.
Experience in FL and LA is that native palms resist hurricane winds
pretty well with natural anchorage. Dep and fibrous rather than
spreading I think. Other trees likely to have wider spread roots
meaning - I think - more area for all that water to press down on. 




----- Original Message ----- From: "bobw" <bobw.enteract@xxxx.com>
To: "UK Tree Care" <uktc@xxxxxx.tree-care.info>
Sent: Monday, January 10, 2005 1:29 PM
Subject: Re: Asian tsunami
Just a question of how water above the roots as weight might have
prevented uprooting? Seems a bit counter-intuitive--then again,
that's what I prides meself upon.
In looking atthe videos of the debris swept along by the tsunami,
that all may be quite the equivailant of enormous top-side loading,
rather than the flow-around quality of wind we're used to places like
Chicago. 

tubs




Scott Cullen wrote:
Just a guess is that the water wrapped around the tunks and the
leeward resistance of that mass might have prevented breakage. And
the weight of the water downward onto the root systems might have
prevented uprooting. 

Scott Cullen

----- Original Message ----- From: "Ken" <ken@xxxxxxxxx.co.uk>
To: "UK Tree Care" <uktc@xxxxxx.tree-care.info>
Sent: Monday, January 10, 2005 7:58 AM
Subject: Asian tsunami
Has anyone else noticed when seeing the footage of the horrific
devastation caused by Boxing Day's tsunami in asia, how many trees
seemed to be left standing when everything else in the path of the
waves was swept away/ demolished?
Obviously the mechanical loads caused by the water will have, for
the most part, been quite different to a gale-force wind which would
put the load on the crown rather than washing around the trunks, but
it still seems incredible especially as so many seem to be growing
in sand. 

Any comments ?

Ken








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