UKTC Archive

Re: Risk of windthrow

Subject: Re: Risk of windthrow
From: Scott Cullen
Date: Dec 20 2007 11:03:25
SC insertions...
  ----- Original Message ----- 
  From: Dominic Scanlon 
  To: UK Tree Care 
  Sent: Thursday, December 20, 2007 4:16 AM
  Subject: RE: Risk of windthrow



  Ron wrote:

  <We have Greensand mixed with gravels and alluvial's here at Dorking and
  it behaves much the same as any other sand.>

  I planted 100 Japanese maples on the greensand ridge at Winkworth
  Arboretum - the easiest digging I ever had, especially compared to the
  London clays I was used to.  

  To get back on topic - There was no more wind throw there than anywhere
  else.

  SC Windthrow is a complex phenomenon with multiple variables.  Very 
basically the wind applies a horizontal force which is transferred and 
multiplied over the tree height (Mattheck's lever arm) and creates a 
"bending," "tipping," or "overtuning moment,"  call it Mo.  The tree is 
anchored into the soil by a root system.  The extent of the root system 
together with the cohesive strength of the soil create a "resistance moment," 
call it Mr.  If Mr>Mo the tree stands.  If Mo>Mr the tree blows over.

  SC.  Sandy soils may have lower cohesive strength than others, but sandy 
soils also drain better.  A soil with high strength when dry may lose 
strength when saturated such that Mr declines enough that it becomes <Mo and 
the tree overturns.

  SC Back to the horizontal wond force, that is a function of wind speed or 
velocity.  When wind speed is quoted by a weather agency it is usually at a 
standard height of 10m.  Below 10m the speed declines... to ~0 at ground 
level.  Above 10m the speed increases to some maximum called the 
"geostrophic" wind (which is nice to know but is in hundreds of m... way 
above tree heights.)  This vertical area in which wind loses speed and energy 
becasue of friction or "stress" with the earth's surface is called the 
atmospheric boundary layer (ABM).  Japanese maples are going to be much 
shorter than say oaks or other "shade" trees, exepreince winds of lower 
velocity and thus present a lower Mo in a given soil than taller trees.  So 
Japanese maples provide a good comparion against trees of similar heights in 
similar soils, but not to the wind stability of taller trees.

  SC

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