Storm damage assessment

Editor's note:

Predicting tree failures with consistent success is an inexact science. The best tool for increasing our success in making such predications is to observe trees that have failed. Through observation and experience we can start to anticipate situations when there is a likelihood that tree failure may occur. The following article is based on observations of many trees after recent storm events. It is being offered for consideration and discussion and not with the endorsement that this is the way trees always fail. Hopefully, this article will motivate the readers to begin observing trees after failure so that they can begin to better understand under what conditions trees fail.

And they call the wind Mariah!

By Dr. Tom Eiber, DNR-FIS Supervisor The last few years in Minnesota have seen some exceptional weather activity. Moreover, if many of the predictions that I have read about the next decade are true, it is likely to continue. Over the past five years, I had the opportunity to "attend" a half dozen serious wind storm events ranging from the 1997 Monticello Event to the Cottage Grove windstorm of several years earlier. Several observations have become very clear and form a logical scheme of damage. The table below is a rating system similar to the Beaufort Scale commonly used to visually rate winds when no wind gauge is available.

This scale rates storm damage on a one to five scale, one being the least and five being the most damage. It has come to light largely through a "back-door" application of the Minnesota Hazard Tree Rating System which works on the premise that trees fail in predictable patterns. They clearly do. And they do it for Mariah. Almost exclusively.

ClassWind speeds (in mph)Damage
150 - 65 Small twigs and branches (mainly cottonwood) strew lawns.
Small silver maple with codominant stems and included bark fold over.
Trees with severe defect or multiple defects may fail.
265 - 75High category* /main stem/ multi-defect trees fail.
Branches with decay and included bark unions fail. 50% of high category cracks fail depending on exposure.
Exposed trees with moderate cracks elongate or widen the cracks.
Medium sized silver maples with included bark fail.
Random green ash and blue spruce root may windthrow if moderately exposed to the west.
Cottonwoods shed moderate to large branches.
385 - 95High category/single defect stems fail particularly in the "hot spot".
Moderate category branch decay and included bark unions fail.
Remaining high category cracks fail. Moderate cracks fail.
Windthrow common in green ash and blue spruce particularly in lawn situations.
495 -110Most high category defects fail.
Many moderate category defects fail.
5 110+ Defect free trees fail.
Trees with defects rarely remain standing.

* = Refers to categories in the manual, "How to detect, assess and correct hazard trees in recreational areas".

Storms take a particularly heavy toll in silver maple due to the weak, brash wood and the long, slender nature of the branches, but silver maple's tendency to form included bark unions with codominant stems is its greatest drawback. This tree is made to dis-assemble in storms and typically makes up at least 50-60% of debris needing to be cleaned up after a Mariah Event. The potential for property damage from silver maple failing from west to east (or thereabouts) is substantial.

The amount of included bark is critical. If the severity of the included bark defect is measured as a percentage of stem diameter, eg. a 3" length of included bark on a 9" diameter stem is 33%, two "break points" become critical. See diagram. Storm breakage becomes "possible" as the percentage approaches 33%. Storm breakage becomes "likely" if it exceeds 50%. Of course, exceptions occur, but a tree, even a silver maple, with a minor amount, perhaps 10%, of included bark will likely survive a direct hit by a Class 2 storm.

Depth of included barkTrees with >50% included bark will likely fail from storms of much lower intensity and easily fail with a Class 1 storm. Included bark is clearly most serious in silver maple, probably due to the weak wood, but is clearly a serious problem in ANY SPECIES. The ease with which silver maples form weak unions probably outweighs its weak wood, but (1) the combination of the two is deadly and (2) other trees can suffer the same defect.

Green ash (>14" DBH) without root flair and blue spruce with ten feet of "sail" above the roof tops or in "wind alleys" are highly prone to windthrow and should be considered for removal if they are west of a "target". Any tree with a high category, main stem defect (particularly in the "hot spot") should be considered for removal, but this issue becomes critical if a major target (eg. a home) lies approximately east of the tree.

Tornados? I have never assessed tornado damage with the above system, only straight line winds and thunderstorms. Tornados, even weak ones (F-1), are likely to cause sound, defect-free trees to fail. It seems likely that any tornado would rate as a Class 5 event.