Speed of Debris Relative to Wind Speed

[Bob Hartig]

As a matter of curiosity, are there any studies on how fast large, weighty objects in a tornado's debris field are capable of moving relative to the actual wind speed?

It takes one second for a 205 mph tornado wind to travel the length of a football field. Smaller, lighter debris borne on that wind are presumably traveling at roughly that same speed. But as the size and mass of debris increases, friction and gravity have more of an effect. It's safe to say that a roof or a pickup truck entrained in that wind isn't moving at the same speed as a piece of insulation, and even a 2 x 4 probably isn't traveling quite that fast. But what percentage of the actual wind speed are such objects capable of fulfilling?

I realize the question is simplistic, given the variables involved, and answers are liable to range from "It depends" to quite complex. I'm mainly interested in what thought and research has gone into the matter and whether it's a factor in damage surveys.

 

[Royce Sheibal]

Three major concepts of physics come to mind.

1. - The lift-off velocities of arbitrary bodies. This is an incredibly complex calculation that makes my head explode, but involves mass, weight, air resistance, etc. all in a simulated 3D environment.

Once a body has lifted off, it is governed by 2. - Terminal Velocity and wind resistance equations again in a simulated environment. The body is not likely to 'circle' the tornado due to the large force required to make a turn (and the vast majority of tornado forces push tangentially or up, not in). It may however make small spiral loops in the violent updraft before it leaves it. From this we also can infer that peak velocities likely occur at 1 of two points, shortly after lift-off and then second upon "falling" impact for larger objects. Constructed large objects will likely be broken down into many smaller ones rapidly due to the violent forces, resulting in either fall due to loss of wind resistance, or uptake into the updraft due to loss of mass relative to resistance. Only solid objects like cars, etc will likely survive in a large piece.

And all of those equations are dependent on 3. - Fluid dynamics of the air debris within the tornado. The "air" in a tornado is not the "air" we think of normally. It's more like an aerosolized version of a debris flow than air. The more dense the air / debris makeup the more force and velocity can be imparted due to fluid dynamics reasons and resistance reasons, all of which has to be calculated in an environment as well. I would like to find some studies that explored the relation between tornadoes becoming more violent shortly after ingesting large amounts of dirt and debris (or do they become violent before the debris response, but the velocities are yet not properly observed? Interesting topic).

There are tons of articles on all 3 of these things but you'll have to get pretty deep into the simulation studies.

Perhaps the best way to actually find the velocities of debris (outside of calculations) is from damage surveys. We can replicate damage in labs to see exactly how fast things had to go to do x amount of damage. Here's a mind blower for you. The Topeka F5 of 1966 was incredibly intense and did some nuts wind damage. Persons trapped in a hallway at the local college reportedly suffered from 'through and through' wounds to their arms from small rocks and pebbles. This would indicate velocities in this wind-funneled area to be approaching that of a bullet, likely over 300MPH, even though the damage surveys showed winds well below that speed on the exterior of the building. There's a book on it, great read.

TLR, small objects (lets say a 2x4 or smaller) may potentially exceed observed winds due to local environmental effects, meanwhile solid large objects may serve as the best proxy for observed wind speeds due to their very high take-off velocities that will tend to be near that of their terminal velocity.

 

[Jeff Duda]

I do not know the answer to your question, Bob. I know back in "the day" that stereo photogrammetry was used to try to estimate tornado wind speeds, but I think those techniques relied on tracking smaller objects, thus not in line with your question.

I suspect the real answer is that there is sooooooooo much variation in debris movement speeds at large debris sizes that it comes down to needing to know the exact dimensions and detailed material composition of each piece to have any reasonable estimate of movement speed.

 

[Alex Elmore]

I'm mainly interested in what thought and research has gone into the matter and whether it's a factor in damage surveys.

Debris usually isn't used to determine tornado intensity in damage surveys. I've heard from someone who takes part in surveys mention that on a couple surveys of violent tornadoes, there were some debris characteristics that helped give them confidence in the rating they gave the damage. Two that come to mind were a safe that had been anchored and was moved a considerable distance, and then a truck that had bounced off the top of a water tower (Smithville, MS EF5 on April 27, 2011). I believe that if debris is factored into damage surveys it's when damage surveyors take into account a structure failing due to debris loading.

 

[Jeff Duda]

Debris usually isn't used to determine tornado intensity in damage surveys.

That's a pretty misleading statement if no context is given. "Degree of damage" for a "damage indicator" is the basis of the EF-scale. Wind speed estimates are derived from the DOD. So debris is most certainly used to determine tornado intensity in damage surveys.

 

[Bob Hartig]

Royce, Jeff, and Alex, thanks for your responses! I appreciate your taking my question seriously, as I know it's a bit impractical. It is, however, fascinating to think about. On a side note, the classic video of the June 8, 1995, Pampa, TX, tornado, shows a flattened pickup truck being flung at 90 mph from the vortex. I'm guessing, though, that the initial "muzzle velocity" had to have been considerably higher in order to eject so large and heavy an object several hundred feet.

 

[Alex Elmore]

That's a pretty misleading statement if no context is given. "Degree of damage" for a "damage indicator" is the basis of the EF-scale. Wind speed estimates are derived from the DOD. So debris is most certainly used to determine tornado intensity in damage surveys.

I think there may be a slight misunderstanding here, because I agree with everything you said except for the last line, and that may be because of the context we're using "debris". When considering damage to a single family residence, if the roof is missing, I consider the roof to be debris, regardless of if/how it stays whole as it goes airborne or hits other structures/objects. However, what happens to the roof of the house after it leaves the house has no impact on the intensity rating of the damage to the house based upon the damage indicators list:

https://www.weather.gov/oun/efscale

I think that if roof/pieces of the roof, along with other types of debris, hit the structure next door and cause damage, it would be hard to determine how much damage the debris was responsible for and how much the wind from the tornado itself caused. The structure may be rated if the surveyors determine that damage to the structure was directly due to the tornado, but I don't think it can be rated if they think the damage was purely due to debris striking the structure. For example, I was on a survey last week where a shed was completely destroyed. The missing shed was obviously rated, but where the structure (debris) ended up was not taken into consideration in the rating. A house that was a hundred-couple hundred yards away was damaged, but that was due to a tree falling on it, so it was not rated. It sustained no visible damage other than from the tree.

For debris loading, if the surveyors determine the damage to a structure is due to tornadic winds and debris loading, they still need to rate the damage based up the degree of damage to the structure itself. However, they may adjust the estimated wind speed within the rating to take into account the impact of debris loading on the damage to the structure. Hope this clears things up.

 

[Royce Sheibal]

https://www.jstor.org/stable/199859?seq=8#page_scan_tab_contents

Ancient (1914) recount of debris damage from the Omaha Easter Tornado Sequence of 1913. Engineers estimated debris velocities of up to 400mph. I understand that 1913 was a long time ago, but engineers still new their stuff back then. They didn't give out F5 ratings much back then, but 3 of the 5 Omaha tornadoes would have probably been high-end EF5's today.