The EF Scale and Asphalt Scouring Caused by the March 2 Henryville Tornado

[Bob Hartig]

In his chase stats for 2012, fellow chaser Simon Brewer notes that the March 2 Henryville tornado caused "damage to a highway southwest of Henryville peeling and tossing large 6-10 inch thick slabs of asphalt, some weighing around 10,000 lbs, 30-50 meters creating large and deep impact craters." Because of the severity of the damage, Simon has considered the tornado to be an EF5, though it was officially rated an EF4. Since highway scouring is not among the DIs listed on the EF scale's rating system, I understand why the lower rating was assigned; there's no mechanism for assigning a higher rating based on asphalt peeling.

It's too bad, though. I realize that a number of factors can work together to peel asphalt, and EF5 winds aren't necessary to get the job done. But in the Henryville case, what Simon has described sounds pretty wild. And it wasn't the work of a slow-moving tornado nibbling away at small sections of asphalt by abrasion; it was a fast-moving tornado ripping up large chunks in seconds.

I take an interest in this because the tornado crossed that stretch of SR 135 less than a mile in front of me. It took mere seconds to do so--it had to have been moving at 60 mph--and it was rapidly intensifying at the time. Bill Oosterbaan and I got cut off by downed powerlines at Dutch Creek Road, just south of the main damage path, so I didn't see the scoured section of highway. But I believe that Simon did (correct me if I'm wrong, Simon). I was surprised to learn of the scouring, but not too surprised, because it was obvious when the tornado crossed that it was entering the violent category--not large at the time, but very intense and growing rapidly.

I'm curious whether highway scouring was ever discussed as a possible DI during the development of the EF Scale. I can see problems associated with using it as such, but those being granted, still, large chunks of asphalt ripped up by a swiftly-moving tornado that took only ten or fifteen seconds to accomplish such a feat seems like something that should at least be given consideration.

 

[Shawn Schuman]

Ha! I was thinking exactly the same thing when I first read Simon's post. I also mentioned something similar about "unconventional" DIs in the El Reno thread. Simon has photos of it on his site. According to the storm survey (and I may be a bit off, I can't recall exact details) the asphalt was scoured from an area roughly four yards square, and it was five to six inches thick. Some of the chunks were apparently thrown as far as 30 yards.

It definitely looked impressive, though by looking at the photos it does appear that the asphalt juts up above the surrounding ground higher and sharper than you'd expect. That may have factored in to some extent. But as I mentioned in the El Reno thread, I think there's a lot of value in considering unconventional DIs if possible. The biggest problem I see is that the wide range of variables with something like asphalt or grass scouring may make it difficult to standardize.

ETA: Here's the survey from LMK: http://www.crh.noaa.gov/Image/lmk/03022012_EF4.htm

The tornado then traveled over a ridge and intensified as it hit State Route 135 at Dutch Creek Road. Here, large chunks of 5- or 6-inch thick asphalt from an approximately 4-by-4-yard section of roadway were blown 10 to 30 yards into the adjacent grass next to the road. Just east of Route 135, tremendous tree damage was observed. At this location, the tornado was estimated to be of EF3 strength with 150 mph winds.

 

[John Olexa]

I wonder how many times it happens though, even in stronger tornados

 

[Rob H]

I'm guessing that asphalt standards aren't nearly as reliable as other construction that DIs apply to. As a child, I remember a time or two where we pulled out large chunks of asphalt just because we could. Maybe it's impractical to test how suction vortices rip up asphalt in a controlled experiment. Maybe the road was flooded two years ago and that asphalt was barely staying in the road even before a tornado came. There are conceivably a lot of maybes.

Tim Marshall might be a good person to get input from if he still lurks here, as I'm certain he's considered asphalt scouring as a DI at least once before.

EF rating is still somewhat flawed in that it can't be used for historical analysis since assessment quality has changed considerably, and it still involves something being hit that can have a DI applied. There was a 5/3/99 tornado that was bigger and possibly stronger than Moore, but it didn't hit anything - so it didn't get an EF-5 rating. It might be a can of worms, but I guess I'm not sure why Simon would consider Henryville to be an EF-5. There's no real important distinction to chasers between a high end EF-4 and an EF-5 other than a difficult-to-obtain punch on an imaginary punch card.

 

[Greg Higgins]

Recently Tim Marshall made a presentation to our local AMS chapter and discussed the Joplin tornado. One of the things that he picked up on was the movement of "anchored" concrete parking curbs (like whatyou would see in a parking lot) that had been moved a considerable distance and in a few cases even lofted a short distance. We also discussed scouring of asphalt and other roadways. He indicated taht he is interested in doing some "research" into this area as has been pointed out, there are not any DI's at this time. Might be interesting to get Tim's thoughts on this subject................

 

[Bob Hartig]

Using asphalt scouring as a DI would no doubt involve a number of variables. I don't know how granular you can get with them, but to my thinking, the extremes would be to make too much of scouring on the one hand--i.e. assuming that it invariably implies a violent-class tornado--versus discounting it entirely on the other rather than considering its implications case by case. The Louisville damage assessment team noted the scouring by Dutch Creek Road, yet considered the tornado to have caused only EF3 damage at that point. I wonder what their thinking was. And I agree, it would be great to get Tim Marshall's thoughts on the matter. He's the guru, and his mentioning the possibility of researching asphalt scouring as a DI piques my curiosity.

 

[Joshua Nall]

After looking at those photos I’m not sure you can call that scouring. Unless i'm not seeing something correctly, I noticed the chunks of asphalt appear to be removed from the downwind side of the roadway. Most pictures of scouring I’ve seen, it appeared that the road came apart little by little and started at the edge on the upwind side. This damage here may be due to lift over the domed surface. There would be nothing for the wind to get under and force the asphalt up on the downwind side of a smooth roadway like this. But there would be lift. You would have to calculate what wind speed would be required to lift the weight of the asphalt considering the area of the “wingâ€. Maybe this has been done before. It would take more than just a guess…. “that’s EF3â€â€¦. Or “that’s EF5â€....

Also the NWS says it was about a 4yd by 4yd section removed that was 6 inches thick. Asphalt weighs around 140lb per cubic foot. That would be about 10,000 pounds for the whole section. A 3 foot by 4 foot section would be around 800 lbs. Not that that matters really.

I would think that the whole section probably lifted and broke apart as it lifted. Here when they lay the rock substrate it is wet, and it sets up and hardens, then they come along and put down some asphalt liquid I guess is what it is, for adhesive, and then lay the asphalt. Not sure about other states, but it may be that the asphalt was more or less just laying on the rock.

 

[Simon Brewer]

After looking at those photos I’m not sure you can call that scouring. Unless i'm not seeing something correctly, I noticed the chunks of asphalt appear to be removed from the downwind side of the roadway. Most pictures of scouring I’ve seen, it appeared that the road came apart little by little and started at the edge on the upwind side. This damage here may be due to lift over the domed surface. There would be nothing for the wind to get under and force the asphalt up on the downwind side of a smooth roadway like this. But there would be lift. You would have to calculate what wind speed would be required to lift the weight of the asphalt considering the area of the “wingâ€. Maybe this has been done before. It would take more than just a guess…. “that’s EF3â€â€¦. Or “that’s EF5â€....
QUOTE]

This is a good guess of possible forces, which caused this significant damage to the highway from the Henryville tornado. The wider damage path associated with the main tornado circulation was easily visible from a forest west of the highway through a field, and past the highway through another forest to the east. Also, an individual suction vortex damage path was easily found starting in the field scouring vegetation and tossing boulders from a drainage ditch, then crossing a section of highway peeling and tossing massive slabs of asphalt, the largest broke upon second contact with the ground (it bounced leaving an significant impact crater) on the downwind side of the highway. Typical sphalt scouring is usually associated with EF3 and stronger tornadoes, but typical asphalt scouring is found on rural roads with relatively thin asphalt 1-2 inches or less thick. It's amazing to think how short a time period the small suction vortex was probably over that section of highway; maybe only a second tops! I usually don't stop to investigate tornado damage, but when I saw the highway damage on March 2nd I was blown away! I took more photos of the road damage than I did the storm and tornado. I consulted Dr. Greg Forbes and he agreed this was one of the most incredible damage cases he's heard, only possibly being eclipsed by the trench created by the Philadelphia MS EF5 on April 27, 2011.

The NWS Louisville office had a massive job on their hands with the extensive damage survey from this large outbreak, so I don't think, or would I say, they did a poor job. I think they did the best they could with the resources available. I don't think they ever did a ground survey at this particular damage location, because they originally listed the asphalt as being 3 inches thick. After I sent several emails detailing the damage they increased the thickness to 5-6 inches thick, which was still incorrect, because the larger slabs from center of highway were actually 8-10 inches thick. All the slabs were peeled from the center and downwind side of the highway. When we arrived at the damage path a few hours after the tornado had passed a road crew had already filled a large portion of the highway with gravel, but hadn't filled the shoulder. If I had not seen the massive slabs of asphalt scattered to the east of the road I would have kept driving.

I'm not surprised the tornado was only rated EF4 in the town of Henryville; it visually appeared to significantly weaken shortly before entering the town losing the rapid rippling motion of the vortex, not displaying the amazing vortex expansion, and losing a significant portion of the condensation funnel. While the strength/wind speed of a tornado cannot be judged by visual appearance certain visual phenomena can suggest a weakening or strengthening pattern in an individual tornado event (example being a tornado exhibiting a large bowl condensation funnel with a light dust debris field appearance and then later showing a rippling stovepipe appearance with violent helical motions; the later usually will have a much more severe damage path).

The true problem with the F and EF-scales is using damage to estimate winds. The F and EF Scales are at the mercy of the quality and quantity of damage indicators and the diameter of the tornado. If a skinny rope 10 meters wide with 250mph winds hits a large house and obliterates half the house it will be rated EF3 based on damage due to its width, not its winds. Also, a large tornado with 200+ MPH winds in open country will get rated EF1 at best if it doesn't hit any man-made structures or trees. Also, do we really know so much about tornado damage to know that a certain structure was destroyed by 198 MPH winds and not 202 MPH winds? I think there should be a damage scale, but don't assign wind speeds. Tired of hearing media say, "...the tornado had 135 MPH winds...". What I'd like to hear media say is, "the tornado caused EF3 damage" and say nothing about the wind speed.

Simon

 

[Bob Hartig]

That's a terrific writeup, Simon. Thank you!

Also, an individual suction vortex damage path was easily found starting in the field scouring vegetation and tossing boulders from a drainage ditch, then crossing a section of highway peeling and tossing massive slabs of asphalt, the largest broke upon second contact with the ground (it bounced leaving an significant impact crater) on the downwind side of the highway. Typical sphalt scouring is usually associated with EF3 and stronger tornadoes, but typical asphalt scouring is found on rural roads with relatively thin asphalt 1-2 inches or less thick. It's amazing to think how short a time period the small suction vortex was probably over that section of highway; maybe only a second tops!

I had wondered whether a suction vortex was the culprit. By the time the tornado crossed the road, it had become a powerful stovepipe and suction vortices were becoming evident. The winding road and rolling, forested terrain hid the bottom of the tornado where it interacted with the pavement from my sight. "Only a second"--could be. I guessed on the conservative side, but I do know the vortex was moving fast and crossed the road in what seemed like the blink of an eye. There was no time for it to do any scouring, only ripping and tearing.

 

[Simon Brewer]

That's a terrific writeup, Simon. Thank you!



I had wondered whether a suction vortex was the culprit. By the time the tornado crossed the road, it had become a powerful stovepipe and suction vortices were becoming evident. The winding road and rolling, forested terrain hid the bottom of the tornado where it interacted with the pavement from my sight. "Only a second"--could be. I guessed on the conservative side, but I do know the vortex was moving fast and crossed the road in what seemed like the blink of an eye. There was no time for it to do any scouring, only ripping and tearing.

Bob, hope you don't mind, but I found your youtube clip from this day, matched it with Google Maps and estimated the time and your location when the tornado crossed the road. Grabbed a low-quality video still and slapped it on Microsoft Paint with Google Maps image and labled your estimate viewing location, tornado location, and possilble road crossing image.

Image below is a google maps image of Highway or Route 135 north of Palmyra, IN and a Youtube video still from Bob Hartig's footage:



Simon

 

[Shawn Schuman]

I think there should be a damage scale, but don't assign wind speeds. Tired of hearing media say, "...the tornado had 135 MPH winds...". What I'd like to hear media say is, "the tornado caused EF3 damage" and say nothing about the wind speed.

Couldn't agree more with this. I've never really understood the purpose of it beyond public curiosity, and it strikes me as causing more confusion than anything else. As an example of how little the general public understands the wind speed portion of the scale, many people believe that EF5 tornadoes are actually less powerful than the F5s of the past because of the lower estimates. It's kind of a mess, but maybe there's something I'm missing.

I think this instance illuminates both the inadequacies of the EF scale and the difficulties that would be encountered in adding to or trying to improve it. I watched a presentation that Tim Marshall gave which sounds like it may have been the same AMS presentation Greg mentioned above, and I was also intrigued by Tim's brief discussion of the EF scale as it related to the Joplin survey. I was left with the impression that he felt the scale still needed some additions and/or refinements. I also recall that he said the overall extent and severity of the event was considered when rating the tornado, which is something I'd never heard before. I don't want to put words in his mouth, but I believe he said something about non-DIs (including those parking curbs and some missing manhole covers as well as the overall extent of the damage) being considered in order to get to an EF5 rating "for historical purposes." I wasn't quite sure how to interpret that, but it was very interesting.

Edit: I found a link to the presentation I was referring to: https://ams.confex.com/ams/26SLS/webprogram/Paper211662.html

 

[Simon Brewer]

Couldn't agree more with this. I've never really understood the purpose of it beyond public curiosity, and it strikes me as causing more confusion than anything else. As an example of how little the general public understands the wind speed portion of the scale, many people believe that EF5 tornadoes are actually less powerful than the F5s of the past because of the lower estimates. It's kind of a mess, but maybe there's something I'm missing.

I think this instance illuminates both the inadequacies of the EF scale and the difficulties that would be encountered in adding to or trying to improve it. I watched a presentation that Tim Marshall gave which sounds like it may have been the same AMS presentation Greg mentioned above, and I was also intrigued by Tim's brief discussion of the EF scale as it related to the Joplin survey. I was left with the impression that he felt the scale still needed some additions and/or refinements. I also recall that he said the overall extent and severity of the event was considered when rating the tornado, which is something I'd never heard before. I don't want to put words in his mouth, but I believe he said something about non-DIs (including those parking curbs and some missing manhole covers as well as the overall extent of the damage) being considered in order to get to an EF5 rating "for historical purposes." I wasn't quite sure how to interpret that, but it was very interesting.

Edit: I found a link to the presentation I was referring to: https://ams.confex.com/ams/26SLS/webprogram/Paper211662.html

Earlier I wrote 135 MPH and compared with EF3, but 136+MPH is designated for low-end EF3...oops.

Thanks for the link; I've seen a few of Tim Marshall's presentations in person and spoke with him about several events, but haven't seen his Joplin presentation. Tim Marshall gave a fascinating presentation discussing how significant unconventional damage indicators can become in a damage survey. As a storm chaser I am indifferent to tornado damage ratings, but as a scientist I do care about data and accuracy. The trench dug by Philadelphia MS tornado, parking curbs tossed by Joplin tornado, pumpjack tossed by Bridge Creek OK tornado, large asphalt slabs peeled and tossed by Henryville IN tornado, and concrete porch slabs tossed by Hackleburg AL tornado will go down as examples of unconventional damage indicators from tornadoes.

 

[Shawn Schuman]

I recall reading a paper that I think was done by some folks from BMX/HUN about some of the extreme, unconventional damage indicators from the April 27 outbreak. I'll see if I can track it down. It provided several interesting examples, including a few of the ones Simon mentioned. The Smithville tornado apparently carried a 40-pound bag of sports equipment more than 30 miles, and of course there's the SUV that was thrown half a mile into the town's water tower (leaving a large dent) and then tossed a further quarter-mile. The Tuscaloosa tornado destroyed a steel train trestle and tossed one of the 30-plus ton trusses 100 feet uphill, and also threw a 30-plus ton train car something like 400 feet. The Rainsville tornado did some extreme damage as well; I'm sure most have heard of the damage along Lingerfelt (sp?) Road where an 800-pound safe was ripped from its anchors and found 200 meters away with its door ripped off.

I think some of the more impressive "unconventional" DIs came from the El Reno tornado. The two million pound Cactus 117 rig was destroyed and the derrick was rolled several times. One of the 10,000-pound-rated steel straps that anchored the site's storm shelter was snapped and I believe another was damaged. And one SUV was thrown nearly 800 yards and completely obliterated. I think vehicles could be another interesting DI, though I'm sure there would be problems with implementing such a thing.

 

[Jeff Duda]

This is just another example of the ambiguities caused by the fact that the (E)F-scale was designed to be a damage scale, then somewhere along the way people tried to also use it as a wind speed scale. That's why tornadoes like the Bowdle monster will end up as an EF4 even though I could bet my grad student stipend it had 200+ MPH winds in it, or that other tornadoes that looked like monsters and had violent motion are rated as EF0 since they really did no apparent damage.

ADD: I should clarify that the original F-scale was designed to connect the Beaufort scale to the mach scale, which is where the F6-F12 designations came from. So I suppose it technically was also a wind scale, but I think the meaning of the scale has been lost between then and now.

 

[Rob Wadsworth]

I've always thought that the EF scale was just applied to building/structure damage.

 

[Bob Hartig]

I should clarify that the original F-scale was designed to connect the Beaufort scale to the mach scale, which is where the F6-F12 designations came from. So I suppose it technically was also a wind scale, but I think the meaning of the scale has been lost between then and now.

Yeah, that's how it started out, and then it got limited to F5. F6 was a theoretical possibility if you went by wind speeds, but once the damage function of the scale got emphasized, F6 became meaningless because it dealt with purely hypothetical winds for which the damage yardstick didn't exist (having been blown away entirely at F5). I initially thought of the scale as Ted Fujita's attempt to extrapolate wind speeds from damage, but eventually I got it hammered home that it was a damage rating only, nothing more, though I suppose someone should have informed the good doctor as well, because he didn't seem to have been aware of that fact when he designed the scale.

Now we've got the EF Scale and we're using Doppler radar in some cases to help determine ratings, and part of the scale's premise in the first place was to more accurately correlate damage with wind speeds. To my mind, this Scale has suffered from identity issues for a long, long time, and if I think about it too much, I get a headache. Thankfully, far more astute minds than mine continue to work through the scale's imperfections and complexities with an eye on increasing its usefulness. But I'll bet even those guys are popping their share of Advil.

 

[Shawn Schuman]

I've always thought that the EF scale was just applied to building/structure damage.

Mostly, yes. Trees are officially included as well. But buildings can't always tell the whole story if there aren't sufficiently well-built structures in the path of the tornado, so sometimes consideration is given to things that aren't officially part of the scale. I don't know how much weight they're given, but it would seem they can be strongly considered given several tornadoes have been rated EF5 largely on the strength of unconventional damage.