Example of man-made pavement scouring

[Mike Stropkovic Jr]

I'm obviously new here, and have been trying to learn everything that I can about severe weather. One of the most amazing things to me is how strong tornadoes can strip layers of pavement off the ground. I remember seeing this brought up in other discussions here and seeing it mentioned in damage reports from various tornadoes over the years. It reminded me of an incident that happened back in 2008 at a NASCAR race in Deleware.

The day's race had been delayed due to rain, and so NASCAR sent out their fleet of track drying vehicles, basically consisting of jet engines with a nozzle to direct their exhaust over the pavement, pulled on trailers behind trucks. One of the vehicles happened to accidentally blow a sizable chunk of asphalt off the surface of pit lane. Amazingly, this was captured on camera. I've posted the video link below, and would like to know everyone's thoughts on how this could relate to tornado-induced scouring. I'm also curious if anyone has done any research into this phenomina to determine what kinds of wind speeds or debris loading, or even road construction cause this to happen.

Watch video >

 

[Todd Rector]

Interesting video. It seems once the air blast got underneath the asphalt at the seam where it met the concrete, it lifted it away from the substrate underneath it more than blew it off from the top. I can't help but wonder how much of a factor the heat from the engine was, since asphalt gets quit soft at very modest temperatures.

It would be interesting to know more about the exhaust speed of that engine. Without knowing the the engine size/type and approximate RPM it would be anybody's guess.

 

[Royce Sheibal]

It's interesting to consider that asphalt and concrete are quite strong when being compressed under normal road conditions, but quite weak when exposed to any sort of force that would lift or bend it. Whilst I've never really though about the physics of scouring before, it does seem to make sense. The soil will most likely be eroded quite easily on the sides of the road (I've heard reports of up to 3 feet of soil being removed in extreme cases, anecdotaly) This will expose the sides of the pavement, allowing the air to push not only on the side, but possibly from underneath the road. Due to the relative weakness of the pavement when lifted or flexed in this way, it begins to break upward and gets sucked into the updraft in small chunks, and once the erosion begins it will likely spread to surrounding pavement due to the numerous exposed edges. Crazy stuff man.

 

[MClarkson]

At full power, at point blank range, the inner(or only with smaller or older models) high pressure section of jet engine exhaust approaches the speed of sound. Now you don't have to get very far away from this for the exhaust air to start mixing with ambient air or air that has been pushed through the more efficient low pressure section. Even with that low pressure mixing, a big turbofan engine will be in the EF5 range of wind speeds directly behind them at take-off power. There are other videos of big jets throwing away chunks of concrete and asphalt as well. I don't know what the throttle is at for the track driers, but even though its a small engine it likely lacks the lower speed section of an efficient modern turbofan: the exhausting being pointed nearly directly at the road surface, at close range it is capable of producing EF5 winds if the throttle is set high.

In this case, a range of speeds from 150 to 400mph seems fairly probable, without knowing the throttle setting and how cleanly that exhaust duct is sealed, I don't think I can be more precise than that.

 

[Dan Robinson]

The force of the air coming from that dryer doesn't look like anything approaching that of a tornado. It's in the back of an apparently unmodified full-size pickup truck. The asphalt layer on that pit track looks VERY thin and flimsy compared to what you'd see on a highway. The pieces flutter away like paper rather than tumbling on trajectories like heavy objects would.

 

[MClarkson]

A small jet engine that could fit on the back of a small truck could produce in the ballpark of 2000lbs of thrust. With the thrust applied fairly low, I could see the truck handling that, or a good portion of that, especially with a bit of ballast added.