# Can 50 mph straight-line winds dislodge a manhole cover?

We had a storm last night with winds of that type, which knocked a big tree over onto our house. That is of course loads of fun; but my SDMB question is about this manhole cover. My wife feels sure that the wind must have done it, but it is your usual heavy-duty circular cover (though it does have slots in it to allow rain to drain through), and I just can’t see it.

No.

Manhole covers are typically around 120+ pounds and flush with the ground. When they’re dislodged accidentally, it’s usually because of pressure from below (e.g. steam pipe or electrical explosions) rather than anything happening above grade. If the winds damaged something connected to underground utilities, they could have indirectly caused it, but the wind by itself itself isn’t taking that thing anywhere.

Maybe someone dislodged it to take shelter from the 50 mph winds down there.

Was there a lot of rain accompanying the storm? Did you see evidence of any flooding? Verses the wind, it’s more likely that storm drain line may have filled and back pressure forced the cover off from below. The rain might not even have to fall on you directly but collect from the watershed above.

Lieu, I think that sounds like the best explanation. At one point, though only for a brief period, the wind was driving the rain horizontally so hard that it was forming puddles inside doors to the outside and dripping from windows. And I have never seen that before, after living in this house for fifteen years.

It’s not as immediately implausible as it might seem. I did a quick back-of-the-envelope calculation to estimate how fast the wind would need to be in order to have a 120-lb manhole cover (with a diameter of 1 m) be able to “lean into the wind” at a 45-degree angle. The answer I came up with was 63 miles per hour, which surprised me; I had expected it to be much larger. Of course, the speed required as the “lean angle” gets close to horizontal would go up dramatically, since the wind is both providing less force (due to the reduced cross-sectional area) and less torque (due to the reduced lever arm) if the cover is nearly flat.

What if the wind found an open, grated tunnel entrance to the sewers which faced directly into the incoming wind? Would there be enough air pressure build up to pop off the manhole covers?

In principle, I imagine the Bernoulli effect could cause this.

Air flowing along the street, parallel to the surface, would exert reduced pressure perpendicular to the direction of flow. But the air pressure beneath the cover, directed upward perpendicular to the cover, would not be reduced. So there will be net upward pressure on the cover.

Without doing the math (which I don’t have the physics/engineering knowledge to do), I can’t guess if that could lift the full weight of a cover. And if the cover is full of holes or slits to let the rainwater run in, that’s all the more questionable.

Interesting point. I too only have a dabbler’s knowledge of physics, and I can’t decide if the slits would make this less or maybe even more possible.

rho * V1[sup]2[/sup] / 2 + P1 = rho * V2[sup]2[/sup] / 2 + P2

Let the “1” denote conditions under the cover, and “2” denote conditions above the cover.

Manhole covers vary in size, but since we’ll be calculating pressure in pounds per square inch, it’s enough to know the manhole cover’s weight per unit area. That varies too, but let’s pick one that’s 3/4" thick and made out of steel (density = 0.282 pounds per cubic inch); the weight per unit area then is 0.2115 pounds per square inch. That’s the difference in pressure between top and bottom that we need in order to lift the cover. In other words, in Bernoulli’s equation above, P1 = P2+0.2115.

And now, because I hate standard units, I’m going to SI: 0.2115 psi = 1458 Pa.

So:

P1 = 101,325 Pa

P2 = P1 - 1458 = 99,867 Pa

rho = 1.2 kg/m[sup]3[/sup] ( I will assume incompressible flow, so rho is the same on both sides of the cover)

V1 = 0

The equation:

101,325 = 1.2*V2[sup]2[/sup]/2 + 99,867

V2 = 24.6 m/s

Going back to standard, this is 55 MPH.

In other words, it seems plausible that high winds could dislodge a 3/4" thick steel manhole cover. Thicker covers would require higher winds.

This is the same principle by which EF5 tornadoes can strip pavement from the ground: the wind is driven into existing cracks in the pavement where it develops stagnation pressure beneath the slab. which (combined with the low pressure above the slab) is enough to crack and tear the pieces of pavement loose. Here’s a photo of damaged pavement in Joplin, MS.

Interestingly, they also have a picture of a coverless manhole, along with the claim that winds in excess of 200 MPH were required to remove the “well-sealed” cover. Maybe those covers were thicker than 3/4"? Why would they calculate such a different number than I did?

Anecdotal only, but we get 50 mph winds fairly often in the spring and fall. It’s also not unusual to get gusts in the 70+ mph range. We have no issues with manhole covers popping up.

Wow! Now I am going to have to tell my wife I should not have been so dismissive of her theory. Huh.

No! Don’t do this! You’re opening yourself up to all manner of future grief and second guessing! Danger, danger, Will Robinson!

Ha, I know what you mean.

Edited because we are not allowed to type in all caps, apparently.

Because over time, the gap between the cover and frame becomes full of compacted crud, making the cover much harder to lift out.
I’ve lifted loads of manhole covers, using all kind of contraptions designed to make it easier, but the vast majority require some attention with a bolster chisel and lump hammer to free them up first.

Fun fact:

In Formula 1 races that take place on public roads, like Monaco and Monte Carlo, they weld the manhole covers shut before the race.

F1 cars are designed to use ground effects to improve handling. This causes a strong suction between the car bottom and the road and in the past has resulted in manhole covers getting lifted and causing an accident.

Dont’t tornadoes have a lot of updraft at the core? Could the Joplin manhole, which appears to be not only lift ted but gone, have been passed directly over by a tornado, rather than just subjected to horizontal winds? It’s not clear (or maybe I missed it) if the OP manhole was similarly gone or just ajar.

My understanding of physics is, shall we say, unfettered by a grasp of the mathematics, but it seems that strong horizontal winds could significantly lower the pressure above the manhole (as others have said) that would then suck air up through any holes in that manhole cover, which would create some lift. There’d probably need to be a fairly narrow range of the ratios of solid surface to holes to wind speed in order to create the right combo of low pressure above and updraft from below to lift the cover. Then once it was ajar, that ratio would be altered and other factors would come into play.

Admittedly a somewhat semi WAG, and I defer to people who actually know what they are talking about.

Just in case it gets lost in the frenzy; all these calculations are almost certainly theoretical, in that, as lieu said, water backing up (either from floodwater in the storm sewer, or floodwater leaking into the sanitary sewer, depending on what the manhole is for) is by far the most likely cause of the OP’s dislodging. So **SlackerInc’**s wife is wrong if she’s asserting the wind “must” have done it.

I have seen manhole covers spinning on the top of a column of water from a storm drain that was over full.
Never seen on lifted by wind.

Just ajar. Probably still covering around 60% of the manhole and tilted.