# Jumping sewer lid - WHY?

Intro: Few hours ago, there was a storm. We heard some constant banging which couldn't be explained by thundering. Then we found out, it was a sewer lid jumping. Maybe it's normal in other parts of the world, but for me it was like the first time in my life.

I've captured the video.

The question is what was causing this to happen. It's kind of clear that it was air pressure so strong that it was capable of lifting this metal lid. But where did this air pressure appear?

• Wind blowing into the sewer? If so, would it be so strong to lift the lid? And shouldn't be the sewers protected against wind somehow?
• Water filling the sewer so quickly, that it made the air pressure this strong? Would this really be the easiest way for the air to leave the sewer?
• Something else (such as bored sewer worked :) )?
• I'd bet to the 2nd. Some pipes that would normally drain the water from the sewers would be totally blocked by the water, maybe even reversed, so the sewer would be effectively sealed everywhere except in the manholes. – rodrigo Jul 22 '14 at 15:37
• It happens in at least one other place. Correction: two other places. What would we do without YouTube? – John Rennie Jul 22 '14 at 16:57
• Consider yourself lucky youtube.com/watch?v=4BkFp5CL4q0 – DJohnM Jul 22 '14 at 17:53
• Thanks for those other videos. I looked for them too, but since I didn't know how the "manhole cover" thing is called, I didn't find anything. – Erlik Jul 22 '14 at 20:12
• wide water entrance + tight pipes inside... – Andrestand Sep 14 '14 at 11:08

## 7 Answers

The jumping manhole lid caused by the Bernoulli`s principle sounds quite reasonable at the first glance. However, I have seen a few storms in my life, but no jumping manhole covers that I can recall. Also, I noticed, that in the video the jumping does not match the wind gusts (observe that small tree).

In the absence of trolling sewer workers, the lid seems to be lifted exclusively by air pressure. A typical manhole cover is rather heavy (50kg), and this particular cover seems to be around 80 cm in diameter. Lifting such a cover would require at least $\Delta{p}=995$ Pa pressure difference between sidewalk level and the sewer. To cause the lid to jump, much grater pressure difference would be required. The simplest form of the Bernoulli's equation claims that wind velocity above the manhole is proportional to square root of the pressure difference, $$v^2=\frac{2 \Delta{p}}{\rho_{air}}$$ With the density of air being $1.2 kg/m^3$ this gives $v\approx 41 m/s$. The storm in the video does not really resemble a terrible hurricane, therefore I would suspect that the jumping is caused by air pressure produced inside the sewer system. Probably a sewage pump is causing all the noise. Although this would only be the case, if the pump was really powerful and the pipes were clogged. If the pipes were clean, the excess air would normally escape through them and would not bother lifting the manhole cover. And combination of sewage pump and too narrow pipes by design would probably cause flooding every time there was a decent rain.

• I am not convinced by any of the answers, but author of this one at least provided reasoning and backed it up by calculation (instead just stating that it's been well researched). – Erlik Sep 19 '14 at 8:01

This phenomenon has been well-researched. If the underground pipe is almost full with rapidly-flowing water the waves in the pipe will cause the pipe to be completely filled in some places. The waves are irregular and the air in between the wave crests gets compressed - when it passes the manhole access the air is vented. 1000Pa is actually a very small amount of pressure - about 0.5% of what is in a car tire, and can easily be created by irregular wave functions in a pipe.

Increase the pressure a bit and even large manhole lids can rocket out of their housing and become a spectacularly dangerous projectile. In areas where this phenomenon is likely you will find the covers bolted down. Not locked, just some easily-removed large bolts to keep it where it should be.

• Could you add some link or reference to the related research? – Erlik Sep 16 '14 at 8:36

I will vote for

Water filling the sewer so quickly, that it made the air pressure this strong? Would this really be the easiest way for the air to leave the sewer?

In the two videos John Rennie linked to it is evident that the air is coming out with pressure aerating the water which is high around the manhole. In your video not enough water is seen around the manhole.

Manholes are usually over wells where several sewer lines join to go down a common exit line. With a severe enough storm all entry routes to the well may be full so they act like pistons, pushing the air ahead of them raising its pressure and part of it is enough to raise the lid, release a bit of pressure and raise it again. If the storm were severe enough filling up the well, the cover would have been lifted by the water. This shows how high the water pressure can get, depending on the design,

The answers in the comments may be possible, but unlikely. Air would be able to escape through other places such as storm drains/gutters. Its seems more likely to me that the lid is not being pushed up by high pressure underneath so much as it is being pulled by low pressure above the hole. It has to do with something called Bernoulli's Principle.

You can read the article to get the full idea of the physics behind this, but the basic idea is that a moving fluid has a lower pressure than a fluid with the same properties and conditions except for being at rest. We use Bernoulli's principle all the time in everyday life without thinking about how it works. For instance, an airplane's wing is designed with a specific shape so that air moving over the top of it moves faster than the air below it, causing a pressure differential that creates lift on the wings.

Thinking about it this way, it makes sense that during a storm, wind blowing over the top of a manhole cover could cause a low pressure area, while the still air underneath the cover would still be at normal pressure. With fast enough winds, it could easily create enough of a pressure differential to lift the cover, making it "jump". Of course, as soon as the lid is lifted, air rushes out and the pressure equalizes, so the lid goes back down.

• Bernoulli might be at work here, but not with respect to airplanes. – user10851 Jul 22 '14 at 18:30
• Interesting, I'd always been taught it was. Thanks! – Alecg_O Jul 22 '14 at 18:34
• In that case, a more accurate example would be the venturi in your car's carburetor – Alecg_O Jul 22 '14 at 18:41
• Having lifted a manhole cover I'd be surprised if anything short of a hurricane could budge it. They must weigh 50kg. – John Rennie Jul 22 '14 at 19:08
• I don't know about the OP's video, but in the two videos posted by @JohnRennie in the comments, there is no appreciable wind at all... This is interesting! Reminds me of a movie I once saw :) – bitsmack Jul 22 '14 at 19:20

Well this happens mainly due to two factors-

1. The pressure difference inside the drain and outside. there is high pressure inside the drain which is balanced by lifting the lid.

2. The excessive increase in water level in the drain. These two factors result in the water gushing out after the air from the air gaps come out.

3. This may also happen due to difference of temperature.

The jump is most likely due to several factors. The main one would probably be similar to what caused the storm in the first place, a high pressure low pressure gradient.

Lifting such a cover would require at least Δp=995 Pa pressure difference between sidewalk level and the sewer.

Someone already derived 995 pascals would be enough to lift a manhole of 50kg and 80cm diameter. Relying on Bernoulli alone at atmospheric pressure would require 41m/s winds.

We also need to account for the fact that the storm reduces the pressure above the manhole.

Air is compressible and water is not at the pressures we're talking about. As the air gets pushed around through the pipes, it is compressed a bit. It will try to escape when it reaches an opening, which could be a manhole or grate. If the manhole is above a junction, then there are also eddies which further change the pressure gradient. Even without a junction, turbulent flow would create eddies. Normally there is lower pressure above an eddy. The eddy below the manhole is in a closed system so it would just allow buildup of more air below the manhole.

All of this could result in a larger pressure than normal atmospheric pressure under the manhole. Storm drains are generally not connected to the sewer system so during heavy rainfall, the inlets are inundated, trapping even more air with the water. Compressed air tries to expand when the manhole finally lifts, resulting in the spray of water.

If the air is really trapped and being compressed, someone can test this by just standing on top of the manhole for a bit and see if it lifts higher or sprays more water. Although that is probably very dangerous...

It could be due to similar principles behind a hydraulic ram, where water is taken at high flowrate, low head and ejected at lower flowrate and higher head.

There needs to be a kink upstream to trap air and form a closed pressure vessel, which is possible as water level rises during heavy rainfalls.

Here the sewer cover would act as the relief labled no. 4.