OT: SS ManholeCover

The density of water doesn't much matter if the water is pushing hard enough. Gasses can blow the cover 20 feet in the air if they are from an explosion.

The holes are small to nonexistant to limit the ingress of stormwater. If enough stormwater enters the sewer system, the treatment plant can't process it all and has to discharge raw sewage. Not a good alternative.

Erm... First concept: Manhole covers are rarely, if ever, "moored". They stay put in normal circumstances because they're heavy, and resting against a lip that's set in such a manner that gravity is constantly pulling the cover into place against it.

Second concept: How hard is it to figure out that Mr. Manhole cover acts in EXACTLY the same manner as the sail on a sailboat, or a 4x8 sheet of plywood you're trying to carry in a strong wind? Water is flowing. If enough of it is flowing to push against the cover with enough force to overcome gravity's influence on it, said cover is, unless it's bolted down (which is rarely the case except in certain high-security locations), going to move, and there ain't a thing to be done about it. WEll, OK, that's not true... You might try parking your car with a wheel on the cover if it worries you that much. But if the water flow is strong enough, you're going to come back to find your car flipped over and the manhole cover gone anyway...

Not much of a mystery...

Most of the manhole covers around here are secured firmly by a coat of asphalt that the city paved over them with.

There is usually a crew that follows the paving crew that raises the manholes. Manholes are there for a purpose, to gain access to the sewer.

It would be easy to rig up two or three catches on each manhole cover that could be rotated via a wrench from above. These would keep the manhole cover in place, but would also keep it from rising to vent pressure in the event of an explosion. I suspect that they are designed the way they are for a reason

Since I am in the communications biz, I can tell you confidently (and thankfully) that not all manholes you see in the street lead to sewers. Some of my manholes (ex-Western Union) are older than myself.

That head you see poke up in the middle of US 1 in South Florida could well be mine. Hopefully always in a barracaded traffic lane.

Vaughn

But now we have the internet.

If the water pressure is sufficient to lift the manhole cover, but someone has locked it down, it simply means the sewer will back up at a higher point and flood a different street.

In that case if the cover averages 1" thick it would only take 6.8" of head for the water to lift it. When a major sewer line stops up there is usually a lot more head than that behind it. If it stops up quickly the enertia of all that water will blow every cover on the line sky high. If the manholes were completely sealed the pressure would blow out the pipe.

That happened here in town. A fabric dye plant flushed some tanks. They started pouring 10,000 gpm through a 36" sanitary line to the treatment plant. They had prearanged with the water department but somebody forgot to tell the computer. When the additional load hit the plant the automated system shut down the line to prevent an overflow. Blew cover for 2 miles but the covers closest to the plant were sealed shut. The W&S department had to dig up half a block of pipe and replace it..

Really ? Here in the UK it's pretty rare to place non-sewer "manholes" in the roadway itself. We stick them in the pavements (sidewalks) instead. The sewers are only still there because that's where the Victorians put them.

Even "manholes" aren't that common for our comms gear either. Most of ours are in shallow jointing chambers where the whole lid comes off for access and you can work comfortably seated on the ground. I've been in a few large underground "rooms" accessed through a manhole and shaft, but these were largely old installations in old cities.

Or bathtub.

I have been told, in casual conversation with a city engineer, that the manhole covers act as safety values to protect the pipes and other parts of the system from damage in the event of heavy runoff.

Plausible, anyway.

--RC If I weren't interested in gardening and Ireland, I'd automatically killfile any messages mentioning 'bush' or 'Kerry'

There's an old engineering tale, that may have some basis in fact, that in the early history of the US Air Force Academy the students in mechanical engineering were asked to model the sewer system. One enterprising soul (or group, depends on version) decided to see what would happen if all the toilets on campus were flushed in sync. The model predicted a 12 foot jump of a manhole cover at the confluence in the center of campus. The prof was skeptical so, in true military academy manner, the students set up a test and invited the prof to witness it. The cover only jumped about 4.5 feet up (probably some turbulent losses) but the student got full marks.

Jim

There was a similar legend at the Citadel. Supposedly all the pleabes turned on the faucets in the barraks and on signal turned them all off. The water hammer effect split pipes all over the building.

Chuckle!!!

Same goes for New Mexico Militry Institute in the early 60's.

However the student brass kaboshed it every time we tried to pull it off. The place is many, many decades old going back to the 30's I believe..... all hell would break loose if it were accomplished today.....

B.

.......

Thanks, everyone for your thoughts on this.

--Winston

If it counts, we had a very heavy rainstorm a couple of months back. My drive slopes towards my garage and has a storm drain manhole at the top of it. I saw that the concrete cover was lifted three inches up and water was gushing out. I went out in the rain and stood on the cover, it made no perceptible difference so I parked the front wheel of my 2 1/2 long ton Transit van on the cover. That slowed the water down! Unfortunately the garage had got 2" of water in it by then.

Mark Rand RTFM

On Thu, 21 Oct 2004 23:35:17 +0100, Mark Rand calmly ranted:

Now you know why most people won't consider purchasing a house on the downhill side of a street.

----- = The wealth of reality, cannot be seen from your locality. =