Water volume from a manhole

[Boomer343]

Had a major rain and hail storm hit our area a couple of days ago and cause some flooding. I'm trying to find some information on how much water flows when the man hole cover is blown off and the geyser is 4 feet or more in the air.

In our subdivision there was a large church parcel set aside for two churches. The two congregations built one church then were supposed to build the second but instead decided thast one church would do and that selling the land for housing would be a good idea. The tie in to the storm drain from this new area is where the man hole is and there wasn't one there before.

I've tried a few different searches but haven't found what I'm looking for so any help appreciated.

We actually got off lucky but this has happened 4 or 5 times since the new area was added and twice in the last 12 months so I need to make some noise about it.

[VirginiaJim]

Well, a typical manhole weighs 110 pounds from my search on the net.  You'd have to do the math to see how much force is required to lift it 4 ft based on the diameter of the hole, volume of the water column, etc..  I didn't do that well in physics so I can't help you with that.

[Mister Tee]

Actually once the manhole cover is off, you would calculate the velocity based on the height of the water column and the flow based on that velocity and the diameter of the manhole (typically 24" or 36" for a storm drain.)

I found a list of tabulated velocity heads (http://www.engineeringtoolbox.com/static-pressure-head-d_610.html) here which would suggest a water column of 4 feet would be a velocity head of about 16 feet per second.  Once it's clear of the confines of the manhole, it's mostly velocity head carrying the water but there will be some pressure head element as well.  It would be fairly difficult to calculate how much pressure head to subtract in an open water column.

Assuming you have a 24" manhole, the cross sectional area is 3.14 square feet (I can do that math in my head, a = pi R^2) so the flow is then 3.14*16 =  49.6 say 50 cubic feet per second assuming it's all velocity head.  That works out to 22,400 gallons per minute.

That's a lot of water and it's probably not nearly that much.  If you could manage to stick a pitot gauge calibrated for water velocity at the top of the manhole, you could measure the actual velocity and then calculate the actual flow.

[Boomer343]

Thanks Mister Tee, I have pictures from a previous storm where the geyser was well over 6 feet in height so that amount of water may not be too far off. I was too busy trying to keep the house from flooding to see how high it got this time but the amount of water moving past and around our house on this storm was greater than it ever was before by a considerable amount.

[Mister Tee]

No problem.  In talking to a couple of my collegues, that calc is actually reasonably accurate.  Regardless, someone really screwed the pooch in designing the intertie to the storm drain if this is a new condition, either by a miscalculation of the drainage basin or underestimation of the downstream hydraulics.  Surcharging a gravity pipeline is a very damaging condition not only to the areas being flooded but to the pipeline itself.

It almost sounds as if someone routed a drainage channel through the storm drain, instead of just collecting the surface drainage of a single parcel.