Pressurization of Building Storm Drain Piping.. Easy Question..

[ziptron]

Hello Everyone,

I am not an expert on this subject, just interested, and I have a question which I am not exactly sure how to Google. I wanted to throw out what I think should be an easy question to people in the field in hopes that I could get some information on where to look for additional learning materials.

I noticed in a building recently that the building has storm drains which carry rain water from the roof, into the city storm system. The drain pipes are all iron, and are connected with rubber MJ couplings. To me, this type of connection cannot not handle any pressure within the pipe, and normally it does not need to since normally drain pipes are not full of water. However, what happens in a larger rain storm situation where we have a lot of water entering the city storm system. I can imagine a situation where the storm pipes from a building will not be able to purge quickly enough into the city's storm system and the water level within the buildings storm drain will rise. If we begin to fill the storm pipe within a building at a higher rate then it can discharge into the city, the storm drain will begin to fill up.

If a storm drain is filled with water 4-5 stories high, would we not start to pressurize the storm drain pipe (simply due to water head)? I've yet to see a storm drain pipe that can withstand much (if any) pressure. Would such pressurization lead to drainage pipe failure and cause rain water to flow into the building?

Is my above scenario realistic? Do situations occur where storm pipes are completely filled up? What sort of solutions are buildings equipped with to prevent such failures?

Any information on the above would be greatly appreciated,

Thanks in advance!

 

[SMIAH]

The drain might be controlled by the entrance (I would think it will) as the capacity flowing full exceed the entrance capacity.
Might be site/condition specific.

 

[psmart]

Agree with SMIAH. And since the pipe inlet will generally be the control point, the only significant chance of pressurizing the inside pipes would be if the connection to the storm sewer is inadequate, or the storm sewer itself is over-capacity. But an over-capacity storm sewer will generally overflow at a nearby grate, thus relieving the pressure and preventing much of a water column from accumulating inside the roof drain system. So the primary concern would seem to be having an adequate connection to the storm sewer, ideally with an overflow provision to eliminate any chance of pressurizing the inside pipes.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

there is a remote possibility that the downspout gets plugged with sticks, leaves, dead animals or what have you, such that the capacity is less than the inlet. it would then pressurize and the rubber gaskets would probably hold 10 - 20 feet of head before they blow out. frequent maintenance would reduce that problem.

 

[beej67]

As luck has it, I was recently "encouraged" to do a 24 hour SCS hydrologic analysis of a 30 year old roof drain system to ensure that the 100 year storm does in fact discharge through the drain system, instead of overtopping it's overflow scuppers and bypassing my new detention vault. I found that the runoff from the roof was indeed controlled by the roof drains themselves to a degree, and the resultant flow through the building system was noticeably less than it would have been using a simple 100 year rational analysis assuming uncontrolled runoff and a short (5 minute) Tc. For the record, I calculated about 3 inches of ponding for the 100 year 24 hour (Georgia) storm.

I'd have to go back and re-analyze to check for sure, but my gut feeling is that the 100 year peak discharge was controlled down to around 10 year levels by the roof, which I think is pretty close to building code anyway. I'm not an MEP, can't speak to those specifics. So my gut tells me that the building codes, which point to a lower design storm, in fact handle the 100 year storm quite nicely given the entrance restrictions commonly seen on buildings.

That makes a kind of sense, as you'd think if pressure flow for event storms was a problem, that the MEP codes would have adjusted to match. Seems to me that the code writers got this right, if not exactly for the reasons they thought they did.

Keep in mind, the design storm intensity for these systems is based on a very low Tc, so even if it did leak it wouldn't be leaking for long.


Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com