Surcharge and Mannings
Surcharge and Mannings
(OP)
I was hoping someone could confirm my method for capacity calculation under surcharge conditions.
First, when calculating the full or half gravity flow in a sewer pipe, I would use Manning's equation, simple enough. The slope is the physical slope of the sewer pipe.
Now, if we have a surcharge condition (headwater in the manhole), the downstream pipe now becomes a pressure pipe. (Also assuming no tailwater with free outflow, not culvert condition).
Would I calculate the pipe capacity based on the slope created by the surcharge headwater. In other words, the headwater elevation minus discharge elevation, divided by length.
That would give the hydraulic slope, and I would substitute that hydraulic slope into Mannings for the capacity under surcharge.
Does this seem correct to you?
(Also ignoring entrance losses)
First, when calculating the full or half gravity flow in a sewer pipe, I would use Manning's equation, simple enough. The slope is the physical slope of the sewer pipe.
Now, if we have a surcharge condition (headwater in the manhole), the downstream pipe now becomes a pressure pipe. (Also assuming no tailwater with free outflow, not culvert condition).
Would I calculate the pipe capacity based on the slope created by the surcharge headwater. In other words, the headwater elevation minus discharge elevation, divided by length.
That would give the hydraulic slope, and I would substitute that hydraulic slope into Mannings for the capacity under surcharge.
Does this seem correct to you?
(Also ignoring entrance losses)





RE: Surcharge and Mannings
"In other words, the headwater elevation minus discharge elevation, divided by length."
For the case you describe, you also need to take in to account any entrance and exit losses using the Bernoulli Eq. ( which is simply a special case of the Law of Comservation of Energy. )
good luck
RE: Surcharge and Mannings
r = A/P = Hydraulic radius (feet) – Note: for pipes full or near full, r = 4/Depth
http://w
RE: Surcharge and Mannings
http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06138/chapt2.cfm
RE: Surcharge and Mannings
However, it has been my experience that it's a rare occasion that an open system will be controlled by the surcharge pressure (since the pressure from a few feet of water is relatively small), it will likely be controlled by inlet or outlet conditions.
You've apparently eliminated outlet control as a possibility, but when you say "no tailwater with free outflow, no culvert condition" you've only eliminated one possible culvert condition, that is, outlet control. It is very likely that it's operating under inlet control. That is, the pipe could flow a heck of a lot more water, if you could get it in the pipe fast enough.
As RWF pointed out, Bernoulli is the most extensive way to analyze the condition. A quick check would be to use the simple orifice equation and measure head from the surcharge elevation to the center of the pipe.