Storm Pipe Capacity Under Surcharge

[GoldDredger]

We have a new development needing to connect into an exiting 18" RCP pipe. The Manning's gravity capacity for this pipe at 3.03%, n=0.013 is equal to 18.3 cfs. There is no tail water.

The inflow point of the pipe is a curb inlet. There is the ability to surcharge the pipe, by assuming the inlet WSE to be about 2-ft above the crown of the pipe.

This creates a hydraulic grade line slope greater than the frictional (gravity) slope of the pipe. I am simply calculating the WSE at entrance, and assuming the crown of pipe as outfall, over the distance. This calculates a hydraulic grade line slope of say, 4.00% now.

Without going into very detailed hydraulic analysis, and losses under pressure conditions, could we just use the 4.00% slope in Manning's to get a ballpark capacity of the pipe under pressure conditions.

Placing the new 4.00% slope into Manning's yields a capacity of 21 cfs.

Does this seem like a reasonable means to roughly estimating the actual capacity of a pipe under moderate pressure conditions?

 

[jgailla]

In general, you should use the hydraulic grade line of the system, not the slope of the pipe.
Using the crown of the pipe for your HGL slope in the pipe should be conservative if there is no tailwater.
However, you will have head loss at the inlet. There will also be turbulence diminishing the flow unless the everything is surcharged over the inlet.
I would be careful using Manning's if you have a drop inlet connected to a short length of pipe. It will be controlled by orifice flow.

 

[gbam]

Yes, it is possible to have an EGL slope greater than the pipe slope; hence the term surcharged. Remember that when you surcharge a system your friction loss increases as a result of increased velocity. If there are any angle points/bends there will be higher losses with higher velocities. Of course, you will need to evaluate the outlet for scour. I recommend that you download the FHWA, Urban Drainage Design Manual (HEC22). Here is a link:

http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=22

To answer your question; yes, it is reasonable to use an approximate slope of your HGL; however, you need to confirm flow regime. If your pipe is supercritical with no tailwater, no losses are carried upstream and your system will be based on inlet control. Here is a link to FHWA's Culvert analysis manual.

http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=7

 

[civilman72]

First two responders nailed it...

I'm certainly no H/H expert, but it's amazing to me how many experienced engineers use Mannings equation to calculate pipe capacity for storm sewer and culverts, even under obvious inlet control situations.

 

[SMIAH]

Manning's Equation = non-pressure flow in pipes.

You could use Hazen-Williams, Darcy-Weibach or Manning for a pressured pipe.

Depending on the equation, you'll need the rugosity coefficient, the singular losses, the diameter, the length and the difference between Upstream and Downstream Elevation.

 

[cvg]

the fact is that none of these three equations [Hazen-Williams, Darcy-Weibach or Manning] will correctly determine the amount of flow through a steep drain with inlet control. they are generally only good at steady state flow through conduits, not a transition between open channel flow to pipe flow.

 

[fel3]

Manning's Equation can also be used for pressurized flow. It is algebraically similar to Hazen-Williams and can be applied in a similar fashion. I have used Manning's many times for low head applications in large diameter concrete pipes (e.g. storm drainage and irrigation).

In my binder of random hydraulic information is a graph from a text book where the author plotted some Hazen-Williams curves (actually lines) and Manning's curves (also lines) on the Moody Diagram. The Hazen-Williams lines are sloped and best matched the parts of the Moody Diagram paralleling the smooth pipe curve, while the Manning's lines are horizontal and best matched the upper right part of the Moody Diagram, which is where large diameter concrete pipe lives. Unfortunately, I only have the one page with the graph and part of his example calculations. I do not have the title of the book or the author. An experienced engineer gave this to me about 30 years ago to help me understand why we use Hazen-Williams for water systems and not Manning's.

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"Is it the only lesson of history that mankind is unteachable?"
--Winston S. Churchill

 

[SMIAH]

The general pipe flow equation is derived from the Bernoulli’s equation and continuity principles.