Hydraulic Elements Table 2

[fatlouis]

I'm trying to put in the numbers for the hydraulic elements table for a circular section into excel so I can use the look up function when doing my pipe sizing. I'm not sure what formulas to use so I can come up with the percentages for V/Vfull and Q/Qfull based on y/D.

 

[DPAJR]

You need to get a copy of the "Civil Engineering Reference Manual". The one that is specifically published for the CE license exam. Most university book stores should have one. I would send you the equations but I am on the wrong computer right now.

Just sending you the equations would not be that helpful though because there is a chart at the end of the chapter that you will also need.

I scanned the chart and then loaded it into Autocad to make locating points on the curves easier. You have to take points off two curves on the the chart and go back to the equations. There are four curves on the chart. One set of two curves assumes Mannings number changes with depth and the other set of two assumes Mannings number does not change with depth. You have to be consistent in using one pair or the other. I use the changing Manning's number assumption.

Apparently the curves are developed from field testing because I have not found any equations to duplicate the curves. I always prefer formulas to curves whenever possible.

Good luck

 

[fatlouis]

Thanks DPAJR

 

[beej67]

Unless I'm totally misreading the question, which I may be, all those curves are developed based on the manning's equation. Depending on which curve you select to use, the curve either assumes a constant n with depth or takes into account slight variations in n with depth based on experimental data. The reason you'd like to use curves for mannings is it's not directly solvable for certain variables that appear on both sides of the equation.



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[TerryScan]

fatlouis,

If you are looking for an accurate representation using Manning's equation, one cannot just use a ratio relating y/D to Q/Qfull in a circular section. This is most evident when observing that maximum flow in a circular section occurs when the depth is less than full flow.

One place I found equations online for open channel flow in a circular section is here:

http://www.lmnoeng.com/CircularCulvert.htm

 

[fatlouis]

TerryScan

Perfect, thanks for the link. Let me bounce this off you folk and tell me if I'm on the correct path. In the equation Q/Qfull, the only varibale that is changing with y is theta (the angle), so in my spreadsheet I would have to set up a column to solve for theta for any given depth y. That will allow me to solve the equation of Q/Qfull and with that information I should be able to plot y/D vs. Q/Qfull? Does that sounds correct?

 

[TerryScan]

fatlouis,

Here is a rather raw spreadsheet with open channel flow calcs for a circular pipe.

 

[gbam]

fatlouis - have you considered coding Visual Basic within Excel? Take the equations that Terryscan provided and code a loop to compute flow depths.

 

[fatlouis]

I wouldn't even know where to begin when it comes to VB. Thanks for the suggestion though.

 

[beej67]

***
If you are looking for an accurate representation using Manning's equation, one cannot just use a ratio relating y/D to Q/Qfull in a circular section. This is most evident when observing that maximum flow in a circular section occurs when the depth is less than full flow.
***

Again, unless I'm totally misunderstanding the discussion here...

...As I understand it, that little back-hook once you're past about 93% full isn't evidence against Manning's equation, it's a direct result *OF* Manning's equation, because that's the point where the additional conveyance you gain from filling the pipe up doesn't help the flow as much as the additional surface friction hinders the flow. It's a sort of saddle point that shows up in the equation itself.

I attached a couple pages from my open channel text, for clarity. It's got a nondimensionalized graph that shows the back-hook, which also shows up in the graphs the top poster is talking about.

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[SMIAH]

Might be out of topic but i'm wondering if you guys design with variable n ?

 

[SMIAH]

We design at +/- 80% full (y/D).

 

[TerryScan]

beej67-

Yes, the changes of flow related to depth in a circular section ARE a result of Manning's equation, which is not a direct y/D Q/Qfull relationship. I was trying to explain that I would ONLY use the equation for every depth in calculations vs. interpolating flows relating depth to full flow.

SMIAH-variable n values
This definately sounds like a new topic. Start a new thread if you like. The only time I have used variable n is a) when the surfaces vary (natural channels & floodplains etc.) b) vegetated channels where low flow analysis is more critical such as biofiltration.

 

[beej67]

I've never used a variable n unless it was built into software behind the scenes. My opinion is that a simple approach with adequate and informed safety factors is generally better than a complicated approach without them.


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

http://www.campbellcivil.com

 

[beej67]

SMIAH -

In this area of the country, we design sanitary sewer for 80% y/D, but storm drain design tends to vary widely by municipality. Many of the less advanced southern states still just go by showing your open channel flow capacity of your storm drain is higher than the rational flow in the pipe. Most Georgia municipalities have shifted to an HGL approach, where they want you to show your HGL isn't out of the pipe, which means you have to take extra time and explain to the reviewer what an HGL actually *is* if you've got a tailwater situation. Most coastal areas fall in line with Florida, which generally doesn't care about full flow depth at all, they just want to know how badly their sites are flooded. From about Tampa south, nobody even lays pipes with positive slope, because everything is pressure driven anyway, and they're almost all under the water table to begin with.

Florida's a strange bird. They don't design new development for 100 year flood conditions in south Florida, because they figure in the 100 year flood they've all evacuated anyway.

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

http://www.campbellcivil.com