Storm Water Culvert Design

[cdneng]

Hello,

I would appreciate some help with a storm water pipe design question:

Given peak flow at 80% capacity, maximum and minimum flow velocities, manning number, slope, how would you calculate the required pipe diameter? I have tried using variations of the manning equation, but can not seem to find the right format?

Regards,

 

[cvg]

try HY8 or the FHWA nomographs

 

[dicksewerrat]

divide peak Q by max vel. Cubic fett divided by feet equals square feet.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[cvg]

for full flow that will work, for a steeper culvert and given your peak flow is only 80% of capacity, the culvert will not flow full and Q/V will not work. tailwater / outlet channel can also control the amount of flow going through the culvert

http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=7&id=13

 

[gbam]

Check out a partial flow nomograph. One can also use direct step method if there is a tailwater influence. As cvg pointed out HY8 will provide you with hydraulic results. look at the profile to see the direct step results.

 

[dicksewerrat]

Maybe you have to do some calcs to find your area and hyd. radius for 80%. Ask your prof. about it.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[beej67]

Show of hands among older engineers: Who else has got "the wheel" for a quick answer to questions like this?

Very Long Answer:

Use the basic form and do trial and error. There is no 'direct solution.'

The only real trick you need to be aware of, when doing this for pipes, is you need to realize that a circular pipe flows at its maximum capacity when it's 93.8% full, in terms of depth of flow. Any deeper, and the extra cross sectional area you get is overcompensated by the extra friction from the pipe wall, and your flow actually goes down a little.

So set yourself up a spreadsheet that determines the cross sectional area and wetted perimeter of a pipe that's flowing at depth = 0.938 * pipe diameter. That takes some trig. Then use Manning's to determine the average velocity given a roughness and a slope. Then multiply that velocity times the cross sectional area determined earlier and you have a capacity flow for that pipe at that slope. Then trial and error until you find the pipe size you need. Depending on how you set your spreadsheet up, you might be able to land on your answer by using the "Goalseek" function, under data analysis. If you're sharp, you should also allow yourself to vary the depth of flow to something other than 93.8% full, so you can test other scenarios.

Since you appear to be new to this, keep in mind that pipes in the United States come in sizes that go by 3 inch increments in interior diameter, up to a point, then 6 inch increments. Check a manufacturer's website. If you present a solution that says you need a 22.4 inch diameter pipe, you won't have a pleasant remainder of your workday.

Also, please do be aware that Manning's equation is only for normal flow, which means you take an infinitely long pipe and measure the depth at the midpoint where entrance and exit losses don't impact the depth. In the really real world, entrance and exit losses do have an effect, and the water surface elevation changes throughout the pipe depending on a lot of weird factors.

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

http://www.campbellcivil.com

 

[beej67]

Worth mentioning, I suppose -

The reason you can't solve for the pipe diameter directly is because it shows up three times in the equation. Say you replace V with (Q/A) on the left, so you're solving for Q directly. Then you plug (A/P) in for Rh on the right hand side. You can't solve for A because it's nonlinear. Making matters worse, in a pipe A and P are related as well, both to pipe diameter (D), and to depth of flow (d). You could solve for both of those using trig, and plug them in to A and P respectively, and what you get is a giant mess of cosines and other trigonometric hooplah, that can't be solved directly. And if the only way to solve it is trial and error anyway, then you might as well not go through the algebraic headache and keep your spreadsheet simple.

As others have mentioned, there are nomographs for these things. The simple solution is go pull up your big blue Civil Engineering Reference Manual you bought for the PE exam, and look up the table that has pipe sizes and roughnesses by slope and capacity, and pick one off the chart.



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

http://www.campbellcivil.com

 

[gbam]

beej67 - I never had a wheel, but, I did have a slide nomograph from a pipe manufacturer, got 15+ years ago. Since I have written visual basic in excel to compute the partial flow depth of a conduit.

cdneng - take a look at a hydraulics text book for the appropriate equations to compute area and perimeter for a partially full pipe. Search these forums as there have been many discussions on partial flow in pipes.

 

[dicksewerrat]

Mix a little trig and geometry together and get the formula for the area of partial circle and wetted perimeter. High school math problem.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com