MATH HELP: Culvert pipe capacity in gpm | Construction and DIY projects | Forums

Making progress. I found...

https://www.concrete-pipe.org/pdfdd/DD_11.pdf

 

Hydraulic Capacity of Culverts

 

The most widely accepted formula for evaluating the hydraulic capacity of non-pressure sewers is the Manning Formula. This formula is:
Q = 1.486/n x A x R^2/3 x S^1/2

 

Where:
Q = discharge in cubic feet per second
n = Manning’s roughness coefficient
A = cross-sectional area of flow in square feet
R = hydraulic radius in feet (equals the area of the flow divided by the wetted perimeter)
S = slope of pipe line in feet of vertical drop per foot of horizontal distance

So labeled as "Q" or as "C₁", that tells me the resulting figures in the table below are in cubic feet per second...

 From there I was able to use this online calculator to convert everything into gallons per minute...

Still not sure how to interpret "S = slope of pipe line in feet of vertical drop per foot of horizontal distance" and these numbers are way different than those in the first table of the first post which puzzles me. (Note: I believe these numbers are for full pipe capacity while the first table's numbers are for half capacity. That said, its still an enormous discrepancy.)

I found these too...

Manning's Formula for Gravity Flow

Manning's Roughness Coefficient

I also need a more reliable number for the average gpm of the stream where the driveway will cross. More and more I'm thinking 2324 gpm is inaccurate and way too low. I also need to take measurements of the potential volume of water at their highest and to get a much better idea of the natural slope leading up to where the driveway will intersect.

You forgot to adjust the roughness coefficient from 140 to 100. That brings it down to 27119 gpm. Also, that Hazen-Williams calculator will output directly to gpm as an option in the drop down menu.

I've since done more reading and had not yet posted an update, but browsed several more formulas and read more about each. While the Manning formula is said in that pdf to be the industry standard, and that is backed up in that it is what was provided with the actual spec sheet from the manufacturer, it is a formula based on an open channel. So it is not a pipe calculation yet it is the pipe industry standard. The Hazen-Williams formula is based on pipe. That does suggest that it could be the better formula to go by.

Unfortunately, I've not been able to determine at what percentage of capacity either formula calculates for. While I have yet to grasp how it differs, reading I've done suggests that the water and flow will behave differently in a full pipe then a partial pipe. Pipe can be partial and full at the same time as well. There is a lot of dynamic and seemingly chaotic variation discussed by the different formulas, and in my own perception as I delve further into the topic.

I noticed another contradiction which I have not yet seen discussed which is that steeper pipe drop appears to consistently described as increasing velocity. But at say 70% capacity, if the pipe angle is steep enough for the bottom lip of the opening to be as high or higher than the top lip of the exit, it adds turbulence which subsequently slows the velocity.

View post on imgur.com

Confusing? I've tried to state all this with clear language, but its all rather convoluted.

Concerning the bridge, I just don't see how I could possibly afford that. I can do the 42" arch culvert for about $2000. Then there is the cost of the hired machinery, gravel, and possibly rip-rap and/or fill. That's probably another $2000 - $5000. Hopefully the fill can come from my property -- ideally the pond hole.

I imagine a bridge could cost $5000 - $7000 on its own. If I am to have a mobile home trucked in, whatever driveway I build, and whatever method of crossing the stream, it must be sturdy enough, and broad enough for that.

A bridge was what I had originally had in mind before deciding on a culvert and filling to elevation, but after I had priced the concrete and other costs, I began searching for a less expensive option.

Here is the video of a stone arch bridge that I had wanted to do for myself. The file appears to have gotten corrupted since I last watched it, but it is still watchable...

As for timing the stream, I don't think I can use the weighted float method. There are too many obstructions and shallow spots to get a good run. I think there is too much flow to use the bucket method. That leaves the weir method and meter method.