Pond Boss Forums INTRO Questions & Observations Water flow through various culverts

I am trying to figure out sizing for culverts in a creek crossing. Currently we have four forty inch culverts in place normally its fine but it has gone over the top four or five times in the last ten years. This year twice. I am looking to put another row of culverts above the fortys. And am trying to figure out the difference in flow between various sizes

Have you talked to your counties NRCS office?

No I have not. Just trying to figure out the best size to use my choices are twelve, fifteen, eighteen and twenty-four inch. Just trying to figure out wether to go with bigger or more

For example does eighteen inch allow much more water than the fifteen inch

Pie are squared. Do the math to see what the difference is in interior area of each pipe. BUT that won't help you unless you know how many gallons of water you will see during a storm, and that's where the NRCS comes into play.

How much water you see is determined by the watershed, and what is or is not growing on that watershed.

THAT will help you determine the water velocity. You will need that number to calculate how much water a certain culvert size will handle.

https://calculator.academy/culvert-capacity-calculator/

Or you can just go to the local NRCS office for the answer.

Keep in mind that two bigger ones are way superior to 4 smaller ones, pipe volume quadruples every time the size doubles, inotherwords, it will take four 8" pipes to carry the amount of one 16" tube.
Its basic math from there, the 18" culvert will carry almost 1 1/2 times the amount of water the 15" one will, its kinda crazy how that works.

That being said, like Esshup said, until you get an engineer to do the math you are basically making a stab at it, so you want to be sure you oversize it, and I have had engineers be wrong too. the county here will calculate what size culvert you need to install when you obtain your permit. I have had a 4' dia CMP (per their recommendations) wash out and get washed a hundred ft down stream after an extremely heavy rain event immediately after installation.

Everyone above is giving you good advice.

Based on your culvert sizes, it looks like you may be buying them at a "big box" store. I was evaluating a similar purchase and decided to calculate Price per Square inch of culvert area. In my case, the largest culverts were clearly the best deal.

Further, the work to lay and pack one 24" culvert is much less than the work to do four 12" culverts.

Finally, hydraulic calculations are very difficult, even just to acquire the proper data. However, you can "poor boy" your calculations a little bit. If the last few floods were flowing over the road in a stretch 10' wide and 6" deep, then that is another 720 sq. inches of flow you need to bypass. (Plus some extra cushion since flow through pipes is a little more restrictive than open flow.)

Two 24" culverts (452 sq. in. each) would cover that estimate.

The final problem is that the largest water flow you have witnessed, is almost certainly NOT the highest possible water event for your crossing - so you still need a little extra cushion on top of your calculations.

P.S. There is a chance that the larger culverts will not "fit" under your current road bed. In that case, look at the elliptical or "pipe arch" shapes for your application. The pipe arch shape really lets through a lot of water while minimizing your height restrictions.

Like others have said, bigger is better, a single 24" will flow more water than two 12's. 12" = 113 sq. in, 24" = 452 sq in.

The county had that problem here on one of their roads. They dug the culverts out. formed a big rectangle out of concrete for a culvert and put the road back over the top of it.

Also, what the culvert is made from will influence flow rate too. Smooth walls on a culvert will flow more than a heavily ribbed wall culvert.

Thanks for the information I figured that was the case. I have plenty of room to go up it is a creek crossing I could go up eight feet and not have a problem