Culvert Design

[Daisy]

Problem 6: Transportation

A one-mile section of highway is to be constructed through a national forest. The highway will have a 24 ft paved surface, with 6 ft gravel shoulders. There is 66 ft of right-of-way, and a decrease in grade of 132 ft. If the runoff is 17 ft3/sec with a velocity of 8 ft/sec, what size storm sewer is needed?

18 in 20 in 22 in 24 in

What size of pipe would you choose? I'm curious, because the pipe size I would select if I was doing this for work is not the correct answer. Thank you!

 

[petobe]

Problem 6: Transportation A one-mile section of highway is to be constructed through a national forest. The highway will have a 24 ft paved surface, with 6 ft gravel shoulders. There is 66 ft of right-of-way, and a decrease in grade of 132 ft. If the runoff is 17 ft3/sec with a velocity of 8 ft/sec, what size storm sewer is needed?

18 in 20 in 22 in 24 in

What size of pipe would you choose? I'm curious, because the pipe size I would select if I was doing this for work is not the correct answer. Thank you!

20 in

 

[jeb6294]

Is that the whole question?

Based on the lack of information they provide, I'm not sure whether you are supposed to assume that you just need to calculate the pipe size for 17cfs in a culvert flowing full or if you are supposed to figure it out based on how much head you can have before the water gets over the road.

On the other hand they are giving you Q and V...so are you just supposed to fing the diameter pipe required to flow 17cfs at 8ft/sec? If that's the case, then it's as simple as plugging your Q and V into the ol' Q=VA, solving for A, and finding the diameter which in this case would be 20-in.

 

[Daisy]

That's the whole question, and it is 20 in, as solved from Q = V*A. I tried to 'cheat', I guess, and use my work nomographs, which for a culvert projecting from the fill, inlet controlled, I come up with a CMP=30" for 17 cfs. Guess I need to just look at what is given instead of using my 'cheat sheet', but that's the type of screw up I'm worred about on the exam.

 

[IlPadrino]

That's the whole question, and it is 20 in, as solved from Q = V*A. I tried to 'cheat', I guess, and use my work nomographs, which for a culvert projecting from the fill, inlet controlled, I come up with a CMP=30" for 17 cfs. Guess I need to just look at what is given instead of using my 'cheat sheet', but that's the type of screw up I'm worred about on the exam.

Yeah... well this isn't really a "culvert design" question - it's open channel flow.

Rule #1: Never make the problem harder than it needs to be!

 

[IlPadrino]

Oh... and not to be discouraging, but I sure hope you don't see a problem like this on the exam - it requires almost no engineering sense, and it certainly isn't the right way to "size" a storm drain.

 

[Daisy]

"Never make the problem harder than it needs to be"....this late in the studying game, my brain is mush and I seem to making lots of problems harder than they need to be!! I had to look up the equation for the area of a circle over the weekend. I can't wait for this to be over!

 

[IlPadrino]

"Never make the problem harder than it needs to be"....this late in the studying game, my brain is mush and I seem to making lots of problems harder than they need to be!! I had to look up the equation for the area of a circle over the weekend. I can't wait for this to be over!

Keep smiling and take a few days off! You've got to keep your batteries charged and every now and again do a reboot.

 

[Daisy]

My daughter has been sick the last couple of days, so I haven't been able to study much in the evening, which is probably a good thing. I really wish she felt better (and HOPE I don't get her cold), but the break from studying has been refreshing. Thanks for the encouragement.

 

[jeb6294]

Oh... and not to be discouraging, but I sure hope you don't see a problem like this on the exam - it requires almost no engineering sense, and it certainly isn't the right way to "size" a storm drain.

I remember seeing a couple similar to this when I took the exam.

 

[IlPadrino]

I remember seeing a couple similar to this when I took the exam.

I'm not talking about simple Q=VA plug-and-chug... I'm talking about pretending that you are really going to calculate storm drain size on [edit: JUST] flow and velocity - that isn't Engineering, it's middle-school math.

 

[jeb6294]

I'm not talking about simple Q=VA plug-and-chug... I'm talking about pretending that you are really going to calculate storm drain size on flow and velocity - that isn't Engineering, it's middle-school math.

You mean that's not how you design a culvert in the real world...surely you jest?!?! :Locolaugh: