Okay, using:

http://www.calctool......williams_g

(Edit: Originally posted April 23, 2014, 6:00 pm -- Corrected May 22, 2014, 8:28 am -- Correction of data to actual depth drop which is 1 foot for every 24 feet.)

With

• Roughness = 100
• Pipe Length = 24 feet
• Drop = 2 inches Drop = 12 inches

For 42 inch pipe I get

• Capacity = 35,733.2 gallons per minute Capacity = 94,031.5 gallons per minute
• Velocity = 8.27507 feet per second Velocity = 21.7758 feet per second
• Pipe Slope = 0.694444% Pipe Slope = 4.16667%

For 48 inch pipe I get

• Capacity = 50,768.0 gallons per minute Capacity = 133,596 gallons per minute
• Velocity = 9.00132 feet per second Velocity = 23.6869 feet per second
• Pipe Slope = 0.694444% Pipe Slope = 4.16667%

I think I am satisfied with the data and reasoning in favor of 48 inch pipe.

Rise (height) which is the internal dimension is listed as 36 inches for 48 inch arch pipe. I'm adding 10 inches to allow for upper and lower wall thickness which is listed as 5 inches each. That's 46 inches tall. Rise for 48 inch round pipe is 48 inches. Add 10 inches to allow for upper and lower wall thickness which is listed as 5 inches each. That's 58 inches tall. So arch pipe would save 14 inches in height.

Span (width) which is the internal dimension is listed as 58 inches for 48 inch arch pipe. For arch pipe, that is the ground level width. For round pipe, ground level width is about 1/3rd of the 48 inch diameter which would be 16 inches. I could potentially get round pipe and then get flared end sections for it later. The flared end section has the same ground profile as arch pipe. But flared end sections are about $1600 each. So buying round now could save about $700 in the short term, but cost $3200 in the long term.

I think wider ground contact is important, and saving on height may prove necessary, so again, arch pipe wins out. 48 inch arch pipe it is.

Now I just need to confirm that 24 feet (3 sections of 8 feet each) will be wide enough. I know that the slope must at a minimum use the 4/3 formula which states that for every 4 feet of height, 3 feet of width must be added for slope. So an 8 foot wide road surface with 2 feet of planted erosion control shoulder on either side would be 12 feet wide. The arch pipe is 3 feet 10 inches tall, rounded to 4 feet, and I need 4 feet of depth above that to bury phone and electric which would be 8 feet tall. That should also be enough height to bring the fill to elevation. At 8 feet high, applying the 4/3 formula tells me I need to add 6 feet on either side for slope. That brings the width of fill at the base to 24 feet and 12 feet at the top. So 24 feet of culvert pipe should work.

Since it looks like I may have high clay content (not performed soil test yet), the reading material on culverts advised against that. If I need to truck in fill, I need to know how much. So...

50' full depth span

30' slope span (80' full span)

12'Wx8'xH50'L=4800x2(side slope)=9600

12'Wx8'Hx15'L=1440(slope span)+9600=11040 cubic feet

11040 cubic feet = 408.888889 cubic yards (rounded to 409) = 456 tons = 24 truckloads

17yds = 19 tons = 1 truckload