Example

Problem: A 12-in.-diameter pipe has water flowing through it at 12 fps. What is the discharge in (a) cfs, (b) gpm, and (c) MGD?

Solution: First, determine the area (A) of the pipe. The formula for the area of a circle is:

where:

n = 3.14159, or 3.14. D = diameter of the circle in feet. r = radius of the circle in feet.

Thus, the area of the pipe is:

(a) Determine the discharge in cfs:

Q = V x A = 12 ft/s x 0.785 ft3/s = 9.42 ft3/s (cfs)

(b) Determine the discharge in gpm. We need to know that 1 cfs = 449 gpm, so 9.42 cfs x 449 gpm/cfs = 4230 gpm.

(c) Determine the discharge in MGD. We know that 1 MGD = 1.55 cfs; thus,

The law of continuity states that the discharge at each point in a pipe or channel is the same as the discharge at any other point (provided water does not leave or enter the pipe or channel). In equation form, this becomes:

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