This picture was taken on the Rio Grande River Valley, on the left side of the image is the river, the right side is the flood control ditch. I am guessing the purpose is when the river rises too high some underground pipes divert the extra water to this so it can drain into the flood control.
This image there is a second, black pipe visible that goes under the ditch road and empties into the ditch.
I would love to know what the grey pipe is called so I can look up a diagram of how they work exactly.
I'm guessing it's a vent for an existing water line.
It serves...as a vent, to prevent a "suctioning" which could cause the pipe to rupture.
It could also act in reverse, as a relief, but that would depend on how it's valved.