The NRCS called these pipes Toe drains. They go about a 1/3 of the way back up the dam then across the entire back of the dam. It is 6 in pipe with a bunch of little 1/8 inch holes in it. One pipe goes to the right one goes to the left. then they are discharged toward the main drain pipe. They are covered in gravel. I would say about a foot or so square in gravel down the entire length.
They are there just incase there is a spring under the dam, the water has a place to go.
