I believe it is drainage culvert cut into pieces.

Oh, I can see that clearly could be it! Wow..thank you!

someone asked about the sharp edges....the response was..."This is the most asked question and rightfully so. Yes the edges are sharp and uneven. These need to be polished with power tools, and covered with any flexible material, such as sheet metal or reinforsed fexipble hose/pipe, experienced carpenter can create a wood edge, but this make take many hours. I hope it helps and thank you for your question"...

What type of materials do you use for drainage--do you alternate rock with the soil?

I have seen cattle feed troughs and water troughs done this way. There's different sizes, won't have to worry about sharp edges. In this area we have Scruggs tractor supplies that would have these.

You could also use foam garage door insulation tubes, they are already slit and have adhesive.