So no soil? Just rocks and wood chips? I'm no expert but that sounds Iike a recipe for bumble foot?
And no they will not stop digging. Ever. You will have rocks mixed in with your wood chips.
My ladies have decided they wanted a basement under their coop so I'm getting a seepage issue as it's below grade.
You may want to go ahead and put in a french drain while its open and the ladies are not trying to help.
They have funny way of dirt moving that will give you a problem you...
You can't over engineer when it comes to drainage.
You won't truly know until it's built and the rains comes. Im also on a slope. I've added ditches, dams, drains, roofs and gutters in the battle of the boot sucking mud. I won.