I have tried this on asphalt and it didn't work too well for me. Gravel would definately help to hold the buggers in place. I just hate handling them until the husks are dried and fall off. Otherwise you get covered with the black dye or yellow dye depending upon how old the nuts are.

I have thought about creating a water powered device that works like an automatic fish scaler. I have a stream with flow that could do some work. My vision is a basket that is anchored in the stream bed and rotates with the flow. You just fill it up and let the stream do the work and carry the mess away.

I suppose it would be easier to just pick up a bag of Black Walnuts for $1.59 at the grocery store, but nowhere near as fun.