Until a few weeks ago, I had never heard of such a thing. Have you ever?
This is something which I think I should like to do, gathering the materials
will take time for me, but the benefit of this outweigh the thoughts. We have one at the Nature Center, that is where the idea came from. Though thought being that the plants which need the least amount of water drain to those who need more.. It looks more compact then the conventional rectangle or square garden, and I'm sure you could make it that way if you choose..the sky is the limit on how you design these. I think they are rather pretty and practical.
a few links for you to paruse...
If you done one of these, I do hope you'll stop and share your experience. I think these are very cool.