Hey Lindsey!
This is my first reply on Permies though I've been lurking for years now.
We have built a earthbag root cellar here in the Southern Willamette Valley two summers ago. This is the second winter. It is still a work in progress. We dug about 8 feet down at the highest point on our property. Last fall the rains came before we could put a roof/floor (we put a pantry/shed above it) and it was flooded halfway up. A great time to see first hand that earthbags would make a great pond! The water eventually seeped back into the ground as it just had a clay floor. This past summer we put in the skeleton of the pantry above including the roof, but once the rains came it was flooded again but not nearly as bad as before. We pumped out the water and it has yet to come back nearly a month later. We'll be putting a concrete floor (we'd prefer not to but we hope that it will help with the water issue ) in there and finally stuccoing the bags as soon as it's feasible. Our next root cellar with definitely be build into a hill on our property. We would put in a french drain and keep the floor angles downhill for water runoff and just make the shelves level. Our earth bags are still very much intact and doing much better than we expected with all of the flooding. So if you build on a hill with the drainage you described, it should do very well. Ours is wonderfully cool in the summer and has not frozen these 2 winters.
Good luck, and I hope you post some pics of your progress.
