1/5 doesn't sound awfully lot, I think olives would be dwarfing enough. Strangely chestnuts grow on the place, but the one very close to the creek has the side to the creek which does not look so good, I reckon all this water recently.
We're in the Upper Mountains.
With all this weather stuff going on I start to plan for the next drought.
Our drainage is miserable, I will have to mound trees.
Do you need to mound the trees to the depth of the roots? And if a tree has a tap root then you would have to mound it to the depth of the tap root? Or do trees adapt?
If I would plant walnuts trees on a mound onto the miserable drainage (maybe 30 cm below surface at least at some points) would I be likely to succeed?
Inspired by the Holzer ideas I would throw first a pile of woody material, then lawn clippings I get for free, then earth and last compost and manure. I would try to surround it by stones, if I find sufficient big ones. I would try to make the pile higher than a meter so I might up with a meter as it sits down over time.
Maybe I'll do the same thing with avocado trees and olive trees.