You might want to consider this link before you do this.
Jack Spirko: Hugul Swale/Don't article Permaculture News.
Now if you have your hugul primarily underground, or your ground is not very sloped that mitigates most of the problem addressed in this link. You could have a pipe going down to a french drain gravel swale in the base of the trench that the hugul logs sit on, as Chadwick H states. I would definitely go with gravel rather than tiles or perforated pipe, so that the woody core has direct access to the water, and as they rot and break down the wood layer and fungi will mesh more and more with the water source. It might be tricky to figure out the depth of gravel that you would need, for the below two reasons:
I think it would depend on your soil. The trench swale underneath a bed might act as drainage, drawing moisture out of your bed via gravity, if the subsoil is coarse/aggregate based
and super draining.
I think it would also depend on your rainfall: whether the roof will provide adequate water to fill the trench and get into the woody material enough to offset the gravity drainage, with an aggregate trench below the wood.
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