The location for the run will probably have some flooding in the rainy months but it is the only suitable location for the run. I think during those times the run would be a very wet mess hence having the concrete or wood base to keep it elevated. I agree that plain dirt may be more attractive...
I have 3 chickens. I already have a coop but want to extend the run. I am thinking of either a concrete base or an untreated wood base, either option covered in sand. How large does a run have to be to comfortably accommodate 3 laying hens?