You will need some form of litter in the run. Gravel is one of the least suitable choices however - although it allows for drainage, it's harder on a chicken's feet and also allows for poop to travel down and start decaying and stinking. Gravel in a trench for drainage is a bit different.
If you have space for a pile of wood chips, that's the best and most economical choice for litter base in PNW coastal areas. Read up on deep litter, that's what I use and what I see most folks in my area using.
Example: after a heavy night of rain, the lawn had noticeable squishy spots and some puddles, but the deep litter in the unroofed run was surface dry enough that I didn't worry about chicks getting chilled or damp.
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