Excellent idea, you definitely don't want runoff seeping into the coop area.
Trench/swale and/or a berm could help a lot.
I have a trench for run off going thru part of my yard, it's not filled with anything...I think it works better this way when the ground is frozen.
Grass grows in it, I mow it very short, and occasionally have to shovel/rake dirt/leaves out of it...and maybe pull some big weeds.
Filling it with gravel would be work, might lighten the more frequent maintenance, but could also fill with silt and be much harder to redo.
I would suggest digging a shallow trench where you think it's needed then watch the area during a heavy rain to see if more might be needed.
