I live in washington state...so it rains and rains and rains. I have tarps on the side that the heavy rains come in, and more than half the runs covered....some places are dry...but water seeps in from surrounding areas. This will be my first winter with chickens. I positioned the runs with enough of a slope they do drain. The dirt is glacial till with the water table only a few inches deep....one acre is actually about 2 inches under water about 1 month of the year....no chickens there! The bark/mulch available would contain mostly fir....but cedar is also a native species here. I have a big pile I can get for free...mom ordered too much for her house! Sand tends to wash away....gravel does not work that well....tends to sink into the muck and disappear!