Determine the depth and height of the water damage. Next, mark at least one or two inches above the rot. Using a circular saw, cut across the post a depth of half the column. Also, determine the height of the joint to be at least two thirds the width of the column. Repeat the same depth on the opposite side at the same height. Finish cutting out the rotted wood with a handsaw, then separate the wood with a chisel. Prepare the new infill material by determining the height of the new material and cutting it to height, then matching the joint together. If necessary, apply wood hardener to the surface of the old wood to add more strength.