I'm cutting down some big Leylandii alongside a ditch. I plan to replace them with a 6 foot close-boarded fence. I'm using 5x4 notched timber posts concreted in at least 2 feet deep. I'm trying to work out how far to set the posts back from the edge of the ditch to make sure high winds can't push them over, or whether they should be braced in some way. Any ideas?