I need to build a little drainage ditch beside my patio to drain water away from the fence. There is only about 12 inches between the patio and the fence post. Is this area large enough to dig a small drainage ditch maybe 6 inches deep and 6 inches wide and feel with little stones? Ideally, how far away should it be from the patio to prevent it from collapsing into the ditch? Also, how far away from the fence post should it be?