Nailing doors shut

The pressure developed by a wall of water is tremendous. If a flood is anticipated, locking, filling cracks under the door and reinforcing doors will buy time. Not much if the water gets too high, but a little.  One lesson of Sandy was the ease and rapidity with which doors buckled to let in water. The reinforcement should involve 3/4 inch plywood that will extend four inches on each side of the door to be bolted, screwed (or nailed) to the door frame, as high as the lower ledge of the lowest window if possible.  If no plywood is available use 2 X 4 's. If you retreat to the upper part of the building close inside doors to the cellar. Use nails with double-heads so if the water comes in, you can eventually remove the plywood to let the water out.