For those times when the storm takes the worst possible turn, and your city has already been out of sandbags for most of a week...dog food or kitty litter in a hefty bag!
The dog food/kitty litter container is often already mostly water proof. Double-layer force flex bags, which you can double up on, add an extra layer of defense. These will shape to any space easier than sandbags will, and the pieces are too small to become meaningful projectiles outside of truly awful luck.
My grocery store still had plenty of supplies, including lots of dog food. Oddly enough, they were completely cleaned out of Gatorade, but still had Powerade in a wide variety of flavors right next to it. Every soup was gone unless it started with "Cream Of" or "New England".
Don't get me wrong, I prefer Gatorade to Powerade like the next red-blooded American, but there are times when brand loyalty is not of paramount importance. (Those times are always, but especially now.)
If you prefer a more serious sign-off, we'll go with this one:
Special Thanks: Glad, for a fine trash bag. Purina One, for not being sold out and using water-resistant packaging.
Copyright: The Office, Boondock Saints