A "wall of water" a few inches in height will wash cars away. 20 feet of water, as a wall, is simply packed with pure kinetic energy. I can't even imagine a 20 foot high wall of water and I've seen plenty of flash floods that are 2, 3 or even 5 feet in height. They are devastating themselves.
I've seen them too and you are right. A foot of water, depending on the speed can knock you off your feet and sweep you away. It's very deceptive.