Aside from the factor of surprise (many people are caught sleeping), people just don’t appreciate the power of moving water. Even six inches of fast-moving flood water can knock you off your feet. Most automobiles will float and can be swept away in only two feet of water. Never try to walk, swim, or drive through the swift currents of a flash flood. Nearly half of all U.S. flash flood fatalities are auto related. Never attempt to drive over a flooded road. The depth of water is not always obvious. Also, the road bed may have been washed out under water. Dry creek beds can go from dusty bone dry to a ten-foot-deep torrent of water within a minute as the thunderstorm rains drain down from surrounding higher terrain. (Source: http://www.crh.noaa.gov/riw/severe/floods.