According to NOAA, most floods start with slow-moving thunderstorms, thunderstorms that travel the same path several days in a row, or hurricanes. Floating debris and ice sometimes form a temporary dam which causes the water to back up until the debris breaks free, releasing a wall of water and causing flooding downstream.
Some rivers flood seasonally with rains. Warm rains in mountain areas melt snow to increase the volume of water in the river. Hurricanes or tropical storms bring large amounts of rain inland and cause the flooding of streams and rivers.
Urbanization increases the flood potential. The covering of formerly grassy pastures with asphalt increases the runoff from two to six times. Urban flooding turns streets into rivers. A basement in an urban setting becomes a potential death trap due to flooding.
NOAA warns residents to know the flood dangers for where they live. Most floods occur with some warning, and it is important to evacuate on the recommendation of officials. Floodwaters of only 6 inches have the ability to knock a human off his feet. If the water is over 2 feet in depth, it will float an automobile. Each additional foot of water adds another 500 pounds of lateral force to move the vehicle downstream.