It usually occurs slowly enough to allow people and property to be moved to safety. Urban and small stream flooding occurs when heavy rain falls in a short period of time. Storm sewers and small streams cannot handle the runoff resulting in flooded underpasses, basements and back-up sewers.
Flash flooding occurs very quickly and is always life threatening.
It happens more frequently in hilly or mountainous areas. Rainfall of two to four inches in a couple of hours can cause a flash flood. Dam failures also can result in flash flooding. Floods and flash floods can occur away from the area of heavy rain. Especially in hilly or mountainous areas, rainfall that occurs upstream can result in flash floods downhill or downstream from where the heavier rain fell.