How floods differ from flash floods?
Both refer to the same thing, but the term "flash flood" implies that the flood came so quickly and unexpected that there was no time to make preparations. In many cases the flash flood causes the water level to rise several metres within a very short period of time.
Flash floods are most likely to be caused by a sudden weather event, such as a storm, while slow-onset floods are caused by prolonged, sustained rain.
Flash floods are deadlier. They are fast-moving floods that occur without warning, and thus do not give people time to get out of the way. They are strong and powerful, often with incredible volumes of water, able to wash cars away and knock down houses. In the United States alone, flash floods are the main weather-related killer.