WHAT ARE FLASH FLOODS? By Corley and Claire. -A sudden flood of great volume , usually caused by a heavy rain, or failure of dam or the thaw of an ice jam . . These fast moving waters can move boulders , tear out trees , and destroy buildings . -Occurs suddenly in a short duration -Little or no advance warning,over a relatively small area Picture: A flash flood after a thunderstorm in the Gobi, Mongolia.