Several factors contribute to flash flooding. The two key elements are rainfall intensity and duration.
Intensity is the rate of rainfall, and duration is how long the rain lasts.
Topography, soil conditions, and ground cover, also play an important role. Flash floods occur within a few minutes or hours of excessive rainfall, a dam or levee failure, or a sudden release of water held by an ice jam. Flash floods can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Rapidly rising water can reach heights of 30 feet or more. Most flash flooding is caused by slow-moving thunderstorms or thunderstorms repeatedly moving over the same area.