Land that has been paved (roads, parking lots, etc.) can not absorb rainfall. Urbanization increases runoff 2 to 6 times more than undeveloped land. Urban flooding can also turn streets into swift-water rivers.
Flash flooding can be caused by excessive rainfall, a dam or levee failure, or the sudden release of water held by an ice jam. They can develop in minutes, moving very rapidly with very little warning.
In addition to fast moving water, flash floods are also dangerous because of debris propelled by the current. Flash flooding can develop from sources outside of the local area.