Flash floods are distinguished from other types of flooding by the short time scales of their development and intensification. They are the leading cause of deaths from natural hazards in the United States. A majority of flash flood deaths are motor vehicle related.
Flash floods are most frequent during the night, and damage annually is greatest in urban areas.
In a natural setting, intense rain, soils that are of low permeability or are in a highly saturated state, impervious ground surfaces, steep slopes that facilitate rapid flood-wave movement, and possibly anchor convection are all conducive to the genesis of these disastrous events. Failure of small to medium-sized dams, including debris dams, and the sudden formation and breakup of ice jams are also responsible for increased damage associated with flash floods. In spite of continuing efforts of government and the private sector to improve observations and warning procedures, flash floods remain one of the elusive natural killers, as the recent disaster in Shadyside, Ohio, has painfully demonstrated. Preceding months of above-normal rainfall had generated soil moisture contents of near saturation. As a result, moderate amounts of rainfall caused large amounts of surface and near-surface runoff.