Typically, flash floods occur primarily at night and when there is an abundance of atmospheric moisture; in addition, there is usually little, if any, vertical wind shear present. Flash flooding can be produced by large, slow-moving storms or as a result of “train effect” storms (i.e., sequential mature storms that release precipitation over the same area). Train effect storms can be part of multicell cluster or squall line storm systems. Flash-flood waves, moving at incredible speeds, can roll boulders, tear out trees, destroy buildings and bridges, and scour out new channels. Killing walls of water can reach 10–20 ft. On small streams, especially near the headwaters of river basins, water levels may rise quickly in heavy rainstorms.

Flash flooding occurs when a barrier holding back water fails or when water falls too quickly on saturated soil or dry soil that has poor absorption ability. The runoff collects in low-lying areas and rapidly flows downhill. Flash floods most often occur in normally dry areas that have recently received precipitation, but may be seen anywhere downstream from the source of the precipitation, even dozens of miles from the source. In areas on or near volcanoes, flash floods have also occurred after eruptions, when glaciers have been melted by the intense heat. Flash floods mainly occur in the highest mountain ranges of the United States.

Flash floods are slightly different from normal floods. Normal floods involve water rising and overflowing from its normal path. However, flash floods appear quickly and move swiftly across land with little warning. Flash floods occur for a variety of reasons including concentrated rainfall during a slow moving thunderstorm, hurricanes, and tropical storms. The most devastating flash floods are from dam and levee failures. When either structure breaks, an enormous amount of water is suddenly unleashed, destroying everything in its path. The water in a flash flood moves at such a high velocity that it can move boulders, uproot trees, demolish buildings, and destroy bridges. The walls of water in such a flood and be anywhere from ten to twenty feet tall and usually carry a substantial amount of debris.

Sources:

Scienceclarified

Wikipedia

Think Quest