Floods occur whenever the surface of a stream rises above the walls of its channel. Water then spreads laterally from the stream channel and submerges everything below a certain elevation. Floods are destructive and deadly. According to the NOAA, floods are responsible for$8.22 billionin damages and95 deathsin an average year in the United States. However, the statistics are skewed because most of the destruction is caused by rare large floods.

There are four types of floods: regional floods, flash floods, ice-jam floods, and dam-failure floods.

Regional and flash floods are covered in the next exercise.

Ice-jam floods occur when ice obstructs the flow of water across a channel, effectively damming the stream. As a consequence, the stream's

water level rises and overflows its banks. The ice jam will eventually break, which may generate an additional destructive surge of water.

Dam-failure floods occur when a human-made dam or levee breaks. These floods are particularly catastrophic because they can rapidly submerge an area without warning.

Part A -Regional and flash floods Regional and flash floods are similar and different in these ways:

Both regional and flash floods result from the release of large quantities of precipitation and/or snow melt.

Regional floods occur over broad areas and may persist for a long time, whereas flash floods are local and brief.

Flash floods are more intense than regional floods because they occur in regions where surface water accumulates more rapidly.

A region's surface conditions and topography determines whether it will be susceptible to regional or flash flooding. Flat regions favor regional floods because the gradual slope of the landscape slowly directs water into a stream channel, whereas hilly or mountainous regions favor flash floods because steep slopes that can quickly funnel water into a stream channel are present.

Ground that is impermeable to water (i.e., asphalt, concrete, and nonporous rock) favors flash floods because it allows surface water to rise rapidly, and ground that is made of porous, unconsolidated sediments (i.e., some soils) favors regional floods.