Floods

Categorization of Floods

Floods can be categorized according to their speed and duration. Slow-onset floods may take days to accumulate, and can last for one or more weeks or even months. Because they last for a relatively long period of time, these floods can cause significant loss of livestock and damage to crops, roads, and railways. Rapid-onset floods occur more quickly and last for a shorter period than slow-onset floods. They pose a greater threat to life because there is less time for preventative action, and they are potentially more damaging due to a faster and more dangerous flow of water. Flash floods have the most rapid onset and are usually the most lethal. The onset of a flash flood is sudden and occurs within a few hours or possibly minutes, usually following intense, heavy rainfall. They can also be caused by sudden and massive snowmelt or by a dam or levee failure.

When heavy rainfall occurs in areas with steep slopes, riverbeds can be quickly overwhelmed by water accumulating on the slopes and accelerating downhill. The area of land covered by a flash flood is usually small relative to other flood types, but greater concentration of water in the area can cause the waters to rise very high. Because of the speed at which flash floods occur and the strength of floodwaters to carry away people and objects like cars and trees, flash floods pose the greatest risk to life and property. Moreover, flash floods can occur even without any localized rain because they can originate several miles away. Flash floods are responsible for the majority of flood-related deaths, and 50 percent of all flash-flood fatalities are vehicle related. In Rapid City, South Dakota in 1972, 15 inches of rainfall in five hours created a flash flood that killed 237 people.

Floods can also be categorized according to their location or geographical factors. Riverine or fluvial floods are the most common type of flooding. Sustained rainfall, excessive snowmelt, or the sudden release of waters held behind a dam can cause the flow of water to exceed a river channel's capacity and to flow outside of the riverbed and onto surrounding areas. Riverine floods can occur gradually or suddenly, depending on the cause of the flooding. Coastal floods occur in coastal areas when the sea floods onto land during severe storms such as hurricanes, which produce heavy rain and wind. Storms form in low-pressure areas, and low pressure causes the sea level to rise. Winds create high waves and push the rising water inland. Coastal floods can also be caused by tidal waves created by earthquakes or volcanoes that drive ocean water onto shore. Arroyo floods occur primarily in mountainous desert regions and are most common in the southwestern United States. An arroyo is a river or creek bed that is usually dry except during seasonal or unexpected rainstorms. Fast-moving rivers form, and can cause damaging floods and other problems through erosion, transportation, and deposition.

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