Floods

Characterization of Floods

Floods are characterized by their duration, intensity and frequency, cause, and subsequent impact. Duration is the length of time an area is inundated and/or a stream is above flood stage (such as the height of the water surface above an established local elevation at which overflow of the stream-banks begins). The intensity or magnitude of a flood is usually measured as the volume of water passing a point per unit of time (such as cubic foot per second), known as discharge.

Alternately, flood magnitudes along streams are often expressed as the height above flood stage. Annual flood magnitudes are compared with historical records using return periods, the average time interval between floods of a given size. Return periods are the reciprocal of the flood frequency or the probability of a flood level being equaled or surpassed in any given year. For example, a flood with a return period (or recurrence interval) of 50 years has a one in 50 probability or 2 percent chance of occurring in any given year. Flood [Page 562]probabilities are statistically independent events, which means that a 50-year flood has the same 2 percent chance of occurring each year and successive 50-year flood events are possible. The definitive cause of floods is excess precipitation, but the timing, persistence, and atmospheric circulation type determine the flood size and damage extent.

Highly localized flooding that occurs extremely quickly with rapid runoff is known as flash flooding. This type of flooding usually occurs within minutes to a few hours after a heavy rainfall event such as a thunderstorm. Flash floods also occur when a human-made structure such as a dam or levee breaks or when water behind an ice jam on a frozen river is quickly released during a spring thaw. They can be extremely hazardous events both to lives and property. Rainfall intensity (the rate of rainfall) and duration (how long the rain lasts) are two of the most important factors that determine the likelihood of flash flooding. When the atmosphere supports the development of slow-moving (long duration) thunderstorms, rainfall quickly saturates the uppermost layer of soil. Once the soil reaches its holding capacity, any excess rain that falls cannot be absorbed by the ground and either evaporates or becomes runoff. It is the runoff that leads to flash floods. Thunderstorms that produce heavy (high intensity) rainfall, upward of 1 to 2 in. or more per hour, can produce flash flooding. Organized, long-lasting, multicellular thunderstorms such as mesoscale convective systems (MCSs) are often responsible for flash-flooding conditions, especially in areas of highly variable terrain.

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