Environmental Hazards: Flooding

Magnitude and Probability of Floods

Floodplains are created naturally by rivers and streams over many years and carry flows during times of high water. When the amount of water in the river or stream reaches its capacity, it spills over onto the floodplain. This constitutes a flood. It is important to recognize that all rivers are prone to flood. The size of potential floods, however, is generally proportional to the size of the stream system: the larger the drainage basin, the larger the prospective flood. Large drainage basins can lead to extensive flooding, as occurred in the U.S. Midwest in 1993 and 2008, Pakistan in 2010, and Australia in 2011. This is not to say that small drainage basins do not create difficulties. To the contrary, flash flood events have generated considerable problems and are often associated with substantial loss of life. The magnitude of the flood, as measured by the discharge (the amount of water flowing past a given point over a given period), therefore, may not be the key determinant of human impact. Instead, it is the human dimension that must be explored to fully understand the flood hazard.

A common way of describing flood magnitude is based on probabilities of the frequency at which different flood discharges are reached and the recurrence or probable return period or recurrence interval. The 100-year flood, for example, has a 1% chance (a 0.01 probability) of occurring in any given year, the 25-year-event a 4% chance (a 0.04 probability). In the United States, it is the 100-year flood that has been adopted for flood management purposes and is used to delineate flood hazard zones—the 100-year floodplain. Thus, such floodplains can expect to be inundated once every 100 years on average. Of course, as with all averages, this does not suggest a regularity of occurrence, and three 100-year events could recur 3 years in a row, or even more often. Also, the 100-year floodplain represents just the outer boundary of the zone; the land and hence buildings closer to the stream and at lower elevations are at even greater risk. The floodplain, therefore, constitutes a gradation from a high probability of flooding adjacent to the stream to lower probabilities as one moves away. Furthermore, land outside this boundary is not flood free; it is just at a lower risk to flooding.

...