## River Floods And Flood Frequency

seasonal variations in rainfall cause stream discharge to rise and sometimes overflow the stream's banks. Both stream discharge and velocity increase during floods, so at floods the streams carry larger particles. Many of the most dramatic changes in river channels occur during floods—meander channels may be cut off, new channels may form, natural levees may be breached, and, occasionally, the river may abandon one channel altogether in favor of another. Floods have a probable interval of recurrence for floods of specific magnitude. small floods occur quite often, typically every year. Larger floods occur less frequently, and the largest floods occur with the longest time interval between them. The time interval between floods of a specific discharge is known as the recurrence interval, and this is commonly cited using statistics for the 50-year flood, 100-year flood, 500-year flood, and so on.

Curves of the discharge versus recurrence interval can be drawn for every stream and river to determine its characteristic flooding frequency. Knowing how likely it is that a flood of a certain height will recur within a certain time frame is important information for everyone living near a stream or on a river flood-plain. For instance, if a flood of 150 cubic feet/second (4.25 m3/s) covered a small town with 10 feet (3 m) of water 30 years ago, is it safe to build a new housing development on the floodplain on the outskirts of town? using the flood frequency curve for that river,

Flood frequency curve for two different periods of different climates, along a river in Africa. The recurrence time (horizontal axis) represents how often a flood with a specific volume of water (vertical axis, showing discharge in cubic feet per second) occurs at a location along a river. Note that the river had a dry climate from 1934 to 1960, and floods were not as large or as frequent as in the wetter period from 1961 to 1970. Changes in climate can severely impact the predicted frequency of floods. (Data from the Ministry of Water Development, Kenya)

100,000

80,000

60,000

40,000

20,000

1961-1970

1961-1970

1934-1960

Recurrence interval (years)

### 50 100

© Infobase Publishing planners could determine that floods of 150 cubic feet/second (4.25 m3/s) are expected on average every 40 years, and floods of two times that magnitude are expected every 100 years. Planners and insurers might (in the best of situations) conclude from that information that it is unwise to build extensively on the floodplain—the new community should be located on higher ground.

understanding flood frequency and the chances of floods of specific magnitude occurring along a river is also essential for planning many other human activities. Engineers must determine how much water bridges and drainage pipes must be built to handle and how to plan for land use across the floodplain. In many cases bigger, more expensive bridges should be built, even if it seems unlikely that a small stream will ever rise high enough to justify such a high bridge. In other cases structures are built with a short lifetime of use expected, and planners must calculate whether the likelihood of flood warrants the extra cost of building a flood-resistant structure.

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