Geoscience Reference
In-Depth Information
River floods
To hydrologists, the term 'flood' refers to a river's annual peak discharge period, whether
the water inundates the surrounding landscape or not. In more common parlance, however,
a flood is synonymous with the river overflowing its banks, and this is the meaning used
here. Rivers flood in the normal course of events. This often occurs on the floodplain, as
the name implies, but flooding can affect almost all of the length of a river.
Extreme weather, particularly heavy or prolonged rainfall, is the most frequent cause of
flooding. The melting of snow and ice is another common cause. These events can often
be predicted to an extent because they are seasonal. Other reasons for river floods are usu-
ally harder to anticipate. They include landslides, log jams, ice jams, avalanches, volcanic
eruptions, and earthquakes.
River floods are one of the most common natural hazards affecting human society, fre-
quently causing social disruption, material damage, and loss of life. Indeed, the largest ever
death tolls from any natural hazards are attributed to flooding along the Yangtze and other
Chinese rivers, with claims that some floods in the 20th century took the lives of millions
of people: floods in 1931 and 1959 accounted for, respectively, 3.7 million and 2 million
fatalities. It should be noted, however, that the death tolls reported for some floods in Ch-
ina vary a great deal, and other estimates for the 1931 and 1959 events put the death tolls
much lower, in the hundreds of thousands. Hence, the statistics for some of these devastat-
ing floods, which are inevitably very difficult to calculate, should be treated with caution.
Conversely, it is also worth noting that the scale of Chinese river flooding is astonishing
and often undoubtedly affects hundreds of millions of people, as on the Yangtze in 1998,
so the worst-case figures may not be so easy to dismiss.
The hazards associated with floods have encouraged the development of many techniques
for predicting them. Flood hazard maps are commonly utilized for land-use zoning, en-
abling an authority to prohibit certain developments on land that is particularly flood-prone,
for instance. Anticipating when a flood will occur can be done in several different ways.
Most floods have a seasonal element in their occurrence and can often be forecast using
meteorological observations, with the lag time to peak flow of a particular river in response
to a rainstorm calculated using a flood hydrograph.
Other flood predictions seek to estimate the probable discharge which, on average, will be
exceeded only once in any particular period, hence the use of such terms as '50-year flood'
