Geoscience Reference
In-Depth Information
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Floods
Floods occur when water from terrestrial run-off leaves a stream chan-
nel and spreads across the surrounding landscape. The definition of a flood can
take on different meanings to different people. Many would consider flooding to
occur only when the level of water has risen to a point where the threat to prop-
erty and infrastructure is unavoidable (Bell, 1999;Chapman, 1999; Smith, 2001;
Bryant, 2005). In this sense flooding is viewed as a hazard. Floods are also the
maintainers of ecosystems and support life in coastal estuaries, lakes, wetlands
and enrich vast floodplains and play an integral role in the geomorphic evolu-
tion of landscapes (Jones, 2002). Flooding is a natural function of river behaviour
and floodplains by definition are innately flood-prone (Chapman, 1999).
Causes of floods
The causes of floods can be broadly divided into physical, such as clima-
tological forces, and human influences such as vegetation clearing and urban
development. The latter can result in an exacerbation of the conditions affect-
ing the run-off of water resulting in flood intensification (Smith, 2001). The
most common causes of floods are intense and/or prolonged storm precipita-
tion, rainfall over areas of snow cover, rapid snow melt, the successive occur-
rence of medium to major size storms and the failure of dams, including ice
dams (Chapman, 1999). The most important causes of floods are atmospheric
hazards, most notably rainfall. Floods can also be associated with oceanic and
atmospheric processes on a large scale such as the El Nino Southern Oscillation
(ENSO) phenomenon. For example, the 1993 USA floods were associated with an
ENSO event and the 1988 flooding in Sudan and Bangladesh were linked to a
La Nina episode (Smith, 2001). The floods of eastern Australia in 1974--75 also
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