Floods - Extreme Events: A Physical Reconstruction and Risk Assessment - page 58

major snow melt flood from a rapid temperature rise after heavy winter snow in
British Columbia, Canada, resulted in more than 2000 people left homeless and
a compensation bill in excess of Can $20 million (Smith, 2001). The 1998 Chinese
floods left a damage bill of US $20 billion (Smith, 2001). The 1993 Mississippi
River floods resulted in a damage bill between US $15--20 billion and saw a 17%
reduction in the national soybean and corn yields (Chapman, 1999). In India,
75% of the direct flood damage is attributed to crop loss (Smith, 2001).
Floods, therefore, are one of the most costly and wide reaching of all natu-
ralhazards. Our ability to model the size and extent of a flood during a heavy
rainfall event has improved greatly in recent years. Because of this, emergency
managers and hydrologists can estimate with a reasonable degree of accuracy
thearea of land likely to be inundated if detailed digital terrain models and
numerical flood models are available. Many lives can be saved in these situa-
tions through effective and timely evacuations. But property damage and agri-
cultural losses can still be high. In many ways our ability to mitigate against
floods is advanced. But as with many hazards, and despite our technological
tools, many urban developments are still allowed to occur in flood prone areas.
Palaeorecords, while often ignored, can provide us with vital information on the
magnitude and frequency of the flood hazard and also the extent of a landscape
likely to be inundated in the future. Many risk assessors are unfamiliar with
these long-term records and the methods by which they can be uncovered. As
a consequence they limit their understanding of the flood hazard by restricting
their analyses to the historical record.
palaeofloods
Avariety of natural phenomena record floods. These include:
slackwater sediments;
plunge pool deposits;
flood transported debris such as tree branches and logs left high on
river banks or valley sides;
deposits of large boulders in river channels and the spacing of boulder
bars and rapids along the length of a river; and
theshape and form of a stream, otherwise known as channel geometry.
Each of these either record the height of the water during a flood or is a measure
of the magnitude of the flood flow and its competence to move objects of a
certain size and density. Some forms of palaeoflood evidence are a direct measure
of the flood height (usually minimum height) and in this sense are not strictly
proxies but an actual record of that event. Other forms of evidence, such as