Geoscience Reference
In-Depth Information
might see it as a signal of Greenhouse warming, it
appears to represent a shift towards extreme rainfall
events in this region. Not only did the climatic patterns
giving rise to one catastrophic flood reappear days later
in the same location, but the patterns also tended to
repeat themselves at other locations along the coast,
and recur several times over the next few years. Since
1984, the 80 km stretch of coast encompassing
Wollongong and Sydney area has experienced at least
twelve similar events. A local coroner's inquiry into one
recent event has shown that local authorities are almost
unaware of this change in the rainfall regime. In New
South Wales, the state government has realized that
urban flooding has become more prevalent, and has
added a $A25 surcharge to ratepayers' assessments to
cover clean-up expenses and recover relief costs. Even
this is a misconception of the nature of the climatic
shift, because the surcharge is being applied to the
western suburbs of Sydney, which in fact have not
experienced the greatest number of flash floods. The
greatest rainfall has occurred during thunderstorms
over the central business district, which has a higher
surface roughness coefficient because of tall buildings
and parallel streets.
These events represent a significant increase in the
occurrence of a natural hazard that has economic and
personal ramifications for most of the four million
residents in the region. Neither the significance of
present flooding, nor its consequences, has been fully
appreciated.
HIGH-MAGNITUDE,
REGIONAL FLOODS
High-magnitude, regional floods usually present a
disaster of national or international importance.
Usually, large-scale drainage basins or many smaller
rivers in the same region are flooded. Flooding appears
to have increased in intensity and extent in recent years.
As higher precipitation is one of the consequences of
global warming, flooding has recently received the
attention of media and relief organizations. Figure 6.10
plots the locations of major floods for fifteen years
between 1985 and 2003. Flooding has occupied an
extraordinary proportion of the world's land surface
concentrated in two main regions: the tropics and the
mid-latitude storm belts. This section describes histori-
cal flooding for three areas - the Mississippi River,
eastern Australia, and China - to illustrate flooding's
pervasiveness and high economic cost.
Miss
issippi River floods
(Bolt et al., 1975; Lott, 1993; Larson, 1996; Trotter et al.,
1998; Public Broadcasting Service, 2000)
The Mississippi River drainage basin is the third
largest in the world, and the largest in North America.
The basin covers 3 224 000 km
2
or 41 per cent of the
conterminous United States (Figure 6.11). Its head-
waters can be divided into two areas, one originating at
the continental divide in the Rocky Mountains, and the
Flood locations 1985-2003, excluding 1989, 1992, 1996, and 1997 (based on Dartmouth Flood Observatory, 2003).
Fig. 6.10
