Geoscience Reference
In-Depth Information
damage. The methods for interpreting river flows
that may be used for flood assessment are discussed
in Chapter 6. They provide some form of objective
flood size assessment, but their value is highly
dependent on the amount of data available.
Floods are a frequently occurring event around
the world. At the time of preparing of this chapter
(June and July 2007) there were eleven large flood
events reported in the news media (see Table 5.4).
These floods were caused by varying amounts of
rainfall, and occurred in different seasons of the year
but all caused significant damage and in many cases
loss of lives. There are numerous reasons why a river
will flood and they almost always relate back to the
processes found within the hydrological cycle. The
main cause of river floods is when there is too much
rainfall for the river to cope with. Other, more
special causes of floods are individual events like
dam bursts,
jökulhlaups
(ice-dam bursts) or snow
melt (see pp. 72-75).
Influences on flood size
The extent and size of the flood can often be related
to other contributing factors that increase the effect
of high rainfall. Some of these factors are described
here but all relate back to concepts introduced in
earlier chapters detailing the processes found within
the hydrological cycle. Flooding provides an excel-
lent example of the importance of scale, introduced
in Chapter 1. Many of the factors discussed here
have an influence at the small scale (e.g. hillslopes
or small research catchments of less than 10 km
2
)
but not at the larger overall river catchment scale.
Figure 5.13
Images of flood inundation in Fiji, 2007.
in the Mississippi and Missouri rivers during 1993
resulted in a similar economic valuation of loss
(US$15-20 billion) but only 48 lives were lost
(USCE, 1996). The flood was the highest in the
hydrological record and had an average recurrence
interval of between 100 and 500 years (USCE,
1996). The difference in lives lost and relative
economic loss (for size of flood) is a reflection of the
differing response to the flood in two economically
contrasting countries.
As described in Chapter 2 for precipitation,
flooding is another example where the
frequency-
magnitude relationship
is important. Small flood
events happen relatively frequently whereas the
really large floods occur rarely but cause the most
Antecedent soil moisture
The largest influence on the size of a flood, apart
from the amount and intensity of rainfall, is the
wetness of the soil immediately prior to the rainfall
or snow melt occurring. As described on p. 59, the
amount of infiltration into a soil and subsequent
storm runoff are highly dependent on the degree
of saturation in the soil. Almost all major flood
events are heavily influenced by the amount of
