Geoscience Reference
In-Depth Information
Flooding as a
Hazard
CHAPTER 6
INTRODUCTION
FLASH FLOODS
The earlier chapter on large-scale storms did not
consider in detail the effects of flooding associated with
tropical cyclones, extra-tropical storms or east-coast
lows. Nor was it appropriate to discuss large-scale
flooding in that chapter because some of the worst
regional flooding in the northern hemisphere has
occurred in spring in association with snowmelt. In
addition, rainfall associated with thunderstorms is a
major cause of flash flooding. This chapter examines
flash flooding events and regional floods. Flash flooding
refers to intense falls of rain in a relatively short period
of time. Usually the spatial effect is localized. Modifica-
tion of the overall landscape during a single event is
minor in most vegetated landscapes; however, in arid,
semi-arid, cultivated (where a high portion of the land
is fallow) or urban areas, such events can be a major
cause of erosion and damage. Steep drainage basins
are also particularly prone to modification by flash
floods because of their potential to generate the highest
maximum probable rainfalls. Large-scale regional
flooding represents the response of a major continental
drainage basin to high-magnitude, low-frequency
events. This regional flooding has been responsible
historically for some of the largest death tolls attribut-
able to any hazard. When river systems alter course as a
response to flooding, then disruption to transport and
agriculture can occur over a large area.
Magnitude and frequency of heavy
rain
fall
(Griffiths, 1976)
Short periods of heavy rainfall occur as a result of very
unstable air with a high humidity. Such conditions
usually occur near warm oceans, near steep, high
mountains in the path of moist winds, or in areas sus-
ceptible to thunderstorms. A selection of the heaviest
rainfalls for given periods of time is compiled in
Figure 6.1. Generally, extreme rainfall within a few
minutes or hours has occurred during thunderstorms.
Events with extreme rainfall over several hours, but less
than a day, represent a transition from thunderstorm-
derived rain to conditions of extreme atmospheric
instability. The latter may involve several thunderstorm
events. In the United States, the Balcones Escarpment
region of Texas (see Figure 6.2 for the location of
major placenames mentioned in this chapter) has expe-
rienced some of the heaviest short-duration rainfalls
in this category. Extreme rainfall over several days
is usually associated with tropical cyclones, while
extreme rainfall lasting several weeks to months usually
occurs in areas subject to seasonal monsoonal rainfall,
or conditions where orographic uplift persists. In the
latter category, Cherrapunji in India, at the base of
the Himalayas, dominates the data (9.3 m rainfall in
