Geoscience Reference
In-Depth Information
1000 W m
-2
are sufficient to move boulders 1.5 m in
diameter. Many flash floods can reach this capacity. If
cavitation levels are reached, bedrock channels can be
quickly excavated. The Missoula floods, in the western
United States, were able to pluck
basaltic
columns
of bedrock, each weighing several tonnes, from
streambeds, and carve canyons through bedrock in a
matter of days. More importantly, as will be discussed
below, present-day flood events are capable of
completely eroding, in a matter of days, floodplains
that may have taken centuries to form.
once floodwaters deepen a small amount, the flow will
then revert to being
subcritical
. Finally, Figure 6.4
plots the region where
cavitation
occurs. Cavitation is
a process whereby water velocities over a rigid surface
are so high that the vapor pressure of water is
exceeded, and bubbles begin to form at the contact. A
return to lower velocities will cause these bubbles to
collapse with a force in excess of 30 000 times normal
atmospheric pressure. Next to impact cratering, cavi-
tating flow is the most erosive process known on Earth
and, when sustained, is capable of eroding bedrock.
Fortunately, such flows are also rare. They can be wit-
nessed on dam spillways during large floods as the
mass of white water that develops against the concrete
surface towards the bottom of the spillway. Note that
the phenomenon differs from the white water caused
by air entrainment due to
turbulence
under normal
flow conditions at the base of waterfalls or other drop
structures. Cavitation is of major concern on spillways
because it will shorten the life span of any dam, and
may be a major process in the back-cutting of water-
falls into bedrock. Only a few isolated flash floods in
channels have generated cavitation. However, some of
the Missoula floods reached cavitation levels over long
distances.
Large flood events have enormous potential to
modify the landscape. The stream powers of cata-
clysmic floods, as well as many other common flash
floods in the United States, are substantial enough to
transport sand-sized material in wash load, suspended
gravel 10-30 cm in size, and boulders several metres in
diameter as bedload. This material can also be moved
in copious quantities. Even stream powers of
Synoptic patterns favoring flash
floo
ding
(Hirschboeck, 1987)
The severity of catastrophic flooding is ultimately
determined by basin physiography, local thunderstorm
movement, past soil moisture conditions, and degree
of vegetation clearing. However, most floods originate
in anomalous large-scale atmospheric circulations that
can be grouped into four categories as follows: (i)
aseasonal occurrence or anomalous location of a defin-
able weather pattern; (ii) a rare concurrence of several
commonly experienced meteorological processes; (iii)
a rare upper atmospheric pattern; or (iv) prolonged
persistence in space and time of a general meteoro-
logical pattern. The degree of abnormality of weather
patterns is very much a function of three types of
pressure and wind patterns that are often part of the
general atmospheric circulation. These patterns are
shown in Figure 6.5. Meridional air flow represents
north-south air movement and is usually associated
with Rossby wave formation linked to the meandering
100
50
Missoula floods
Pecos R., Texas
20
10
Cavitation
No cavitation
Yangze R.
Katherine Gorge
5
Mississippi R.
2
Amazon R.
1
0.1
1
10
100
1000
Mean depth (m)
Channel depth versus current velocity for various floods plotted together with boundaries for cavitation and supercritical flow (adapted from
Baker & Costa, 1987).
Fig. 6.4
