Geoscience Reference
In-Depth Information
these large rainfall amounts over this shorter time
span. The line of best fit probably represents a physical
limit to the rate of condensation and droplet formation
possible under the Earth's present temperature and
pressure regime. The amounts of rainfall possible in
less than one day are staggering. Within an hour,
0.42 m of rain is possible, and within a day, 1.91 m of
rain can fall. The latter value represents more than the
average yearly rainfall of any Australian capital city.
The large amounts of rainfall at Cherrapunji, India
(or, for that matter, at Tully, Queensland), while excep-
tional, probably do not disturb the environment nearly
as much as these intense but short falls. Over long time
periods, locations normally receiving heavy rainfall
have evolved dense vegetation that can absorb the
impact of falling rain, and have developed drainage
patterns that can handle the expected runoff. Most
extreme short-term rainfalls occur in places where veg-
etation is sparser, and where drainage systems may not
be best adjusted to contain large volumes of runoff.
Here, dislodgement of topsoil by raindrop impact
can also be high. Sheet flow occurs within very short
distances of drainage divides and overland channel
flow will develop within several meters downslope. As
a result, sediment erosion and transport is high and
rapid. For these reasons, flash flooding in arid and
semi-arid regions can become especially severe.
In urban areas, where much of the ground is made
impervious by roads or buildings and where drainage
channels are fixed in location, flash flooding becomes
more likely with much lower amounts of rainfall than
indicated in the above graph. Since 1970, flash flooding
in semi-arid or urban catchments appears to be
increasing worldwide, including in the United States
and Australia.
10-1000 times larger. This is particularly so where
channels are narrow, deep and steep. Stream power
per unit area of a channel is defined by the following
equation:
=
v
(6.2)
where
= power per unit boundary area
v
= velocity
= boundary shear stress
=
R S
where
= specific weight of the fluid
(9800 N m
-3
)
S
= the energy slope of the flow
R
= the hydraulic radius of the
channel
=
A
(2
d
+
w
)
-1
where
A
= the cross-sectional area of the
wetted channel
d
= the mean water depth
w
= the width of the channel
The parameters in Equation 6.2 are not that difficult to
measure, because many rivers and streams have
gauging stations that measure velocity and water depth
during floods. Once the height of a flood has been
determined, it is relatively simple to calculate the
hydraulic radius. The slope of a channel is normally
used in place of the energy slope; however, this may
increase errors in discharge by 100 per cent during
catastrophic flash floods. Problems may also arise in
determining the specific weight of the fluid. The value
quoted above is standard for clear water. During
floods, however, waters can contain high concen-
trations of suspended material that can double the
specific weight of floodwater.
Figure 6.3 plots stream power against drainage basin
area for small flash floods, mainly in the United States,
and for the largest floods measured in recent times.
Some of these floods, such as the Teton River flood
in Idaho on 5 June 1976, can be associated with the
collapse of dams following heavy rains. The collapse of
a dam can greatly increase the magnitude of stream
power, because of its effect on hydraulic radius and the
energy slope of the flow. In fact, dam collapses have led
to the largest flash flood death tolls. For example, the
Johnstown, Pennsylvania, flood of 31 May 1889 killed
over 2200 people. The failure of dams during heavy
rains can be attributed to neglect, inadequate design
for
Floo
d power
(Baker & Costa, 1987)
The amount of work that a flood can perform, and the
destruction caused by a flash flood, are not necessarily
due to the high amounts of rainfall described above.
Nor are high flood discharges a prerequisite for
erosion. Rather, it is the amount of
shear stress
, and the
stream power - the amount of energy developed per
unit time along the boundary of a channel - that is
more significant. It is because of stream power that
floods in small drainage basins, as small as 10-50 km
2
,
can be more destructive than major floods on the
Mississippi or Amazon rivers, with discharges
high-magnitude events, geological location (for
