Geoscience Reference
In-Depth Information
Figure 16.32 Stream hydrograph before
and after urbanization.
The response of
the urbanized stream is much faster and
greater than during pre-urban conditions.
Lag time before
urbanization
Storm peak
Lag time after
urbanization
Stream flow
after
urbanization
Discharge
Stream flow
before
urbanization
Stream
base flow
Precipitation
event (storm)
Time
type of storm event occurs in both places. In contrast to river
floods, which are slow-evolving events such as the 1993 Mis-
sissippi basin floods that lasted for weeks, flash floods are brief
but intense and can result in both significant property damage
and loss of life. Although the scene in Figure 16.33 suggests
otherwise, it is not a good idea to drive across a flooded road-
way, even if the water appears to be shallow. After all, many
automobiles become buoyant and can be carried away in only
about 0.6 m (2 ft) of water. Of all the human deaths attributed
to flooding, approximately half occur when people are in their
vehicles.
Artificial Levees
Given the increasing number of people living on floodplains
around the world, big river floods and flash floods can be eco-
nomically disastrous. As noted previously, for example, the
1993 floods in the Mississippi basin caused $30 billion in dam-
age. In the context of this kind of cost, humans have engineered
a variety of control structures to mitigate the effects of flooding.
One kind of flood-control structure is an artificial levee.
Recall that a natural levee is a small ridge that forms along
the stream channel where coarse sediments accumulate during
a flood. This ridge increases the height of the stream bank so
that the next flood must have a greater discharge, and thus be
deeper, in order to reach flood stage. An
artificial levee
is based
on the same principle as a natural levee, with the difference be-
ing that an artificial levee is designed by engineers and is usually
built to a height such that a given city or valuable farmland is
protected from a major event such as a 50-year or 100-year flood
(Figure 16.34). In the United States, most of the large artificial
levees along major rivers such as the Mississippi, Ohio, and Mis-
souri are built and maintained by the United States Army Corps
of Engineers. For example, in the Memphis district of the Corps
of Engineers, levees are built to a height of about 7.6 m (25 ft)
above the natural elevation of the Mississippi River bank.
Although levees certainly provide a societal benefit with
respect to flood protection, they also have negative side effects.
One problem with levees is that they keep the river from re-
plenishing the sensitive wetland ecosystem that borders most
rivers. This ecosystem is intimately related to the frequent but
Figure 16.33 An example of flash flooding.
A flash flood is a
brief but intense flood that occurs more frequently in urban areas
where stream response is rapid. About half of flood-related fatali-
ties occur when people try to drive across flooded roadways. This
Artificial levee
An engineered structure along a river that
effectively raises the height of the river bank and thus confines
