Geoscience Reference
In-Depth Information
the river is said to be at
flood stage
. In humid regions, most
high-order streams are bordered by low, relatively level ter-
rain commonly referred to as the
floodplain
. Although flood-
plain formation will be discussed in greater detail later in this
chapter, it is sufficient here to say that the stream creates this
surface, which stands only a few meters above the channel wa-
ter level at base flow. During the wet season, the combination
of significant surface runoff and groundwater influx can cause
the floodplain to become inundated by excess discharge. In
most humid locations floods are annual to semiannual events
and are part of the natural evolution of river systems. In ad-
dition, they are important in the maintenance of the flora and
fauna that live along a river (which, as you recall from Chapter
10, is called the
riparian zone
).
Occasionally, periods of extremely heavy rains occur in
a watershed, causing the streams to rise to spectacular lev-
els that result in great damage to human property and even
sometimes significant loss of life. Fortunately, these kinds
of floods are rare, with a close statistical relationship be-
tween the
return period
—in other words, the frequency of
the event—and its magnitude. To put it more simply, more
extreme floods happen less frequently than do lower-magni-
tude floods. Hydrologists who monitor flood frequency may
describe a particular large flood as being, for example, a 30-
year or 50-year event; in other words, it is expected statisti-
cally to occur once every 30 or 50 years. They also refer to
the probability of a particular discharge being exceeded by
an even larger one.
To see how flood magnitude, return period, and statisti-
cal probability can be used to predict stream behavior, ex-
amine Figure 16.12, which shows data from the Skykomish
River at Goldbar, Washington. The Skykomish has a base
flow of about 200 m
3
/sec (about 7060 ft
3
/sec), which statis-
tically occurs every year and has a 99% probability of being
equaled or exceeded on an annual basis. According to the
graph, the 10-year peak discharge in this stream is about
1700 m
3
/sec (
Ʈ
60,000 ft
3
/sec), and this flow has a 20%
chance of being equaled or exceeded each year. A more
unlikely event is the 50-year flood, which has a discharge
of approximately 2500 m
3
/sec (about 88,300 ft
3
/sec). This
discharge has only a 2% probability of being equaled or
exceeded on a yearly basis. This is not to say that a 50-year
flood or even a larger one could not happen in successive
years—it could. It is just not very likely from a statistical
perspective.
3000
50-year flood
2500
2000
10-year flood
1500
Mean
annual
flood
1000
500
0
1
2
5
10
20
50
100
Recurrence interval (years)
Figure 16.12 Flood frequency data for the Skykomish
River at Goldbar, Washington.
Each data point is the mea-
sured maximum discharge over a 53-year period.
as a 500-year flood. It caused about $30 billion in damage and
easily
remains the costliest flood in U.S. history. The 1993
flood dominated the national media during June and July of
that year because of the catastrophic impact it had on people
living along the Mississippi River and its tributaries. The
causes of this flood, and associated stream response in the ba-
sin, nicely demonstrate how environmental variables interact
in a holistic way.
The story of the 1993 flood actually begins in the pre-
ceding winter when unusually heavy rain and snow saturated
the soils in the Midwest. Following this wet winter, an un-
usual weather pattern developed during June and July of that
year, one that resulted in the formation of a strong subtropi-
cal high-pressure system that stalled over the southeastern
United States (Figure 16.13). Recall from Chapter 8 that at-
mospheric pressure systems in the midlatitudes rarely stay
in one place but instead typically migrate from west to east
in association with the westerly winds. The year 1993 was
unusual because a high over the southeastern United States
remained stationary for about 2 months. This stalled high
caused two significant things to occur. First, the midlatitude
jet stream persistently flowed over the Midwest because it
was blocked from moving east by the southeastern high. Sec-
ond, water vapor from the Gulf was continuously pumped
into the Midwest by the clockwise rotation of the southeast-
ern high. Given that cool/dry cP air was steadily colliding
with warm/moist mT air along the stalled jet stream in the
upper Midwest, prodigious rains fell during June and July.
1993 Mississippi Basin Floods
Perhaps the best example
of a major flood in the fairly recent past within the United
States is the Mississippi River flood of 1993, which affected
a large part of the upper Midwest. This flood was at least a
100-year event and has been classified by some hydrologists
