Geology Reference
In-Depth Information
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People have tried to control floods for thousands
of years. Common practices are to construct dams that
impound reservoirs and to build levees along stream banks
(
What Would You Do
Figure 12.17a, b). Levees raise the banks of a stream,
thereby restricting flow during floods. Unfortunately,
deposition within the channel raises the streambed, making
the levees useless unless they, too, are raised. Levees along
the banks of the Huang He in China caused the streambed
to rise more than 20 m above its surrounding fl oodplain in
4000 years. When the Huang He breached its levees in 1887,
more than 1 million people were killed. Sacramento, Cali-
fornia, lying at the junction of two rivers, is among the most
fl ood-prone cities in the United States. Some of the levees
that protect the city are 150 years old and in poor condition;
the cost of repairing them has risen to as much as $250,000
per 100 m.
Dams and levees alone are insuffi cient to control large
fl oods, so fl oodways are also used in many areas. A fl ood-
way is a channel constructed to divert part of the excess
water in a stream around populated areas or areas of eco-
nomic importance (Figure 12.17c). Some communities
build floodwalls to protect them from floods. Floodwalls
have gates that permit access to the waterway, but can be
closed when the water rises (Figure 12.17d). Reforestation
of cleared land also reduces the potential for fl ooding be-
cause vegetated soil helps prevent runoff by absorbing
more water.
When flood-control projects are well planned and
constructed, they are functional. What many people fail to
realize is that these projects are designed to contain fl oods
of a given size; should larger floods occur, rivers spill
onto floodplains anyway. Furthermore, dams occasion-
ally collapse, and reservoirs eventually fi ll with sediment
unless dredged. In short, fl ood-control projects not only
are initially expensive, but also require constant, costly
maintenance.
The largest part of many states' mineral revenues comes
from sand and gravel, most of which is used in construction.
You become aware of a sand and gravel deposit that you
can acquire for a small investment. How would the proximity
of the deposit to potential markets infl uence your decision?
Assuming the deposit was stream deposited, would it be
important to know whether deposition took place in braided or
meandering channels? How could you tell one from the other?
As for predicting fl oods, the best that can be done is
to monitor streams, evaluate their past behavior, and an-
ticipate floods of a given size in a specified period. Most
people have heard of 10-year fl oods, 20-years fl oods, and so
on, but how are such determinations made? The U.S. Geo-
logic Survey, as well as state agencies, record and analyze
stream behavior through time and anticipate floods of a
specifi ed size. So a 20-year fl ood, for example, is the period
during which a fl ood of a given magnitude can be expected.
It does not mean that the river in question will have a fl ood
of that size every 20 years, only that over a long period of
time, it will average 20 years. Or we can say that the chances
of a 10-year fl ood taking place in any one year are 1 in 10
(1/10). In fact, it is possible that two 10-year fl oods could
take place in successive years, but then not occur again for
several decades.
DRAINAGE SYSTEMS
Thousands of waterways, which are parts of larger drainage
systems, fl ow directly or indirectly into the oceans. The only
exceptions are some rivers and streams that fl ow into desert
basins surrounded by higher areas. But even these are parts
of larger systems consisting of a main channel with all its
Figure 12.16 Alluvial Fans and Their Deposits
a These alluvial fans at the base of the Panamint Range in Death
Valley, California, were deposited where streams discharged from
mountain canyons onto adjacent lowlands.
b Ancient alluvial fan deposits in Montana are made up of
sand and gravel.
 
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