Environmental Engineering Reference
In-Depth Information
two-thirds want freight trains to carry an increased share of the nation's
freight. Trucks ranked below air freight as second choice.
Unsafe Dams
Most people who think of dams envision the giant federal dams, which
are less than 5 percent of all dams in the United States: Hoover Dam in
Nevada, Grand Coulee in Washington, or one of the thirty-four dams
along the Tennessee River and its tributaries. But there are 85,000 dams
more than six feet high in the United States, and tens of thousands of
smaller dams. In the 1950s and 1960s, a dam went up in the United
States every six minutes. They were built for a variety of reasons: fl ood
control, hydroelectric power, water supply and irrigation, navigation,
tourism, and recreation in the lakes that form behind the dam. But dams,
like the other infrastructure, require maintenance and replacement as
they age. Although a rock-fi lled dam in Syria built around 1300 B.C.E.
is still in use, dams in general have a much shorter life expectancy, both
because they weaken with age and because they fi ll with sediment carried
into the reservoir behind the dam. Human agriculture and construction
have greatly increased erosion rates, and 13 percent of the river's sedi-
ment load is trapped in reservoirs, signifi cantly reducing their useful life
span.
48
According to the American Society of Civil Engineers, the average
life expectancy of a dam is fi fty years, and the average age of the 85,000
dams in the United States in 2009 was fi fty-one years. By 2020, 85
percent will be older than fi fty years (fi gure 2.4).
49
Antiquated dams become increasingly vulnerable to such events as
seismic shifts from below, water pressures that scour them from behind,
invasive species that choke intake and outfl ow pipes, and the effects of
a fast-changing climate that bring fl ooding on scales for which the dams
were not designed. A study in 2009 by the American Society of Civil
Engineers reported that the number of high-hazard dams—dams whose
failure would cause loss of human life—was 1,819.
50
The cost to repair
them was estimated at more than $10 billion. As downstream land
development increases, so will the number of dams with high hazard
potential. States reported that 3,300 dams were unsafe.
51
A 2003 study
by the Association of Dam Safety Offi cials placed the cost of bringing
U.S. dams into safety compliance and to remove obsolete dams at $36.2
billion.
52
The urgency of the problem was emphasized by two storms in New
England in 2005 and 2007 that caused the overtopping or breaching of
