Environmental Engineering Reference
In-Depth Information
without letup; rainfall totals in the upper Mississippi River basin were
60 percent higher than average.
4
Added to this was the fact that the
previous year was cooler than average, so the ground was still partly
saturated and unable to absorb much of the new rainfall. Over 500 river
hydrologic stations in the Midwest were above fl ood stage at the same
time. At St. Louis, the river crested at more than 19 feet above fl ood
stage and remained above fl ood stage at St. Louis for more than two
months.
Clearly, there is no way that humans could have stopped the unending
rainfall in the spring and summer of 1993. Technology could not have
made 1992 warmer so the soil could have absorbed more water, and
technology cannot control either the location of the jet stream or the
normal northward movement of wet Gulf air.
It is noteworthy that the National Climatic Data Center has docu-
mented a 10 percent increase in precipitation and a 20 percent increase
in extreme precipitation events (more than 2 inches of rainfall in 24
hours) in the United States since 1900. Since 1958, the number of days
with very heavy precipitation have increased 58 percent in the Northeast,
27 percent in the Upper Midwest, 18 percent in the Southeast, 16
percent in the Southwest, 13 percent in the band from the Dakotas to
Texas, and 12 percent in the Pacifi c Northwest.
5
These increases can
be attributed to climatic change, an indication that normal fl oods
(those not related to a stagnant jet stream) are likely to become more
common.
In populous New York, portions of Lower Manhattan and coastal
areas of Brooklyn, Queens, Staten Island, and Nassau County on Long
Island will be fl ooded more often. Much of the city's critical infrastruc-
ture, including tunnels, subways, and airports, lies well within the range
of projected storm surge and would be fl ooded during such events.
Attempts at Containment
The chief type of structure along the river that might contain the fl ood-
waters is the levee. The U.S. Army Corps of Engineers over the years has
built 3,500 miles of levees (embankments) extending 1,000 miles along
the Mississippi and its tributaries in an attempt to prevent the river from
spilling onto the fl oodplain it had built over the centuries.
6
During the
1993 fl ood, over 1,000 of the levees failed despite continual efforts by
people in affected areas to increase levee heights, which average 24 feet,
by adding sandbags.
