Environmental Engineering Reference
In-Depth Information
of years because the data on which predictions are made are inadequate.
The records do not match the pattern today in terms of the weather.
What have been called 100-year fl oods may now be 10-year fl oods due
to a combination of changing conditions over time. The fl oodplain maps
issued by the Federal Emergency Management Agency (FEMA) badly
need updating.
Nature is usually more powerful than humankind's feeble efforts to
control it. As Mark Twain wrote 125 years ago, “You can plan or not
plan and it doesn't make a hell of a lot of difference. What makes a dif-
ference is how much it decides to rain.”
3
On average, fl oods kill about
140 people each year and cause $6 billion in property damage. Loss of
life to fl oods during the past half-century has declined, mostly because
of improved warning systems, but economic losses have continued to rise
due to increased urbanization and coastal development.
Floods are expensive calamities that affect all Americans through their
taxes. The federal government uses tax money to subsidize reconstruc-
tion after fl oods, so fl oods affect the wallets of all Americans, whether
they live in fl ood-prone areas or not.
River Floods Are Terrible
Floods can be conveniently divided into two varieties: those that origi-
nate on land (excess precipitation) and those that originate at sea (hur-
ricanes). The fl ooding of the Midwest in the summer of 1993 was the
most devastating river fl ood in U.S. history. More than 50,000 homes
were damaged or destroyed in nine states in the Upper Midwest, 74,000
people were displaced, about 50 people died, over 12 million acres of
farmland (19,000 square miles) were inundated and rendered useless,
and hundreds of thousands of people were without safe drinking water
for many weeks. Overall damages totaled $20 billion. The Mississippi
River was closed to traffi c for two months.
The cause of the deluge of water from the atmosphere was a confl u-
ence of atmospheric conditions that occurs occasionally and unpredict-
ably. During the summer of 1993, the jet stream that moves from west
to east stalled over the Midwest and formed a barrier to moisture-laden
air moving northward from the Gulf of Mexico. The moisture could not
continue its northward movement and so dumped its water repeatedly
on the hapless people below. Wave after wave of storms rumbled across
the river basin. From April through August, the deluge continued almost
