There are several kinds of floods, including those
associated with hurricanes and tidal surges in coastal areas,
those caused by rare large thunderstorms in mountains and
canyon territory, and those caused by prolonged rains over
large drainage basins.
Flash floods result from short periods of heavy rainfall
and are common near warm oceans, along steep mountain
fronts that are in the path of moist winds, and in areas prone
to thunderstorms. They are well-known in the mountain and
canyon lands of the southwest desert in the United States
and many other parts of the world. Some of the heaviest rainfalls
in the United States have occurred along the Balcones
escarpment in southeastern Texas. Atmospheric instability in
this area often forms along the boundary between dry desert
air masses to the northwest and warm moist air masses rising
up the escarpment from the Gulf of Mexico to the south and
east. Up to 20 inches (50 cm) of rain have fallen along the
Balcones escarpment in as little as three hours from this
weather situation. The Balcones escarpment also seems to
trap tropical hurricane rains, such as those from Hurricane
Alice, which dumped more than 40 inches (100 cm) of rain
on the escarpment in 1954. The resulting flood waters were
65 feet (20 m) deep, one of the largest floods ever recorded in
Texas. Approximately 25 percent of the catastrophic flash
flooding events in the United States have occurred along the
Balcones escarpment. On a slightly longer time scale, tropical
hurricanes, cyclones, and monsoonal rains may dump several
feet of rain over periods of a few days to a few weeks, resulting
in fast (but not quite flash) flooding.
Flash floods typically occur in localized areas where
mountains cause atmospheric upwelling leading to the development
of huge convective thunderstorms that can pour several
inches of rain per hour onto a mountainous terrain,
which focuses the water into steep walled canyons. The
result can be frightening, with flood waters raging down
canyons as steep, thundering walls of water that crash into
and wash away all in their paths. Flash floods can severely
erode the landscape in arid and sparsely vegetated regions
but do much less to change the landscape in more humid,
heavily vegetated areas.
Many of the canyons in mountainous regions have fairly
large parts of their drainage basins upriver. Sometimes the
storm that produces a flash flood with a wall of water may
be located so far away that people in the canyon do not even
know that it is raining somewhere, or that they are in immediate
and grave danger.
The severity of a flash flood is determined by a number
of factors other than the amount of rainfall. The shape of the
drainage basin is important, because it determines how
quickly rainfall from different parts of the basin converge at
specific points. The soil moisture and previous rain history
are also important, as are the amounts of vegetation, urbanization,
and slope.
The national record for the highest, single-day rainfall is
held by the south Texas region, when Hurricane Claudette
dumped 43 inches (110 cm) of rain on the Houston area in
1979. The region was hit again by devastating floods during
June 8–10, 2001, when an early-season tropical storm suddenly
grew off the coast of Galveston and dumped 28–35
inches (70–89 cm) of rain on Houston and surrounding
regions. The floods were among the worst in Houston’s history,
leaving 17,000 people homeless and 22 dead. More than
30,000 laboratory animals died in local hospital and research
labs, and the many university and hospital research labs
experienced hundreds of millions of dollars in damage. Fifty
million dollars were set aside to buy out the properties of
homeowners who had built on particularly hazardous flood
plains. Total damages have exceeded $5 billion. The standing
water left behind by the floods became breeding grounds for
disease-bearing mosquitoes, and the humidity led to a dra
The Mississippi River is the largest river basin in the United States
and the third largest river basin in the world. The river basin is the
site of frequent floods that can be devastating because of the millions
of people that live there. All of the 11 major tributaries of the
Mississippi River have also experienced major floods, including
events that have at least quadrupled the normal river discharge in
1883, 1892, 1903, 1909, 1927, 1973, and 1993.
Floods along the Mississippi River in the 1700s and 1800s
prompted the formation of the Mississippi River Commission,
which oversaw the construction of high levees along much of the
length of the river from New Orleans to Iowa. These levees were
designed to hold the river in its banks by increasing their natural
heights. By the year 1926 more than 1,800 miles (2,900 km) of levees
had been constructed, many of them more than 20 feet (6 m)
tall. The levees gave people a false sense of security against the
floodwaters of the mighty Mississippi and restricted the channel,
causing floods to rise more quickly and forcing the water to flow
faster.
Many weeks of rain in the late fall of 1926 followed by high
winter snow melts in the upper Mississippi River basin caused the
river to rise to alarming heights by the spring of 1927. Residents all
along the Mississippi were worried, and they began strengthening
and heightening the levees and dikes along the river, in the hopes
of averting disaster. The crest of water was moving through the
upper Midwest and had reached central Mississippi, and the rains
continued. In April levees began collapsing along the river sending
torrents of water over thousands of acres of farmland, destroying
homes and livestock and leaving 50,000 people homeless.
One of the worst-hit areas was Washington County, Mississippi,
where an intense late April storm dumped an incredible 15
inches (38 cm) of rain in 18 hours, causing additional levees along
the river to collapse. One of the most notable was the collapse of
the Mounds Landing levee, which caused a 10-foot deep lobe of
water to cover the Washington County town of Greenville on April
22. The river reached 50 miles (80.5 km) in width and flooded
approximately one million acres, washing away an estimated 2,200
buildings in Washington County alone. Hundreds of people perished
while they were trying to keep the levees from collapsing and
were washed away in the deluge. The floodwaters remained high
for more than two months, and people were forced to leave the
area (if they could afford to) or live in refugee camps on the levees,
which were crowded and unsanitary. An estimated 1,000 people
perished in the floods of 1927, some from the initial flood and more
from famine and disease in the unsanitary conditions in the months
that followed.
Once again, in 1972 the waters began rising along the Mississippi,
with most tributaries and reservoirs filled by the end of the
summer. The rains continued through the winter of 1972–73, and the
snowpack thickened over the northern part of the Mississippi
basin. The combined snowmelts and continued rains caused the
river to reach flood levels at St. Louis in early March, before the
snow had even finished melting. Heavy rain continued throughout
the Mississippi basin, and the river continued to rise through April
and May, spilling into fields and low-lying areas. The Mississippi
was so high that it rose to more than 50 feet (15 m) above its average
levels for much of the lower river basin, and these river heights
caused many of the smaller tributaries to back up until they too
were at this height. The floodwaters rose to levels not seen for 200
years. At Baton Rouge, the river nearly broke through its banks and
established a new course to the Gulf of Mexico, which would have
left New Orleans without a river. The floodwaters began peaking in
late April, causing 30,000 people to be evacuated in St. Louis by
April 28, and close to 70,000 people were evacuated throughout the
region. The river remained at record heights throughout the lower
drainage basin through late June. Damage estimates exceeded
$750 million (1973 dollars).
In the late summer of 1993 the Mississippi River and its tributaries
in the upper basin rose to levels not seen in more than 130
years. The discharge at St. Louis was measured at more than one
million cubic feet per second. The weather situation that led to
these floods was remarkably similar to that of the floods of 1927
and 1973, only worse. High winter snowmelts were followed by
heavy summer rainfalls caused by a low-pressure trough that
stalled over the Midwest because it was blocked by a stationary
high-pressure ridge that formed over the East Coast of the United
States. The low-pressure system drew moist air from the Gulf of
Mexico that met the cold air from the eastern high-pressure ridge,
initiating heavy rains for much of the summer. The rivers continued
to rise until August, when they reached unprecedented flood
heights. The discharge of the Mississippi was the highest recorded,
and the height of the water was even greater because all the levees
that had been built restricted the water from spreading laterally
and thus caused the water to rise more rapidly than it would have
without the levees in place. More than two-thirds of all the levees
in the Upper Mississippi River basin were breached, overtopped, or
damaged by the floods of 1993. Forty-eight people died in the 1993
floods, and 50,000 homes were damaged or destroyed. Total damage
costs are estimated at more than $20 billion.
The examples of the floods of 1927 and 1993 on the Mississippi
reveal the dangers of building extensive levee systems along
rivers. Levees adversely affect the natural processes of the river
and may actually make floods worse. The first effect they have is to
confine the river to a narrow channel, causing the water to rise
faster than if it were able to spread across its floodplain. Additionally,
since the water can no longer flow across the floodplain it
cannot seep into the ground as effectively, and a large amount of
water that would normally be absorbed by the ground now must
flow through the confined river channel. The floods are therefore
larger because of the levees. A third hazard of levees is associated
with their failure. When a levee breaks, it does so with the force of
hundreds or thousands of acres of elevated river water pushing it
from behind. The force of the water that broke through the Mounds
Landing Levee in the 1927 flood is estimated to be equivalent to the
force of water flowing over Niagara Falls. If the levees were not in
place, the water would have risen gradually and would have been
much less catastrophic when it eventually came into the farmlands
and towns along the Mississippi River basin.
matic increase in the release of mold spores that cause allergies
in some people, and some of which are toxic.
The Cherrapunji region in southern India at the base of
the Himalayan Mountains has received the world’s highest
rainfalls. Moist air masses from the Bay of Bengal move
toward Cherrapunji, where they begin to rise over the high
Himalayan Mountains. This produces a strong orographic
effect, where the air mass can not hold as much moisture as it
rises and cools, so heavy rains result. Cherrapunji has received
as much as 30 feet (9 m) of rain in a single month (July 1861)
and more than 75 feet (23 m) of rain for all of 1861.
A final type of flood occurs in areas where rivers freeze
over. The annual spring breakup can cause severe floods, initiated
when blocks of ice get jammed behind islands, bridges,
or along bends in rivers. These ice dams can create severe
floods, causing the high spring waters to rise quickly, bringing
the ice-cold waters into low-lying villages. When ice dams
break up, the force of the rapidly moving ice is sometimes
enough to cause severe damage, knocking out bridges, roads,
and homes. Ice-dam floods are fairly common in parts of New
England, including New Hampshire, Vermont, and Maine.
See also DRAINAGE BASIN; RIVER SYSTEM; URBANIZATION
AND FLASH FLOODING.
Tidak ada komentar:
Posting Komentar
Catatan: Hanya anggota dari blog ini yang dapat mengirim komentar.