 Now, in 2008, even greater flooding has occurred.
Record rains and flash flooding have occurred month after month
since the start of the year. Measured annual, seasonal, monthly and
weekly rainfalls have broken half-millennium records.
In June, state climatologists documented that Illinois rainfall
amounts for 2008 had broken an 80-year record with 22.2 inches, 7
inches above the average, measured from January to May.
Locally, farmers have held their breath as their crops have gone
under water again and again. Residents groaned when their basements
turned into swimming pools multiple times. A quarter-mile from
rivers and streams, water has repeatedly lapped at patio doors.
Emergency responders have gone on standby numerous times and
rescued a few stranded homeowners and vehicle occupants. A teen lost
his life in the flooded Salt Creek in May.
Regionally, entire towns found their buildings knee-deep in
waters. Economies ground to a halt as the waters infiltrated, soaked
and washed away Midwest homes and businesses, leaving behind only
debris and despair.
Flooding on the great rivers bordering Illinois to the west and
south has been documented as exceeding the flood of '93.
Could it be another 500-year flood, 485 years sooner than
expected?
The official answer, yes, has everyone shaking their heads as to
what is going on.
Understanding begins with the defining the terms "100-year flood"
and "500-year flood." Neither term means a specific timetable.
Neither term is intended to be used as a rain train due to stop
every "X" number of years in our community. In applying the terms,
these floods would be seen on average 100 or 500 years apart if a
person lived several thousand years or so.
What this means as applied to this year's flooding is that we
current humans just were unlucky enough to fall into an aberration
in the mathematical scheme of things.
The U.S. Geological Survey offers further explanation:
Large floods are
often given designations as a "one-hundred-year flood" but a
100-year flood does not mean that such a flood occurs once every 100
years; instead it means that there is a one in one-hundred (or 1%)
chance of such a flood occurring in a given year. Two 100-year
floods could occur a year apart or even a month apart -- it all
depends on how much rain is falling or how quickly the snow melts. A
"20-year flood" has a one in twenty (or 5%) chance of occurring in a
particular year so it would be a less destructive flood than a
100-year flood while a "500-year flood" has a one in 500 (0.2%)
chance of occurring so it would be much more catastrophic than a
100-year flood.
It also is explained that great floods often are encompassed by a
time frame of above-ordinary rainfalls -- rainfalls that not only
caused the flood, but prepared an area for flooding through ground
saturation and then kept an area flood-prone for a protracted period
of time after.
The USGS also explains a little further, "Midwesterners who
experienced the Great Flood of 1993 -- said to be a 500-year flood
at the time -- can hardly be faulted for thinking they were off the
hook for seeing that designation again for, say, a few hundred
years."
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"The term ‘500-year flood' can be a little misleading," said Robert
Holmes, USGS national flood specialist. "We hydrologists realize the
term has instant public recognition and we use it to point to the
extraordinary nature of such floods. However, the occurrence of a
500-year flood doesn't depend on what happened last year or 15 years
ago or 100 years ago. It's based on the annual likelihood of the
degree of flooding -- in other words, the odds. A 500-year flood
magnitude has a 0.2 percent -- 1 in 500 -- of being exceeded in any
one year. A 100-year flood magnitude has a 1 percent chance -- 1 in
100 -- of being exceeded in the same place, and so on. To come up
with these statistics, we take our annual peak flow values from our
network of USGS streamgages and feed that data into highly developed
probability models."
"Streamgages"
provide long-term stream flow data that scientists need to better
understand floods and define flood-prone areas.
[To download Adobe Acrobat Reader for
the PDF file, click
here.]
"This is why we make every effort to keep our streamgages active
for long periods of time," Holmes continued. "The better the
historical record you have, the deeper the background you'll have to
develop more accurate probability predictions for floods. A
longer-term record provides a better estimate of the probability of
a certain size flood occurring. Changes due to land use and climate,
for example, could affect the accuracy of those estimates."
"Since we constantly refine our models and revise our statistics
based on the observed record," Holmes said, "this year's data in the
affected region is likely to raise the value of the flow rate
required to designate a 500-year flood. That means it's possible
that the current flood could lose its standing as a 500-year flood
as the data is further analyzed."
USGS plays a critical role in reducing flood losses by operating
a nationwide streamgage network that monitors the water level and
flow of the nation's rivers and streams. USGS streamgage data are
used by the National Weather Service to develop flood forecasts, by
the U.S. Army Corps of Engineers to manage flood control, and by
state and local agencies in their flood response activities.
If there is one thing to be concluded about this summer, it is
that the odds are now stacked against us seeing another great flood
in our lifetimes. Of course that doesn't mean we won't get socked by
heavy rains again. It doesn't even mean we won't see another great
flood this summer. It just means that the odds are on our side. Of
course they were this year too, but that didn't help.
[U.S.
Geological Survey; LDN]
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