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The construction of buildings, dams and other manmade structures can affect
a river’s flood, according to Nicholas Pinter, a geologist at Southern
Illinois University. But, he says, that link is not universally accepted. Presenting
a new technique at the Geological Society of America’s North-Central Section
and Southeastern Section Joint Meeting in Lexington, Ky., last month, Pinter
reported that by ignoring the effects of human construction on flood hazards,
engineers and flood managers have underestimated flood risks.
Pinter and one of his students, Reuben Heine, isolated the effects of engineering
efforts at the Mississippi River and Missouri River. Their technique holds discharge
constant to analyze flood levels over time. “What we’re trying to
do is to nail down, to quantify, what some of the impacts of human actions within
the drainage system have been on the flooding,” Pinter says. They found
that the same quantities of water at two different points in time have created
dramatically different flood magnitudes, with, year-to-year increases in flooding.
In 1882, for example, 712,000 cubic feet per second flooded St. Louis, Mo.,
at 31.5 feet, “barely popping out over the banks.” Pinter says. However,
during a 1993 flood, the same water flow created a much larger flood, at a level
of 40 feet. Pinter says this 8.5-foot difference is because the river now conveys
the water less efficiently, allowing it to flood over. “The story is that
it is navigation structures, primarily wing dams, that are acting to decelerate
flow.”
Pinter says his team translated these revised flood levels into changes in the
flood statistics. Still looking at St. Louis, the current official level for
the 100-year flood is 47.1 feet. Pinter’s research indicates that this
level should be at least 4 feet higher. At the same time, as part of a $14 million-dollar
effort to reevaluate flood numbers for the upper Mississippi, Missouri and Illinois
rivers, the Army Corps of Engineers has issued preliminary results that suggest
lowering the level by half a foot. Pinter says that these traditional methods
“do not recognize long term changes over time in the river system.”
That, he says, needs to change. In his study, Pinter stresses the need for updated
flood probabilities to create a new generation of U.S. floodplain maps.
Lisa M. Pinsker
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