The construction of buildings, dams and other manmade structures can affect
a rivers 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 Americas 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 were 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. Pinters 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