At least 19 levees in Iowa, Illinois and Missouri have failed over the past week after heavy rains, according to the U.S. Army Corps of Engineers—and workers swarmed to shore up others with sandbags to prevent rising waters in the Mississippi and its tributaries from overflowing and washing away surrounding communities, farmland and soil. Engineers blame overdevelopment—and potentially poor maintenance of the levees—for the flooding.
"Whenever you have development, you are going to increase the runoff, increase how much the rivers and streams have to carry," says civil engineer W. Gene Corley, a senior vice president at CTLGroup, an engineering firm that evaluates infrastructure, such as levees. "The other side of that is that if you don't have development, you don't have housing for people or business or manufacturing."
In addition, by paving over previously open space—or farming previously reserved lands—communities in these watersheds contribute to record high waters through increased runoff.
This extra runoff is why waters in Cedar Rapids, Iowa, for example, rose to a record 30-plus feet (over nine meters). "The amount of development that has happened in communities up and down the river way creates less opportunity for water to be absorbed back into the earth as opposed to just running off," notes structural engineer Jeffrey Garrett, president and CEO of CTLGroup. "Suddenly, you've got a lot more water that has to flow between the levees."
Efforts to carve a controlled channel for the Missouri, Mississippi and their tributary rivers also contributed to the problem. The reason: the Mississippi now flows through a conduit roughly half the width of its channel that existed before most of the levees were built, which means more water passes faster through a narrower area. "Channeling the river means you are going to have higher crests in the river," Corley notes, that are sometimes higher than the levees built previously to contain them.
Ultimately, levee breaches in places like Gulfport, Ill., help preserve communities farther downriver—by siphoning off some of the raging waters. (When a levee breaks, some water remains and inundates the land behind the breached embankment. This lowered volume reduces the pressure downstream.)
The majority of levees in the Midwest have held so far during the torrential downpours, although those that failed, Corley says, may also have been poorly maintained. But, he adds, "it's too early to tell" the exact reasons the levees failed.
Failure to build strong enough levees—and fix known flaws in them—was blamed for catastrophic flooding in New Orleans, Mississippi and Alabama after Hurricane Katrina in 2005, which killed more than 1,800 people and caused more than $81 billion in damage.