Nicholas Pinter, a geologist and the associate director of the Center for Watershed Sciences at the University of California, Davis, explains the problem with levees: "What you're doing in many cases is taking a flood plain out there — it can be 5 [or] 6 miles wide — and you're forcing the water that would otherwise spread across that area to go through a narrow passageway." The water flows higher and faster as the passageway gets narrower, Hersher reports.
The idea that levees could cause worse problems than they prevent isn't new; in 1852 an engineer's report to the federal government warned that levees on the Mississippi made the waters higher and faster. But after an epic flood in 1927, Congress required the Army Corps of Engineers to build a system of levees and dams on the Lower Mississippi, Hersher reports. A recent report found that more severe and frequent flooding over the past 150 years correlated with levee-building.
Residents near levees can face disastrous consequences if a flood breaches a levee, since they're less likely to have enough flood insurance. The federal government doesn't require homeowners living behind levees to carry such insurance. Likewise, farmers living behind levees are less likely to adequately insure their fields.