More Flood Analysis

The Washington Post reports sensibly this morning about how the Mississippi River flood is causing us to rethink how we manage flood-prone river basins. Long overdue. How overdue? After Hurricane Katrina I wrote a long piece pointing out that the hue and cry about climate change in the weeks after Katrina was taking after the wrong environmental factor. A couple sections of that paper seem relevant to the current moment:

The most significant long-term problem is the erosion of the Gulf Coast near the mouth of the Mississippi River. Even if we were to wave a magic wand and make all prospective climate change disappear, the erosion of the Gulf Coast would continue and the vulnerability of New Orleans would increase. As is well known, the Louisiana coastline has been shrinking rapidly for decades. The Louisiana coastline has been losing an estimated thirty-four square miles of land a year for the last fifty years; the U.S. Geological Survey estimates that Katrina erased thirty square miles of coastal land.[16] Over the last seventy-five years Louisiana has lost a land area the size of Delaware. Last year’s report of the U.S. Commission on Ocean Policy notes that Louisiana accounts for 80 percent of the total annual coastline erosion in the entire United States. A contrast of an older map of Louisiana with a current satellite photo of the actual coastline is jarring.
The erosion of the low-lying coastal marshlands eliminated natural barriers to storm surges and has reduced the river basin’s ability to dilute and filter out pollution naturally, which has contributed to the worsening problem of hypoxia (nitrogen runoff that depletes dissolved oxygen and creates a “dead zone”) in the Gulf of Mexico. It also accelerated subsidence in and around New Orleans, increasing the area’s hurricane risk. The cause of this erosion is not mysterious: the vast system of dams, levees, canals, artificial channels, and flood control projects stretching all the way to the headwaters of the Mississippi River in the upper Midwest has reduced the amount of sediment reaching the mouth of the Mississippi by two-thirds. In addition, the channelization of the Mississippi around New Orleans and its mouth into the Gulf of Mexico ensures that what sediment still flows ends up mostly out in the gulf, where it disappears beyond the continental shelf. For thousands of years the Mississippi River’s mouth could be likened to a loose garden hose, changing its course rapidly and dispersing sediment widely. Restoring the region’s ecosystem requires finding ways to mimic this long-term dynamic process. . .
A long-term strategy for restoring the natural balance of the Mississippi River basin and delta will need to consider nonstructural alternatives to the problem along the entire course of the river, and not just in and around New Orleans. Rather than building and reinforcing levees, some low-lying areas along the entire course of the Mississippi should be allowed to become functional flood plains again. Low value lands could be purchased, and private landowners could be indemnified from periodic flood damage. This is likely to be cheaper than building and maintaining levees and relying on sediment pipelines. Natural flood plains also offer the benefit of absorbing and neutralizing nitrogen runoff, and hence contributing to the solution of the hypoxia problem.

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