Rethinking, Then Rebuilding New Orleans

RICHARD E. SPARKS

Rethinking, Then Rebuilding New Orleans

This time around, science should contribute to a systemic long-term plan that will better accommodate the natural forces that shape the Mississippi Delta.

New Orleans will certainly be rebuilt. But looking at the recent flooding as a problem that can be fixed by simply strengthening levees will squander the enormous economic investment required and, worse, put people back in harm’s way. Rather, planners should look to science to guide the rebuilding, and scientists now advise that the most sensible strategy is to work with the forces of nature rather than trying to overpower them. This approach will mean letting the Mississippi River shift most of its flow to a route that the river really wants to take; protecting the highest parts of the city from flooding and hurricane-generated storm surges while retreating from the lowest parts; and building a new port city on higher ground that the Mississippi is already forming through natural processes. The long-term benefits—economically and in terms of human lives—may well be considerable.

To understand the risks that New Orleans faces, three sources need to be considered. They are the Atlantic Ocean, where hurricanes form that eventually batter coastal areas with high winds, heavy rains, and storm surge; the Gulf of Mexico, which provides the water vapor that periodically turns to devastatingly heavy rain over the Mississippi basin; and the Mississippi River, which carries a massive quantity of water from the center of the continent and can be a source of destruction when the water overflows its banks. It also is necessary to understand the geologic region in which the city is located: the Mississippi Delta.

The Mississippi Delta is the roughly triangular plain whose apex is the head of the Atchafalaya River and whose broad curved base is the Gulf coastline. The Atchafalaya is the upstream-most distributary of the Mississippi that discharges to the Gulf of Mexico. The straight-line distance from the apex to the Atchafalaya Bay is about 112 miles, whereas the straight-line distance from the apex to the mouth of the Mississippi is twice as long, about 225 miles. (These distances will prove important.) The Delta includes the large cities of Baton Rouge and New Orleans on the Mississippi River, and smaller communities, such as Morgan City, on the Atchafalaya. (Although residents along the Mississippi River at many places considerably to the north of New Orleans commonly refer to their floodplain lands as “the Delta,” the smaller rivers and streams here empty directly into the Mississippi River, not the Gulf of Mexico, and hence geologists more properly call this region the alluvial plain of the Mississippi River.)

The Mississippi River builds, then abandons, portions (called “lobes”) of the Delta in an orderly cycle: six lobes in the past 8,000 years (Fig. 1). A lobe is built through the process of sediment deposition where the river meets the sea. During seasonal floods, the river spreads over the active lobe, depositing sediment and building the land higher than sea level. But this process cannot continue indefinitely. As the lobe extends further into the sea, the river channel also lengthens. A longer path to the sea means a more gradual slope and a reduced capacity to carry water and sediment. Eventually, the river finds a new, shorter path to the sea, usually down a branch off the old channel. The final switching of most of the water and sediment from the old to the new channel may be triggered by a major flood that scours and widens the new channel. Once the switch occurs, the new lobe gains ground while the old lobe gradually recedes because the sediment supply is insufficient to counteract sea level rise, subsidence of the land, and wave-generated coastal erosion.

FIGURE 1.

Mississippi Delta switching. The succession of different river channels and delta lobes during the past 5,000 years are numbered from oldest (1) to youngest (7). (Meade, 1995 U.S. Geological Survey Circular 1133, fig. 4C; also see Törnqvist et al., 1996, for an updated chronology.)