Argument: New Orleans sinks further each year, increasing costs/risks
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Klaus Jacob. "Time for a Tough Question: Why Rebuild?". Washington Post. 6 Sept. 2005 - First, all river deltas tend to subside as fresh sediment (supplied during floods) compacts and is transformed into rock. The Mississippi River delta is no exception. In the early to mid-20th century, the Army Corps of Engineers was charged with protecting New Orleans from recurring natural floods. At the same time, the Corps kept the river (and some related canals) along defined pathways. These well-intended defensive measures prevented the natural transport of fresh sediments into the geologically subsiding areas. The protected land and the growing city sank, some of it to the point that it is now 10 feet below sea level. Over time, some of the defenses were raised and strengthened to keep up with land subsidence and to protect against river floods and storm surges. But the defenses were never designed to safeguard the city against a direct hit by a Category 5 hurricane (on the Saffir-Simpson scale) or a Category 4 hurricane making landfall just west of the city.
Joel K. Bourne, Jr. "New Orleans: A Perilous Future". National Geographic. August 2007 - THE REALITY REMAINS DAUNTING for those trying to rebuild, or trying to decide whether to come back at all. The risk of catastrophic flooding is rising year by year, with no end in sight—in no small part because the city is sinking.
Even before it was covered by millions of tons of floodwater, New Orleans had sunk well below sea level, because of the draining and compacting of the backswamp and the pumping of groundwater. According to the latest satellite measurements, the city continues to sink at around two-tenths of an inch (0.5 centimeters) each year. The rate is faster in Lakeview and fastest of all in neighborhoods to the east and west. In St. Bernard Parish, subsidence tops out at nearly an inch (2.5 centimeters) a year. Some sections of the MRGO levees have sunk up to four feet (1.2 meters) since they were built, according to Roy Dokka, an LSU geologist who co-authored the satellite study, and Katrina breached many of the low spots.
"This is a place where people shouldn't be living, yet we're here," says Dokka. "But subsidence isn't going to kill people. It's the ever increasing vulnerability to storm surges and our inability to prepare for them."
"The Case Against Rebuilding New Orleans". The Unplanning Journal. 8 Sept. 2005 - should we rebuild New Orleans?
From a Geological stand point, the quick answer would have to be no.
New Orleans, as everyone has now learned, is situated on the ground between the Mississippi River and Lake Ponchetraine in a natural delta. Constant deposition of sediment is required to sustain any delta. Without it, erosion from tidal and wave forces will eventually erode away the formation. Hurricanes can accelerate this process dramatically. Worldwide, this erosion can be observed in almost every river delta that has had its flooding action halted by damming of the upstream head waters or channelization via artificial levee construction. In New Orleans, both man made interventions affected the Mississippi River’s flow.
As concerning as that may be, the pumping of groundwater and later, oil and natural gas, further increased the rate of subsidence to where portions of the city are now more than 20 feet below sea level.
With land subsiding in the interior and eroding at the fringes, the conclusion is clear. This area is not inhabitable without significant protection and mitigation, with each year requiring greater measures (higher levees, stronger flood walls and bigger pumps).
[...] Geologically, the cards are stacked against New Orleans. Given the passage of time, the area known as New Orleans (and Baton Rouge) would ordinarily be relegated to the annals of history, destined to become the next lost city, overrun by wild life and studied by future civilizations (if there are to be any). It is only through the application of massive amounts of energy, is nature held at bay. But nature can only be delayed, not denied. For without constant supervision, work and physical investment, the forces of nature will triumph in the end. In fact they are already beginning overwhelm our best laid plans.
Walter Youngquist. "Should New Orleans be Rebuilt?". NPG Internet Forum - New Orleans on a sinking delta
New Orleans is built on part of the Mississippi River delta complex. As man-made levees have kept sediments from being deposited in the area, much of the city is now below sea level. It will continue to sink as the sediments compact. Withdrawals of groundwater by numerous wells in the area also cause the sediments to compact and the land surface to subside, a situation known in many other areas. To compound the problem, sea level is slowly rising. A study from Texas A & M University projects that New Orleans in 100 years will be three feet lower than it is today if present trends persist.
Walter Youngquist. "Should New Orleans be Rebuilt?". NPG Internet Forum - Levees needed with constant care
To maintain the river through New Orleans to the sea, levees have to continually be maintained along the Mississippi from well above New Orleans, at New Orleans, and below New Orleans farther and farther out into the Gulf of Mexico. Also sediments are not now being distributed back and forth along the coast by the river as would normally happen. So the brackish water marshes, nursery to so much aquatic life, are not being replenished by sediment, but are being invaded by the sea. This is destroying the once prolific fisheries around the delta. Hurricane damage and flooding is now more severe because in Louisiana some 25 square miles of wetlands become seawater every year as the marsh areas slowly disappear from lack of sediments. That means less protection from storm surges (Bunch, 2005). Special levees have been built around New Orleans to replace the protection, which would otherwise have been afforded by the marshes.
River on stilts
As you walked down the streets of New Orleans you looked UP to the ships on the river. In effect the Mississippi River is supported on stilts, carrying it out into the Gulf beyond what clearly can be seen are the normal limits of the delta. Sooner or later that situation and other levees protecting New Orleans from adjacent bodies of water are doomed to collapse--an environmental disaster waiting to happen. In late August 2005, Katrina, a category four hurricane, with heavy rain and a more than 30-foot storm surge, breached two major levees protecting New Orleans and flooded 80 percent of the city. The disaster was no longer waiting. As much of New Orleans is below sea level, the water cannot flow out but has to be pumped out. Clogged sewage systems throughout the city have to be cleared. Houses have either floated off their foundations or are infiltrated with mud, many damaged beyond repair. The water supply system was contaminated. It is said by those on the scene to be a catastrophe beyond description. The cost in human tragedy is immeasurable.
[...] Even if New Orleans is rebuilt to proposed "new standards," whatever they may be, it is doing it in defiance of geologic history and environmental principles. In the longer term, or perhaps even in the shorter term, it is an unsustainable situation. Southwest Pass would have to be built farther out into the Gulf. Upstream, levees would have to continue to be built higher. Around New Orleans as it continues to sink, the other levees which also protect the city have to be raised. This is a continual problem for, as one engineer noted, for every five feet added to a levee, the levee sinks three feet deeper into the muck. Sooner or later the levees will break again. As the U. S. Army Corps of Engineers is a Federal Agency, the continuing and increasing cost of maintaining a city where the environment clearly says it should not be, will be borne by all U. S. taxpayers. And inevitably the natural regimen of the Mississippi River will prevail, permanently. Residents of the city can never feel secure.