Layers: New Orleans

LayersNOLAdetailironworkbassplayerNew Orleans shows many faces and has many pasts.  Common to all has been wind and water.  Just above where the Mississippi empties into the subtropical waters of the Gulf of Mexico, on a long curve of a river defined by curves, just above or just below water, New Orleans is a city in peril.  Hurricanes and river floods punctuated periods of prosperity from the river that connects the North American heartland with the Gulf, and the world. That riverine link brought together a variety of people and their cultures, and even more come for Mardi Gras and Rue Bourbon, the music, the food, and the frivolity.

Today, Hurricane Katrina defines peril in New Orleans, though the river’s rise in flood is itself a major threat.  The raw natural history of flood and hurricane is told elsewhere but New Orleans is unique because the hazard is more human than natural, caused from attempts by people and their institutions to hold the hazards at bay.

Through natural geomorphic processes, flood barriers are built by flooding itself.  Energy in moving water erodes and transports earth material, the greater the water or the faster it flows, the more work is done.  Water dissolves and disaggregates rock and sediment, which is carried as sediment by the river and joins the water in breaking apart additional material. Its strength depends on the energy of gravity, the hydraulic pressure pushing against the surface, and the energy of wind impressed on the surface of the water.  If water is suddenly slowed, it drops the largest and heaviest portions of the sediment it is carrying.  Large volumes of swiftly flowing water may carry sediment as large as houses and deposit them while still a torrent while microscopic clay particles are suspended by gentle breezes stirring a still lake.   Where the water’s speed diminishes suddenly, dropped sediment becomes a barrier to flooding.  In the Gulf, this happens when the Mississippi River overflows onto its banks.  Leaving its channel, water slows because of the shallowness and friction from the new bottom, causing it to deposit a sediment load atop the banks as it overflows.  That further slows the water and more sediment is dropped and the layered depositions build higher.  Constructed over centuries, this natural levee contains smaller floods while the largest floods overflow it and build it higher.  Natural levees, which run the length of the river, are made of the fine silts and clays that compact when the water goes down.  Eventually the levee defines the channel. In the Gulf of Mexico, the same geomorphic processes form barrier islands. With greater energy from the wind, such as during a hurricane, waves can carry sediment inland then deposit it in the shallower coast.  The dropped sediment forms the long, linear barrier islands. Much as the natural levees, barrier islands stop subsequent small storms while larger storms build them higher.

These natural processes create the shoreline and riverbanks.  The topography of the land is the result of deposition and erosion over geologic time where land and water form equilibrium. This is how the natives came to know the river valley and coast, which made for ample hunting and fishing, and easy travel on the barrier islands and levees.  Europeans also understood its natural advantage, particularly with regard to transportation of goods from the vast interior.  They built New Orleans on the natural levee that protected it from Mississippi River floods and with protection from the Gulf of Mexico hurricanes by wetlands and barrier islands.

But a growing population needed more space.  Levees were built higher for additional flood protection while drainage canals and pumps produced dry land from swamp.  Then additional barriers and drainage systems organized land and water to maximize commerce.  These human built structures were not as robust, nor in equilibrium with the natural environment.  Initially hailed as strong, they proved to be weak and disaster prone.  “Better” construction followed each defining disaster; fear was replaced by confidence and complacency, which was broken by the next disaster.  Attempts to defeat and tame land, river and Gulf only made the city more vulnerable.

Up the river, sediment was dropped in the quiet waters behind thousands of dams in the vast Mississippi drainage.  Within the Gulf, ship traffic, dug canals, drainage and depleted sediment nourishment ate away at the protecting wetlands so salty waters approached the city.  Within the city itself, though drainage created huge tracts of land for housing, the soil was dry, oxidized, and compact. The city sunk below sea level, so levees had to reach higher and pumps had to be larger and more numerous.  Oil and sulfur removed from beneath the Gulf also caused land to subside. Globally, increasing temperatures warmed the sea surface, increasing evaporation and precipitation over the Mississippi watershed and the water flowing into the river system. Then came Hurricane Katrina.

A very powerful storm while in the Gulf, Hurricane Katrina shoved sea water landward in a colossal storm surge that inundated the remaining wetlands, cascaded over the barrier islands, and surged up drainage and navigation canals.  The storm winds wreaked havoc on infrastructure, cutting electricity to pumps and movable flood barriers.  The deluge of rain quickly saturated the ground then worked into the porous sands of levees built along drainage and navigation canals.  One by one, the raging surge overtopped them, or they were undermined at the base by rising water.  Each failing levee insured the failure of the next as the surge mounted and rain fell.  Without power, the pumps were silent in the howling wind.

Even as the wind died the walls failed and the city below sea-level began to fill.  Within a day, three quarters of one of America’s most vibrant cities lay roof deep in ugly water.  Polluted waters lapped at eaves and over rooftops for almost a month.  Thousands died in the flood, thousands of dwellings were destroyed.  The very essence of the city was drowned, but this was not a natural disaster.  A natural event overwhelmed weak, human-built structures, meant to save the city, and a human caused disaster ensued.

(From New Orleans)  Text prepared by Dr. James Brey

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Layers: Places in Peril

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