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How Katrina came to be so damaging to New Orleans

 

Entergy Truck on the bank of former cypress swamp east of New Orleans

Entergy Truck on the bank of former cypress swamp east of New Orleans

Start with the geology: When the Muddy Mississippi floods, it washes over its banks, dropping its heaviest sands and silts closest to its channel. Flood after flood it builds a natural levee to an average height of fifteen feet above the adjacent floodplain. Relieved of the heavy sands, the floodwaters wash a carpet of fine silts across the low-lying floodplain and create a poorly drained, marshy backswamp in which cypress and tupelo flourish.

Now a little bit of history: In 1718 Sieur de Bienville founded New Orleans on the natural levee the Mississippi north of its mouth. Over the objections of engineers he laid out the town on the left bank of the river at the point where the river swings nearest to Bayou St. John and Lake Pontchartrain to the north. Residents built their houses on high ground, on the natural levee of the river and near the head of Bayou St. John, which gave them a shortcut to the Gulf of Mexico through Lake Pontchartrain. A cypress forest fronted by a marsh lay between the village and the lake and protected the village from storm surges in the lake. New Orleans held to the natural levee until the 1920s when the developers cleared the cypress forest, drained the marsh, reclaimed land from the lake, and built a floodwall to keep the lake from flooding neighborhoods that followed. The newly drained land compacted and sank below sea level.

More geology: When a fast moving stream meets a still body of water, it almost comes to a halt and deposits its load of sediment in the still body of water. In this way the Mississippi built the Louisiana Coast.

And: When the river becomes too long and its slope to the sea too flat, it finds a shorter, steeper course. The Mississippi, south of Baton Rouge has done this at least five times in the last 7,000 years. First it deposited its sediment on the western edge of the delta, where Hurricane Rita touched in mid-September 2005. Then it shifted to the eastern side of the delta, southeast of New Orleans, where Hurricane Katrina tore up the landscape. Then the river shifted directly south of New Orleans. Then, a little to the west of there, and finally, about 1300 years ago, it began to deposit the modern delta, laying it down between two older deltas and contributing sediment to them also.

More delta building: A distributary channel is one that breaks off from the trunk channel of a river and carries its sediment to the sea. Such a channel functions much like a trunk channel. Like the trunk channel, it builds a natural levee, adding to it with each flood. Like the trunk channel, it washes fine silts and clays over its natural levee into a poorly drained backswamp. And, it can split into a splay of branching channels: each one pushing into the Gulf, each one forming a natural levee, each one washing fine silts and clays into a poorly drained marsh. Together they form a delta lobe laced together by a network of natural levees.

Basins, which form between the branching channels, fill with marshes–saline marshes closest to the delta front, freshwater inland swamps farthest away from the delta front. In between, the marshes progress from freshwater to brackish to saline as the lobe pushes out into the Gulf of Mexico.

Finally, there is a natural process called subsidence: As long as a distributary continues to deliver sediment to its delta front, and as long as flood deposits feed the basins between the separate channels, mud flats form in the basins and are quickly colonized by fresh water vegetation. Once the distributary ceases to deliver sediment to its delta front, once flood deposits no longer feed the basins between separate channels, and once the cycle of growth and decay in the marsh can no longer sustain the marsh, sea water seeps in and begins to eat away at the marsh, replacing it with shallow lakes or bays.

There is a rule of thumb: Every mile or two of marshland reduces a storm surge from a hurricane by as much as a foot. At the beginning of the twenty-first century Louisiana was disappearing, subsiding, at the rate of one acre every twenty-four minutes. Project that rate over a year, and Louisiana was disappearing at the rate of 20,000-25,000 acres per year, which correlates to twenty-five to thirty-five square miles per year. In the eighty years before 2000 Louisiana lost 600,000 acres of vegetated wetlands. If this continued, the Louisiana shoreline would creep inland some thirty-three miles in some areas of the coast. Louisiana was disappearing and New Orleans was threatened by hurricanes, as coastal marshes receded, a process that was aggravated by other changes to the Mississippi and its Delta.

The process started with the first levee built at New Orleans in 1723. Every change we have made since to the natural functions of the river has decreased the amount of sediment delivered to the Gulf of Mexico for the construction of coastal marshes.

Artificial levees on the Mississippi, extending clear to the Gulf of Mexico, may have made human habitation of the delta south of Cape Girardeau possible, but prevented the Mississippi from refreshing its marshes with its sediment when it did flood. Revetments built of concrete mats may have stabilized the river’s navigation channel, but reduced erosion of the banks, a source of sediment in the marshes. James Eads’ jetties at the mouth of the river may have allowed the river to cut a thirty-foot navigation channel through the sandbar blocking the South Pass of the modern delta, but delivered sediment carried by the river to very deep water at the continental shelf, where it washed away, never to be used for marsh building. Closure of the old distributaries of the river may have prevented flooding in the bayou country of Louisiana, but cut the flow of Mississippi sediment to the coastal marshes. Channel dams may have made navigation on the Upper Mississippi profitable, but they retained its sediment north of Alton, Illinois. Dams on the Missouri, from which the Mississippi drew sixty percent of its sediment, did the same. In short at the beginning of the twenty-first century, the coastal marshes south of New Orleans received eighty percent less sediment than they had at the beginning of the twentieth and eroded away.

Deteriorated fringe marsh off the Mississippi River near Port Sulphur, Louisiana

Deteriorated fringe marsh off the Mississippi River near Port Sulphur, Louisiana

The destruction of the marshes was further aggravated by the discovery of oil in the early twentieth century. Oil companies dredged long, straight navigation and exploration canals–an urban grid laid over the meandering bayous that laced the coastal marshes together. They dropped their dredge material along the edge of their canals, created spoil banks, levees that disrupted the natural flow of water across the marshes. They prevented water and sediment from refreshing the marshes; they created open lakes. Each vessel passing through the canals dragged a wake behind it, which spread out and eroded the banks, creating still more open water. Finally, the canals broke down the zone between fresh water and salt water, allowing salt to bleed into fresh water marshes, killing the native plants and trees, forcing wildlife to adapt or go elsewhere.

The Mississippi River Gulf Outlet barge canal may have shortened the distance between the Port of New Orleans and the Gulf of Mexico by 40 miles, but its construction decimated the marshes southeast of New Orleans, marshes that would have absorbed the brunt of hurricanes like Katrina. On August 29 MR GO, as the locals call it, funneled Katrina’s storm surge into its shared channel with the Gulf Intracoastal Waterway, into the heart of New Orleans, where the surge breached the floodwalls of the Industrial Canal and flooded the Lower Ninth Ward in the eastern part of the city.

Finally, because the city lies in a bowl surrounded by levees, every drop of rain that falls on New Orleans must be pumped out through drainage canals built at the beginning of the twentieth century. The design and/or the construction of the floodwalls along the 17th street and London drainage canals was flawed. Rather than overtopping the floodwalls along the canals, Katrina’s storm surge undermined them. They collapsed, flooding the rest of the city, all but the French Quarter, which was built on the natural levee.

That is how New Orleans came to the flooded on August 29, 2005, 287 years after its founding on a semi-dry spit of land in the coastal marshes of Louisiana.

Former cypress swamp destroyed by MRGO east of New Orleans

Former cypress swamp destroyed by MRGO east of New Orleans

 

 

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