New Orleans - Subsidence

How Humans Sank New Orleans

Engineering put the Crescent City below sea level. Now, its future is at risk.

• 2/6/2018 - The Atlantic - Story by Richard Campanella

https://www.theatlantic.com/technology/archive/2018/02/how-humans-sank-new-orleans/552323/

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What was beginning to happen was anthropogenic soil subsidence—the sinking of the land by human action. When runoff is removed and artificial levees prevent the river from overtopping, the groundwater lowers, the soils dry out, and the organic matter decays. All this creates air pockets in the soil body, into which those sand, silt, and clay particles settle, consolidate—and drop below sea level.

Construction of the new drainage system began in 1896 and accelerated in 1899, when voters overwhelmingly approved a two-mill property tax to create the New Orleans Sewerage and Water Board. By 1905, 40 miles of canal had been excavated, hundreds of miles of pipelines and drains had been laid, and six pumping stations were draining up to 5,000 cubic feet of water per second. System efficacy improved dramatically after 1913, when a young engineer named Albert Baldwin Wood designed an enormous impeller pump that could discharge water even faster. Eleven “Wood screw pumps” were installed by 1915, and many are still in use today. By 1926, over 30,000 acres of land had been “reclaimed” via 560 miles of pipes and canals with a capacity of 13,000 cubic feet of water per second. New Orleans had finally conquered its backswamp.

That wasn’t nearly enough to drain the swamp, but it was enough to begin permanently altering the New Orleans’s land surface. We know this because in 1893, when the city finally got serious and funded expert engineers to figure out how to solve this problem, surveyors set out to map local elevations as had never been done before. The resulting topographical map of New Orleans (1895) would inform the engineering of what would become a world-class system.

The 1895 map also revealed something curious: The rear precincts of one downtown faubourg had, for the first time, dipped slightly below sea level. The sinkage would not bode well for things to come.

What was beginning to happen was anthropogenic soil subsidence—the sinking of the land by human action. When runoff is removed and artificial levees prevent the river from overtopping, the groundwater lowers, the soils dry out, and the organic matter decays. All this creates air pockets in the soil body, into which those sand, silt, and clay particles settle, consolidate—and drop below sea level.

Construction of the new drainage system began in 1896 and accelerated in 1899, when voters overwhelmingly approved a two-mill property tax to create the New Orleans Sewerage and Water Board. By 1905, 40 miles of canal had been excavated, hundreds of miles of pipelines and drains had been laid, and six pumping stations were draining up to 5,000 cubic feet of water per second. System efficacy improved dramatically after 1913, when a young engineer named Albert Baldwin Wood designed an enormous impeller pump that could discharge water even faster. Eleven “Wood screw pumps” were installed by 1915, and many are still in use today. By 1926, over 30,000 acres of land had been “reclaimed” via 560 miles of pipes and canals with a capacity of 13,000 cubic feet of water per second. New Orleans had finally conquered its backswamp.

The change in urban geography was dramatic. Within a decade or so, swampland became suburbs. Property values soared, tax coffers swelled, and urbanization sprawled onto lower ground toward Lake Pontchartrain. “The entire institutional structure of the city” reveled in the victory over nature, wrote John Magill, a local historian. “Developers promoted expansion, newspapers heralded it, the City Planning Commission encouraged it, the city built streetcars to service it, [and] the banks and insurance companies underwrote the financing.” The white middle class, eager to flee crumbling old faubourgs, moved into the new “lakefront” neighborhoods en masse, to the point of excluding black families through racist deed covenants. And in a rebuke of two centuries of local architectural tradition, new tract housing was built not raised on piers above the grade, but on concrete slabs poured at grade level. Why design against floods if technology has already solved that problem?

The change in topographic elevation was more subtle, but equally consequential. A city that had been entirely above sea level into the late 1800s, and over 95 percent in 1895, had by 1935 fallen to about 70 percent above sea level.

Subsidence continued even as more and more people moved into subsiding areas. While the vast majority of New Orleans’s 300,000 residents lived above sea level in the early 1900s, only 48 percent remained above the water in 1960, when the city’s population peaked at 627,525. That year, 321,000 residents lived on former swamp, over which time they dropped into a series of topographical bowls four to seven feet below sea level.

The average New Orleanian of this era perceived being below sea level as something of a local curiosity. Then as now, most folks did not understand that this was a recent man-made accident, or that it could become hazardous. But streets increasingly buckled and buildings cracked. When Hurricane Betsy ruptured levees and flooded the bottoms of four sunken urban basins in 1965, the curiosity became more of a crisis.

Soil subsidence made frightful headlines in the 1970s, when at least eight well-maintained houses in a suburban subdivision exploded without warning. “Scores of Metairie residents,” The New Orleans Times-Picayune reported, “wondered whether they are living in what amounts to time bombs.” The affected subdivision, low-lying to begin with and positioned on an especially thick layer of peat, had been drained just over a decade earlier. With so much “wet sponge” to dry out, the soils compacted rapidly and subsided substantially, cracking slab foundations. In some cases, gas lines broke and vapors leaked into the house, after which all it took was a flicked light switch or a lit cigarette to explode.

The emergency was abated through ordinances requiring foundational pilings and flexible utility connections. But the larger problem only worsened, as gardens, streets, and parks continued to subside, and those neighborhoods that abutted surrounding water bodies had to be lined with new lateral levees and floodwalls. Many of those and other federal structures proved to be under-engineered, underfunded, and under-inspected, and all too many failed in the face of Hurricane Katrina’s storm surge on August 29, 2005. The rest is topographic history, as seawater poured through the breaches and filled bowl-shaped neighborhoods with up to 12 feet of saltwater.

. . .

What to do? Urban subsidence cannot be reversed. Engineers and planners cannot “reinflate” compacted soils if city dwellers have built lives upon them. But they can reduce and possibly eliminate future sinkage by slowing the movement of runoff across the cityscape and storing as much water as possible on the surface, thus recharging the groundwater and filling those air cavities. The Greater New Orleans Urban Water Plan, conceived by a local architect, David Waggonner, in dialogues with Dutch and Louisiana colleagues, lays out a vision of how such a system would work. But even if executed fully, the plan would not reverse past subsidence. This means that greater New Orleans and the rest of the nation must be committed to maintaining and improving structural barriers to prevent outside water from pouring into “the bowl. "