Eyewitness to Geology

A non-scientific account of coastal subsidence in Louisiana

One day in 1958 I was involved in a December activity common in southeast Louisiana, duck hunting in a marsh pond. At 12 years old I was already an experienced hunter, having been given my first pirogue and shotgun at the age of ten. The pirogue was a plywood imitation of the Cypress dugouts of the pre-Columbian Louisianans. The shotgun was a Stevens bolt action 0.410, a Sears catalog purchase. With these gifts, came freedom. If I wanted to hunt, all I need do was pick up my 0.410, launch my pirogue in the canal behind my house, and paddle or pole to the marsh. This particular December 1958 hunt was different: it was my first experience with coastal subsidence.



A small clump of wooded land stood not far from the pond I was hunting. The small waterway leading to my blind passed near the shore of this little stand of trees. On my way out of the pond, I noticed that a small stream was flowing out of the trees. This was highly unusual because I was in an area where only the rise and fall of the tide produced flow, and the little island of trees was just above the normal tidal range. I landed to investigate.

The little stream was less than a foot wide and several inches deep. The water was crystal clear and flowed rapidly over a bed of red algae. It was hot enough to produce a visible steam that wafted over the water surface until it met with the marsh. The little wooded hump was the only feature that was not grass or water for miles in any direction. To the north, about two miles or more away, was a levee, and beyond that was the skyline of New Orleans. To the west, three or four miles away, stood the smoking stack of the Kaiser Aluminum refinery. To the south and east were miles of marsh. There were no pipelines in the area, no navigation canals for miles. There was nothing that could have produced a constant flow of hot, clear water. I shrugged my shoulders and left.

Evidence

Decades later I recounted this observation to a couple of geologists. We had been discussing the existence of geological fault lines in heavy clays resulting from movement, much like fault lines in bedrock. The geologists explained to me that water trapped deep in the Earth could be squeezed out by geological movement. Water expelled in this manner was commonly hot and very salty. One of the geologists asked me to point on a map where I had encountered that “spring.” I pointed to a spot now in the middle of open water.

“It was about here,” I said. “The trees and land are all gone now, subsided into water deep enough to trawl for shrimp.”

“That’s right on top of a major fault line!” he said.

The geologists were convinced that large areas of coastal Louisiana were slipping down and southward in separate “slip” zones, in addition to the more recognized causes of subsidence. Another piece to the puzzle had been found.

In 1960 I began working summers and weekends with my father. He was an engineer and land surveyor and, after a few years as the parish (county) engineer, he had opened his own private consulting business. We were running a traverse along the protection levee between St. Bernard Parish and the marsh to the east. I noticed that the marsh and water on the flood side appeared as close to the elevation of the crown of the levee as did the rooftops on the protected side. When I mentioned this to my father, he explained that since the drainage was improved on the protected side, the water table had lowered and the soil subsequently had shrunk from dehydration and decomposition of organics. Once a levee had been built to protect it from seasonal flooding, the shrinkage of the surface meant that the levee had to remain.

He reminded me of several square-shaped lakes, such as the Pen near Lafitte or the Big Mar near Caernarvon. These areas were once large sections of freshwater marsh that were surrounded by levees in the 19th century as they were converted into rich farmland. When the farms failed, the levees were not maintained, and water flooded back into subsided farmland.

That same year, we had a survey project that included a traverse across one such shallow water body in an area known as Florissant. I had a difficult time wading across the eighteen-inch deep lake because I kept tripping over submerged crop rows. The land used to be a sugar cane farm until about 1860, a crop that does not tolerate flooding very well. Now, one hundred years later, it was open water.

In 1964 the condition of the levees protecting St. Bernard Parish and the City of New Orleans became a center of attention. Before the levees could be repaired and upgraded, Hurricane Betsy struck the city, resulting in widespread flooding. The water was pumped out, and bigger, better levees were planned and built.

Enter the Government

I began my professional surveying and engineering career in southeast Louisiana in 1971. I went away in the Army for a couple of years, and my first re-introduction to working in the New Orleans area involved elevation work. We were having a problem determining the datum to use on a small project. It seemed that we could not close with level lines from earlier runs. We were reduced to selecting the benchmark that best matched local “normal” sea level and going with that value.

By the time 1975 rolled around the federal government had created the Federal Emergency Management Authority (FEMA) and Flood Insurance Rating Maps (FIRMS) and had published Base Flood Elevations (BFE). The result of these new programs was pure chaos.

Surveyors across the New Orleans metropolitan area began to complain that the benchmarks established by the National Geodetic Survey (NGS) and other governmental agencies were in disagreement. NGS attempted to rectify the problem with new level runs and adjustments. Nothing seemed to work. Initially it was assumed that the problem stemmed from the well-understood problem of subsidence due to development and groundwater draw-down.

In an attempt to counteract the “perceived” problem, benchmarks were placed on deep-pile-supported buildings or long steel rods, to counteract the effects of surface de-watering. The results were dismal. Deep marks faired little better than shallow ones. The changes in elevation were never constant, and any attempt to predict future values based upon past performance failed. Benchmarks in the unprotected marsh moved, marks on pile-supported buildings moved, marks on long rods moved, marks inside the levee system moved, marks outside the levee system moved. They were all moving down, and fast. The most stable marks moved down at the rate of 0.2 foot a decade. Some moved as much as a foot a year.

Every time I visited the marsh the change was evident. Wooded lowlands were becoming marshlands. Marshlands were becoming ponds. Ponds were becoming lakes. Lakes were becoming open water. Every year there was less land, more water, and higher tides. Small storms caused large tidal surges as the Gulf of Mexico crept northward. It wasn’t just the surveyor who noticed something was changing.

During this time, FEMA was requiring flood elevation certificates, and their regulations were based upon the premise that the land surface stood still. I remember one particular meeting with representatives of FEMA to “clarify” FEMA policy on elevation certification. The official FEMA policy was, “a surveyor was allowed to rely on the published value of a benchmark if it was within six inches of the true value.” FEMA assumed that surveyors “knew” what the true value of a mark should be. These were the people in charge of the program!

Finally, the development of GPS technology provided the tools needed to quantify this movement.

Subsistence Causes

The cause of this movement is many faceted, but I have narrowed it to a few categories. Geologists and coastal engineers could expand on this list or provide a more academic treatise on the subject, but here is the blue-collar explanation.

Near-surface subsidence. Levees that protect land and property from seasonal flooding caused a lowering of the water table and shrinkage of the land’s surface. This subsidence is significant but short term. In some places it has been as much as two meters. People could have avoided this by not reclaiming land and building levees, homes, business, and roads. If people didn’t live in southeast Louisiana the river could flood the land every year and add new soil.

Deep compaction. The mass of material deposited by the Mississippi River is slowly compacting. In a few million years, it will be rock. This movement is gradual, about 5 mm per year, but very long term. There is nothing that can be done or undone to change deep compaction.

Shifting land mass. Large sections of land are gradually sliding into the Gulf of Mexico. The quantity of this movement has not been measured. It’s called gravity. Nothing can be done to change this.

Sea level rise. The sea level has been rising steadily for the last 10,000 years. This rise might be due to material eroding into the sea, or sinking continental plates, or something else. It has risen about 1 mm per year for the last 5,000 years. Nothing can be done to change this.

Global warming. There is much debate over how much modern sea level rise is the result of climate change and how much is the result of other factors. Global-warming advocates claim that sea level rise could increase to as much as 2 mm per year. Deniers claim that there is no such thing as global warming. It doesn’t matter who is right. If global cooling began, resulting in 5-mm-per-year sea level fall, the change would not be noticed in southeast Louisiana.

The people of southeast Louisiana are faced with two choices to counteract the effects of deltaic subsidence. They can begin to build homes, businesses, and infrastructure that can accommodate or quickly recover from periodic inundation, or they can move. There is no other choice that I can see.

For a more scholarly discussion of the subject of coastal subsidence I recommend the article, “Subsidence and Sea Level Rise in Louisiana: a Study in Disappearing Land,” in NOAA Magazine and the NGS report, “Sea-Level Rise and Subsidence: Implications for Flooding in New Orleans, Louisiana,” by Burkett, Zilkoski and Hart.

About the Author

  • Stephen Estopinal, PE, PLS
    Stephen Estopinal, PE, PLS
    Stephen Estopinal, PE, PLS is assistant division manager and senior project manager at SJB Group, LLC in Louisiana. He has been involved in the practice of land surveying for more than 30 years and is the author of "A Guide to Understanding Land Surveys 3rd ed." John Wiley & Sons, Inc. 2009.

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