Teresa Harbison carries belongings through flood waters outside of her home in St. Amant, Louisiana on Sunday. Jonathan Bachman/Reuters




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The Geological Society of America on Wednesday published a study by a team of Tulane University geologists that found Louisana's coastline is sinking 50 percent faster than was estimated two years ago. They discovered the coast is sinking at an average rate of 9 millimeters per year (with a margin of error of 1 millimeter). In some areas—including along the Mississippi River south of New Orleans—the rate is 12 millimeters per year.

The sinking coast doesn't tell the whole story. The study did not take into account the rising sea level, which creeps up the coast at a rate of 3 millimeters per year. This means the net impact on the coast, or the "relative sea level rise," averages 12 millimeters per year, and reaches 15 millimeters per year in the most highly impacted areas.

The study was conducted by testing 274 locations across the state's coast. At each, researchers placed in the ground steel rods containing pins. The differences in the heights of the pins were measured over the course of six to 10 years to determine how much the surface had subsided. Researchers would also lay minerals such as white chalk on the ground, and later take core samples to determine how much sediment had been deposited on top of the chalk layer.

These surface-level changes are largely due to sediment deposits, or the lack thereof. In some cases, so much new sediment was deposited on parts of the coast that areas sank under its weight. In others, areas that would have had a base strong enough to support new sediment deposits were not receiving enough, causing land loss. As the study notes, the state's coastline has experienced wetland loss "equivalent in area to the state of Delaware" over the past 50 years.

Sediment delivery isn't all that is causing Louisiana's coast to sink. Researchers charted subsistence at depths of 50 feet or more by planting anchors and then charting their rise or fall using GPS. Sinking at this depth is largely created by a continental "hinge effect," which results from land rising at the Arctic Circle. Why is land rising at the Arctic Circle? Because weight on arctic landforms is being lifted as the ice caps melt.

These melting ice caps are also the cause of the rising sea level, which is responsible for 3 millimeters of the coast's 12 millimeters of relative sea level rise.

The study found that the state's coastline is subsiding at a higher rate than was previously thought. Recent studies that were conducted mostly with tide gauges found that the subsistence was occurring at 1-6 millimeters per year over the past few decades. These studies predicted "worst case scenarios" of 8-10 millimeters per year.

As the study published Wednesday notes, "perhaps worst case scenarios should be considered the new normal."


6/15/17 at 1:20 PM


original story HERE











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