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Land along Louisiana's coastline is sinking 50 percent faster than was estimated just two years ago, according to a new map published Wednesday (June 14) as part of a study by Tulane University geologists. It says the average subsidence is 9 millimeters a year, more than one third of an inch.
In the worst spots, the subsidence rate is closer to 12 millimeters, or almost a half inch. Among them:
- Along the Mississippi River below New Orleans, including near Caernarvon
- Onshore just north of the Isles Dernieres, at the Atchafalaya and Wax Lake deltas west of Morgan City
- In an inverted triangle of land between Kaplan and Lake Arthur along the eastern part of the Chenier Plain.
The new study measures only the sinking of ground and not the sea level rise of an additional 3 millimeters, or one 10th of an inch, says Torbjorn Tornqvist, a geology professor at Tulane and one of the authors of the study. The combination of subsidence and sea level rise is called "relative sea level," which in this case would average 12 millimeters across the coast and 15 millimeters, approaching two thirds of an inch, in areas with the fastest subsidence.
The study was published in the online journal GSA Today by the Geological Society of America. The lead author of the study was Jaap Nienhuis, a post-doctoral fellow at Tulane.
Tornqvist said the most important conclusion of the study is the average subsidence rate for the coast. It was identified by collecting information at 274 locations.
The study said the new estimate is significantly higher than 2011 and 2015 studies that indicated the rate averaged between 1 and 6 millimeters per year. But the study does not predict whether subsidence rates will stay the same or increase in future years.
Tornqvist suggested the average number was more important than the higher and lower subsidence rates at various locations. That's because of the wide margin of error identified by the researchers at the individual locations where the data was gathered, and the difficulty of determining the effects of a variety of other factors on the significance of those measurements.
Scientists who helped the state Coastal Protection and Restoration Authority update Louisiana's coastal master plan this year have taken a similar cautious approach, said Denise Reed, a scientist and vice president for strategic research initiatives at The Water Institute of the Gulf. State officials gathered together a group of scientists to give "moderate" and "less optimistic" subsidence scenarios for large polygon-shaped areas of the coast. After determining that not enough new information, or new measurement methods, had been developed since 2012, the same estimates were used for the 2017 plan update.
Those estimates were equally varied, ranging from 3.6 milllimeters to 6 millimeters a year in a polygon labeled Barataria and Terrebonne Marsh, to between 11.8 millimeters and 20.5 millimeters for the Golden Meadow region. Unlike the Tulane study, which represents subsidence rates today based on recent measurements, the master plan projects subsidence rates over the next 50 years.
But like the Tulane study, they do not include sea level rise.
Tornqvist said factors that must be considered in determining the accuracy of readings include:
- Whether sediment is being delivered naturally or through human processes to an area
- Whether a location may be subject to geological faulting that could result in unusual drops in the land surface
- Whether the area is seeing little sediment added to its surface.
For instance, high rates of subsidence near the Caernarvon freshwater diversion in wetlands of Plaquemines Parish 's east bank might actually be caused by higher rates of sediment being deposited there over recent years. The added weight of new sediment likely increases the speed of sinking, Tornqvist said.
That role of increased sediment deposits -- either by dredging material from the Mississippi or Atchafalaya rivers and using it to build new wetlands, or by building sediment diversions that deliver sediment-rich water to open-water areas -- should be taken into account by state officials as they plan restoration projects, he said. More sediment likely will be needed to take into account the speedier subsidence.
Along the state's westernmost coastline, an opposite cause-and-effect must be considered, Tornqvist said. There, less sediment is being delivered to wetlands that actually are situated over more stable land forms. The result is that while subsidence rates are lower, it's an area seeing an increased land loss rate; there's not enough new sediment to keep pace with even the slower sinking rate.
The new map is based on measurements made at 274 sites participating in the federal-state Coastwide Reference Monitoring System program. They consisted of different measurements made at different levels in the soil.
To measure the rise and fall of the shallow surface level, researchers place a steel rod into the ground; the rod has a table containing pins that lower to the ground surface. Over time, differences in the heights of the pins measure whether the land has risen or fallen.
Often, a layer of white chalk or other mineral is placed on the ground and over several years, a core sampler is used to determine how deep that layer has become covered with new sediment and organic material, such as decaying wetland grasses.
The researchers determine deeper subsidence, about 50 feet or lower, by monitoring the rise and fall of a structure anchored at that depth, using global positioning equipment. That measurement also can capture a less obvious continental "hinge effect" sinking caused by the underlying rock and sediment formations dropping in response to rising land forms at the Arctic Circle, the result of melting ice. The deeper measurements can also pick up the effects of faulting, where blocks of soil move as a result of tiny earthquakes.