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Sunday, August 25, 2019



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Study: La. land sinking and rising

Study: La. land sinking and rising

Some areas of Louisiana are sinking faster while other areas appear to rising, according to studies from 2002 and last year.

For example, Alexandria sank 49 millimeters and Cocodrie dropped 20 millimeters while Thibodaux’s reading at Nicholls State University suggested it had risen by 7 millimeters.

“Which doesn’t make sense,” said Cliff Mugnier, the chief of Louisiana State University’s geodesy program that participated in the studies. “That will be subject to an explanation by a geologist.”

In 2002, a federal agency conducted elevation surveys at different sites across the state in collaboration with LSU. In 2018, researchers returned to measure the same sites again.

In the 16 years that passed, the data showed how the elevation of areas changed, in many cases lowering, providing more recent evidence of Louisiana’s ongoing subsidence, or sinking.

The rate wasn’t consistent across the state, with some areas sinking faster than others. Some even rose slightly. Mugnier said these measurements are only the initial results of the multi-step process required to measure and understand the state’s changing landscape.

These initial measurements released on Tuesday were collected by the National Geospatial-Intelligence Agency in coordination with LSU using instruments that measure the elevation using the absolute gravity at a site. The absolute gravity can be used to show how far a point is from the center of the Earth.

“It doesn’t truly mean that it’s sinking,” Mugnier said.

Another LSU professor, Ahmed Abdalla, is taking on the next step of the process -- using GPS to confirm whether the elevation has changed. Once both the current and historical GPS data is collected, the National Geodetic Survey will create models based on both sets of data.

If the GPS data confirms the gravity readings, Mugnier said, “that will be a major scientific breakthrough because no one’s ever done that before.”

Scientists don’t have an explanation for why areas like Cocodrie have lowered and Thibodaux has slightly risen. The focus is on gathering the data before turning it over to other scientific fields to better understand the causes of elevation changes.

If an elevation change is unrelated to subsidence in Louisiana, Mugnier said typically it suggests that there was a change to the water table in the area over the time period.

“When industry or oil wells are pumping water and hydrocarbons under the ground, then it changes the amount of density in the surface,” he said.

Mugnier also noted that Louisiana pumps out a lot of groundwater for drinking purposes.

Aside from the National Geospatial-Intelligence Agency’s assistance with measuring, Mugnier said LSU has also increased the number of GPS antennas that it has set up across the state to measure elevation. The number is at 130, including a site in Houma.

While Houma wasn’t a site for the measurements collected in 2002 and 2018, the antenna will now allow its elevation changes to be recorded.

Ultimately, Mugnier said the goal is to use this data to help planners in communities.

“If there is subsidence occurring at a certain rate, this is information that planners need to have so they know how low the evacuation routes are and how long they need to give cities and people to evacuate before they flood,” he said.

Eventually, a few millimeters worth of changes add up.

“The sinking’s not very fast,” he said. “But it accumulates.”

The measurement of the changes are becoming instrumental to land surveyors and civil engineers in the state, said Mugnier. It’s also something that scientists will never have stop measuring.

“I call it the perfect research question,” he said. “Because it never ends.”

 

-- Daily Comet Staff Writer Halle Parker can be reached at hparker@houmatoday or 857-2204. Follow her on Twitter, @_thehalparker.