Underground map of La. to help fight coastal loss at sea level

Published: Nov. 11, 2015 at 12:52 AM CST|Updated: Nov. 11, 2015 at 2:37 AM CST
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Chris McLindon surveys a little piece of history along Paris Road in Chalmette.

McLindon, an oil industry geologist who has become active in coastal research, wades into 2 feet of water in search of the remnants of the Old Paris Road.

"You can feel the road bed," McLindon said, as he carefully shuffled through marsh in search of asphalt remnants.

"It's definitely a hardened road bed," McLindon said, using a shovel as a measuring stick.  "And that's about a little over 2 feet deep."

Built in the 1930s, most likely 2-3 feet above sea level, the roadway sank.

"A fairly discreet hot spot, but the impacts are obvious," McLindon.

A study by the National Oceanic and Atmospheric Administration ranks parts of South Louisiana among the most rapidly sinking spots on earth. While opinions are sharply divided about the causes, McLindon argues the primary culprits of subsidence in the Delta are underground faults.

"The changes that are happening in coastal Louisiana are just outside our perception," McLindon said. "They happen at a rate that's slow enough that we don't really perceive them on a day-to-day basis."

Fellow geologist Eric Broadbridge witnesses the effects daily on his computer screen.

"We can see the earth basically underneath in a 3-D cube," Broadbridge said, as he surveyed seismic data that provides a sort of sonogram of the earth.

For centuries, scientists tell us, the Mississippi River belched its annual spring flood, crafting new land. Over time, as the river changed course, the sea struck back, devouring parts of the Delta.

However, nature waged this war in three dimensions, depositing mountain-sized loads of sediment over the centuries in 16 separate deltas over the last 6,000-8,000 years. Deep underground, the soil compacts as the sheer weight of the land compresses the earth. By 1932, the river had built the modern Louisiana Delta before man put the Mississippi in a straight jacket of levees.

While the levees robbed the marsh of silt, McLindon argues the earth kept moving and compacting.

"All of these faults moving at the same time reached sea level at about the same time and you see this explosion of land loss in the 1970s. That's what caused the initial response that we're dealing with today," McLindon said.

McLindon points to one conspicuous area near Norco, just west of I-310, where a cypress swamp suddenly converts to marsh along a tree line.

"Live, healthy trees on one side," he said pointing, "and then, this patch of dying trees and marsh on the other side."

Broadbridge first noticed the change by studying seismic data, then plotting it on aerial photos of the same spot.

"If the levees weren't here, this would probably be covered with sediment and you wouldn't see this demarcation where the actual surface fault was because it would've filled in with sediment," Broadbridge said.

The site is among a number of hot spots of subsidence, such as LA. Highway 1, where the state has built an elevated roadway to the critical oil facilities at Port Fourchon.

NOAA predicts the old Highway 1, the original two-lane roadway, will go underwater south of Golden Meadow in daily high tides by the year 2040.

In Madisonville, during frequent times of high water, Mike Benjamin uses an old military truck to access the T-Rivers bar he owns.

"(Hurricane) Isaac was the last time I saw it this high," Benjamin said as he drove his truck through a few feet of floodwater in late October.

However, many coastal scientists argue natural faults solve only part of the subsidence puzzle.

"I think where we're having problems right now is understanding the contribution or the magnitude of each one of these mechanisms," said Mead Allison, Director of Physical Processes & Sediment Systems for the Water Institute of the Gulf.

Allison said many of the factors influencing subsidence are not well understood, including the role of fluids sucked from earth, from drinking water to oil and gas deposits. As fluids are extracted, many geologists believe earth fills in the gaps.

"Deltas are also areas of large hydrocarbon reservoirs that we've extracted a lot of oil and gas from to the benefits of society, but there is some side effect to that, potentially," Allison said.

He cites research showing the world's great river deltas disappear most rapidly where man has the greatest presence. However, geologists in the oil industry believe they are sitting on a treasure trove of information about the role of underground faults.

"The best way to do it is to get that data into the hands of universities and have them do independent research projects using the data," McLindon said.

The New Orleans Geological Society recently approved a proposal to share data with university researchers. The work could be used to create a sort of underground atlas of South Louisiana, pinpointing hundreds of faults.

"We see a need to inform the public about something only we can see," McLindon said.

With billions of dollars at stake in the state's coastal restoration efforts, the results could tell scientists and engineers what coastal projects are most worthwhile with the greatest chances for success.

"Knowing if there are hot spots that are associated with faulting, knowing where those areas are kind of allows you to design your program around that," said the Water Institute's Allison.

McLindon believes there is a somewhat brighter side to the story of faults.

"Areas like this (Paris Road) are not going to experience significantly more land loss because the land has already subsided below the surface," McLindon said.

But, he believes, living in coastal Louisiana will require "flexing and bending" to forces shaping lives from far underground.

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