Worst-case sinkhole scenario would reroute Bayou Corne
Texas Brine Co. has developed a backup plan to replace the cracked southern section of a protective levee surrounding the sinkhole in northern Assumption Parish and may look to reroute Bayou Corne if conditions deteriorate further, records show.
The company’s new draft plan, filed this month with regulators who are still reviewing it, proposes “triggers” that would prompt the levee replacement. It also outlines an alternative of rerouting Bayou Corne if a replacement levee proves too unstable to maintain due to sinking of the remaining land between the sinkhole and the bayou.
Company officials and regulators say it is unlikely there would be a need to reroute the bayou based on current projections for the sinkhole’s expansion but need to be prepared.
Bayou Corne runs just south of the levee’s southern segment and forms half of a semi-circular arc of waterways south of La. 70. Bayou Corne ultimately joins Grand Bayou, which flows south to Lake Verret.
A popular getaway and fishing spot for Baton Rouge-area residents, the scenic cypress and tupelo gum swamps north of Lake Verret that the levee aims to protect are part of the largest swath of cypress in the Terrebonne Basin, state reports say.
The sinkhole’s brackish water contains high salt concentrations, posing an ecological threat to the surrounding areas if not contained by protective levees.
The sinkhole was dormant for weeks this fall, but has rumbled back to life twice since late October. Spikes in “micro-earthquakes” have resulted in cracks and sinking in a section of the levee’s southern arm.
As the 26-acre, lakelike hole has edged toward the southern levee and the bayou beyond, Texas Brine had been under growing pressure from regulators, parish government officials and the remaining residents in the Bayou Corne community to lay out contingency plans in the event the sinkhole expands farther to the south.
In the early months of the sinkhole, Texas Brine and state regulators used expedited permitting processes to build a levee, also known as a containment berm, as part of its response efforts.
Officials with responding agencies said they do not want that to happen again, especially if Bayou Corne has to be rerouted. Texas Brine would need a key wetlands permit from the U.S. Army Corps of Engineers and other regulatory approvals to reroute the bayou.
Tom Killeen, administrator for state Department of Environmental Quality’s Inspection Division, said that office has urged Texas Brine officials to start moving on the permit process.
“It’s a point we have made to them,” Killeen said. “It’s not long enough off for them not to be penciling in information in the application right now. We want to have enough time to vet it and to public notice it and treat it through a normal regulatory process.”
Though laying out plans for worst-case contingencies in the Dec. 13 plan, Texas Brine officials said they believe they can safely repair cracks in the existing levee, maintain it and monitor any changes with an array of sensors.
State projections, company officials note, do not show the sinkhole will ever reach the bayou even if the sinkhole reaches what is considered a worst-case size of 40 acres.
“The berm is working,” Texas Brine officials said in a written statement about its plans. “It is constantly monitored to make sure containment is maintained.”
Louisiana Conservation Commissioner James Welsh ordered Texas Brine in November 2012 to contain the sinkhole’s oily and salty contents and protect the surrounding freshwater swamp.
The levee is the primary line of defense.
Scientists think the breach of an underground salt dome cavern operated by Texas Brine last year unleashed percolating methane gas from natural deposits and caused the sinkhole to emerge between the Bayou Corne and Grand Bayou communities probably on Aug. 3, 2012.
The sinkhole and the partially collapsed Texas Brine cavern are linked underground. As a result, the ultimate size of the sinkhole is linked to how much rock and sediment it takes to fill the cavern until an equilibrium is reached.
While state and company officials still can’t say how long that will take, projections suggest the rate of growth has slowed since the sinkhole formed although it is not done expanding.
Texas Brine designed the containment levee with a 20-year lifespan in mind.
Testing has shown the sinkhole’s surface water is brackish, too salty to drink, while just 100 feet down, the water is saltier than the open ocean, state reports say.
Texas Brine attained early containment of the sinkhole with a first layer of the levee in February while the company finished the 5- to 6-foot wall of earth, limestone and special liners.
An extension of the western levee had to be built, and the breaches or sunken areas in the southern levee were repaired. The entire levee was finished in September, Texas Brine officials said.
State and parish officials agree with Texas Brine that the containment system, once completed, has successfully held back the sinkhole’s contents.
“We have not seen any data to suggest that water from the inside is finding its way to the outside,” Killeen said.
Texas Brine’s new plan does not have detailed designs for a new southern levee or bayou rerouting but proposes conditions when detail work would begin.
For example, a new southern levee farther from the sinkhole, would be built if the existing levee drops 4 feet in 30 days even after planned repairs or if testing shows the underlying earth is too unstable for repairs, Texas Brine’s plan says.
Once the company determined it had to move the southern levee, it would then develop plans to reroute the bayou. The actual rerouting of the bayou would proceed if the replacement southern levee hit the same stability triggers set out for the existing levee.
Patrick Courreges, spokesman for the state Department of Natural Resources, said the plan and a separate report on the existing levee’s stability are under review.
An earlier company plan focused only on continued maintenance of the levee, said John Boudreaux, director of the Assumption Parish Office of Homeland Security and Emergency Preparedness.
Though the new plan still needs tweaking, it seems to address most of the contingency concerns, he said.
“Bayou Corne is probably the biggest question,” Boudreaux said.
However, he said, scientific research on the sinkhole indicates little likelihood of it reaching Bayou Corne.
A way to think of the sinkhole cavity is to compare it with the hollow space inside a tuba — a deep, cavernous middle surrounded by a broad shallow area at the top where the horn edges flair out.
Worst-case projections from August show the sinkhole, now more than 200 feet deep, and an outer area of sunken earth around the sinkhole between 2 and 10 feet deep could grow to a combined 80 acres.
The hole is an oval shape, longer toward the northeast and La. 70 and toward the southeast and Bayou Corne.
Under a worst case scenario, planning documents say, the combined area would be 2,500 feet across at its widest, stopping 160 feet from Bayou Corne at the closest point. Texas Brine’s proposed replacement levee would contain the hole and be about 120 feet from the bayou at the closest, maps show.
Nick Romero, 65, who remains in Bayou Corne despite a standing nearly 17-month evacuation order, said he is glad to see there is plan but doesn’t think it is enough. He said he has seen the sinkhole defy predictions before.
“I believe they need to begin their plan to reroute Bayou Corne. Waiting for something to happen is a plan to fail,” said Romero, who was provided a copy of Texas Brine’s plan. “They need a plan that contains contractors, equipment, materials, owner agreements they can act on immediately.”
Jim Looney, 71, who lived in Bayou Corne for six years before a Texas Brine buyout and fished the area for decades, said he does not think a rerouting of the bayou is feasible given its role in draining the swamps into Lake Verret.
“Too many things intertwine together in the water movement out there to try to say we’re going to reroute it,” said Looney, a local columnist and author on fishing. “That’s the best way I can put it.”