BY DAVID J. MITCHELL
River Parishes bureau
November 02, 2012
BAYOU CORNE — Assumption Parish officials said a helicopter flight over a sinkhole east of the Bayou Corne community confirmed on Thursday their estimate of how much territory the slurry-filled hole swallowed Tuesday.
But Texas Brine Co. officials defended Thursday their smaller estimate of the collapse zone as the more accurate figure.
Depending on whom you talk to, the sinkhole gobbled either 1,500 square feet of land or 30,000 square feet of land along the eastern rim of the funnel-shaped hole, estimated to have a 5.5-acre surface area prior to this week’s sloughing-off incident.
The 449-foot-deep sinkhole has undergone periodic edge collapses since its formation sometime overnight on Aug. 2-3. On Tuesday, more earth fell in, pulling down 15 more trees.
State Department of Natural Resources and Shaw Environmental and Infrastructure scientists believe Texas Brine’s Oxy Chem No. 3 cavern, which is 200 feet to the southeast of the sinkhole, had a wall failure that resulted in the slurry hole.
The salt cavern is located inside but near the western edge of the Napoleonville Dome. The cavern was solution-mined since the early 1980s from inside the 1-by-3-mile solid salt deposit reaching thousands of feet underground.
The sinkhole, found early on Aug. 3 in swamps between Bayou Corne and Grand Bayou, prompted the evacuation of 150 homes the same day. The evacuation order continues.
John Boudreaux, director of the parish Office of Homeland Security and Emergency Preparedness, said he produced his estimate with a pre-collapse, ground-based photograph of the site and Google Earth-based mapping software that used aerial photographs of the site before and after the collapse, which occurred at 4:45 p.m. Tuesday.
He said with the newly available aerial photographs taken Thursday, he was able to verify those measurements — roughly an area 300 feet along the eastern bank and 100 feet inland — are accurate within 10 feet for the irregularly shaped collapse zone.
The widest part extends about 110 feet inland. The narrowest is about 88 feet, for example.
“Although not perfect, it’s pretty close,” Boudreaux said.
He demonstrated on Thursday at the parish command post in Bayou Corne how he used landmarks such as an oil retardant boom, a now-missing stand of trees and the location of an access road on Texas Brine’s site to establish the amount of this week’s land loss.
He said aerial photographs of the current sinkhole were compared with a photograph taken Oct. 12. He said no collapses, or sloughing-off incidents, occurred between then and Tuesday.
Boudreaux said he is waiting for an aerial photograph from Sunday but does not expect any differences.
Texas Brine’s estimate gives the collapse zone dimensions of 75 feet along the sinkhole rim by 20 feet inland and hews closer to Boudreaux’s initial estimate issued Tuesday afternoon before Boudreaux was able to compare aerial photographs.
“We feel we have been pretty accurate in the past, and, in the past, we have concurred with the parish’s assessment. We just disagree in this particular case,” Cranch said.
In other developments, Texas Brine conducted sonar testing of its damaged cavern Thursday to get another look at the inside.
The floor of the cavern rose from 5,650 feet to 4,000 feet underground after 3.3 million cubic yards of sediment from outside the salt dome partially filled the cavern after the sidewall breach, experts have reported.
Texas Brine officials noted in public statements that the floor rose another 10 feet recently.
Cranch said the sonar equipment was lowered gradually into the cavern to take 360-degree images of the interior. Results are expected to be released in a few days