June 5, 2005, 6:28PM
IS TEXAS NEXT?
Coast leaving scientists with a sinking feeling
Controversial NOAA report says Louisiana's shores plunging fast — are Texas' next?
By ERIC BERGER
Copyright 2005 Houston Chronicle
GRAPHICS
• Houston-area subsidence
• How fast is Louisiana sinking?
By century's end, much of southern Louisiana may sink into the Gulf of Mexico. The Texas coastline, including Galveston, could soon follow.
That's the sobering — and controversial — conclusion of a new report published by the National Oceanic and Atmospheric Administration that finds the northern Gulf of Mexico is sinking much faster than geologists thought.
The report centers on the humble benchmark, a small metal disk bolted to the ground, that provides a standard elevation above sea level for land surveying and mapping as well as determining flood-prone areas.
But there's one problem with benchmarks: They don't give reliable elevation readings if they're sinking along with everything else.
That's what the geologists who wrote the NOAA report say is happening in Louisiana: The yardstick is broken. Instead of minimal geologic subsidence along most of the Louisiana coast, as previously thought, the state's entire coastal region is sinking at least 5 feet every century.
And although a number of local officials disagree with the report's conclusions about Texas, here's a scary thought: Similar forces could well be at work just a few miles south of Houston.
"Subsidence doesn't stop at the Texas border," said Roy Dokka, a co-author of the NOAA report and a Louisiana State University geologist.
A colleague of Dokka's in Houston, the editor of the Houston Geological Society Bulletin, is more blunt in his assessment of the report. "Galveston," says geologist Arthur Berman, "is history."
Flooding a major threat
The report already has ignited debate in Louisiana. If that state's coast continues to sink, its multibillion-dollar plan to protect coastal cities and wetlands from flooding has targeted the wrong problem, erosion. Every building on land certified as safe from flooding may, in fact, be in danger if Louisiana's benchmarks are flawed. And levees thought to protect New Orleans from a Category 3 hurricane might fail even if a moderate Category 2 storm struck the Big Easy.
Texas could have similar problems if its benchmark elevations are flawed. The National Hurricane Center bases its storm-surge models on benchmarks, as do emergency planners trying to determine when key evacuation routes might flood.
Houston felt the problem acutely during Tropical Storm Allison when benchmarks indicated that certain areas, such as some Texas Medical Center buildings, should not have flooded even in the torrent of rain produced by that storm.
"We know that a lot of benchmarks in Texas are inaccurate," said Gary Jeffress, a mapping specialist at Texas A&M University-Corpus Christi.
Shifting land
Subsidence — the sinking or settling of land — comes in two basic forms. One is man-made, caused by groundwater pumping or oil and gas extraction. The other, which Dokka says is causing nearly all of the problems along Louisiana's coast, is natural, or geologic, subsidence.
Houston has grappled with man-made subsidence for the past 30 years with help from the Harris-Galveston Coastal Subsidence District, which has spent about $10 million studying and measuring the problem.
As a result of this research, the Houston region has undergone a multibillion-dollar conversion from ground wells to surface water for its consumer and industrial needs. In the next decade or so, north and west Harris County will spend as much as $3 billion more for a similar conversion.
Ron Neighbors, general manager of the subsidence district, said he thinks this conversion has largely solved the area's subsidence problems. He attributes little of the region's sinking to geologic subsidence and doesn't think the area's coastal regions may be sinking as much as 4 or 5 feet a century.
"We realize there is some amount of natural subsidence that will occur over time," Neighbors said. "But we believe that to be about three quarters of a foot per century."
If he has little confidence in the report by Dokka, Neighbors has even less in the scientist himself. Neighbors said the Louisiana geologist has incentive to cause fear.
"You don't get any grants if you say everything is OK," Neighbors said.
Berman said that, although conclusions drawn by Dokka's report are controversial, they were checked by NOAA for 18 months and wouldn't have been published if they were bogus. The controversy comes, he says, because the findings challenge existing models.
Every geologist agrees that three main factors contribute to coast loss: rising sea levels, coastal erosion and subsidence. What Dokka and his supporters suggest is that the role of natural subsidence is nearly as important, or even more important, than man-made subsidence. And the problem for coastal planners is that, unlike with man-made subsidence, there's nothing that can be done to stop natural subsidence.
Forces of nature
Dokka and others think geologic forces during millions of years are acting to sink the Gulf Coast. The Texas coast rests upon a mobile layer of shale about 20,000 feet below ground. But because lots of water is mixed in with the shale, it cannot be fully compacted into rock, so it behaves something like toothpaste, geologists say.
A second observation is that several rivers, principally the Mississippi but also the Colorado, Brazos and many others, drain into the Gulf of Mexico, bringing untold tons of sediment over millions of years.
"You're draining almost the entire North American continent into the Gulf," Berman said. "The cumulative weight of that is immense."
So immense, in fact, that the Gulf of Mexico is pushing down on the Earth's crust, making an indent. For coastal regions along the Gulf, it's like being at the edge of a trampoline with a bowling ball weighing down the center: The natural inclination is to slide toward the center. The toothpastelike shale layer facilitates the slide, Berman said. Geologists call it gravity gliding.
Roberto Gutierrez, a geophysicist at the University of Texas at Austin's Bureau of Economic Geology, said Dokka's results were not unexpected among the geosciences community. "The Earth is a dynamic place," he said.
State's efforts fledgling
But even after the Dokka study, the central question — What is the dominant factor driving coastal land loss in Texas? — remains unanswered, he said. Like other geoscientists contacted for this article, Gutierrez said there simply isn't enough data to know how natural subsidence is affecting Texas.
Texas' General Land Office estimates that the state loses about 235 acres of Texas Gulf shoreline each year. Like Louisiana, the state's Coastal Texas 2020 program has focused on coastal erosion.
The coastal program has received funding only since 1999 and has just begun to study the issues, said Lorrie Council, a team leader. During meetings with coastal landowners and public officials, she said, they pointed to beach loss from erosion, primarily when strong storms cause big waves and wash sand out to sea, as the biggest problem.
The state recently created a committee to look at subsidence as a factor, she said.
"It's something we're aware of, but we don't have enough information on it, and we're working actively to get it," she said.
The information may well come from Jeffress, the geographer at Texas A&M-Corpus Christi. Because of efforts by U.S. Sen. Kay Bailey Hutchison, Jeffress says he will receive $665,000 to modernize the height of benchmarks along the Texas coast. He hopes the grant will be renewed annually.
Expensive work
Using a mix of traditional surveying and the Global Positioning System, Jeffress said, he will work with private contractors to begin updating the network of benchmarks installed in Texas shortly after World War II.
It's not cheap. The basic premise of the work Dokka did in Louisiana, which Jeffress will emulate to some degree in Texas, is to start at some point well inland that rests on bedrock. For Dokka that meant the upper reaches of Louisiana. For Jeffress, it means Austin. Then, like traditional surveyors, they take level readings by sight all the way to the coast. It's a time-consuming, costly process, requiring about $1,500 per mile.
One of Jeffress' first tasks, he says, will be checking the seawall road in Galveston and the Gulf Freeway for low-lying spots that could serve as choke points during an evacuation.
He said he has no doubt that many of the existing benchmarks used by highway planners are incorrect. And although it's not his job to interpret why, he says he thinks natural subsidence must play some role.
"I think there's a general trend here," he said. "Considering how many people live along the coast, and how close they live to the water, this could be a problem in the future."
Area well documented
But Neighbors says subsidence in Harris and Galveston counties is the most well studied in the country. Height modernization of benchmarks began here in the mid-1980s, he said, and when communities, including those in Louisiana, wanted to learn how to improve their elevation measurements, they come here.
The subsidence district's records, he said, indicate that subsidence has slowed or even halted in Harris and Galveston county areas where groundwater pumping has ceased.
To justify billions of dollars in conversion from groundwater to surface water, he said, the science had to be sound.
Simply put, Neighbors said, if natural subsidence were a problem here he would know about it. Then he paused, in thought. Geology, with all of the Earth's chaotic, unpredictable and violent processes, is not a science of absolutes.
"I'm not going to say that we've learned it all," he added. "We don't know everything. But I don't think we're going to fall into the sea anytime soon."
eric.berger@chron.com
whew.
Good one!