The Times Picayune published a 3 part series written by Bob Marshall and Mark Schliefstein on the disappearing Louisiana Coastline – an intriguing read on the future of the gret stet
Part 1: Because of subsidence and global warming, Louisiana is slowly disappearing
by Bob Marshall, The Times-Picayune
Saturday December 13, 2008, 8:36 PM
Seventy miles south of New Orleans, on the eastern end of Grand Isle, a small tide gauge records the Gulf of Mexico rising against the surrounding land. The monthly increases are microscopic, narrower than a single strand of hair.
Climate scientists recording those results think they add up to something huge. The gauge, they say, may be quietly writing one of the first big stories in the age of global warming: the obituary for much of southeast Louisiana.
In 50 to 100 years, the numbers tell them, rising seas caused by global warming, combined with the steady subsidence of Louisiana’s coast, will lift the Gulf of Mexico two to six feet higher in many areas surrounding New Orleans.
Such a rise would overwhelm the most ambitious coastal restoration plans now under way and submerge almost everything in southeast Louisiana outside hurricane levees. And that means the areas inside the levees essentially would become coastline, far more vulnerable to hurricanes and continuing coastal erosion, and in need of a far more drastic and expensive flood protection apparatus.
“The delta of the Mississippi River is the most vulnerable location in the nation to global warming, because it is sinking at the same time sea level is rising, ” said Virginia Burkett, a senior researcher at the National Wetlands Research Center in Lafayette and one of the nation’s foremost experts on climate change. “And it’s only going to get worse.
“This area is facing big trouble from climate change. I think there’s consensus on that point.”
As the scientific forecasts of global warming gain popular acceptance, many Americans now ponder how their lives might change.
Longer, hotter summers. Shorter, warmer winters. Less rain, or more. Lifestyle adjustments ranging from different light bulbs to hybrid-powered cars.
But climate scientists now say residents in low-lying, fast-sinking southeast Louisiana will have a more serious concern: survival.
“People who live here have a lot more at stake in what happens in the Antarctic and Greenland than any people in this country, ” Tulane researcher Torbjorn Tornqvist said. “We know we’re sinking, and we know sea level is rising. . . . If either gets much worse, we’ll be among the first to experience disaster.”
These predictions come on top of already dire warnings that the traditional forces of coastal erosion — sediment deprivation and canal dredging — have left the state with less than a decade to fix that problem or face permanent land loss.
The additional threat from global warming not only reinforces the need to speed coastal restoration efforts, scientists say, it also raises critical questions about many vital hurricane protection and coastal restoration projects in the planning stages or already under way.
Are the projects being designed to meet the increased threats from sea-level rise, including higher storm surges and expanding areas of open water? Can the planned structures be adapted to meet increased threats, as the Gulf rises and the land sinks? Will pumping stations that battle rainwater flooding have enough power to lift water two to four feet higher? Are the causeways and bridges that link the region’s communities high enough to survive the gradually rising tides — not to mention stronger storm surges?
Are the state and city even planning for the changes the world’s scientific community says are heading our way?
Scientists involved in global warming research speak with confidence about the threats to coastal Louisiana, because they are based on three factors that generate little debate:
— Subsidence in Louisiana, documented for decades, will continue at alarming rates for the foreseeable future.
— Sea-level rise is one of the most widely accepted, easily measurable effects of the warming climate.
— Even if the world moves aggressively to reduce suspected causes of global warming, sea levels would continue to rise for centuries as the oceans slowly respond to temperatures that have been rising since the 1800s.
“The debate within the scientific community is no longer ‘if’ this will happen. It’s now ‘when and how quickly, ‘ ” Burkett said.
University of New Orleans researcher Shea Penland, in one of his last interviews before he died this year, summed up the scenario: “Without some really huge and immediate steps to meet this new challenge, we’re just S.O.L.”
Yet scientists are concerned that a threat growing by only fractions of an inch each year will be underestimated by decision-makers. In contrast to an instant and overwhelming disaster like Katrina, sea-level rise will proceed slowly, almost imperceptibly — until it’s too late to address.
“We’re like that frog in the pot of water on the stove — if we wait until it starts boiling, we won’t be able to jump out, ” said Burkett, echoing a sentiment common in the scientific community.
Worldwide, the scientific community speaks through the Intergovernmental Panel on Climate Change , established in 1988 by the World Meteorological Organization and the United Nations. Consisting of scientists and government agencies from dozens of nations including the United States, the panel sought to determine whether the planet was warming, the causes and potential impacts, and how governments might adapt.
In 1990, the panel began releasing a series of reports confirming global warming, and outlining the primary cause: Greenhouse gases, the carbon-based pollutants released in the burning of fossil fuels, are trapping heat inside the atmosphere. The panel has recommended immediate dramatic reductions in these pollutants to begin curbing the problem. However, the panel admits the process is so advanced that many changes already under way will continue through this century even in the face of an aggressive cleanup. One of those changes is accelerating sea-level rise.
Because the forecasts from the climate change panel rely on complicated computer modeling, researchers can’t predict impacts with certainty. But as the panel moves toward its third decade, members’ confidence has increased. More sophisticated models have been supported by real-world events, such as the recent rapid melting of glaciers and polar ice fields. Predictions that once were termed “possible” are now made with “high confidence.”
Although serious scientific debate remains about some of those projections, the predictions for sea-level rise, which could slowly drown coastal communities worldwide, has drawn wider agreement.
That consensus rests on two indisputable events that will occur as oceans continue to warm:
— Sea-water will expand as it warms, encroaching into land masses worldwide.
— The runoff from melting glaciers and ice fields will increase the total volume of the oceans.
The latter impact has recently become a grave concern because ice fields and glaciers have started melting more rapidly than the climate panel’s models predicted just two years ago. Climate scientists, alarmed by the increase, are struggling to understand the causes.
“If these rates continue in Greenland and the Antarctic, then all bets are off, ” Burkett said. “Then we’re not talking about two to four to six feet of rise, we’re talking about something much greater and even more rapid.”
Worldwide, the midrange estimate predicts oceans will rise 18 inches by 2100. Louisiana faces a far more alarming forecast. Here, those same models predict that, relative to land, water levels will rise 2 to 6 feet, with the highest rates in the southeastern coast surrounding New Orleans.
Louisiana falls victim to what scientists term “relative” sea-level rise: the net result when water rises at the same time land sinks. And the southeast portion of the state’s coast, the vast delta of the Mississippi River, is subsiding at one of the fastest rates in the world.
Healthy coastal wetlands could probably handle a rise of 18 inches over 100 years, scientists say, because they have a natural ability to gain elevation through the regular arrival of new building material from three sources: sediment from spring river floods, storm surges that carry offshore sediments onshore, and the steady deposit of new soil created from decaying plants in healthy wetlands. This is the process called “accretion.”
Wetlands can also adjust to rising sea level by migrating northward in their basins and colonizing higher ground.
“And, in fact, there are wetlands in this region that have been doing quite well against current levels of relative sea-level rise, ” said Denise Reed, a wetlands researcher at UNO. “So, by itself, the projections of sea-level rise we’re seeing published are no reason to think healthy wetland ecosystems can’t keep pace.”
An example of such a healthy wetland is the delta of the Atchafalaya River on the central Louisiana coast. Unrestrained by levees, the Atchafalaya has built more than 27 square miles of new land in the past 40 years.
But the health of southeast Louisiana’s wetlands began to fail in the early 1900s when federal and state levees shut off river sediment from flowing into the wetlands. Erosion accelerated in the mid-1900s with extensive canal dredging for oil, gas, shipping and housing development, cutting through healthy wetlands and ultimately creating vast expanses of open water.
Those problems alone make it difficult for much of the region to keep pace with the 18-inch rise in sea level expected by the end of the century just from rising surface temperatures. When subsidence is added to the equation, natural adaptation becomes impossible, coastal experts think.
The numbers are grim. Southeast Louisiana is expected to sink between two and five feet by the end of the century — one of the fastest subsidence rates on the planet. Those estimates are supported by real-life measurements that show sea level has been rising one inch every 30 months in some sections of the southeast coast. That rate would result in a 16-inch rise by 2050.
The Intergovernmental Panel on Climate Change asked its computers how high the Gulf of Mexico would rise along the Louisiana coast. Researchers calculated varying scenarios of subsidence and rates of sea-level rise, determined by how quickly the world moved to reduce greenhouse gases.
The best-case scenario, which includes a rapid atmospheric cleanup and slower subsidence, shows rises of 12 inches in 50 years and 24 inches in 100 years.
The worst-case scenario, using little change in greenhouse gas build-up, shows a 38-inch rise in 50 years and more than 6 feet in 100 years, a rate that could drown many areas surrounding New Orleans and make the city all but an island.
And studies completed since those 2007 projections trend away from any best-case endings, indicating greenhouse gasses are accumulating much faster than predicted just 12 months ago. This latest research, Burkett said, showed the amount of carbon in the atmosphere by the end of this century could be double the pre-industrial levels of the late 1800s.
“The elephant in the room remains the rate of ice sheet declines, either in Greenland or western Antarctica, ” Burkett said. “If they were to disintegrate, we could see a sea level rise” of 16 to 19 feet.
Researchers familiar with southeast Louisiana’s rapidly deteriorating coastal wetlands agree that even the best-case scenarios threaten to inundate all areas outside of hurricane levees during the next century — unless rapid and aggressive coastal restoration starts within a few years.
“Most of that area (outside the Atchafalaya) is struggling to stay even with the old rates of sea-level rise, so I don’t think they stand much chance of surviving what the models are forecasting, ” said Don Cahoon, a U.S. Geological Survey researcher who wrote some of the most detailed studies of accretion in Louisiana marshes. “They just can’t gain enough elevation under the present conditions to adjust.”
UNO’s Penland laughed off hopes that healthy marshes in the region could survive even the low-range sea-level rise predictions.
“When you add subsidence to rates of sea-level rise we know are coming due to global warming, ” he said, “the scenario goes from threatening to disastrous.”
Part 2: Southeast Louisiana is sinking under its own weight
by Bob Marshall, The Times-Picayune
Sunday December 14, 2008, 7:09 PM
LEEVILLE — From atop the bridge soaring over Bayou Lafourche, a sweeping panorama of the southeast Louisiana coast unfolds. Scattered strings of green marsh break up wide expanses of open water. Pelicans swing on the breezes. Fish jump across the waves as crabbers and oyster harvesters pursue their livelihoods in a postcard scene of a rich life close to nature.
But Windell Curole, whose family has lived here for five generations, can’t find the beauty in it. He sees tragedy. “When my grandfather was a boy, there were cotton fields here,” he said waving his hand in a 180-degree arc that took in mostly water. “But in just 50 years, it became marsh, then it became open water.”
The culprit: subsidence of soft marsh soils, combined with coastal erosion. “This whole area is sinking — and we’re sinking at the same time the Gulf of Mexico is rising. You don’t need to be a scientist to understand what that means,” said Curole, general manager of the South Lafourche Parish Levee District.
Of all the threats facing southeast Louisiana in the age of global warming, subsidence is the most daunting, because it is a problem without a solution.
Sea-level rise can be addressed by cutting the emissions of greenhouse gases that warm the climate. Coastal erosion can be battled with river diversions and stopping the canal dredging that has allowed the salty Gulf of Mexico to eat through thousands of miles of wetlands, marching ever closer to population centers. But no one can fathom a cure for the steady sinking of southeast Louisiana.
Even if the most ambitious restoration projects come to fruition — hardly a sure thing — and millions of tons of new sediment get poured across the troubled landscape, subsidence will continue unabated. As certain and unstoppable as the next California earthquake, subsidence stems from geological forces far beyond human control. We live on an ancient delta that will forever sink and compress under its own weight.
These processes are measured in tiny fractions of an inch per year, not in the sudden, catastrophic movements of earthquakes. New Orleanians have long accepted this shifting, sinking land as part of the bargain of living in their favorite city. They experience it daily in the roller-coaster roads and sidewalks, the tilting houses, the spider-web cracks in walls, the lawns that must be fed truckloads of new soil every few years.
But to scientists contemplating the future prospects for southeast Louisiana, the cumulative impact of subsidence represents more than an annoyance — it’s the reason our neighborhood faces the most dire threat from sea-level rise in the nation. The Intergovernmental Panel on Climate Change estimates that global warming will cause the sea to rise about 1.5 feet around most of the world by 2100.
But in southeast Louisiana, the sea is expected to rise between 2-1/2 and 6 feet relative to the land, swamping the coast. Researchers with the United States Geological Survey reported wetlands in southern Lafourche Parish are losing one inch of elevation every 30 months. That would be almost 1.5 feet in 50 years — twice the rate of the climate panel’s forecasts worldwide.
In most of the nation — even in low-lying southern Florida — oceans are expected to rise against stable landmasses. In Louisiana, the seas will rise as the land sinks, doubling the threat. Wetlands in healthy deltas — unfettered by levees and canal dredging — could add enough sediment to keep up with the predicted pace of sea-level rise, scientists said. But the only area in south Louisiana gaining enough land to match those forecasts is the delta of the Atchafalaya River. “Of course we’re subsiding: We live on a delta,” said Steve Nelson, chairman of the department of earth and environmental sciences at Tulane University. “That shouldn’t be news to anyone.”
Yet some experts worry that the public and politicians don’t fully grasp the role of perpetual subsidence in coastal restoration and hurricane protection. The perception, they say, is that the coast can be permanently fixed with a few one-time projects that would move sediment from rivers to rebuild lands that have sunk.
But any new land will need constant renourishing with sediment and fresh water — possible only through massive river diversions that require perpetual maintenance budgets — because subsidence will never stop pulling the coast below the level of the Gulf. “When we build a project to rebuild the marsh and build levees to protect from hurricanes, it can’t be a one-shot deal,” Curole said.
Some geologists worry coastal planners have not emphasized that dynamic in their restoration plans. “Look, no one is against rebuilding wetlands — they’re important for a variety of reasons that don’t necessarily have anything to do with storm protection,” said Roy Dokka, an LSU researcher who has helped construct a GPS system that effectively remapped southeast Louisiana elevations over the past five years. “But I think this whole coastal restoration apparatus has been negligent in telling people the real nature of the threat here, and that is subsidence.”
Like most problems with the state’s disappearing coast, subsidence has been caused largely by the interventions of man, state and federal scientists said. The southeast Louisiana landscape, settled by Europeans beginning in the 1700s, was created by the delta-building action of the Mississippi River since the end of the last ice age. Melt water from retreating glaciers that swept across the continent also carried a heavy load of silt into the Mississippi River valley, where it flowed downhill toward the Gulf of Mexico.
When the river reached the flatter coastal plain, the current slowed, and billions of tons of sediment began drifting to the bottom, covering a harder, older layer of land that geologists call the “Pleistocene basement.” This foundation gently loses elevation as it slopes southward into the depression that is filled by the Gulf of Mexico.
The delta-building continued in fits and starts over seven millennia. Layers of silt were topped by marsh and forested swamp, which were then covered by more silt, or sunk below the Gulf when the river changed course. This cycle repeated over the centuries, resulting in a soft, multilayered land mass composed of sand, marsh, sunken swamps and clay, interwoven in unpredictable patterns. Viewed in cross-section, the land that became present-day southeast Louisiana would show a multilayered sponge cake — the delta — resting on the older, firmer, more stable Pleistocene material.
When Bienville founded New Orleans in 1718, the delta was more than 400 feet deep at the river’s mouth and extended northward on top of that Pleistocene foundation to midway between New Orleans and Baton Rouge. It continued growing southward until the late 1800s, when construction of flood-control levees prevented the river from resupplying the delta with new sediment. And that’s when subsidence became a serious problem.
Geologists generally point to three major contributors to what has become the fastest-sinking land in North America. The sheer weight of delta deposits constantly pushes downward, compressing all those moist, loose soils against the solid base below. In healthy deltas, new soils usually arrive fast enough to outpace the rate of compression. But when levees cut off the supply of new sediment on the Mississippi delta, compression gained the upper hand.
Drainage projects also speed subsidence, because removing water from moist delta soils causes them to compact. At the same time, draining exposes highly organic soils — such as those composed of old marshes and swamps — to decomposition, which can remove inches and even feet of elevation from some areas in a matter of decades. That’s why communities developed behind protective levees have a lower elevation than the wetlands on the other side.
“Look at any of the (elevation) mapping we’ve done, and you’ll see every community is lower than the wetlands they are being protected from,” Dokka said. “And, in general, that is only going to get worse, especially as sea levels rise.”
And while subsidence occurs naturally, industry activity can accelerate the process. Extraction of mineral deposits from beneath the delta has also been linked to subsidence by state and federal studies. Removal of oil and gas can create empty spaces beneath the surface, allowing layers above to collapse into the holes, adding to the subsidence. New laws require companies to fill these empty holes.
But the causes of subsidence are not restricted to the delta layers. Slippage along the web of fault lines in the Pleistocene layer below the southern half of the state has also been linked to dramatic increases in subsidence rates in specific locations, Dokka said. Research also indicates slight but ongoing compression in the Pleistocene basement.
Folks in southern Lafourche Parish say they understand subsidence. They have watched it transform their landscape for the past 70 years. When their French ancestors arrived in the 1800s on the southern reaches of Bayou Lafourche — once the main stem of the Mississippi River — the land was high and dry, a patchwork of forests and open space. The town of Golden Meadow got its name from the vast fields of goldenrod that dominated the landscape each spring. Katherine Richardelle, 71, remembers growing up on the east side of Bayou Lafourche in a Golden Meadow that was a child’s dream playground. “There were flowers and trees, and high ground,” she said. “It was just a beautiful place.”
But the major causes of coastal subsidence would all come together here with calamitous results, beginning in the 1940s. Richardelle remembers when the oil rigs began sprouting in her neighborhood, when canals were dredged across the fields to facilitate drilling barges and supply boats, and occasional blow-outs sprayed sticky showers of crude oil on houses, laundry and residents.
“Back then, there were no restrictions on the oil companies,” she recalled. “Everything changed with the invasion of the oil companies.” Eventually, her father put their house on a barge and moved the family up the bayou to Cut Off. Today, those once-thriving neighborhoods on the east bank of Bayou Lafourche are ghost towns of abandoned frame homes sitting in newly forming wetlands.
Norma Cheramie, 71, and her cousin Linda Cheramie, 60, are two of the few holdouts. They can stay because their property rests atop an ancient Indian midden — a native American garbage dump of clam shells — that raises the property maybe a foot higher than the surrounding acres.
But they need elevated boardwalks to reach the road when rain falls or the tide rises. “Twenty years ago, there was still 30 or 40 families living within a half-mile of here,” Linda Cheramie said. “But the water just keeps coming. The land just keeps sinking.” Without quick help from massive rebuilding projects, said Windell Curole, Golden Meadow won’t have long.
“Last year the scientists started saying we have 10 years left to save the coast — but, hell, we didn’t have 10 years left for Leeville 30 years ago!” said Curole. “We already lost most of Leeville to subsidence, and now they’re telling us the oceans are going to rise even faster? “If you live down here … You don’t need to be a scientist to figure this out. Just look out across the levee and all you see is water.”
Part 3: Protecting southeast Louisiana will be extraordinarily expensive
by Mark Schleifstein, The Times-Picayune
Monday December 15, 2008, 9:17 PM
On a hot summer afternoon, as laughing gulls, terns and brown pelicans glided above, two front-end loaders dredged sediment from a channel through rapidly eroding marsh, piling it high onto a newly created barrier island.
The island, just north of Port Fourchon on Louisiana’s central coast, is part of a strategy aimed at protecting the nationally significant port, which is the jumping-off point for supplies to most of the 600 offshore oil platforms nearby. Those supplies come south to the port via Louisiana 1, a skinny highway bisecting a thin mesh of disappearing marshland.
In the coming years, Port Fourchon will become an island. And Louisiana 1 will become a 20-mile-long bridge. The port itself will remain above water, but only because its tenants already have raised their properties to an average 6 feet above today’s sea level. Ted Falgout, director of the port, worries that 6 feet won’t be enough. “We’ve got a minimum of 20 to 30 years with the way we’re built today, but looking further out from that . . . they don’t have a real good handle on how rapidly sea-level rise will occur, ” Falgout said.
The port’s strategy — raising facilities, strengthening a barrier island, building a bridge — underscores the potentially crippling effects of sea-level rise on the economy of Louisiana and the nation. Further, it highlights the massive cost of protecting valuable coastal assets as seas continue to rise because of global warming, according to the best available predictions. Ultimately, the problem might demand far more drastic and politically dicey projects than now planned — including changing the course of the Mississippi River.
Meanwhile, the U.S. Department of Transportation has warned communities and businesses along the entire Gulf Coast that they will face similar threats to roads, railroads, airports and seaports. Along Louisiana’s coastline, the threat of sea-level rise is inseparably intertwined with the crisis presented by the many causes of coastal erosion, including storm surges like those caused by Hurricanes Katrina and Rita in 2005. In the space of a single month, surge destroyed more of the state’s wetlands and barrier islands than had been lost in the previous 10 years.
But with sea level predicted to rise by at least another 2 feet — and possibly 6 feet — during the next century, the demands to immediately launch sweeping coastal restoration projects have gained a new urgency.
Those same rising seas should also shape two different levels of hurricane protection being planned by both federal and state officials:
— Levee improvements costing about $14 billion that are scheduled for completion by 2011 and are meant to protect the New Orleans area from flooding by moderate-size hurricanes, with a theoretical 1-in-100 chance of occurring in any year.
— A combination of even larger levee and coastal restoration projects aimed at protecting the entire Louisiana coast from flooding caused by “the equivalent of a Category 5 hurricane, ” or intense hurricanes with a theoretical 1-in-400 or 1-in-1,000 chance of occurring in any year.
The “Category 5” improvements — the plan for which has already been delayed more than a year by the corps and faces still more delays — could cost as much as $80 billion just for the New Orleans area, and take 20 or more years to complete.
Preliminary corps cost estimates don’t specify the costs added to both sets of projects by sea-level rise. But the phenomenon certainly will add substantial cost and complexity, and could become a far more threatening factor if worst-case scenario sea-level predictions come to pass.
In Plaquemines and Cameron parishes, on opposite corners of the state’s coast, the few residents who have returned to their hurricane-devastated communities must struggle with new requirements to raise rebuilt homes and businesses above potential flood levels — sometimes as much as 19 feet above sea level. In Lafourche and Terrebonne parishes, officials are struggling to find ways to pay for a new levee — one that’s tripled in design height and increased more than tenfold in price — to protect Houma and other communities from hurricanes.
Some think such measures ultimately will fall short. It’s possible to create a sustainable coast, said Denise Reed, a professor of geology and geophysics at the University of New Orleans. But it will require endless reserves of cash and political will to construct — and perpetually maintain — radical engineering projects such as changing the course of the Mississippi River to harness its sediment in rebuilding the marsh. “This is not about keeping it the way it is now, ” Reed said. “It’s about designing a sustainable, dynamic system.”
One way to do that would be to create a new mouth of the river, to the north and east of Southwest Pass, and turn it into a “slackwater” navigation channel, a method recommended for additional study by the National Research Council. Sediment-rich water exiting the new mouth would build wetlands east of the river and allow sand to drift west, creating new barrier islands and nourishing old ones.
Such an alternative would require appeasing powerful shipping interests on the river, who fear it would result in continuous shoaling — the collection of mud that can block shipping channels — and would snarl maritime traffic. Letting the river loose also could mean the abandonment of traditional communities in either lower Plaquemines Parish or the central part of the coast. Both concerns are steep political challenges that neither the state nor the federal government has shown any interest in tackling, Reed said.
Congress has ordered the corps to study such river alternatives as part of the Louisiana Coastal Area plan, but it remains unclear when any such study would be completed or acted upon. “Changing the course of the river is serious business because it is the lifeline to the middle of this country, ” said Troy Constance, a corps coastal project manager. “But that’s not to say that, in the end, at some time in the future, such a study won’t be done.”
At Port Fourchon, Falgout has a head start on preparing for sea-level rise, but the struggle to adapt continues. While the new barrier island was being built north of Port Fourchon, Falgout and members of the Louisiana 1 Coalition lobbied federal officials and the Governor’s Office of Homeland Security and Emergency Preparedness, seeking support for $238 million in federal money, on top of $300 million already in hand, to finish the new bridge that will connect Port Fourchon to Golden Meadow. When completed, vehicles will travel 22 feet above the water.
If that seems like a lot of money, consider that Port Fourchon’s needs are but a fraction of what’s needed to adapt to rising Gulf seas in the coming decades. In Louisiana, future responses to global warming are being factored into dozens of hurricane protection and coastal restoration projects.
And yet, the cost of doing nothing could dwarf those numbers. For instance, corps predictions, which factor in sea-level rise, estimate the damage from future hurricanes of Katrina’s strength at $152.8 billion. That projection assumes completion of the 100-year levee improvements now under way, but no further improvements.
The corps already is accounting for sea-level rise and subsidence in its 100-year-storm levee designs. The loss of wetlands and barrier islands means deeper water will settle in front of levees, increasing the power of waves that accompany storm surge, said Nancy Powell, chief of hydraulics for the corps’ New Orleans District office. The new earthen levees are assumed to be able to last 50 years, the traditional lifetime of corps-built projects, but clay will have to be periodically added during that time to protect against expected sea-level rise.
More problematic are the hard structures — floodgates and concrete-and-metal floodwalls — that are an integral part of the levee system. To account for rising sea level during their 50-year design life, designers are adding between 2 1/4 feet and 3 feet to their heights during initial construction, depending on whether they face Lake Pontchartrain, Breton Sound or the Gulf of Mexico.
As part of the “Category 5” protection study, which will include recommendations for building higher levees and gates in combination with major wetlands and coastline reconstruction, engineers are looking at several scenarios for the effects of sea level rise and subsidence on Louisiana’s coast. The alternatives are expected to deal with increased water heights of between 1.9 feet and 3.2 feet along the coast.
The result is levee heights varying from 35 feet above sea level to 45 feet above sea level. But because corps rules allow it to build projects to last only 50 years, the draft report warns that at the end of the 50-year project life, continuing sea level rise and subsidence will pose problems.
For instance, one alternative construction technique calls for the use of hollow-core levees, in which a concrete shell would be covered with clay, which both speeds construction and reduces costs. But the engineers warn the shells can’t be easily raised to account for future conditions — which could mean rebuilding the entire levee.
Scientists also are debating whether proposed coastal restoration projects — most of which involve channeling the Mississippi or Atchafalaya rivers and their sediment into marshes, to build or sustain coastal land masses — have accounted for predictions of rising water and a decrease in the amount of sediment carried by the river.
More fundamentally, neither the corps nor the state has clearly identified what areas of the coastline can and should be saved, according to a peer review by the National Academy of Engineers. In part, the problem is predicting which areas will be too susceptible to sinking below future rising seas to justify it.
The debate about which areas should be saved centers on two major sections of the state’s coastline: lower Plaquemines Parish, along the Mississippi River, and the central part of the state, defined as the Barataria and Terrebonne estuaries.
“There is not enough money, and there is not enough mud, to do everything we want to do, ” Reed said during an April meeting of the state Coastal Protection and Restoration Authority.
At Port Fourchon, Falgout could never more clearly see the fragility of the coast and his port than on Aug. 29, 2005, when Hurricane Katrina roared into Buras, 45 miles to the west. As the storm moved north, high winds pushed water from the north toward the port.
The new barrier island, built as an afterthought using sediment moved from even bigger construction projects in the port, reduced the storm surge slightly. But even as the island grows, the wetlands surrounding it will continue to disappear and the Gulf will continue to rise. “We are not going to be able to sustain that marsh, and one day, ” he said, “Fourchon is going to be an island.”