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Grand threat to the low-lying coast could
one day be under water for half of every year...

and similar low-lying locations along Louisiana's central coastline could be inundated with floodwaters for half of each year by the end of the century, thanks to the effects of rapidly sinking soils and rising water levels in the Gulf of Mexico, according to new estimates from the National Oceanic and Atmospheric Administration.

The effects of relative sea level rise, the combination of subsidence and rising water, will be significant by 2050, according to the study, with as many as 79 flooding events lasting a total of 20 days.

The new study is based on five years of measurements at a benchmark on a bulkhead at the U.S. Coast Guard station on the inland side of the island's eastern end. But the results of that benchmark are similar to recent readings at other sea-level benchmarks in that part of the coast — and could spell major problems for Port Fourchon, the major jumping-off point for deepwater oil and gas production.

The predictions — as dramatic as they seem — may actually underestimate the extent of future flooding because the study does not include even larger rates of sea level rise that are expected to be caused by global warming, said Stephen Gill, chief scientist for the National Ocean Service's Center for Operational Oceanographic Products and Services.

The study found that relative sea level rise, the combination of rising water levels and sinking land beneath the island, equaled 9.24 millimeters a year -- about a third of an inch -- between 2005 and 2009. While there were four high-water events lasting 53 hours that inundated the benchmark between 2005 and 2009, the study found that between 2046 and 2050, sea level would have risen by 1.2 feet at the marker, resulting in 79 flood events lasting 478 hours, or 20 days. Between 2096 and 2100, sea level is expected to rise by 2.7 feet, which would cause the number of events to skyrocket to 1,241 and to last as long as 22,229 hours, or over half of the five-year period.

Port plans for changes

The results assume that the speed in which land is sinking today — about 6 millimeters a year — beneath that part of the coast will continue unabated throughout the 21st century. Subsidence in the area is occurring as sediment deposited along older courses of the Mississippi River and its tributaries sinks, releasing oxygen and water from between grains of silt and sand deep beneath the surface.

But they also assume that the sea level rise portion of the equation, which makes up about 3 millimeters a year, remains steady. Various estimates of the effects of global warming could add as much as another 2 millimeters of rise a year to those totals, especially in the later years of the century.

"I don't think anybody's really surprised by this," said Wayne Keller, executive director of the Grand Isle Port Commission and a member of the island's levee board.

He said the port is already factoring in expected sea level rise into its development plans. For instance, a new state Department of Wildlife and Fisheries laboratory building was elevated with sea level rise in mind, he said.

And the island already is attempting to alleviate problems caused by subsidence, including the repeated inundation of Louisiana 1 from high tides.

"One of our biggest concerns when a storm is coming in is that there's a low spot between Grand Isle and Port Fourchon," Keller said. "Once water gets above a certain point, you're stuck on the island. That's going to be more and more of a concern."

Staying high and dry

The state and federal government already have built an elevated highway between Port Fourchon and Golden Meadow, and Grand Isle and Jefferson Parish officials continue to lobby to get an additional stretch of elevated highway to the island, said Marnie Winter, environmental director for the parish.

"We also need to get sediment put into the system, which we've been pushing for 20-some years," Winter said. The Army Corps of Engineers has just announced plans to designate the western half of Grand Terre Island, just east of Grand Isle, as a disposal area for sediment dredged from the Barataria Waterway. Once placed there, the sediment could wash west to help keep Grand Isle's beaches above the rising sea level, she said.

The statewide building code, adopted in 2006, following Hurricane Katrina, also should assist in requiring new construction to be built with sea level in mind, Winter said.

The island's levee district also is working on plans to expand the levee system around more of the island's populated areas, and the town also is developing plans for pump stations that would help remove water from inside the new levee system, she said.

Farther inland, Winter said, the town of Lafitte is working with zoning experts to develop structural recommendations for residents, including standards for raising houses to avoid inundation.

Global warming kicks in

Louisiana is not alone in facing the effects of sea level rise, especially as global warming kicks in, said Denise Reed, a coastal research scientist at the University of New Orleans.

"This is what's going to happen to the built environment all around the coast of the U.S.," she said. "It's going to flood more and more often."

The natural environment can keep up with sea level rise by depositing sediment along shorelines and the growth of vegetation in wetlands, she said. But in areas where sediment has been cut off, such as in Louisiana, where levees and navigation channels block the introduction of silt and sand into wetlands, survival will be more difficult, she said.

"That has real implications for the facilities at Port Fourchon and Grand Isle," she said. "This tells us that the future has to be different. What we do for Grand Isle depends on what future Grand Isle believes it will have. They can build levees and hide behind them from the rising water, but the issues are really going to be access, roads and water supply.

"I think there are engineering solutions for many of these issues," she said. "But whether or not they're the common-sense thing to do is something we need to consider. Whether the level of investment they require is something we can afford is something we need to consider."

Visit the Gulf State Insurance Information Center at www.GulfStateInsuranceCenter.org.

Written by Mark Schleifstein, Times Picayune — New Orleans, La.
November 29, 2010

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