Ticking Away the Tides

If you put your ear up to the tidal gauge sitting at the U.S. Coast Guard station on Grand Isle, you can hear a faint ticking, like a clock. In reality, it’s the instruments within that are sending sound waves into the water to measure its depth at different times throughout the day. The “granddaddy of tidal stations,” as Osborn calls it, is the longest running tide station in Louisiana. Over the last 70 years, the tidal gauge has recorded one of the fastest rates of relative sea level rise in the world—approximately a rise of 3.0 feet every hundred years.

Following at least 2,000 years of little change, global sea level has risen by roughly 8 inches over the past century, as global temperatures began to increase. There are two principal ways in which global warming causes sea level to rise: the melting of land-based ice that increases the volume of ocean water, and thermal expansion—as water warms, it expands. Climate scientists say that there is strong evidence that global sea level is currently rising at an increased rate, and that a warming global climate will cause further sea level rise over this century and beyond.

While it is important to monitor global changes, site-specific measurements and projections of sea level rise are needed for realistic local planning efforts, whether you’re trying to elevate a highway or a levee or build a new hurricane shelter. In Southern Louisiana, local geologic factors such as subsidence also need to be considered.

Tim Osborn and David Camardelle, mayor of Grand Isle, walk up the levee/dune structure that serves as a critical line of defense for protecting the people and structures on the barrier island from storm surge. Photo by Caitlyn Kennedy, NOAA Climate.gov.

“If you go to NOAA’s Pensacola, Florida, tide station, you’ll see that sea levels are rising at two millimeters a year [0.7 foot every hundred years],” Osborn explains. “That’s because Pensacola isn’t moving—it’s made up of limestone and sand. The land isn’t sinking. You come here and it is five times that rate. That jump is due to the subsidence. Global sea level rise is a part of that whole picture here, but it’s only about one-fifth of the actual rate we’re seeing right here, right now.”

Data collected from NOAA’s Grand Isle tidal gauge shows that relative sea level has been rising approximately 9.2 millimeters per year since the 1940s. This observed rate of relative sea level rise is assumed to be about the same throughout the local region, which includes the vicinity of the area around the LA-1. But this information alone wasn’t helpful for local leaders trying to anticipate how the at-grade highway—the part that isn’t elevated—will be impacted in the coming century. Osborn enlisted the help of Stephen Gill, Senior Scientist for NOAA’s Center for Operational Oceanographic Products and Services (CO-OPS), to get a better picture of the future.

Gill used previous published studies of potential sea level change through 2100 to develop three illustrative relative sea level rise scenarios for the LA-1 region. The scenarios are the 20th century relative sea level rise rate and then either a 2 or 7 millimeter per year potential increase in the rate of global sea level rise.

The first scenario shows relative sea level rise continuing to rise at its current rate of 9.2 mm per year through the end of the century. The second scenario shows the twentieth century rate increasing by 2 mm per year (11.2 mm per year) starting around 2008. A third scenario, in which the twentieth century rate increases by 7 mm per year (16.2 mm per year), kicks in after 2050.

The graph shows potential sea level change through 2100. Three sea level rise scenarios were developed for the region surrounding the LA-1. The first scenario shows relative sea level rise continuing to rise at its current rate of 9.2 mm per year through the end of the century. The second scenario shows the 20th century rate increasing by 2 mm per year (11.2 mm per year) starting around 2008. The third scenario, in which the 20th century rate increases by 7 mm per year (16.2 mm per year), kicks in after 2050. Data courtesy of Stephen Gill, NOAA Center for Operational Oceanographic Products and Services (CO-OPS); graph by NOAA Climate.gov team.

“These are just illustrative scenarios for purposes of estimating potential impacts,” says Gill. “We’re not saying that this is how it’s going to play out. We do know that the published climate change studies estimate sea level rates are going to accelerate up to a certain point by the middle of the century, and then really accelerate toward the end of the century. The purpose of these scenarios was to illustrate how potential impacts might look.”

Gill added that even without any acceleration in sea level rise there will be a significant increase in the frequency and duration of flooding in the future. The present local relative rate of sea level rise is so high, he says, that today’s higher tides are slowly becoming tomorrow’s minor flooding events. Accelerated sea-level rise will just make it worse.

The Costs of Closure

On August 28 of this past summer, Port Fourchon took a direct hit from Hurricane Isaac. While port infrastructure and the elevated portion of the LA-1 suffered only minor damage, the entire highway was closed for 78 hours, largely due to the fact that the land-based section was inundated with storm surge.

“This was the first major test of the elevated highway and it faired pretty well,” Boulet reported in the aftermath of the storm. “But had Hurricane Isaac slowed or approached the region from another direction, the northern section of land-based LA-1 could have washed out, and every household in the country could have felt the impacts.”

Tropical storm Isaac approaches the Louisiana and Mississippi coasts on August 28, 2012. The storm made landfall as a category 1 hurricane near the mouth of the Mississippi River in southwestern Louisiana that evening, and hit Port Fourchon around 4 a.m. the following day. NASA image by Jeff Schmaltz, LANCE/EOSDIS Rapid Response.

After a hurricane sweeps through the Gulf, Boulet explains, the industry’s ability to efficiently repair port production facilities is critical to its immediate recovery. If the industry doesn’t recover quickly enough, Americans feel the impact at the gas pump. During the five most recent named hurricanes that caused LA-1 road closures, average gas prices rose 13 cents per gallon across the nation.*

Given these stakes, in 2011, the Department of Homeland Security’s NISAC Laboratory and the University of Louisiana-Lafeyette’s NIMSAT Institute built upon the future sea level rise projections to provide an assessment of the national consequences of a potential disruption to LA-1, and the resulting local, regional, and national economic impacts. The study considered the likelihood of two scenarios in which damage to the LA-1 would require 90 days to repair, effectively closing down the highway and Port Fourchon facilities in the meantime.

Using the predicted rate of 11.2 mm per year from 2007 through 2050, the study concluded that a 90-day closure could occur as early as 2030, 8 or 9 years earlier than it would using the present-day 9.2 mm per year trend. By 2066, at least five percent of the road could be submerged year round.

Most worrisome, though, was the finding that the highway could experience a total washout even before 2030 in the event of a major hurricane. A halt in port operations due to a 90-day closure could result in up to a $7.8 billion loss in American gross domestic product due to cascading impacts on the oil and gas industry across the United States.