New report in Nature addresses subsidence, flooding in New Orleans

Most of New Orleans is sinking at an average rate of 6mm a year. In some areas, subsidence is occurring at a rate of as much as 29mm/year. That's according to research published in this week's edition of the journal Nature by scientists from the Rosenstiel School. Titled, “Subsidence and Flooding in New Orleans,” the authors conclude that when global sea level rise is factored into their analysis, the average rate of subsidence of the city relative to sea level is even higher — 8mm on average per year.

“When you multiply this over 20, 30, or even 100 years, you can see that New Orleans will be lower, and this information should be factored into reconstruction plans, as we look at subsidence that is up to 3 feet in 40 years,” said the lead author of the paper, Dr. Tim Dixon, Rosenstiel School geophysics professor. “What we found is that some of the levee failure in New Orleans were places where subsidence was highest. These levees were built over 40 years ago and in some cases, the ground had subsided a minimum of 3 feet which probably put them lower than their design level.”

Through analysis of satellite radar imagery, and using structures in the city that strongly reflect the radar signal, the researchers were able to see where land is subsiding the most in New Orleans. The team generated a map from space-based synthetic-aperture radar measurements, and note in their paper that it “revealed that parts of New Orleans underwent rapid subsidence in the three years before Hurricane Katrina struck in August 2005. One such area was next to the Mississippi River-Gulf Outlet (MRGO) canal: levees failed here during the peak storm surge and the new map indicates that this could be explained by subsidence of a meter or more since the levee's construction.”

To make the map, the team used 33 scenes recorded from Canada's RADARSAT satellite. The technique involves phase comparison of 33 radar images taken at different times along the same orbit and exploits points on the ground that strongly reflect radar, termed “permanent scatterers.”

“While it may not trouble people that the ground is nearly one inch lower each year in places, in the long term, the impacts could be rather significant,” said Dr. Falk Amelung, one of the paper's co-authors. “While most people aren't accustomed to thinking about 100 years out, it's important to recognize that a large part of New Orleans is sitting on sediments that will only continue to sink into the Gulf of Mexico, and it will only get harder and harder to ensure the levees' durability. By 2106, for example, the ground will be nearly three feet lower on average.”

“Global warming poses further challenges to this issue, as well,” said Shimon Wdowinski another co-author from the Rosenstiel School. “As the larger Mississippi Delta slowly slides into the Gulf of Mexico, the levees will be further tested if global warming increases the intensity and frequency of hurricanes.”

The researchers conclude that their subsidence estimates for the levees “are probably minimum estimates when considered over the lifetime of the levees, given that subsidence was most rapid in the first few years after their construction in the 1960s. Levee failure may have resulted from overtopping because the levees were too low.

“Data from the U.S. Army Corps of Engineers collected after hurricanes Katrina and Rita confirm that water overtopped some levees that subsequently failed. Alternatively, the high subsidence rates the team observed might reflect active faulting or a weak, easily compacted soil, promoting failure at or near the levee base.”

The Office of Naval Research and NASA provided funding for this project. Additional authors on the paper include: Alessandro Ferretti and Fabrizio Novali of Tele-Rilevamento Europa; Fabio Rocca of Dipartimento di Elettronica e Informazione, Politecnico di Milano; Roy Dokka of Louisiana State University; Giovanni Sella from the National Geodetic Survey; Sang-Wan Kim from the Rosenstiel School; and Dean Whitman from the Florida International University Department of Earth Sciences.

Scientists embark on biennial census with high-tech map, deeper dives

A team of 38 research divers from the Rosenstiel School, NOAA Fisheries, the State of Florida, the National Park Service, and the National Undersea Research Center celebrated the Tortugas Ecological Reserve's fifth birthday of protected status in true scientific fashion. Employing even deeper mixed-gas dives and working from a new high-tech, high-resolution LIDAR (laser-based) topographic map, they embarked on their biennial census to measure how well the protected status is helping the Florida Keys ecosystem rebound from decades of overfishing. Reporters were invited to spend a day on the R/V Spree with the scientists to learn more about this unique process. The made first-hand observation and interviews with field scientists.

The Dry Tortugas, a remote area about 70 miles west of Key West, is known for its extensive coral reefs, fish, sharks, lobsters, and other marine life. In 2001, the Florida Keys National Marine Sanctuary was allowed to set aside no-take areas, forming the Tortugas Ecological Reserve, to help address the critical state of over fishing in the region.

In August 2005, the Florida governor and cabinet unanimously approved to implement a management plan for a no-take marine reserve in the Dry Tortugas National Park. The Florida Fish and Wildlife Conservation Commission concurred in early February 2006 with the proposed National Park Service regulations related to marine fishing in the park. The park's marine reserve, coupled with that in the Florida Keys National Marine Sanctuary, is designed to protect precious coral reefs, fishery, and cultural resources, and to ensure sustainability of intensely exploited regional reef fisheries resources — benefiting the Tortugas, the Florida Keys and beyond.

Southern Florida coral reefs generated an estimated 91,000 jobs and US$6 billion in economic activity in 2005. These ecosystem goods and services, however, are threatened by increased exploitation and environmental changes from a rapidly growing regional human population.

Dr. Jerry Ault, led the team of researchers to monitor ongoing population changes and observe the effects of hurricanes on the coral reef habitat in the Reserve. Ault and his team conduct biennial fish abundance surveys on the more than 220 species that comprise the Tortugas' reef fish community in the Florida Keys. This year, his team documented changes in fish abundance and habitat quality in a region that got hit by six major hurricanes since the last survey in June 2004. By statistically comparing this year's findings to previous survey data collected since 1999, scientists can determine what effects two years of intense hurricanes have on the marine environment. Collecting scientific data on a regular basis is imperative to understand the dynamics of the ecosystem and to provide sustainable fishery management recommendations. The Tortugas region plays a critical role in the health of the overall Florida Keys coral reef ecosystem thus making this study important to understanding the overall functioning of tropical marine habitats.

Hurricane Plan 2006

The 2006 RSMAS Hurricane Plan is located on the Rosenstiel School website in the myRSMAS section. The general plan for the school as well as any specific plans for each division and research center are detailed in it. If the campus goes into a serious hurricane alert you may want to print the plan and take it with you if you evacuate or just in case the power shuts down.

Campus Safety

All RSMAS faculty, staff and students are asked to please save the following date/time for a campus-wide evacuation drill:

Wednesday August 30th at 10:00 am.

Please try not to schedule important meetings, events/work for that date and time.

We will be providing detailed information within the next few weeks by campus e-mail.