Hurricane Isaac's legacy: wetland destruction, and a test of the New Orleans levees

The hurricane season of 2012 will rightfully be remembered for the legacy left behind by Hurricane Sandy. But in Louisiana, the other hurricane to affect the U.S. in 2012--Hurricane Isaac--left a legacy of its own. Isaac hit Louisiana as a Category 1 hurricane with 80 mph winds on August 28, but the storm's massive wind field brought a storm surge characteristic of a Category 2 hurricane to the coast. A storm surge of 11.1 feet was measured at Shell Beach, LA and higher surges were reported in portions of Louisiana. The surge levels experienced along portions of the New Orleans levee system were similar in magnitude to the surge of Hurricane Katrina in 2005. Fortunately, the new $14.5 billion upgrade to the New Orleans levee system kept the city dry, and we can now be confident that the city will stay protected from Category 2-level storm surges. The new levee system has yet to be tested against a full Category 3-level storm surge, the maximum it is designed to handle.


Figure 1. Hurricane Isaac lit up by moonlight as it spins towards the city of New Orleans, LA, on August 26, 2012. The Suomi National Polar-orbiting Partnership (NPP) satellite captured these images with its Visible Infrared Imaging Radiometer Suite (VIIRS). The "day-night band" of VIIRS detects light in a range of wavelengths from green to near-infrared and uses light intensification to enable the detection of dim signals. Image Credit: NASA/NOAA, Earth Observatory.

Environmental impacts of Isaac
One major long-term environmental impact of Isaac will be the erosion and destruction of wetlands along the Southern Louisiana coast. A 2011 study by the USGS found that four hurricanes in the past seven years--Katrina, Rita, Gustav, and Ike--have together destroyed about 250 square miles of Louisiana marshland--an area 20% the size of Rhode Island. Researchers have yet to quantify how great Isaac's impact was on Louisiana's wetlands, but given Isaac's large size, large storm surge, and the extended battering it gave the coast, I expect 2012 will be one of the state's highest years on record for wetland loss. Over the past 25 years, Louisiana has lost an average of 17 square miles per year of wetlands, and I expect Louisiana lost 30 - 70 square miles of wetlands in 2012, primarily due to Isaac. As I explain in my detailed article on Storm Surge Reduction by Wetlands, wetlands can help significantly reduce the storm surge from a hurricane, though the degree of protection wetlands provide from storm surges is extremely complicated and is largely unknown. The general rule of thumb is that each 2.7 miles of marsh reduces the storm surge by one foot, though wetlands will provide almost no protection from a slow-moving storm like Isaac, which had enough time to completely inundate the coast, despite the presence of wetlands. Louisiana's wetlands have other huge benefits besides hurricane protection, though--they filter out nutrients that would contribute to the huge Gulf of Mexico dead zone, they support 25% of the nation's total commercial fishing haul, and provide storm protection to five of the nation's largest ports.


Figure 2. Hurricane Isaac inundated large areas of Southeastern Louisiana due to storm surge levels in excess of 10'. Image credit: USGS.

One way in which Isaac may have helped the marshlands of Louisiana and Mississippi, though, is that the storm drowned tens of thousands of Nutria, the large semi-aquatic South American rodents released in Louisiana and Mississippi in the 1930s by fur trappers looking for new stock. Nutria can severely reduce overall wetland biomass, and lead to the conversion of wetland to open water. Populations were kept in check as long as fur prices were high, but a fur price collapse in the 1980s led to a nutria population explosion into the millions. The Louisiana Department of Wildlife and Fisheries emphasizes that without effective, sustainable nutria population control, coastal wetland restoration projects will be greatly hindered. The Louisiana's 2012 Coastal Master Plan, unanimously approved by the Louisiana State Legislature in May 2012, called for 109 projects costing $50 billion over 50 years to use a combination of restoration, nonstructural, and targeted structural measures to provide increased flood protection. Louisiana needs to build a series of engineered structures called diversions along the lower Mississippi River in order to restore river sediment to Louisiana's marshlands, said a report co-authored by 22 prominent scientists and engineers in April 2012.

Jeff Masters