Subsidence Sea Level Rise

Tropical Storms Floods and Storm Surge

Subsidence is currently causing a steady sinking of the land in New Orleans, making it more vulnerable to the effects of sea level rise and flooding, as well as structurally weakening important levee structures.  Geological studies show that the city is currently sinking by an average rate of 8 mm/ year. (Dixon et al., 2006; Dokka, 2006) This is due in part to a natural bending of the tectonic plate because of sediment deposition from the Mississippi river, and partly because of compaction caused by the drying of the organic soils that New Orleans was built on. (Dokka, 2005; Dixon et al., 2006; McCulloh, Heinrich, Good, 2006) As this last factor has been a result of drawing from the water table and construction, it could potentially be slowed by management of these hazards.  In the 1960s and 70s, studies have shown that the Michoud fault’s movement was causing an additional 7-16 mm/ year of subsidence. (Dokka, 2006) Subsidence is especially troublesome in certain areas; it has recently occurred at rates of 2-3 centimeters/ year in areas such as Lakeview, and under the levees surrounding the MR-GO canal.  This last phenomenon in particular was a leading cause of flooding during Hurricane Katrina. (Dixon et al., 2006)

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Sea Level in the Context of New Orleans

In context to New Orleans, an approximate one meter rise in sea level will flood most of southern Louisiana, including the city. David Pugh (2004) estimates that if the sea level were to rise 1 m, the United States alone would lose 1500 homes per year and that loss in property would equal to some $20 to $150 billion. An IPCC report quotes (Yohe et al., 1999) that an increase of sea levels through 2065 found losses of US $370 million for dry land, US $893 million for wetlands, and US $57-524 million in transient cost (McCarthy, 2001).

Tropical Storms in New Orleans
Written by Aubrey Samost

New Orleans has a precarious location.  Located on the coast, it is highly susceptible to hurricanes and tropical storms.  Tropical storms are fueled by the warm water of the Gulf of Mexico, the Caribbean Sea, and the central waters of the Atlantic Ocean.  Storms that enter the Gulf of Mexico stand a heightened chance of heading straight towards the city and strengthening as they travel.  This is due to the Loop Current.  The Loop Current is a precursor to the Gulf Stream.  Some years the Loop Current is strong and close to the surface, while other years, the current is hardy noticeable.  The current also moves around between Cuba, Florida, and the Gulf Coast.  Hurricanes, like Katrina, are likely to hit the Loop Current and strengthen.  A lot of storms hit category 5 intensity levels because of this strip of warm deep water.  When a hurricane passes over the ocean, it uses the top warm layer of water as fuel.  This stirs up the ocean and cools the temperature of the water, weakening the hurricane.  When the hurricane hits the Loop Current, it uses the surface warm water as fuel, but when the water mixes, the layers underneath are still warm.  This large supply of warm water allows the storm to reach such strong intensities (Gyory, Mariano, Ryan, 2005).

New Orleans is also at risk for tropical storms because of the rain that they carry.  With its low elevation and complex levee system, the city is at a high risk for flooding.  Much of the city is located below the Base Flood Elevation (BFE)and far below sea level.  Other natural factors, such as subsidence, further increase the city’s susceptibility to tropical storms and the damage that they bring. Overall, New Orleans is at a high risk for damage from tropical storms and hurricanes due to the unlucky combination of all of these factors.

Floods and Storm Surge
Written by Samantha Fox

    Wetlands play a huge role in retaining flood waters and decreasing storm surge.  They are a major line of defense against natural disasters such as hurricanes and flooding.  However, the annual net wetland loss for Southern Louisiana is estimated at 75 square kilometers per year (USGS Fact Sheet: Louisiana’s Coastal Resources).  If this continues, approximately half of the wetlands in the area will be destroyed in 100 years. 

    Without this protection, New Orleans will be more susceptible to dangerous flooding and storm surges.  Furthermore, storm strength is expected to increase as a result of global warming leading harsher conditions that the wetlands must battle.  Fewer wetlands and stronger storms creates a potentially disastrous situation that could destroy homes and lives. 

    Moving people out of the city is a way of preventing such an event.  While wetland restoration would aid in protecting the city, with the predicted sea level rise and increased storm strength, they might not be repaired quickly enough to significantly counter such negative phenomenon.