In Louisiana, there is no room for Complacency

By Greg Holland, C3WE Director NCAR

A decade after Katrina, New Orleans has regrouped and more than recovered. The levee protections are back stronger and reinforced, and pumps have been refurbished and made safe from flooding. Reports from the area indicate a new thriving culture that has built on their colorful history and are adding a new age of entrepreneurial activity. The city is buzzing and life is good.

But how safe is the city? And what of the surrounding communities?

Let's consider some history.

In geological terms the Louisiana wetlands have regularly moved around and changed shape as the Mississippi dumps mud and silt; plants colonized this new land, which leads to further expansion; subsidence and storms erode them away. By 1950 the progressive buildup had taken the river miles out into the Gulf of Mexico and the wetlands covered over 7,000 square miles or around 40% of all wetlands in the United States. They now produce around 16% of all the seafood consumed in the country along with about one third of the gas and oil.

Native Americans arrived here around 1300 years ago and New Orleans arose from a French settlement in 1718 on a natural levee inside a bend in the Mississippi. Four years later the settlement was destroyed by a hurricane. In 1791 it was again destroyed along with a Spanish fleet. Since then an average of one major hurricane has had some impact on the city every 7-10 years. Many of the modern hurricanes are straight from the rogues gallery: Betsy 1965, Camille 1969, Andrew 1992, Rita and Katrina 2005, Gustav and Ike 2008.

The last half-century has seen massive changes and not all have been good. Extensive levees and shipping channels have been built, failed, and then been reinforced. The wetlands have shrunk by 30% with over 2,000 square miles of land lost, which is around 80% of the loss of land throughout the entire country.

This wetland loss has occurred largely from human influences: a combination of land subsidence and hurricane damage, the blocking of the supply of mud and silt by the river levees, and vegetation die-off from the incursion of salt water through the crisscrossing 10,000 mile mass of commercial canals.

Wetland loss already is having a severe disruptive impact on local communities as is documented in “You can’t stop the water”The wetlands also provide an effective buffer to storm surge and especially to ocean waves, so their loss reduces the safety margin for New Orleans. Left as is and including the steady rise of sea levels from our warming earth, much of the wetlands could be lost in the next 50 years.

The current levee protection is for a 1:100 year stormThis tends to evoke a sense of safety but the reality can be different. 1:100 simply means there is a 1% chance of such a storm occurring in any one year. Viewed from this perspective, there is actually only around a 65% chance of such a storm occurring in the next 100 years. The bad news is that there is a 25-30% chance of one or more occurring in the next 30 years!

The return period also is almost universally quoted as a single value, without any accompanying assessment on the level of uncertainty. Consider for example the return period for hurricanes at Gulfport, where published estimates vary from 1:10 to 1:17 years. Estimates for major hurricane impacts at locations within 100 nm of New Orleans - which is well below the granularity of hurricane observations in the area - vary from 1:26 to 1:52 years. Similar results are found if one compares return periods over longer sDamage potential highlighting risk in the Gulf of Mexicotretches of coastlines.

Along the same lines, say we reduce the 100 y return period for a Katrina-like storm to 50% than we now have a 45% probability of one or more in the next 30 years and a 25-30% chance of a 1:200 year storm.

Experience has shown that although intense storms are dangerous and can cause great damage, intensity is not the only story. Katrina arrived as a middle-level Cat 3 - with only Cat 1 winds over New Orleans - yet it generated over 25 ft of storm surge. Ike in 2008 was only a Cat 2 at landfall, yet it generated substantial surges along the coast west from Louisiana into Texas that peaked at around 17 ft.

In Katrina and Ike it was the size that counted. This and the translation speed are often-neglected components of damage that can be of greater importance than the intensity.

One of the tools in the Global Risk, Resilience and Impacts Toolbox provides a quick way of assessing Cyclone Damage Potential (CDP) by combining intensity, size and translation speed into a single index that varies from one (little damage) to ten (extreme damage). The accompanying spatial plot of maximum CDP for cyclones during the period 1980-2010 emphasizes the danger that Louisiana and New Orleans face compared to the rest of the United States. 

Obviously Gulf communities are doing the best they can, but there is no room for complacency. Greater protection is not the only answer, given its enormous cost and the history of such protection causing severe problems elsewhere. One of the major themes of the Engineering for Climate Extremes Partnerships is one of including response and recovery into the planning stages, a process called “Graceful Failure”.

New Orleans has adopted some graceful failure components. Its extensive pumping facilities provide a way of removing water up to a point. Other cities also incorporate holding basins and even the use bike and walking paths as escape routes for floodwaters. Suggestions have been made to relocate the mouth of the MississippiPerhaps there are other complementary activities that can be accomplished and perhaps this can be done at much reduced cost. It surely is worth the effort to find out.

As the locals say “You can’t stop the water”.