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New Orleans' Achilles Heel: a Storm Surge on the Mississippi River?

By: Dr. Jeff Masters, 7:23 PM GMT on June 27, 2013

In the wake of the unthinkable devastation wrought by Hurricane Katrina in New Orleans in 2005, Congress approved a $14.5 billion upgrade to the city's flood defenses--the Hurricane and Storm Damage Risk Reduction System (HSDRRS), a 139-mile system of levees, walls and gates designed to protect against a 1-in-100-year storm surge, equivalent to what a Category 3 hurricane would bring. The last link of this formidable barrier system was completed on June 17, when the Army Corps of Engineers announced that they had finished a $3 billion flood-control ring around the West Bank of Orleans, Jefferson, Plaquemines and St. Charles parishes, on the opposite side of the Mississippi River from New Orleans. The only work remaining on the $14.5 billion Hurricane and Storm Damage Risk Reduction System is a $615 million project to install three permanent pumps to replace temporary pumps installed on canals leading from New Orleans into Lake Pontchartrain. The new flood defense system already has undergone a stern test, thanks to Hurricane Isaac of 2012. Isaac was a large, slow-moving Category 1 storm with 80 mph winds that brought a storm surge characteristic of a Category 2 storm to New Orleans. The new flood defenses performed admirably, giving confidence that the city's new flood defenses can indeed withstand the 15' storm surge that a 1-in-100 year Category 3 hurricane might bring. The Louisiana coastline has experienced 59 hurricane strikes since 1851 (including Isaac), but only one of these hurricanes brought a storm surge to New Orleans capable of overwhelming the 2013 levee system--if it behaves as designed. That storm was the deadliest hurricane in Louisiana history, the 1893 Chenier Caminanda hurricane, which hit the coast south of New Orleans as a Category 4 storm. Over 2,000 people died in the storm, mostly due to storm surge.

Figure 1. The Gulf Intracoastal Waterway West Closure Complex, part of the $14.5 billion upgrade to New Orleans' flood defenses in the wake of Hurricane Katrina.

New Orleans' Achilles Heel: the Mississippi River levee system?
But the New Orleans levee system has an Achilles' heel, one that makes it vulnerable to a mere Category 1 hurricane storm surge. That weakness is the levees that protect the city from the Mississippi River. Although these levees lie more than 100 miles upriver from the Gulf of Mexico, hurricane storm surges from five storms over the past 50 years--Betsy (1965), Katrina (2005), Georges (1998), Isaac (2012), and Gustav (2008)--have pushed storms surges of seven feet or higher upriver to New Orleans. These large surges can occur because strong sustained easterly winds push water into shallow Breton Sound, overtopping the river?~@~Ys east bank south of Pointe ?| la Hache, Louisiana. The surge then penetrates into the Mississippi River, and is confined by levees on the west bank. The main channel?~@~Ys width and depth allow the surge to propagate rapidly and efficiently upriver. Normally, these storm surges moving up the Mississippi River are not a concern for New Orleans, since hurricanes typically arrive in August, September, and October, when the river is at its lowest flow levels, more than 15' below the tops of the levees. But if any of these five hurricanes had occurred when the Mississippi River was in flood--when the river is just 3' below the tops of the levees in New Orleans--the storm surge coming up the river would have easily pushed water over the tops of the levees, filling the below-sea level bowl that much of the city lies in. For example, Hurricane Katrina pushed the highest storm surge on record up the Mississippi River, reaching 13' in New Orleans. But since the river was only at a 3' water level prior to the hurricane, the surge stayed 4' below the tops of the levees. But it wouldn't take a Category 3 hurricane to flood New Orleans if the Mississippi River were at flood stage. A large, slow-moving storm like Hurricane Isaac of 2012 would be able to overwhelm New Orleans' flood defenses. During Hurricane Isaac, a storm surge estimated at 12' moved up the Mississippi in Plaquemines Parish, south of New Orleans. Since salt water is more dense than fresh water, the surge traveled along the bottom of the river, with the fresh water flow of the river lying on top. The surge continued upriver, and before reaching New Orleans, encountered an underwater barrier in Plaquemines Parish. This barrier was constructed by the Army Corps of Engineers in the summer of 2012, in order to keep salt water from moving upstream and contaminating the drinking water for Plaquemines Parish and New Orleans (salt water had made it 90 miles upriver to the outskirts of New Orleans, due to the river being 7' below average in height due to the drought of 2012.) The barrier was able to completely block the flow of salt water upriver, and no salt water made it as far as New Orleans. However, the massive intrusion of ocean water into the river channel caused the mighty Mississippi's fresh water flow to back up for hundreds of miles. Water levels were elevated by 10' in New Orleans (103 miles upstream from the mouth of the Mississippi), 8' in Baton Rouge (228 miles upstream), and 1.4' at Knox Landing, an amazing 314 miles upstream.

Figure 2. Carrollton gage on the Mississippi River at New Orleans. Flood stage on the gage is at 17', which is 3' below the top of the city's levees. Image credit: USACE.

Figure 3. Water level at the Carrollton gage on the Mississippi River at New Orleans during 2013 (solid black line) and during 2012 (dashed black line.) The river is currently 9' higher than it was in 2012. The passage of Hurricane Isaac August 28 - 30, 2012 is apparent as a sharp spike from two to nearly ten feet (the water level actually hit eleven feet, but the averaging in this plot obscures this fact.) Prior to Isaac's arrival, the river was running just two feet above its record low. Isaac drove a storm surge of ten feet to New Orleans, raising the river level to a stage height of eleven feet. Levees on the Mississippi protect New Orleans to a stage height of twenty feet, so Isaac's storm surge was nine feet below the tops of the levees. Image credit: rivergages.com.

Figure 4. Water level at the Carrollton gage on the Mississippi River at New Orleans during the passage of Hurricane Isaac in August 2012. Prior to Isaac's arrival the river was running just two feet above its record low. Isaac drove a storm surge of ten feet to New Orleans. Levees on the Mississippi protect New Orleans to a stage height of 20 feet, so Isaac's storm surge was nine feet below the tops of the levees. Image credit: NOAA Tides and Currents.

The risk of an early-season hurricane hitting during a Mississippi River flood
A huge pulse of flood waters began heading southwards down the Mississippi River in late May, thanks to runoff from torrential rains associated with the deadly thunderstorms that killed 23 people in Oklahoma. The river crested at major flood stage in St. Louis on June 4, and that crest propagated steadily downstream, reaching New Orleans on June 20. The river crested at 12.9', which is about 7' below the tops of the levees protecting the city from the river. The river is will slowly fall over the next few weeks. However, the Upper Mississippi River will reach major flood stage during the first week of July, due to heavy rains over the past week. This pulse of flood water will arrive in New Orleans in late July, keeping the river level higher than average. A slow-moving Category 1 hurricane like Hurricane Isaac hitting during July or August this year could potentially have a storm surge capable of overwhelming the river's levees and flooding the city, due to the relatively high water in the Mississippi River this summer.

While I expect that the risk of storm surge flooding in New Orleans due to an early season hurricane hitting when the Mississippi River is high is only about a 1-in-250 year event, we should expect an increase in this risk in future years, due to climate change. As I blogged about earlier this June, the Atlantic hurricane season is getting longer in association with increased ocean temperatures, and more early-season named storms are occurring in May and June. Heavy precipitation events--predicted by climate models to increase due to more heat and moisture in the atmosphere from global warming--are also on the increase in the U.S., raising the odds of unusually high water in the Mississippi River. Fortunately, the latest 2-week forecast from the GFS model predicts high wind shear over the Gulf of Mexico well into July, and there are no predictions from our reliable models of a potential tropical storm forming in the Atlantic through at least July 3.

A 20% chance that New Orleans will suffer a catastrophic flood in the next 30 years
According to the Army Corps of Engineers, New Orleans' flood defenses currently protect the city from a 1-in-100 year hurricane storm surge, and from a 1-in-750 year Mississippi River flood. A 1-in-100 year flood has a 26% chance of occurring in a 30-year period, so I rate New Orleans' odds of suffering a catastrophic flood that overwhelms its flood defenses at about 20% over the next 30 years. However, these odds are gradually increasing. Coastal Louisiana is sinking, and sea level is rising. These twin effects have given Southern Louisiana the highest levels of relative sea level rise of anyplace in the U.S.: 3' over the past 100 years. While the state of Louisiana has recently approved an ambitious $50 billion plan to help slow down subsidence by diverting sediment from the Mississippi River into coastal marshlands, sea level rise will not slow down this century unless strong action is taken by the nations of the world to cut down Earth's fever by limiting burning of coal, oil, and natural gas. The strongest Atlantic hurricanes are expected to grow stronger and dump up to 20% more precipitation by century's end, increasing the odds of a catastrophic New Orleans flood. New Orleans will need additional multi-billion dollar flood defenses in coming decades if we hope to keep the city. And the U.S.--and the world--need New Orleans. There has to be a port at the end of the Mississippi River. Between Baton Rouge and New Orleans, the Lower Mississippi River has four of the fifteen largest ports in America, which handle over 60% of all U.S. grain exports to the world, thanks to the barges moving downriver. Going upriver, those barges transport the petrochemicals, fertilizers, and raw materials essential for the functioning of U.S. industry and agriculture. When those ports have been closed due to barge accidents or high water levels, the cost has been $300 million per day to the U.S. economy. If we lose the Port of New Orleans for an extended period of time, there are major impacts to the U.S. economy, global economy, and world food supplies. Though the cost would be huge--in excess of $100 billion-- we should consider building at least 1-in-1,000 year flood defenses for New Orleans. The Netherlands has built 1-in-10,000 year flood defenses for their important cities--why are we building only 1-in-100 year defenses, which will only offer more like 1-in-50 year protection by mid-century?

Recent storms surges on Mississippi River at New Orleans
Below is a look at the the tracks of all of the named storms during the period 2008 - 2012 that affected Louisiana, and the level of storm surge experienced on the Mississippi River at New Orleans.

Hurricane Isaac brought a storm surge of 10' to the Mississippi River at New Orleans. Isaac made landfall in Louisiana on August 29, 2012, as a Category 1 hurricane with top winds of 80 mph.

Hurricane Gustav brought a storm surge of 7' to the Mississippi River at New Orleans. Gustav made landfall in Louisiana on September 1, 2008, as a Category 2 hurricane with top winds of 105 mph.

Tropical Storm Ida brought a storm surge of 3' to the Mississippi River at New Orleans. Ida made landfall on November 10, 2009, in Alabama with top winds of 60 mph.

Tropical Storm Lee brought a storm surge of 2' to the Mississippi River at New Orleans. Lee made landfall on September 3, 2011, on the Central Louisiana coast with top winds of 60 mph.

Other recent named storms that have brought a storm surge of 2 - 3' to the Mississippi River at New Orleans include: Tropical Storm Bill (2003), Tropical Storm Isidore (2002), Hurricane Lili (2002), Tropical Storm Hanna (2002), Hurricane Danny (1997), Hurricane Andrew (1992), Hurricane Florence (1988), and Tropical Storm Beryl (1988).

Thanks go to wunderground member Patrick Pearson for providing some of the links used in this article.

This will be my last "live" post until Tuesday, as I'll be taking a few days off. There's going to be a wicked hot heat wave over the Western U.S. and Western Canada while I'm gone, and wunderground's weather historian, Christopher C. Burt, will be updating his blog over the next few days if some notable all-time extremes are observed.

Jeff Masters

Tropical Hurricane

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.