Can We Protect New Orleans?

Background

Sources are cited by numbers. The numbers refer to categories in our References list.

 

What is a levee?

A levee is a mound of earth or some artificial material, such as concrete, that is built up along a boundary with some body of water to keep the river or lake from overflowing onto what the levee is protecting behind it.

(10)

 

What are some types of levee reinforcements?

A levee can have some special reinforcements, such as I-walls, T-walls, and aprons. I-walls are basically walls built into the ground around which the levee is built up. T-walls are basically the same, but they have a weighted, wider base; thus, they are somewhat more stable. Aprons are coverings that are put on top of the levees to help prevent erosion. They can either be artificial, like concrete, or natural, such as a pile of rocks. They are especially useful to prevent erosion on the backside of the levee in the case that overtopping occurs.

(10)

 

What is the history of the Mississippi River levee system?

The Mississippi River levees have been built in stages since the 1700s. At first farmers built them to protect their own farm land from being flooded along the Mississippi River. Then the government started getting involved in the construction and maintenance of the levees when the port city of New Orleans became increasingly important to the nation’s economic health. The canals were built into the system in 1878 to drain water from the city. The primary function of the levees was to protect from flooding from the Mississippi River, not from a hurricane. More recently, the focus has shifted to hurricane protection.

(3)

 

What is the effect of subsidence on the levee system?

But New Orleans has become too dependent on its levee and pumping systems because New Orleans itself is sinking. Much of it is already well below sea level. The city was built on wetlands that have been pumped dry and over a fault that is gradually swallowing up land. Consequently, levee failures are even more catastrophic because the water will just sit in the city. Subsidence also causes the levees to sink, thus decreasing the effective height of the levees.

(10, 11)

 

What have been the effects of past floods on the levee system?

In 1927, the Mississippi was flooding so the people of New Orleans destroyed some of the upstream levees to save the city before the flood reached New Orleans.  After Hurricane Betsy in 1965 the Army Corps of Engineers rebuilt the levee system by constructing I-walls, which are the cheapest and least stable form of protection. That was the last time the New Orleans levee system was seriously updated.

(3)

 

What was the state of the Mississippi River levee system pre-Katrina?

A map of this is shown in the maps section of the webpage. The levee system was supposedly built for a Category 3 hurricane. Levees are present on either side of the Mississippi River to prevent flooding from its rising river bed, and levees are present along Lake Pontchartrain, the Industrial Canal, and the Mississippi River Gulf Outlet to prevent flooding from rising water and storm surges from hurricanes. An ample Hurricane Protection System was proposed in 1992 and construction had begun on it, but by the time of Hurricane Katrina only 45% of it was completed.

(3)

 

Where were the levee breaches caused by Hurricane Katrina and what caused them?

 

17^th Street Canal and London Avenue Canal Breaches:

At one point along the 17^th Street Canal Breach and at two points along the London Avenue Canal, the levees were not overtopped but failed. The levees in this area  are about 14 feet, but the water inside the canals was only at 8.5 feet. The canals were backed up with water from the storm surge coming from Lake Pontchartrain. The pumps in the canals were unable to pump the water out because the pumps were located below sea level in the section of the canal in the interior of the city, not near the lake. The levees along both canals are concrete I-walls atop earthen levees. The concrete walls were actually pushed aside by the water pressure building up inside the canals. The steel pilings driven into the soil were too shallow, and the soil foundations in which the concrete walls were anchored in were poor, too soft, and permeable. Water was able to seep through and undermine the foundations and wedge the wall from its foundations, causing the whole wall to be pushed over and water to enter the city. The canals, which are supposed to pump water out of the city actually caused much of New Orleans to flood by letting water into the city. The breaches at the 17^th St. Canal and London Avenue Canal were caused by engineering failures. The levees were built of poor soil, the pilings were not deep enough, and the pumping system was designed poorly.

 

Funnel Effect, Industrial Canal Breach, and Flooding in St. Bernard Parish and New Orleans East:

A storm surge many feet higher than predicted for a Category 3 storm preceded Hurricane Katrina into New Orleans. It came down the Mississippi River Gulf Outlet (MR-GO). It then entered the Intracoastal Waterway. While in the MR-GO and Intracoastal Waterway, the storm surge overtopped the earthen levees along St. Bernard Parish to the south and New Orleans East to the north. The overtopping of the levees caused erosion on the backside of the earthen levees as water came spilling over, weakening the system even more.  The Intracoastal funneled the storm surge into the Industrial Canal where it met up with storm surge coming in from Lake Pontchartrain. The levees with concrete floodwalls on the Industrial Canal were breached once on the west side into Bywater and twice on the east side into the Ninth Ward, where some of the worst flooding occurred. All of the levees in this area were overtopped. The water was 1.7 feet above the 13-foot levee. The overtopping eroded the levee on the protected side, letting more water pour in. However, some evidence shows that the initial flooding of the Ninth Ward occurred before the levees were overtopped. The levee was built on top of marsh which was on top of clay which in turn was on top of sand. However, the sand was at a depth too great to affect the integrity of the levee. A gap, or wedge, formed on the canal side of the levee between the wall and the marsh foundation increased the water pressure on the wall and probably caused it to fall over and flood the Ninth Ward.

 

Many of the levee breaches occurred where two different types of levees were joined together. At these joints of the two kinds of materials the weaker one would fail. This is why standardization of levees will be key in the solution. Different groups built on top of each other’s work at different times causing discontinuities in the system.

(4)