Levee OvertoppingI think one of the most interesting parts of the Hurricane Katrina story from a geo

Levee OvertoppingI think one of the most interesting parts of the Hurricane Katrina story from a geologist’s perspective is how the levees actually failed. These key pieces of infrastructure are literally all that stands between New Orleans and another Katrina-level disaster, so understanding how they failed in this case is key to understanding future risks.At a most basic level, a human-built levee is a wall designed to keep water on one side and not the other. Levees can be made of many things; some are just earthen levees, made of piles of dirt and vegetation, while others can be reinforced by concrete and steel.New Orleans and the surrounding areas use a variety of levee types. This image comes from near one of the main failures along the Industrial Canal, a path through New Orleans that allows ships to move from Lake Pontchartrain to the North to the Mississippi River in the South.The Industrial Canal originally began construction in the early part of the 20th century and it has been gradually strengthened over time. In 2005, it was protected by levees several meters above the normal water level, but these levees proved totally inadequate during the storm. The failures along the Industrial Canal flooded the area known as the Lower Ninth Ward, directly hitting some of the poorest sections of New Orleans where the people could least afford to evacuate or rebuild. This area still is recovering from these breaches today.This image comes from the National Science Foundation’s report on the levee failures prepared right after the storm. It illustrates the importance of on-site investigations immediately after a failure; right here you can see both reasons why this levee failed and this evidence is only available because photographs were taken rapidly, before the site was repaired.First, see the ditch filled with water immediately behind the levee? Prior to Hurricane Katrina’s strike, this ditch wasn’t there. The ground should be flat or even elevated as you approach the back of the levee to give it support. This ditch shows one of the reasons why this levee failed.When Katrina struck, the storm surge flowed into the Mississippi River, Lake Pontchartrain, and the Industrial Canal. Even though New Orleans did not receive the strongest storm surge, the water level in this canal overtopped the levee.Water poured over the top of the levee, creating a waterfall several meters high behind the concrete wall. That waterfall eroded the dirt just behind the levee, removing the material the levee needs to hold back the weight of the water.The waterfall ate away at the levee’s support. Enough erosion of the ground behind a levee and the levee will fail, which is exactly what happened at several spots along the Industrial Canal.In the distance at this spot you can see one other feature that damaged this levee system; there’s a barge outside the canal. This barge broke away from its moorings and hit the levee right where that breach formed. Combined with the erosive force of the levee overtopping, the impact of this barge formed one of the major breaches in this levee system.The breaches along the industrial canal testify to why it’s so important to build levees that are tall enough. If a levee is overtopped, even if it is constructed well, the levee will fail. The levees along this canal simply weren’t prepared for the size of the storm surge they took.Over the past few days there have been several press reports stating that New Orleans is now “prepared for a 100-year storm”. However, based on everything I’ve read, they’re defining a 100-year storm as about a category 3 hurricane, when it’s still possible for New Orleans to be hit by storms even stronger than that with much bigger storm surges.In 2006, at the request of the U.S. Congress, a report was prepared stating that protecting against a category 5 storm, a “200-300 year event” in New Orleans, would require levees that were about 10 meters tall. The estimated cost at the time of building such a system was $11 billion. Since then, no action has been taken to begin building such a system.The Army Corps of Engineers, which manages the levee system, says that unlike in 2005 the levee system now present should not fail even if it is overtopped in a storm, and significant overtopping has happened in near-miss hurricanes since 2005 so there is some evidence that they’ve improved the construction. However, a single breach, even a surprise breach caused by something like the impact of that barge, could leave these areas vulnerable to the same combination as in 2005: overtopping of levees, erosion, and levee failure.-JBBImage credit: Rune Storesund/NSFhttp://www.ce.berkeley.edu/projects/neworleans/report/CH_6.pdfFull report and summaries:http://www.ce.berkeley.edu/projects/neworleans/http://bit.ly/1Kq0xUkhttp://bit.ly/1KSaEx0 -- source link

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