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Author: Paul F. Mlakar
The U.S. Army Corps of Engineers is analyzing what went wrong (and right) during and after Hurricane Katrina.
In terms of flooding, Hurricane Katrina was one of the strongest storms to hit the coast of the United States in the past century. Katrina first made landfall on the southeast coast of Florida as a Category 1 hurricane, then crossed Florida into the Gulf, where it grew in size and strength and became a Category 5 hurricane, then weakened again before it made a second landfall in southeast Louisiana. The storm then moved northward, pushing storm surge into coastal areas of Louisiana, Mississippi, and Alabama, and finally made a third landfall as a Category 3 storm in Mississippi.
Katrina caused the greatest loss of life and damage to property to the New Orleans metropolitan area, St. Bernard Parish, Plaquemines Parish, and the Mississippi Gulf Coast in recorded history. The hurricane created breaches in the floodwalls along the 17th Street Canal, the London Avenue Canal, and the Inner Harbor Navigation Canal (IHNC). Water flowed from Lake Pontchartrain through these breaches and inundated large urban areas in New Orleans to depths of as much as 20 feet. The levees in St. Bernard Parish and Plaquemines Parish were also overtopped, causing the inundation of substantial additional urban areas. The magnitude of the destruction, the extensive damage to the hurricane-protection system, and the catastrophic failure of a number of structures raised significant questions about the integrity of the flood-protection system prior to the storm and the capacity of the system to provide future protection, even after repairs.
Immediately following Katrina, the U.S. Army Corps of Engineers (USACE) undertook four important missions. The first was to rescue survivors and “unwater” the city so that it could begin to recover. Second, the chief of engineers established an Interagency Performance Evaluation Task Force (IPET) to provide credible, objective scientific and engineering answers to questions about the reliability of hurricane protection. Third, the reconstruction of the damaged protection infrastructure to the authorized level began as soon as possible based on the best information available. Finally, Congress directed USACE to study the feasibility of providing a higher level of protection in the future.
This article describes the progress of IPET on the second mission. The task force had to act swiftly to collect ephemeral data necessary for its study, such as water levels as a function of space and time in the affected region inferred from observations of residual evidence and eyewitness accounts. IPET also had to document the physical condition of the damaged infrastructure before it was changed by cleanup and reconstruction activities.
IPET collected this information in a completely open and transparent way. When approached by teams from the American Society of Civil Engineers (ASCE), National Science Foundation, and Louisiana Department of Transportation and Development, who had similar purposes, IPET integrated these teams into a joint endeavor to collect information for all of the interested parties.
This article includes (1) a general description of the hurricane-protection system in New Orleans and preliminary observations about the effectiveness of three representative cases of infrastructure and (2) a discussion of IPET’s study of these cases, the review by ASCE, and further scrutiny by the National Research Council (NRC).
New Orleans was founded in 1718 by the French explorers Iberville and Bienville. According to one anecdote, their engineer advised against settling on this particular parcel of land, which lies below sea level. Nonetheless, by the middle of the nineteenth century, the settlement had grown into the “Crescent City” surrounded by swampland and the Mississippi River. The environment was characterized by muddy streets, stagnant water, pathological conditions, frequent flooding, and limited room to grow.
In the progressive years around the turn of the twentieth century, a public consensus was reached in support of a serious drainage effort, and by the 1920s, a comprehensive system of gutters, drains, sewer mains, outfall canals, and pumping stations was in place. These measures saved lives, improved the quality of life, and became a model for the protection of low-lying regions worldwide. These improvements also allowed the city to expand outward from the higher ground close to the river into the lower area near Lake Pontchartrain.
By 1965, further enhancements were made in response to this growth. The major features of the hurricane and flood-protection system were substantial levees along the Mississippi River and the shore of Lake Pontchartrain (Figure 1- see PDF version for figures). A number of other levees were planned along the lakefront and the recently completed Mississippi River Gulf Outlet. Gates to control the inflow of hurricane surges into Lake Pontchartrain and a lakefront lock on the IHNC were also authorized but were not yet funded.
On September 9, 1965, Hurricane Betsy struck the Louisiana area, causing major flooding, loss of life, and property damage in New Orleans. A higher level of hurricane protection was subsequently authorized by Congress. In the ensuing four decades, the construction of those improvements had progressed (Figure 2). Many of the levees planned in 1965 were in place, but the gates and lock on Lake Pontchartrain had not been constructed. Additional protection was provided through floodwalls atop the levees along the canals at 17th Street, London Avenue, Orleans Street, and the IHNC Canals.
The “saucer-like” topography of metropolitan New Orleans is shown clearly in Figure 3, a cross section of the city from Lake Pontchartrain to the Mississippi River on September 20, 23 days after Katrina struck. The ground slopes gradually down from the river flood protection to the lakefront hurricane protection. At that time, the lake level was at an elevation of 1.5 feet, and the river was at 3.5 feet. The flooding in the bowl of the city was still higher than 5.0 feet. Protection along the London Canal had been restored to 10.0 feet.
The Michoud Generating Plant of Entergy Corporation, located near the intersection of the Gulf Intracoastal Waterway and the Mississippi River Gulf Outlet (Figure 3), is protected from hurricane surge by a levee along the waterfront. Levees around the world are generally constructed of soil (Figure 4) and designed to provide protection to lowlands for a short time during seasonal flooding, high tides, or tropical storms. Basically, a levee is an embankment of soil that has been placed and compacted to form a barrier against high water. The soil for the levee is generally taken from shallow pits adjacent to the levee, termed “borrow pits.” Unlike a dam, which is usually sited on a favorable foundation, levees are placed according to flood control needs regardless of the suitability of the foundation conditions. The grass on levee slopes helps protect the soils against erosion during flooding.
In a photo at the Michoud levee (Figure 5) taken by an Entergy security camera during Hurricane Katrina, the levee is invisible under the severe overtopping surge from the storm. Preliminary observations of residual high-water marks and hydrodynamic analyses of hurricane surge by IPET are consistent with the degree of overtopping shown in the photo.
Figure 6, taken by the teams jointly collecting ephemeral data in early October, shows a photo of the same levee from a slightly different angle. The chain link security fence has been damaged, and the back slope of the levee shows minor erosion, but the section of the levee itself is largely intact and still capable of providing protection from lesser surges.
This levee is representative of many that were overtopped by a loading higher than their design level but remained relatively intact. We often associate lessons learned with poor performance, but there is also something to be learned from cases of excellent performance. What was it about the embankment material, slope, protective vegetation, surge, and other conditions at the Michoud levee that resulted in its remaining intact? This will be a subject of comprehensive study as IPET proceeds.
Inner Harbor Navigation Canal Floodwall
The IHNC is an important commercial waterway that links Lake Pontchartrain, the Gulf Intercoastal Waterway, the Mississippi River Gulf Outlet, and the Mississippi River (Figure 2). In 1965 when Hurricane Betsy hit the area, the earthen levees that lined the IHNC were overtopped and breached. In an attempt to provide a higher level of protection, a floodwall was built atop the existing levees (Figure 7). These I-walls, so named because of their shape in cross section, are cantilevered structures that derive support solely from the soil in which they are founded. An I-wall consists of sheet pile embedded in the foundation that protrudes into a monolithic concrete wall above.
Like the Michoud levee, the initial indication is that the loading from Hurricane Katrina exceeded the level for which the I-wall was designed. This conclusion is based on observations of residual high-water marks and accounts by eyewitnesses, and corroborated by preliminary hydrodynamic calculations of the hurricane surge.
Unfortunately, the response of this wall was catastrophic (Figure 8). Significant stretches of the wall were breached on both sides of the canal, contributing to the flooding of the metropolitan area. Figure 9, taken shortly after the region was unwatered in October, shows the condition of the breached I-wall and the missing levee. Figure 10, immediately adjacent to the largest breach, shows that the remaining wall clearly tilts away from the canal. The significant trench was probably scoured by the water overtopping the wall. The extent of this scour diminishes with distance from the breach.
This wall is representative of walls throughout the system that were overtopped and failed to some degree and probably exacerbated the flooding. Thus, this I-wall will be the focus of intense study by IPET to determine how various hydraulic, geotechnical, and structural factors affected its performance. The study will include both state-of-the-art physical hydraulic modeling and analytic modeling on high-performance computers.17th Street Canal Floodwall
17th Street Canal Floodwall
The 17th Street outfall canal is on the boundary of Orleans and Jefferson Parishes (Figure 2). Water that collects in the interior basin of metropolitan New Orleans is evacuated to Lake Pontchartrain through this canal with the assistance of one of the largest pumping stations in the world located at the interior end of the canal. After Hurricane Betsy, an I-wall was constructed on top of the existing levee to increase protection against hurricane surges entering the canal from the lake.
Unlike the Michoud levee and the IHNC floodwalls, indications at this time are that the water in the 17th Street Canal did not reach the top of the I-wall. This conclusion, supported by hydrodynamic calculations of the storm surge and wave height, has been further substantiated by observations of the high-water marks following Katrina.
Regrettably, a portion of the wall lining the east side of the canal was breached. A photo (Figure 11) taken during the emergency closure of the breach shows that substantial flooding ensued. There is some evidence that the levee and floodwall may have translated as a unit away from the canal (Figure 12). The location and condition of the floodwall monoliths in the breach (Figure 13) are consistent with this hypothesis.
A comprehensive study by IPET will include state-of-the-art calculations of soil structure interaction on high-performance computers and physical modeling in large centrifuges. This will be a high-priority item for IPET, because some other parts of the system behaved similarly.
IPET is working toward completing a comprehensive analysis before the start of the next hurricane season on June 1, 2006. The work of IPET includes a number of interrelated tasks, each of which is being investigated by a team led jointly by an expert from USACE and an expert from an external organization. IPET is also working with other organizations conducting related studies and analyses to maximize its effectiveness within the short time frame of the study. Comprised of individuals from more than 40 organizations, IPET teams have a diversity and depth of knowledge and experience.
The activities of IPET are focused on answering five strategic questions: