New Orleans Levee System Performance during Hurricane Katrina: London Avenue and Orleans Canal South

Hurricane Katrina was one of the worst natural disasters in U.S. history. The effects of the hurricane were particularly devastating in the city of New Orleans. Most of the damage was due to the failure of the levee system that surrounds the city to protect it from flooding. This paper presents the results of centrifuge models conducted at Rensselaer Polytechnic Institute and the U.S. Army Corps of Engineers simulating the behavior of the levees at London Avenue North and South that failed during Hurricane Katrina. Those levees failed without being overtopped by the storm surge. Also included are the results of a centrifuge model of one levee section at Orleans Canal South, which did not fail during the hurricane. The key factor of the failure mechanism of the London Avenue levees was the formation of a gap between the flooded side of the levee and the sheetpile. This gap triggered a reduction of the strength at the foundation of the protected side of the levee. The results are fully consistent with field observations.     

New Orleans Levee System Performance during Hurricane Katrina: 17th Street Canal and Orleans Canal North

Centrifuge modeling of the 17th Street Canal and Orleans Canal North levees was performed in this study. During hurricane Katrina the levees on the 17th Street Canal failed, leading to breaches in the outfall canal in the city. Two mechanisms were observed in the centrifuge modeling that could cause a breach. First, a water-filled crack formed in front of the floodwall as the water in the canal rose above the top of the levee. The levees on the 17th Street Canal, which were supported on clay foundations, failed when this cracking led to a translational (sliding) failure in the clay layer commencing at the toe of the floodwall. The levees at Orleans Canal North, where failure did not occur, were also modeled to demonstrate that the model tests could successfully simulate failure and nonfailure conditions. The centrifuge model tests identified the importance of the crack formation in relation to the stability of the floodwall. These tests also confirmed that levee geometry, floodwall depth of penetration, and the underlying soil profile were all critical to the performance of the system under flood loading.     

Stability of I-Walls in New Orleans during Hurricane Katrina

Failures of I-walls during Hurricane Katrina were responsible for many breaches in the flood protection system in New Orleans. Six breaches were examined in detail by Task Group 7 of the Interagency Performance Evaluation Taskforce. Four of these failures and breaches, which occurred before the water levels reached the top of the wall, were not caused by overtopping erosion. The failure of the I-wall at the 17th Street Canal resulted from shear through the weak foundation clay. The south failure of the London Avenue I-wall was caused by subsurface erosion, which carried massive amounts of sand inland, and removed support for the wall, leading to catastrophic instability. At the north breach on London Avenue, the failure was caused by high pore pressures, combined with a lower friction angle in the loose sand, which resulted in gross instability of the I-wall under the water pressure load from the storm surge. Looking back, with the benefit of 20-20 hindsight, these stability and erosion failures can be explained in terms of modern soil mechanics, exploration techniques, laboratory test procedures, and analysis methods. An important factor in all of the cases investigated was development of a gap behind the wall as the water rose against the wall and caused it to deflect. Formation of the gap increased the load on the wall, because the water pressures in the gap were higher than the earth pressures that had acted on the wall before the gap formed. Where the foundation soil was clay, formation of a gap eliminated the shearing resistance of the soil on the flood side of the wall, because the slip surface stopped at the gap. Where the foundation soil was sand, formation of the gap opened a direct hydraulic connection between the water in the canal and the sand beneath the levee. This hydraulic short circuit made seepage conditions worse, and erosion due to underseepage more likely. It also increased the uplift pressures on the base of the levee and marsh layer landward of the levee, reducing stability. Because gap formation has such important effects on I-wall stability, and because gaps behind I-walls were found in many locations after the storm surge receded, the presence of the gap should always be assumed in I-wall design studies.     

New Orleans and Hurricane Katrina. III: The 17th Street Drainage Canal

The failure of the levee and floodwall section on the east bank of the 17th Street drainage canal was one of the most catastrophic breaches that occurred during Hurricane Katrina. It produced a breach that rapidly scoured a flow pathway below sea level, so that after the storm surge had largely subsided, floodwaters still continued to stream in through this breach for the next two and a half days. This particular failure contributed massively to the overall flooding of the Metropolitan Orleans East Bank protected basin. Slightly more than half of the loss of life, and a similar fraction of the overall damages, occurred in this heavily populated basin. There are a number of important geotechnical and geoforensic lessons associated with this failure. Accordingly, this paper is dedicated solely to investigating this single failure. Geological and geotechnical details, such as a thin layer of sensitive clay that was laid down by a previous hurricane, proper strength characterization of soils at and beyond the toe of the levee, and recognition of a water-filled gap on the inboard side of the sheet pile cutoff wall are judged to be among the most critical factors in understanding this failure. The lessons learned from this study are of importance for similar flood protection systems throughout other regions of the United States and the world.     

Erosion Study of New Orleans Levee Materials Subjected to Plunging Water

During Hurricane Katrina, overtopping water caused erosion and subsequent failure of several sections of I-type flood walls in New Orleans. Erosion stemmed from the kinetic energy of water falling from the top of the flood wall, unlike the typical surface erosion caused by shear flow. This study evaluated the effects of important parameters of levee soils—fines content, degree of compaction (DOC), clay mineralogy, and water content in relation to the erosion behavior of New Orleans levees subjected to the plunging water. In general, test results showed that a higher fines content contributed to greater erosion resistance. The trend became unclear when fines content exceeded 20–25%. A higher degree of compaction did not necessarily contribute to greater erosion resistance. Underwater soaked soils showed much less erosion resistance than nonsoaked soils. Soils containing expansive clay minerals showed less erosion resistance than soils containing nonexpansive clay minerals.     

Geologic Conditions Underlying the 2005 17th Street Canal Levee Failure in New Orleans

A careful program of subsurface sampling and cone penetration test soundings was employed to characterize the geologic conditions beneath the failed portion of the 17th Street Canal levee in New Orleans, where a 150?m long section of the levee and floodwall translated up to ～ 16?m when flood waters rose to 1–2?m of the wall’s crest on August 29, 2005, during Hurricane Katrina. The subsurface conditions are characterized by discrete layers of fill placed upon the historic cypress swamp, which is underlain by a deeper, prehistoric cypress swamp. These swamp deposits were consolidated beneath the levee, and in the area of the 2005 failure, the swamp materials infilled a natural depression believed to be an old slough, which dipped below the sheetpile tips for a distance of about 50?m, which corresponds to where the breach appears to have initiated. Detailed examination of the recovered soils suggest that recent hurricanes periodically inundated the swamps with saline and/or brackish water, which cause a mass dieoff of swamp vegetation and flocculation of suspended clays, due to the sudden increase in salinity. These conditions promote deposition of discontinuous clay seams beneath layers of organics, which are then covered by fresh water swamp deposits. This sequence is repeated, like a series of tree rings, throughout the swamp deposits. The cypress swamp deposits lying beneath the levee also exhibit high hydraulic conductivity. These materials contain corky wood, and recovered samples often exhibited densities less than water. Nine of the post-Katrina borings recovered intact samples of a basal rupture surface comprised of organic silty clay exhibited near zero residual shear strength after shearing 80 to 100 mm.     

Development of the New Orleans Flood Protection System prior to Hurricane Katrina

The system of flood protection surrounding New Orleans and its adjoining parishes prior to Hurricane Katrina evolved over a period of 280?years. The earliest drainage works sought to elevate the river’s natural levees and excavate drainage canals leading towards Bayou St. John, the only natural break across the Metairie-Gentilly distributary ridge. An extensive zone of Cypress Swamps occupied the levee flank depression between the ridge and Lake Pontchartrain. 58?km of drainage canals were excavated across the natural levee backslope and through the swamp depressions bordering the lake between 1833 and 1878. These canals sought to drain the lower portions of the city, which suffered periodic outbreaks of yellow fever, which killed more than 100,000 people during the 19th century. The city has not suffered flooding from the Mississippi River since 1895, most damaging floods having emanated from hurricane surge off of Lake Pontchartrain. Since 1559, 177 hurricanes have struck the Louisiana coastline. A system of pump stations was constructed between 1895 and 1927, which pump water into the river, the lake, and adjacent bayous. The cypress swamps were replaced by the Lakeview and Gentilly residential districts, built after 1945. This old swamp zone has settled as much as 3+?m since 1895. After 1927 the Army Corps of Engineers assumed a leadership role in providing flood control infrastructure, supervising the Mississippi River & Tributaries Project in 1931–1972. In 1955 the Corps role was expanded to include the City of New Orleans, which included maintaining capacity and freeboard of the old drainage canals. After a series of lawsuits between 1961 and 1977, the Corps was forced to employ concrete flood walls along the subsiding drainage canals. These walls were constructed in the 1990s, though some transition elements remained incomplete when Hurricane Katrina struck in August 2005.     

New Orleans and Hurricane Katrina. IV: Orleans East Bank (Metro) Protected Basin

This paper addresses damage caused by Hurricane Katrina to the main Orleans East Bank protected basin. This basin represented the heart of New Orleans, and contained the main downtown area, the historic French Quarter, the Garden District, and the sprawling Lakefront and Canal Districts. Nearly half of the loss of life during this hurricane, and a similar fraction of the overall damages, occurred in this heavily populated basin. There are a number of important geotechnical lessons, as well as geo-forensic lessons, associated with the flooding of this basin. These include the difficulties associated with the creation and operation of regional-scale flood protection systems requiring federal and local cooperation and funding over prolonged periods of time. There are also a number of engineering and policy lessons regarding (1) the accuracy and reliability of current analytical methods; (2) the shortcomings and potential dangers involved in decisions that reduced short-term capital outlays in exchange for increased risk of potential system failures; (3) the difficulties associated with integrating local issues with a flood risk reduction project; and (4) the need to design and maintain levees as systems; with each of the many individual project elements being required to mesh seamlessly. These lessons are of interest and importance for similar flood protection systems throughout numerous other regions of the United States and the world.     

New Orleans and Hurricane Katrina. I: Introduction, Overview, and the East Flank

The failure of the New Orleans regional flood protection systems, and the resultant catastrophic flooding of much of New Orleans during Hurricane Katrina, represents the most costly failure of an engineered system in U.S. history. This paper presents an overview of the principal events that unfolded during this catastrophic hurricane, and then a more detailed look at the early stages of the event as the storm first drove onshore and then began to pass to the east of the main populated areas. The emphasis in this paper is on geotechnical lessons and it also includes broader lessons with regard to the design, implementation, operation, and maintenance of major flood protection systems. This paper focuses principally on the early stages of this disaster, including the initial inundation of Plaquemines Parish along the lower reaches of the Mississippi River as Katrina made landfall, and the subsequent additional early levee breaches and erosion along the eastern flanks of the regional flood protection systems fronting Lake Borgne that resulted in the flooding of the two large protected basins of New Orleans East and St. Bernard Parish. Significant lessons learned include (1) the need for realistic assessment of risk exposure as an element of flood protection policy; (2) the importance of considering erodibility of embankment and foundation soils in levee design and construction; (3) the importance of considering all potential failure modes; and (4) the problems inherent in the construction of major regional systems over extended periods of multiple decades. These are important lessons, as they are applicable to other regional flood protection systems in other areas of the United States, and throughout much of the world.     

New Orleans and Hurricane Katrina. II: The Central Region and the Lower Ninth Ward

The failure of the New Orleans regional flood protection systems, and the resultant catastrophic flooding of much of New Orleans during Hurricane Katrina, represents the most costly failure of an engineered system in U.S. history. This paper presents an overview of the principal events that unfolded in the central portion of the New Orleans metropolitan region during this hurricane, and addresses the levee failures and breaches that occurred along the east–west trending section of the shared Gulf Intracoastal Waterway/Mississippi River Gulf Outlet channel, and along the Inner Harbor Navigation Channel, that affected the New Orleans East, the St. Bernard Parish, and the Lower Ninth Ward protected basins. The emphasis in this paper is on geotechnical lessons, and also broader lessons with regard to the design, implementation, operation, and maintenance of major flood protection systems. Significant lessons learned here in the central region include: (1) the need for regional-scale flood protection systems to perform as systems, with the various components meshing well together in a mutually complementary manner; (2) the importance of considering all potential failure modes in the engineering design and evaluation of these complex systems; and (3) the problems inherent in the construction of major regional systems over extended periods of multiple decades. These are important lessons, as they are applicable to other regional flood protection systems in other areas of the United States, and throughout much of the world.     

Geology of the New Orleans Area and the Canal Levee Failures

The geologic history of the New Orleans area significantly influences the engineering properties of the foundation soils beneath the levees. Geologic and engineering data gathered from the levee breaches identify a spatially complex geomorphic landscape, caused by Holocene sea level rise, lateral changes in depositional environments, development of Mississippi River delta lobes, and the distributary channels associated with delta development. Overlying the Pleistocene surface beneath New Orleans are predominantly fine-grained, shallow water sediments associated with bay sound (or estuarine), nearshore-gulf, sandy beach, lacustrine, interdistributary, and paludal (marsh and swamp) environments. These environments define the New Orleans area history during the Holocene and comprise the levee foundation beneath the failure areas. A barrier beach ridge is present in the subsurface along the southern shore of Lake Ponchartrain, which blocked the filling of the lake with fluvial-deltaic sediments. This buried beach impacted the supply and texture of sediment being deposited by advancing distributary channels and influenced the engineering properties of these soils. Marsh and swamp soils beneath the failure area at the 17th Street Canal are much thicker in comparison to those beneath the London Avenue Canal failures because of the influence of the beach complex, and are thickest in the Industrial Canal area. Additionally, human activities in the New Orleans area during historic time contributed to the spatial complexity and affected the engineering properties of the foundation soils. These activities include construction of drainage and navigation canals, groundwater pumping, hydraulic filling of the Lake Ponchartrain lake front, and construction of levees to prevent river flooding. Human activities, combined with the geologic setting and subsidence in this region, are responsible for the unique landscape that was impacted by Hurricane Katrina.     

Analysis of the Stability of I-Walls with Gaps between the I-Wall and the Levee Fill

Following Hurricane Katrina an extensive investigation of the performance of floodwalls in the New Orleans area was undertaken by the U. S. Army Corps of Engineers and others. This investigation included detailed study of failures of cantilevered sheet pile “I-walls” during the hurricane. An important lesson from this investigation was that gaps can form on the canal side of I-walls as the water rises in the canal and causes the I-wall to deflect. Once formed, these gaps filled with water, resulting in significantly higher loads on the walls. Gap formation was a key factor in several I-wall failures, and modeling such gaps correctly is clearly an important aspect of analyzing I-wall stability. This paper describes simple procedures for estimating the depths of gaps behind I-walls, for calculating the loads to which they are subjected, and for including them in stability analyses. The effects of gaps on the stability of the 17th Canal and the London Avenue Canal I-walls are discussed.     

Evaluation of a T-Wall Section in New Orleans Considering 3D Soil-Structure Interaction

T-walls in New Orleans survived Hurricane Katrina whereas I-walls obviously failed in several sections. However, it is still unclear whether these T-walls truly survived the hurricane with a fair amount of safety margins or barely survived it with undetected damages. The initial design of T-walls was based on simplified loading conditions with limited consideration of soil-structure interaction. In this study, three-dimensional (3D) numerical analyses were conducted, incorporating realistic loading conditions and soil-structure interactions but with time-saving techniques to evaluate the detailed behavior of T-walls. This paper addressed the procedure of innovative 3D numerical analyses and important findings by using special structural elements in FLAC3D. From this study, T-walls were found to have adequate stress levels in H-piles and concrete walls. However, it showed that the major factor that may cause the instability of the T-wall was the slope instability-type unbalanced force. This unbalanced force, however, was counteracted by the batter piles so that the overall stability of the T-wall system could be maintained.     

Pavement Structures Damage Caused by Hurricane Katrina Flooding

In September of 2005, Hurricane Katrina devastated New Orleans and caused sustained flooding. Limited pre- and postflooding tests indicated that the pavement structures tested were adversely impacted by the flood water. Consequently, the Louisiana Dept. of Transportation and Development hired an independent contractor to structurally test approximately 383 km (238?mi) of the region’s federally aided urban highway system both inside and outside of the flooding area. Falling weight deflectometer (FWD) tests were performed every 161 m (0.1?mi) over each selected roadway, along with other field tests. The FWD data were imported into a geographical information system and plotted against a USGS geo-referenced map. Comparative analyses were made possible through the use of extensive flood maps made available through NOAA and FEMA. This arrangement made it possible to classify spatially and graphically all test points on the basis of flooding versus nonflooding, short flooding duration versus longer flooding duration, shallow flooding versus deep flooding, and thin pavements versus thick pavements. Three pavement types, asphaltic concrete, Portland cement concrete, and composite, were considered in this analysis. The statistical inference about the difference in the means of compared data groups was conducted with 95% confidence.     

Hurricane Katrina Storm Surge Reconnaissance

Hurricane Katrina (August 23–30, 2005) was one of the costliest and deadliest hurricanes to ever strike the United States, impacting low-lying coastal plains particularly vulnerable to storm surge flooding. Maximum storm surges, overland flow depths, and inundation distances were measured along the Gulf Coast of Florida, Alabama, Mississippi, and Louisiana. The vehicle-based survey was complemented by inspections with the reconnaissance boat along the Gulf Coast and the Mississippi Barrier Islands. The survey covered both the impact on the built and the natural environments. The storm surge peaked to the east of Katrina’s path exceeding 10?m in several locations along the Mississippi coastline. The storm surge measurements show that the lower floors of specially designed buildings were damaged by the surge of seawater and associated wave action, while the upper floors sustained minimal wind damage. The storm surge measurements along New Orleans Lakeshore allowed the investigators to exclude overtopping as failure mechanism for the 17th Street outfall canal levee. Hurricane Katrina’s storm surge distribution (Category 3 at landfall) is compared against Hurricane Camille’s storm surge distribution (Category 5 at landfall). The land loss on the barrier islands and the increased vulnerability of the US Gulf Coast to future hurricane storm surges is discussed.     

Hydrodynamic Investigation of Coastal Bridge Collapse during Hurricane Katrina

A number of U.S. coastal bridges have been destroyed by hurricanes, including three highway bridges in Mississippi and Louisiana during Hurricane Katrina (2005). This paper addresses three fundamental questions on the coastal bridge failures: (1) what were the hydrodynamic conditions near the failed bridge during the hurricane; (2) what was the cause of the bridge collapse; and (3) what was the magnitude of the hydrodynamic loading on the bridge under the extreme hurricane conditions. Guided by field observations of winds, waves, and water levels, two numerical models for storm surges and water waves are coupled to hindcast the hydrodynamic conditions. Fairly good agreement between the modeled and measured high watermarks and offshore wave heights is found, allowing an estimate of the surge and wave conditions near the bridges in nested domains with higher resolutions. The output of the coupled wave-surge models is utilized to determine the static buoyant force and wave forces on the bridge superstructure based on empirical equations derived from small-scale hydraulic tests for elevated decks used in the coastal and offshore industry. It is inferred that the bridge failure was caused by the wind waves accompanied by the storm surge, which raised the water level to an elevation where surface waves generated by strong winds over a relatively short fetch were able to strike the bridge superstructure. The storm waves produced both an uplift force and a horizontal force on the bridge decks. The magnitude of wave uplift force from individual waves exceeded the weight of the simple span bridge decks and the horizontal force overcame the resistance provided by the connections of the bridge decks to the pilings. The methodology for determining the hydrodynamic forcing on bridge decks can be used to produce a preliminary assessment of the vulnerability of existing coastal bridges in hurricane-prone areas.     

Reducing Erosion of Earthen Levees Using Engineered Flood Wall Surface

Erosion was one of the major causes for the failure of New Orleans levee system during Hurricane Katrina. Protection of flood walls from erosion failure can be achieved in many different ways. This study consisted of experimental research to develop engineered flood wall surfaces that can reduce the erosion energy of the plunging water before the water hits the levee surface, so that the flood protection system becomes more resilient. Test results were compared to hydrodynamics simulation results by using FLOW3D. The results revealed that the erosion resistance of levees can be substantially reduced by providing protective structures at the surface of the flood walls. An effectively designed protective structure could reduce the erosion depth as much as 40% and extend the erosion time as much as 400%.     

Case Histories in Soil and Rock Erosion: Woodrow Wilson Bridge, Brazos River Meander, Normandy Cliffs, and New Orleans Levees

This lecture presents four case history examples of erosion processes. Because the topic of soil and rock erosion is relatively underdeveloped in geotechnical engineering, an introduction precedes the case histories to describe some fundamental aspects of erosion. Erosion involves the soil or rock through its erodibility, the water through its velocity, and the geometry of the obstacle through its size and shape. Knowledge of these three components is needed for any erosion problem to be studied and solved. A set of fundamental issues are addressed in the first part including an erodibility classification for soils and rocks, an explanation of the stresses imposed by the water on the soil-water or rock-water interface, and an explanation of how the geometry impacts the problem. The Woodrow Wilson Bridge case history outlines a new and less conservative method to compute the scour depth and gives examples of bridge scour calculations. The Brazos River meander case history outlines a new method to predict meander migration and gives an example of migration calculations. The Pointe du Hoc case history gives an explanation of a process of rock cliff erosion. The New Orleans levees case history gives an example of erosion of levees by overtopping and proposes an erosion design chart for levee overtopping. Whenever possible the results are presented in a probabilistic fashion. All case histories make use of the erosion function apparatus, an apparatus developed to quantify the erodibility of a soil or rock and to give the constitutive law for erosion problems: the erosion function. The power point slides for the lecture including many photos of the case histories are available at ?http://ceprofs.tamu.edu/briaud/? under “Lectures” and the video (DVD) of the lecture is available from the author, free of charge.