Hurricane Katrina and New Orleans: Anatomy of a Disaster

Anatomy of a Disaster: Hurricane Katrina and New Orleans

Abstract

Hurricane Katrina was one of the worst natural disasters in US history.
Resulted in over 1600 fatalities and $30B in direct economic losses in southern Louisiana.</li><li>Louisiana and Mississippi coastlines experienced the highest surge level recorded in North America.</li><li>Katrina-generated waves equaled the highest previously measured by NOAA buoys.</li><li>New Orleans exemplifies the risk of living below sea level.</li><li>The Interagency Performance Evaluation Task Force (IPET) was established to examine the performance of the hurricane protection system (HPS).</li><li>IPET provided real-time input to repairs and rebuilding.</li><li>Forensic analysis depended on modeling and simulation of hurricane surge and waves.</li><li>Existing computational tools were used to define future surge and wave hazards.</li></ul><h4 id="5d5b8003-fe0f-4f0b-9664-0bf864973f6d" data-toc-id="5d5b8003-fe0f-4f0b-9664-0bf864973f6d" collapsed="false" seolevelmigrated="true">1. Introduction</h4><h5 id="3276b653-eeea-4cc1-b3b1-b8f66d894dc3" data-toc-id="3276b653-eeea-4cc1-b3b1-b8f66d894dc3" collapsed="false" seolevelmigrated="true">1.1. Background</h5><ul><li>Four decades since Hurricane Betsy caused major flooding in New Orleans.</li><li>Complacency was a factor.</li><li>The HPS in 2005 was based on a 1965 hazard definition.</li><li>Compromised by legal and fiscal battles.</li><li>Some structures were at lower than authorized design elevations and remained incomplete.</li><li>Hurricane Katrina, a Category III storm, caused massive flooding and losses.</li><li>Katrina created the highest measured surge and equaled the highest wave height recorded on a NOAA buoy in North America.</li><li>Extensive overtopping of earthen levees caused most of the 50 structural breaches.</li><li>Failure of four floodwalls before water reached their design limits added most of the floodwaters to downtown New Orleans.</li><li>Insured losses made Katrina the number one disaster globally since 1970.</li><li>Over 200 miles of the 350 miles of levees and floodwalls were damaged.</li><li>Loss of life exceeded 1600, and direct property losses approached$ 30B $. Total losses were estimated at around$ 200B $.</li></ul><h5 id="c424187b-8874-4a61-baf1-ed453799d9d8" data-toc-id="c424187b-8874-4a61-baf1-ed453799d9d8" collapsed="false" seolevelmigrated="true">1.2. Investigation</h5><ul><li>The Interagency Performance Evaluation Task Force (IPET) was a primary investigative effort.</li><li>IPET was established by the Chief of Engineers.</li><li>Task Force included experts from 25 universities, 25 private companies, and 10 government agencies.</li><li>Peer review by the American Society of Civil Engineers External Review Panel and the National Research Council Committee on New Orleans Regional Hurricane Protection Projects.</li><li>ASCE review was continuous to facilitate immediate transition of findings to the design of repairs.</li><li>NRC review was a more strategic and traditional independent review.</li><li>The IPET was charged with answering five principal questions:<ul><li>The System: What were the pre-Katrina characteristics of the HPS components; how did they compare to the original design intent?</li><li>The Storm: What was the surge and wave environment created by Katrina and the forces incident on the levees and flood walls?</li><li>The Performance: How did the levees and floodwalls perform, what insights can be gained for the effective repair of the system, and what is the residual capability of the undamaged portions? What was the performance of the interior drainage system and pump stations and their role in flooding and un- watering of the area?</li><li>The Consequences: What were the societal-related conse- quences of the flooding from Katrina to include economic, life and safety, environmental, and historical and cultural losses?</li><li>The Risk: What was the residual risk of flooding and losses in New Orleans prior to Hurricane Katrina, and what will it be following the planned repairs and improvements?</li></ul></li><li>The first four questions were traditional ‘‘what happened’’ analyses.</li><li>The results of their investigation were directly used to inform the repair and rebuilding process to mitigate those pre- Katrina vulnerabilities as possible.</li><li>The initial IPET draft report documenting these investigations and their findings and the repairs were simultaneously completed on 1 June 2006.</li><li>The risk assessment used the body of knowledge gained from the forensic analysis coupled with development of a new risk assessment methodology to hind-cast probability of flooding and both economic and life-safety risk prior to Katrina, estimate the near- term (2007) risk after repairs and rebuilding had been completed and project risk for the future (2011) when a planned system, designed to withstand the 100-year surge and wave event, is scheduled to be in place.</li><li>Following the 2006 release of the first draft, refinements and additional analyses were accomplished, in response to both ASCE and NRC comments, were made and final versions of the IPET report dealing with this first four questions were released in 2007.</li><li>The fifth question, dealing with analysis of the vulnerability of flooding and the residual risk of losses, used the forensic analyses as a launching platform and was largely addressed in the 2006 and 2007 time frame.</li><li>Draft documentation of the risk analysis was released in 2008, and final documentation in 2009 after modifications and enhancements based on ASCE (2007) and NRC (2009) reviews.</li><li>Interim flood vulnerability and risk maps were first released to the public in 2007.</li><li>The IPET work is chronicled in a 9 volume report, Performance Analysis of the New Orleans and Southeast Louisiana Hurricane Protection System (IPET, 2007/2009) and a supplemental report, A General Assessment of Vulnerability to Flooding and Risk for New Orleans and Vicinity, Past, Present, and Future (IPET 2, 2009).</li><li>The IPET Released individual Volumes I-IX.</li><li>The IPET investigation was complemented by two other investigative teams that worked separately and under different sponsorships.<ul><li>Team Louisiana was centered at Louisiana State University and sponsored by the state of Louisiana.</li><li>The Independent Levee Investigative Team (ILIT) was centered at the University of California at Berkeley and funded in part by the National Science Foundation.</li></ul></li><li>The three studies had significant overlap in their analyses but differed in approach and scope.</li><li>The results of the three efforts that address the areas of common evaluation are largely in agreement with the exception of three findings.<ul><li>The fundamental failure mechanisms for the canal floodwalls were identical, but the location of the failure plane at the 17th Street site was postulated to be in different subsurface layers.</li><li>One of the floodwall failures in the Inner Harbor Navigation Canal was specified by IPET to be similar to that of the 17th Street Canal failure, and as under seepage failure by the ILIT (2006).</li><li>The final difference involved the breaching of the exterior levees along St. Bernard Parish. Team Louisiana proposed that the levees ‘‘disintegrated’ before overtopping because of external erosion (Team Louisiana, 2007) while the IPET analysis demonstrated that the levees breached after overtopping due to erosion on their back side.</li></ul></li><li>While these differences are far from trivial, the majority of the findings were in close agreement.</li></ul><h4 id="920e0013-0a24-434e-bdab-bf94def48711" data-toc-id="920e0013-0a24-434e-bdab-bf94def48711" collapsed="false" seolevelmigrated="true">2. Analysis</h4><h5 id="f1232405-0b08-49bc-8779-ce4346dd9949" data-toc-id="f1232405-0b08-49bc-8779-ce4346dd9949" collapsed="false" seolevelmigrated="true">2.1. The System</h5><ul><li>The 350 miles of levees and floodwalls comprised the Hurricane Protection System (HPS).</li><li>HPS Included over 70 pumping stations and innumerable transitions, gates, and closures.</li><li>Over 200 miles of structures and approximately half of the pumping stations were damaged by the surge, waves, and flooding created by Katrina.</li><li>Elevation issues in New Orleans were addressed in Volume II of the IPET report.</li><li>Because of the complex and variable subsidence in Southeast Louisiana, establishing an accurate vertical reference for measurements has been a constant challenge.</li><li>Efforts by the Corps of Engineers and the NOAA National Geodetic Survey (NGS) accelerated to establish accurate elevations for reference points using GPS technology.</li><li>The relationship of local mean sea level (LMSL) to the geodetic datum was also established to provide a complete reference system for all analysis, repair, and planning activities.</li><li>Additional surveys established the post-Katrina elevation of all critical features and structures that comprise the HPS as well as perishable data such as high-water marks resulting from flooding.</li><li>Digital elevation data obtained by various methods, including airborne LIDAR, have been corrected to the geodetic datum and have been made available through the IPET Web site.</li><li>The data provided a foundation for analyses and the application of the results in the HPS repairs.</li><li>Accurately defining the elevations of the current structures provided a clear definition of the changes needed to achieve authorized protection elevations for the system and ultimately achieve 100-year or higher levels of protection.</li><li>Guidance has been provided to update Corps of Engineers criteria and methods for managing the reference data in areas of rapid and variable subsidence.</li><li>Volume III of the IPET report documents the hurricane protection system (HPS) as it existed prior to Katrina.</li><li>The HPS comprised three separate authorized projects, designed and constructed in multiple steps over time, starting in 1965 and continuing through August 2005.</li><li>The design criteria for the HPS was developed using the Standard Project Hurricane (SPH) concept.</li><li>SPH defined as a hypothetical hurricane intended to represent the most severe combination of hurricane parameters that is reasonably characteristic of a specified region, excluding extremely rare combinations.</li><li>The maximum surge elevations created by the SPH traveling along a few select tracks was used as the basis of design.</li><li>These surge levels were translated into required structure elevations (including freeboard) using methods accepted at the time.</li><li>This defined performance analysis of individual structures to better under- stand the causes of catastrophic breaching.</li><li>Pumping stations and interior drainage facilities are not part of the official hurricane protection project; their purpose is to remove rainwater and groundwater from low elevation areas.</li><li>During Katrina, most were evacuated because of a lack of safe havens for operators or alternative power supplies.</li><li>However, they are integrated physically and operationally to the HPS and as such were considered in the examination of flooding.</li><li>A brief description of the drainage system and pump stations and their condition and capacities is included in Volume III.</li><li>Bottom line: The HPS in place in 2005 could not perform as a system.</li><li>In some areas, it was not completed (especially to the south and west of the Mississippi River), and in others, the lower than authorized elevations reduced its intended protection.</li><li>The capacity for protection also varied because many of the levee sections had little resilience to overtopping, thus performing adequately until overtopped and then failing catastrophically.</li><li>Sections of I-walls failed because designs did not consider the failure modes that caused them to breach.</li><li>The designs of the levee-floodwall structures along the outfall canals were particularly vulnerable.</li><li>Pumping stations were inoperable due to lack of backup power, little or no flood proofing, and evacuation of operators.</li><li>A series of incremental decisions systematically increased the inherent risk in the system without recognition or acknowledgment.</li></ul><h5 id="01da635d-7613-46b6-989c-4cbfa1ad11ab" data-toc-id="01da635d-7613-46b6-989c-4cbfa1ad11ab" collapsed="false" seolevelmigrated="true">2.2. The Storm</h5><ul><li>Volume IV of the IPET report documents the analysis of Hurricane Katrina and the hydrodynamic environment it created.</li><li>A time-history of Katrina-generated forces, by location around the HPS, coupled with an understanding of when specific structural breaching occurred was essential to conducting a credible performance analysis.</li><li>The timeline of events was developed by combining eyewitness interviews with a wide variety of physical information and evidence.</li><li>The timeline provided estimates of when overtopping, breaching, and flooding occurred in the individual drainage basins and along the various reaches of the HPS.</li><li>Since Katrina effectively wiped out all water level measuring devices around New Orleans, modeling was the only avenue for recreating the surge and wave conditions that the HPS structures experienced.</li><li>The modeling effort involved two major components:<ul><li>Regional modeling of surge and waves generated by Katrina.</li><li>Local, high-resolution modeling and analysis to estimate the time-history of forces each component experienced.</li></ul></li><li>The regional modeling provided a time-history of the surge and wave environments for all locations around the HPS.</li><li>The regional modeling employed the advanced circulation model, ADCIRC, for surge and both WAM and STWAVE for waves.</li><li>WAM was used for regional wave modeling and STWAVE for shallow and near shore environments, to include Lake Pontchartrain.</li><li>The model applications required a very high resolution representation of the geospatial character of the near shore environment and the HPS structures, resulting in a computational grid with over 2.2 million nodes.</li><li>The WAM and ADCIRC codes were driven by a time-history of Katrina wind fields developed by NOAA based on measured data.</li><li>The near-field impacts of the HPS structures themselves and confined channels of the canals were addressed with high-resolution hydrodynamic modeling using Bousinesq analysis.</li><li>This created a more refined time-history of water levels and forces in the confined spaces of the outfall canals, the Inner Harbor Navigation Canal (IHNC), and the Gulf Intra-coastal Waterway (GIWW).</li><li>Katrina created a record surge because of its unique combination of relatively high intensity (low central pressure) and relatively large physical size (radius to maximum wind speed).</li><li>Katrina’s lowest central pressure deficit (in reference to reference pressure of 1013 mbar) was$ -92 $mbar, and its radius to max wind speed was approximately 25 nmi.</li><li>Camille, a category V storm at landfall, had a central pressure deficit of approximately 102 mbar and a radius of 12 nmi, and Hurricane Betsy (Category III) had a central pressure deficit of approximately 70 mbar and a radius of approximately 40 nmi.</li><li>Bottom line: Katrina created record surge and wave conditions along the East side of New Orleans and the coast of Mississippi.</li><li>Peak water levels along the Plaquemines and St. Bernard levees and within the Inner Harbor Navigation Canal (IHNC) were significantly higher than the structures, leading to massive overtopping and catastrophic breaching.</li><li>Wave heights during Katrina were typically similar to those assumed for the design of the structures, except for Plaquemines Parish, where they were higher than the design assumptions.</li><li>Wave periods, however, were three times longer than the design assumptions, particularly along the east side of St. Bernard and Plaquemines Parishes.</li><li>The longer period, more energetic waves created greater potential for run-up and overtopping.</li><li>Conditions within Lake Pontchartrain were roughly equal to the HPS design criteria for the shoreline structures.</li><li>The Mississippi River Gulf Outlet (MRGO) channel, presumed to be a major factor in propagating storm surge into the IHNC, was demonstrated to have little impact on storm water levels for large storms.</li></ul><h5 id="69a14c8b-e007-412f-915a-c315754b7739" data-toc-id="69a14c8b-e007-412f-915a-c315754b7739" collapsed="false" seolevelmigrated="true">2.3. The Performance</h5><ul><li>Volume V of the IPET report documents the structural performance analysis of the levees and floodwalls.</li><li>The analysis addresses the floodwall breach sites on 17th Street and London Avenue Outfall Canals and the IHNC individually, describing the field investigations, computer modeling, and physical modeling used to determine the most likely failure mechanism.</li><li>Numerical models for stability and seepage, along with detailed field investigations, were used to determine the most likely failure mechanisms for each site.</li><li>Both mechanisms involved deflection of the floodwall as water levels rose, allowing high hydrostatic pressures to reach deep into the foundation of the wall.</li><li>In the case of the 17th Street canal, the deflection fundamentally split the levee-floodwall structure into two halves.</li><li>This resulted in only the back half resisting the lateral forces of the water and a large displacement of the structure along a failure plane in the underlying weak clay soils.</li><li>For the London Avenue canal, the hydrostatic pressure entered relic beach sand underlying the levee, caused uplift, and failure of the system through massive blowout of the subsurface sand.</li><li>Centrifuge testing at both the Rennsalear Polytechnic Institute and the Engineer Research and Development Center confirmed these mechanisms.</li><li>It also describes the analysis of Orleans Outfall Canal, which provided un-breached analogs for both the 17th Street and London Avenue sites.</li><li>A broad analysis of the impact of overtopping and scour on the St. Bernard and Plaquemines Levees was accomplished to understand the massive breaching that occurred in those parishes in terms of the types of materials used in the levees and the forces to which they were exposed.</li><li>The failure mechanisms determined for the I-wall breach sites, coupled with the knowledge gained by studying the Orleans Canal non-breach analog sites, were used to develop criteria for investigating the remaining undamaged I-wall sections for performance integrity and to develop approaches to strengthen I-wall sections as necessary.</li><li>Analysis for levee breaching was used to determine the primary causes of breaching and specify repair and rebuilding strategies that would be more robust.</li><li>The knowl- edge gained was used to develop operating rules for managing water in the outfall canals after the temporary surge gates and pumps are installed and operating, as well as input to Corps of Engineers assessments of changes needed in engineering guide- lines and design criteria.</li><li>Volume VI of the IPET report describes the second major component of the performance analysis, that of the interior drainage and pump stations.</li><li>This volume describes the character of pump stations in each parish and documents their performance during and after Katrina.</li><li>The performance curves, including those describing backflow, were critical inputs to the drainage models that were used to recreate Katrina flooding and to assess the likely degree of flooding for a number of ‘what if’ scenarios.</li><li>The interior drainage models used (HEC-RAS and HEC-HMS) are the operational river analysis and hydrologic modeling systems developed by the Corps’ Hydrologic Engineering Center and widely used in drainage analyses.</li><li>The interior drainage modeling included characterization of the impact of pump station performance and the relative impacts of breaching and overtopping on resultant flooding.</li><li>Hypothetical scenarios were examined using the interior drainage and pumping modeling capability to explore a number of important questions for the future.</li><li>Hypothetical condition of all structures being at authorized levels and no breaching examined the potential flooding if the system had been completed as authorized.</li><li>Bottom line: With the exception of four foundation design failures, all of the major breaches were caused by overtopping and subsequent erosion.</li><li>Reduced protective elevations increased the amount of overtopping, erosion, and subsequent flooding, particularly in Orleans East.</li><li>Ironically, the structures that ultimately breached performed as designed, providing protection until overtopping occurred and then becoming vulnerable to catastrophic breaching.</li><li>The levee-floodwall designs for the 17th Street and London Avenue Outfall Canals and IHNC were inadequate for the complex and challenging environment.</li><li>In four cases, the structures failed catastrophically prior to water reaching design elevations.</li><li>A significant number of structures that were subjected to water levels beyond their design limits performed well.</li><li>Typically, in the case of floodwalls, they represented more conservative design assumptions and, for levees, the use of higher quality, less erodible materials.</li><li>Pumping had little to no impact on the level of flooding, and only about 16 percent of the total pumping capacity for the region was operable during or immediately after Katrina.</li></ul><h5 id="a6834337-12db-43ff-b367-738f79c28106" data-toc-id="a6834337-12db-43ff-b367-738f79c28106" collapsed="false" seolevelmigrated="true">2.4. The Consequences</h5><ul><li>Volume VII of the IPET report describes the efforts to define the losses that occurred because of Katrina and to consider the potential losses from future hurricanes.</li><li>The consequences from Hurricane Katrina flooding have been characterized in economic, human health and safety, social and cultural, and environmental terms.</li><li>The assessment of flood consequences has several purposes integral to understanding the dimensions of the Hurricane Katrina event as well as other possible hurricane and storm events.</li><li>Consequences are one of the dimensions of risk necessary to understand the level of safety provided by the HPS.</li><li>To achieve these objectives, a number of hypothetical scenarios were examined as well as the consequences of the Katrina event:<ul><li>Actual: Katrina with actual system performance—representing the actual flooding in greater New Orleans resulting from Hurricane Katrina.</li><li>Hypothetical: Four hypothetical flooding scenarios are examined to explore consequences of alternative scenarios of flood control and HPS performance in Greater New Orleans.<ul><li>Hypothetical Katrina Scenario 1 (Resilient Levees): Levees and floodwalls crest elevations are at their pre-Katrina levels. Katrina overtops portions of the flood protection system, the levees and floodwalls maintain their integrity and do not breach, and interior pumping is as occurred during Katrina.</li><li>Hypothetical Katrina Scenario 2 (Resilient Levees and Pumps): Levees and floodwalls crest elevations are at their pre-Katrina levels. Katrina overtops portions of the flood protection system, the levees and floodwalls maintain their integrity and do not breach, and interior pumping is at 100% availability.</li><li>Hypothetical Katrina Scenario 3 (Resilient Floodwalls): Levees and floodwalls crest elevations are at their pre-Katrina levels. Katrina overtops portions of the flood protection system. Overtopped levees incur scour as in Katrina, but the floodwalls maintain their integrity and do not breach, and interior pumping is as occurred during Katrina.</li><li>Hypothetical Katrina Scenario 4 (Authorized System): Levees and floodwalls are at authorized elevations, and no breaching occurs. Overtopping can occur, and pumps operate at 100%.</li></ul></li></ul></li><li>The methods used and the specific consequences determined are documented for economic (direct and indirect), human safety and health, cultural and historical, and environmental losses.</li><li>Direct property damages represent monetary damages to residential, commercial, industrial, public buildings, vehicles, and infrastructure.</li><li>Indirect economic consequence was focused on estimating local and regional economic impacts and examining possible changes in the structure of the regional economy from pre-Katrina levels into the future.</li><li>Indirect economic consequences are represented in terms of possible repopulation of the area, capital stock formation, and employment levels.</li><li>The types of human health and safety consequences considered varied by event scenario.</li><li>For the actual Katrina scenario, the effects considered include recorded mortality as well as actual and potential morbidity, including both physical and mental health impacts.</li><li>For the hypothetical Katrina scenario (without system failure), the assessment of human health effects focused only on potential mortality.</li><li>The population and property census date were combined with elevation data to create loss of life – elevation relationships and direct economic damage – elevation relationships by sub-basin for the risk assessment.</li><li>The social and cultural consequences assessment considered social, cultural, and historical indicators as expressed in both quantitative and qualitative terms.</li><li>These include indicators of populations, neighborhoods, communities, institutions, and geographic points and locales, at local, regional, and national scales.</li><li>For the actual Katrina scenario, these indicators were used to assess social and cultural changes from pre-Katrina levels.</li><li>The environmental consequences assessment considered quantitative measures of contamination and loss of significant ecological resources.</li><li>The specific ecological resources addressed include ecological support, fisheries, wildlife, pests, and special status species.</li><li>For the actual and hypothetical Katrina scenarios, post-Katrina ecological resource conditions were compared against pre-Katrina conditions for those resources.</li><li>Bottom line, approximately 80% of New Orleans was flooded, in many areas depth of flooding exceeding 15 ft.</li><li>The majority, approximately two-thirds overall in areas such as Orleans East Bank and St. Bernard, of the flooding and half of the economic losses can be attributed to water flowing through breaches in floodwalls and levees.</li><li>There were at least 727 fatalities in the five parishes in and around New Orleans, and over 75% of the fatalities were people over age 60.</li><li>The poor, elderly, and disabled, the groups least likely to be able to evacuate without assistance, were disproportionately impacted.</li><li>Direct property losses exceeded$ 20 billion $, and 78% of those losses were in residential areas.</li><li>There was an additional loss of over$ 7 billion $in public structures and utilities.</li><li>The indirect consequences were equally disastrous.</li><li>The breakdown in New Orleans’ social structure, loss of cultural heritage, and dramatically altered physical, economic, political, social, and psychological character of the area are unprecedented in the United States.</li><li>In themselves, these create a formidable barrier to recovery.</li><li>Where water depths were small, recovery has been almost complete. In areas where water depths were greater, recovery or reinvestment has been very difficult.</li></ul><h5 id="cdc5c311-0b39-4148-a7f4-77bea6e05370" data-toc-id="cdc5c311-0b39-4148-a7f4-77bea6e05370" collapsed="false" seolevelmigrated="true">2.5. The Risk</h5><ul><li>Volume VIII of the IPET report documents the risk and reliability assessment for the HPS.</li><li>This was an effort to accomplish a system-wide perspective of the past and near-term capability of the HPS.</li><li>This volume documents the methodology for defining the future hurricane hazard to New Orleans and vicinity, the methodology for characterizing and assessing the reliability (performance) of the HPS structures and related features against that hazard, the methods used for determining the likelihood of flooding in the various sub-basins and protected areas, the approach for estimating potential losses as a result of flooding as well as the risk model itself.</li><li>It also presents detailed results of these analyses and the findings and lessons learned from the analysis.</li><li>The results are provided in three major forms.<ul><li>First, the vulnerability to flooding is presented as inundation frequency depth estimates displayed as maps.</li><li>Risk is provided in terms of both mean values of expected loss of life and property losses at the specific exceedance frequencies (2%, 1%, and 0.2%) or return periods (50-, 100-, and 500-year).</li></ul></li><li>In addition to being available within the IPET report and IPET web site, the risk maps have been made available in a limited interactive mode through Google Earth.</li><li>Maps including the West Bank areas were published in July 2007, and maps showing the impact of pumping were published in March 2008.</li><li>The hazard definition was accomplished through a partnership between the Corps of Engineers, FEMA, NOAA, and IPET and adopted the rigorous surge and wave modeling capability assembled for the IPET work reported in Volume IV.</li><li>This allowed development of a single body of knowledge on storm-generated water levels (surge, waves, and wave setup) and frequency of occurrence for a large number of locations around the study area.</li><li>It also provided a single information resource for IPET risk assessment, FEMA flood mapping, Corps levee and floodwall design, and planning efforts to examine higher levels of protec- tion.</li><li>Over 150 historical and hypothetical storms were run through a quasi-coupled version of the ADCIRC and STWAVE models to define the storm water level—frequency of occurrence relation- ships for a wide variety of locations around the region.</li><li>This ensemble of storms represents the wide range of storms that can impact New Orleans and provides a joint probability model for defining the hazard (surge and wave environment) that each reach or structure might experience in the future.</li><li>Because the nature of the structures comprising the HPS impacts the ultimate surge and wave levels, a number of the storms were modeled using three different geometries for the HPS.<ul><li>The first represented pre-Katrina conditions.</li><li>The second represented the repaired HPS.</li><li>The third represented the projected 100-year certified structures planned for the future.</li></ul></li><li>The reliability of the system is modeled by first characterizing the types and properties of the structures and features associated with the HPS. This includes gates, levees, floodwalls, pump stations, and bridges.</li><li>The entire HPS is divided into reaches, sections considered to be reasonably uniform in properties and performance. In addition, specific structures are considered separately.</li><li>The expected performance of each reach or specific structure is defined using fragility curves, relating the probability of failure to storm-generated water levels.</li><li>The fragility curves consider failure by multiple mechanisms, such as potential for breaching before overtopping or as a direct result of overtopping.</li><li>The fragility curves are based on the foundation conditions (soils) of the structures, character of the structure or feature (design, construction, and maintenance), and the type of forces created by different water levels (storm surge plus waves).</li><li>Each fragility curve includes the uncertainties of different types of performance based on unknowns and possible variability in performance.</li><li>Combining the hazard with the reliability analysis provides estimates of the likelihood of area being flooded to different depths.</li><li>As such, for each sub-basin, one ends up with depth of flooding—frequency curves.</li><li>When these relationships are combined with the loss-depth of flooding curves generated in the consequence analyses as described earlier (see Volume VII), it is possible to compute risk for either loss of life or loss of property.</li><li>Since the loss relationships represent the pre-Katrina situation with respect to property (distribution and value) and population (distribution and demographics), changes in risk derived from these data is a reflection of changes in the performance of the HPS.</li><li>Bottom line: Given the improvements in the HPS accomplished by June 2007, the residual vulnerability to flooding was reduced significantly in a few areas and moderately in others.</li><li>Large areas of New Orleans and Southeast Louisiana remain very vulnerable to flooding from large hurricanes.</li><li>The areas gaining the most benefit were those whose primary source of vulnerability was the outfall canals.</li><li>The relative levels of risk to loss of life and property remain closely associated with the property values and population densities in the sub-basins and the land elevation (potential for deep flooding).</li><li>The areas in proximity to the IHNC, where HPS structures in 2007 have a reduced likelihood to catastrophically breaching but are very vulnerable to overtopping, remain vulnerable.</li><li>Similarly, the West Bank (region of New Orleans to the west and south of the Mississippi River), where significant areas existed in 2007 without completed HPS structures in a number of areas, remains highly vulnerable.</li><li>Risk remains significant for the 2% or the 50-year return period flood; however, it is primarily due to rainfall, not hurricane surge and wave conditions.</li><li>Pumping can have a substantial impact at this level to reduce flood depths in most areas; however, the impact of pumping is highly dependent on the efficiency of the drainage system and the operational reliability of the pumping stations.</li><li>All of New Orleans and Southeast Louisiana is highly vulnerable to catastrophic flooding for flood events that are in the neighborhood of 0.2% or the 500-year return period.</li><li>At this time evacuation is the only effective means to substantially reduce loss of life for large hurricane events.</li><li>Property risk can be reduced through elevation and flood proofing of structures, continued strengthening and improvement of the HPS components to include reliability of the pumping capability and resilience of levees and floodwalls, and appropriate land-use management of the most vulnerable areas.</li><li>Completion of the risk reduction measures currently under construction (if they perform as modeled) will dramatically reduce vulnerability of flooding at the 1% or 100-year level and significantly reduce the severity of flooding at the 0.2% or 500- year frequency level.</li></ul>The central argument is that Hurricane Katrina exposed critical vulnerabilities in New Orleans' hurricane protection system (HPS). It highlights the complex interplay of factors that led to the disaster, including design flaws, incomplete construction, lack of maintenance, and inadequate understanding of the region's unique risks. The analysis emphasizes the need for a comprehensive, system-wide approach to risk assessment and mitigation, considering structural integrity, environmental factors, and socio-economic vulnerabilities to enhance resilience against future disasters. Ultimately, it calls for improved engineering practices, better emergency preparedness, and informed land-use management to reduce the risk of catastrophic flooding and loss of life in vulnerable areas like New Orleans. Also, the HPS in place in 2005 could not perform as a system. In some areas, it was not completed (especially to the south and west of the Mississippi River), and in others, the lower than authorized elevations reduced its intended protection. The capacity for protection also varied because many of the levee sections had little resilience to overtopping, thus performing adequately until overtopped and then failing catastrophically. Sections of I-walls failed because designs did not consider the failure modes that caused them to breach. The IPET investigation was complemented by two other investigative teams that worked separately and under different sponsorships: Team Louisiana and The Independent Levee Investigative Team (ILIT). These three studies had significant overlap in their analyses but differed in approach and scope. The results of the three efforts that address the areas of common evaluation are largely in agreement with the exception of three findings: The fundamental failure mechanisms for the canal floodwalls, One of the floodwall failures in the Inner Harbor Navigation Canal and The final difference involved the breaching of the exterior levees along St. Bernard Parish. The majority of the findings were in close agreement. Bottom line: With the exception of four foundation design failures, all of the major breaches were caused by overtopping and subsequent erosion. Reduced protective elevations increased the amount of overtopping, erosion, and subsequent flooding, particularly in Orleans East. Ironically, the structures that ultimately breached performed as designed, providing protection until overtopping occurred and then becoming vulnerable to catastrophic breaching. Direct property losses exceeded$ 20 billion $, and 78% of those losses were in residential areas. There was an additional loss of over$ 7 billion$$ in public structures and utilities. Given the improvements in the HPS accomplished by June 2007, the residual vulnerability to flooding was reduced significantly in a few areas and moderately in others. The areas gaining the most benefit were those whose primary source of vulnerability was the outfall canals. Property risk can be reduced through elevation and flood proofing of structures, continued strengthening and improvement of the HPS components to include reliability of the pumping capability and resilience of levees and floodwalls, and appropriate land-use management of the most vulnerable areas. Completion of the risk reduction measures currently under construction (if they perform as modeled) will dramatically reduce vulnerability of flooding at the 1% or 100-year level and significantly reduce the severity of flooding at the 0.2% or 500- year frequency level.
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