Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

C O R P O R A T I O N

Jordan R. Fischbach, David R. Johnson, Edmundo Molina-Perez For more information on this publication, visit www.rand.org/t/RR1988

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www.rand.org Preface

In October 2013, the State of Louisiana asked the RAND Corporation to conduct an investigation of the potential coastal flood damage reduction benefits and the potential induced damage effects to both Louisiana and Mississippi from a proposed barrier across the mouth of Lake Pontchartrain in southeast Louisiana. In response, this report describes an initial evaluation of proposals for a Lake Pontchartrain barrier using the Coastal Louisiana Risk Assessment (CLARA) model. We conducted this evaluation jointly with the engineering firm Arcadis, and Arcadis produced a parallel report (Atkinson and Roberts, 2015) that describes the preliminary design assessment and screening process used to identify a subset of barrier options to consider. We developed results from this analysis to guide the selection of a barrier option for formal consideration in the 2017 Coastal Master Plan analysis. The Coastal Protection and Restoration Authority (CPRA) of Louisiana funded the research, and we initially developed the report to guide CPRA staff and the CPRA board in determining the appropriate next steps for Lake Pontchartrain barrier project proposals. We later determined that this research would also be of interest to a public audience and elected to independently publish this report in 2017, timed with the release of the final master plan. This report is intended for CPRA, as well as stakeholders and residents in southeast Louisiana, particularly communities surrounding Lake Pontchartrain. It is also intended for similar audiences in Mississippi whom a proposed barrier might affect. The research described in this report directly applies an updated version of the CLARA model developed as part of CPRA’s 2017 Model Improvement Plan process, and the analysis is closely linked with the CLARA analysis conducted in support of the overall research effort to develop the 2017 Coastal Master Plan. Related publications include the final master plan (CPRA, 2017), appendixes to the master plan describing the updated CLARA model (Fischbach, Johnson, Kuhn, et al., 2017), and the final project analysis and comparison (Alymov et al., 2016). CPRA funded this effort under a master service agreement with Brown and Caldwell, contract 2503213245.

RAND Infrastructure Resilience and Environmental Policy

The research reported here was conducted in the RAND Infrastructure Resilience and Envi- ronmental Policy program, which performs analyses on urbanization and other stresses. This includes research on infrastructure development; infrastructure financing; energy policy; urban planning and the role of public–private partnerships; transportation policy; climate response,

iii iv Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier mitigation, and adaptation; environmental sustainability; and water resource management and coastal protection. Program research is supported by government agencies, foundations, and the private sector. This program is part of RAND Justice, Infrastructure, and Environment, a division of the RAND Corporation dedicated to improving policy- and decisionmaking in a wide range of policy domains, including civil and criminal justice, infrastructure protection and homeland security, transportation and energy policy, and environmental and natural resource policy. Questions or comments about this report should be sent to the project leader, Jordan Fischbach ([email protected]). For more information about RAND Infrastruc- ture Resilience and Environmental Policy, see www.rand.org/jie/irep or contact the director at [email protected]. Contents

Preface...... iii Figures and Tables...... vii Summary...... ix Acknowledgments...... xiii Abbreviations...... xv

CHAPTER ONE Introduction...... 1 Motivation...... 1 Goals of This Study...... 2

CHAPTER TWO Methods and Data Sources...... 5 Overview of the CLARA Model...... 5 Estimating Flood Risk for Coastal Mississippi...... 7 Selecting a Suitable Statistical Storm Set...... 7 Asset Inventory Updates for Mississippi...... 9 Other Assumptions...... 12 Experimental Design...... 14

CHAPTER THREE Results from Proposed Barrier Alignments...... 17 Introduction...... 17 Risk-Reduction Benefits for Louisiana Parishes...... 20 Potential Induced Flood Damage in Coastal Mississippi...... 30

CHAPTER FOUR Discussion and Next Steps...... 37 Conclusion...... 38

References...... 39

Figures and Tables

Figures

1.1. Map of the Lake Pontchartrain Region with the Study Domain Highlighted in Color.... 2 2.1. CLARA Model Structure...... 6 2.2. Maps Showing Two Proposed Alignment Configurations...... 15 3.1. Future Without Action Flood Depths for the Study Region, 100-Year Flood Interval, No Fragility...... 18 3.2. Future Without Action Flood Depths for the East Bank of Greater New Orleans, 500-Year Flood Interval...... 19 3.3. Future Without Action Expected Annual Damage, by Region and Asset Class, in Billions of Constant 2015 Dollars...... 19 3.4. Changes in 100-Year Depths for Four Barrier Alignment Options...... 21 3.5. Change in Hurricane and Storm Damage Risk Reduction System Surge and Wave Heights for Highway 90 with Its Existing Crown and Low Gates, 100-Year Flood Interval...... 23 3.6. Expected Annual Damage, by Alignment and Louisiana Region, in Billions of Constant 2015 Dollars...... 26 3.7. Reduction in Expected Annual Damage, by Alignment and Louisiana Region, in Millions of Constant 2015 Dollars...... 27 3.8. Increase in Expected Annual Damage, in Millions of Constant 2015 Dollars, by Mississippi County...... 31 3.9. Map of 100-Year Damage and Damage Increase from the Alignment in Mississippi That Keeps Highway 90’s Existing Crown and Adds 2-Foot Gates...... 34

Tables

S.1. Lake Pontchartrain Barrier Configurations Considered in This Analysis...... ix S.2. Expected Annual Damage Reduction, by Parish or County, for Four Proposed Barrier Configurations, in Millions of Constant 2015 Dollars...... x 2.1. Mississippi Assets at Risk, Year 50, in Thousands of Assets...... 10 2.2. Mississippi Road-Miles per Road Type...... 10 2.3. Parcel-Level Data Crosswalk Between Occupation Type and General Building Stock Code...... 11 2.4. Asset Values at Risk in Coastal Mississippi, Year 50, in Billions of Constant 2015 Dollars...... 12 2.5. Lake Pontchartrain Barrier Configurations Considered in This Analysis...... 14

vii viii Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

3.1. Average Flood Depth Change, by Louisiana Parish, 100-Year Flood Interval, 50th Percentile...... 22 3.2. Average Hurricane and Storm Damage Risk Reduction System Change in Surge and Wave Height, 100-Year Flood Interval, in Feet...... 24 3.3. Expected Annual Damage Reduction, by Parish or County, for Four Proposed Barrier Configurations, in Millions of Constant 2015 Dollars...... 28 3.4. Damage Reduction or Inducement, by Annual Exceedance Probability Interval, Louisiana Study Region, in Billions of Constant 2015 Dollars...... 29 3.5. Increase in Expected Annual Damage Induced, by County and Asset Class, for Coastal Mississippi...... 32 3.6. Increase in Damage, by Annual Exceedance Probability Interval for Coastal Mississippi...... 33 Summary

Louisiana’s coastal communities face a range of challenges and risks posed by flooding from extreme coastal storms. The Coastal Protection and Restoration Authority (CPRA) of Loui- siana has committed to the goal of reducing net economic losses from extreme coastal storms and, in recent years, has considered a wide range of proposed project types designed to better protect coastal communities and assets. In 2013, CPRA asked RAND to conduct a preliminary investigation of the coastal flood damage reduction benefits and induced damage effects from five different options for a barrier across the mouth of Lake Pontchartrain. To address this question, the RAND team applied an updated version of the Coastal Louisiana Risk Assess- ment flood risk model to estimate flooding and damage with or without options in place for a barrier. This report describes the results of this initial evaluation, conducted in advance of the analysis supporting Louisiana’s 2017 Coastal Master Plan. It includes estimates of the potential benefits for coastal Louisiana from such a barrier, including storm surge and wave heights, flood depth, and direct economic damage reduction from different barrier options in one future scenario reflecting sea level rise and coastal land subsidence conditions 50 years from today. It also includes new estimates of the potential induced economic damage (disbenefits) from the proposed barrier options to Hancock, Harrison, and Jackson Counties in Mississippi, as well as to St. Bernard and Plaquemines Parishes in Louisiana. The analysis considers five proposed configurations for a barrier enclosing Lake Pontchartrain. Table S.1 summarizes the barrier options evaluated.

Table S.1 Lake Pontchartrain Barrier Configurations Considered in This Analysis

Name Location Road Height, in Feet Gate Height, in Feet

FWOA Not applicable No change Not applicable

Hwy90/0/2 Highway 90 No change 2.0

Hwy90/10/0 Highway 90 to Slidell 10.0 Not applicable

CSX/10/10 CSX railroad to Slidell 10.0 10.0

Hwy90/10/10 Highway 90 to Slidell 10.0 10.0

Hwy90/24.5/24.5 Highway 90 to Slidell 24.5 24.5

NOTE: Hwy90/10/0 = Highway 90 with 10-ft. crown, no gates, and Slidell extension. Hwy90/0/2 = Highway 90 with existing crown and 2-ft. gates. CSX/10/10 = CSX railroad with 10-ft. crown, 10-ft. gates, and Slidell extension. Hwy90/10/10 = Highway 90 with 10-ft. crown, 10-ft. gates, and Slidell extension. Hwy90/24.5/24.5 = Highway 90 with 24.5-ft. crown, 24.5-ft. gates, and Slidell extension.

ix x Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Results showed that a Lake Pontchartrain barrier could provide substantial damage reduction benefits for southeastern Louisiana, with median expected annual damage reduction benefits ranging from $1.2 billion to $1.4 billion per year (Table S.2). A barrier could also lower future design height requirements for the Greater New Orleans hurricane protection system along the south shore of Lake Pontchartrain, although it could also increase design heights for the eastern face of the system or otherwise necessitate additional levee armoring in other locations. All barrier alignments that we considered increased flood damage in coastal Mississippi compared with a future without action (Table S.2), but damage inducement was typically low for the best-performing alignment options (1 to 2 percent) compared with total flood damage risk for the three Mississippi coastal counties. Preliminary results show that the Hwy90/10/10 and CSX/10/10 alignments yield the highest net damage reduction for the overall region, while the Hwy90/0/2 alignment produces damage reduction nearly as high, with the lowest induced damage effects on neighboring Louisiana parishes and Mississippi coastal counties. In general,

Table S.2 Expected Annual Damage Reduction, by Parish or County, for Four Proposed Barrier Configurations, in Millions of Constant 2015 Dollars

Alignment State Parish or County Hwy90/0/2 Hwy90/10/10 CSX/10/10 Hwy90/24.5/24.5 Louisiana St. Tammany 620 712 696 539 St. John the Baptist 209 258 252 267 Ascension 86 105 103 107 Jefferson 150 149 140 152 Orleans 142 155 175 129 Livingston 17 28 27 32 Tangipahoa 6 11 10 15 St. James 5 5 5 6 St. Charles –1 –1 –3 0 Plaquemines 0 0 0 0 St. Bernard –14 –20 –25 –48 Total 1,220 1,403 1,380 1,199 Mississippi Hancock –14 –23 –22 –42 Harrison –5 –10 –11 –23 Jackson –2 –3 –4 –18 Total –22 –35 –38 –84

Change in EAD (millions of constant 2015 $) –50 700

NOTE: Because of rounding, totals might not sum precisely. This table shows the reduction, or inducement, in expected annual damage for coastal parishes and counties in Louisiana and Mississippi from four proposed Lake Pontchartrain barrier alignments as estimated in this analysis: For clarity, we omit Hwy90/10/0 results because this option provided no depth or damage reduction benefit. We project the values for each scenario 50 years into the future, representing one set of assumptions about uncertain future sea level rise, landscape subsidence, asset growth, and performance of hurricane protection levees during flood events. The table shows the median (50th percentile) values. Green shading indicates positive damage reduction (benefit); red shading indicates induced damage (disbenefit). RAND RR1988-TS.2 Summary xi this analysis strongly supported moving one of these high-performing alignments forward for formal consideration as part of the 2017 Coastal Master Plan analysis. Subsequently, CPRA included the Hwy90/0/2 alignment as part of the 2017 analysis and ultimately selected this project for implementation as part of the final 2017 Coastal Master Plan.

Acknowledgments

This document describes a preliminary investigation that was designed to guide analysis in the 2017 Coastal Master Plan. We gratefully acknowledge the support and feedback received throughout the analysis and writing process from the Coastal Protection and Restoration Authority (CPRA) of Louisiana and our partners at Arcadis. Specifically, we would like to thank Mark Leadon of CPRA for his tireless efforts as project manager for the overall investigation. Other CPRA participants, including Mandy Green, Karim Belhadjali (now with Abt Associates), Jas Singh, Ed Haywood, and Rickey Brouillette, greatly improved the final results by providing constructive guidance and feedback during the analysis process and in response to early results. We partnered and worked closely with John Atkinson, Brian Lindberg, and Hugh Roberts of Arcadis throughout the multiyear investigation, and we especially appreciate the effective communication and timely feedback we received from the Arcadis team. We also appreciate the support and responsiveness from Stephanie Hanses of Brown and Caldwell while navigating a tricky analysis and model production schedule. We would like to thank the U.S. Army Corps of Engineers Mobile District for providing parcel-level asset data for coastal Mississippi to support this analysis. Our colleague Debra Knopman provided helpful initial feedback, and we gratefully acknowledge the constructive peer reviews provided by Carter Price (RAND Corporation) and Robert A. Dalrymple (Johns Hopkins University). Their comments were insightful and greatly improved the final document.

xiii

Abbreviations

ADCIRC Advanced Circulation AEP annual exceedance probability CLARA Coastal Louisiana Risk Assessment CPRA Coastal Protection and Restoration Authority CSX/10/10 CSX railroad with 10-ft. crown, 10-ft. gates, and Slidell extension EAD expected annual damage FEMA Federal Emergency Management Agency FWOA future without action GBS general building stock HSDRRS Hurricane and Storm Damage Risk Reduction System HSDRRS East Greater New Orleans Hurricane and Storm Damage Risk Reduction System east of the Mississippi River HSIP Homeland Security Infrastructure Program Hwy90/0/2 Highway 90 with existing crown and 2-ft. gates Hwy90/10/0 Highway 90 with 10-ft. crown, no gates, and Slidell extension Hwy90/10/10 Highway 90 with 10-ft. crown, 10-ft. gates, and Slidell extension Hwy90/24.5/24.5 Highway 90 with 24.5-ft. crown, 24.5-ft. gates, and Slidell extension MARIS Mississippi Automated Resource Information System mb millibar MsCIP Mississippi Coastal Improvements Program MTTG Morganza to the Gulf SWAN Simulating Waves Nearshore USACE U.S. Army Corps of Engineers

CHAPTER ONE Introduction

Motivation

Louisiana’s coastal communities face a range of challenges and risks posed by flooding from extreme coastal storms. Tropical storms and hurricanes making landfall nearby can push storm surge and high waves into populated areas, damaging or destroying assets, disrupting economic activity and basic services, and threatening the health and safety of coastal residents. Louisiana has been at increasing risk from coastal flooding caused by a variety of factors, including rising sea levels, subsiding (sinking) coastal land, loss of wetlands and other coastal ecosystems, and the continued development of resources and assets along the coast. Hurricanes Katrina, Rita, Gustav, Ike, and Isaac all made landfall in Louisiana in just the past decade, for instance, producing devastating storm surge floods. As a result, Louisiana’s Coastal Protection and Restoration Authority (CPRA) has identified a primary planning objective to “[r]educe economic losses from storm based flooding to residential, public, industrial, and commercial infrastructure” (CPRA, 2007, p. 37). CPRA evaluated a range of proposed structural and nonstructural options to include in its 2012 Louisiana’s Comprehensive Master Plan for a Sustainable Coast (CPRA, 2012a) to address this risk-reduction goal. One important option considered was to build a storm surge barrier with gates across the mouth of Lake Pontchartrain, designed to keep storm surge from tropical storms and hurricanes from flowing into the lake. Such a barrier would provide primary protection to communities on the north shore of Lake Pontchartrain (St. Tammany Parish), including the towns of Covington, Mandeville, and Slidell. It would also provide an additional line of protection for the levees and gates along the south shore that make up part of the Greater New Orleans Hurricane and Storm Damage Risk Reduction System (HSDRRS), potentially reducing storm surge and wave heights before they reach the protected areas of Orleans and Jefferson parishes. Figure 1.1 shows the region of focus surrounding Lake Pontchartrain. The 2012 Coastal Master Plan analysis considered two options for a Lake Pontchartrain barrier: high (33 ft.) and low (24.5 ft.), respectively, with both drawn from prior investiga- tions by the U.S. Army Corps of Engineers (USACE). The Coastal Louisiana Risk Assessment (CLARA) analysis in 2012 showed that both options could provide substantial risk-reduction value to the Louisiana coastal region, with the barriers yielding a reduction in expected annual damage (EAD) ranging from $1.1 billion to $2.2 billion in a future scenario (CPRA, 2012b). Both options also ranked highly among structural flood protection projects considered in terms of their cost-effectiveness. However, stakeholders involved in master plan development raised concerns during the process that such a barrier could divert storm surge from the lake only to flood neighboring

1 2 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Figure 1.1 Map of the Lake Pontchartrain Region with the Study Domain Highlighted in Color

Mississippi Louisiana Jackson Harrison County County Tangipahoa Hancock County Biloxi Gulfport Livingston St. Tammany Pascagoula Bay St. Louis Slidell Pearl Ascension River St. John Lake Pontchartrain the Baptist St. James St. Charles Orleans Jefferson New Orleans St. Bernard

Plaquemines

0 10 miles

RAND RR1988-1.1 populated areas. In particular, given the proximity to the border with Mississippi, a primary concern was that a new barrier could induce additional flooding and flood damage to Mis- sissippi coastal communities, such as Bay St. Louis, Gulfport, Biloxi, and Pascagoula, during coastal storms. These concerns could not be addressed through the previous quantitative risk and damage analysis, however, because the 2012 analysis did not include counties in coastal Mississippi likely to be affected by induced flooding. As a result, CPRA did not include the barrier in the 2012 Coastal Master Plan but instead chose to invest in additional research and evaluation of proposed barrier options leading to the 2017 update of the master plan.

Goals of This Study

This report describes an initial evaluation of proposals for a Lake Pontchartrain barrier designed to provide additional risk reduction to lake-adjacent Louisiana communities and Greater New Orleans using the CLARA model. The report provides estimates of the potential benefits for coastal Louisiana from a series of proposed barrier options enclosing the mouth of Lake Pon- tchartrain with different protection structure heights and alignments. Results for each barrier option include storm surge and wave heights, flood depth, and direct economic damage reduction in one future scenario 50 years from today. It also includes new estimates of the potential induced economic damage (disbenefits) from the proposed barrier options to Hancock, Har- Introduction 3 rison, and Jackson counties in Mississippi, as well as to St. Bernard and Plaquemines parishes in Louisiana. A separate document prepared by Arcadis serves as a complement to this report (Atkinson and Roberts, 2015). The Arcadis team conducted a preliminary screening of storm surge reduction from a much wider range of proposed barrier options to identify the subset evaluated in the CLARA analysis. It also ran the detailed hydrodynamic storm surge and wave simulations for a subset of simulated coastal storms, with or without the barrier options in place, to support the CLARA risk and damage analysis. This report builds on the Arcadis investigation and presents final flood depth and damage outcomes for selected barrier alignments. The results described here draw on assumptions from both the 2012 Coastal Master Plan process and the recent CLARA model improvement effort conducted in preparation for the 2017 analysis (Fischbach, Johnson, Kuhn, et al., 2017 [hereafter FJK, 2017]). We developed results to guide CPRA’s selection of one or more barrier alignments to consider in the 2017 Coastal Master Plan analysis. As a result, we describe only damage reduction benefits and potential induced damage disbenefits; we did not consider capital and operation and maintenance costs, environmental effects, and other metrics relevant to project selection and implementation. CPRA considered these metrics as part of the 2017 analysis once the selected barrier alignment was moved forward to the next phase.

CHAPTER TWO Methods and Data Sources

Overview of the CLARA Model

This evaluation applies an updated version of the CLARA model to estimate surge and wave heights, flood depths, and direct economic damage from coastal storms to assets on the Lou- isiana and Mississippi coasts. The original version of the CLARA model (CLARA 1.0) is described in detail in Fischbach, Johnson, Ortiz, et al., 2012, and Johnson, Fischbach, and Ortiz, 2013. During this study, we developed an updated version of the model, CLARA 2.0, and applied it in this analysis, as well as in the subsequent 2017 Coastal Master Plan flood risk and damage analysis (FJK, 2017; Alymov et al., 2016).1 We restricted the study region to portions of coastal Louisiana east of the Mississippi River that a Lake Pontchartrain barrier could affect, positively or negatively. CLARA uses principles of quantitative risk analysis, which describe risk as the product of the probability or likelihood of a given event occurring (in this case, the annual probability of storm surge–based and wave-based flooding at different depths) and the consequences of that event (the damage that results from the flooding).2 In CLARA, references to flood risk are best understood as risk to structures, physical infrastructure, and other local economic assets. CLARA uses several types of information to estimate flood depths and resulting damage. First, the model estimates peak storm surge and wave heights. To produce an empirical estimate of the cumulative distribution function associated with flood depths in a given scenario, the model combines the surge and wave characteristics associated with a set of different possible storms and estimates of the relative likelihood of those storms. It runs the same set of storms through hydrodynamic simulations of various landscapes that reflect a future in which no barrier alignment is implemented or futures in which the different proposed alignments have been constructed on the landscape. In “Selecting a Suitable Statistical Storm Set” later in this chapter, we describe the storm set, and we describe other assumptions affecting future conditions (e.g., sea level rise) in “Other Assumptions” after that. The model also incorporates data that characterize the landscape, hurricane protection systems, and assets at risk along the Louisiana and Mississippi coastlines. Along the coast, CLARA labels different areas as unenclosed (those with no levees, floodwalls, or other barriers or with structures that do not fully enclose the population at risk) or enclosed (those with hurricane protection that fully encloses the area in a ring and creates a polder).

1 Readers interested in further technical detail on the methods, assumptions, and additional analysis used to develop CLARA 2.0 should consult the technical appendixes to the 2017 Coastal Master Plan, especially Attachment C3-25 (FJK, 2017). For brevity, we omit these detailed descriptions from this report. 2 We adapted the CLARA description in this section from that in FJK, 2017.

5 6 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Figure 2.1 illustrates the structure of the CLARA model. The input preprocessing module combines the parameters associated with each storm in the sampled set of hypo- thetical storms with the characteristics from observed historical storm events affecting the Gulf Coast to produce statistical estimates of the likelihood of different flood elevations. The preprocessing module also directly estimates flood depths in unenclosed areas by subtracting the prevailing ground elevation from simulated hydrodynamic storm surge and wave heights. Finally, the module records surge and wave conditions along protection structures and estimates statistical still-water storm surge and significant wave heights at a selected set of annual exceedance probabilities (AEPs) for each location.3 In the flood depth module, CLARA estimates flood depths for enclosed areas, with a particular focus on storm surge and wave overtopping and system fragility. Specifically, for areas surrounded by protection systems, storm surge and waves that coastal storms produce can lead to overtopping, protection structure failures (breaches), or both. The model estimates the volume of water introduced to each polder through overtopping, structure failures, and rainfall from each synthetic storm. After subtracting any water removed by pumping systems, the module determines still-water flood depths by assessing how water flows between enclosed polders and then comparing the resulting water volumes with the storage capacity of each polder. It uses Monte Carlo simulation and bootstrapping techniques, as described in FJK, 2017, to account for the random chance associated with possible structure breaches, as well as other parametric uncertainties. Finally, the module combines the sampling results from different synthetic storms to estimate cumulative distribution functions for flood depths, as well as confidence bounds around these estimates. The depth of the flood directly determines the amount of damage that occurs, so flood depths are inputs to the economic module. In this step, CLARA estimates the number and value of the assets at risk from flooding and calculates damage in dollars as a function of

Figure 2.1 CLARA Model Structure

Input preprocessing Flood depth Economic module module module Outputs

Asset Overtopping Flood depth Spatial data inventory and damage preprocessing exceedances

System Asset fragility valuation

Storm data EAD preprocessing Interior Economic drainage damage

RAND RR1988-2.1

3 AEPs are statistical estimates of the flooding and damage expected to recur with a certain probability in each year. For example, the 1-percent or “100-year” AEP is the flood depth that has a 1-percent chance of occurring or being exceeded in each year. This is commonly referred to as the 1-in-100 or 100-year flood. Methods and Data Sources 7 flood depths (Fischbach, Johnson, Ortiz, et al., 2012). The methods used build closely on the Federal Emergency Management Agency (FEMA) Hazus approach (FEMA, undated), com- bining a projection of assets at risk and current levels of structure mitigation (e.g., structure elevation above ground level, first-floor flood-proofing) with curves relating depth to damage for each structure type to produce damage estimates. The inventory of assets at risk in future scenarios is generally assumed to track population change, with the exception of certain asset types, such as roads or agricultural crops. Model outputs include summaries of flood depth and damage exceedance values, at a selected set of AEPs, and EAD from storm surge–based flooding events. These metrics are generated at each grid point in the model’s spatial domain and aggregated by Louisiana parish, Mississippi county, or other levels of spatial aggregation as needed to support the analysis. The version of CLARA applied in this analysis, CLARA 2.0, has been updated to support the 2017 Coastal Master Plan (FJK, 2017). Key improvements include an expanded model domain to account for a growing floodplain, higher spatial resolution, an updated inventory of Louisiana assets at risk, and a new approach to estimate parametric and model uncertainty surrounding estimates of flood depths and damage. FJK, 2017, describes the updated methods in detail. Except where noted, the approach for this investigation is the same as in the test analysis conducted for the 2017 Model Improvement Plan, and detailed descriptions can be found in the separately referenced report (FJK, 2017).

Estimating Flood Risk for Coastal Mississippi

The next two sections of this report describe a series of augmentations made to the CLARA model to allow for flood depth and damage estimates in coastal Mississippi. For the most part, these updates entailed obtaining and incorporating data describing assets at risk in the three Mississippi coastal counties in order to support economic damage assessment. As a first step, we expanded the study domain for the CLARA model to incorporate Hancock, Harrison, and Jackson counties in Mississippi. During the model improvement process, we added grid points for these counties, and Figure 1.1 in Chapter One shows the combined Louisiana and Mississippi domain considered. We identified no enclosed areas for coastal Mississippi, so we modeled flood depths in these areas as unenclosed using the process described above.

Selecting a Suitable Statistical Storm Set

To estimate the statistical recurrence of flooding and damage, CLARA uses a sample of simulated coastal storms, each run through the detailed Advanced Circulation (ADCIRC) and Simulating Waves Nearshore (SWAN) hydrodynamic surge and wave models. These detailed hydrodynamic storm runs are computationally intensive and require supercomputing resources to complete. As a result, CPRA must resolve an important trade-off between the precision of the statistical estimates and the computational cost of the hydrodynamic simulations, especially when evaluating multiple barrier alignments. To better inform the set of trade-offs, the CLARA team conducted a storm selection investigation early in this process. The goal was to identify a storm set with sufficient precision 8 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

(i.e., low bias and variance) across the study region that was small enough to allow the Arcadis team to evaluate all alignments with ADCIRC and SWAN in a reasonable time frame. This analysis mirrored the more detailed storm selection investigation described in Sec- tion 6.5 of FJK, 2017, and drew from the same universe of 446 FEMA-defined simulated coastal storms for the Louisiana coastal region. In this case, however, we considered only the easternmost tracks for coastal Louisiana; the western tracks were not expected to meaningfully change results when we compared conditions with or without a Lake Pontchartrain barrier. In addition, we considered nine additional synthetic storms that the USACE Mississippi Coastal Improvements Program (MsCIP) identified for coastal Mississippi (USACE, 2009).4 This yielded a total of 232 storms as a maximum number of storms to which to compare smaller subsets. Arcadis next provided storm surge and wave results from a single future without action (FWOA) scenario run for all 232 storms. In this case, the FWOA represents conditions on the coast 50 years from today if no additional policy actions or investments are made (i.e., no barrier or other hurricane protection is constructed in the interim). With the exception of the MsCIP storms, which the Arcadis team ran through ADCIRC and SWAN to support storm selection, we drew all other storm inputs directly from Arcadis’ model improvement analysis for this initial comparison. We then used CLARA 2.0 to estimate the potential flood depth and damage bias and variance for the study region from a range of possible smaller storm sets, comparing with the full 232-storm set as a baseline. We conducted two iterations of the analysis and shared and discussed results with CPRA and Arcadis through in-person and phone meetings. Conclusions from the analysis can be summarized as follows:5

• The inclusion of additional MsCIP storms did not meaningfully change the statistical estimates of flood depth and damage in the study region, so we did not include them in the final set. • The same torms sets identified as producing low bias in the CLARA Model Improvement Plan report also yielded low bias in this analysis, suggesting that the eastern half of the sets identified for the 2017 Coastal Master Plan would be suitable for this separate investigation. • The storm selection analysis considered only an FWOA with no barrier, but CPRA wanted to ensure that the final storm set had sufficient variation to capture a barrier’s effects on neighboring areas, particularly Mississippi. In particular, this entailed including “off- angle” storms that make landfall at angles 45 degrees less or greater than the mean angle and that inclusion of a barrier could especially affect.

Informed by this initial assessment and the subsequent discussions, CPRA selected a storm set, including the eastern half of set 11 from the CLARA Model Improvement Plan report (FJK, 2017; see also Atkinson and Roberts, 2015). This subset includes nine storms per track for all five mean-angle tracks, varying the central pressure from 900 to 960 millibars (mb) and the storm size (radius of maximum winds) from 6 to 35.6 nautical miles. Set 11

4 These storms varied in their central pressure and radius of maximum wind speed and were drawn from MsCIP’s storm track 18 (see USACE, 2009, Figure 2.4-7d). 5 The storm selection analysis reached the same conclusions as the 2017 Coastal Master Plan Attachment C3-25, so we do not show specific analysis results in this document. For details of this analysis, see FJK, 2017, Section 6.5. Methods and Data Sources 9 also includes 960 and 975 mb storms on the off-angle tracks for the eastern region per the discussion above; the lower-intensity 975 mb storms are designed to represent storms down to approximately category 1 wind speeds on the Saffir–Simpson scale. The eastern half of set 11 includes a total of 77 storms, and we used this set to compare all barrier alignment options with an FWOA in the CLARA analysis for both Louisiana and Mississippi.

Asset Inventory Updates for Mississippi

The most substantial update to CLARA needed for this investigation was the incorporation of economic and asset data for the Mississippi coastal counties. In this section, we describe data sources used and methods applied to develop the database of assets at risk in coastal Mississippi. Specifically, we performed the following tasks, on which the rest of this section elaborates:

• We updated the inventory of assets at risk to include coastal Mississippi, drawing on the most-recent data sources available. • The CLARA 2.0 geospatial grid requires a higher level of geospatial fidelity than the previous analysis, motivating a move away from data sources based on U.S. census blocks to those containing information on individual parcels or geospatial point, line, or polygon data.

Structure Inventory and Vehicles We adopted residential structure counts for Mississippi from several sources. Both residential and commercial structure counts for Mississippi are taken from the MsCIP socioeconomic database that USACE provided (USACE, 2009), in combination with geodatabases accessed from the Mississippi Automated Resource Information System (MARIS) (MARIS, 2016). The resulting database contains information for more than 200,000 tax parcels in Hancock, Har- rison, and Jackson counties. This data set has been recently updated and includes substantial detail, and its spatial resolution exceeds that of the inventory used for many areas of coastal Louisiana in CLARA. No clear directionality of bias, however, emerges from these resolution differences. We integrated these data into a single parcel database for incorporation in CLARA 2.0. The structures in each parcel are described by location (longitude and latitude coordinates), number of units, occupation type, damage category, square footage, and foundation type. Vehicle count estimates follow the same methodology as in previous work. Specifically, privately owned vehicle counts come from the FEMA Hazus model (FEMA, undated) and are based on an average number of privately owned vehicles per household from 2000 U.S. census data updated to reflect 2010 average counts (USACE, 2009). Table 2.1 shows a summary of the count of structures and vehicles by CLARA asset class for each Mississippi coastal county. This summary shows year 50 counts used in the future scenario analysis, which we projected forward from current counts using the 2012 Coastal Master Plan growth assumption of 0.67 percent per year. 10 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Table 2.1 Mississippi Assets at Risk, Year 50, in Thousands of Assets

Hancock Harrison Jackson Asset Class County County County

Single-family residential 13.2 70.4 52.3

Manufactured homes 0.8 4.1 3.0

Small multifamily residential 0.9 4.9 3.6

Large multifamily residential 1.5 7.9 5.8

Commercial 0.9 4.6 3.5

Industrial 0.2 1.1 0.8

Public 0.0 0.1 0.1

Total structures 17.5 93.0 69.1

Vehicles 13.2 70.5 52.3

NOTE: Because of rounding, totals might not sum precisely. Hancock County includes fewer than 100 public buildings in this inventory, shown as 0 in the table because of rounding.

Transportation Infrastructure The inventory of roads and bridges used in the analysis described later in this report comes from the NAVTEQ (now HERE) navigation data included in the Homeland Security Infra- structure Program (HSIP) Gold database (HSIP, 2014). This data set provides more-detailed road data for the entire study region. Using the NAVTEQ database, we assembled a new inventory of roads for CLARA 2.0 (Table 2.2). For this inventory, we partitioned each of the road lines included in the NAVTEQ geodatabase across all the grid points considered in the study region. Road types in this database include highways, mains, streets, and bridges, which we classified into the categories shown in Table 2.2. For each road type, the updated inventory includes the length of road (in road-miles) that falls within the vicinity of each grid point.

Agriculture For this study, we did not include crops grown in the coastal region in the updated asset inventory for Mississippi. The CLARA team analyzed the possibility of using the agricultural data contained in the HSIP Gold database but found that this data source did not provide adequate data fidelity or resolution for crop coverage in the Mississippi counties considered. Previous work that focused on coastal flood risk in Louisiana using the CLARA model indicates that direct damage to crops from flooding represents a very small fraction (less than 0.2 percent)

Table 2.2 Mississippi Road-Miles per Road Type

Hancock Harrison Jackson Road Type County County County

Highway 103 178 169

Street 982 1,675 1,711 Methods and Data Sources 11 of overall damage, and induced damage itself is small compared with total flood risk, so we expect the omission to introduce little to no bias in the estimates of induced damage for coastal Mississippi.

Converting Data to CLARA 2.0 Grid Point Resolution The codes describing structure characteristics in the parcel-level data sets from MsCIP and MARIS were not equivalent to the general building stock (GBS) codes used in CLARA 2.0. As a result, it was necessary to map the parcel-level data codes to the appropriate GBS codes. Table 2.3 lists the equivalent GBS codes for a subset of the combinations of occupation type and damage category codes in the parcel-level database. Using this crosswalk, we were able to map the structures contained in the integrated parcel-level database to the standard GBS codes used in CLARA 2.0.

Assets Values at Risk in Coastal Mississippi Table 2.4 shows a summary of the total value of assets at risk (in constant 2015 dollars) for each county and asset class in Mississippi in the year 50 future condition. The table shows that asset values at risk from flooding are much higher in total in Harrison County and Jackson County than in Hancock County. In all three counties, commercial assets are the largest category of values at risk, followed by large multifamily structures (e.g., apartment buildings) and single- family homes.

Table 2.3 Parcel-Level Data Crosswalk Between Occupation Type and General Building Stock Code

Occupation Type Occupation Type Code Damage Category Code Equivalent GBS Code

One story 1STY Residential RES1

Two story 2STY Residential RES1

Mobile home MOBHOME Mobile home RES4

Multiunit MULTI-UNIT Multiunit RES2

Commercial COM Commercial COM

Industrial IND Industrial IND

Professional PROF Commercial COM

Warehouse WARE Industrial IND

Grocery GROC Commercial COM

Dining EAT Commercial COM

Public PUBL Government GOV

Repair REPA Commercial COM

Retail RETA Commercial COM 12 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Table 2.4 Asset Values at Risk in Coastal Mississippi, Year 50, in Billions of Constant 2015 Dollars

Hancock Harrison Jackson Asset Class County County County

Single-family residential 5.8 30.7 22.8

Manufactured homes 0.1 0.5 0.4

Small multifamily residential 0.4 2.1 1.6

Large multifamily residential 6.2 33.1 24.6

Commercial 11.5 61.5 46.1

Industrial 1.7 9.2 6.9

Public 0.0 0.5 0.4

Roads 0.3 0.5 0.5

Vehicles 3.5 1.9 1.4

Total 26.4 139.9 104.5

NOTE: Because of rounding, totals might not sum precisely. Public assets at risk in Hancock County are less than $100 million in value and appear as 0.0 in the table because of rounding.

Other Assumptions

Future Scenario Conditions This investigation considers the performance of proposed barrier alignments under one plausible future scenario condition 50 years into the future adapted from the 2012 Coastal Master Plan analysis. Specifically, we adopted rates of land subsidence and sea level rise directly from the 2012 Less Optimistic scenario (Peyronnin et al., 2013), with sea level rise totaling 1.47 ft. (0.45 m) over the analysis time frame. Both the Arcadis and RAND teams used the same set of scenario assumptions. Similarly, we applied the 0.67-percent annual population growth rate used in the 2012 analysis to project assets at risk forward to year 50. Note that, after reviewing the subsidence rates assembled for the 2012 analysis and consulting with the Arcadis team, we found little evidence for land subsidence in the Mississippi coastal area, so we set the rate of subsidence in these areas to 0 mm per year over the period of analysis. Note also that all estimates of surge, wave, and flood elevations described in this document use the North American Vertical Datum of 1988 (NAVD 88) as a common baseline for comparison. Other scenario assumptions from the Less Optimistic scenario that are relevant to CLARA, however, were not applied. For this investigation, CPRA decided that the flood depth and damage statistics should be based on estimates of coastal storm frequency and intensity based on historical observation alone, rather than using the storm frequency and intensity modifiers applied for the Less Optimistic scenario. It made this decision because the frequency and intensity modifiers to be applied in the 2017 Coastal Master Plan analysis dif- fered substantially from the 2012 values because of advances in the scientific literature over the Methods and Data Sources 13 intervening time span. In addition, this simplifying assumption allows for easier interpretation of analysis results when looking at a single scenario alone.

Enclosed Area Hurricane Protection Heights Data sets describing levee, floodwall, and gate heights for different reaches (portions of a hurricane protection system surrounding a protected area) are important components of the CLARA analysis for enclosed protected areas. Parishes in the study area that have enclosed protection systems include portions of St. Tammany Parish (small ring levees in Slidell) and Plaquemines Parish. But the most significant protected area in terms of geographic size and assets at risk is the portion of HSDRRS east of the Mississippi River (HSDRRS East), which includes portions of Orleans, Jefferson, and St. Bernard parishes. We adapted reach heights for Slidell and Plaquemines Parish directly from the 2012 analysis for this study. In 2012, we modeled these reach heights as subsiding over time per the assumed rate for each region. But CPRA requested that the Arcadis and RAND teams update the values and subsidence assumptions made for HSDRRS East, using a more recent data set that USACE provided. Specifically, we updated all design heights for HSDRRS East using a cleaned data set that Arcadis provided, and, per CPRA guidance, we assumed that these design heights are maintained over time. In effect, this approach assumes an ongoing series of levee lifts and other improvements to HSDRRS East that the responsible levee district, the Southeast Louisiana Flood Protection Authority—East, will undertake over the course of the next 50 years. These assumed improvements would be intended to maintain current target design heights and offset the effects of land subsidence. In practice, it means that areas within HSDRRS East play a relatively small role in this comparative analysis because the assumed maintained levee heights prevent HSDRRS East from flooding during most storm events considered. The inclusion of a barrier thus leaves these areas largely unaffected, except under more-extreme events in which overtopping or levee failure for the HSDRRS East system could begin to occur. Lastly, for this analysis, we artificially raised the HSDRRS East levees along the Missis- sippi River to approximately 32 ft. (10 m) in CLARA, mirroring the Arcadis approach. We did this in response to some anomalies that the Arcadis team detected related to flooding from the river and to ensure that no flooding was entering the system from the river erroneously.

Parametric Uncertainty in Enclosed Areas As noted above, we conducted this analysis using the version of CLARA 2.0 developed during the 2017 model improvement process. This version of the model includes a new approach to parametric uncertainty developed for 2017 and documented in Attachment C3-25 to the 2017 Coastal Master Plan (FJK, 2017). In this instance, parametric uncertainty refers to uncertainty or noise associated with flood depth and damage assessment that emerges from sources other than the key drivers of uncertainty in the master plan. For example, noise surrounding estimates of storm surge and wave elevations or associated with landscape elevations that serve as inputs to CLARA’s algorithms for estimating flood risk can now be captured in the model and presented as percentile ranges within each scenario. Accounting for parametric uncertainty enables us to more appropriately report results in terms of a range of damage and damage reduction benefits rather than as a single point estimate. Further information on the methods and model implementation can be found in FJK, 2017. 14 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

However, during early phases of this analysis, we identified unexpected upward bias occurring for flood depth estimates in enclosed areas, particularly HSDRRS East. We traced this bias back to a step in the parametric uncertainty approach, which uses a stylized Monte Carlo simulation process to incorporate uncertainty associated with surge and wave heights on the exterior of the system into flood depth exceedance estimates on the interior (FJK, 2017, pp. 58–66). Subsequent testing confirmed that this uncertainty step was not working as intended; although we subsequently corrected it for the 2017 analysis, a fix was not available in time for the Lake Pontchartrain barrier analysis. For the results described in Chapter Three, we disabled the Monte Carlo uncertainty process for enclosed areas in CLARA to eliminate this source of bias. The effect of this simplifying assumption is that the 10th- and 90th-percentile uncertainty bounds for depth and damage within enclosed areas are narrower than they otherwise would be with the full uncertainty approach implemented. However, given the limited flooding noted on the interior of HSDRRS East in the results in Chapter Three, we do not believe that this assumption meaningfully affects any of the rankings or conclusions described in this report.

Experimental Design

The flood depth and damage analysis using CLARA evaluated an FWOA condition, as well as five proposed configurations for a barrier enclosing Lake Pontchartrain. Table 2.5 summarizes the final set of alignments that the Arcadis team identified through a preliminary screening analysis and evaluated with CLARA. Figure 2.2 shows the alternative locations proposed for two of the configurations, as well as a summary of the barrier crown and gate heights for the Hwy90/10/10 and CSX/10/10 options. In the table, we present the alignments roughly by the scale of intervention, ranging from a no-gate option that somewhat raises the existing road grade to a large 24.5-ft. barrier with gates. The lower alignments would be designed to reduce the volume of storm surge entering the lake but not necessarily to prevent surge and wave overtopping (i.e., they would deliberately allow some overtopping). In contrast, the 24.5-ft.

Table 2.5 Lake Pontchartrain Barrier Configurations Considered in This Analysis

Name Location Road Height, in Feet Gate Height, in Feet

FWOA Not applicable No change Not applicable

Hwy90/0/2 Highway 90 No change 2.0

Hwy90/10/0 Highway 90 to Slidell 10.0 Not applicable

CSX/10/10 CSX railroad to Slidell 10.0 10.0

Hwy90/10/10 Highway 90 to Slidell 10.0 10.0

Hwy90/24.5/24.5 Highway 90 to Slidell 24.5 24.5

NOTE: Hwy90/0/2 = Highway 90 with existing crown and 2-ft. gates. Hwy90/10/0 = Highway 90 with 10-ft. crown, no gates, and Slidell extension. CSX/10/10 = CSX railroad with 10-ft. crown, 10-ft. gates, and Slidell extension. Hwy90/10/10 = Highway 90 with 10-ft. crown, 10-ft. gates, and Slidell extension. Hwy90/24.5/24.5 = Highway 90 with 24.5-ft. crown, 24.5-ft. gates, and Slidell extension. Methods and Data Sources 15

Figure 2.2 Maps Showing Two Proposed Alignment Configurations

Hwy90/10/10 CSX/10/10

Tie into the existing Slidell protection levee Road crest set to a constant elevation of 10 ft. Tie into the existing Slidell protection levee

SOURCE: Arcadis. NOTE: Locations are approximate, and the scales of the maps are not equal. RAND RR1988-2.2

alignment would be intended to prevent surge up to a 100-year return interval from entering the lake. As described above, we conducted the evaluation for a single set of assumptions about future conditions, drawing from the 2012 Coastal Master Plan’s Less Optimistic future scenario in year 50. For each of the FWOA and barrier alignments considered, the ARCADIS team ran ADCIRC and SWAN for 77 simulated coastal storms to provide suitable storm surge and wave inputs for the CLARA model. We then used the CLARA 2.0 model to estimate the statistical return intervals for storm surge and wave heights, flood depths, and direct economic damage at 10-, 50-, 100-, and 500- year AEP return intervals and in terms of EAD. For enclosed areas, we considered two possible structure fragility scenarios:

1. No Fragility allows surge and waves to overtop protection structures but assumes that no structure failures can occur (overtopping only). 2. Morganza to the Gulf (MTTG) is one of the fragility probability scenarios described in FJK, 2017, that allows structure failure to occur during storm events and uses fragility probabilities derived from the recent USACE MTTG Post Authorization Change study (USACE, 2013, pp. 38–57).

In addition, we used the new CLARA 2.0 parametric uncertainty approach to estimate uncertainty surrounding flood depth, damage by return interval, and EAD in each scenario. Results in enclosed areas also include limited parametric uncertainty bounds but do not include uncertainty associated with exterior surge and wave heights, as previously noted. Results generated and provided to CPRA include the 10th, 50th, and 90th percentiles across the range of parametric uncertainty. Chapter Three describes these results in detail.

CHAPTER THREE Results from Proposed Barrier Alignments

Introduction

In this chapter, we summarize the CLARA flood depth and damage analysis results for the Lake Pontchartrain barrier investigation. The narrative includes figures and tables highlighting selected results by location, alignment, scenario, and percentile. Using Tableau visualization software, we also provided CPRA with complete analysis results across all dimensions, not necessarily highlighted in this chapter. This chapter begins with a brief description of flood depths and EAD in the study region, including Louisiana parishes and the Mississippi coastal counties, in an FWOA. Next, we show the degree to which the barrier alignment options reduce (or increase) flood depths in different areas at the 100-year return interval and further describe how a barrier could change the height of 100-year storm surge and waves along affected portions of HSDRRS East. Next, using EAD (the key flood risk decision metric for the 2012 and 2017 Coastal Master Plan analyses) and damage by return interval, we describe changes in economic damage with a barrier in place in Louisiana. Specifically, we estimate the net change in risk in different regions of the Louisiana coastline and rankings of damage reduction by alignment. We then focus on coastal Mississippi and describe induced damage in the Mississippi counties from the five alignments in terms of both EAD and damage by return interval. Finally, the chapter concludes with a brief summary of the results and preliminary recommendations for CPRA emerging from the analysis.

Flood Risk in a Future Without Action At present, both coastal Louisiana and Mississippi are at substantial risk from coastal flood events. This risk is expected to increase in an FWOA, in large part because of global sea level rise trends, ongoing land subsidence, and coastal wetland loss across much of the Louisiana study area. Figure 3.1 illustrates the future risks to the study region. The figure shows an estimate of median 100-year (1-percent AEP) flood depths in the FWOA generated from CLARA. This figure shows that many coastal areas surrounding Lake Pontchartrain are facing upward of 10 to 15 ft. of flood depths at the 100-year interval. These high 100-year depths extend into Mississippi, including such areas as Bay St. Louis and Pass Christian in the west and extending to Pascagoula in the eastern portion of the state. We also observe depths exceeding 18 ft. at the mouth of the Pearl River and extending throughout portions of Plaquemines Parish. Enclosed areas outside HSDRRS East, including Slidell and Plaquemines, also show extensive 100-year flood depths in the FWOA.

17 18 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Figure 3.1 Future Without Action Flood Depths for the Study Region, 100-Year Flood Interval, No Fragility

New Orleans

Flood depth, in feet <1 1–31 4–64 7–97 10–1210 1313–15 1616–18 More than 18

NOTE: The map shows only grid points with flood depths greater than 0.5 ft. RAND RR1988-3.1

Figure 3.1 also shows 100-year flood depths within HSDRRS East assuming a No Fra- gility scenario. These results confirm that the system would continue to provide 100-year risk reduction for New Orleans—the design target for the system as a whole—and the protected portions of Jefferson and St. Bernard parishes if the HSDRRS East design heights are maintained over the 50-year time span. Only minor overtopping from waves is observed at the 100- year interval, and areas that do show minor depths (1 to 3 ft.) are mostly uninhabited (e.g., Violet Marsh). CLARA results also show that HSDRRS East continues to perform well in scenarios with return intervals more extreme than 100 years (1-percent AEP). For instance, Figure 3.2 shows 500-year flood depths for HSDRRS East in the No Fragility (left pane) and MTTG (right pane) scenarios. Without fragility, flooding of 1 to 3 ft. can be observed for selected areas, including Kenner and Metairie in Jefferson Parish, the Lower Ninth Ward in Orleans Parish, portions of neighboring St. Bernard, and St. Charles. The extent of flooding grows in scenarios that consider fragility (MTTG scenario), with extensive flooding in eastern New Orleans and along the south shore of Lake Pontchartrain for the main portion of the city. Nevertheless, 500-year flood depths remain low in this scenario, most often 1 to 3 ft., and large portions of central and uptown New Orleans are not flooded in this case. The extent and depth of flooding shown here are lower than the flooding that occurred during Hurricane Katrina, for example, in which 80 percent of the city of New Orleans was inundated with depths that exceeded 6 ft. CLARA also estimates substantial flood damage for the study region in an FWOA. For example, Figure 3.3 shows a summary of EAD by region and asset class under the MTTG Results from Proposed Barrier Alignments 19

Figure 3.2 Future Without Action Flood Depths for the East Bank of Greater New Orleans, 500-Year Flood Interval

No Fragility MTTG Flood depth, in feet <1 1–31 4–64 6 7–97 10–1210 2 13 13–15

NOTE: The map shows only grid points with flood depths greater than 0.5 ft. RAND RR1988-3.2

Figure 3.3 Future Without Action Expected Annual Damage, by Region and Asset Class, in Billions of Constant 2015 Dollars

St. Tammany 2.1

Orleans and Jefferson 0.5

Other Lake Pontchartrain parishes 0.5 Region Asset class Plaquemines and All other St. Bernard 0.1 Industrial Residential Coastal Mississippi Commercial 1.6 0.0 0.2 0.4 0.6 0.81.0 1.2 1.4 1.6 1.82.0 2.2 2.4 2.6 2.8 3.0 EAD, in billions of constant 2015 dollars NOTE: The stacked bars show the 50th percentiles (medians), and the lines show the 10th- to 90th-percentile ranges. RAND RR1988-3.3 scenario.1 Key summary regions include (see Figure 1.1 in Chapter One) St. Tammany, Orleans, and Jefferson parishes (mostly within HSDRRS East); portions of other Louisiana parishes surrounding the lake (Livingston, St. Charles, St. James, St. John the Baptist, and Tangipahoa); Plaquemines and St. Bernard Parishes; and coastal Mississippi. In the figure,

1 Throughout the remainder of the chapter, we show MTTG damage results to better illustrate potential effects on HSDRRS East in the event of failure. 20 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier median values are shown with the stacked bars, with the lines extending around these bar plots showing the 10th- to 90th-percentile ranges of EAD values estimated. EAD for St. Tammany Parish—including areas along the north shore of the lake with a growing population—is estimated at $2.1 billion ($1.4 billion to $2.8 billion) in the FWOA. Densely populated Orleans and Jefferson parishes face lower average annual damage of $500 million because HSDRRS East is present, with the sum total of EAD for other parishes surrounding the lake also totaling approximately the same amount. EAD in Plaquemines and St. Bernard parishes is lower still, in large part because they have fewer and less valuable assets than other regions. Finally, the sum total of estimated EAD in coastal Mississippi, including affected portions of Hancock, Harrison, and Jackson counties, is $1.6 billion ($1.1 billion to $2.1 billion). Importantly, although this damage risk could increase with a barrier, it should be noted that the baseline for these areas is high in the FWOA scenario. Finally, Figure 3.3 also shows the relative contribution to EAD by type of asset. In most locations, the largest source of economic losses and damage is commercial assets, followed by single and multifamily residences. Industrial and public buildings, roads, vehicles, and agricultural crops make up a comparatively small fraction of damage totals in the CLARA estimates, whether in Louisiana or Mississippi.

Risk-Reduction Benefits for Louisiana Parishes

Next, we describe the estimated benefits that a barrier could provide in coastal Louisiana. This section describes, in turn, benefits in terms of flood depth reduction behind the barrier, storm surge and wave height changes at points around HSDRRS East, and damage reduction in terms of both EAD and damage by AEP interval. The subsequent section then details potential negative effects on Mississippi coastal parishes. Note that this section summarizes results for four of the five barrier alignments considered. We do not describe the remaining alignment, Hwy90/10/0, in detail because it was shown to provide essentially no depth or damage reduction benefit (no change). Excluding gates from the alignment allows storm surge to flow unimpeded through the Rigolets and Chef Menteur passes during simulated storm events, and raising the highway elevation does not appear to meaningfully reduce surge propagation around the lake. As a result, the other approaches out- perform this option, and, for clarity, we omit it in the summary figures and tables.

Flood Depth Reduction The remaining barrier alignment options that we considered all reduce flood depths for the Lake Pontchartrain region behind the barrier and correspondingly increase flood depths on the gulf side of the barrier. A summary of the change in 100-year flood depths, for example, is shown in Figure 3.4 (50th percentile). The figure shows the change in flood depths by CLARA grid point for all four alignments. Table 3.1 further provides a shaded tabular summary of flood depth changes by Louisi- ana parish, expressed as average change in flood depths by parish. Parishes inside or outside of enclosed protection systems (such as HSDRRS East) are also split to show results for areas inside or outside of the protection system. In general, all of the remaining barrier alignment options reduce flood depths for the areas targeted for protection. Both Figure 3.4 and Table 3.1 show that Hwy90/0/2 yields the Results from Proposed Barrier Alignments 21 More than –8 –8 to –7 –6 to –5 –4 to –3 –2 to –1 0 1 to 2 3 to 4 5 to 6 7 to 8 More than 8 Change in depth, feet CSX/10/10 Hwy90/24.5/24.5 Hwy90/0/2 Hwy90/10/10 NOTE: The fi gure shows the change in fl ood depths by CLARA grid point for all four alignments, with green points indicating depth reduction and red showing depth increase relative to the FWOA. The fi gure shows only grid points with changes in fl ood depths greater than 0.5 ft. bins are set at midpoint of each range. do not show Hwy90/10/0 because it does reduce So, for example, a depth delta of 1 to 2 ft. is actually 0.5 2.49 ft., while 3 4 between 2.5 and 4.49 We depth. Fragility scenario = MTTG . RAND RR1988-3.4 Figure 3.4 Changes Depths in 100-Year for Four Barrier Alignment Options 22 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Table 3.1 Average Flood Depth Change, by Louisiana Parish, 100-Year Flood Interval, 50th Percentile

Parish Protection Status Hwy90/0/2 CSX/10/10 Hwy90/10/10 Hwy90/24.5/24.5 St. Tammany Unenclosed –1 0 –1 0 Enclosed 0 0 0 0 St. John the Unenclosed –2 –1 –1 –1 Baptist Ascension Unenclosed –1 –2 –2 –2 Jefferson Unenclosed –2 –4 –4 –6 Enclosed 0 0 0 0 Orleans Unenclosed 0 0 –1 –1 Enclosed –1 –1 –1 0 Livingston Unenclosed –2 –3 –2 –3 Tangipahoa Unenclosed –2 –3 –3 –4 St. James Unenclosed –1 –2 –2 –2 St. Charles Unenclosed –2 –3 –3 –3 Enclosed 0 0 0 0 Plaquemines Unenclosed 0 0 0 1 Enclosed 0 0 0 0 St. Bernard Unenclosed 0 1 1 1 Enclosed 0 0 –1 0

Average change in depth (ft) –7 3

NOTE: The table shows average flood depth change, in feet, by parish, for grid points with at least 1 ft. of depth in FWOA conditions. Fragility scenario = MTTG. RAND RR1988-T3.1 least 100-year depth reduction, on average, with results typically ranging from 1 to 2 ft. of reduction relative to an FWOA. In contrast, Hwy90/24.5/24.5 yields the most depth reduction for points behind the barrier, with results in some areas reaching 4 to 6 ft. Results from this high alignment are mixed in parishes that are partially inside and partially outside of the barrier, however. For example, average 100-year depth reduction in portions of St. Tammany Parish without an enclosed protection system (i.e., nearly the whole parish except for a portion of the town of Slidell, which has locally built ring levees) is shown as 0 in Table 3.1, but this actually indicates depth reduction upward of 6 ft. for points behind the barrier and depth inducement of 4 to 6 ft. for locations in front. A simple parishwide average cancels each effect out, but the difference is clearly visible in the mapped version. Both Hwy90/10/10 and CSX/10/10 yield depth reduction that generally falls between the other two alignments. Both alignments show about 3 to 4 ft. of depth reduction for grid points behind the barrier (Figure 3.4) and 1 to 2 ft. of inducement for areas in front, shown to be influenced by the barrier. Results from both are very similar, on average (Table 3.1), except in the immediate vicinity of the alignment itself (Figure 3.4). Here, the CSX alignment is angled further outward toward the gulf and thus produces depth reduction for a slightly larger area around the mouth of the lake. Results from Proposed Barrier Alignments 23

For all alignment options, 100-year depth reduction results show little change within protected areas of HSDRRS East because of the assumption about future levee lifts and maintenance previously described. Similarly, we observe no increases for enclosed areas of Plaquemines Parish (East Bank). This is because enclosed areas of Plaquemines already overtop and flood extensively in the FWOA with subsided levees at the 100-year interval, with depths upward of 6 to 8 ft. in some locations. The presence of a barrier to the north does not appear to meaningfully alter these existing flood patterns.

Surge and Wave Changes for the Greater New Orleans Protection System Maintaining the HSDRRS East protection structures at their current 100-year design heights or higher, relative to ground level, is rightfully recognized as a key state and local priority in future decades. But future levee lifts and structure improvements designed to offset the effects of land subsidence could be costly and present implementation challenges. Although the HSDRRS East design heights are assumed to be maintained at current levels in this 50-year analysis, CPRA also wanted to consider how the 100-year still-water surge heights and associated significant wave heights might change for HSDRRS East with a barrier in place.2 This provides a rough, first-order estimate of the barrier’s potential benefits for this important and already-protected area, although not yet quantified in terms of a change in construction or maintenance costs associated with a lowering of the design heights of the HSDRRS East levees. Figure 3.5 and Table 3.2 summarize these potential benefits for HSDRRS East. Figure 3.5, for example, shows the change in 100-year still-water elevation (left pane) and associated signif-

Figure 3.5 Change in Hurricane and Storm Damage Risk Reduction System Surge and Wave Heights for Highway 90 with Its Existing Crown and Low Gates, 100-Year Flood Interval

Change in still-water surge elevation, FWOA Change in significant wave height, FWOA

New Orleans New Orleans East East New Orleans Main New Orleans Main St. Bernard St. Bernard

Change in still-water surge height, in feet Change in significant wave height, in feet –2.0 2.0 –1.0 1.0

NOTE: The figure shows 50th-percentile values for HSDRRS. Green indicates a reduction in surge or wave height, while red indicates an increase relative to the FWOA. Note that the color scales differ. RAND RR1988-3.5

2 Significant wave height is defined as the mean height (trough to crest) of the highest one-third of waves. 24 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier 3 0.6 0.6 0.1 –0.5 –0.5 –0.3 –1.4 –1.5 –1.3 –1.0 –1.1 –1.1 24.5 Hwy90/24.5/ 0.3 0.4 0.1 –0.5 –0.4 –0.3 –1.2 –1.1 –1.1 –0.9 –0.5 –1.0 0.4 0.5 0.1 –0.5 –0.5 –0.3 –1.3 –1.1 –1.1 –1.0 –0.5 –1.0 Signiﬁ Delta cant Wave Signiﬁ Hwy90/10/10 CXS/10/10 0.2 0.2 0.1 –0.3 –0.3 –0.2 –0.8 –0.6 –0.7 –0.6 –0.3 –0.6 Hwy90/02 1.9 2.2 0.4 –3.6 –3.4 –3.6 –4.3 –5.3 –4.3 –3.4 –3.4 –4.8 24.5 Hwy90/24.5/ 1.1 1.2 0.4 –3.2 –3.0 –3.4 –2.9 –2.7 –2.5 –2.7 –1.2 –2.6 1.3 0.4 1.2 –3.5 –3.1 –3.5 –3.2 –3.2 –2.9 –2.8 –1.6 –3.0 Still-Water Surge Delta Still-Water Hwy90/10/10 CXS/10/10 0.8 0.7 0.2 –1.8 –1.7 –2.0 –1.9 –1.8 –1.8 –1.7 –1.0 –1.8 Hwy90/02 St. Rose St. Charles, remainder St. Charles, Norco Metairie West Lake West Forest Lakeview City Park Six Flags Kenner Marsh Violet New Orleans East Gentilly Poydras Subunit Average change in surge and wave elevation (ft) Average geographic NOTE: The table shows 50th-percentile values for points surrounding and outside HSDRRS. Subunits represent different locations in each region (polder). Region New Orleans East New Orleans Main St. Bernard –5 Table 3.2 Average Hurricane and Storm Damage Risk Reduction System Change in Surge Height, and Wave Flood 100-Year Interval, in Feet Results from Proposed Barrier Alignments 25 icant wave height (right pane) for points surrounding the protection system with Hwy90/0/2 in place.3 Figure 3.5 shows that the alignment yields 1 to 2 ft. of surge height reduction and approximately 0.5 ft. of wave height reduction at the 100-year interval for the portions of HSDRRS East behind the barrier, including the entire stretch along the south shore of the lake. However, at points in front of the barrier tie-in (along the eastern face of the New Orleans East polder), the reverse is true: Surge heights increase by 0.5 to 1 ft. for portions of the St. Bernard and New Orleans East polders, with wave heights also increasing by up to 0.5 ft. We note similar patterns for the other alignment options, in which the magnitude of the change increases with the height of the barrier alignment. Table 3.2 summarizes these changes for all four alignments, averaging the results for different areas within each polder. These summary results show that Hwy90/10/10 and CSX/10/10 yield 2 to 3 ft. of surge height and 0.5 to 1 ft. of wave reduction, on average, for portions of the system inside the barrier; outside of the barrier, the inducement is slightly higher than that observed with Hwy90/0/2. The high (Hwy90/24.5/24.5) barrier alignment yields the greatest surge reduction for the south shore areas, ranging from 3 to 5 ft. on average, but also surge height inducement of 2 ft. in some portions of New Orleans East and St. Bernard. These results, although preliminary and not intended to support design calculations, suggest that a new barrier could lead to revisions in the future 100-year design heights for HSDRRS East. Whether the reduced cost associated with lower design heights on the south shore would offset any increases indicated for areas outside of the barrier, however, is outside the scope of this report.

Damage Reduction We next consider the change in coastal flood damage and damage reduction benefits from a proposed barrier for Louisiana parishes in the study area. In this section, we generally describe damage change in terms of EAD; for ease of reference, however, we also provide results by AEP interval (50, 100, and 500 years) in a summary table (Table 3.4 at the end of this section). First, Figure 3.6 shows a bar-plot summary of EAD with or without the four proposed barrier alignments in place. Total FWOA EAD for the Louisiana study region (MTTG fragility scenario, 50th percentile) is estimated at $3.2 billion, with a parametric uncertainty range of $2.1 billion to $4.4 billion. The results show that all four compared alignments reduce total EAD compared with this FWOA. For instance, Hwy90/0/2 reduces total EAD to $2.0 billion ($1.4 billion to $2.7 billion), while Hwy90/10/10 yields $1.8 billion in remaining damage ($1.2 billion to $2.5 billion). Despite a much taller barrier, Hwy90/24.5/24.5 shows remaining EAD similar to that of Hwy90/0/2. This is caused by induced damage in some areas, particularly outlying portions of St. Tammany Parish and in St. Bernard Parish, which does not occur to the same degree with the lower alignment options. Importantly, EAD is not eliminated (i.e., reduced to 0) under any barrier configuration, meaning that residual or remaining flood risk is present for all options. In part, this is because the barrier does not protect any of the targeted areas from Lake Pontchartrain itself. It therefore cannot protect communities from storm surge and waves pushed directly from the lake. As shown in Figure 3.6, damage reduction varies substantially by region. Figure 3.7 focuses on these differences with a bar plot specifically showing the change in EAD by align-

3 Still-water elevations include astronomic tides and storm surge but, by definition, exclude the effects of waves. 26 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Figure 3.6 Expected Annual Damage, by Alignment and Louisiana Region, in Billions of Constant 2015 Dollars

4.5

Region 4.0 Plaquemines and St. Bernard Other Lake Pontchartrain parishes 3.5 Orleans and Jefferson St. Tammany 0.1 3.0 0.5

2.5 0.5

2.0 0.1 0.1 0.2 0.1 0.1 0.1 0.2 0.2 0.1 0.1 1.5 0.2 0.2

1.0 2.1 EAD, in billions of constant 2015 dollars 1.6 1.5 1.4 1.4 0.5

0.0 FWOA Hwy90/0/2 Hwy90/10/10 CSX/10/10 Hwy90/24.5/24.5 NOTE: Each bar is colored by Louisiana region and represents the 50th-percentile EAD estimate; the lines show the 10th- to 90th-percentile ranges for all regions. Fragility scenario = MTTG. RAND RR1988-3.7 ment for each region. The bar plots show damage reduction values at the median, while the lines show the change in EAD at the 10th and 90th percentiles.4 Results shown here confirm the previous observation that EAD reduction in St. Tam- many Parish yields the highest overall monetary benefit across the alignments, followed by the other parishes surrounding Lake Pontchartrain and Orleans and Jefferson parishes (mostly within HSDRRS East). For St. Tammany Parish, Hwy90/10/10 yields the greatest EAD reduction ($712 million), followed closely by CSX/10/10 ($696 million). At the median, Hwy90/0/2 yields somewhat lower benefit ($620 million), while Hwy90/24.5/24.5 lags because of the induced damage previously noted. We note, however, that the 10th- to 90th-percentile benefits represented by the error bars on the plot are very similar and overlap for many or most of the alignments. EAD reduction is also notable in other surrounding parishes, ranging from $321 million (Hwy90/0/2) to $426 million (Hwy90/24.5/24.5). In these areas further west and further inland, Hwy90/24.5/24.5 yields the most benefit because of the higher degree of depth reduction and lack of induced flooding. In addition, substantial damage reduction is also noted for

4 Note that the 90th percentile can be interpreted as change in EAD at the 90th percentile of flood depths, which is not necessarily equivalent to the 90th percentile of change in EAD. The same is true for the 10th percentile. A comparison of the benefits from each alignment using a formal statistical test would instead suggest estimating the variance of the difference in EAD. The CLARA team developed an approach to directly estimate the variance of differences in EAD between cases for the final 2017 analysis subsequent to finalizing the analysis in this report, but we did not apply it here. Results from Proposed Barrier Alignments 27

Figure 3.7 Reduction in Expected Annual Damage, by Alignment and Louisiana Region, in Millions of Constant 2015 Dollars

Region Alignment

St. Tammany Hwy90/0/2 620 Hwy90/10/10 712 CSX/10/10 696 Hwy90/24.5/24.5 539 Orleans and Hwy90/0/2 292 Jefferson Hwy90/10/10 304 CSX/10/10 315 Hwy90/24.5/24.5 281 Other Lake Hwy90/0/2 321 Pontchartrain Hwy90/10/10 parishes 407 CSX/10/10 394 Hwy90/24.5/24.5 426 Plaquemines Hwy90/0/2 –14 and St. Hwy90/10/10 –20 Bernard CSX/10/10 –25 Hwy90/24.5/24.5 –48 9008007006005004003002001000–100–200 1,000

EAD reduction NOTE: The bars indicate 50th-percentile values, and the lines show the 10th- to 90th-percentile ranges. Fragility scenario = MTTG. RAND RR1988-3.7

Orleans and Jefferson parishes in the MTTG scenario. These benefits emerge largely from the reduction in the chance of levee fragility or failure along the south shore of Lake Pontchartrain for AEPs more extreme than the 100-year interval. In contrast, both EAD and EAD reduction within HSDRRS East under the No Fragility scenario (not shown) are an order of magnitude lower, with the reduction in EAD ranging from $16 million to $22 million across the alignments at the median. Finally, we note some damage inducement for the region encompassing areas of St. Ber- nard Parish and Plaquemines Parish outside of the barrier. The increase in EAD is low compared with the damage reduction described previously. It is due entirely to changes in the surge and wave heights along the St. Bernard portion of HSDRRS that, on average, tend to increase flood depths and damage somewhat on the interior of the system compared with an FWOA. Damage inducement in St. Bernard ranges from $14 million to $48 million per year, depend- ing on the alignment (MTTG scenario, median value). Table 3.3 summarizes EAD reduction by parish. The parishes are generally sorted from the highest to lowest damage reduction, with areas showing damage inducement at the bottom. Note that this table also includes a summary of induced damage in the three Mississippi coastal counties; in the next section, we discuss these results in detail. Looking at the total results for the Louisiana study region, we see that EAD reduction at the median across the Louisiana study area ranges from $1.2 billion to $1.4 billion from the different alignments, with Hwy90/10/10 ranking highest in terms of net EAD reduction. Interestingly, Hwy90/0/2, which could end up as the least-cost alignment option (not esti- 28 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Table 3.3 Expected Annual Damage Reduction, by Parish or County, for Four Proposed Barrier Configurations, in Millions of Constant 2015 Dollars

NOTE: Because of rounding, totals might not sum precisely. This table shows the reduction, or inducement, in EAD for coastal parishes and counties in Louisiana and Mississippi from four proposed Lake Pontchartrain barrier alignments as estimated in this analysis: Hwy90/0/2, Hwy90/10/10, CSX/10/10, and Hwy90/24.5/24.5. We project the values for each scenario 50 years into the future, representing one set of assumptions about uncertain future sea level rise, landscape subsidence, asset growth, and performance of hurricane protection levees during flood events. The table shows the median (50th percentile) values. Green shading indicates positive damage reduction (benefit); red shading indicates induced damage (disbenefit). Fragility scenario = MTTG. RAND RR1988-T3.3 mated in this analysis), performs nearly as well in terms of net economic benefit and yields the lowest level of induced damage in affected areas of Louisiana and Mississippi. Finally, Table 3.4 shows damage reduction by alignment and by AEP interval rather than EAD. For each parish, we provide damage reduction (or inducement) at the 50-, 100-, and 500-year intervals (millions of dollars), again highlighting the median estimate and the MTTG fragility scenario. Damage reduction results by AEP interval are at least an order of magnitude higher than the EAD results: Total 100-year damage reduction, for example, ranges from $25.6 billion to $30.6 billion across alignments. In contrast with EAD, which represents an average value across a range of possible events weighted by likelihood, these results can instead be thought of as the damage reduction associated with an extreme flood event affecting each parish. For instance, Table 3.4 shows that damage from a 50-year flood event could be reduced by $8 billion to $15 billion in St. Tammany Parish with a barrier in place. Of course, these results still represent a statistical construct from many simulated storms rather Results from Proposed Barrier Alignments 29

Table 3.4 Damage Reduction or Inducement, by Annual Exceedance Probability Interval, Louisiana Study Region, in Billions of Constant 2015 Dollars

Return Alignment Interval, in Years Parish Hwy90/0/2 Hwy90/10/10 CSX/10/10 Hwy90/24.5/24.5 50 St. Tammany 13.0 13.4 14.8 8.7 Orleans 0.0 0.1 0.1 0.6 St. John the Baptist 5.5 6.0 6.0 6.2 Jefferson 0.0 0.0 0.0 0.0 St. Bernard 0.0 0.0 0.0 0.0 Ascension 1.7 2.1 2.0 2.1 St. Charles 0.0 0.0 0.0 0.0 Livingston 0.3 0.5 0.4 0.5 Plaquemines 0.0 0.0 0.0 0.0 Tangipahoa 0.1 0.1 0.1 0.2 St. James 0.1 0.1 0.1 0.1 Total 20.7 22.4 23.6 18.5 100 St. Tammany 8.7 11.3 10.7 9.7 Orleans 6.2 6.2 6.2 6.4 St. John the Baptist 7.3 8.7 8.6 8.8 Jefferson 0.0 0.0 0.0 0.0 St. Bernard 0.0 0.0 0.0 –0.4 Ascension 3.0 3.5 3.4 3.5 St. Charles 0.0 0.0 0.0 0.0 Livingston 0.3 0.6 0.5 0.7 Plaquemines 0.0 0.0 0.0 0.0 Tangipahoa 0.1 0.2 0.2 0.2 St. James 0.1 0.1 0.1 0.1 Total 25.6 30.6 29.7 29.0 500 St. Tammany 11.0 14.0 12.2 12.2 Orleans 18.4 17.4 24.6 9.8 St. John the Baptist 7.7 11.6 11.1 12.5 Jefferson 13.4 12.2 13.3 13.0 St. Bernard –1.3 –3.1 –3.2 –10.3 Ascension 3.7 4.3 4.2 4.4 St. Charles 0.0 0.0 0.0 – 0.1 Livingston 0.3 0.6 0.5 0.8 Plaquemines 0.0 0.0 0.0 0.0 Tangipahoa 0.1 0.2 0.2 0.3 St. James 0.1 0.1 0.1 0.1 Total 53.4 57.4 63.0 42.6 NOTE: Because of rounding, totals might not sum precisely. The table shows the median (50th-percentile) values. Positive numbers = damage reduction. Negative numbers = damage inducement. Fragility scenario = MTTG. 30 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier than damage reduction from a specific event, but they are nevertheless useful to understanding how a barrier might benefit the study area during an extreme flood event. Results by AEP interval generally show the same ranking by alignment as results by EAD do. The Hwy90/10/10 and Hwy90/24.5/24.5 alignments produce about the same total damage reduction at the 100-year interval; in contrast, the CSX/10/10 alignment yields the highest net benefit at the 50-year and 500-year intervals, particularly from St. Tammany Parish (50-year) and enclosed Orleans and Jefferson parishes (500-year), compared with the Hwy90/10/10 alignment. Note also that substantial damage inducement exceeding $10 billion occurs in St. Bernard Parish at the 500-year interval from the Hwy90/24.5/24.5 alignment. Once again, the Hwy90/0/2 alignment yields substantial damage reduction across these intervals. This result is somewhat surprising; one might expect that the low gates would perform well at higher frequencies (e.g., 10- or 50-year AEP) and not necessarily from more-extreme flood events, but these results show that the low gates yield notable damage reduction even at the 500-year interval.

Potential Induced Flood Damage in Coastal Mississippi

One of the primary concerns about a new Lake Pontchartrain barrier is the potential that a new alignment, by preventing storm surge from entering the lake, would instead shift the floodwaters to neighboring communities outside of the barrier. This is of particular concern for coastal Mississippi, which lies to the east of the proposed alignment location and is especially vulnerable to storm surge piling up on a barrier from the counterclockwise rotation of North- ern Hemisphere tropical storms and hurricanes that affect the Gulf Coast. This preliminary analysis therefore seeks to quantify the potential for direct economic damage inducement in coastal Mississippi from the proposed alignments. In this section, we discuss the potential for direct economic damage inducement in coastal Mississippi, using both EAD and damage by AEP interval as evaluation metrics. We present results in absolute (dollar) terms, as well as a percentage comparison to the baseline damage risk to coastal Mis- sissippi estimated in a future in which no barrier is constructed (FWOA). As a baseline, recall that total FWOA EAD in coastal Mississippi was estimated at $1.6 billion in the year 50 future scenario condition considered here (see Figure 3.3). Figure 3.8 shows a bar-plot summary of induced EAD in Mississippi from each proposed alignment, detailed by county. This figure shows the EAD increase noted with a barrier in place compared with damage by county if no barrier were constructed. The figure yields several notable insights:

• EAD inducement is highest in Hancock County—which includes the Pearl River basin and is closest to the Louisiana border—followed consecutively by Harrison and Jackson counties, from west to east. • Increased damage from the 24.5-ft. barrier is much higher than from the other alignments considered, at least doubling the inducement from the Hwy90/10/10 and CSX/10/10 alignments, for instance. • The scale of EAD inducement, although notable, ranges from millions to tens of millions of dollars, several orders of magnitude lower than the $1.6 billion in estimated EAD for the Mississippi coastal region as a whole. As such, the scale of damage inducement Results from Proposed Barrier Alignments 31

Figure 3.8 Increase in Expected Annual Damage, in Millions of Constant 2015 Dollars, by Mississippi County

Parish or County Alignment

Hancock Hwy90/0/2 14 Hwy90/10/10 23 CSX/10/10 22 Hwy90/24.5/24.5 42 Harrison Hwy90/0/2 5 Hwy90/10/10 10 CSX/10/10 11

Hwy90/24.5/24.5 23 Jackson Hwy90/0/2 2 Hwy90/10/10 3 CSX/10/10 4 Hwy90/24.5/24.5 18 4035302520151050–5 45 Increase in EAD

NOTE: The bars show 50th-percentile values, and lines indicate the 10th- to 90th-percentile ranges. RAND RR1988-3.8

is small compared with total expected flood damage for these coastal counties averaged across a range of event types and likelihoods.

These results are confirmed in Table 3.5, which shows the same median EAD inducement results as Figure 3.8 but also includes percentage increase and is broken out further by type of asset. According to Table 3.5, the percentage increase in EAD from a barrier in Hancock County ranges from 3.2 percent (Hwy90/0/2) to 9.5 percent (Hwy90/24.5/24.5), compared with FWOA damage totals. The percentage is lower in Harrison County (0.9 to 4.2 percent, same alignments) and lower yet in Jackson County (0.4 to 3.2 percent) for locations farther away from the barrier influence. To summarize across all counties, Hwy90/0/2 could increase Mississippi EAD by about 1.4 percent ($22 million) at the median estimate, while Hwy90/10/10 would increase EAD by approximately 2.3 percent ($35 million). The error bars confirm that, in most cases, induced damage is not expected to vary widely and stays within a relatively narrow range for each alignment considered. The breakdown by asset class largely mirrors the breakdown in total FWOA damage by asset class noted previously (Figure 3.3). In the Mississippi study region, damage inducement most affects commercial assets, followed by residential and industrial assets. Induced EAD for residential structures totals $7.4 million under Hwy90/0/2, for instance, and $12.3 million with Hwy90/10/10. We can also consider damage inducement in Mississippi according to AEP interval. Once again, this provides a snapshot of induced damage at extreme (low-likelihood) intervals rather than an average across a range of tropical depression likelihoods. Table 3.6 shows a breakdown for 50-, 100-, and 500-year damage inducement. For each alignment, return interval, and county, the table shows total damage (FWOA plus damage increase), induced damage, and inducement as a percentage of the FWOA total. Once again, the damage numbers are 32 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier 5 3 4 3 3 3 3 9 4 4 5 4 9 8 12 10 Percentage 5 1 0 8 1 1 3 1 Hwy90/24.5/24.5 11 15 14 18 24 23 42 84 Amount 2 2 1 1 1 1 1 2 2 2 2 5 5 5 5 5 Percentage CSX/10/10 2 1 0 0 4 7 4 1 0 8 1 1 11 13 22 38 Amount Alignment 2 0 1 1 1 1 2 2 2 2 2 5 5 5 5 5 Percentage Hwy90/10/10 2 1 0 0 3 6 3 1 0 8 1 1 13 10 35 23 Amount 1 0 0 1 0 0 1 1 1 1 1 3 3 3 3 3 Percentage Hwy90/0/2 1 1 0 0 2 3 2 0 0 5 5 1 0 8 14 22 Amount Asset Class Asset Commercial Residential Industrial All other Total Commercial Residential Industrial All other Total Residential Industrial All other Total Table 3.5 Increase in Expected Annual Damage Induced, County by and Asset Class, for Coastal Mississippi NOTE: Because of rounding, totals might not sum precisely. The table shows the median (50th percentile). Amounts are in millions of constant dollars. 2015 Total Jackson Harrison Hancock Commercial County Results from Proposed Barrier Alignments 33 8 6 1 5 6 5 7 7 2 1 3 13 Percentage 1.2 1.2 1.2 1.7 0.5 0.5 0.1 0.8 0.6 0.4 0.2 Delta Hwy90/24.5/24.5 7.4 8.8 17.1 11.9 10.2 10.5 14.2 26.4 23.5 23.3 63.9 36.6 2.6 Total 3 0 0 1 5 1 1 2 3 1 0 1 Percentage 0.2 0.0 0.0 0.3 0.4 0.2 0.1 0.4 0.7 0.2 0.1 0.7 Delta CSX/10/10 7.1 9.7 9.8 8.7 11.4 13.6 16.3 25.5 23.3 23.2 62.9 34.8 Total Alignment 4 0 0 1 5 1 0 2 3 1 0 1 Percentage 0.3 0.0 0.0 0.5 0.3 0.2 0.1 0.5 0.7 0.2 0.1 0.7 Delta Hwy90/10/10 7.1 9.8 9.8 8.7 11.4 13.6 16.4 25.6 23.3 23.2 62.9 34.8 Total 2 0 0 1 4 1 1 1 2 1 0 1 Percentage 0.2 0.0 0.0 0.3 0.1 0.2 0.1 0.2 0.5 0.2 0.5 Delta Hwy90/0/2 7.0 9.7 9.7 8.7 11.4 13.5 16.1 25.4 23.3 23.2 0.0 62.6 34.6 Total Hancock County Harrison Jackson Total Hancock Harrison Jackson Total Hancock Harrison Jackson Total 50 Return Return Interval, in Years Table 3.6 Coastal Mississippi for Interval Probability Exceedance Annual by Damage, in Increase 100 500 NOTE: Because of rounding, totals might not sum precisely. The table shows the median (50th percentile). Amounts are in billions of constant dollars. 2015 Percentagesare percentages of increase in damage. 34 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier several orders of magnitude higher in total than with EAD inducement because of the change in metric (extreme values versus likelihood-weighted average). But the increase in percentage terms is largely similar to that observed with EAD for all three intervals, typically ranging from a 1- to 8-percent increase relative to FWOA across the entire coastal area and 0 to 13 percent for specific counties. Aside from Hwy90/24.5/24.5, total percentage increase from the other alignments is estimated at 1 to 2 percent of total estimated damage by AEP interval in coastal Mississippi. Finally, CPRA was also interested in the pattern of damage and damage inducement that CLARA estimates for coastal Mississippi. This information could help target mitigation resources toward specific areas of Mississippi that might be significantly affected by damage inducement with a particular barrier configuration in place. Figure 3.9, for instance, shows a map of median 100-year damage in the FWOA (top pane) and change in median 100-year damage with Hwy90/0/2 in place (bottom pane). The color scales are logarithmic to better distinguish low- and high-damage regions. The top pane of Figure 3.9 shows the general pattern of Mississippi coastal flood damage at the 100-year interval. In terms of geographic coverage, Hancock County shows the most

Figure 3.9 Map of 100-Year Damage and Damage Increase from the Alignment in Mississippi That Keeps Highway 90’s Existing Crown and Adds 2-Foot Gates

FWOA damage (100-year)

Damage (2015) <10,000 Jackson 10,000 to Harrison 99,999 100,000 to Hancock 999,999 Pearl River 1 million to 9.99 million 10 million to 99.99 million ≥100 million

Change in 100-year damage with Hwy90/0/2

Damage Harrison Jackson increase (2015) <100,000 Hancock 100,000 to 999,999 Pearl River 1 million to 9.99 million ≥10 million

NOTE: The color scales are logarithmic to better distinguish low- and high-damage regions. The maps show 50th-percentile values. The results reflect damage at the 100-year return period rather than EAD. Color scales differ between top and bottom maps. Amounts are in constant 2015 dollars. RAND RR1988-3.9 Results from Proposed Barrier Alignments 35 area damaged, but smaller areas with denser concentrations of assets in Harrison County (Pass Christian, Long Beach, Gulfport, and Biloxi) and Jackson County (Pascagoula) suffer a substantial fraction of the total damage. Also notable is the large fraction of flood damage occurring because of tributary rivers backing up with storm surge during the flood event simulations in ADCIRC, such as the Biloxi River in eastern Harrison County or the Pascagoula River system in Jackson County. In terms of change in damage, the most-notable concentrations of damage increase are in the immediate vicinity of the barrier and assets adjacent to the Pearl River in Hancock County, including the John C. Stennis Space Center and such towns as Pearlington, Ansley, and Lake- shore. Damage inducement also occurs around Bay St. Louis, with some concentrated areas noted in Gulfport (see Figure 1.1 in Chapter One). Moving eastward, more scattered damage inducement is noted in the Biloxi region, with nearly no change noted for Pascagoula in eastern Jackson Parish. In general, these patterns of inducement largely reflect the existing patterns of damage in the top pane, with inducement effects diminishing steadily for communities farther to the east.

CHAPTER FOUR Discussion and Next Steps

Results from this analysis confirm what was observed in the 2012 Coastal Master Plan analysis: A barrier across the mouth of Lake Pontchartrain could provide substantial flood damage reduction benefits for many coastal Louisiana communities, especially those areas surrounding the lake currently lacking any structural protection. This is true both in damage reduction terms and for potential future design height changes for the portions of the New Orleans HSDRRS behind the barrier. However, these results further show that the high-barrier alignments previously considered in 2012 for this project option—either 24.5 or 33 ft.—likely did not adequately balance damage reduction behind the barrier and induced damage for regions outside. Instead, we show that a lower, 10-ft. barrier with gate structures—following Hwy90/10/10 or CSX/10/10— could yield even greater net damage reduction than a high barrier by reducing induced damage effects and still providing nearly equivalent damage reduction for communities surrounding the lake. We note only minor differences in performance between Hwy90/10/10 and CSX/10/10, however, and these differences occur largely in the immediate vicinity of the barrier. The dis- tinction among the project benefits is reduced further when taking parametric uncertainty into account, as the 10th- to 90th-percentile ranges across the study area mostly overlap. As a result, a choice between these alignments would likely need to be based on feasibility, cost differences, or other CPRA considerations rather than on damage reduction benefits alone. Furthermore, an option that reinforces the existing Highway 90 crown and adds gate clo- sures at each of the passes also appears to produce the lowest levels of induced damage and perform nearly as well as the 10-ft. alignments, despite a much smaller footprint (and presumably lower cost). This initial investigation, however, does not include estimates of the construction or operation and maintenance costs associated with each alignment option. It also does not address the potential environmental impact from different options, which could profoundly influence their performance rankings and the feasibility of moving a project forward to selection and construction. But if we assume that Hwy90/0/2 might also have the lowest cost and have the least negative effect on the ecosystems and ecohydrology of the lake, followed by Hwy90/10/10, CSX/10/10, and Hwy90/24.5/24.5, respectively, the results suggest that even such a minimal-footprint low-gate option could nevertheless yield substantial damage reduction benefits. This investigation also builds on past work by providing initial estimates of induced damage effects in Mississippi in terms of both EAD and damage by AEP interval. In general, all alignments have a detrimental effect on Mississippi and increase coastal flood damage compared with an FWOA. But the amount of inducement is relatively small and could be minimized by the selection of Hwy90/0/2, Hwy90/10/10, or CSX/10/10. Apart from

37 38 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Hwy90/24.5/24.5, the other alignments increase Mississippi flood damage (EAD or by AEP interval) by 1 to 2 percent in total, and this inducement effect is several orders of magnitude smaller than the damage reduction benefits estimated for southeastern Louisiana. As a result, and given the levels of EAD inducement noted, Louisiana could set up and fund a program to support additional flood hazard mitigation for affected areas of coastal Mississippi or otherwise provide more direct assistance to Mississippi homeowners and business owners to offset this damage increase if a barrier were constructed. This would still represent a fraction of the cost associated with future flood damage in Louisiana if no barrier were constructed. Overall, this investigation is not yet conclusive because it tests the barrier options in only one uncertain future scenario reflecting plausible sea level rise, land subsidence conditions, and asset growth 50 years from today. It also does not formally consider other decision metrics, such as project cost and environmental impact. However, the analysis led us to recommend that at least one alignment option be carried forward and considered in the 2017 Coastal Master Plan analysis because of the substantial net damage reduction noted. The analysis shows that the Hwy90/0/2 alignment performs best in terms of balancing damage reduction, project footprint, and induced damage effects. For these reasons, CPRA included this option in the 2017 analysis and subsequently selected it for implementation in the final master plan. Subsequent analysis for the 2017 Coastal Master Plan confirmed the findings in this report, with similar levels of damage reduction and net benefit noted for Hwy90/0/2 when tested across a wider range of scenario assumptions.

Conclusion

CPRA asked RAND to conduct an investigation of the damage reduction benefits and induced damage effects from five different options for a barrier across the mouth of Lake Pontchartrain. To address this question, the RAND team applied an updated version of the CLARA model to estimate surge and wave heights, flood depths, and damage with or without barrier alignment options in place. Results showed that a Lake Pontchartrain barrier could provide substantial damage reduction benefits for southeastern Louisiana, with median EAD reduction benefits ranging from $1.2 billion to $1.4 billion per year. A barrier could also lower future design height requirements for HSDRRS East along the south shore of Lake Pontchartrain, although it could also increase design heights for the eastern face of the system or otherwise necessitate additional levee armoring in other locations. All barrier alignments considered increased flood damage in coastal Mississippi compared with an FWOA, but damage inducement was typically low for the best-performing alignment options (1 to 2 percent) compared with total flood damage risk for the three Mississippi coastal parishes. Results show that the Hwy90/10/10 and CSX/10/10 alignments yield the highest net damage reduction for the overall region, while the Hwy90/0/2 alignment produces damage reduction nearly as high with the lowest induced damage effects on neighboring Louisiana parishes and Mississippi coastal counties. In general, this analysis strongly supported moving one of these high-performing alignments forward for formal consideration as part of the 2017 Coastal Master Plan analysis. Subsequently, CPRA included the Hwy90/0/2 alignment as part of the formal 2017 analysis and ultimately selected this project for implementation as part of the final 2017 Coastal Master Plan. References

Alymov, Vadim, Zachary Cobell, Kim de Mutsert, Zhifei Dong, Scott Duke-Sylvester, Jordan R. Fischbach, Kevin Hanegan, Kristy Lewis, David Lindquist, J. Alex McCorquodale, Michael Poff, Hugh Roberts, Jenni Schindler, Jenneke M. Visser, Zhanxian Wang, Yushi Wang, and Eric White, 2017 Coastal Master Plan, Appendix C, Chapter 4: Model Outcomes and Interpretations, version II, Baton Rouge, La.: Coastal Protection and Restoration Authority, November 2016. Atkinson, John, and Hugh Roberts, Lake Pontchartrain Barrier Evaluation: Technical Report, Highlands Ranch, Colo.: Arcadis, October 30, 2015. Coastal Protection and Restoration Authority of Louisiana, Integrated Ecosystem Restoration and Hurricane Protection: Louisiana’s Comprehensive Master Plan for a Sustainable Coast, Baton Rouge, La., April 30, 2007. As of April 13, 2017: http://sonris-www.dnr.state.la.us/dnrservices/redirectUrl.jsp?dID=4063376 ———, Louisiana’s Comprehensive Master Plan for a Sustainable Coast, Baton Rouge, La., March 20, 2012a. As of April 13, 2017: http://coastal.la.gov/a-common-vision/2012-coastal-master-plan/ ———, Louisiana’s Comprehensive Master Plan for a Sustainable Coast, Appendixes A–J, Baton Rouge, La., 2012b. ———, Louisiana’s Comprehensive Master Plan for a Sustainable Coast, 2017 draft plan release, Baton Rouge, La., 2017. As of March 29, 2017: http://coastal.la.gov/wp-content/uploads/2016/08/2017-MP-Book_Single_Combined_01.05.2017.pdf CPRA—See Coastal Protection and Restoration Authority of Louisiana. Federal Emergency Management Agency, Mitigation Division, Hazus® MH Technical Manual, Washington, D.C., undated. As of April 13, 2017: http://www.fema.gov/media-library-data/20130726-1820-25045-8292/hzmh2_1_fl_tm.pdf FEMA—See Federal Emergency Management Agency. Fischbach, J. R., D. R. Johnson, K. Kuhn, M. Pollard, C. Stelzner, R. Costello, E. Molina, R. Sanchez, J. Syme, H. Roberts, and Z. Cobell, 2017 Coastal Master Plan Modeling, Attachment C3-25: Storm Surge and Risk Assessment, version 3, Baton Rouge, La.: Coastal Protection and Restoration Authority, 2017, pp. 1–202. Fischbach, Jordan R., David R. Johnson, David S. Ortiz, Benjamin P. Bryant, Matthew Hoover, and Jordan Ostwald, Coastal Louisiana Risk Assessment Model: Technical Description and 2012 Coastal Master Plan Analysis Results, Santa Monica, Calif.: RAND Corporation, TR-1259-CPRA, 2012. As of April 13, 2017: http://www.rand.org/pubs/technical_reports/TR1259.html Homeland Security Infrastructure Program, HSIP Gold, 2014, not available to the general public. HSIP—See Homeland Security Infrastructure Program. Johnson, David R., Jordan R. Fischbach, and David S. Ortiz, “Estimating Surge-Based Flood Risk with the Coastal Louisiana Risk Assessment Model,” Journal of Coastal Research, Special Issue 67, 2013, pp. 109–126. MARIS—See Mississippi Automated Resource Information System.

39 40 Reducing Coastal Flood Risk with a Lake Pontchartrain Barrier

Mississippi Automated Resource Information System, “County Data Download,” updated April 2016. Referenced August 15, 2014. As of August 15, 2014: http://www.maris.state.ms.us/HTM/DownloadData/County.html Peyronnin, Natalie, Mandy Green, Carol Parsons Richards, Alaina Owens, Denise Reed, Joanne Chamberlain, David G. Groves, William K. Rhinehart, and Karim Belhadjali, “Louisiana’s 2012 Coastal Master Plan: Overview of a Science-Based and Publicly Informed Decisionmaking Process,” Journal of Coastal Research, Special Issue 67, 2013, pp. 1–15. USACE—See U.S. Army Corps of Engineers. U.S. Army Corps of Engineers, Mississippi Coastal Improvements Program (MsCIP) Comprehensive Plan and Integrated Programmatic Environmental Impact Statement, June 2009. As of June 2, 2017: http://www.sam.usace.army.mil/Missions/Program-and-Project-Management/MsCIP-Program/ MsCIP-Downloads/ ———, Final Post Authorization Change Report and Revised Programmatic Environmental Impact Statement: Morganza to the Gulf of Mexico, Louisiana, New Orleans, La.: U.S. Army Corps of Engineers, Louisiana Coastal Protection and Restoration Authority Board, and Terrebonne Levee and Conservation District, May 2013. As of April 13, 2017: http://www.mvn.usace.army.mil/Portals/56/docs/PD/Projects/MTG/FinalRevisedProgrammaticEISMtoG.pdf