The New Orleans East-Bank Hurricane Surge Residual Risk Reduction Report (Feb 2016) was prepared for the Southeast Louisiana Flood Protection Authority—East under a federal Community Development Block Grant (Appendix A). The Report evaluates residual risks from hurricane surge to the New Orleans East-Bank polders—risks beyond the capacity of the recently refurbished post-Katrina perimeter floodwall/levee system—and alternatives for reducing these residual risks, particularly compartmentalization projects. The East-Bank includes the Metro, New Orleans East, and St. Bernard Polders (forced drainage areas encircled by levees), plus the Inner Harbor Navigation Canal (IHNC) Basin. The perimeter floodwall/levee system was essentially constructed to meet the requirements of the National Flood Insurance Program—within its limited surge risk management purposes—and is known as the Hurricane and Storm Damage Risk Reduction System (HSDRRS). The Report includes:
The Executive Summary (with Table of Contents and Introduction)—reviews content of the report's four parts and presents twenty major conclusions and four recommendations.
Part I. East-Bank Surge Inundation Hazard—summarizes the East-Bank 100- and 500-yr exterior surge hazard estimates; recent East-Bank hurricane surge events; the post-Katrina HSDRRS (Appendix B) and its sustainability issues; and the surge inundation hazard for the three East-Bank polders. The exterior and polder hazards have been partially revised in accordance with Four Priority Issues with the USACE Surge Hazard and HSDRRS Overtopping Analysis, Bob Jacobsen PE LLC, March 2015 (Appendix C). Exterior surge and overtopping hazard estimates are partially revised for 20 East-Bank HSDRRS locations, along with reasonably conservative treatments of uncertainty, as appropriate for comprehensive risk management purposes. This Part then presents an overview of eight approaches to reducing East-Bank surge inundation hazard.
Part II. East-Bank Compartmentalization Alternatives—examines physical, economic, and demographic conditions in each of the three East-Bank polders affecting the compartmentalization approach to surge inundation risk reduction (Appendix D); reviews previous post-Katrina investigations of East-Bank compartmentalization; and identifies—with the additional assistance of regional stakeholders (Appendix E)—36 potential compartmentalization project alternatives.
Part III. Modeling of Class D Surge Inundation and Compartmentalization Features—evaluates East-Bank polder flooding associated with an extreme HSDRRS breach scenario with 30,000 acre-ft of inflow volume. This scenario is estimated to have a regional East-Bank return period—with future HSDRRS sustainability issues—on the order of 250 years. ADCIRC high resolution two-dimensional models were developed for each of the three East Bank polders and validated with Hurricane Katrina flood data (Appendix F). Appendix G depicts the Hurricane Katrina inundations for the three polder models. The breach scenario was then simulated at 31 different locations around the three polders to assess the influence of the compartmentalization features identified in Part II. Appendix H depicts the 31 inundation scenarios: Metro Polder St. Charles 1, 2 & 3; Metro Polder East Jefferson 1 & 2, Metro Polder East Jefferson 3 & 4, Metro Polder Orleans 1 & 2, Metro Polder Orleans 3 & 4, Metro Polder Orleans 5, New Orleans East Polder 1, 2, 3 & 4, New Orleans East Polder 5, 6, 7 & 8, St. Bernard Polder-Upper 1, 2 & 3, St. Bernard Polder-Upper 4, 5 & 6, St. Bernard Polder-Lower 1, 2 & 3, St. Bernard Polder-Lower 4 & 5.
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Part IV. Evaluation of Compartmentalization Alternatives—screens the 36 compartmentalization alternatives identified in Part II and modeled in Part III, yielding five remaining priority projects. Part IV then presents a feasibility evaluation of engineering, risk reduction, and costs for these five projects. (Appendix I provides additional hydrologic/hydraulic engineering analysis related to alternatives for the IHNC Basin. Appendix J provides 33 CFR § 165.838 Regulated Navigation Area; Gulf Intracoastal Waterway, Inner Harbor Navigation Canal, New Orleans, LA.)
Hurricane Surge Hazard Primer—readers are strongly encouraged to peruse this supplement to ensure they are familiar with important concepts regarding hurricane surge, surge hazard and risk, and surge risk management. The Supplement provides a basic discussion of:
Hurricane climatology, including the dozen storm conditions affecting surge;
Three phases of surge—forerunner, filling, and tilting;
The effect of landscape features on hurricane surge;
The general state of surge science, forecasts, and statistics; the critical "scientific guesstimate" nature of surge hazard values such as the 100- and 500-year surge;
The role of the National Flood Insurance Program in shaping surge hazard analysis and surge protection; and
The additional demands of comprehensive surge risk management for higher quality surge hazard estimates, more rigorous treatments of uncertainty, and more conservative Factors of Safety in surge protection design than provided under the NFIP.
Readers desiring a detailed discussion of surge hazard analysis methodologies are referred to Hurricane Surge Hazard Analysis: The State of the Practice and Recent Applications for Southeast Louisiana, May 2013.