April 12, 2006   ADVISORY Base Flood Elevations for Orleans Parish, Louisiana Hurricanes Katrina and Rita were both strong Category 5 hurricanes for several days in the Caribbean and Gulf of Mexico before pushing waters toward the Louisiana coast. Katrina made landfall on August 29, 2005, near the Mississippi-Louisiana border, and Rita made landfall on September 23, 2005, at the Texas-Louisiana border. These hurricanes caused extensive damage in the parishes of Louisiana along the Gulf Coast and Lake Pontchartrain. To minimize the flood impacts of future events, the U.S. Department of Homeland Security’s Federal Emergency Management Agency (FEMA) is providing advisory information concerning coastal flood elevations and interior levee ponding elevations that can be used to guide recovery efforts. This guidance is necessary because Hurricanes Katrina and Rita, along with other recent storms, have created concerns about the accuracy of the flood risk information for Orleans Parish (including incorporated areas) and whether the risk may be understated. Assessing flood hazards in Orleans Parish is challenging due to the existence of numerous flood control facilities. These facilities experienced damage of varying degrees throughout southeastern Louisiana as a result of Hurricanes Katrina and Rita, and the U.S. Army Corps of Engineers (USACE) is on an aggressive path to repair and improve the flood control system. The USACE is on schedule to have repairs to damaged areas completed by June 2006, to have all federal levees constructed to authorized heights by September 2007, and to have fully authorized levels of protection and improvements to the system completed by 2010. Although USACE improvements to the flood control system will make Orleans Parish safer than it was before the storms, they will not eliminate the potential for flooding. In fact, based on analyses recently completed by the USACE, the flood control system will not meet the standards necessary for providing protection against the 1-percent-annual-chance (100-year) flood, which is also referred to as the base flood. The National Flood Insurance Program (NFIP) uses the base flood as the standard for floodplain management. FEMA and the USACE have worked together to develop flood hazard data and formulate recommendations to be considered by State and local governments as they begin to make recovery decisions. This information is both reliable and current, and is aimed at assisting in the recovery process as it moves forward. Owing to the differences in flood risk information for areas inside and outside of levees, this Flood Recovery Guidance has been organized below to treat these two physical settings separately. Inside of Levee-Protected Areas For areas in the Parish located within existing levees, FEMA has determined that eventual levee certification is likely. In the levee areas of Sub-Basins “a” to “h” of the Parish (see Figure 1), FEMA recommends the following: new construction and substantially damaged homes and businesses within a designated FEMA floodplain should be elevated to either the Base Flood Elevation (BFE) shown on the current effective Flood Insurance Rate Map (FIRM) or at least 3 feet above the highest adjacent existing ground elevation at the building site, whichever is higher; and new construction and substantially damaged homes and businesses not located in a designated FEMA floodplain should be elevated at least 3 feet above the highest adjacent existing ground elevation at the building site. For the Parish Advisory BFE (ABFE) inside levees, this Guidance is similar to NFIP rules for areas protected by levees being restored to provide 1-percent-annual-chance base flood protection. Should the requirements needed for application of these rules fail to materialize, flood elevations in this area would be based on a “without levee” scenario and could exceed elevations of 8 feet (west and south of Mississippi River) or 13 to 14 feet (east and north of Mississippi River) referenced to the National Geodetic Vertical Datum of 1929 (NGVD29). In addition to the recent USACE storm surge modeling, FEMA has also developed these recommendations based on the height and integrity of the levee system expected to be in place by September 2007. Although FEMA is confident in the results from this current assessment, the agency will continue to monitor progress made with regard to levee improvements, findings from other ongoing studies, and enhancements to the agency’s understanding of the probability of flooding in this area. FEMA will adjust the recommended flood elevations as necessary as the agency prepares updated FIRMs for Orleans Parish and its incorporated areas. Outside of Levee-Protected Areas USACE, in close coordination with FEMA, has completed a preliminary analysis of the 1-percent-annual-chance flood elevations for all areas of the Parish outside of levees along the Gulf of Mexico shorelines east and south of Interstate 10 and Lake Pontchartrain. This analysis considered storm data from the past 155 years (including Hurricanes Katrina and Rita), new and existing long-term tidal gage records, and other existing engineering studies. The results of the USACE storm data analysis indicate that the new 1-percent-annual-chance flood elevations in areas impacted by coastal storm surge are higher that those shown on the current, effective FIRMs for Orleans Parish. As a result of the storm data analysis, FEMA has developed ABFEs that incorporate freeboard above the BFEs shown on the FIRMs. “Freeboard” is defined as follows (from 44 CFR 59.1): Freeboard means a factor of safety usually expressed in feet above a flood level for purposes of floodplain management. ‘‘Freeboard’’ tends to compensate for the many unknown factors that could contribute to flood heights greater than the height calculated for a selected size flood and floodway conditions, such as wave action, bridge openings, and the hydrological effect of urbanization of the watershed. FEMA recommends that for the Gulf of Mexico shoreline outside-levee areas in Orleans Parish (see Figure 1), a freeboard of 1 foot be applied. That is, structures should be elevated at least 1 foot above the current BFE shown on the effective FIRM for the building site. Community Adoption FEMA is encouraging local officials and citizens to adopt the elevation and freeboard recommendations for inside and outside of levee-protection made in this Guidance Document and to elevate structures accordingly. These added precautions will take into account increased flood risk due to subsidence, provide extra flood protection to the structure, reduce nuisance flooding, and may result in lower flood insurance premiums. Using elevation and freeboard are prudent measures for ensuring structures are rebuilt using the best available information to protect lives and property, and is also a sound floodplain management practice that communities are encouraged to adopt and enforce. Updated Flood Risk Information for Orleans Parish A FEMA coastal model study of hurricane storm surge flooding and levee flood protection is already underway at USACE, and FEMA intends to have an updated preliminary Flood Insurance Study (FIS) and updated FIRMs for coastal areas of Orleans Parish as soon as possible. The updated FIS and FIRMs may show an increase of the 1-percent annual-chance stillwater elevations (SWELs), Special Flood Hazard Areas (SFHAs), and BFEs over existing flood data (including the storm data analysis and engineering studies used for this Flood Recovery Guidance), and may result in the coastal high hazard area (V Zone) moving further landward. Until the restudy is completed, FEMA is encouraging communities within Orleans Parish to use the Flood Recovery Guidance described herein. This Guidance method can be used during the recovery and reconstruction of the Louisiana coastal and levee-protected areas by determining the site-specific ABFEs as described below. Flood Recovery Guidance Method Inside of Levee-Protected Areas To apply the Flood Recovery Guidance provided above to determine an ABFE for inside of levee-protected areas, individuals must review the current, effective FIRM and detailed topographic data (ground elevations) for the building site. In the Parish levee Sub-Basins “a” to “h”, the first floor of new construction is recommended to be elevated to the BFE shown on the FIRM or at least 3 feet above the highest adjacent existing ground elevation at the building site, whichever is higher. (A professional surveyor may need to be consulted to accurately determine the highest adjacent existing grade for the proposed site.) Outside of Levee-Protected Areas To apply the Flood Recovery Guidance provided above to determine an ABFE in areas outside of levees, the first step is to determine the SFHAs and BFEs from the effective FIRM that apply to the structure on the building site. Once the BFE applicable to the building has been determined, the ABFE can be calculated (see Figure 2) using the appropriate freeboard amount specified above for the Parish. Specifically, the ABFE is the current BFE plus a freeboard of 1 foot. For structures located in Zone VE on the effective FIRMs, the bottom of the lowest horizontal structural member is recommended to be at the ABFE. Other Pertinent ABFE Information Although the information provided here is advisory, communities should consider its use for rebuilding in a safer manner. For additional information, community officials, residents, and other interested parties can access the FEMA website for these flood recovery advisories at http://www.fema.gov/hazard/flood/recoverydata/index.shtm. In addition to determining site-specific ABFEs, community officials should consider additional protective measures to reduce future flood risks. These measures could include using additional freeboard and using the FEMA Coastal Construction Manual (CCM) (FEMA Publication 55). The CCM recommends the use of V Zone building standards in all areas subject to waves and velocity floodwaters caused by hurricane storm surges. For additional information on recommended practices, see the Coastal Construction Fact Sheet Series available at http://www.fema.gov/fima/mat/fema499.shtm. Ultimately it will be local officials, working with property owners, who will make final decisions regarding construction type and elevations that will apply during the recovery and rebuilding process. The ABFEs will be a valuable tool until new model studies can be developed and incorporated into the FIS and FIRMs. Within the next one to two months, FEMA will also publish a set of maps that will show detailed event information for Hurricanes Katrina and Rita, including flood inundation boundaries and high-water elevations. Datum Conversion Considerations Conversion of orthometric height measurements (elevations) from the NGVD29 to North American Vertical Datum of 1988 (NAVD88) is of importance to surveyors and building officials using this Guidance. Studies show some variability of the conversion factor between NGVD29 and NAVD88 over the geographic extent of Orleans Parish, and it would appear that a Parish average conversion factor of -0.20 foot would be appropriate for application. For site-specific determinations, a tool such as CORPSCON, developed by the USACE (http://crunch.tec.army.mil/software/corpscon/corpscon.html), can be used. The latest information on NAVD88 elevations in Louisiana can found on the National Oceanic and Atmospheric Administration’s (NOAAs) National Geodetic Survey (NGS) website at http://www.ngs.noaa.gov/heightmod/LouisianaControl.shtml. Future updates to the FIS and FIRM by FEMA will include a conversion of all flood data and BFEs within the Parish from NGVD29 to NAVD88. Figure 1. ABFE guidance and levee sub-basin locations for Orleans Parish. 1. Method for Calculating ABFE Outside of Levee-Protected Areas: ABFE = FIRM BFE + Freeboard FIRM BFE = 100-year SWEL + wave height Freeboard = 1 foot 2. Example: For Orleans Parish FIRM BFE = Zone VE (EL 12 feet) and Freeboard = 1 foot: ABFE = 12 + 1 = 13 feet NGVD29 Compare ABFE to the lowest adjacent grade (LAG) elevation. Building LAG (z) = 4 feet; the building is recommended to be elevated 9 feet above ground surface. 1. Method for Calculating ABFE Inside of Levee-Protected Areas: ABFE = The greater of either the FIRM BFE or the highest existing adjacent grade (HEAG) at the building site + 3 feet 2. Example: Consider a site where: Orleans Parish FIRM BFE = Zone AE (EL 5 feet) (relative to NGVD29) Site HEAG = 4 feet (NGVD29) Compare FIRM BFE to site HEAG + 3 feet: BFE of 5 feet < 7 feet (site HEAG of 4 feet + 3 feet) ABFE at this site is 7 feet (NGVD29). Therefore, the structure’s first floor (including basement) is recommended to be elevated to 7 feet (NGVD29) or higher. Figure 2. How to determine the ABFE based on the site’s effective BFE and recommended freeboard.