This page provides information about how a coastal engineering analysis is performed and how coastal elevations are calculated and used in preparing a Flood Insurance Rate Map (FIRM) for the National Flood Insurance Program (NFIP). This page also provides information on Federal Emergency Management Agency (FEMA) and partner efforts in each of the six States that make up the Southeastern United States to produce Flood Insurance Rate Maps (FIRMs) and associated Flood Insurance Study (FIS) reports. First, it is important to understand some key coastal terms.
Key Terms Defined
Key coastal analysis and mapping terms used on this page are defined below. Additional definitions are provided on the Glossary page.
- Area of Limited Wave Action (MiWA) - An area of special flood hazard landward of the Area of Moderate Wave Action that is subject to wave heights less than 1.5 feet. These areas are designated Zone AE on a FIRM.
- Area of Moderate Wave Action (MoWA) - An area of special flood hazard landward of the Coastal High Hazard Area that is subject to wave heights that are less than 3 feet, but greater than or equal to 1.5 feet. These areas are designated Zone AE on a FIRM and are sometimes referred to as "coastal AE zones."
- Base Flood Elevation (BFE) - The elevation of a flood having a 1-percent chance of being equaled or exceeded in any given year.
- Coastal BFEs - The 1-percent-annual-chance flood elevations shown on a FIRM within the Coastal High Hazard Area (CHHA). Coastal BFEs can be calculated using the following equation:
Stillwater Elevation + Wave Height = Coastal BFE
- Coastal High Hazard Area (CHHA) - An area of special flood hazard extending from offshore to the inland limit of a primary frontal dune along an open coast and any other area subject to high-velocity wave actions from storms or seismic sources.
- High-Velocity Wave Action - A condition in which wave heights or wave runup depths are greater than or equal to 3.0 feet.
- Limit of Moderate Wave Action (LiMWA) - A line within the SFHA designated Zone AE on a FIRM that marks the inland limit of the area inundated by the 1-percent-annual-chance, 1.5- foot breaking wave. The LiMWA is provided on the FIRM, for informational purposes, because these moderate waves can cause damage to structures; the damage would not be as severe as the damage caused by the 1-percent-annual-chance, 3- foot breaking waves.
- Preliminary BFEs - The BFEs that are shown on the Preliminary FIRM and in the Preliminary FIS report before the 90-day appeal period begins.
- Primary Frontal Dune (PFD) - A continuous or nearly continuous mound or ridge of sand with relatively steep seaward and landward slopes immediately landward and adjacent to the beach and subject to erosion and overtopping from high tides and waves during major coastal storms. The inland limit of the PFD occurs at the point where there is a distinct change from a relatively steep slope to a relatively mild slope.
- Stillwater Flood Elevation (SWEL) - Projected elevation that floodwaters would assume, referenced to National Geodetic Vertical Datum of 1929, North American Vertical Datum of 1988, or other datum, in the absence of waves resulting from wind or seismic effects.
- Stillwater Flood Level (SWFL) - Rise in the water surface above normal water level on the open coast due to the action of wind stress and atmospheric pressure on the water surface.
- Storm Surge - The water that is pushed toward land from the high winds of a major storm, such as a hurricane. For additional information, see the National Hurricane Center's Introduction to Storm Surge.)
- Transect - Cross section taken perpendicular to the shoreline to represent a segment of coast with similar characteristics.
- Wave - A ridge, deformation, or undulation of the water surface.
- Wave Height - The vertical distance between the highest part of a wave (wave crest) and the lowest part of the wave (wave trough).
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Overview of Analysis and Mapping Process
Coastal flooding on the Atlantic and Gulf coasts is a product of combined offshore, nearshore, and shoreline processes. The interrelationships of these processes are complex, and their relative effects vary significantly with coastal setting. These complexities present significant challenges to the partners and contractors who determine coastal flood elevations and flood zones in support of the NFIP.
To provide guidelines for performing the coastal engineering analyses, FEMA issued Atlantic Ocean and Gulf of Mexico Coastal Guidelines Update. These guidelines offer insight and recommended methods to analyze complex coastal processes in a reasonable way. (The project teams working in each State use their best technical judgment in deciding how to perform the analyses for each section of the Atlantic or Gulf shoreline.)
A general explanation of how these analyses were performed for the Mississippi coastal counties is provided below. A short video illustrates some of the concepts discussed below. Full-length videos for the Mississippi coastal mapping project also are available in English, Spanish, and Vietnamese.
To learn more about the approach taken in each State or Study Area, interested parties should read the sections below and visit the referenced Websites or documents.
Generally speaking, the analyses for the Mississippi coastal mapping project were carried out in three phases, summarized below.
Phase I - Data Gathering
During Phase I, the project team gathered accurate terrain and storm data from a wide array of sources using the best technology available. Sources of data included State agencies, the National Oceanic and Atmospheric Administration, and the U.S. Army Corps of Engineers.
The technology used included airborne Light Detection and Ranging (LIDAR) technology to paint an accurate picture of the study area. The project team also used data from satellite-based Global Positioning Systems, weather satellites, land-based meteorological stations, ocean data buoys, and hurricane-forecasting data collected from specially equipped NOAA aircraft, known as "Hurricane Hunters."
To process the large volume of data collected during Phase I, the project team needed a high-speed, high-capacity computer system. The team developed the 17th fastest parallel "supercomputer" of this type in the world. The U.S. Department of Energy also allowed the project team to use the agency's supercomputer to expand computing capacity further.
The highly accurate data collected during Phase1 were just right to define the long-term coastal flood hazards and risks. They set the stage for Phase II.
Phase II - Coastal Flood Analysis
During Phase II, the project team established the statistical flood levels across the coastal area being studied. These levels, also referred to as "stillwater elevations", were established using historical flood data that the project team tabulated and statistically evaluated using tried and true scientific methods.
Plugging the data into highly accurate computer models, the study team was able to determine the maximum coastal flood height for hundreds of representative storms.
From the thousands of data points used in the statistical analysis, the project team determined the extent of the area that would be flooded by the 1-percent-annual-chance flood. The project team then compared the newly mapped 1-percent-annual-chance floodplain to the area covered during Hurricane Katrina to determine the reasonableness of the mapped floodplain.
To complete Phase II, the project team then compared the new extent of the 1-percent-annual-chance floodplain to the Special Flood Hazard Area (SFHA) shown on the previously effective FIRM(s) for the study area.
Phase III - Localized Wave Modeling and Floodplain Mapping
During Phase III, the project team calculated the location and height of waves as coastal floodwaters move in over normally dry land. The effort during Phase II yielded the 1-percent-annual-chance stillwater levels. During Phase III, the project team used detailed wave modeling to determine the 1-percent-annual-chance wave heights. Using both sets of information, the team is able to calculate the coastal Base Flood Elevations (BFEs), as shown in the graphic depiction below.
The team then adds the BFEs to the affected FIRM panels and in the tables that appear in the FIS report. (For additional information, see Appendix J of FEMA's Guidelines and Specifications for Flood Hazard Mapping Partners.)
The topography of the Mississippi Gulf coast is complex, and development patterns and vegetation vary greatly along the shore. Therefore, the project team completed the detailed modeling of wave conditions along more than 150 transects across the Mississippi coast.
The detailed wave modeling undertaken during Phase III yielded Preliminary BFEs, the flood insurance risk zones that make up the high-risk SFHAs, and the moderate-risk and low-risk flood zones that are added to the FIRMs during Phase IV.
Within the SFHAs, two types of flood insurance risk zones are shown: VE zones and AE zones. VE zones, also referred to as Coastal High Hazard Areas, are zones where high-velocity wave action accompanies the storm surge and can cause severe damage to buildings. AE zones are areas that are affected by storm surge, but where wave action is diminished.
During Phase III, the project team also established the Limit of Moderate Wave Action (LiMWA). (See the diagram below and the FEMA animation that explains all of the coastal zones shown.) Although not as damaging as the waves expected in VE zones, the waves in these Areas of Moderate Wave Action (MoWA), sometimes referred to as "coastal AE zones," can still cause damage to buildings. This is critical information for community officials and citizens as they decide whether special design considerations should be applied to new or renovated buildings in the affected coastal areas to reduce the risk to life and property.
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Gulf Coast Analyses in Mississippi
Coastal flooding from the Gulf of Mexico affects communities in Hancock, Harrison, and Jackson Counties. The project team involved in the analysis of the flooding and the preparation of digital FIRMs and FIS reports included representatives of FEMA, the Mississippi Emergency Management Agency (MEMA), the Mississippi Department of Environmental Quality (MDEQ), and their contractors. Additional project information is provided on the Mississippi Coastal Mapping Project website
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Gulf Coast Analyses in Alabama
Coastal flooding from the Gulf of Mexico affects communities in Baldwin and Mobile Counties. The project team involved in the analysis of the flooding and the preparation of digital FIRMs, FIS reports, and related products includes representatives of FEMA, the Alabama Department of Economic and Community Affairs (ADECA), and their contractors.
To learn more about the ADECA and FEMA coastal engineering analysis and mapping efforts for the Gulf coast areas of Alabama, please visit the Alabama page.
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Gulf Coast Analyses in Florida
Coastal flooding from the Gulf of Mexico directly affects communities in 23 counties, which are separated into project study areas. The project teams involved in the analysis of the flooding and the preparation of digital FIRMs, FIS reports, and related products include representatives of FEMA, the Florida Division of Emergency Management (FDEM), the Northwest Florida Water Management District (NWFWMD), the Suwanee River Water Management District (SRWMD), and their contractors.
To learn more about the coastal engineering analysis and mapping efforts for the Gulf coast areas of Florida, please visit the Florida page or the individual pages for the Northwest Florida Study Area (2008 Study), the Northwest Florida Study Area (2009 Study), the Big Bend Florida Study Area, and the West Florida Florida Study Area. The individual page for the Southwest Florida Study Area will be launched when the coastal flood risk study begins later this year.
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Atlantic Coast Analyses in Florida
Coastal flooding from the Atlantic Ocean directly or indirectly affects communities in 17 counties, which are separated into project study areas. The project teams that are actively involved in the analysis of the flooding and the preparation of digital FIRMs, FIS reports, and related products include representatives of FEMA, FDEM, and their contractors.
To learn more about the coastal engineering analysis and mapping efforts for the Atlantic coast areas of Florida, please visit the Florida page or the individual pages for the Northeast Florida Study Area and the East Coast Central Florida Study Area. The individual page for the South Florida Study Area will be launched when the coastal flood risk study begins later this year.
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Atlantic Coast Analyses in Georgia
Coastal flooding from the Atlantic Ocean affects communities in nine counties. The project team involved in the analysis of the flooding and the preparation of digital FIRMs, FIS reports, and related products for these counties includes representatives of FEMA; the Floodplain Management Office of the Georgia Department of Natural Resources (GA DNR); and their contractors.
To learn more about the coastal engineering analysis and mapping efforts for the Atlantic coast areas of Georgia, please visit the Georgia page.
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Atlantic Coast Analyses in South Carolina
Coastal flooding from the Atlantic Ocean affects communities in six counties. The project team involved in the analysis of the flooding and the preparation of digital FIRMs, FIS reports, and related products for these counties includes representatives of FEMA, the South Carolina Department of Natural Resources (SC DNR), and their contractors.
To learn more about the coastal engineering analysis and mapping efforts for the Atlantic coast areas of South Carolina, please visit the South Carolina page.
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Atlantic Coast Analyses in North Carolina
The coastal engineering analyses in North Carolina affect communities in 19 counties listed below. The project team involved in the analysis of the flooding and the preparation of FIRMs and FIS reports includes representatives of FEMA, the North Carolina Floodplain Management Program (NCFMP), the North Carolina Division of Emergency Management (NCEM), and their contractors.
To learn more about the coastal engineering analysis and mapping efforts for the Atlantic coast areas of North Carolina, please visit the North Carolina page.
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