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.
- Base Flood Elevation (BFE) - The elevation of a flood having a 1-percent chance of being equaled or exceeded in any given year.
- Coastal A Zones - Areas of special flood hazards landward of the Coastal High Hazard Area that are subject to wave heights that are less than 3 feet but greater than or equal to 1.5 feet. These moderate waves can cause significant damage to structures, although the damage would not be as severe as the damage caused by the 3-foot or greater breaking waves in the Coastal High Hazard Area. These areas are designated Zone AE on the Flood Insurance Rate Map.
- 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 Flood Elevation + Wave Height = Coastal BFE
- Coastal High Hazard Areas - Areas of special flood hazard extending from offshore to the inland limit of the Primary Frontal Dune along an open coast and any other area subject to high-velocity wave actions from storms.
- 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. FEMA's LiMWA animation describes the LiMWA and other coastal zones.
- Preliminary BFEs - The BFEs that are shown on the Preliminary FIRMs and in the Preliminary FIS reports 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. (A FEMA Fact Sheet [PDF] provides additional information about PFDs and how they are assessed and accounted for in FEMA coastal flood risk studies.)
- Stillwater Flood Elevation (SWEL) - The 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 winds.
- Stillwater Flood Level (SWFL) - The 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, NOAA's storm surge with floodwall protection animation, or FEMA's storm surge analysis video.)
- Transect - A 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).
- Wave Runup - The rush of water up a slope or structure.
- Wave Setup - The increase in the stillwater surface near the shoreline due to the presence of breaking waves.
Back to top
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 (PDF).These guidelines offer insight and recommended methods to analyze complex coastal processes in a reasonable way. (Note: The project teams working in each State/Study Area 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 web pages 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 (NOAA), and the U.S. Army Corps of Engineers.
The technology used included airborne LIght Detection And Ranging, or 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 flood elevations, or SWELs, 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 1-percent-annual-chance floodplain extent to the Special Flood Hazard Area (SFHA) shown on the previously effective FIRMs 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 SWELs. 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 added the BFEs to the affected FIRM panels and in the tables that appear in the FIS report. (For additional information on the FIS report, 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 were added to the FIRMs later in the study process.
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, or CHHAs, 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, referred to as coastal A zones, can still cause damage to buildings.
Post-storm field visits and laboratory texts have confirmed that wave heights as small as 1.5 feet can cause significant damage to buildings when constructed without consideration to the coastal hazards. Additional flood hazards associated with coastal waves include floating debris, erosion, and scour, all of which can cause structural damage unless buildings are built to Zone VE standards.
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.
Back to top
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
Back to top
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, Alabama Department of Economic and Community Affairs (ADECA), Northwest Florida Water Management District (NWFWMD), 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 County Status for Baldwin County page, the County Status for Baldwin County page, or the Information for Alabama page.
Back to top
Gulf Coast Analyses in Florida
Coastal flooding from the Gulf of Mexico directly affects communities in 24 counties, which are separated into project study areas. The project teams involved in the analysis of the flooding and the preparation of new or updated digital FIRMs, FIS reports, and related products include representatives of FEMA, Florida Division of Emergency Management (FDEM), NWFWMD, 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 Information for Florida page or the individual Information for Northwest Florida Study Area (2008 Study), Information for Northwest Florida Study Area (2009 Study), Information for Big Bend Florida Study Area, Information for West Florida Study Area, or Information for Southwest Florida Study Area pages.
Back to top
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 Information for Florida page or the individual Information for Northeast Florida Study Area, Information for East Coast Central Florida Study Area, or Information for South Florida Study Area pages.
Back to top
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 new or updated 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 Flood Mapping, Assessment, and Planning website or the Information for Georgia page.
Back to top
Atlantic Coast Analyses in South Carolina
Coastal flooding from the Atlantic Ocean affects communities in 10 counties. The project team involved in the analysis of the flooding and the preparation of new or updated 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 Information for South Carolina page.
Back to top
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 new or updated 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 NCFMP website or the Information for North Carolina page.
Back to top