Mapping Methods & Data Sources
The Hurricane Ivan surge inundation maps show a variety of information concerning the hurricane's impact, information that was developed through numerous post-storm surveys and data analyses. The Ivan surge inundation maps also show current FEMA flood map data, which are used by local communities to regulate development in mapped floodplains.
The data sources and analysis methods used to develop the Ivan surge inundation maps are described in the sections that follow. A more detailed discussion of the data and methods is provided in a summary report, “Hurricane Ivan Surge Inundation Maps, Summary of Methods” (dated December 6, 2004) (PDF 0.9 MB)| TXT 0.03 MB). To download GIS data for the Ivan maps, see the GIS Data page.
The base map for the Ivan surge inundation maps is pre-storm, digital orthophotography that was provided by each county.
FEMA Flood Map Data:
In addition to data pertaining to Hurricane Ivan, Base Flood Elevations (BFEs) and flood zones (A and V Zones) from each community’s effective Flood Insurance Rate Map (FIRM) are shown on the Ivan surge inundation maps. FIRMs depict flood elevations and velocity conditions that have a 1% chance of being equaled or exceeded in any given year, also referred to as the 100-year flood. In coastal areas, these FIRM flood elevations include additional effects due to wave height and/or wave runup. Ivan map users should note: The Ivan surge inundation limits and elevation contours do not include wave effects, and therefore may not be directly comparable to the FIRM flood elevations.
Hurricane Ivan High Water Marks:
Within days of the storm, field observers and survey crews were deployed by FEMA and various state, county, and local governments to interview residents, find evidence of coastal high water levels, take digital photographs, and survey coastal high water marks (HWMs) from Hurricane Ivan. These field crews collected detailed information about each HWM, including physical basis of the mark, such as a mud line inside the building, a mud line on the outside of the building, or debris. Wherever possible, crews also noted the coastal flooding characteristics captured by the coastal HWM, including storm surge, wave runup, and wave height. These designations were based on a combination of physical flood evidence and interviews with witnesses at the time of collection.
The survey crews used static Global Positioning System (GPS) methods to determine an accurate elevation for each coastal HWM. Coastal HWM locations were surveyed horizontally in North American Datum of 1983 (NAD83), State Plane feet, and vertically in North American Vertical Datum of 1988 (NAVD88) US survey feet. Coastal HWM locations have been surveyed to within accuracies of 0.25 foot vertically and 10 feet horizontally with a 95% confidence level. Results from FEMA’s HWM surveys will be published and made available to the public and local officials.
Please note: To facilitate comparison to FEMA’s effective FIRMs, all elevation data for Baldwin County , Alabama are referenced to the vertical datum of NAVD88, and elevation data for the Florida counties are referenced to NGVD29.
Hurricane Ivan Preliminary Debris Lines:
FEMA mapped the inland limit of waterborne debris between Fort Morgan (Baldwin County), Alabama to Navarre Beach (Santa Rosa County), Florida. The debris lines were delineated based on an interpretation of digital aerial color photography that was flown by the U.S. Army Corps of Engineers a few days after the storm. Photo-interpreters inspected the aerial imagery to identify high concentrations of waterborne debris, with some interpolation necessary in areas of dense vegetation or standing water.
At the time of this project, the U.S. Army Corps of Engineers aerial photography had been georeferenced but not orthorectified. As a result, the inland debris limits are accurate to ±75 feet in any direction. Lastly, the debris limits shown on the Ivan surge inundation maps are preliminary; this information may change as FEMA completes the final debris mapping.
Hurricane Ivan Surge Inundation Limits:
Flood inundation limits for Hurricane Ivan were created by mapping the coastal HWM elevations onto digital, pre-storm topographic contour data. These inundation limits represent the inland extent of flooding caused by Hurricane Ivan’s storm surge. In areas where the coastal HWMs were close together but elevations differed significantly (more than 2-3 feet), engineering judgment was used to interpolate the inundation limit between coastal HWMs.
Topographic data from the sources listed below were used in the inundation mapping. At the time of this project, post-hurricane topography was not available for the study area.
|Location||Topographic Data Sources|
|Baldwin County, AL||Woolpert, LLC, 1-ft and 5-ft contour interval mapping, dated 2001, prepared for Baldwin County, Alabama, Scales 1:12000 and 1:24000, NAVD88|
|Escambia County, FL||Analytical Surveys, Inc., 1-ft and 2-ft contour interval mapping, dated 2003, prepared for Escambia County, Florida, NAVD88|
|Okaloosa County, FL||Okaloosa County GIS Department, 2-ft contour interval mapping compiled from aerial photographs dated, February 1999 (Fort Walton Beach and Niceville areas), Scale 1”=100’, NAVD88|
Okaloosa County GIS Department, 1-ft contour interval mapping compiled from aerial photographs dated, September 2001 (Destin area), NAVD88
Florida Department of Environmental Protection, 5-ft and 10-ft contour interval mapping digitized from U.S. Geological Survey 7.5-Minute Series Topographic Maps dated 1970 - 1987, Scale 1:24000, NGVD29
|Santa Rosa County, FL||Florida Department of Environmental Protection, 5-ft and 10-ft contour interval mapping digitized from U.S. Geological Survey 7.5-Minute Series Topographic Maps dated 1969 and 1973, Scale 1:24000, NGVD29|
In some areas, the mapped waterborne debris line was landward of the initial mapped inundation limit. This generally occurred in areas between widely spaced coastal HWMs, where the inundation limit had been interpolated. It was assumed that the debris line represented the minimum extent of surge-related flooding. As a result, the inundation limit was adjusted to match the debris line in these locations.
Hurricane Ivan Surge Elevation Contours:
Surge elevation contours, based upon the surveyed coastal HWM elevations, were mapped at one-foot intervals across the project area. The coastal HWM elevations were used to find patterns in Hurricane Ivan’s storm surge as it pushed against the open coast and into the inland bays. Knowing the path and landfall location of Hurricane Ivan, together with knowledge of how storm surge propagates inland, allowed surge contours to be drawn across areas where the coastal HWMs indicate a change in storm surge elevation.
Assumptions are made in some locations to allow the contours to “step” up or down at one-foot intervals. Because of the inherent uncertainty and the random and irregular spacing of coastal HWMs, the surge contours represent a generalized maximum storm surge elevation, and required professional judgment in their creation. Within certain surge contours, coastal HWMs may be higher or lower than the contours if they did not fit the overall pattern assessed from the coastal HWMs.